Phospholipase A2 and phospholipase C activities of platelets. Differential substrate specificity, Ca2+ requirement, pH dependence, and cellular localization.

Deoxycholate treatment of horse platelets previously labeled in their phospholipids with [14C]arachidonate produces selective conversion of [14C]phosphatidylinositol (PI) to [14C]1,2-diacylglycerol. This phospholipase C activity, which has a pH optimum of 7.5, is specific for phosphatidylinositol since other phospholipids or neutral lipids are not affected. Although exogenous Ca2+ is not required for activity, ethylene glycol bis(beta-aminoethyl ether)N,N,N',N-tetraacetic acid or EDTA abolishes phosphatidylinositol degradation. However, in the presence of added Ca2+, other phospholipids such as phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylserine (PS) are also degraded but by a phospholipase A2 activity. This activity generates the respective lyso-derivatives as well as various [14C]arachidonate metabolites. The phospholipase A2 activity is further enhanced by increasing the pH (7.5 to 9.5), a condition which severely suppresses the phospholipase C activity. Most of the platelet phospholipase A2 activity is associated with the particulate fractions of the cell, while the phospholipase C activity appears to be almost completely restricted to the soluble fraction. Deoxycholate treatment of the particulate fractions results in cleavage by phospholipase A2 of phosphatidylcholine and phosphatidylethanolamine but not of phosphatidylinositol. The preferred substrates for platelet phospholipase A2 appear to be phosphatidylethanolamine, phosphatidylcholine, and phosphatidylserine, while phosphatidylinositol seems to be degraded nearly exclusively by phospholipase C.