Chemical and Biological Effects
Chemically, oxidative stress is associated with increased production of oxidizing species or a significant decrease in the effectiveness of antioxidant defenses, such as glutathione. 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.
Production of reactive oxygen species is a particularly destructive aspect of oxidative* stress. Such species 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. The major portion of long term effects is inflicted by damage on DNA. Most of these oxygen-derived species are produced at a low level by normal aerobic metabolism. Normal cellular defense mechanisms destroy most of these. Likewise, any damage 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.
| 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-sulfur 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. |
| ONOO-, 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. |
Table adapted from.
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