Eyer P
Walther-Straub-Institut für Pharmakologie und Toxikologie der Ludwig-Maximilians-Universitat München, Germany.
Chem Biol Interact. 1991;80(2):159-76. doi: 10.1016/0009-2797(91)90022-y.
During autoxidation of 1,4-hydroquinone (H2Q, less than 1 mM) at pH 7.4 and 37 degrees C, stoichiometric amounts of 1,4-benzoquinone (Q) and hydrogen peroxide were formed during the initial reaction. The reaction kinetics showed a significant induction period which was abolished by minute amounts of Q. Hydrogen peroxide and catalase were without effect on the autoxidation process. Transition metals apparently were not involved, since chelators like EDTA, DETAPAC, and desferrioxamine or FeSO4 had no influence on the autoxidation kinetics. Superoxide dismutase (SOD) did not abolish the induction period but dramatically enhanced the autoxidation rate by more than two orders of magnitude. The stimulatory effect was first-order in SOD concentration but showed saturation kinetics. The dependence of Q and hydrogen peroxide formation rates on H2Q concentration shows a biphasic behaviour: dependence on the square at low H2Q, but on the square root at high H2Q concentration. As revealed by calculatory simulations the results can be adequately described by the known reaction rate constants. The reaction starts with the comproportionation of H2Q and Q to yield two semiquinone molecules which autoxidize to give two superoxide radicals and two molecules of Q which enter into a new cycle of comproportionation. Because of unfavourable equilibria the autocatalytic reaction soon comes to steady state, and the further reaction is governed by the rate of superoxide removal. At excess SOD, the comproportionation reaction is rate-limiting, thus explaining the saturation effects of SOD. The experiments do not allow a decision between the two functions of SOD; the conventional action as a superoxide:superoxide oxidoreductase or as a semiquinone:superoxide oxidoreductase. In the latter reaction SOD is thought to be reduced by semiquinone with Q formation. In the second step the reduced enzyme would be re-oxidized by a superoxide radical which is formed during autoxidation of the second semiquinone molecule generated in the comproportionation reaction. From thermodynamic considerations, the latter function of SOD appears to be plausible.
在pH 7.4和37℃条件下，1,4 - 对苯二酚（H2Q，浓度小于1 mM）自氧化过程中，初始反应阶段会按化学计量比生成1,4 - 苯醌（Q）和过氧化氢。反应动力学显示出明显的诱导期，极微量的Q可消除该诱导期。过氧化氢和过氧化氢酶对自氧化过程无影响。过渡金属显然未参与其中，因为像EDTA、DETAPAC和去铁胺或硫酸亚铁等螯合剂对自氧化动力学没有影响。超氧化物歧化酶（SOD）并未消除诱导期，但使自氧化速率显著提高了两个多数量级。刺激作用在SOD浓度上呈一级反应，但表现出饱和动力学。Q和过氧化氢生成速率对H2Q浓度的依赖性呈现双相行为：在低H2Q浓度下依赖于平方关系，而在高H2Q浓度下依赖于平方根关系。经计算模拟表明，已知的反应速率常数能够充分描述这些结果。反应起始于H2Q和Q的歧化反应，生成两个半醌分子，这两个半醌分子自氧化生成两个超氧自由基和两个Q分子，后者进入新的歧化循环。由于平衡不利，自催化反应很快达到稳态，后续反应受超氧自由基清除速率的控制。在过量SOD存在时，歧化反应成为限速步骤，从而解释了SOD的饱和效应。这些实验无法在SOD的两种功能之间做出抉择；即作为超氧化物：超氧化物氧化还原酶的传统作用，或作为半醌：超氧化物氧化还原酶的作用。在后一种反应中，SOD被认为会被半醌还原并生成Q。在第二步中，还原态的酶会被在歧化反应中生成的第二个半醌分子自氧化过程中产生的超氧自由基重新氧化。从热力学角度考虑，SOD的后一种功能似乎是合理的。
