Oxidation of alpha-tocopherol during the peroxidation of dilinoleoylphosphatidylcholine in liposomes

Liposomal suspensions of dilinoleoylphosphatidylcholine (DLPC) containing alpha-tocopherol (0.1 mol%, based on DLPC) were oxidized at 37 degrees C. The oxidation,vas initiated by a lipid-soluble or water-soluble free radical initiator, or by the addition of CuSO4 and fructose, in all the oxidation systems, alpha-tocopherol suppressed the formation of DLPC hydroperoxides until all the alpha-tocopherol had been depleted. The oxidation products of alpha-tocopherol were 8a-alkyldioxy-alpha-tocopherones, 5,6-epoxy-alpha-tocopherylquinone, 2,3-epoxy-alpha-tocopherylquinone, and alpha-tocopherylquinone. The 8a-alkyldioxy-alpha-tocopherones were decomposed in the liposomes primarily by being hydrolyzed to produce alpha-tocopherylquinone. The results indicate that alpha-tocopherol can trap peroxyl radicals to form 8a-alkyldioxy-alpha-tocopherones which are hydrolyzed to alpha-tocopherylquinone in phospholipid bilayers. In another oxidation pathway, a-tocopherol may be oxidized by peroxyl radicals to form isomeric epoxy-alpha-tocopherylquinones.