Belikova Y O, Kotlyar A B, Vinogradov A D
Department of Biochemistry, School of Biology, Moscow State University, U.S.S.R.
Biochim Biophys Acta. 1988 Oct 26;936(1):1-9. doi: 10.1016/0005-2728(88)90245-9.
The purified succinate-ubiquinone reductase catalyzes the L- (or D-) malate: acceptor oxidoreductase reaction with Km for malate of about 2.10(-3) M and initial Vmax of 50 and 100 nmol per min per mg of protein for L- and D-stereoisomers, respectively (25 degrees C, pH 7.0). The reaction rate rapidly decreases both in the absence and presence of L-glutamate and L-glutamate-oxaloacetate transaminase added for trapping of oxaloacetate. Both keto and enol forms of oxaloacetate were found to be strong, slowly dissociating inhibitors of succinate dehydrogenase; the first-order rate constant for the enzyme inhibition by the enol form is about 3 times as high as that by the keto form. Oxidation of malate by succinate dehydrogenase in the presence of the oxaloacetate trapping system occurs at an indefinitely constant rate when enoloxaloacetate, which is an immediate product of the reaction, is rapidly converted into the keto isomer--a substrate for transaminase. A quantitative kinetic scheme for malate oxidation by succinate dehydrogenase which includes two kinetically distinct enzyme-oxaloacetate complexes is proposed, and the specific role of the mitochondrial oxaloacetate keto-enol-tautomerase (EC 5.3.2.2) in the regulation of succinate dehydrogenase is suggested.
纯化的琥珀酸 - 泛醌还原酶催化L -（或D -）苹果酸：受体氧化还原酶反应，苹果酸的Km约为2.1×10⁻³ M，L -和D -立体异构体的初始Vmax分别为每毫克蛋白质每分钟50和100 nmol（25℃，pH 7.0）。在添加L -谷氨酸和L -谷氨酸 - 草酰乙酸转氨酶以捕获草酰乙酸的情况下，无论有无这些物质，反应速率都会迅速下降。发现草酰乙酸的酮式和烯醇式都是琥珀酸脱氢酶的强抑制剂，且解离缓慢；烯醇式对酶的抑制一级速率常数约为酮式的3倍。当反应的直接产物烯醇草酰乙酸迅速转化为酮式异构体（转氨酶的底物）时，在草酰乙酸捕获系统存在下，琥珀酸脱氢酶催化苹果酸的氧化以无限恒定的速率发生。提出了一个琥珀酸脱氢酶催化苹果酸氧化的定量动力学方案，该方案包括两种动力学上不同的酶 - 草酰乙酸复合物，并暗示了线粒体草酰乙酸酮 - 烯醇互变异构酶（EC 5.3.2.2）在琥珀酸脱氢酶调节中的特定作用。
