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KINETICS OF BROMODICHLOROMETHANE METABOLISM BY CYTOCHROME P450 ISOENZYMES IN HUMAN LIVER MICROSOMES
Citation:
Allis, J W. AND G. Zhao. KINETICS OF BROMODICHLOROMETHANE METABOLISM BY CYTOCHROME P450 ISOENZYMES IN HUMAN LIVER MICROSOMES. CHEMICO-BIOLOGICAL INTERACTIONS 140(2002):155-168, (2002).
Description:
Kinetics of Bromodichloromethane Metabolism by
Cytochrome P450 Isoenzymes in Human Liver Microsomes
Guangyu Zhao and John W. Allis
ABSTRACT
The kinetic constants for the metabolism of bromodichloromethane (BDCM) by three cytochrome P450 (CYP) isoenzymes have been measured in human liver microsomes. BDCM is a drinking water disinfection byproduct normally found in chlorinated drinking water at the second highest concentration of all byproducts. The three CYP isoenzymes, CYP2E1, CYP1A2 and CYP3A4, have been identified previously as important in the metabolism of this compound. To measure the constants for each isoenzyme, enzyme-specific inhibitory antibodies were used to block the activities in two of the three isoenzymes. CYP2E1 was found to have the lowest Km, 2.9 M, and the highest catalytic activity, kcat. The Km for the other isoenzymes, CYP1A2 and CYP3A4, were about 60 M with lower values of kcat. Apparent kinetic constants for two microsomal samples that were not inhibited were consistent with these results. In addition, 11 human microsome samples characterized for 10 CYP activities were correlated with the metabolism of 9.7 M BDCM by each sample; statistical analysis showed a correlation with CYP2E1 activity only. This result is consistent with the finding that CYP2E1 is the only isoenzyme with a Km lower than the BDCM concentration used. The kinetic constants were compared to similar results from recombinant human isoenzyme preparations containing only one CYP isoenzyme. The results for CYP2E1 were very similar, while the results for CYP1A2 were somewhat less similar and there was a substantial divergence for CYP3A4 in the two systems. Possible reasons for these differences are differing levels of CYP reductase and/or differing makeup of the membrane lipid environment for the CYPs. Because of the low levels of BDCM exposure from drinking water, it appears likely that CYP2E1 will dominate hepatic CYP-mediated BDCM metabolism in humans.