Substances that can prevent or delay the oxidation of substances such as proteins, lipids, carbohydrates and DNA in biological systems are called antioxidants. Antioxidants can be described as a system that protects biomolecules and the organism against the harmful effects of free radicals, reduces or repairs the damage done by ROS to the target molecule, and this is called antioxidant defense [ 11 , 73 ].

The level and composition of antioxidant defense systems differ from tissue to tissue and cell to cell. This reaction is the cellular source of H 2 O 2. SODs detoxify superoxide anions, preventing their reaction with NO and preventing the formation of peroxynite.

In mammals, there are three isoforms of superoxide dismutase found in the cytosol: copper, zinc superoxide dismutase Cu, ZnSOD or SOD1 , manganese superoxide dismutase MnSOD or SOD2 found in the mitochondrial matrix, and superoxide dismutase SOD3 found in the extracellular space [ 75 ].

The enzyme catalase, which is a hemoprotein, can detoxify hydrogen peroxide into water under high concentration hydrogen peroxide conditions. In mammals, catalases can also catalyze peroxidase-type reactions, provided that substrates have limited access to heme. It is mostly found in peroxisomes. Its activity is high in the liver, kidney, myocardium, striated muscle and erythrocytes [ 76 , 77 ].

Glutathione peroxidase is the multiple isozymes responsible for the reduction of hydrogen peroxides. There are six GPx isozymes in mammalian tissues, expressed as GPx1, 2, 3, 4, 5, and 6. GPx1, 2, 3 and 4 isozymes are selenoprotein.

The interlocking enzyme system GPx and glutathione reductase GR catalyze the reduction of H 2 O 2 by consuming glutathione. GPx enzymes catalyze the GSH-dependent reduction of fatty acid hydroperoxides other than H 2 O 2 and various synthetic hydroperoxides such as cumene and t -butyl hydroperoxides [ 78 , 79 ].

Glutathione γ-Glutamyl Cysteinyl Glycine is an intracellular antioxidant and a tripeptide found in low concentrations in the extracellular distance. In mammalian cells and tissues, GSH is involved in reactions with reactive oxygen species, electrophiles, non-enzymatic antioxidants, and protein degluthionylase.

It protects the cell against oxidative damage by creating an environment with a high redox potential inside the cell. It protects cells against oxidative damage against glutathione, hydrogen peroxide, hydroxyl radical, superoxide anion and alkoxyl radicals and prevents inactivation of proteins and enzymes by keeping the sulfhydryl groups of proteins in a reduced state [ 80 ], [ 81 ], [ 82 ].

A family of enzymes that catalyze the detoxification of low concentration hydrogen peroxide and the conjugation of GSH to a wide variety of xenobiotics.

In mammalian tissues, cytosolic, mitochondrial and microsomal GSTs are membrane-associated proteins in eicosanoid and glutathione metabolism. Some GSTs show GPx-like activity with organic hydroperoxides, previously called non-selenium glutathione peroxidase activity.

They catalyze the reaction of organic peroxides with GSH to form GSSG and alcohols [ 83 ], [ 84 ], [ 85 ]. Glutathione GSSG , which is oxidized by reactions with reactive oxygen species, is converted back into reduced form by using NADPH as a cofactor by the glutathione reductase enzyme.

This reaction is important in regulating cellular redox homeostasis and detoxification reactions of ROS [ 86 ]. In addition, thioredoxin reductase TRX , thioredoxin peroxidase PRX , which is characterized in human cells, is important in the detoxification of hydrogen peroxide.

There are studies showing that PRX acts as peroxynitrite reductase and may have functionality as a protective molecule in ROS-mediated lung injury [ 87 , 88 ]. The human diet contains a number of different compounds with antioxidant capacity [ 89 ]. Ascorbic acid, a water-soluble vitamin, neutralizes hydrogen peroxide, superoxide, and hydroxyl radicals.

The monoanion form ascorbate, which is dominant at physiological pH, scavenges thiyl, nitroxide and oxysulfide radicals. Protects lipids against oxidation by neutralizing radicals that initiate lipid peroxidation.

It reduces the tocopheroxyl radical, which is responsible for the regeneration of vitamin E, to α-tocopherol.

It prevents LDL oxidation with vitamin E [ 90 , 91 ]. α-Tocopherol, a fat-soluble vitamin, reacts with lipid peroxyl radicals.

Tocopherols and tocotrienols inhibit lipid peroxidation. Carotenoids such as β-carotene and lycopene act as antioxidants by scavenging singlet oxygen and inhibiting lipid peroxidation, preventing the formation of β-carotene peroxide radicals, protecting the cell from oxidative stress [ 94 ].

Polyphenols prevent the formation of ROS by chelating the free Fe and Cu involved in the Habern—Weiss and Fenton reactions. There is increasing evidence that polyphenols protect cells against oxidative damage, limiting the risk of various degenerative diseases associated with oxidative stress [ 95 ].

For example, Thymoquinone is the main component of the essential oil of Nigella sativa , has been reported to have many properties such as antioxidant, anti-inflammatory, antineoplastic, and antiviral [ 96 ].

The resulting unstable, reactive and unpaired valence-electron radicals have a high ability to react with biomolecules. Free radicals cause deterioration in the structure and functions of proteins, damage to the cell membrane structure by lipid peroxidation, nucleic acid base modifications and chromosomal changes, causing oxidative damage to cellular structure and components.

Antioxidants, on the other hand, are defense systems that protect biomolecules and the organism against oxidative damage and reduce the damage done by reactive oxygen species. In order to be protected from oxidative damage caused by free radicals, seasonal vegetables and fruits should be consumed regularly.

Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.

Pizzino, G, Irrera, N, Cucinotta, M, Pallio, G, Mannino, F, Arcoraci, V, et al.. Oxidative stress: harms and benefits for human health. Oxid Med Cell Longev ; Search in Google Scholar PubMed PubMed Central.

Juan, CA, Pérez de la Lastra, JM, Plou, FJ, Pérez-Lebeña, E. The chemistry of reactive oxygen species ROS revisited: outlining their role in biological macromolecules DNA, lipids and proteins and induced pathologies.

Int J Mol Sci ; Arfin, S, Jha, NK, Jha, SK, Kesari, KK, Ruokolainen, J, Roychoudhury, S, et al.. Oxidative stress in cancer cell metabolism. Antioxidants ; Hao, Y, Zhu, YJ, Zou, S, Zhou, P, Hu, YW, Zhao, QX, et al.. Metabolic syndrome and psoriasis: mechanisms and future directions. Front Immunol ; Allegra, M.

Redox systems, oxidative stress, and antioxidant defences in health and disease. Taysi, S, Tascan, AS, Ugur, MG, Demir, M. Mini Rev Med Chem ;— Search in Google Scholar PubMed. Guler, C, Toy, E, Ozturk, F, Gunes, D, Karabulut, AB, Otlu, O. Evaluation of salivary total oxidant-antioxidant status and DNA damage of children undergoing fixed orthodontic therapy.

Angle Orthod ;— Halliwell, B, Gutteridge, JMC. Free radicals in biology and medicine , 5th ed. Oxford: Oxford University Press; —2 pp. Gulcin, İ. Antioxidants and antioxidant methods: an updated overview. Arch Toxicol ;— Pacher, P, Beckman, JS, Liaudet, L.

Nitric oxide and peroxynitrite in health and disease. Physiol Rev ;— Halliwell, B. Biochemistry of oxidative stress. Biochem Soc Trans ;— Search in Google Scholar. Davies, MJ. Singlet oxygen-mediated damage to proteins and its consequences. Biochem Biophys Res Commun ;— Koppers, AJ, De Iuliis, GN, Finnie, JM, McLaughlin, EA, Aitken, RJ.

Significance of mitochondrial reactive oxygen species in the generation of oxidative stress in spermatozoa. J Clin Endocrinol Metab ;— Schneider, CD, Oliveira, AR. Oxygen free radicals and exercise: mechanisms of synthesis and adaptation to the physical training.

Rev Bras Med Esporte ;—8. Forman, HJ. Hydrogen peroxide: the good, the bad, and the ugly. In: Oxidants in biology: a question of balance. New York: Springer; —17 pp. Benchoam, D, Cuevasanta, E, Möller, MN, Alvarez, B.

Hydrogen sulfide and persulfides oxidation by biologically relevant oxidizing species. Pompella, A, Visvikis, A, Paolicchi, A, De Tata, V, Casini, AF. The changing faces of glutathione, a cellular protagonist.

Biochem Pharmacol ;— Underbakke, ES, Surmeli, NB, Smith, BC, Wynia-Smith, SL, Marletta, MA. Nitric oxide signaling. In: Compherensive inorganic chemistry II, from elements to applications. Oxford: Elsevier; —62 pp. Bruckdorfer, R. The basics about nitric oxide. Mol Aspect Med ;— Shnayder, NA, Petrova, MM, Popova, TE, Davidova, TK, Bobrova, OP, Trefilova, VV, et al..

