Rat liver acyl-coenzyme A:cholesterol acyltransferase: its regulation in vivo and some of its properties in vitro.

To gain insight into the role of the enzyme acyl-coenzyme A:cholesterol acyltransferase (ACAT) in cellular cholesterol homeostasis, its regulation in rat liver was investigated both in vivo and in vitro. In vitro assay conditions were optimized and some properties of the microsomal enzyme in vitro was also studied. Arrhenius plots of microsomal ACAT activity showed discontinuities at about 28-29 degrees C and 16-17 degrees C. Detergents above their critical micelle concentrations and organic solvents both inhibited the enzyme. Addition of progesterone or SC 31769, a 7-keto-cholesterol analogue, to microsomes inhibited activity while addition of 25-hydroxycholesterol increased the rate of cholesterol esterification, suggesting that the enzyme is susceptible to both negative and positive modulation by steroids, steroid analogues, or their metabolic products. Increasing the rate of cholesterol biosynthesis had a variable effect on ACAT activity. It was higher at the circadian peak of sterol biosynthesis than at the nadir. Increasing sterol biosynthesis by intragastric administration of mevalonolactone resulted in increased activity. In contrast, increasing the rate of sterol biosynthesis by feeding cholestyramine or administration of Triton WR 1339 had little effect on ACAT. Increasing hepatic cholesterol content by feeding cholesterol, cholate, or an atherogenic diet, fasting or intragastric administration of mevalonolactone all resulted in increased ACAT activity. ACAT activity showed a positive correlation with changes in microsomal free and esterified cholesterol contents. The response of ACAT to changes in hepatic cholesterol concentration in vivo and its response to changes in the rate of cholesterol synthesis support the hypothesis that this enzyme plays an important role in maintenance of hepatic cholesterol homeostasis.