Modulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity with cAMP and wth protein fractions of rat liver cytosol

3-Hydroxy-3-methylglutaryl coenzyme A reductase activity is diminished in several $\text{in vitro}$ liver systems preincubated in the presence of cAMP. Reductase activity in isolated, washed liver microsomes is inactivated by ATP, Mg⁺⁺, and a protein fraction separated from the liver cytosol. This effect is augmented by 3′–5′ cyclic AMP. Reductase activity in previously inactivated microsomes can be partially restored by incubation with a second protein fraction of the cytosol.