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Oxidative stress

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Oxidative stress is caused by an imbalance between the production of reactive oxygen and a biological system's ability to readily detoxify the reactive intermediates or easily repair the resulting damage. All forms of life maintain a reducing environment within their cells. This reducing environment is preserved by enzymes that maintain the reduced state through a constant input of metabolic energy. Disturbances in this normal redox state can cause toxic effects through the production of peroxides and free radicals that damage all components of the cell, including proteins, lipids, and DNA.

In humans, oxidative stress is involved in many diseases, such as atherosclerosis, Parkinson's disease and Alzheimer's disease, but it may also be important in prevention of ageing by induction of a process named mitohormesis. Reactive oxygen species can be beneficial, as they are used by the immune system as a way to attack and kill pathogens. Reactive oxygen species are also used in cell signaling. This is dubbed redox signaling.

Chemical and biological effects

In chemical terms, oxidative stress is a large increase (becoming less negative) in the cellular reduction potential, or a large decrease in the reducing capacity of the cellular redox couples, such as glutathione.[1] The effects of oxidative stress depend upon the size of these changes, with a cell being able to overcome small perturbations and regain its original state. However, more severe oxidative stress can cause cell death and even moderate oxidation can trigger apoptosis, while more intense stresses may cause necrosis.[2]

A particularly destructive aspect of oxidative stress is the production of reactive oxygen species, which include free radicals and peroxides. Some of the less reactive of these species (such as superoxide) can be converted by oxidoreduction reactions with transition metals or other redox cycling compounds including quinones into more aggressive radical species that can cause extensive cellular damage.[3] Most of these oxygen-derived species are produced at a low level by normal aerobic metabolism and the damage they cause to cells is constantly repaired. However, under the severe levels of oxidative stress that cause necrosis, the damage causes ATP depletion, preventing controlled apoptotic death and causing the cell to simply fall apart.[4][5]

Oxidant Description
•O2-, superoxide anion One-electron reduction state of O2, formed in many autoxidation reactions and by the electron transport chain. Rather unreactive but can release Fe2+ from iron-sulphur proteins and ferritin. Undergoes dismutation to form H2O2 spontaneously or by enzymatic catalysis and is a precursor for metal-catalyzed •OH formation.
H2O2, hydrogen peroxide Two-electron reduction state, formed by dismutation of •O2- or by direct reduction of O2. Lipid soluble and thus able to diffuse across membranes.
•OH, hydroxyl radical Three-electron reduction state, formed by Fenton reaction and decomposition of peroxynitrite. Extremely reactive, will attack most cellular components
ROOH, organic hydroperoxide Formed by radical reactions with cellular components such as lipids and nucleobases.
RO•, alkoxy and ROO•, peroxy radicals Oxygen centred organic radicals. Lipid forms participate in lipid peroxidation reactions. Produced in the presence of oxygen by radical addition to double bonds or hydrogen abstraction.
HOCl, hypochlorous acid Formed from H2O2 by myeloperoxidase. Lipid soluble and highly reactive. Will readily oxidize protein constituents, including thiol groups, amino groups and methionine.
OONO-, peroxynitrite Formed in a rapid reaction between •O2- and NO•. Lipid soluble and similar in reactivity to hypochlorous acid. Protonation forms peroxynitrous acid, which can undergo homolytic cleavage to form hydroxyl radical and nitrogen dioxide.

Production and consumption of oxidants

The most important source of reactive oxygen under normal conditions in aerobic organisms is probably the leakage of activated oxygen from mitochondria during normal oxidative respiration.

Other enzymes capable of producing superoxide are xanthine oxidase, NADPH oxidases and cytochromes P450. Hydrogen peroxide is produced by a wide variety of enzymes including monoxygenases and oxidases. Reactive oxygen species play important roles in cell signalling, a process termed redox signaling. Thus, to maintain proper cellular homeostasis, a balance must be struck between reactive oxygen production and consumption.

The best studied cellular antioxidants are the enzymes superoxide dismutase (SOD), catalase, and glutathione peroxidase. Less well studied (but probably just as important) enzymatic antioxidants are the peroxiredoxins and the recently discovered sulfiredoxin. Other enzymes that have antioxidant properties (though this is not their primary role) include paraoxonase, glutathione-S transferases, and aldehyde dehydrogenases.

