42-KDA PROTEIN AS A SUBSTRATE FOR PROTEIN PHOSPHATASE(S) IN INTACT HUMAN BLOOD-PLATELETS

The level of phosphorylation of any cellular protein depends on the ba lance of the activities of protein kinases and protein phosphatases th at act on the protein, In this study, we have characterized, in intact human blood platelets, the activity of protein phosphatase(s) that re verse the action of protein kinase C (PKC), using as a substrate, endo genous 42 kDa protein which has been previously phosphorylated by PKC. In this study 1,2-dihexanoyl-sn-glycerol (DHG) was used to stimulate PKC, diacylglycerol kinase inhibitor-R59022 was used to maintain the a ctivity of PKC and staurosporine and okadaic acid were used to inhibit PKC and protein phosphatases respectively. Our observations indicate that: (1)protein phosphatase 1 (PP1) and/or protein phosphatase 2A (PP 2A) are likely to be the enzymes that reverse the phosphorylation acti vity of PKC on the 42 kDa protein; (2) PP1 and/or PP2A dephosphorylate sites which have been previously phosphorylated by PKC; and (3) PP1 a nd/or PP2A dephosphorylate, on the 42 kDa protein, both serine and thr eonine residues, which have been previously phosphorylated by PKC.