Antioxidant defences that protect the body from free-radical damage include the enzymes and the antioxidant vitamins. Free radicals are generated endogenously under physiological and pathological conditions but also upon exposure to exogenous challenge.
Vitamin E was discovered in 1922 by Evans and Bishop as a necessary dietary factor for reproduction in rats. Of the four tocopherols and four tocotrienols (designated as α-, β-, γ-, and δ-) found in food, only α-tocopherol meets human vitamin E requirements. The a-form contains 3 methyl groups, whereas the b- and g- have two and the d-form only one methyl group. The term vitamin E is used as a generic description for all tocol and tocotrienol derivatives qualitatively exhibiting the biological activity of α-tocopherol.
Tocopherol isomers are chain-breaking antioxidants. The antioxidative activity of the tocopherols is related to scavenging the free radicals of unsaturated lipids.
The free radicals of most significance in systems living in aerobic environments are oxygen (or oxy) radicals. These include superoxide anion F, its conjugate acid HOO- (the simplest form of peroxyl radical), lipid alkoxyl and peroxyl radicals (derived from polyunsaturated fatty acids), and the very reactive hydroxyl radical HO-.
Tocopherol, the most biologically active and abundant form of vitamin E in vivo, efficiently transfers a hydrogen atom to a lipid free radical, such as peroxyl, alkoxyl, and carbon-centred radicals, giving the corresponding non-radical product of the lipid and an α-tocopheroxyl radical.
The α-tocopheroxyl radicals, once formed, react with a second free radical or each other to form a non-radical product. Each molecule of α-tocopherol consumes thus two lipid free radicals and terminates the free-radical chain reaction.
Antioxidant of vitamin E
