HMG-CoA reductase and cholesterol 7-hydroxylase in human liver

The activities of HMGCoA reductase and cholesterol 7α-hydroxylase were assayed in liver biopsies of patients with or without cholestyramine treatment. The active dephosphorylated form of HMGCoA reductase and the activity of cholesterol 7α-hydroxylase were under the detection limits in untreated subjects. After cholestyramine treatment activities of both enzymes were stimulated and the active form of HMGCoA reductase became detectable in four out of the five tested patients. In two subjects who received cholestyramine, the effect of exogenously added [4-14C] cholesterol on cholesterol 7α-hydroxylase was tested. In the presence of Tween 80, the detergent by which [-14C] cholesterol was suspended, the enzyme activity was profoundly inhibited and synthesis of 7α-hydroxycholesterol was extremely low.