The effect of cholestyramine on liver HMG-CoA reductase and cholesterol 7 alpha-hydroxylase in various laboratory animals.

The activity of HMG-CoA reductase and cholesterol 7 alpha-hydroxylase was assayed in the liver of rats, rabbits, hamsters and guinea pigs at the minimum of the day cycle and after one night fasting. The amount of HMG-CoA reductase, as determined after its complete dephosphorylation in vitro was of the same order of magnitude in the tested species. The dephosphorylated active form of the enzyme was detectable only in the rat. Cholesterol 7 alpha-hydroxylase activity was also much higher in the rat. Cholestyramine treatment stimulated the activity of both enzymes. In particular, the ratio between active and inactive HMG-CoA reductase in rabbits, hamsters and guinea pigs became of the same order of magnitude of that found in rats.