Rat‐Liver Acyl‐CoA: Cholesterol Acyltransferase

The activity of acyl‐CoA:cholesterol acyltransferase was measured in rat‐liver microsomes from the rate of incorporation of [1‐14C]oleate into cholesteryl esters, in the presence of ATP and coenzyme A. Endogenous cholesterol in the microsomal fraction and exogenously added cholesterol are available as susbstrate for acyl‐CoA:cholesterol acyltransferase. The kinetics of cholesterol esterification in the absence and in the presence of exogenously added cholesterol suggest that there is, in liver microsomes, a relatively small pool of cholesterol acting as substrate for acyl‐CoA:cholesterol acyltransferase that may support a constant rate of esterification for the assay of acyl‐CoA:cholesterol acyltransferase activity. The activity of acyl‐CoA:cholesterol acyltransferase at theendogenous cholesterol concentration was measured in preparations from rats in various experimental conditions, associated with changes in hepatic cholesterol metabolism, together with the activity of hydroxymethylglutaryl‐CoA reductase, of cholesterol 7α‐hydroxylase and the microsomal concentration of free and of esterified cholesterol. Feeding rats with a diet supplemented with cholesterol for 12 h resulted in a 2–3‐fold increase in the activity of acyl‐CoA:cholesterol acyltransferase, a decrease in Km value of the enzyme for oleate and an increase in the concentration of cholesteryl esters. There was also an increase in the activity of cholesterol 7α‐hydroxylase while the activity of hydroxymethylglutaryl‐CoA reductase declined considerably following the treatment. The diversion of bile acids from the enterohepatic circulation, achieved by feeding rats a diet supplemented with cholestyramine, resulted in a several‐fold increase in the activity of hydroxymethylglutaryl‐CoA reductase and of cholesterol 7α‐hydroxylase in the microsomal fraction, while there was no change in the activity of acyl‐CoA:cholesterol acyltransferase or in the concentration of cholesteryl esters. Moreover, there are considerable changes in the microsomal activity of hydroxymethylglutaryl‐CoA reductase and of cholesterol 7α‐hydroxylase associated with the light cycle, while the activity of acyl‐CoA:cholesterol acyltransferase and the concentration of cholesteryl esters did not exhibit a diurnal variation. These results are discussed in relation to the physiological mechanism underlying the regulation of acyl‐CoA:cholesterol acyltransferase activity.