Acyl-coenzyme A:Cholesterol acyltransferase in human liver. In vitro detection and some characteristics of the enzyme

Abstract

The enzyme, acyl-coenzyme A:cholesterol acyltransferase (ACAT), is responsible for the intracellular esterification of cholesterol. Although it has been detected in the liver from a variety of animals and in human skin fibroblasts and human intestine, it has been reported to be absent from human liver. Since this enzyme may play an important role in cholesterol homeostasis, evidence for its presence in human liver was again sought. Using labeled oleoyl CoA and the endogenous cholesterol as reactants, ACAt was detected in fresh samples of human liver obtained from patients undergoing staging laparotomy for Hodgkin's disease. The enzyme is present almost exclusively in membrane fractions with little activity detected in cytosol. Microsomal ACAt activity was linear with incubation time for up to 10 min. After this, the rate of cholesterol esterification remained constant despite the fact that adequate acyl CoA was present as judged by the continued incorporation of oleate into triglyceride. ACAT activity is destroyed by heating at 100°C for 10 min. It was inhibited only up to 20%–30% by 1 m M 5,5′-dithiobis-(2-nitrobenzoic acid), which completely inactivates the serum cholesterol esterifying enzyme, lecithin:cholesterol acyltranferase (LCAT). Like ACAT in human skin fibroblasts, human liver ACAT was also inhibited by progesterone in vitro. ACAT activity averaged 10.3 ± 5.1 pmole cholesteryl oleate/min/mg microsomal protein for 3 normal livers and 39.0 ± 12.5 for 2 fatty livers. Thus, the level of ACAT activity estimated for the whole liver was 2.1–35.8 μmole/hr in the fasting state. This activity may account for some portion of the cholesterol esters present in plasma VLDL in fasting normolipidemic individuals. However, it is likely that the major role of hepatic ACAT is in the regulation and maintenance of hepatic cholesterol homeostasis.