Landsperger W J, Harris B G
J Biol Chem. 1976 Jun 25;251(12):3599-602.
The regulatory properties of the NAD-dependent malic enzyme from the mitochondria of Ascaris suum have been studied. The malate saturation curve exhibits sigmoidicity and the degree of this sigmoidicity increases as the pH is increased. Fumarate was the only compound tested that stimulated the enzyme activity, whereas oxalacetate was the most powerful inhibitor. Activation by low levels of fumarate was found to be competitive with malate. It is proposed that this stimulation has physiological significance in controlling the dismutation reaction in the parasite. The branched-chain volatile fatty acid excretion products, tiglate, 2-methylbutanoate, and 2-methylpentanoate, inhibited the enzyme activity and this inhibition was competitive with malate. The Ki values for these compounds are in the physiological range of their concentrations; therefore, it is suggested that they may aid in controlling the malic enzyme activity in vivo. Oxalacetate inhibition of malic enzyme activity was competitive with malate, and the Ki values decreased with an increase in pH. Two alternatives are proposed which could account for the lack of oxalacetate decarboxylation by the ascarid malic enzyme.
对猪蛔虫线粒体中依赖NAD的苹果酸酶的调节特性进行了研究。苹果酸饱和曲线呈S形，且随着pH值升高，这种S形程度增加。富马酸是所测试的唯一能刺激该酶活性的化合物，而草酰乙酸是最有效的抑制剂。发现低水平富马酸的激活与苹果酸具有竞争性。有人提出这种刺激在控制寄生虫中的歧化反应方面具有生理意义。支链挥发性脂肪酸排泄产物，惕各酸、2-甲基丁酸和2-甲基戊酸，抑制了该酶活性，且这种抑制与苹果酸具有竞争性。这些化合物的Ki值在其浓度的生理范围内；因此，有人认为它们可能有助于在体内控制苹果酸酶的活性。草酰乙酸对苹果酸酶活性的抑制与苹果酸具有竞争性，且Ki值随pH值升高而降低。提出了两种可能解释蛔虫苹果酸酶缺乏草酰乙酸脱羧作用的替代方案。
