Vitamin E: Mechanism of Its Antioxidant Activity

The antioxidant activity of vitamin E (α-tocopherol) during the peroxidation of unsaturated lipids has been reviewed based on its reaction products. Free-radical scavenging reactions of α-tocopherol take place via the α-tocopheroxyl radical as an intermediate. If a suitable free radical is present, a non-radical product can be formed from the coupling of the free radical with the α-tocopheroxyl radical. The reaction products of α-tocopherol with lipid-peroxyl radicals are 8a-(lipid-dioxy)-α-tocopherones which are hydrolyzed to α-tocopherylquinone. If the supply of oxygen is insufficient, α-tocopherol can trap the carbon-centered radicals of lipids to form 6-O-(lipid-alkyl)-α-tocopherols. On the other hand, the dimer and trimer of α-tocopherol is formed by the bimolecular self-reaction of the α-tocopheroxyl radical in a reaction mixture containing a large amount of α-tocopherol. The other product-forming pathway yields isomeric epoxy-α-tocopherylquinones and their precursors, epoxyhydroperoxy-α-tocopherones, but the mechanism of this pathway remains unknown. The reaction products of other vitamin E compounds (γ- and δ-tocopherols) during lipid peroxidation are almost the same as those formed from the α-tocopherol. The tocopheroxyl radicals of γ- and δ-tocopherols prefer to react with each other to form dimeric products that are still effective as antioxidants.