Evidence for predominance of phospholipase A2 in release of arachidonic acid in thrombin-activated platelets: phosphatidylinositol-specific phospholipase C may play a minor role in arachidonate liberation.

Human platelets prelabeled with [3H]glycerol exhibited a trasient increase in radioactivity (1.5-fold gain) in 1,2-diacylglycerol when they were exposed to thrombin. An alteration in radioactivity in monoacylglycerol which is derived from diacylglycerol by diacylglycerol lipase, however, was not observed during the whole period of incubation with thrombin. Lysophosphatidylcholine and lysophosphatidylethanolamine gained radioactivity. By contrast, the level of lysophosphatidylinositol plus lysophosphatidylserine did not show any change. When the effects of thrombin on platelet lipids were examined for [3H]arachidonate-labeled platelets, thrombin-activation induced a 15-fold increase in radioactivity in 1,2-diacylglycerol, a subsequent decrease of which was accompanied by accumulation of radioactivity in phosphatidic acid. There was a concurrent release of free arachidonic acid. These findings, taken together with phospholipid alteration analyzed by phosphorus assay upon thrombin-activation, indicate evidence than newly produced diacyglycerol in thrombin-activated platelets may be immediately converted to phophatidic acid by a diacylglycerol kinase rather than metabolized to monoacylglycerol or arachidonic acid by diacylglycerol lipase, and also that arachidonic acid would be mainly released from phosphatidylcholine and phosphatidylethanolamine by a phospholipase A2 activity.