Stimulation of the microsomal acyl-CoA: cholesterol acyltransferase by high-cholesterol diets in rabbit liver and small intestine.

Modulation of acyl-CoA: cholesterol acyltransferase (ACAT; EC 2.3.1.26) activity by different diets or coincubation with serum lipoproteins was studied in rabbit liver and intestinal mucosa microsomal preparations. The total amount of cholesterol in the serum lipoproteins, hepatic cells and small intestine mucosa changes in response to changes in cholesterol dietary period. The concentration of either esterified or unesterified cholesterol present in liver and small intestine microsomes remains within a relatively narrow range, whereas the activity of ACAT increased remarkably. In order to clarify the mechanism by which the activity of ACAT was stimulated, liver microsomes of normal rabbits were preincubated with lipoproteins. All lipoprotein fractions stimulated microsomal ACAT activity, among which the intermediate lipoprotein (IDL) showed most effective. These results suggest that ACAT activity in microsomes was affected not only by the amount of microsomal cholesterol, but also by an extra-microsomal soluble protein such as lipoproteins.