Bromobenzene Toxicity Research Articles

Chronotoxicity of bromobenzene-induced hepatic injury in mice.

The aim of the present study is to investigate whether or not bromobenzene (BB) toxicity varies with circadian periodicity. Seven-week-old male ICR mice were injected with 900 mg/kg (5.73 mmol/kg) BB intraperitoneally at 4 different time points of a day (zeitgeber time [ZT]: ZT0, ZT6, ZT12, and ZT18). Mortality was then monitored for 7 days after injection. