You are here:
QUANTITATIVE EVALUATION OF BROMODICHLOROMETHANE METABOLISM BY RECOMBINANT RAT AND HUMAN CYTOCHROME P450S
Citation:
Allis, J W. AND G. Zhao. QUANTITATIVE EVALUATION OF BROMODICHLOROMETHANE METABOLISM BY RECOMBINANT RAT AND HUMAN CYTOCHROME P450S. CHEMICO-BIOLOGICAL INTERACTIONS 140(2002):137-153, (2002).
Description:
ABSTRACT
We report quantitative estimates of the parameters for metabolism of bromodichloromethane (BDCM) by recombinant preparations of hepatic cytochrome P450s (CYPs) from rat and human. BDCM is a drinking water disinfectant byproduct that has been implicated in liver, kidney and intestinal cancers in rodents and bladder and intestinal tumors, pregnancy loss, and low birth weight effects in humans. BDCM is acutely toxic to liver and kidney in rat, and requires metabolic activation for both toxic and carcinogenic effects. Earlier work identified CYP2E1, CYP2B1/2 and CYP1A2 as activating enzymes necessary for hepatotoxicity in rat. In order to extend an existing physiologically based pharmacokinetic model for rat to include a capability for extrapolation to humans, it is necessary to evaluate quantitatively the principal metabolic pathways in both species. We have conducted in vitro experiments using recombinant preparations of the three rat CYP isoenzymes mentioned above and for CYP2C11 and CYP3A1 as well. Similar experiments have been performed with human recombinant isoenzymes for CYP2E1, CYP1A2, CYP2A6, CYP2B6, CYP2D6 and CYP3A4. Results indicate that the principal metabolizing enzymes in rat are those previously identified, CYP2E1, CYP2B1/2 and CYP1A2. CYP3A1 may also have some activity. In human, CYP2E1, CYP1A2 and CYP3A4 show substantial activity, and CYP2A6 also measurably metabolizes BDCM. In both species, CYP2E1 is the low Km isoenzyme, with Km approximately 27-fold lower than those for the isoenzymes with the next lowest Km. In addition, the metabolic parameters, Km and kcat, for rat and human CYP2E1 were nearly identical. The metabolic parameters for CYP1A2, the only other isoenzyme active in both species, were not similar across species. In addition, calculations based on the kinetic constants obtained are compared to results from three in vivo experiments to show that the in vitro kinetic data is relevant to in vivo exposures. We conclude that although several CYPs metabolize BDCM, at low concentration/exposure, BDCM metabolism is dominated by CYP2E1 in both rat and human, but that other isoenzymes can be important at higher concentrations. We further conclude that the kinetic data are consistent with in vivo experiments.