Differences in mechanisms of modulation between rat liver cholesterol 7 alpha-hydroxylase and HMG-CoA reductase

The effects of microsomal HMG-CoA reductase kinase, cytosolic phosphoprotein phosphatase and cytosolic, thiol-dependent cholesterol 7 alpha-hydroxylase stimulatory protein on purified cholesterol 7 alpha-hydroxylase and HMG-CoA reductase from rat liver were compared. Neither HMG-CoA reductase kinase nor phosphoprotein phosphatase had any significant effect on cholesterol 7 alpha-hydroxylase activity. They inhibited and stimulated, respectively, the activity of HMG-CoA reductase. The purified cytosolic protein which stimulated cholesterol 7 alpha-hydroxylase threefold in the presence of glutathione had no effect on HMG-CoA reductase. The results show that there are separate intracellular systems for modulation of cholesterol 7 alpha-hydroxylase and HMG-CoA reductase.