Prospects for the personalized multimodal therapy approach to pain management via action on NO and NOS. Molecules ; Radi, R. Oxygen radicals, nitric oxide, and peroxynitrite: redox pathways in molecular medicine.

Proc Natl Acad Sci U S A ;— Slater, TF. Free-radical mechanisms in tissue injury. Biochem J ;— Garcia, V, Sessa, WC. Endothelial NOS: perspective and recent developments. Br J Pharmacol ;— Ally, A, Powell, I, Ally, MM, Chaitoff, K, Nauli, SM.

Role of neuronal nitric oxide synthase on cardiovascular functions in physiological and pathophysiological states. Nitric Oxide ;— Xue, Q, Yan, Y, Zhang, R, Xiong, H. Regulation of iNOS on immune cells and its role in diseases.

Gensert, JM, Ratan, RR. The metabolic coupling of arginine metabolism to nitric oxide generation by astrocytes. Antioxidants Redox Signal ;— Thomas, DD, Ridnour, LA, Isenberg, JS, Flores-Santana, W, Switzer, CH, Donzelli, S, et al..

The chemical biology of nitric oxide: implications in cellular signaling. Free Radic Biol Med ;— Free radical properties, source and targets, antioxidant consumption and health. Oxygen ;— Liaudet, L, Vassalli, G, Pacher, P. Role of peroxynitrite in the redox regulation of cell signal transduction pathways.

Front Biosci ;— Fujii, J, Homma, T, Osaki, T. Superoxide radicals in the execution of cell death. Hotamisligil, GS. Endoplasmic reticulum stress and the inflammatory basis of metabolic disease. Cell ;— Hajam, YA, Rani, R, Ganie, SY, Sheikh, TA, Javaid, D, Qadri, SS, et al..

Oxidative stress in human pathology and aging: molecular mechanisms and perspectives. Cells ; Fleury, C, Mignotte, B, Vayssière, JL. Mitochondrial reactive oxygen species in cell death signaling. Biochimie ;— Villamena, FA. Chemistry of reactive species. In: Moleculer basis of oxidative stress.

ch1 Search in Google Scholar. Li, X, Fang, P, Mai, J, Choi, ET, Wang, H, Yang, XF. Targeting mitochondrial reactive oxygen species as novel therapy for inflammatory diseases and cancers. J Hematol Oncol ; Zeeshan, HM, Lee, GH, Kim, HR, Chae, HJ. Endoplasmic reticulum stress and associated ROS.

Konno, T, Melo, EP, Chambers, JE, Avezov, E. Schrader, M, Fahimi, HD. Peroxisomes and oxidative stress. Biochim Biophys Acta ;— Sandalio, LM, Rodríguez-Serrano, M, Romero-Puertas, MC, del Río, LA.

Role of peroxisomes as a source of reactive oxygen species ROS signaling molecules. Subcell Biochem ;— Cho, KJ, Seo, JM, Kim, JH. Bioactive lipoxygenase metabolites stimulation of NADPH oxidases and reactive oxygen species.

Mol Cell ;—5. Phaniendra, A, Jestadi, DB, Periyasamy, L. Free radicals: properties, sources, targets, and their implication in various diseases. Indian J Clin Biochem ;— Biller, JD, Takahashi, LS. Oxidative stress and fish immune system: phagocytosis and leukocyte respiratory burst activity.

An Acad Bras Cienc ;— Niki, E, Yoshida, Y, Saito, Y, Noguchi, N. Lipid peroxidation: mechanisms, inhibition, and biological effects. Ito, F, Sono, Y, Ito, T. Measurement and clinical significance of lipid peroxidation as a biomarker of oxidative stress: oxidative stress in diabetes, atherosclerosis, and chronic inflammation.

Le, NA. Lipoprotein-associated oxidative stress. In: Reactive oxygen species in biology and human health. Betteridge, DJ. What is oxidative stress? Metabolism ;—8.

Karabulut, AB, Karadag, N, Gurocak, S, Kiran, T, Tuzcu, M, Sahin, K. Apricot attenuates oxidative stress and modulates of Bax, Bcl-2, caspases, NFκ-B, AP-1, CREB expression of rats bearing DMBA-induced liver damage and treated with a combination of radiotherapy.

Food Chem Toxicol ;— Avery, SV. Molecular targets of oxidative stress. Hougland, JL, Darling, J, Flynn, S. Protein posttranslational modification.

In: Molecular basis of oxidative stress: chemistry, mechanisms, and disease pathogenesis. ch3 Search in Google Scholar. Pisoschi, AM, Pop, A, Iordache, F, Stanca, L, Predoi, G, Serban, AI. Oxidative stress mitigation by antioxidants — an overview on their chemistry and influences on health status.

Eur J Med Chem ; Schallreuter, KU, Gibbons, NC, Zothner, C, Abou Elloof, MM, Wood, JM. Hydrogen peroxide-mediated oxidative stress disrupts calcium binding on calmodulin: more evidence for oxidative stress in vitiligo.

Biochem Biophys Res Commun ;—5. Baek, J, Lee, MG. Oxidative stress and antioxidant strategies in dermatology. Redox Rep ;—9. Hauck, AK, Huang, Y, Hertzel, AV, Bernlohr, DA. Adipose oxidative stress and protein carbonylation.

J Biol Chem ;—8. Chattopadhyay, M, Khemka, VK, Chatterjee, G, Ganguly, A, Mukhopadhyay, S, Chakrabarti, S. Enhanced ROS production and oxidative damage in subcutaneous white adipose tissue mitochondria in obese and type 2 diabetes subjects.

Mol Cell Biochem ;— Manna, P, Jain, SK. Obesity, oxidative stress, adipose tissue dysfunction, and the associated health risks: causes and therapeutic strategies. Metab Syndr Relat Disord ;— Zhou, Y, Li, H, Xia, N. The interplay between adipose tissue and vasculature: role of oxidative stress in obesity.

Front Cardiovasc Med ; Sahiner, UM, Birben, E, Erzurum, S, Sackesen, C, Kalayci, Ö. Oxidative stress in asthma: part of the puzzle. Pediatr Allergy Immunol ;— Dubois-Deruy, E, Peugnet, V, Turkieh, A, Pinet, F. Oxidative stress in cardiovascular diseases. Vona, R, Gambardella, L, Cittadini, C, Straface, E, Pietraforte, D.

Biomarkers of oxidative stress in metabolic syndrome and associated diseases. Singh, A, Kukreti, R, Saso, L, Kukreti, S. Oxidative stress: a key modulator in neurodegenerative diseases. Perrotte, M, Pincemail, J, Haddad, M, Ramassamy, C.

In: Inflammation, aging, and oxidative stress. New York: Springer International Publishing; —99 pp. Bjørklund, G, Meguid, NA, El-Bana, MA, Tinkov, AA, Saad, K, Dadar, M, et al.. Oxidative stress in autism spectrum disorder.

Mol Neurobiol ;— Mateen, S, Moin, S, Khan, AQ, Zafar, A, Fatima, N. Increased reactive oxygen species formation and oxidative stress in rheumatoid arthritis.

PLoS One ;e Zargari, F, Rahaman, MS, KazemPour, R, Hajirostamlou, M. Arsenic, oxidative stress and reproductive system. J Xenobiot ;— De Luca, MN, Colone, M, Gambioli, R, Stringaro, A, Unfer, V. Oxidative stress and male fertility: role of antioxidants and inositols. Panner Selvam, MK, Durairajanayagam, D, Sikka, SC.

Molecular interactions associated with oxidative stress-mediated male infertility: sperm and seminal plasma proteomics. Adv Exp Med Biol ;— Mohammadi, M. Oxidative stress and polycystic ovary syndrome: a brief review. Int J Prev Med ; Daenen, K, Andries, A, Mekahli, D, Van Schepdael, A, Jouret, F, Bammens, B.

Oxidative stress in chronic kidney disease. Pediatr Nephrol ;— Gyurászová, M, Gurecká, R, Bábíčková, J, Tóthová, Ľ.

Oxidative stress in the pathophysiology of kidney disease: implications for noninvasive monitoring and identification of biomarkers. Ansari, MY, Ahmad, N, Haqqi, TM. Oxidative stress and inflammation in osteoarthritis pathogenesis: role of polyphenols.

Biomed Pharmacother ; Sabharwal, SS, Schumacker, PT. Nat Rev Cancer ;— Pham-Huy, LA, He, H, Pham-Huy, C. Free radicals, antioxidants in disease and health. Int J Biomed Sci ;— This reaction occur in vivo, but the situation is complexed by the fact that superoxide anion the main source of hydrogen peroxide in vivo normally also be present [ 13 ].

