Benzo[a]pyrene metabolism and macromolecular binding in strains of Ah responsive and Ah non-response mice

Using liver microsomes as the enzyme source in in vitro assays, benzo[a]pyrene (BP) metabolism was studied in eight inbred strains of mice (C57BL/6J, DBA/2HaD, BALB/cCR, AKR/Sn, RF/J, CBA/J, C57L/J and 129/J). BP metabolite formation was monitored by high pressure liquid chromatography (HPLC) and by following aryl hydrocarbon hydroxylase (AHH) activity. Other parameters measured in cluded the formation of alkali-extractable radioactivity due to [3H] BP metabolites, microsomal protein binding, DNA binding and epoxide hydrase activity. The induction of BP metabolism and binding to macromolecules was studied after treatment of animals with phenobarbital (PB) or 3-methylcholanthrene (MC). Four of the mouse strains (C57BL/6J, BALB/cCr, CBA/J and C57L/J) were highly inducible with respect to liver AHH when pretreated with MC. The induction of AHH by MC in these strains correlated well with the radioactive metabolites of [3H] BP remaining in the alkali extract derived from the AHH assay mixture and with the increased binding of [3H] BP to microsomal protein and DNA. In addition to the eight strains listed above, eight recombinant inbred lines showed a positive correlation between AHH induction and induction of DNA-binding metabolites. PB pretreatment resulted in less than two-fold induction of AHH and alkali-extractable radioactivity. However, DNA and microsomal protein binding were induced by PB pretreatment more than AHH. Ratios of MC-induced/basal activity for BP-phenols were very similar to induction ratios of AHH activity determined by the fluorometric method. BP-quinone formation was induced to the same extent as phenols. This relationship did not hold for dihydrodiol formation; dihydrodiol induction was often higher than AHH or phenol induction. For MC-pretreated mice, dihydrodiol induction, as determined by HPLC, did not parallel macromolecular binding induction as closely as did AHH. For PB-pretreated mice, dihydrodiol induction was as poor as indicator of binding induction as AHH. Epoxide hydrase activity, using styrene oxide as substrate, was induced markedly by PB-pretreatment, but very little by MC-pretreatment. Epoxide hydrase induction did not parallel BP-dihydrodiol induction when microsome preparations from MC- or PB-treated mice were used. These data suggest this enzyme is not rate limiting in this system.