Methoxyresondin: An inappropriate substrate for CYP1A2 in the mouse

Hepatic microsomes derived from Cyp1a2(-/-) knockout (KO) and parental stra ins of mice, C57BL/6N and 129Sv, were used to examine the specificity of me thoxyresorufin and acetanilide as substrates for CYP1A2 activity. In additi on, animals from each group were exposed to CYP1-inducing compounds. As exp ected, microsomes from untreated 1a2 KO mice did not have immunodetectable CYP1A2 protein; however, methoxyresorufin-O-demethylase (MROD, 25.5 +/- 6.1 pmol/min/mg protein) and acetanilide-4-hydroxylation (ACOH, 0.64 +/- 0.04 nmol/min/mg protein) activities were still present. Furthermore, induction of ethoxyresorufin-O-deethylase (EROD) by 2,3,7,8-tetrachlorodibenzo-p-diox in (TCDD) in 1a2 KO mice was accompanied by a greater than 70-fold increase in MROD activity. In contrast, ACOH was only induced 2-fold by TCDD. As wi th 1a2 KO mice, the parental strains exposed to TCDD or 2,3,4,7,8-pentachlo rodibenzofuran (4-PeCDF) showed substantial EROD and MROD induction, wherea s ACOH activity was induced to a lesser degree. PCB153 (2,2',4,4',5,5'-hexa chlorobiphenyl) resulted in low levels of both EROD and MROD induction. Res ults indicate that both substrates are subject to metabolism by non-CYP1A2 sources, and the apparent contribution of CYP1A1 activity to methoxyresoruf in metabolism makes MROD unsuitable for differentiating CYP1A1 and CYP1A2 a ctivities in the mouse. BIOCHEM PHARMACOL 56;12:1657-1660, 1998. (C) 1998 E lsevier Science Inc.