CORRELATION OF METABOLISM, COVALENT BINDING AND TOXICITY FOR A SERIES OF BROMOBENZENE DERIVATIVES USING RAT-LIVER SLICES IN-VITRO

For many acute-acting chemicals, toxic responses observed in vivo correlate strongly with metabolic activation and macromolecular covalent binding (CVB) observed in vitro and often in vivo; bromobenzene (BB) is a classic example of this behavior. Substituent groups modulate the toxicity of bromobenzene in vivo and in liver slices cultured in vitro in parallel fashion [Fisher, R., Hanzlik, R.P., Gandolfi, J.A., and Brendel, K. (1992), In Vitro Toxicology, 4, 173 - 186]. In the present study we used the liver slice system to examine the relationship between toxicity, metabolism and covalent binding amongst a series of [H-3/C-14] dual labelled BB derivatives including (in order of increasing hepatotoxicity) o-bromoanisole (BA), o-bromotoluene (BT), o-bromobenzonitrile (BBN), BB and o-dibromobenzene (DBB). Among these congeners apparent relative rates of metabolism varied only 4-fold, but the most extensively metabolized compounds were the least toxic. CVB varied 7-fold across the series, and those compounds which bound the most frequently were the most toxic. For each compound the relative binding index (RBI = pmol bound/ nmol metabolized) and the average retention of tritium relative to carbon-14 in the CVB fraction were constant throughout the 24 h incubations, suggesting that the metabolic profile of each compound remained constant with time. The RBI values, however, did not reflect relative toxicity as well as total CVB values. The T/C ratios of the CVB residues varied from 0.36 (for BA) to 0.81 (for BBN), indicating that ortho-substitution on BB exerts important qualitative as well as quantitative effects on overall metabolism and reactive metabolite formation. The finding that relative toxicity among a series of bromobenzene congeners is paralleled by their relative covalent binding measured in the same system in which toxicity is assessed adds support to the hypothesis that covalent binding contributes to the observed toxicity, rather than merely being a correlated epiphenomenon.