The fatty acid composition of phosphatidylinositol from thrombin-stimulated human platelets

The level of phosphatidylinositol (PI) in human platelets falls abruptly when they are stimulated with thrombin and then returns to baseline within 15 min. PI is enriched in arachidonate and our recently proposed pathway for arachidonate release (Bell, R. L., Kennerly, D. A., Stanford, N., and Majerus, P. W. (1979) Proc. Natl. Acad. Sci. U. S. A. 76, 3238-3241) provides a link between the unique fatty acid composition of PI and the increased turnover of PI seen in many secretory tissues when they are stimulated. We measured the fatty acid composition of PI at time points following stimulation with thrombin as a means of defining the route of PI resynthesis which occurs, and the mechanism for the arachidonate enrichment. In resting platelets, the fatty acid content (per cent of total fatty acid contributed by each) of PI is: palmitate, 5.1; stearate, 39.9; oleate, 8.6; linoleate, 5.6; and arachidonate, 40.8. At 30 s after thrombin treatment, when the mass of PI has fallen from 18.7 +/- 2.3 to 12.5 +/- 2.4 nmol/10(9) platelets, there are no significant changes in per cent fatty acid composition. However, at 8 min, when PI has returned to 16.8 +/- 2.9 nmol/10(9), the per cent fatty acid composition is: palmitate, 7.8 (p > 0.05); stearate, 34.0 (p < 0.005); oleate, 10.7 (p < 0.05); linoleate, 16.1 (p < 0.001); and arachidonate, 31.3 (p < 0.05). The altered fatty acid composition excludes the "Pi cycle" as the mechanism for resynthesis as it would result in an unchanged pattern. The evidence suggests that PI is synthesized without a specific fatty acid composition, and then undergoes a deacylation-reacylation cycle to result in the unique 1-stearyl-2-arachidonyl pattern.