Evidence against in vitro modulation of rat liver cholesterol 7 alpha-hydroxylase activity by phosphorylation-dephosphorylation: comparison with hydroxymethylglutaryl CoA reductase.

The activity of cholesterol 7 alpha-hydroxylase in rat liver microsomes was investigated under conditions favourable for phosphorylation-dephosphorylation. The enzyme activity was similar in the presence or absence of sodium fluoride during preparation. Preincubation with ATP and magnesium did not affect the enzyme activity. Cholesterol 7 alpha-hydroxylase was inhibited by alkaline phosphatase, but this inhibition was similar also after inactivation of the phosphatase. Under similar conditions, rat hepatic hydroxymethylglutaryl CoA reductase activity was clearly modulated in agreement with phosphorylation-dephosphorylation. The absence of such a modulation of cholesterol 7 alpha-hydroxylase argues against involvement of phosphorylation-dephosphorylation in the regulation of this enzyme.