The effect of cholestyramine and Mevinolin on the diurnal cycle of rat hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase.

Rats were fed powdered rat chow or a rat chow diet containing 5% cholestyramine or 5% cholestyramine with Mevinolin (112 mg/100 g food, 200 mg/kg body weight per day). The specific activity of hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase was determined at different times during the diurnal cycle of the enzyme. Animals fed cholestyramine had higher specific activities of HMG-CoA reductase at all time points tested when compared to controls. The specific activity at the peak in the diurnal cycle was approximately 8-fold higher in cholestyramine-treated animals. Rats administered the cholestyramine-Mevinolin diet had higher specific activities of the enzyme than either cholestyramine-treated or control animals. In the cholestyramine-Mevinolin-treated animals the peak in the diurnal cycle was shifted to D-12 (12th hour of the dark cycle) and the specific activity at this point was approximately 133-fold greater than the basal (L-6) activity in control animals. Optimal conditions for immunotitration studies were determined such that valid conclusions could be drawn from these data. Based on immunotitration experiments, the increased hepatic HMG-CoA reductase activity in cholestyramine-treated animals resulted in part from a 3-fold activation of the enzyme, while the increased specific activity in the cholestyramine-Mevinolin-treated animals was due solely to increased enzyme mass. Hence the administration of Mevinolin blocked the activation of the enzyme induced by feeding cholestyramine alone. This was confirmed by purifying the enzyme to apparent homogeneity from cholestyramine-Mevinolin-treated animals; the specific activity of this enzyme was 4,000-7,700 nmol NADPH oxidized/min per mg protein. Cholestyramine-Mevinolin treatment affords a novel system for generating milligram quantities of rat hepatic HMG-CoA reductase.-Tanaka, R. D., P. A. Edwards, S. Lan, E. M. Knöppel, and A. M. Fogelman. The effect of cholestyramine and Mevinolin on the diurnal cycle of rat hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase.