Intact human platelets loaded with 32PO4 contain multiple phosphorylated proteins. Thrombin treatment of intact 32PO4-loaded platelets results in a 2-6-fold increase in phosphorylation of a platelet protein (designated "peak 7" protein) of approximately 40,000 mol wt as determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis and by gel filtration on Sephadex G-150. A similar increase in phosphorylation was observed in a platelet protein (designated "peak 9" protein) of approximately 20,000 mol wt. The time for half-maximal phosphorylation of peak 7 and peak 9 protein was 10-14 s. The concentration of thrombin at half-maximal phosphorylation was 0.25 U/ml for both proteins. Prior incubation of platelets with dibutyryl cyclic adenosine 3',5'-monophosphate or prostaglandin E1 inhibited thrombin-induced peak 7 and peak 9 protein phosphorylation. The erythroagglutinating phytohemagglutinin of Phaseolus vulgaris, a non-proteolytic release-inducing agent, induced peak 7 and peak 9 protein phosphorylation. Thus, the characteristics of peak 7 and peak 9 protein phosphorylation are similar to those of the platelet release reaction, suggesting that the phosphorylation of these proteins may play a role in the platelet release reaction. When platelet sonicates or the supernatant fraction from platelet sonicates were incubated with [gamma-32P]ATP there was phosphorylation of both peak 7 and peak 9 proteins. This phosphorylation was unaffected by either added thrombin or adenosine 3',5'-cyclic monophosphate (cAMP) despite the presence of the phosphodiesterase inhibitor 1-methyl-3-isobutylxanthine. Thus, the thrombin-dependent phosphorylation depends upon intact platelets. When the supernatant fraction from platelet sonicates was fractionated by histone-Sepharose affinity chromatography, two distinct protein kinase enzymes were resolved, one a cAMP-dependent holoenzyme and the other a cAMP-independent enzyme. The isolated cAMP-dependent enzyme fraction catalyzed the cAMP-(but not thrombin-) stimulated phosphorylation of a protein that co-electrophoresed with peak 7 protein.
In human platelets stimulated by thrombin and collagen, diacylglycerol is rapidly produced from phosphatidylinositol. Concurrently, an endogenous protein having a molecular weig...
SUMMARY Fresh plasma freed of intact platelets can be shown to contain minute particulate material (platelet‐dust) which can be separated by ultracentrifugation. Much of this ma...
Platelet membrane glycoproteins IIb and IIIa were reconstituted into liposomes containing phosphatidylcholine. The reconstituted vesicles bound antiplatelet antibodies and showe...
We have investigated the composition and function of membrane microparticles released from platelets exposed to the C5b-9 proteins of the complement system. Gel-filtered human p...
Secretion of platelet granule constituents is closely associated with the phosphorylation of a cytosol polypeptide of Mr = 47,000 that we have called P47 (Haslam, R. J., Lynham,...
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