Regulation of rat liver microsomal cholesterol 7 alpha-hydroxylase: reversible inactivation by ATP + Mg2+ and a cytosolic activator

Modulation of cholesterol 7 alpha-hydroxylase catalytic activity by adenine nucleotides was studied in rat liver microsomal preparations. Inactivation of cholesterol 7 alpha-hydroxylase showed specific requirements of ATP and ADP. AMP and cyclic AMP were stimulatory and cyclic AMP had no effect in the ATP inactivation. The inactivation reactions by ATP were dependent on Mg2+ ions, a cytosolic factor, and time. Ca2+ ions were less effective whereas Mn2+ ions were highly inhibitory to hydroxylase activity. The inactivation could be reversed in a time-dependent reaction requiring a cytosolic activator that was precipitable by ammonium sulphate of saturation up to 65%. The current data suggest that cholesterol 7 alpha-hydroxylase can exist in two catalytic forms that are reversible.