Methoxyresorufin: an inappropriate substrate for CYP1A2 in the mouse.
Hepatic microsomes derived from Cypla2(-/-) knockout (KO) and parental strains of mice, C57BL/6N and 129Sv, were used to examine the specificity of methoxyresorufin and acetanilide as substrates for CYP1A2 activity. In addition, animals from each group were exposed to CYP1-inducing compounds. As expected, 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-dioxin (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 with 1a2 KO mice, the parental strains exposed to TCDD or 2,3,4,7,8-pentachlorodibenzofuran (4-PeCDF) showed substantial EROD and MROD induction, whereas ACOH activity was induced to a lesser degree. PCB153 (2,2',4,4',5,5'-hexachlorobiphenyl) resulted in low levels of both EROD and MROD induction. Results indicate that both substrates are subject to metabolism by non-CYP1A2 sources, and the apparent contribution of CYP1A1 activity to methoxyresorufin metabolism makes MROD unsuitable for differentiating CYP1A1 and CYP1A2 activities in the mouse.
Hamm
J T
JT
Curriculum in Toxicology, University of North Carolina, Chapel Hill 27599-7270, USA. hamm.jonathan@epamail.epa.gov
Ross
D G
DG
Richardson
V M
VM
Diliberto
J J
JJ
Birnbaum
L S
LS
D000818:Animals; D001189:Aryl Hydrocarbon Hydroxylases / Q000096:biosynthesis / Q000378:metabolism; D001572:Benzofurans; D019363:Cytochrome P-450 CYP1A1 / Q000096:biosynthesis / Q000378:metabolism; D019388:Cytochrome P-450 CYP1A2 / Q000096:biosynthesis / Q000378:metabolism*; D003577:Cytochrome P-450 Enzyme System / Q000096:biosynthesis / Q000378:metabolism; D004790:Enzyme Induction; D008297:Male; D051379:Mice; D008815:Mice, Inbred Strains; D018345:Mice, Knockout; D008862:Microsomes, Liver / Q000187:drug effects / Q000201:enzymology*; D010078:Oxazines / Q000378:metabolism*; D010088:Oxidoreductases / Q000096:biosynthesis / Q000378:metabolism; D011078:Polychlorinated Biphenyls; D000072317:Polychlorinated Dibenzodioxins; D013379:Substrate Specificity