Characterization of phospholipase A2 secretion from human platelets

Human platelets secreted phospholipase A2 in a dose- and time-dependent manner when challenged with thrombin, 12-O-tetradecanoyl-phorbol-13-acetate (TPA), or collagen. Enzyme release was maximal at concentrations of 0.1 units/ml of thrombin, 100 nM TPA, or 2 micrograms/ml of collagen; and complete by 2 min in platelets treated with thrombin or TPA. Cells challenged with collagen required up to 5 min for maximal secretion. Besides dose and time functions, phospholipase A2 secretion was also dependent on platelet concentration and the levels of bovine serum albumin in the incubation medium. The secreted enzyme was soluble and exhibited substrate and Ca2+ requirements similar to a detergent-solubilized, partially purified phospholipase A2 from whole platelets [Kramer et al., Biochim. Biophys. Acta (1988) 959, 269-279]. The pH optimum of the secreted enzyme, however, was 1-2 units lower than the pH optimum of the phospholipase A2 from whole cells. Secreted phospholipase A2 hydrolyzed phosphatidylethanolamine at 5-12 times the rate of phosphatidylcholine when the substrates were present in pure form. These apparent differences in activity were greatly diminished, though, when 1:1 molar mixtures of the two substrates were employed. Because phospholipase A2 catalyzes a key reaction during the formation of bioactive arachidonate metabolites, the secretion of this enzyme from platelets may be important in the regulation of thrombosis.