Superoxide anion and hydrogen peroxide react together directly to produce the hydroxyl radical, but the rate constant for this reaction in aqueous solution is actually zero. However, if transition metal ions are present a reaction sequence is established that can proceed at a rapid rate:.

The net result of the reaction series illustrated above is known as the Haber-Weiss reaction. Although most iron and copper in the body are secluded in forms that are not available to catalyse this reaction sequence, it is still of importance as a mechanism for the formation of the hydroxyl radical in vivo.

Such conditions are found in areas of active inflammation and various pathologic situations such as stroke, septic shock, ischaemia-reperfusion injury, and it is therefore likely that hydroxyl radicals contribute to tissue damage in these settings.

Iron is released from ferritin by reducing agents including ascorbate and superoxide itself, and hydrogen peroxide can release iron from a range of haem proteins. Therefore, although the iron binding proteins effectively chelate iron and prevent any appreciable redox effects under normal physiological conditions, this protection can break down in disease states.

The role of copper is analogous to that described above for iron [ 14 — 17 ]. Nitric oxide NO. is generated in biological tissues by specific nitric oxide synthases NOSs , which metabolise arginine to citrulline with the formation of NO.

via a five electron oxidative reaction [ 18 ]. acts as an important oxidative biological signalling molecule in a large variety of diverse physiological processes, including neurotransmission, blood pressure regulation, defence mechanisms, smooth muscle relaxation and immune regulation [ 19 ].

has a half-life of only a few seconds in an aqueous environment. However, since it is soluble in both aqueous and lipid media, it readily diffuses through the cytoplasm and plasma membranes [ 20 ]. has effects on neuronal transmission as well as on synaptic plasticity in the central nervous system.

In the extracellular milieu, NO. reacts with oxygen and water to form nitrate and nitrite anions. An important route of NO. degradation is the rapid reaction with superoxide anion to form the more reactive product, peroxynitrite ONOO —. Peroxynitrite reacts with proteins to form nitrotyrosine 3-NT [ 21 ].

Immune cells, including macrophages and neutrophils, simultaneously release NO. and superoxide into phagocytic vacuoles as a means of generating peroxynitrite to kill endocytosed bacteria [ 22 ].

Other inflammatory cells can also produce reactive chemicals that can result in 3-NT formation, including the peroxidases in activated neutrophils and eosinophils.

Increased levels of NO and 3-NT have been reported in a variety of human skin diseases such as skin cancers, systemic lupus erythematosus, psoriasis, urticaria, and atopic dermatitis [ 22 ].

It consists by endogenous and exogenous factors. Then, both reactive species are produced by strictly regulated enzymes, such as nitric oxide synthase NOS , and isoforms of NADPH oxidase, or as by-products from not so well regulated sources, such as the mitochondrial electron-transport chain.

Moreover, lipid peroxidation may contribute to and amplify cellular damage resulting from generation of oxidized products, some of which are chemically reactive and covalently modify critical macromolecules [ 26 ].

Compared with free radicals, the aldehydes are relatively stable and can diffuse within or even escape from the cell and attack targets far from the site of the original event.

Some of these aldehydes have been shown to exhibit facile reactivity with various biomolecules, including proteins, DNA, and phospholipids, generating stable products at the end of a series of reactions that are thought to contribute to the pathogenesis of many diseases.

Modification of amino acids by α, β-unsaturated aldehydes occurs mainly on the nucleophilic residues Cys and, to a lesser extent, His and Lys [ 29 , 30 ]. Lipid hydroperoxides and aldehydes can also be absorbed from the diet and then excreted in urine.

It follows that measurements of hydroxy fatty acids in plasma total lipids as well as plasma or urinary MDA and HNE can be confounded by diet and should not be used as an index of whole-body lipid peroxidation unless diet is strictly controlled [ 31 ]. Furthermore, the validity of many biomarkers remains to be established.

Cells communicate with each other and respond to extracellular stimuli through biological mechanisms called cell signalling or signal transduction. Signal transduction is a process enabling information to be transmitted from the outside of a cell to various functional elements inside the cell [ 37 ].

A biochemical basis for transducing extracellular signals into an intracellular event has long been the subject of enormous interest. Being initiators, transmitters, or modifiers of cellular response, free radicals occupy a significant place in the complex system of transmitting information along the cell to the target sensor.

The effects of most extracellular signals are promoted via receptor ligation on either cell surface or cytoplasmic receptors. In a given signaling protein, oxidative attack induces either a loss of function or a gain of function or a switch to a different function.

The ability of oxidants to act as second messengers is a significant aspect of their physiological activity. The incorporation of free radicals into a complex cascade of transducing the signal to the effectors modifies and alters the order of events: numerous second messengers acquire the properties of third messengers, while intermediaries of free radical activity often function in both initiating and terminating signal transduction.

These sequential events ultimately lead to either normal cell proliferation or development of cancer inflammatory conditions, aging, and two common agerelated diseases — diabetes mellitus and atherosclerosis [ 40 — 43 ].

Some cellular signaling pathways in mammals. Under normal conditions elevated intracellular reduced potential , nuclear factor erythroid 2-related factor 2 Nrf2 is stabilized through binding to Keap-1 in the cytoplasm.

Depending upon the binding site present in the promoter region, different antioxidant genes are induced. Many hydrogen peroxide sensors and pathways are triggered converging in the regulation of transcription factors including AP-1, Nrf2, CREB, HSF1, HIF-1, TP53, NF-kB, Notch, SP1 and CREB-1, which induce the expression of a number of genes, including those required for the detoxification of oxidizing molecules and for the repair and maintenance of cellular homeostasis, controlling multiple cellular functions like cell proliferation, differentiation and apoptosis.

In addition, the family of FoxO-related transcription factors plays an important role in redox responses. Antioxidant enzymes destroy free radicals by catalysis, whereas phasedetoxifying enzymes remove potential carcinogens by converting them to harmless compounds for elimination from the body [ 45 ].

Recently, it was reported that Nrf2 provides a new therapeutic target for treatment of diabetic retinopathy and acetaminophen-induced liver injury [ 45 , 46 ].

In addition, many chronic neurodegenerative diseases i. The antioxidant responsive element ARE is a cis-acting regulatory element in promoter regions of several genes encoding phase II detoxification enzymes and antioxidant proteins [ 54 ].

The ARE plays an important role in transcriptional activation of downstream genes such as NAD P H:quinone oxidoreductase NQO1 , glutathione S-transferases GSTs , glutamate-cysteine ligase previously known as γ-glutamylcysteine synthetase , heme oxygenase-1 HO-1 , thioredoxin reductase-1 TXNRD1 , thioredoxin, and ferritin [ 55 — 59 ].

Several lines of evidence suggest that Nrf2 binds to the ARE sequence, leading to transcriptional activation of downstream genes encoding GSTs [ 61 — 64 ], glutamate-cysteine ligase [ 65 ], HO-1 [ 63 — 66 ], and thioredoxin [ 59 ].

Previously, it was demonstrated that Nrf2 is a critical transcription factor for both basal and induced levels of NQO1 expression in IMR human neuroblastoma cells [ 55 , 56 ]. In contrast to the clear evidences for a role of Nrf2 in ARE activation, the upstream signaling pathway is controversial.

For example, mitogen-activated protein kinase [ 67 ], protein kinase c [ 68 ], and phosphatidylinositol 3-kinase [ 69 — 72 ] have been suggested to play an important role in ARE activation.

Keap1 Kelch-like ECH-associated protein 1 , an adaptor subunit of Cullin 3-based E3 ubiquitin ligase, regulates Nrf2 activity. Keap1 retains multiple sensor cysteine residues that detect various stress stimuli [ 74 ]. Post-translational modifications at the level of Keap1 that prevent its interaction with Nrf2 are another mechanism leading to Nrf2 activation.

Indeed, Keap1 phosphorylation at Tyr renders the protein highly stable and its dephosphorylation induced by hydrogen peroxide results in rapid Keap1 degradation and Nrf2 activation [ 63 — 65 ].

Studies challenging the molecular basis of the Keap1-Nrf2 system functions are now critically important to improve translational studies of the system. Indeed, recent studies identified cross talk between Nrf2 and other signaling pathways, which provides new insights into the mechanisms by which the Keap1-Nrf2 system serves as a potent regulator of our health and disease [ 74 ].

Inducers of this system have been revealed in different studies [ 76 ]. The administration of mitochondria-targeted antioxidant and changes in expression profiles of Nrf2 and Nrf2-controlled genes encoding antioxidant enzymes occur together with changes in their activity in the blood leukocytes of rats in hyperoxia.

Under these conditions, the activity of superoxide dismutase and glutathione-S-transferase was found to be normal and the activity of catalase and glutathione peroxidase was found to increase.

Pretreatment with SkQ1 normalized the activity of pro-oxidant enzymes NADPH-oxidase and myeloperoxidase, which was significantly higher in hyperoxia [ 77 ].

