Inhibition of microsomal cholesterol ester hydrolase by okadaic acid in isolated rat hepatocytes.

Okadaic acid, a potent and specific inhibitor of protein phosphatases 1 (IC50 10-20 nM) and 2A (IC50 0.05-2 nM) caused early and sustained inhibitions of microsomal cholesterol ester hydrolase activity in hepatocyte suspensions. The changes in the kinetic properties of the esterase and its response to exogenous alkaline phosphatase and cyclic AMP-dependent protein kinase after cell exposure to 1 microM or 1 nM okadaic acid differed markedly among themselves, which suggests the involvement of both protein phosphatases 1 and 2A in the regulation of the microsomal hydrolysis of cholesterol esters. Furthermore, the inhibitory effect of okadaic acid is likely to be independent of the dibutyryl-cyclic AMP promoted cell events leading to stimulation of esterase activity.