Phospholipase A₂-catalyzed hydrolysis of plasmalogen phospholipids in thrombin-stimulated human platelets

In the present study, phospholipase A₂ (PLA₂)-catalyzed hydrolysis of platelet membrane phospholipids was investigated by measuring PLA₂ activity, phospholipid hydrolysis, arachidonic acid release and choline lysophospholipid production in thrombin-stimulated human platelets. Thrombin-stimulated platelets demonstrated selective hydrolysis of arachidonylated plasmenylcholine and plasmenylethanolamine, with little change in diacyl phospholipids. Accelerated plasmalogen hydrolysis was accompanied by increased arachidonic acid and thromboxane B₂ release and increased lysoplasmenylcholine production. Thrombin stimulation caused an increase in PLA₂ activity measured in the cytosolic fraction with plasmenylcholine only; no increase in activity was measured with phosphatidylcholine. No change in membrane-associated PLA₂ activity was observed with either substrate tested. Pretreatment with the Ca²⁺-independent PLA₂-selective inhibitor, bromoenol lactone, inhibited completely any thrombin-stimulated phospholipid hydrolysis. Thus, thrombin stimulation of human platelets activates a cytosolic PLA₂ that selectively hydrolyzes arachidonylated plasmalogen phospholipids.