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Cis-unsaturated fatty acids activate soluble protein kinase C (PKC) in vitro and in intact platelets. The following studies were conducted to determine the effects of oleate on individual isoenzymes of PKC in human platelets. Human platelets were found to contain predominantly PKC alpha, beta I, beta II, and delta with minor immunoreactivity for PKC epsilon, zeta, and eta. In intact platelets, sodium oleate caused a time-dependent redistribution of PKC alpha, beta II, and delta from cytosol to membrane fractions with little effects on PKC beta I. On the other hand, PMA and thrombin induced translocation of all four isoenzymes of PKC. In vitro, oleate partially activated (50% of Vmax) purified calcium-dependent PKC (alpha, beta I, and beta II) with an EC50 of 50 microM whereas it fully activated (100% of Vmax) purified calcium-independent PKC (predominantly delta) with an EC50 of 5 microM. The selective effects of oleate on PKC isoenzymes were investigated in platelet cytosol which contains endogenous PKC and its physiologic substrates. Under these conditions, oleate potently activated calcium-independent PKC causing the phosphorylation of the 40-kDa substrate. Activation of calcium-dependent isoforms occurred only at higher concentrations of oleate. Thus, oleate activates multiple isoenzymes of PKC with predominant effects on calcium-independent PKC.
Khan et al. (Mon,) studied this question.