Modulation of rat liver 3-hydroxy-3-methylglutaryl coenzyme A reductase by lipid inhibitors, substrates, and cytosolic factors.

Abstract

Oleic acid and oleoyl coenzyme A were found to be inhibitors of microsomal and purified 3-hydroxy-3methylglutaryl coenzyme A (HMG-CoA) reductase (EC 1.1.1.34)activities.Inhibition of the microsomal enzyme was time-dependent and complete in 10 min.The addition of oleic acid, but not of oleoyl-CoA, resulted in gross morphological changes in the microsomal vesicles.Purification of the enzyme increased its sensitivity to both inhibitors, indicating that the microsomal membrane was not a necessary component of the inhibition.In all instances, oleoyl-CoA was a more potent inhibitor than oleic acid.Either of the two substrates, HMG-CoA o r NADPH, could prevent the inhibition significantly when present prior to addition of the lipids.However, the substrates could not reverse the inhibition once it had occurred.3-Hydroxy-3-methylglutaric acid, coenzyme A, or both together had no preventive effect.The presence of HMG-CoA did not prevent the morphological changes seen in the presence of oleic acid, yet the enzyme activity was protected.Phosphatidylcholine or bovine serum albumin, both capable of binding to the lipids, were able to prevent the inhibition but were unable to reverse it.By contrast, crude rat liver cytosol reversed the inhibition completely.In the absence of NADPH, the incubation of purified enzyme with HMG-CoA resulted in inhibition of the enzyme; on the other hand, incubation of the purified reductase with NADPH, in the absence of HMG-CoA, resulted in an activation of the enzyme.These data strongly indicate that substrates (or inhibitors) may induce conformational changes in the enzyme molecule and that a regulatory mechanism possibly exists in the cell involving substrate levels, cytosolic factors, and naturally occurring inhibitors such as fatty acids and their acyl-CoA derivatives.A close correlation between hypercholesterolemia and atherosclerosis has spurred investigations of the mechanism of regulation of cholesterol biosynthesis in mammals.