[Interaction of cytotoxin and Ca2+ with phospholipase A2]

The effects of Ca2+ and cytotoxin from Middle Asian cobra venom on the hydrolytic activity of phospholipase A2 from the same source and on its action on erythrocyte membrane phospholipids and mitochondria were studied. The necessity of phospholipase A2 for Ca2+ depends on the type of substrates (phosphatidyl methanol, phosphatidyl ethanol, cephalin, phosphatidyl ethyleneglycol, phosphatidyl inositol), whereas enzymatic cleavage of lecithin can also occur in the absence of Ca2+. Cytotoxin Vc" I had no effect on the rate of phospholipid hydrolysis in the absence of Ca2+. However, in the presence of Ca2+ cytotoxin at low concentrations activated phospholipase A2, while its high concentrations inhibited the phospholipid hydrolysis with the exception of phosphatidyl ethyleneglycol and phosphatidyl inositol. In experiments with purified phospholipids no synergistic effect of cytotoxin and phospholipase A2 was observed. The activating effect of cytotoxin and phospholipase A2 was comparable to that of the cationic detergents. The effect of phospholipase A2 on erythrocyte hemolysis and succinate oxidation by mitochondria was more complicated: the activating or inhibiting effect depended on the state and ratio of Ca2+, cytotoxin and phospholipase A2. It is suggested that the synergistic effect observed in membrane systems is primarily due to the structural organization of the membranes and the phase condition of the membrane phospholipids.