[Effect of Ca2+ ions on phospholipase D interaction with mitochondrial membrane phospholipids]

Effect of phospholipase D and Ca2+ ions on mitochondrial polyenzyme systems is studied. Phospholipase D is found to have a lytic effect on mitochondrial membrane phospholipids both in the presence and in the absence of Ca2+ ions. Low Ca2+ concentrations stimulate the phospholipase D-membrane interaction, thus increasing the efficiency of the enzyme action. The degradation of mitochondrial membrane phospholipids or their heating result in a decrease of membrane binding with phospholipase D. The presence of Ca2+ protects the membranes from such a degradation. High Ca2+ concentrations cause a "pseudo" stabilization of mitochondrial polyenzyme systems with respect to phospholipase D. In fact, a decrease in phospholipase binding with mitochondrial membranes takes place at 30 mM and higher concentrations of CaCl2, which is due to the intensive phospholipid degradation at the initial reaction step. Optimal Ca2+ concentration range for the activation of phospholipase D is 2-5 mM. It does not coincide with Ca2+ concentration values necessary for the phospholipase activation with "disorganized substrates" (30-60 mM). Combined effect of Ca2+ and phospholipase D on outer and inner NADH oxidation pathway manifests differently. The inner pathway (the respiration chain) is more subjected to the lytic action of the enzyme, while the oxidation rate through the outer pathway increases under low phospholipase D concentrations, and the oxidation stops only under high enzyme concentration in the presence of Ca2+ ions. The data are also given, demonstrating that some effects, which have been early considered to be related to the effect of Ca2+ on mitochondria, are really due to changes under the lytic effect of endogenous phospholipases, Ca2+ being the enzyme activator.