Free-Radical Scavenging Reactions of Vitamin E on Biological Membrane Systems

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The addition products of alpha-tocopherol (alpha-TH) with peroxyl radicals of phosphatidylcholines (PCs) ; 1-palmitoyl-2-linoleoyl-PC,1-palmitoyl-2-linolenoy-PC,1-palmitoyl-2-arachidonoyl-PC,and 1-stearoyl-2-arachidonoyl-PC,were isolated and identified as 8a-(PC-dioxy)-alpha-tocopherones. Large-unilamellar liposomal suspensions of PC containing alpha-TH (0.1 mol%, based on PC) were oxidized at 37ﾟC.The oxidation was initiated by a water-soluble free radical initiator or by Fe (III) -EDTA and ascorbate. In all the oxidation systems, alpha-TH suppressed the formation of PC hydroperoxides until all the alpha-TH had been depleted. The oxidation products of alpha-TH were 8a- (PC-dioxy) -alpha-tocopherones, 5,6-epoxy-alpha-tocopherylquinone, 2,3-epoxy-alpha-tocopherylquinone, and alpha-tocopherylquinone. The 8a- (PC-dioxy) -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- (PC-dioxy) -alpha-tocopherones which are hydrolyzed to alpha-tocopherylquinone in PC bilayrs. In another oxidation pathway, alpha-TH is oxidized by peroxyl radicals to form isomeric epoxy-alpha-tocopherylquinones. alpha-Tocopherol also reacted with peroxyl radicals produced from the iron-catalyzed decomposition of PC hydroperoxides to form 8a-alkyldioxy-alpha-tocopherones. The results indicate that alpha-tocopherol can react with lipid-peroxyl radicals to stop lipid peroxidation in liposomes.The lipid peroxidation was performed in the system of rat liver homogenate using a water-soluble free radical initiator. Endogenous alpha-TH in the homogenate could suppress the lipid peroxidation, and alpha-tocopherylquinone was detected as the oxidation products. However, the free-radical scavenging mechanism of alpha-TH in biological systmes is still unclear.