In vitro activation of 1,2-dichloroethane by microsomal and cytosolic enzymes

Abstract

[1,2- 14C]1,2-Dichloroethane was metabolized by rat liver enzyme systems to nonvolatile products and to products irreversibly bound to protein and calf thymus DNA. Cytosolic metabolism to all three types of metabolites was dependent upon the presence of reduced glutathione (GSH), suggesting the role of GSH transferases. Microsomal metabolism to all three types of products occurred via mixed function oxidation; microsomal GSH transferase(s) catalyzed the formation of metabolites irreversibly bound to DNA. GSH blocked microsomal mixed function oxidase (MFO)-catalyzed binding to protein but stimulated binding to DNA in a synergistic manner. 2-Chloroacetaldehyde, S-(2-chloroethyl)-GSH, and 1-chloroso-2-chloroethane are proposed as major species involved in irreversible binding but vinyl chloride, 2-chloroethanol, and chloroethyl radicals are not. The microsomal MFO, synergistic microsomal MFO-GSH transferase, and cytosolic GSH transferase systems differed in their preferences for irreversible binding of label from dichloroethane to various homopolyribonucleotides and only the latter system produced metabolites mutagenic to Salmonella typhimurium TA 1535. We postulate that several 1,2-dichloroethane activation pathways are operative which produce different adducts; the relative contribution of each pathway to total nonvolatile metabolites, mutagenic metabolites, and DNA and protein adducts under these in vitro assay conditions was estimated.