Induction of DNA repair in rat spermatocytes and hepatocytes by 1,2-dibromoethane: the role of glutathione conjugation

Abstract

1,2-Dibromoethane (EDB) is a widely used industrial chemical, and a well-known mutagen and carcinogen. EDB is biotransformed either by cytochrome P450-dependent oxidation, leading to the formation of bromoacetaldehyde, or by enzyme-catalyzed conjugation with glutathione, giving rise to reactive half-sulfur mustard compounds and their derivatives. In vitro mutagenicity and DNA binding studies suggest that the latter pathway is the primary source of genotoxic metabolites from EDB. In this study we have examined EDB-induced unscheduled DNA synthesis (UDS) in F-344 rat pachytene spermatocytes and hepatocytes. EDB (10-100 microM) induced UDS in both hepatocytes and spermatocytes in vitro. In contrast, only hepatocytes exhibited UDS when isolated from rats given EDB (100 mg/kg) 2 h earlier, and only then if the compound was given i.p. rather than orally. Preincubation of hepatocytes or spermatocytes with inhibitors of cytochrome P450-mediated oxidation had no effect on EDB induction of UDS in vitro. In contrast, depletion of cellular glutathione strongly inhibited EDB-induced UDS in both cell types in vitro. Treatment of rats with 175 mg metyrapone/kg (an inhibitor of hepatic mixed-function oxidases) 1 h prior to administration of EDB in vivo had no effect on EDB-induced UDS in hepatocytes, but led to a positive UDS response to EDB in spermatocytes in vivo. This suggests that the mixed-function oxidase pathway of metabolism is the primary route of clearance of EDB and that inhibition of cytochrome P450-mediated oxidation led to a more extensive tissue distribution of the parent compound. These data also suggest that the pathway which produces genotoxic metabolites from EDB in hepatocytes and spermatocytes, in vitro and in vivo, involves the conjugation of EDB to glutathione and its subsequent metabolism.