Effect of membrane environment on inhibition of acyl-CoA:cholesterol acyltransferase by a range of synthetic inhibitors.

The effect of the membrane environment of acyl-CoA:cholesterol acyl transferase (ACAT), an important intracellular enzyme of cholesterol metabolism, on the properties of a range of inhibitors of varying potencies was studied. ACAT activity from rat liver was solubilised with 3% deoxycholate (97% solubilised activity). After dilution into cholesterol/phosphatidylcholine liposomes (molar ratio 0.35), the assay of this reconstituted system showed linearity with protein and time. Saturation with oleoyl-CoA was achieved at 10 microM. Comparison of the potency of the ACAT inhibitors in the reconstituted assay and in a microsomal assay revealed a relationship between the lipid content of the assay and the inhibitory activity for potent inhibitors of ACAT (CI976, CL277,082, YMI7E and DuP128). This relationship was unrelated to lipophilicity of the drugs. Octimibate, lovastatin and progesterone, none of which is a potent ACAT inhibitor but which have all been described as ACAT inhibitors in the literature, all had low potencies in both assay systems. These results suggest that the lipid concentration must be taken into account when comparing potencies of ACAT inhibitors. The present data also indicate that some compounds which inhibit cholesterol esterification may do so by an indirect mechanism.