The activation of Nrf2 can be also mediated by additional signal transduction pathways, e. Mechanisms by which the Nrf2 signaling pathway is constitutively activated in several types of cancer include 1 somatic mutations of Keap1 disrupting the binding capacity to Nrf2, 2 epigenetic silencing of Keap1 and 3 transcriptional induction of Nrf2 by oncogenes such as K-ras, B-raf or c-myc [ 87 ].

Furthermore, increased levels of hydrogen peroxide and increased Nrf2 activity in tumor cells, result in an enhanced anaerobic glycolysis and utilization of the pentose phosphate pathway activity to generate NAD P H equivalents necessary for the Trx and GSH-based antioxidative systems [ 88 , 89 ].

Since NAD P H generating enzymes are Nrf2 targets, the energy metabolism is directly connected with the redox homeostasis. In contrast, knock down of Nrf2 suppresses tumor growth, inhibits cell proliferation and promotes increased apoptosis [ 85 , 91 ].

The fact, that several cancers exhibit induced Nrf2 levels associated with enhanced tumor progression and chemotherapy resistance, whereas the lack of Nrf2 has opposite effects, Nrf2 represents a promising target for cancer therapies.

Since its discovery in , NF-kB nuclear factor kappa B transcription factor has aroused a wide interest in its unusual regulation, diverse stimuli that activate it, and its apparent involvement in a variety of human diseases, including atherosclerosis, asthma, diabetes, cancer, arthritis, AIDS, inflammatory diseases, etc.

NF-kB belongs to the REL-family of pluriprotein transcription activators. It is a regulatory protein that controls the expression of numerous inducible and tissue-specific NF-kB responsible genes and participates in the regulation of pro-inflammatory and immune cellular responses, the regulation of cell proliferation, and apoptosis [ 92 — 94 ].

The IkB-kinase complex IKK complex catalyzes phosphorylation of IkBs with the result that IkBs are targeted for degradation by the 26S proteasome thereby freeing NF-kB.

Activation of IKK occurs also by phosphorylation and is catalyzed by an IKK kinase, including TGF-β-activated kinase TAK1 or NF-kB inducing kinase-1 NIK1 , all of which can be regulated by hydrogen peroxide [ 96 ].

Alternatively, the reduced form of the dynein light chain protein LC8 binds to IkB and inhibits its phosphorylation by IKKs. Hydrogen peroxide induces dimerization of LC8 by a disulfide bond, promoting dissociation from IkB, and NF-kB activation [ 97 ].

On the other hand, inhibition of activation of NF-kB by hydrogen peroxide can be mediated by Keap1-dependent degradation of IKKβ [ 98 ]. NF-kB target genes mainly include enzymes involved in the antioxidant response such as ferritin heavy chain [ 99 ] and SOD2 [ ].

Another NF-kB target gene that contributes to both survival and innate immune functions is the HIF-1α gene, encoding the oxygen-regulated subunit of the hypoxia responsive transcription factor HIF-1 [ ].

NF-kB activation is stimulated by pro-oxidative cell status, especially by an increased presence of hydrogen peroxide. The exact signaling cascade seems to be due to the activation of MAP kinase pathway.

On the other side, the activation of NF-kB is blocked by thiol components such as N-acetyl-L-cysteine glutathione precursor and antioxidants. It has been demonstrated that a low concentration of thiol compounds in the cell, primarily glutathione as the most widespread thiol compound, plays a key role in positive regulation of NF-kB activity [ — ].

Therefore, the mechanisms that regulate and control the level of glutathione in the cell indirectly participate in regulating the expression of the genes with an NF-kB binding site in the promoter. Since NF-kB has a ubiquitous role in controlling cytokine activity and immunoregulatory genes, the inhibition of NF-kB activity by steroid hormones, antioxidants, non-steroid anti-inflammatory drugs, and protease inhibitors represents a pharmacological basis for the intervening adjuvant therapy in numerous diseases, including cancer, diabetes mellitus, AIDS, and diverse inflammatory disorders [ ].

The mechanism for activating AP-1 activator protein 1 transcription factor by free radicals is one of the best-explained mechanisms. They regulate several cellular processes, including cell proliferation, apoptosis, survival, and differentiation. AP-1, when upregulated, concentrates in the nucleus to activate gene expression [ ].

Hydrogen peroxide was shown to induce transcription of both c-Jun and c-Fos via activating the JNK, p38 MAPK and ERK signaling cascades [ ], while antioxidants like butylated hydroxyanisole BHA and pyrrolidine dithiocarbamate PDTC induce AP-1 binding activity and APdependent gene expression including glutathione S-transferase [ ].

The AP-1 activity is regulated not only at the genetic level regulation of transcription but also at both posttranscriptional and post-translational levels [ ].

The exposure of HeLa cells to hydrogen peroxide or UV radiation leads to a significant increase in DNK-binding activity of AP-1, irrespective of Fos and Jun protein synthesis. Under the conditions, AP-1 is activated by phosphorylation of specific residues of AP-1 subunits.

For example, the activation of cascade phosphorylation of MAP kinase family c-Jun N-terminal kinase [JNK] i. stress-activated protein kinase [SAPK] leads to the phosphorylation of two serine residues Ser and Ser- 73 in the Jun subunit of AP-1 to promote the activation of this subunit [ ].

Fos protein in AP-1 is also activated, by the phosphorylation of threonine residue Thr due to fos-regulatory kinase activated by p21ras protein [ ]. On the other hand, the phosphorylation of Jun protein Thr, Ser, and Ser by constitutive protein kinases, casein kinases II, and DNK-dependent protein kinase results in inhibiting the binding of AP-1 to DNK.

Dephosphorylation of threonine and serine residues of Jun protein increases the affinity of AP-1 active transcription factor for binding to DNK. This transcription factor is activated due to PKC that initiates dephosphorylation of the Jun subunit of AP-1 protein following activation of phosphatases.

P21ras itself is a signaling target of radicals generated by hydrogen peroxide and nitric oxide. Their overexpression is also responsible for the activation of PKC and dephosphorylation of serine and threonine residues in DNA binding domain of c-Jun [ , ].

Dimer complex Fos and Jun products interacts with DNA regulatory element known as AP-1 binding site or with CRE. These elements are present in the regulatory domain of AP-1 inducible genes [ ]. However, this increases the oxidative state in the cell, and resulting in a homeostatic disruption of the redox balance, cell damage, and apoptosis [ , ].

miRNAs are short strands of noncoding RNA that posttranscriptionally regulate gene expression and are being considered key elements in the pathogenesis of various disease [ ]. There are current studies indicating that the miRNAs expression can be sensitive to the presence of intracellular hydrogen peroxide levels.

Epigenetic regulation at the DNA level is an important mechanism involved in hydrogen peroxide-mediated expression changes of multiple genes, indicating that miRNA expression is very sensitive to hydrogen peroxide stimulation [ ].

For example, in smooth muscle cells, the cellular treatment with hydrogen peroxide resulted in an upregulation of microRNA [ ]. In addition, the expression of miRa in hydrogen peroxide-treated H9c2 cells cell line derived from rat heart tissue was markedly upregulated [ ].

In that context, miRNAs could be modulating intracellular pathways formed by the participation of multiple proteins. These radicals interact with redox-sensitive signaling molecules including protein tyrosine phosphatases, protein kinases and ion channels, that contain cysteine residues whose SH groups are oxidized, causing a change in their biological activity, regulating cellular processes like growth factor signaling, hypoxic signal transduction, autophagy, immune responses, and stem cell proliferation and differentiation [ , ].

All tyrosine phosphatases have a conserved amino acid domain that contains a reactive and redox-regulated cysteine, which catalyzes the hydrolysis of protein phosphotyrosine residues by the formation of a cysteinyl-phosphate intermediate, that later is hydrolyzed by an activated water molecule.

Oxidation of this residue to sulfenic acid by hydrogen peroxide renders the tyrosine phosphatases inactive. This oxidation of cysteine to sulfenic acid is reversible, while oxidation by the addition of two sulfinic acid or three sulfonic acid oxygens to the active site cysteine is irreversible.

These modifications can be further stabilized by the formation of inter or intramolecular disulfide S—S or sulfenyl—amide bonds. Protein kinase C PKC contains a cysteine rich domain susceptible to oxidation, while oxidation of Cys and Cys in the kinase domain of the nonreceptor tyrosine kinase Src results in the activation of the protein [ ].

The activity of MAP kinases ERK, c-Jun and p38 is regulated by phosphorylation cascades: MAPKs activation is induced through the phosphorylation of their threonyl and tyrosyl residues within a tripeptide motif TXY by a dual specificity kinase termed MAP kinase kinase MKK , which in turn is phosphorylated and activated by an upstream kinase called MAPK kinase kinase MAPKKK [ ].

