Fatty acid composition of phosphatidylinositol and phosphatidic acid in stimulated platelets. Persistence of arachidonyl-stearyl structure.

The fatty acid composition of phosphatidylinositol (PI), phosphatidic acid (PA), and the free fatty acid pool of human platelets was studied as a function of time following thrombin stimulation. Upon addition of thrombin, the total amount of fatty acids in PI decreased sharply, then rose toward basal levels, while that of PA showed an inverse pattern. However, the percentage distribution of fatty acids in stimulated as well as unstimulated PI and those in stimulated PA remained relatively constant: stearic and arachidonic acids accounted for 90 and 80% of the total in PI and PA, respectively. These data suggest that in stimulated human platelets PI and PA are interconverted via the "PI cycle." The time course of changes in PI and PA may suggest that the levels of arachidonyl-stearyl PA are involved in regulation of the resynthesis of arachidonyl-steryl PI. In contrast to PI and PA, the free fatty acid pool showed sharp increases in the five major platelet fatty acids: arachidonate, stearate, palmitate, oleate, as well as linoleate. These data suggest that most of the fatty acids liberated upon platelet stimulation are not derived via PI metabolism but by other mechanisms.