Inactivation of cytochrome P-450 (CYP2E1) and carboxylesterase (hydrolase A) enzymes by vinyl carbamate in murine pulmonary microsomes

Abstract

We tested the hypothesis that vinyl carbamate (VC) is metabolized in vitro by cytochrome P-450 and carboxylesterase enzymes in murine lung. Incubations with VC and an NADPH-generating system produced a 50% decrease in N-nitrosodimethylamine (NDMA) demethylation and a corresponding loss in the amounts of immunodetectable CYP2E1. Preincubation of microsomes with a CYP2E1 inhibitory antibody or the CYP2E1-selective inhibitor diallyl sulfone (DASO2) inhibited demethylase activity; no alterations were detected upon subsequent exposure to VC. Carboxylesterase-mediated hydrolysis of p-nitrophenyl acetate was reduced by 22% in microsomes incubated with VC. Decreased carboxylesterase activity also was detected in microsomes incubated with phenylmethylsulfonyl fluoride (PMSF), an inhibitor of hydrolase A, a carboxylesterase isozyme. No change in enzyme activity was detected when microsomes were subsequently incubated with VC. The loss in carboxylesterase activity correlated with decreased immunodetectable hydrolase A in microsomes incubated with VC, PMSF, or PMSF and VC. The reduction in VC-induced NDMA demethylase activity was increased to 85% of the control in microsomes previously incubated with PMSF, and this corresponded with a marked decrease in CYP2E1 immunoreactivity in the immunoblots. Covalent binding of VC to proteins was detected in microsomes incubated with VC and an NADPH-generating system. Binding was inhibited in microsomes preincubated with either an inhibitory CYP2E1 antibody or DASO2. In contrast, binding levels were augmented in microsomes preincubated with PMSF. These data supported VC metabolism by CYP2E1 and hydrolase A in murine lung microsomes and is consistent with involvement of CYP2E1 and hydrolase A in the activation and detoxication of VC, respectively.