ASK-1, a member of the MAP3K superfamily for JNK and p38, binds to reduced thioredoxin in nonstressed cells. Son et al. reported that hydrogen peroxide inactivates MKPs by oxidation of their catalytic cysteine, which leads to sustained activation of the MAPK pathway [ ]. However, Zhou et al.

found that upregulation of MKP-1 expression by hydrogen peroxide correlates with inactivation of JNK and p38 activity [ ]. Big mitogen-activated protein kinase-1 BMK-1 , also known as Erk5, is a MAPK identified in Similar to other three MAPKs, BMK-1 has been shown to be activated various extracellular stimuli such as epidermal growth factor, IL-6, and hypoxia and regulated by MAPK cascade.

As a member of MAPK family, BMK-1 has also been linked to various cellular events including proliferation, migration, and apoptosis [ ]. The activity of several complexes of the electron transport chain is modulated by post-translational modifications such as S-nitrosylation, S-glutathionylation or electrophile additions.

Complex I NADH ubiquinone oxidoreductase is modified by nitric oxide or its derivatives and glutathione [ , ]. The S-nitrosylation of complex I correlates with a significant loss of activity that is reversed by thiol reductants.

S-nitrosylation was also associated with increased superoxide production from complex I. The fact that mitochondrial superoxide formation can be regulated by S-nitrosylation of complex I may play an important role in mitochondrial redox signalling.

Complex I has two transitional states, the active A and the deactive D states, and the complex is S-nitrosylated in the D state [ ]. The A to D transition may take place during hypoxia and this might be important in the setting of ischemia—reperfusion.

Other studies have reported nitrotyrosine modification on the complex [ ]. In addition, reversible glutathionylation of complex I increases mitochondrial superoxide production [ ]. Other electron transport complexes, in particular, complex II succinate dehydrogenase and complex V ATP synthase , have been shown to be modified by reactive species.

This process is called retrograde response. Some examples are the regulation of cytosolic stress kinases, modulation of hypoxic signalling, and activation of macroautophagy [ — ]. An antioxidant substance in the cell is present at low concentrations and significantly reduces or prevents oxidation of the oxidizable substrate.

Humans have developed highly complex antioxidant systems enzymatic and non-enzymatic , which work synergistically, and together with each other to protect the cells and organ systems of the body against free radical damage Fig.

The antioxidants can endogenous or obtained exogenously as a part of a diet or as dietary supplements. Some dietary compounds that do not neutralize free radicals, but enhance endogenous activity may also be classified as antioxidants.

An ideal antioxidant should be readily absorbed and eliminate free radicals, and chelate redox metals at physiologically suitable levels. Endogenous antioxidants play a critical role in keeping optimal cellular functions and thus systemic health and well-being.

The most efficient enzymatic antioxidants contain glutathione peroxidase, catalase and superoxide dismutase. Non-enzymatic antioxidants include Vitamin E and C, thiol antioxidants glutathione, thioredoxin and lipoic acid , melatonin, carotenoids, natural flavonoids, and other compounds.

Glutathione peroxidases catalyse the oxidation of glutathione at direction of a hydroperoxide, which may be hydrogen peroxide or another species such as a lipid hydroperoxide:.

Other peroxides, including lipid hydroperoxides, can also act as substrates for these enzymes, which may hence play a role in repairing damage resulting from lipid peroxidation. There are two forms of this enzyme, one which is selenium-dependent GPx, EC1. The differences rise from the number of subunits, catalytic mechanism, and the bonding of selenium at the active centre, and glutathione metabolism is one of the most important antioxidative defense mechanisms present in the cells.

There are four different Se-dependent glutathione peroxidases present in humans, and these are known to add two electrons to reduce peroxides by forming selenole Se-OH and the antioxidant properties of these seleno-enzymes allow them to eliminate peroxides as potential substrates for the Fenton reaction.

Selenium-dependent glutathione peroxidase acts in association with tripeptide glutathione GSH , which is exist in high concentrations in cells and catalyzes the conversion of hydrogen peroxide or organic peroxide to water or alcohol while simultaneously oxidizing GSH.

Catalase EC 1. Catalase consists of four subunits, each containing a haem group and a molecule of NADPH. This enzyme is present in the peroxisome of aerobic cells and is very efficient in promoting the conversion of hydrogen peroxide to water and molecular oxygen.

Catalase has one of the highest turnover rates for all enzymes: one molecule of catalase can convert approximately 6 million molecules of hydrogen peroxide to water and oxygen each minute.

The greatest activity is present in liver and erythrocytes but some catalase is found in all tissues [ ]. Superoxide dismutase EC 1. The hydrogen peroxide is removed by catalase or glutathione peroxidase, as described above. Superoxide dismutase exists in several isoforms, which differ in the nature of active metal centre, amino acid composition, co-factors and other features.

There are three forms of SOD present in humans: cytosolic Cu, Zn-SOD, mitochondrial Mn-SOD, and extracellular-SOD. Superoxide dismutase neutralizes superoxide ions by going through successive oxidative and reductive cycles of transition metal ions at its active site.

This enzyme has two similar subunits and each of the subunit includes as the active site, a dinuclear metal cluster constituted by copper and zinc ions, and it specifically catalyzes the dismutation of the superoxide anion to oxygen and water. The mitochondrial Mn-SOD is a homotetramer with a molecular weight of 96 kDa and includes one manganese atom per subunit, and it cycles from Mn III to Mn II , and back to Mn III during the two-step dismutation of superoxide anion.

Extracellular superoxide dismutase contains copper and zinc, and is a tetrameric secretary glycoprotein having a high affinity for certain glycosaminoglycans [ ].

This is a fat-soluble vitamin existing in eight different forms [ ]. The tocopherols α, β, γ, and δ have a chromanol ring and a phytyl tail, and differ in the number and position of the methyl groups on the ring Fig. These compounds are lipid soluble and have pronounced antioxidant properties.

They react more rapidly than polyunsaturated fatty acids with peroxyl radicals and hence act to break the chain reaction of lipid peroxidation [ ].

In addition to its antioxidant role, vitamin E might also have a structural role in stabilising membranes. Vitamin E deficiency is rare in humans, although it might cause haemolysis and might contribute to the peripheral neuropathy that occurs in abetalipoproteinaemia.

In cell membranes and lipoproteins the essential antioxidant function of vitamin E is to trap peroxyl radicals and to break the chain reaction of lipid peroxidation.

α-Tocopherol is the most effective antioxidant of the tocopherols and is also the plentiful in humans. It quickly reacts with a peroxyl radical to form a relatively stable tocopheroxyl radical, with the excess charge associated with the extra electron being dispersed across the chromanol ring.

This resonance stabilised radical might subsequently react in one of several ways. α-Tocopherol might be regenerated by reaction at the aqueous interface with ascorbate or another aqueous phase chain breaking antioxidant, such as reduced glutathione or urate.

As another option, two α-tocopheroxyl radicals may combine to form a stable dimer, or the radical may be completely oxidised to form tocopherol quinone. The main function of Vitamin E is to protect against lipid peroxidation, and there is also evidence to suggest that α-tocopherol and ascorbic acid function together in a cyclic-type of process.

During the antioxidant reaction, α-tocopherol is converted to an α-tocopherol radical by the donation of a variable hydrogen to a lipid or lipid peroxyl radical, and the α-tocopherol radical may hence be reduced to the original α-tocopherol form by ascorbic acid [ ].

This is an important antioxidant and thus works in aqueous environments of the body. Furthermore, ascorbic acid can be oxidized in the extracellular environment in the presence of metal ions to dehydroascorbic acid, which is transported into the cell through the glucose transporter Fig.

Its primary antioxidant partners are Vitamin E and the carotenoids as well as working alone with the antioxidant enzymes. Vitamin C cooperating with Vitamin E to regenerate α-tocopherol from α-tocopherol radicals in membranes and lipoproteins, and also raises glutathione levels in the cell, thus it is playing an important role in protein thiol group protection against oxidation.

In cells, it is maintained in its reduced form by reaction with glutathione, which catalyzes by protein disulfide isomerase and glutaredoxins. Vitamin C is a reducing agent and can reduce and thereby neutralize, ROS such as hydrogen peroxide [ ]. The oxidation-reduction redox reaction of vitamin C, molecular forms in equilibrium.

L-dehydroascorbic acid also possesses biological activity, due to that in the body it is reduced to form ascorbic acid. The most thiol antioxidant is the tripeptide glutathione GSH , which is a multifunctional intracellular antioxidant. It is noticed to be the major thiol-disulphide redox buffer of the cell.

It is abundant in cytosol, nuclei, and mitochondria, and is the major soluble antioxidant in these cell compartments. Also, it is considered to be playing a role in cell senescence since studies involving human fibroblasts have shown that the intracellular glutathione level has an important influence on the induction of a post-mitotic phenotype, and that by implication depletion of glutathione plays a significant role in the cellular aging in human skin.