Oxidative stress contributes to tissue injury following irradiation and hyperoxia. It is suspected (though not proven) to be important in neurodegenerative diseases including Lou Gehrig's disease (aka MND or ALS), Parkinson's disease, Alzheimer's disease, and Huntington's disease. Oxidative stress is thought to be linked to certain cardiovascular disease, since oxidation of LDL in the vascular endothelium is a precursor to plaque formation. Oxidative stress also plays a role in the ischemic cascade due to oxygen reperfusion injury following hypoxia. This cascade includes both strokes and heart attacks.

Antioxidants as supplements

The use of antioxidants to prevent disease is controversial.[9] In a high-risk group like smokers, high doses of beta carotene increased the rate of lung cancer.[10] In less high-risk groups, the use of vitamin E appears to reduce the risk of heart disease.[11] In other diseases, such as Alzheimer's, the evidence on vitamin E supplementation is mixed.[12][13] However, AstraZeneca's radical scavenging nitrone drug NXY-059 shows some efficacy in the treatment of stroke.[14]

Oxidative stress (as formulated in Harman's free radical theory of aging) is also thought to contribute to the aging process. Various human clinical trials have studied the anti-aging properties of antioxidants. One study of hyperbaric, oxygen-related cell damage, illustrates that an orally-effective form of superoxide dismutase known as glisodin may help protect against DNA damage.[15] There is also good evidence to support this idea in model organisms such as Drosophila melanogaster and Caenorhabditis elegans,[16][17] recent evidence from Michael Ristow's laboratory suggests that oxidative stress may also promote life expectancy of Caenorhabditis elegans by inducing a secondary response to initially increased levels of reactive oxygen species.[18] This process was previously named mitohormesis or mitochondrial hormesis on a purely hypothetical basis.[19]. The situation in mammals is even less clear.[20][21][22] Recent epidemiological findings support the process of mitohormesis, and even suggest that antioxidants may increase disease prevalence in humans.[23]

Metal catalysts

Metals such as iron, copper, chromium, vanadium and cobalt are capable of redox cycling in which a single electron may be accepted or donated by the metal. This action catalyzes reactions that produce reactive radicals and can produce reactive oxygen species. The most important reactions are probably Fenton's reaction and the Haber-Weiss reaction, in which hydroxyl radical is produced from reduced iron and hydrogen peroxide. The hydroxyl radical then can lead to modifications of amino acids (e.g. meta-tyrosine and ortho-tyrosine formation from phenylalanine), carbohydrates, initiate lipid peroxidation, and oxidize nucleobases. Most enzymes that produce reactive oxygen species contain one of these metals. The presence of such metals in biological systems in an uncomplexed form (not in a protein or other protective metal complex) can significantly increase the level of oxidative stress. In humans, hemochromatosis is associated with increased tissue iron levels, Wilson's disease with increased tissue levels of copper. and chronic manganism with exposure to manganese ores.

Non-metal redox catalysts

Certain organic compounds in addition to metal redox catalyts can also produce reactive oxygen species. One of the most important classes of these are the quinones. Quinones can redox cycle with their conjugate semiquinones and hydroquinones, in some cases catalyzing the production of superoxide from dioxygen or hydrogen peroxide from superoxide. Oxidative stress generated by the reducing agent uric acid may be involved in the Lesch-Nyhan syndrome, stroke, and metabolic syndrome. Likewise, production of reactive oxygen species in the presence of homocysteine may figure in homocystinuria, as well as atherosclerosis, stroke, and Alzheimers.

Immune defence

The immune system uses the lethal effects of oxidants by making production of oxidizing species a central part of its mechanism of killing pathogens; with activated phagocytes producing both ROS and reactive nitrogen species. These include superoxide (•O2-), nitric oxide (•NO) and their particularly reactive product, peroxynitrite (OONO-).[24] Although the use of these highly reactive compounds in the cytotoxic response of phagocytes causes damage to host tissues, the non-specificity of these oxidants is an advantage since they will damage almost every part of their target cell.[8] This prevents a pathogen from escaping this part of immune response by mutation of a single molecular target.

