Two separate tyrosine protein kinases in human platelets

Abstract

Tyrosine protein kinase activities were detected in the cytosolic fraction (PC‐TPK) and the particulate fraction (PM‐TPK) in human platelets using the synthetic peptide, E 11 G 1 , (Glu‐Asp‐Ala‐Glu‐Tyr‐Ala‐Ala‐Arg‐Arg‐Arg‐Gly) as a substrate. PC‐TPK and PM‐TPK were different in substrate specificities, divalent cation requirements and apparent M r values. These results strongly suggest that in platelets there exist at least two separate tyrosine protein kinases; one is present in cytosol and the other might be associated with membranes.