The reduced form of glutathione is GSH, glutathione, while the oxidized form is GSSG, glutathione disulphide. The antioxidant capacity of thiol compounds is due to the sulphur atom, which can easily accommodate the loss of a single electron. It has been well established that a decrease in GSH concentration may be associated with aging and the pathogenesis of many diseases, including rheumatoid arthritis, AIDS, alcoholic liver disease, cataract genesis, respiratory distress syndrome, cardiovascular disease, and Werner syndrome.

Furthermore, there is a drastic depletion in cytoplasmic concentrations of GSH within the substantia nigra of patients with Parkinson. Another thiol antioxidant is the thioredoxin TRX system; these are proteins with oxidoreductase activity and are universal in both mammalian and prokaryotic cells.

It also contains a disulphide and possesses two redox-active cysteins within a conserved active site Cys-Gly-Pro-Cys. Thioredoxin contains two adjacent —SH groups in its reduced form that are converted to a disulphide unit in oxidized TRX when it undergoes redox reactions with multiple proteins.

Thioredoxin levels are much less than GSH, however, TRX and GSH may have overlapping as well as compartmentalized functions in the activation and regulation of transcription factors. The third valuable thiol antioxidant is the natural compound α-Lipoic acid ALA , which is a sulfur-containing antioxidant with metal-chelating and antiglycation capabilities.

In contrast to many antioxidants, which are active only in lipid or aqueous phase, lipoic acid is active in both lipid and aqueous phases. Lipoic acid LA is readily digested, absorbed and is rapidly converted to Dihydrolipoic acid DHLA by NADH or NADPH in most tissues Fig.

Researches have demonstrated superior antioxidant activity of DHLA as compared to LA. Since DHLA neutralizes free radicals it is known to regenerate Vitamin C which is even better than GSH and Vitamin E from their oxidized forms.

DHLA has metal chelating properties which help the body to get rid of accumulated ingested toxins. It has been shown previously that oxidants lead to cell death via lysosomal breaking away and that this latter event may involve intralysosomal iron which catalyzes Fenton-type chemistry and resultant peroxidative damage to lysosomal membranes.

Packer et al. As an antioxidant, LA directly terminates free radicals, chelates transition metal ions e. Exogenous administration of LA has been found to have therapeutic potential in neurodegenerative disorders also. Furthermore, LA crosss the blood-brain barrier and is enclosed by all areas of the central and peripheral nervous system.

Lipid peroxides LPO are biomarkers of free radical-associated oxidative stress. Free radical attack on poly unsaturated fatty acids PUFA in the biological system is thought to produce a sequence of reactions, which lead to the formation of both conjugated dienes and lipid hydroperoxides [ ].

Hence the possible mechanisms for the protecting effects of LA against oxidative stress may be as follows: a LA can be reduced to dihydrolipoic acid by NADH, b DHLA is a potent antioxidant to scavenge excess oxidants, and recycle other antioxidants such as vitamin E, C and glutathione, c DHLA chelate metals to prevent free radical generation, thus to diminish oxidant attacks on bio-macromolecules, d LA is the key co-factor of pyruvate dehydrogenase and α-ketoglutarate dehydrogenase the enzymes sensitive to oxidative stress, e supplementation of sufficient LA can stimulate activities of enzymes, thereby promoting and ameliorating oxidative phosphorylation and mitochondrial respiration and f LA promotes the antioxidant defense by inducing phase two enzymes, such as glutathione synthetase to increase antioxidant GSH.

It reduces liver damage caused by paracetamol over dosage in human, and attenuates liver damage and prevents liver and plasma GSH depletion in mice [ ]. Furthermore, NAC is generally used for the treatment of acetaminophen-induced hepatotoxicity, although it has a drawback as it must be given within 8 h after acetaminophen intoxication, and it can also cause other side-effects including vomiting, nausea, and even shock.

Therefore, the need for alternative, more effective, and widely applicable antidotes for acetaminophen-induced liver injury is warranted [ , ].

NAC, is quickly deacetylated to cysteine and thus may increase GSH levels by providing the substrate for the rate limiting step in GSH synthesis. Structure of NAC along with possible chelating sites is presented in Fig. NAC is known to have metal-chelating properties and has been used in several clinical conditions.

Thiol groups present in NAC act to decrease free radical and provide chelating site for metals. Thus, NAC has a potent ability to renovate the impaired prooxidant antioxidant balance in metal poisoning and various diseases [ ]. Structure of N-acetyl cysteine NAC depicting 1 two chelating sites thiol and hydroxyl and 2 deacetylation responsible for its antioxidant potential due to the generation of glutathione.

Melatonin N-acetylmethoxytryptamine is a chief secretory product of the pineal gland in the brain which is well known for its functional versatility.

In hundreds of investigations, melatonin has been documented as a direct free radical scavenger and an indirect antioxidant, as well as an important immunomodulatory agent Fig.

Furthermore, melatonin stimulates a number of antioxidative enzymes including superoxide dismutase, glutathione peroxidase, glutathione reductase, and catalase. Additionally, melatonin experimentally enhances intracellular glutathione another important antioxidant levels by stimulating the rate-limiting enzyme in its synthesis, gamma-glutamylcysteine synthase.

Melatonin also inhibits the pro-oxidant enzymes such as nitric oxide synthase, xanthine oxidase and lipoxygenase. Finally, there is evidence that melatonin stabilizes cellular membranes, thereby probably helping them resist oxidative damage.

Most recently, melatonin has been shown to increase the efficiency of the electron transport chain and, as a consequence, to reduce electron leakage and the generation of free radicals.

These multiple actions make melatonin a potentially useful agent in the treatment of neurological disorders that have oxidative damage as part of their etiological basis [ ].

The carotenoids are a group of lipid soluble antioxidants which based around an isoprenoid carbon skeleton. The most important of these is α-carotene, although these present in membranes and lipoproteins. They are particularly efficient scavengers of singlet oxygen, but can also trap peroxyl radicals at low oxygen pressure with an efficiency at least as major as that of α-tocopherol.

Because these conditions prevail in many biological tissues, the carotenoids play a role in preventing in vivo lipid peroxidation. Carotenoids with bring are characterized with pro-vitamin A activity; the highest activity is represented by β-carotene because it contains two brings on both ends of the carbon chain Fig.

Vitamin A also has antioxidant properties, which do not, but, show any dependency on oxygen concentration [ ]. These are a broad class of low molecular ubiquitous groups of plant metabolites and are an integral part of the human diet.

Flavonoids are benzo-γ-pyrone derivatives consisting of phenolic and pyrane rings and during metabolism hydroxyl groups are added, methylated, sulfated or glucuronidated Fig. There is intense interest in flavonoids due to their antioxidant and chelating properties and their possible role in the prevention of chronic diseases.

Flavonoids are present in food mainly as glycosides and polymers and these comprise a substantial fraction of dietary flavonoids.

The biological properties of flavonoids are determined by the extent, nature, and position of the substituents and the number of hydroxyl groups. These factors also determine whether a flavonoid acts as an antioxidant or as a modulator of enzyme activity, or whether it has antimutagenic or cytotoxic properties.

Thus flavonoids can scavenge peroxyl radicals, and are effective inhibitors of lipid peroxidation, and can chelate redox-active metals, and therefore prevent catalytic breakdown of hydrogen peroxide Fenton chemistry. On the other hand, under certain conditions, flavonoids can also display pro-oxidant activity and this is thought to be directly proportional to the total number of hydroxyl groups, and they have also been reported to modulate cell signaling [ ].

The last 60 years have been characterized by the understanding of the impact of nutrition and dietary patterns on health. An important part of the population is exposed to the risk of trace element and vitamin deficiency for multiple reasons e.

Children, young women and elders are the most exposed. Antioxidants balances the cell-damaging effects of free radicals. Furthermore, people eat fruits and vegetables, which happen to be good sources of antioxidants, have a lower risk for various diseases such as heart, neurological diseases and there is evidence that some types of vegetables and fruits in general, protect against a number of cancers [ — ].

However, this hypothesis has now been tested in many clinical trials and does not seem to be true, since antioxidant supplements have no clear effect on the risk of chronic diseases such as cancer, diabetes mellitus and heart disease.

Under such conditions supplementation with exogenous antioxidants is required to provide the redox homeostasis in cells. Therefore, antioxidant supplementation has become an increasingly popular practice to maintain optimal body function [ — ]. Much debate has arisen about whether antioxidant supplementation alters the efficacy of cancer chemotherapy.

Others suggest antioxidants may mitigate toxicity and thus allow for uninterrupted treatment schedules and a reduced need for lowering chemotherapy doses. Drugs with free radical mechanisms include but are not limited to alkylating agents e.

topotecan, irinotecan [ ]. None of the trials reported evidence of significant decreases in efficacy from antioxidant supplementation during chemotherapy. Many of the studies indicated that antioxidant supplementation resulted in either increased survival times, increased tumor responses, or both, as well as fewer toxicities than controls [ ].

