Involvement of CYP2E1 and carboxylesterase enzymes in vinyl carbamate metabolism in human lung microsomes.

Previous studies have shown that CYP2E1 and carboxylesterase enzymes contributed to vinyl carbamate (VC) metabolism in murine lung. Moreover, these studies have implicated CYP2E1 and the carboxylesterases in bioactivation and detoxication, respectively. Here we have tested the hypothesis that CYP2E1 and carboxylesterase enzymes are involved also in VC metabolism in human lung. Demethylation of N-nitrosodimethylamine (NDMA) is an enzyme activity associated with CYP2E1, and was used as a catalytic marker for this P450 in human lung microsomes. NDMA demethylase activity in lung microsomes from 10 patients ranged from 36.9 +/- 1.0 to 82.4 +/- 2.4 pmol/mg protein/min. Significant decreases (40-65%) in demethylase activity were detected in lung microsomes incubated with VC and NADPH, compared with the controls in which incubations were performed with only VC or only NADPH. Preincubation with the CYP2E1 inhibitor diallyl sulfone also significantly decreased demethylase activity, and abrogated the VC-induced effect. Similarly, preincubation of lung microsomes with a human CYP2E1 inhibitory monoclonal antibody ameliorated the VC-induced reduction in demethylase activity. Microsomal carboxylesterase activity in lung microsomes from 10 patients ranged from 19.02 +/- 2.28 to 48.18 +/- 4.34 nmol/mg protein/min, and was significantly decreased (25-45%) in microsomes incubated with phenylmethylsulfonyl fluoride, an inhibitor of the carboxylesterase enzyme. Preincubation of lung microsomes with phenylmethylsulfonyl fluoride and subsequent incubation with VC and NADPH exacerbated the reduction (60-80%) in demethylase activity evoked by reaction with VC and NADPH. These results are consistent with a role for the CYP2E1 enzyme and microsomal carboxylesterases in VC metabolism.