Bisby R H, Parker A W
Department of Biological Sciences, University of Salford, United Kingdom.
Arch Biochem Biophys. 1995 Feb 20;317(1):170-8. doi: 10.1006/abbi.1995.1150.
The reaction by which the antioxidant capacity of alpha-tocopherol is sustained by recycling of the alpha-tocopheroxyl radical in membranes or lipoproteins by aqueous ascorbate has been studied by laser flash photolysis in model micellar and membrane systems. In bilayers of dimyristoylphosphatidylcholine at 35 degrees C the measured second-order rate constant was 3 x 10(5) M s-1, or about five times slower than previously reported in a solvent system. The rate of reaction was decreased on addition of negatively charged lipid (dipalmitoylphosphatidic acid) and increased by addition of positively charged lipid (didodecyldimethylammonium bromide). These effects of bilayer charge were suppressed by increasing the ionic strength of the aqueous medium. Micellar charge also had an effect on the pH dependence of the reaction rate. Arrhenius data showed that the enthalpy of activation was effectively zero for the reaction in solution between ascorbate and radicals of water-soluble tocopherol analogues, but was positive in membrane and micellar systems. In all cases the entropy of activation was strongly negative. The kinetic deuterium isotope ratio varied between 3 and 8. The data strongly support a concerted electron and proton transfer mechanism for the reaction between alpha-tocopheroxyl radical and ascorbate.
通过激光闪光光解在模型胶束和膜系统中研究了α-生育酚的抗氧化能力通过水相抗坏血酸在膜或脂蛋白中对α-生育酚氧基自由基的循环而得以维持的反应。在35℃的二肉豆蔻酰磷脂酰胆碱双层中，测得的二级反应速率常数为3×10⁵ M⁻¹ s⁻¹，比先前在溶剂系统中报道的慢约五倍。添加带负电荷的脂质（二棕榈酰磷脂酸）会降低反应速率，而添加带正电荷的脂质（十二烷基二甲基溴化铵）则会提高反应速率。双层电荷的这些影响通过增加水相介质的离子强度而受到抑制。胶束电荷也对反应速率的pH依赖性有影响。阿累尼乌斯数据表明，抗坏血酸与水溶性生育酚类似物的自由基在溶液中的反应活化焓实际上为零，但在膜和胶束系统中为正值。在所有情况下，活化熵都强烈为负。动力学氘同位素比率在3到8之间变化。这些数据有力地支持了α-生育酚氧基自由基与抗坏血酸之间反应的协同电子和质子转移机制。
