Some medical interest since dioxygen is used

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some medical interest, since dioxygen is used therapeutically for patients experiencing difficulty breathing, or for those suffering from infection by anaer- obic organisms. 6
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262 5 / DIOXYGEN REACTIONS B. Biological Targets The major biochemical targets of O 2 toxicity appear to be lipids, DNA, and proteins. The chemical reactions accounting for the damage to each type of target are probably different, not only because of the different reactivities of these three classes of molecules, but also because of the different environment for each one inside the cell. Lipids, for example, are essential components of membranes and are extremely hydrophobic. The oxidative damage that is ob- served is due to free-radical autoxidation (see Reactions 5.16 to 5.21), and the products observed are lipid hydroperoxides (see Reaction 5.23). The introduc- tion of the hydroperoxide group into the interior of the lipid bilayer apparently causes that structure to be disrupted, as the configuration of the lipid rearranges in order to bring that polar group out of the hydrophobic membrane interior and up to the membrane-water interface. 6 DNA, by contrast, is in the interior of the cell, and its exposed portions are surrounded by an aqueous medium. It is par- ticularly vulnerable to oxidative attack at the base or at the sugar, and multiple products are formed when samples are exposed to oxidants in vitro. 6 Since ox- idation of DNA in vivo may lead to mutations, this type of damage is potentially very serious. Proteins also suffer oxidative damage, with amino-acid side chains, particularly the sulfur-containing residues cysteine and methionine, appearing to be the most vulnerable sites,6 C. Defense and Repair Systems The biological defense systems protecting against oxidative damage and its con- sequences are summarized below. 1, Nonenzymatic oxidant scavengers Some examples of small-molecule antioxidants are a-tocopherol (vitamin E; 5.24), which is found dissolved in cell membranes and protects them against lipid peroxidation, and ascorbate (vitamin C; 5.25) and glutathione (5.26), which are found in the cytosol of many cells. Several others are known as well. 6,29 HO H ,YH 3 H .GH3 CH 3 CH 3 ascorbic acid (5.24) (5.25)
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III. DIOXYGEN TOXICITY 263 (5.26) glutathione 2. Detoxification enzymes The enzymatic antioxidants are (a) catalase and the various peroxidases, whose presence lowers the concentration of hydrogen peroxide, thereby pre- venting it from entering into potentially damaging reactions with various cell components (see Section VI and Reactions 5.82 and 5.83), and (b) the super- oxide dismutases, whose presence provides protection against dioxygen toxicity that is believed to be mediated by the superoxide anion, O 2 - (see Section VII and Reaction 5.95). Some of the enzymatic and nonenzymatic antioxidants in the cell are illus- trated in Figure 5. I.
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