COMPARATIVE-STUDIES ON COUMARIN AND TESTOSTERONE-METABOLISM IN MOUSE AND HUMAN LIVERS - DIFFERENTIAL INHIBITIONS BY THE ANTI-P450COH ANTIBODY AND METYRAPONE

We have studied coumarin 7-hydroxylase (COH) and testosterone 15-alpha-hydroxylase (15-alpha-OH) activities in human liver microsomes and compared them with corresponding activities catalysed by members of the P450IIA sub-family in DBA/2N mouse liver microsomes. Human liver contained low levels of 15-alpha-OH (about 5-30 pmol/min/mg protein) when compared with control mouse liver microsomes (about 200 pmol/min/mg protein). The anti-P450Coh antibody efficiently inhibited mouse liver 15-alpha-OH, also 7-alpha-OH (which is a member of the P450IIA sub-family), but it did not inhibit human 15-alpha-OH or other testosterone hydroxylases. In mouse liver microsomes, metyrapone preferentially inhibited 15-alpha-OH, but in human liver microsomes it inhibited all testosterone hydroxylations measured, including 15-alpha-OH (IC50 = 2.0-5.0-mu-M). Metyrapone clearly inhibited COH in mouse liver microsomes, but interestingly it had no effect on COH activity in human liver microsomes, although these two isozymes have earlier been shown to be immunologically similar. On the basis of available evidence human and mouse P450Coh isozymes seem to be orthologous enzymes whereas the present results indicate that the human 15-alpha-OH is different from the mouse P450(15-alpha).