Studies on phospholipase A inhibitor in blood plasma. II. Interaction of phospholipase A inhibitor with phospholipase A and its specificity.

Non-competitive inhibition of snake venom phospholipase A2 which has been exhibited by bovine plasma phospholipase A inhibitor, a kind of lipoprotein, was not observed unless the inhibitor was preincubated with the enzyme. The inhibition seemed to be due to the formation of the enzyme-inhibitor complex, which was identified by immunoelectrophoresis. The enzyme-inhibitor interaction was observed maximally on incubation at physiological pH, but not below pH 5. The inhibitor was inactivated by trypsin digestion and heat treatment. It suppressed the phospholipase A2 activities of rat blood plasma as well as of the snake venom and porcine pancreas, but not the enzyme activities such as those of phospholipase C of Bacillus cereus, lipase of porcine pancreas, trypsin, and papain. The inhibitor also showed the ability to decrease membrane-bound phospholipase A1 and A2 activities in intracellular organelles such as plasma membranes, mitochondria, lysosomes, and microsomes. In view of these facts, it was concluded that the plasma inhibitor is specific for phospholipase A.