Vitamin E is the most potent, lipid-soluble antioxi-dant in human plasma and tissues. Thus, vitamin E protects polyunsaturated fatty acids within membrane phospholipids and plasma lipoproteins. When a peroxyl radical forms in a membrane, it is 1000 times more likely to attack a vitamin E molecule than a polyunsaturated fatty acid (Figure 2). The hydroxyl group on the chromanol ring of vitamin E reacts with the peroxyl radical to form the corresponding lipid hydroperoxide and tocopheroxyl radical. Thus, vitamin E acts as a chain-breaking antioxidant, preventing further auto-oxidation of lipids.
The tocopheroxyl radical has a number of possible fates. It can react with another radical to form non-reactive products. Alternatively, it can be further oxidized to the tocopheryl quinone, a two-electron oxidation product. Another possibility is 'vitamin E recycling,' where the tocopheroxyl radical is restored to its unoxidized form by other antioxidants such as vitamin C, ubiquinol, or thiols, such as glutathione. This process will deplete these other antioxidants. For this reason, it is important to maintain a good intake of other dietary antioxidants.
Biologic activity is a term that has been used historically to indicate a disconnection between vitamin E antioxidant activities and in vivo activities. Observations in rodent experiments carried out in the 1930s formed the basis for determining the 'biologic activity' of vitamin E. Although the various vitamin E
R-OO-H Lipid hydroperoxide
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