Allylic Free Radicals and Vitamin

Free radicals are highly reactive species that wreak havoc on cellular molecules. The antioxidant vitamin E, also called a-tocopherol, reacts with cellular free radicals and converts them to resonance-stabilized free radicals that are much less harmful. 

For example, a free radical can react with an unsaturated fatty acid in a membrane lipid. First, the radical abstracts an allylic hydrogen atom from the fatty acid to give a resonance-stabilized allylic radical. 

An allylic free radical can also react with oxygen to produce a hydroperoxy radical (HOO-), an even more reactive intermediate. Hydroperoxy radicals can be generated by many pathways. For example, they are sometimes by-products of drug metabolism, and they are formed as intermediates in enzyme-catalyzed reactions. 

Disproportionation of the hydroperoxy radical produces an alkoxy radical and a hydroxyl radical, a very dangerous species in biological systems. Free radicals such as these react with many cellular proteins and nucleic acids, causing extensive cellular damage. Free radicals may also play a significant role in the aging process. 

Many food additives have antioxidant properties similar to those of vitamin E. These additives, much maligned by persons who object to unnatural food, not only preserve food by preventing free radical oxidation but also preserve the humans who eat them. 

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