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

分离得到α-生育酚(α-TH)与磷脂酰胆碱(PC)过氧基的加成产物：1-palmitoyl-2-linoleoyl-PC，1-palmitoyl-2-linolenoy-PC，1-palmitoyl-2-arachidonoyl-PC，和1-硬脂酰基-2-花生四烯基-PC，鉴定为8a-(PC-二氧基)-α-生育酚。含α-TH(0.1mol%，以PC为基准)的PC大单层脂质体悬浮液在37ﾟ℃下被氧化。氧化由水溶性自由基引发剂或Fe(III)-EDTA和抗坏血酸引发。在所有的氧化体系中，α-TH抑制了PC过氧化氢的形成，直到所有的α-TH都被耗尽。α-TH的氧化产物为8a-(PC-二氧基)-α-生育酚、5，6-环氧基-α-生育基醌、2，3-环氧基-α-生育基醌和α-生育基醌。8a-(PC-dioxy)-α-生育酚在脂质体中主要通过水解制得α-生育酚。结果表明，α-生育酚能捕获过氧化自由基生成8a-(PC-二氧基)-α-生育酚，后者在PC双分子层中被水解为α-生育酚。在另一种氧化途径中，α-TH被过氧基氧化生成同分异构体环氧α-生育苯二酚。α-生育酚还与铁催化的PC氢过氧化物分解产生的过氧基反应生成8a-烷基二氧-α-生育酚。结果表明，α-生育酚能与脂质过氧化自由基反应，阻止脂质过氧化。匀浆中内源性α-TH可抑制脂质过氧化，氧化产物为α-生育苯二酚。然而，α-TH在生物系统中清除自由基的机制尚不清楚。

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.