Dubnova E B, Baĭkov A A
Biokhimiia. 1991 Dec;56(12):2181-7.
Intact rat liver mitochondria possess a very low hydrolytic activity, if any, towards exogenous pyrophosphate. This activity can be unmasked by making mitochondria permeable to PPi by toluene treatment or by disrupting them with detergents or ultrasound, thus indicating that the active site of pyrophosphatase is localized in the matrix. The initial rates of PPi hydrolysis of toluene-permeabilized mitochondria and purified pyrophosphatase were found to depend, in a similar manner, on PPi and Mg2+ concentrations. The simplest model consistent with these data in both cases implies that the reaction proceeds via two pathways and requires MgPPi as substrate and at least one Mg2+ ion as activator. In the presence of 0.4 mM Mg2+ (physiological concentration) the inhibition constant for Ca2+ is 12 microM and the enzyme activity is no less than 50% of the maximal one. The data obtained suggest that the activity of pyrophosphatase in mitochondria is high enough to keep free PPi concentration at a level close to the equilibrium one.
完整的大鼠肝脏线粒体对外源性焦磷酸的水解活性非常低，即便有也微乎其微。通过甲苯处理使线粒体对焦磷酸通透，或用去污剂或超声破坏线粒体，这种活性就会显现出来，这表明焦磷酸酶的活性位点位于线粒体基质中。研究发现，甲苯通透处理的线粒体和纯化的焦磷酸酶的焦磷酸水解初始速率，以相似的方式取决于焦磷酸和镁离子的浓度。在这两种情况下，与这些数据相符的最简单模型表明，反应通过两条途径进行，需要焦磷酸镁作为底物，至少一个镁离子作为激活剂。在0.4 mM镁离子（生理浓度）存在的情况下，钙离子的抑制常数为12 microM，酶活性不低于最大活性的50%。所获得的数据表明，线粒体中焦磷酸酶的活性足够高，能够将游离焦磷酸的浓度维持在接近平衡的水平。
