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.
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