Modulation of reconstituted cholesterol 7 alpha-hydroxylase by phosphatase and protein kinase.

Cholesterol 7 alpha-hydroxylase activity was completely inhibited by incubation with alkaline phosphatase in a reconstituted enzyme system containing a cytochrome P-450, NADPH-cytochrome P-450 reductase and phospholipid. On the other hand, cAMP-dependent protein kinase stimulated cholesterol 7 alpha-hydroxylase activity by 2.5-fold. The modulation of cholesterol 7 alpha-hydroxylase activity was dependent on the amount of phosphatase or kinase added. The phosphatase inhibited enzyme activity was partially reversed by the treatment with protein kinase. These experiments indicate that the reconstituted cholesterol 7 alpha-hydroxylase activity is reversibly regulated by phosphorylation/dephosphorylation mechanism.