Arachidonoyl Transacylase in Human Platelets

Human platelets contain an enzyme that catalyzes CoA-independent release of arachidonic acid from phosphatidylcholine with concomitant incorporation into plasmenylethanolamine. Addition of lysoplasmenylethanolamine (10-80 PM) to a crude membrane preparation of prelabeled platelets (0.24 mg of proteinlml) induces transfer of [3H]arachidonate from endogenous phosphatidylcholine to lysoplasmenylethanolamine (0.8 nmol of arachidonic acid/min/mg of protein). The transacylation reaction occurs in the absence of Ca2+, has a broad pH optimum from 7 to 8, is not affected by excess unlabeled arachidonic acid, and is inhibited by I?-ethylmaleimide (0.2 mM) and Triton X-100 (0.1 mg/ ml). The enzyme shows a high specificity toward the acyl donor (phosphatidylcholine), transfers fatty acids in the order: arachidonic > eicosatrienoic > oleic, and preferentially acylates lysoplasmenylethanolamine but also other lysophosphatides (lysophosphatidylethanolamine > lysophosphatidylserine > lysophosphatidylinositol = 0). Platelet acyltransferase, on the other hand, acylates ethanolamine lysophosphatides with free arachidonic acid in the order: lysophosphatidylethanolamine > lysoplasmenylethanolamine. These results suggest that a distinct acylation mechanism exists for introduction of arachidonic acid into plasmalogen phosphatides. In stimulated platelets, the transacylase may play an additional role in the controlled release of esterified arachidonic acid for synthesis of the biologically active oxygenated metabolites.