CYP2E1-dependent bioactivation of 1,1-dichloroethylene to reactive intermediates in murine and human lung and liver microsomes

1,1-Dichloroethylene (DCE), a chemical used in the manufacturing of flexible films and a widespread water contaminant, has been shown up be pneumotoxic and hepatotoxic in animal studies. Although results of previous investigations have showed an important role for cytochrome P450-dependent metabolism in mediating these toxic effects, the ultimate toxic intermediate(s) and isozyme selectivity in DCE bioactivation in lung and liver are unknown. The metabolites of DCE formed in rat liver microsomes have been previously identified as 2,2-dichloroacetaldehyde, DCE-epoxide, and 2-chloroacetyl chloride. These were identified indirectly by trapping them as stable GSH conjugates. However, it was not completely clear which DCE-metabolites gave rise to these conjugates. In the present investigation, we have examined the reaction of these metabolites with GSH using chromatographic and spectrophotometric techniques. We have also identified the reactive intermediates formed in murine and human lung and liver microsomal incubations. The DCE-epoxide reacted efficiently with GSH and formed the mono- and di-glutathione adducts, 2-S-glutathionyl acetate [C] and 2-(S-glutathionyl)acetyl glutathione) [B]. The equilibrium constant between the hydrate of 2,2-dichloroacetaldehyde (acetal) and the GSH conjugate S-(2,2-dichloro-1-hydroxy)ethyl glutathione heavily favored the acetal, indicating that this metabolite does not react appreciably with GSH and will not likely contribute significantly to GSH depletion in vivo. The major metabolite formed in microsomal incubations was the DCE-epoxide, as estimated from formation of conjugates [B] and [C]. Lower levels of the acetal of 2,2-dichloroacetaldehyde were also formed. Levels of the DCE-epoxide in liver microsomal incubations were higher than those in lung. The DCE-epoxide was produced at nearly 2-fold higher rates in murine lung compared with human lung. In contrast, liver microsomes from some patients yielded levels of the DCE-epoxide that were 2.5 to 3-fold higher than those in mice. Our data supported a strong role for CYP2E1 in catalyzing the formation of the DCB-metabolites. The formation of the DCE-epoxide, and the acetal of 2,2-dichloroacetaldehyde was inhibited by 50% by a CYP2E1 inhibitory monoclonal antibody. Induction of murine liver CYP2E1 with acetone caused a significant increase in DCE-epoxide formation, while inhibition of human liver CYP2E1 with diallyl sulphone (DASO2) caused a 50% reduction in levels of this metabolite. Our results supported the proposal that DCE-induced toxicity is mediated by P450-dependent metabolism, that the DCE-epoxide may be the most important reactive species, and that CYP2E1 plays a role in this activation. Furthermore, our data suggested that humans exposed to DCE may be at potential risk to deleterious effects in both lung and liver, and humans possessing high expression of CYP2E1 may be particularly susceptible to DCE-induced hepatotoxicity.