Activation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase in vitro

Hydroxymethylglutaryl coenzyme A reductase, the regulatory enzyme of cholesterol biosynthesis, was activated by in-vitro incubation of mouse liver microsomes with an artificial system consisting of Na2SO3, MgCl2 and EDTA. The extent and the time course of the activation were very similar to those observed with a 105 000 x g liver supernatant. Throughout its diurnal rhythm hydroxymethylglutaryl-CoA reductase was activated about threefold, while after cholesterol feeding the extent of activation was diminished but was not abolished. The activation could be inhibited by 0.1M fluoride, indicating that a phosphatase might be involved in the activation process.