Microsomal metabolism and covalent binding of [³H/¹⁴C]-bromobenzene. Evidence for quinones as reactive metabolites

Abstract

1. The metabolism and covalent binding of [3H/14C]bromobenzene has been investigated using liver microsomes from untreated and phenobarbital (PB)-pretreated rats. A model has been developed to relate the observed 3H/14C ratios in the covalently bound residues to the type of metabolite (epoxide versus quinone) responsible for their formation. 2. With control microsomes metabolism was linear for 60 minutes, but with PB microsomes the time course showed a short-lived burst of rapid metabolism followed by a long phase with an overall rate comparable to control. With both types of microsomes covalent binding was synchronous with metabolism. 3. The normalized 3H/14C ratios of recovered substrate and water-soluble metabolites was 1.0, whereas that of the covalently bound material was only 0.5. Such extensive loss of tritium implies that a considerable portion of the covalent binding arises from bromobenzene metabolites more highly oxidized than an epoxide (e.g. quinones). 4. The normalized 3H/14C ratios for bromobenzene metabolites covalently bound to liver proteins in vivo (total and microsomal) was the same as with microsomes in vitro (0.5). However, for the lung and kidney the 3H/14C ratios were considerably higher (0.71 and 0.62), indicating that differences between tissues in vivo may be greater than between liver microsomes in vitro and in vivo.

Activation and toxicity of bromobenzene in nasal tissues in mice

“…1988;Narasimhan et al 1988) and not to the formation of reactive epoxides as previously believed (Joilow et al 1974). No attempts were made in the present study to reveal the chemical nature of the reactive metabolite formed in the olfactory mucosa.…”

Species differences in short term toxicity from inhalation exposure to bromobenzene

“…The hepatotoxicity has been believed to be produced as a result of the formation of a reactive 3,4-epoxide (Jollow et al 1974). Recently, reactive quinones have also been shown to be involved in the in vivo and in vitro covalent binding of bromobenzene (Narasimhan et al 1988). The latter study also indicated structural differences between bromobenzene metabolites bound to the liver and to the lung.…”

Changes in hepatic drug metabolizing enzymes and lipid peroxidation by methanol extract and major compound of Orostachys japonicus

“…Bromobenzene is a toxic industrial solvent that elicits toxicity predominantly in the liver, where it causes centrilobular necrosis. Although, formation of secondary quinone metabolites Narasimhan et al, 1988;Slaughter and Hanzlik, 1991) and hydrogen peroxide (Wu et al, 1994) have been proposed to play an important role in the toxicity of bromobenzene, the major toxicity of the bromobenzene has been known to be associated with cytochrome P450-mediated phase I metabolism to reactive epoxide intermediates. The bioconversion to reactive intermediates capable of reacting to critical cellular macromolecules and destroying cellular function is a common toxicological pathway shared by a wide variety of xenobiotics (Cohen et al, 1997;Pumford and Halmes, 1997), and bromobenzene is an example of such hepatotoxic compounds whose conversion to reactive intermediates is integral to its toxicity.…”