Selective Release of Phospholipase A2 and Lysophosphatidylserine- Specific Lysophospholipase from Rat Platelets1

Abstract

Rat platelets released phospholipase A₂ and lysophospholipase upon activation with thrombin or ADP. The release of phospholipases was energy-dependent and was not in parallel with that of a known lysosomal marker enzyme, N-acetyl-β-D-glucosaminidase. The phospholipases are derived from other granules (dense granules or α-granules) rather than lysosomal granules of the cells. All of the activities of both phospholipases in the cell free fraction obtained from the activated platelet reaction mixture was recovered in the supematant after centrifugation at 105,000×g. The degree of hydrolysis of phospholipids by the phospholipase A followed the order: phosphatidylethanolamine (PE) > phosphatidylserine (PS)> phosphatidylcholine (PC). Phospholipase A shows a broad pH optimum (>pH 7.0) and absolutely requires Ca²⁺ Lysophospholipase was specific to lysophos phatidylserine (lysoPS), and neither lysophosphatidylethanolamine (lysoPE) nor lysophosphatidylcholine (lysoPC) was hydrolyzed appreciably. Both 1-acyl- and 2-acyl-lysophosphatidylserine were equally hydrolyzed. Lysophospholipase activity shows similar pH optimum to phospholipase A₂ The lysophospholipase activity was lost easily at 60°C. The activity was reduced by the presence of EDTA, though low but distinct activity was observed even in the presence of EDTA. Addition of Ca²⁺ to the mixtures restores the full activity.