Regulation of the intracellular calcium level in human blood platelets: cyclic adenosine 3',5'-monophosphate dependent phosphorylation of a 22,000 dalton component in isolated Ca2+-accumulating vesicles.

Two protein kinase activities have been separated from the supernatants of homogenized human blood platelets by DEAE cellulose chromatography. One of them (peak I enzyme) is an efficient stimulator of the uptake of Ca2+ into isolated membrane vesicles in the presence of cyclic AMP and ATP. The second (peak II enzyme), although equally active towards histone, exerts only about one third of the activity of the peak I enzyme. The stimulation of Ca2+ uptake is accompanied by the phosphorylation of a membrane protein with an apparent molecular weight of 22 000, which appears to play an essential role in the regulation of the intracellular Ca2+ level and hence of platelet activity.