The sterol substrate specificity of acyl CoA: :cholesterol acyltransferase from rat liver

Rat liver microsomes were incubated with various sterols suspended in Triton WR-1339, and the extent of esterification of these sterols by acyl CoA:cholesterol acyltransferase was determined. A 3 beta-hydroxyl group was required for esterification to occur. Furthermore, the rate of ester formation of campesterol was only 20% that of cholesterol, and the rates for sitosterol and stigmasterol were below detectable limits indicating that the structure of the alkyl side chain plays an important role in the interaction between substrate and enzyme. Additional evidence concerning the importance of the side chain was obtained by following the esterification of a series of linear side chain analogues of cholesterol. Maximal ester formation was obtained when the longest chain on C-20 had five carbons (the same as cholesterol) and either an increase or decrease in the number of carbons reduced the amount of ester formed. Sterols containing a 4-gem-dimethyl group were not esterified, while 4 alpha-methylcholest-7-en-3 beta-ol showed significant esterification. Lathosterol, cholestanol, and desmosterol were esterified 41%, 70%, and 62%, respectively, as well as was cholesterol. The relationship between the specificity of acyl CoA:cholesterol acyltransferase and the occurrence of sterol esters in tissues is discussed.