Rat hepatic cholesterol 7 alpha-hydroxylase: biochemical properties and comparison to constitutive and xenobiotic-inducible cytochrome P-450 enzymes.

Cytochrome P-450Ch7 alpha (cholesterol 7 alpha-hydroxylase) catalyzes the first and rate-limiting step in the conversion of cholesterol to bile acids in mammalian liver. The properties of this cytochrome P-450 (P-450) form were studied in rat hepatic microsomal preparations in comparison to those of several well characterized constitutive and xenobiotic-inducible rat hepatic P-450s. Administration of the bile acid-sequestering resin cholestyramine [4% (w/w) in the diet] to male or female rats maintained on a reverse light cycle led to a 10- to 15-fold induction of P-450Ch7 alpha activity relative to untreated, standard light cycle controls. By contrast, the levels of four hepatic steroid hormone hydroxylating P-450 enzymes, designated 2a, 2c, 3, and PB-4 [Waxman, D.J. (1984) J. Biol. Chem. 259, 15481-15490], were not significantly affected by cholestyramine treatment. Antibody inhibition experiments established that P-450Ch7 alpha is immunochemically distinct from nine other rat hepatic P-450s, including P-450 3, a highly regio- and stereoselective steroid hormone 7 alpha-hydroxylase. P-450Ch7 alpha was shown to be selectively inactivated by micromolar concentrations of the disulfide-containing reagents disulfiram (Antabuse) and 2,2'-dithiopyridine. This inactivation was readily reversed upon incubation with 2-mercaptoethylamine, suggesting the presence of a highly reactive thiol group at the active site of P-450Ch7 alpha. These findings demonstrate that P-450Ch7 alpha corresponds to a unique P-450 enzyme exhibiting inductive, biochemical, immunochemical, and regulatory properties distinct from those of nine well-characterized rat hepatic P-450 forms.