Inhibition of cholesterol 7α-hydroxylase by an antibody to a male-specific form of cytochrome P-450 from subfamily P450IIC

The absence of antibodies to cholesterol 7 alpha-hydroxylase (EC 1.14.13.17), the rate-determining enzyme for bile acid synthesis, has significantly compromised studies on this protein. Nine antibodies raised against proteins from the cytochrome P-450 gene families (P450I, P450IIA, P450IIB, P450IIC and P450III) were tested as inhibitors of 7 alpha-hydroxylase activity. An antibody raised against a male-predominant P-450 (PB2a, P450h) from the P450IIC gene subfamily was an effective inhibitor of activity in liver microsomal fractions from rat, mouse and hamster. The inhibition could be reversed by the addition of PB2a antigen, indicating structural similarity between cholesterol 7 alpha-hydroxylase and proteins within the P450IIC subfamily. Western blot analysis of hepatic microsomal fractions with the PB2a antibody gave three bands, two of which, like cholesterol 7 alpha-hydroxylase, did not inhibit sexual dimorphism. The intensity of one of the bands (apparent Mr 54,000) correlated with changes observed in activity due to diet [Spearman correlation of 0.800 (P less than 0.01)]. These findings suggest that cholesterol 7 alpha-hydroxylase is a form of P-450 which shares structural similarity with cytochromes P-450 in the P450IIC gene subfamily and that its feedback regulation by bile acid involves protein induction rather than simply post-translational modification.