See also

References

Footnotes

  1. ^ Schafer F, Buettner G (2001). "Redox environment of the cell as viewed through the redox state of the glutathione disulfide/glutathione couple". Free Radic Biol Med. 30 (11): 1191–212. PMID 11368918.
  2. ^ Lennon S, Martin S, Cotter T (1991). "Dose-dependent induction of apoptosis in human tumour cell lines by widely diverging stimuli". Cell Prolif. 24 (2): 203–14. PMID 2009322.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  3. ^ Valko M, Morris H, Cronin M (2005). "Metals, toxicity and oxidative stress". Curr Med Chem. 12 (10): 1161–208. PMID 15892631.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  4. ^ Lelli J, Becks L, Dabrowska M, Hinshaw D (1998). "ATP converts necrosis to apoptosis in oxidant-injured endothelial cells". Free Radic Biol Med. 25 (6): 694–702. PMID 9801070.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  5. ^ Lee Y, Shacter E (1999). "Oxidative stress inhibits apoptosis in human lymphoma cells". J Biol Chem. 274 (28): 19792–8. PMID 10391922.
  6. ^ Sies, H. Oxidative stress: introductory remarks. In Oxidative Stress, H. Sies, (Ed.) London: Academic Press Inc,(1985) pp. 1-7.
  7. ^ Docampo, R. . Antioxidant mechanisms. In Biochemistry and Molecular Biology of Parasites, J. Marr and M. Müller, (Eds.) London: Academic Press, (1995) pp. 147-160
  8. ^ a b Rice-Evans C, Gopinathan V. "Oxygen toxicity, free radicals and antioxidants in human disease: biochemical implications in atherosclerosis and the problems of premature neonates". Essays Biochem. 29: 39–63. PMID 9189713.
  9. ^ Meyers D, Maloley P, Weeks D (1996). "Safety of antioxidant vitamins". Arch Intern Med. 156 (9): 925–35. PMID 8624173.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  10. ^ Ruano-Ravina A, Figueiras A, Freire-Garabal M, Barros-Dios J (2006). "Antioxidant vitamins and risk of lung cancer". Curr Pharm Des. 12 (5): 599–613. PMID 16472151.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  11. ^ Pryor W (2000). "Vitamin E and heart disease: basic science to clinical intervention trials". Free Radic Biol Med. 28 (1): 141–64. PMID 10656300.
  12. ^ Boothby L, Doering P (2005). "Vitamin C and vitamin E for Alzheimer's disease". Ann Pharmacother. 39 (12): 2073–80. PMID 16227450.
  13. ^ Kontush K, Schekatolina S (2004). "Vitamin E in neurodegenerative disorders: Alzheimer's disease". Ann N Y Acad Sci. 1031: 249–62. PMID 15753151.
  14. ^ Fong J, Rhoney D (2006). "NXY-059: review of neuroprotective potential for acute stroke". Ann Pharmacother. 40 (3): 461–71. PMID 16507608.
  15. ^ C. Muth, Y. Glenz, M. Klaus, P. Radermacher, Guenter Speit, X. Leverve (2004). "Influence of an orally effective SOD on hyperbaric, oxygen related cell damage". Free Radical Research. 38 (9): 927–932. PMID: 15621710.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  16. ^ Larsen P (1993). "Aging and resistance to oxidative damage in Caenorhabditis elegans". Proc Natl Acad Sci U S A. 90 (19): 8905–9. PMID 8415630.
  17. ^ Helfand S, Rogina B. "Genetics of aging in the fruit fly, Drosophila melanogaster". Annu Rev Genet. 37: 329–48. PMID 14616064.
  18. ^ Publication demonstrating that oxidative stress is promoting life span
  19. ^ Publication that first used the term mitohormesis
  20. ^ Sohal R, Mockett R, Orr W (2002). "Mechanisms of aging: an appraisal of the oxidative stress hypothesis". Free Radic Biol Med. 33 (5): 575–86. PMID 12208343.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  21. ^ Sohal R (2002). "Role of oxidative stress and protein oxidation in the aging process". Free Radic Biol Med. 33 (1): 37–44. PMID 12086680.
  22. ^ Rattan S (2006). "Theories of biological aging: genes, proteins, and free radicals". Free Radic Res. 40 (12): 1230–8. PMID 17090411.
  23. ^ Publication demonstrating negative effects of antioxidants on human health
  24. ^ Nathan C, Shiloh M (2000). "Reactive oxygen and nitrogen intermediates in the relationship between mammalian hosts and microbial pathogens". Proc Natl Acad Sci U S A. 97 (16): 8841–8. PMID 10922044.

General references

  • Current Medicinal Chemistry, Volume 12, Number 10, May 2005, pp. 1161-1208(48) Metals, Toxicity and Oxidative Stress
  • Strand, What Your Doctor Doesn't Know about Nutritional Medicine May Be Killing You.
  • Parker, The Antioxidant Miracle.
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