Studies are now attempting to develop new antioxidants either of natural or synthetic origin. The use of biomarkers provides a logical scientific basis for major intervention trials of antioxidants; such trials could, in turn, eventually validate or disprove the biomarker concept.

Any intervention trial that does take place should be accompanied by measurements of one or more relevant biomarkers at intervals during the study. If the endpoint of the trial is disease incidence or mortality, such studies could help to validate or disprove the biomarker concept.

On the contrary, free radical-mediated lipid peroxidation proceeds randomly without specificity. Lipid peroxidation can neither be programmed nor regulated.

Furthermore, some negative effects of antioxidants when used in dietary supplements ascorbic acid, flavanoids, carotenoids, α-lipoic acid and synthetic compounds have came out in the last few decades [ ].

For example, Ascorbic acid has both antioxidant and pro-oxidant effects, depending upon the dose [ ]. Low electron potential and resonance stability of ascorbate and the ascorbyl radical have enabled ascorbic acid to enjoy the privilege as an antioxidant [ , ].

In ascorbic acid alone treated rats, ascorbic acid has been found to act as a CYP inhibitor. Similar activity has also been observed for other antioxidants-quercetin and chitosan oligosaccahrides [ ], which may act as potential CYP inhibitors.

Specifically, Phase I genes of xenobiotic biotransformation, namely, CYP1A1, CYP2E1, and CYP2C29, have been previously reported to be downregulated in female rats in the presence of a well known antioxidant, resveratrol [ ]. At higher oxygen tension, carotenoids tend to lose their effectiveness as antioxidants.

In a turn around to this, the pro-oxidant effect of low levels of tocopherol is evident at low oxygen tension [ ]. Moreover, α-lipoic acid exerts a protective effect on the kidney of diabetic rats but a prooxidant effect in nondiabetic animals [ ].

The pro-oxidant effects have been attributed to dehydroxylipoic acid DHLA , the reduced metabolite of α-lipoic acid owing to its ability to reduce iron, initiate reactive sulfur-containing radicals, and thus damage proteins such as alpha 1-antiproteinase and creatine kinase playing a role in renal homeostasis [ ].

An increase in α-lipoic acid and DHLA-induced mitochondrial and submitochondrial production in rat liver and NADPH-induced and expression of p47phox in the nondiabetic kidney has also been observed [ ].

Depending on the type and level of ROS and RNS, duration of exposure, antioxidant status of tissues, exposure to free radicals and their metabolites leads to different responses—increased proliferation, interrupted cell cycle, apoptosis, or necrosis [ ].

There has been ever increasing knowledge in the role of oxygen derived pro-oxidants and antioxidants that play crucial role in both normal metabolism and several clinical disease states. Antioxidants exhibit pro-oxidant activity depending on the specific set of conditions. Furthermore, while antioxidants may have reduced free-radical damage to normal tissues leading to diminished toxicity, the non-oxidative cytotoxic mechanisms of the drugs may remain unaffected by antioxidant supplementation.

Further, the significant reductions in toxicity may alleviate dose-limiting toxicities to such an extent that more patients successfully complete prescribed regimens. Advances in the field of biochemistry including enzymology have led to the use of various enzymes as well as endogenous and exogenous antioxidants having low molecular weight that can inhibit the harmful effect of oxidants.

Kurutas EB, Ciragil P, Gul M, Kilinc M. The effects of oxidative stress in urinary tract infection. Mediators Inflamm. Article PubMed CAS Google Scholar. Valko M, Leibfritz D, Moncol J, Cronin MT, Mazur M, Telser J. Free radicals and antioxidants in normal physiological functions and human disease.

Int J Biochem Cell Biol. Article CAS PubMed Google Scholar. Barbacanne MA, Souchard JP, Darblade B, Iliou JP, Nepveu F, Pipy B, Bayard F, Arnal JF.

Detection of superoxide anion released extracellularly by endothelial cells using cytochrome c reduction, ESR, fluorescence and lucigenin-enhanced chemiluminescence techniques. Free Radic Biol Med. Vidrio E, Jung H, Anastasio C. Generation of Hydroxyl Radicals from Dissolved Transition Metals in Surrogate Lung Fluid Solutions.

Atmos Environ. Article CAS PubMed Central Google Scholar. Murphy E, Steenbergen C. Mechanisms underlying acute protection from cardiac ischemia-reperfusion injury. Physiol Rev. Article CAS PubMed PubMed Central Google Scholar. Bortoletto P, Lyman K, Camacho A, Fricchione M, Khanolkar A, Katz BZ.

Chronic granulomatous disease. Pediatr Infect Dis J. Article PubMed PubMed Central Google Scholar. Tsao PS, Heidary S, Wang A, Chan JR, Reaven GM, Cooke JP.

Protein kinase C-epsilon mediates glucose-induced superoxide production and MCP-1 expression in endothelial cells. FASEB J. Google Scholar. Masters CJ. Cellular signalling: the role of the peroxisome.

Cell Signal. Cederbaum AI. Molecular mechanisms of the microsomal mixed function oxidases and biological and pathological implications. Redox Biol. Dröge W. Free radicals in the physiological control of cell function. Article PubMed Google Scholar.

Valko M, Rhodes CJ, Moncol J, Izakovic MM, Mazura M. Free radicals, metals and antioxidants in oxidative stress-induced cancer. Chem Biol Interact. Tatsuzawa H, Maruyama T, Hori K, Sano Y, Nakano M. Singlet oxygen 1 Delta g O 2 as the principal oxidant in myeloperoxidase-mediated bacterial killing in neutrophil phagosomes.

Biochim Biophys Res Commun. Article CAS Google Scholar. Lloyd RV, Hanna PM, Mason RP. The origin of the hydroxyl radical oxygen in the Fenton reaction. Stohs SJ, Bagchi D.

Oxidative mechanisms in the toxicity of metal ions. Pepper JR, Mumby S, Gutteridge JC. Sequential oxidative damage, and changes in iron binding and iron oxidizing plasma antioxidants during cardiopulmonary bypass surgery. Free Radic Res. Jomova K, Valko M. Importance of iron chelation in free radical-induced oxidative stress and human disease.

Curr Pharm Des. Saito H, Hayashi H. Transformation rate between ferritin and hemosiderin assayed by serum ferritin kinetics in patients with normal iron stores and iron overload.

Nagoya J Med Sci. PubMed PubMed Central Google Scholar. Ghafourifar P, Cadenas E. Mitochondrial nitric oxide synthase. Trends Pharmacol Sci. Bergendi L, Benes L, Durackova Z, Ferencik M. Chemistry, physiology and pathology of free radicals. Life Sci.

Chiueh CC. Neuroprotective properties of nitric oxide. Ann NY Acad Sci. Beckman JS, Koppenol WH. Nitric oxide, superoxide, and peroxynitrite: the good, the bad, and ugly. Am J Physiol. CAS PubMed Google Scholar.

Pacher P, Beckman JS, Liaudet L. Nitric oxide and peroxynitrite in health and disease. Young IS, Woodside JV. Antioxidants in health and disease. J Clin Pathol. Flora SJ. Structural, chemical and biological aspects of antioxidants for strategies against metal and metalloid exposure.

Oxid Med Cell Longev. Finkel T, Holbrook NJ. Oxidants, oxidative stress and the biology of ageing. Uchida K. Prog Lipid Res. Carini M, Aldini G, Facino RM.

Mass spectrometry for detection of 4-hydroxy- trans onenal HNE adducts with peptides and proteins. Mass Spectrom Rev. Cracowski JL, Durand T, Bessard G. Isoprostanes as a biomarker of lipid peroxidation in humans: physiology, pharmacology and clinical implications.

Montuschi P, Barnes PJ, Roberts LJ. Isoprostanes: markers and mediators of oxidative stress. Histidine and lysine as targets of oxidative modification. Amino Acids. Draper HH, Csallany AS, Hadley M. Urinary aldehydes as indicators of lipid peroxidation in vivo.

Wilson R, Lyall K, Smyth L, Fernie CE, Riemersma RA. Basu S. Isoprostanes: novel bioactive products of lipid peroxidation. Miraloglu M, Kurutas EB, Ozturk P, Arican O.

Evaluation of local trace element status and 8-Iso-prostaglandin F2α concentrations in patients with Tinea pedis. Biol Proced Online. Morrow JD. Quantification of isoprostanes as indices of oxidant stress and the risk of atherosclerosis in humans.

Arterioscler Thromb Vasc Biol. Moret-Tatay I, Iborra M, Cerrillo E, Tortosa L, Nos P, Beltrán B. Poli G, Leonarduzzi G, Biasi F, Chiarpotto E.

