PHOSPHOLIPID-DEACYLATING ENZYMES OF RABBIT PLATELETS

Phospholipase activities in sonicates of rabbit platelets were studied as these activities may be candidate enzymes for the release of arachidonic acid from platelet membrane phospholipids. Platelets contained phospholipases A1 (EC3.1.1.32), A2 (EC3.1.1.4), and lysophospholipase (EC 3.1.1.5) activities that can be distinguished with respect to optimum pH, dependence on calcium ion, heat lability, and response to detergent. Phospholipase A1 activity had an optimum pH of 4.5, and calcium ion did not affect the activity. Lysophospholipase activity was optimal at pH 8.5, relatively heat labile, and strongly inhibited by sodium deoxycholate. Calcium ion and EDTA had little effect on the activity. Platelets contained relatively high phospholipase A2 activity compared with the above-mentioned enzymes. The activity was optimal at pH 9.5 and relatively heat stable in acidic medium. Calcium ion is specifically required for the activity and EDTA strongly inhibited the activity. The addition of sodium deoxycholate strongly enhanced the activity. Because of its high activity and calcium ion dependency, phospholipase A2 with an optimum in alkaline pH range seemed worthy of further study. The enzyme was recovered mostly in a particulate fraction, preferred phosphatidylethanolamine to phosphatidylcholine, and showed no strict acyl chain specificity. Once sonicated, thrombin and other platelet activating agents and cyclic nucleotides had no direct effect on the activity. © 1979.