Species and Strain Differences in the Hepatic Cytochrome P450-Mediated Biotransformation of 1,4-Dichlorobenzene

Abstract

Our goal was to characterize possible species and strain differences in the hepatic microsomal biotransformation of 1,4-dichlorobenzene (1,4-DCB). Experiments compared extent of labeled 1,4-DCB conversion to oxidized metabolites, glutathione conjugates, and covalently bound metabolites by hepatic microsomes from humans, from male B6C3F1 mice, and from males of three rat strains (Fischer 344, Sprague–Dawley (SD), and Wistar). These rodents were selected for comparison because of their dissimilar responses to 1,4-DCB, notably, hepatocarcinogenicity in the B6C3F1 mouse but not the Wistar or Fischer rat, and nephrotoxicity and carcinogenicity in the Fischer rat. The species rank order for totalin vitroconversion of 1,4-DCB was mouse > rat >> human. Conversion by microsomes from Fischer and Wistar rats was similar, whereas SD rats showed less biotransformation than the other two strains. Microsomes from the mouse produced most of the reactive metabolites as indicated by covalent binding to macromolecules (>20% of total metabolites formed). This covalent binding by mouse microsomes was extensively inhibited by ascorbic acid (AA), with a concomitant increase in hydroquinone formation, indicating an important role for benzoquinones as reactive metabolites. Phenobarbital pretreatment of rats enhanced thein vitroconversion of 1,4-DCB and the amount of covalent binding. Covalent binding for all rat microsomes was partly (33–79%) inhibited by AA. Addition of glutathione (GSH) plus AA further diminished the covalent binding with concomitant increased formation of the GSH-conjugated epoxide. Human microsomes produced the least reactive metabolites, with the majority (>70%) of this covalent binding prevented by GSH addition. The observed species differences, notably the more pronounced biotransformation of 1,4-DCB to reactive species including benzoquinones, could be factors in this compound's liver carcinogenicity in B6C3F1 mice but not other rodent species.