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In human platelets, the Ca2+ ionophore A23187 stimulated the phosphorylation of a 40 kDa protein and myosin light chain (MLC) to the same extents as those induced by thrombin, but the doses of A23187 for 40 kDa protein phosphorylation were higher than those for MLC phosphorylation, although the doses of thrombin for both reactions were nearly the same. Moreover, A23187 produced much less diacylglycerol than thrombin. However, the sites of the 40 kDa protein phosphorylated by the action of A23187 and thrombin were identical, and the 40 kDa protein phosphorylation induced by A23187 and thrombin was inhibited by tetracaine, an inhibitor for protein kinase C. Neither A23187 nor thrombin induced the production of a catalytic fragment of protein kinase C which might be generated by limited proteolysis with Ca2+-dependent protease. These results indicate that A23187 induces protein kinase C activation which phosphorylates the 40 kDa protein, but higher doses of A23187 are required for the activation of this enzyme than for the activation of MLC kinase.
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