Oxidative stress and cell signalling. Curr Med Chem. Eizirik DL, Manndrup-Poulsen T. A chice of death - the signaltransduction of immuneimediated beta-cell apoptosis. Buttke TM, Sandstrom PA. Oxidative stress as mediator of apoptosis. Immunol Today. Article Google Scholar.

Niki E. Klaunig JE, Kamendulis LM, Hocevar BA. Oxidative stress and oxidative damage in carcinogenesis. Toxicol Pathol. Pavlovic D, Kocic R, Kocic G, Jevtovic T, Radenkovic S, Mikic D, Stojanovic M, Djordjevic PB.

Effect of a four week metformin treatment on plasma and erythrocyte antioxidative defense enzymes in newly diagnosed obese NIDDM patientes.

Diabetes Obes Metab. Diaz MN, Frei B, Vita JA, Keaney JF. Antioxidants and atherosclerotic heart disease. N Engl J Med. Marinho HS, Real C, Cyrne L, Soares H, Antunes F. Hydrogen peroxide sensing, signaling and transcription factors regulation. Nabavi SF, Barber AJ, Spagnuolo C, Russo GL, Daglia M, Nabavi SM, Sobarzo-Sánchez E.

Nrf2 as molecular target for polyphenols: A novel therapeutic strategy in diabetic retinopathy. Crit Rev Clin Lab Sci. Pang C, Sheng Y, Jiang P, Wei H, Ji L.

Chlorogenic acid prevents acetaminophen-induced liver injury: the involvement of CYP metabolic enzymes and some antioxidant signals.

J Zhejiang Univ Sci B. Prasad KN. Simultaneous Activation of Nrf2 and Elevation of Dietary and Endogenous Antioxidant Chemicals for Cancer Prevention in Humans. J Am Coll Nutr. Chan K, Han XD, Kan YW. An important function of Nrf2 in combating oxidative stress: detoxification of acetaminophen.

Proc Natl Acad Sci U S A. High sensitivity of Nrf2 knockout mice to acetaminophen hepatotoxicity associated with decreased expression of ARE-regulated drug metabolizing enzymes and antioxidant genes.

Toxicol Sci. Chan K, Kan YW. Nrf2 is essential for protection against acute pulmonary injury in mice. Fahey JW, Haristoy X, Dolan PM, Kensler TW, Scholtus I, Stephenson KK, Talalay P, Lozniewski A.

Sulforaphane inhibits extracellular, intracellular, and antibiotic-resistant strains of Helicobacter pylori and prevents benzo[a]pyrene-induced stomach tumors. Ramos-Gomez M, Kwak MK, Dolan PM, Itoh K, Yamamoto M, Talalay P, Kensler TW. Sensitivity to carcinogenesis is increased and chemoprotective efficacy of enzyme inducers is lost in nrf2 transcription factor-deficient mice.

Braun S, Hanselmann C, Gassmann MG, Auf Dem Keller U, Born-Berclaz C, Chan K, Kan YW, Werner S. Nrf2 transcription factor, a novel target of keratinocyte growth factor action which regulates gene expression and inflammation in the healing skin wound.

Mol Cell Biol. Dhakshinamoorthy S, Long DJ, Jaiswal AK. Antioxidant regulation of genes encoding enzymes that detoxify xenobiotics and carcinogens.

Curr Top Cell Regul. Lee JM, Moehlenkamp JD, Hanson JM, Johnson JA. Nrf2-dependent activation of the antioxidant responsive element by tert-butylhydroquinone is independent of oxidative stress in IMR human neuroblastoma cells. Biochem Biophys Res Commun. Lee JM, Hanson JM, Chu WA, Johnson JA. Phosphatidylinositol 3-kinase, not extracellular signal-regulated kinase, regulates activation of the antioxidant-responsive element in IMR human neuroblastoma cells.

J Biol Chem. Alam J, Wicks C, Stewart D, Gong P, Touchard C, Otterbein S, Choi AM, Burow ME, Tou J. Mechanism of heme oxygenase-1 gene activation by cadmium in MCF-7 mammary epithelial cells.

Role of p38 kinase and Nrf2 transcription factor. Kim YC, Masutani H, Yamaguchi Y, Itoh K, Yamamoto M, Yodoi J. Hemin-induced activation of the thioredoxin gene by Nrf2. A differential regulation of the antioxidant responsive element by a switch of its binding factors.

Orino K, Lehman L, Tsuji Y, Ayaki H, Torti SV, Torti FM. Ferritin and the response to oxidative stress. Biochem J. Integration and diversity of the regulatory network composed of Maf and CNC families of transcription factors. Chanas SA, Jiang Q, McMahon M, McWalter GK, McLellan LI, Elcombe CR, Henderson CJ, Wolf CR, Moffat GJ, Itoh K, Yamamoto M, Hayes JD.

Loss of the Nrf2 transcription factor causes a marked reduction in constitutive and inducible expression of the glutathione S-transferase Gsta1, Gsta2, Gstm1, Gstm2, Gstm3 and Gstm4 genes in the livers of male and female mice. Hayes JD, Chanas SA, Henderson CJ, McMahon M, Sun C, Moffat GJ, Wolf CR, Yamamoto M.

The Nrf2 transcription factor contributes both to the basal expression of glutathione S-transferases in mouse liver and to their induction by the chemopreventive synthetic antioxidants, butylated hydroxyanisole and ethoxyquin.

Biochem Soc Trans. Ishii T, Itoh K, Takahashi S, Sato H, Yanagawa T, Katoh Y, Bannai S, Yamamoto M. Transcription factor Nrf2 coordinately regulates a group of oxidative stress-inducible genes in macrophages.

Nguyen T, Huang HC, Pickett CB. Transcriptional regulation of the antioxidant response element. Activation by Nrf2 and repression by MafK. Wild AC, Moinova HR, Mulcahy RT. Regulation of gamma-glutamylcysteine synthetase subunit gene expression by the transcription factor Nrf2.

Alam J, Stewart D, Touchard C, Boinapally S, Choi AM, Cook JL. Yu R, Lei W, Mandlekar S, Weber MJ, Der CJ, Wu J, Kong AN. Role of a mitogen-activated protein kinase pathway in the induction of phase II detoxifying enzymes by chemicals. Huang HC, Nguyen T, Pickett CB. Regulation of the antioxidant response element by protein kinase C-mediated phosphorylation of NF-E2-related factor 2.

Li J, Lee JM, Johnson JA. Microarray analysis reveals an antioxidant responsive element-driven gene set involved in conferring protection from an oxidative stress-induced apoptosis in IMR cells. Li J, Johnson JA. Time-dependent changes in ARE-driven gene expression by use of a noise-filtering process for microarray data.

Physiol Genomics. Johnson DA, Andrews GK, Xu W, Johnson JA. Activation of the antioxidant response element in primary cortical neuronal cultures derived from transgenic reporter mice. J Neurochem.

Kang KW, Cho MK, Lee CH, Kim SG. Activation of phosphatidylinositol 3-kinase and Akt by tert-butylhydroquinone is responsible for antioxidant response element-mediated rGSTA2 induction in H4IIE cells. Mol Pharmacol. Lee JM, Johnson JA. An important role of Nrf2-ARE pathway in the cellular defense mechanism.

J Biochem Mol Biol. Suzuki T, Yamamoto M. Molecular basis of the Keap1-Nrf2 system. Niture SK, Khatri R, Jaiswal AK. Regulation of Nrf2 — an update.

Forman HJ, Davies KJA, Ursini F. How do nutritional antioxidants really work: nucleophilic tone and para-hormesis free radical scavenging in vivo. Vnukov VV, Gutsenko OI, Milutina NP, Ananyan AA, Danilenko AO, Panina SB, Kornienko IV.

Influence of SkQ1 on expression of Nrf2 transcription factor gene, ARE-controlled genes of antioxidant enzymes, and their activity in rat blood leukocytes.

Biochemistry Mosc. Itoh K, Mimura J, Yamamoto M. Discovery of the negative regulator of Nrf2, Keap1: a historical overview. Antioxid Redox Signal. Kensler TW, Wakabayashi N. Nrf2: friend or foe for chemoprevention? Solis LM, Behrens C, Dong W, Suraokar M, Ozburn NC, Moran CA, Corvalan AH, Biswal S, Swisher SG, Bekele BN, Minna JD, Stewart DJ, Wistuba II.

Nrf2 and Keap1 abnormalities in non-small cell lung carcinoma and association with clinicopathologic features. Clin Cancer Res. Singh A, Misra V, Thimmulappa RK, Lee H, Ames S, Hoque MO, Herman JG, Baylin SB, Sidransky D, Gabrielson E, Brock MV, Biswal S.

Dysfunctional KEAP1-NRF2 interaction in non-small-cell lung cancer. PLoS Med.