Regulation of acyl coenzyme A:cholesterol acyltransferase in the luteinized rat ovary: observations with an improved enzymatic assay.

Abstract

The corpus luteum of the rat stores large quantities of cholesteryl ester which is synthesized by acyl coenzyme A:cholesterol acyltransferase (ACAT), a microsomal enzyme. In previous studies of ACAT, assays that were probably substrate limited were employed. To reevaluate the regulation of ACAT in the ovaries of PMSG-hCG-primed rats, we used an improved assay in which exogenous cholesterol is provided as a dispersion in Triton WR-1339. Assayed in the absence of exogenous cholesterol, ACAT activity increased between days 1--8 post-hCG treatment and then declined between days 8--15. When exogenous cholesterol was included, a similar pattern of ACAT activity was found, but rates of cholesteryl ester formation were 2.5- to 4.5-fold greater. These changes in ACAT paralleled the previously reported levels of ovarian cholesteryl esters. Treatment of rats with 4-aminopyrazolopyrimidine, a drug that lowers blood cholesterol levels and reduces ovarian sterol ester stores, resulted in a lowered ACAT activity measured in the absence of exogenous sterol. However, enzyme activity was similar to that in controls when assayed in the presence of cholesterol. Inhibitors of steroidogenesis (aminoglutethimide and cycloheximide) promoted, within 4 h, ovarian sterol ester storage and resulted in increased ACAT activities measured in the absence of cholesterol. However, in the presence of exogenous sterol, the ACAT activities of controls were equal to those of drug-treated animals. When PMSG-hCG-primed animals received iv injections of hCG on day 8 post-hCG, ovarian sterol ester stores were markedly depleted within 2 h. The ovarian ACAT activity of hCG-treated rats measured without cholesterol was significantly lower than that of controls. With cholesterol, ACAT activities of hCG-treated rats were similar to those in controls. Our findings indicate that the entry of cholesterol into the ACAT substrate pool may be a major factor controlling the rates of cholesteryl ester synthesis in the rat corpus luteum.