The involvement of human placental microsomal cytochrome P-450 in aromatization.

The aromatization of androstenedione is inhibited in the presence of aminoglutethimide, diethylaminoethyl-2,2-diphenyl valerate HCl (SKF 525-A), and 1W3 M KCN, but not carbon monoxide or metyrapone. The role of cytochrome P-450 in this reaction could not, therefore, be unequivocally established by inhibition studies. However, it was determined that steroid binding to placental microsomal cytochrome P-450 is absolutely specific for aromatase substrates, intermediates, and inhibitors. Moreover, the substrate 19nortestosterone, aromatization of which is inhibited by CO, is a competitive inhibitor of both the aromatization and cytochrome P-450 binding of androstenedione, and vice versa. These results suggest that a single species of cytochrome P-450 is responsible for the aromatization of both steroids. Furthermore, both 19-hydroxyandrostenedione and 19-0x0androstenedione also compete with androstenedione and 19-nortestosterone for the same species of cytochrome P-450. This suggests that all three hydroxylation reactions of Cl9 aromatization as well as the aromatization of 19-nortestosterone are carried out at the same enzyme site. In addition, an antibody raised against porcine hepatic NADPH-cytochome c reductase, which specifically inhibits cytochrome P-450 mediated reactions, is a potent inhibitor of the aromatization of both Cl9 and Cl8 estrogen precursors. The extent of inhibition of the CO-sensitive aromatization of CL8 substrates by the antibody is similar to that observed for CO-sensitive drug hydroxylations occurring in liver microsomes. On the other hand, inhibition of the aromatization of the non-CO-sensitive substrate, androstenedione, by the antibody was atypical in that a much greater inhibitory