and the Muscular Dystrophy Association (1985-1986).

Protein kinase C is generally accepted to be a receptor protein of tumor-promoting phorbol esters. The binding of PHlphorbol-n.lS-dibutyrate to protein kinase C can be assayed by a rapid filtration procedure using a glass-fiber filter that has been treated with a cationic polymer, polyethylenimine. The phorbol ester specifically binds to the protein kinase only in the presence of phosphatidylserine and calcium. Non-specific binding is less than 10%, at most, of the total binding. The binding is linear with respect to the concentration of protein kinase C, is dependent on the concentrations of phorbol ester and phosphatidylserine in a saturative manner, and is inhibited by diacylglycerol (an endogenous activator of the protein kinase). Tumor-promoting phorbol esters have a variety of biological activities such as induction of cell pro-