3-Hydroxy-3-methylglutaryl-coenzyme A reductase. The difference in the mechanism of the in vitro modulation by phosphorylation and dephosphorylation to modulation of enzyme activity by non-esterified cholesterol.

Incubation of rat liver microsomal fraction in the presence of increasing concentration of a serum preparation and the re-isolation of the treated microsomal vesicles resulted in a progressive increase in the concentration of non-esterified cholesterol, a progressive decrease in the activity of hydroxymethylglutaryl-CoA reductase and progressive changes in the characteristics of the Arrhenius plots of the enzyme. The changes in the characteristics of the Arrhenius plots of the enzyme in the serum-treated preparations are consistent with a progressive increase in the concentration of non-esterified cholesterol in the environment of the hydroxymethylglutaryl-CoA reductase in endoplasmic reiticulum vesicles. The serum-treated preparations with high non-esterified cholesterol content showed a constant activation energy between 37 and 20 degrees C, whereas the enzyme in the non-treated microsomal fraction, the buffer-treated and the lipoprotein-deficient serum-treated preparations showed breaks in the activation energy at about 29 degrees C. The microsomal fraction from rats fed on the standard, cholesterol- or cholestyramine-supplemented diet showed considerable differences in the activity of hydroxymethylglutaryl-CoA reductase and differences in the characteristics of their Arrhenius plots. However, the incubation of the microsomal fraction from the rats in the three experimental conditions with ATP and Mg2+ and the further incubation of the inactivated enzyme with a preparation of cytosolic phosphoprotein phosphatase resulted in Arrhenius plots with similar characteristics to those of the corresponding original microsomal fraction. These results suggest that changes in the concentration of non-esterified cholesterol in the endoplasmic reticular membrane are responsible for the differences in the activity of hydroxymethylglutaryl-CoA reductase in the microsomal fraction from the rats in these dietary conditions.