Oxidation of α-Tocopherol during the Peroxidation of Dilinoleoylphosphatidylcholine in Liposomes

Abstract

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