Purification of cholesterol 7α-hydroxylase from human and rat liver and production of inhibiting polyclonal antibodies

Cholesterol 7α-hydroxylase, the cytochrome P-450-dependent and rate-controlling enzyme of bile acid synthesis, was purified from rat and human liver microsomes. The purified fractions were assayed in a reconstituted system containing [4-¹⁴C]cholesterol, and cholesterol 7α-hydroxylase activities in these fractions increased 500-600-fold relative to whole microsomes. Polyacrylamide gel electrophoresis of rat microsomes followed by immunoblotting with polyclonal rabbit antisera raised against purified cholesterol 7α-hydroxylases revealed two peaks at molecular masses of 47,000 and 49,000 daltons for both rat and human fractions. Increasing amounts of rabbit anti-rat and anti-human antibodies progressively inhibited rat microsomal cholesterol 7α-hydroxylase activity up to 80%. In contrast, monospecific antibodies raised against other purified cytochrome P-450 enzymes (P-450f, P-450g, and P-450j) did not inhibit rat or human cholesterol 7α-hydroxylase activity. Immunoblots of rat microsomes with the rabbit anti-rat cholesterol 7α-hydroxylase antibody demonstrated that the antibody reacted quantitatively with the rat microsomal enzyme. Microsomes from cholesterol-fed rats showed increased cholesterol 7α-hydroxylase mass, whereas treatment with pravastatin, an inhibitor of hydroxymethylglutaryl-coenzyme A reductase, reduced enzyme mass. Microsomes from starved rats contained slightly less cholesterol 7α-hydroxylase protein than chow-fed control rats. These results indicate a similarity in molecular mass, structure, and antigenicity between rat and human cholesterol 7α-hydroxylases; demonstrate the production of inhibiting anti-cholesterol 7α-hydroxylase antibodies that can be used to measure the change in cholesterol 7α-hydroxylase enzyme mass under various conditions; and emphasize the unique structure of cholesterol 7α-hydroxylase with respect to other cytochrome P-450-dependent hydroxylases.