EFFECT OF SUBSTITUENTS ON INVITRO METABOLISM AND COVALENT BINDING OF SUBSTITUTED BROMOBENZENES

The relative hepatotoxicity of o-bromobenzonitrile, bromobenzene, o-bromotoluene, and o-bromobenzotrifluoride decreases in the order mentioned. In this study, the nature of the substrate-enzyme interaction was investigated by determining spectral binding constants and by following the disappearance of several substituted bromobenzenes in vitro in the presence of the hepatic monooxygenase system. The disappearance rates generally parallel substrate electronic character; all the compounds display spectral binding constants of similar magnitude, and differences are not obviously attributable either to substituent solubility or electronic properties. In addition, the extent to which metabolites of these materials become covalently bound to microsomal proteins following incubation in vitro was determined. Despite an apparently low rate of metabolism, the very toxic o-bromobenzonitrile was covalently bound to a greater extent than bromobenzene. Surprisingly, the less toxic o-bromotoluene was the most efficiently covalently bound substrate studied. Whereas the extent of covalent binding of bromobenzene is increased ninefold by pretreating the animals with phenobarbital, only a 2.4-fold enhancement of o-bromobenzonitrile binding was observed. Glutathione reduced the covalent binding observed for all substrates in concentrations as low as 0.01 mm (except o-bromobenzotrifluoride, which required 0.1 mm). The in vivo metabolism of o-bromotoluene was also studied, and it was determined that the major urinary metabolites are phenols and mercapturic acids, products of the arene oxide metabolic pathway. There does not emerge a simple correlation between macromolecular covalent binding and toxicity among these compounds. © 1979.