Platelet protein phosphatases and their endogenous substrates.

One p-nitrophenyl phosphate phosphatase (A) and five protein phosphatases (B, C, D, E, F) with neutral pH optimum (7.0-7.5) were partially purified from human platelets. Protein phosphatases were activated by Mn2+ (B-F), Mg2+ (D, F) or Ca2+ (F) but all of them had substantial activity even in the presence of EDTA. The activity of phosphatase D was predominant when assayed in the presence of EDTA. Phosphatase F was significantly enhanced by Ca2+ and calmodulin and therefore considered to be calcineurin. Without strict substrate specificity, all protein phosphatases (B-F) dephosphorylated phosphoproteins like actin binding protein, 47k protein and myosin light chain. Thus, it was suggested that protein phosphatases might play a role in the down regulation of platelet function not only in the resting but agonist-stimulated platelets.