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We are reporting a novel pathway of arachidonic acid metabolism in the phosphatides of thrombin-activated platelets. For kinetic studies of arachidonic acid turnover, platelet phosphatides were labeled by incubation of platelet rich plasma with (3H)-arachidonic acid for 15 min. Unincorporated isotope was removed during subsequent gel-filtration. Platelet phosphatides were resolved and quantitated following two-dimensional silica paper chromatography of chloroform/methanol extracts of incubated platelets. Plasmalogen phosphatidylethanolamine (PPE) was examined on paper chromatograms after its breakdown to lysoPPE with HgCl2. In other experiments, gel-filtered platelets were incubated with (14C)-glycerol to monitor de novo phosphatide synthesis. (3H)-Arachidonic acid was released from phosphatidylcholine and phosphatidylinositol of pre-labeled platelets exposed to thrombin and appeared increasingly in PPE in acyl linkage at glycerol-C-2. (3H)-Arachidonic acid was not found in PPE of resting cells. Maximum transfer occurred with 5 U/ml of thrombin and 15 min, of incubation, with t½ of 2½ min., and was Ca+2 dependent. The presence of aspirin, indomethacin, or eicosatetraynoic acid did not prevent the thrombin-activated transfer of (3H)-arachidonic acid to PPE. The stimulated incorporation of (3H)-arachidonic acid into PPE was not accompanied by a stimulation of (14C)-glycerol uptake into this phosphatide. We suggest that perturbation of the platelet may activate a phospholipase A2 leading to turnover of arachidonic acid in PPE, which is rich in this fatty acid. Such turnover may provide substrate for conversion by cyclo-oxygenase and lipoxydase to biologically active metabolites, and therefore, may offer a locus for regulation of prostaglandin synthesis in the human platelet.

A New Pathway for Arachidonic Acid Metabolism in Human Platelets