Inositol Lipids, Phosphatidate and Diacylglycerol Share Stearoylarachidonoylglycerol As a Common Backbone in Thrombin-stimulated Human Platelets

Gel-filtered human platelets were stimulated with 5i.u. of thrombin/ml for times up to 1 min. The fatty acid composition of inositol-containing phospholipids, phosphatidic acid and diacylglycerol was determined by g.l.c. in control and thrombin-stimulated platelet suspensions. Inositol phospholipids were found to have similar proportions of stearic and arachidonic acids, the sum of these representing 86.6% of the total fatty acids in phosphatidylinositol (PtdIns), 76.9% in phosphatidylinositol 4-phosphate (PtdIns4P) and 85.4% in phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2]. However, arachidonic and stearic acids were less abundant in phosphatidic acid (PtdA) and diacylglycerols in non-stimulated platelets. A transient decrease in the mass of PtdIns(4,5)P2 was observed after 5-10s of thrombin stimulation, followed by an increase after 30s. The amounts of PtdIns4P and PtdIns decreased throughout the experiment. A transient accumulation of stearoylarachidonoylglycerol was observed at 5s, whereas stearoylarachidonoylglycerol 3-phosphate (PtdA) was produced in increasing amounts throughout the experiment. The decrease in inositol-containing phospholipids was not fully compensated for by the production of diacylglycerol or PtdA [or PtdIns(4,5)P2] at 1 min. All the changes in inositol phospholipids, as well as those observed in diacylglycerols and PtdA, were due to a parallel reduction or increase in the contents of stearic and arachidonic acids, with a stoichiometry equal to 1. Taken together, this suggests an interconversion of all these lipids with the utilization of a common backbone, stearoylarachidonoylglycerol. The deacylation of this diacylglycerol could account for up to 4-5nmol of arachidonate/10(9) platelets after 1 min stimulation by thrombin.