Chemical modification of acyl-CoA:cholesterol-O-acyltransferase. 2. Identification of a coenzyme A regulatory site by p-mercuribenzoate modification

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Abstract
Acyl-CoA:cholesterol O-acyltransferase (EC 2.3.1.26, ACAT) is the major intracellular cholesterol-esterifying activity in vascular tissue and is potentially a key regulator of intracellular cholesterol homeostasis during atherogenesis. We have previously reported inhibition of microsomal ACAT by histidine and sulfhydryl-selective chemical modification reagents and present here a more detailed analysis-of the effect of sulfhydryl modification on ACAT activity. This analysis indicated two effects of sulfhydryl modification on ACAT activity. Modification of aortic microsomes with relatively low concentrations of p-mercuribenzoate (PMB) (100-200 .mu.M) identified an inhibitory coenzyme A binding site on ACAT which contains a modifiable sulfhydryl group. This site binds CoA tightly (Ki = 20 .mu.M), and PMB modification prevented subsequent ACAT inhibition by CoA without itself inhibiting enzyme activity. At higher concentrations (1-2 mM), PMB inhibited ACAT activity, indicating the presense of a modifiable sulfhydryl group necessary for cholesterol esterification by ACAT. Modification of both sites by PMB was reversible by thiols, and protection against modification was afforded in both cases by oleoyl-CoA, indicating that these sites may also bind oleoyl-CoA. Thus, at least two sulfhydryl groups influence ACAT activity: one is necessary for cholesterol esterification by ACAT, and one is at or near an inhibitory CoA binding site, which may be occupied at intracellular concentrations of CoA.