Lipid peroxides formed by radical action on unsaturated fatty acids in membranes and LDL can be reduced to unreactive alcohols by the enzyme glutathione peroxidase, with the oxidation of the tripeptide glutathione (y-glutamyl-cysteinyl-glycine, GSH) to the disulphide-linked GSSG. In turn, oxidized glutathione is reduced back to the active peptide by glutathione reductase, as shown in Figure 5.15.
Glutathione reductase has selenium at the catalytic site, as a selenocysteine residue (section 126.96.36.199); this explains the role of selenium as an antioxidant nutrient. Glutathione reductase is a flavoprotein, and is especially sensitive to riboflavin (vitamin B2) depletion; as discussed in section 188.8.131.52, measurement of glutathione reductase is used as a means of assessing riboflavin status.
Vitamin E (section 11.4) forms a stable radical that can persist long enough to undergo reaction to yield non-radical products. As shown in Figure 7.17, tocopherol reacts with lipid peroxides to form stable fatty acids and the tocopheroxyl radical. In turn,
tocopheroxyl radical Figure 7.17 The antioxidant role of vitamin E.
There is epidemiological evidence that intakes of vitamin E higher than those required to prevent deficiency, and probably higher than can be achieved from normal diets, may have significant protective action against the development of atherosclerosis and cardiovascular disease (see Figure 7.2), and possibly some forms of cancer. However, intervention trials with vitamin E supplements have been disappointing, with little evidence of decreased mortality from cardiovascular disease among people taking supplements.
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