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 degrees C for 10 min. It was inhibited only up to 20%-30% by 1 mM 5,5'-dithiobis-(2-nitrobenzoic acid), which completely inactivates the serum
cholesterol esterifying enzyme,
lecithin:cholesterol acyltransferase (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 mumol/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.