Induction of cytochrome P-450 in congenic C57BL/6J mice by isosafrole: lack of correlation with the Ah locus.

Isosafrole induction of cytochrome P-450 was compared in congenic strains of C57BL/6J mice, one of which expresses normal levels of the Ah receptor [B6(Ahb)], and another that does not contain a measurable receptor concentration [B6(Ahd)]. Using sucrose gradient analysis of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) binding, an Ah receptor concentration of 69.1 +/- 3.8 fmol/mg protein was measured in the hepatic cytosol from B6(Ahb) mice, while no receptor could be detected in the cytosol from B6(Ahd) mice. Isosafrole treatment (75 mg/kg X 3 days) increased the total hepatic microsomal cytochrome P-450 content to the same extent in the two congenic strains. The level of microsomal monooxygenase induction in the isosafrole-treated B6(Ahd) mice was greater than that of B6(Ahb) mice for ethylmorphine N-demethylase and isosafrole metabolite-complex formation, the latter a measure of cytochrome P2-450. In the case of 7-ethoxycoumarin O-deethylase only the isosafrole-treated B6(Ahd) mice had elevated microsomal activity. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) also revealed a similar induction pattern for the two congenic strains, following isosafrole treatment. Thus, the isosafrole treated B6(Ahd) mice produced an equivalent or slightly larger induction of cytochrome P-450 than the B6(Ahb) mice, suggesting that there is no direct role for the Ah receptor in the regulation of these cytochrome P-450 monooxygenase activities by isosafrole.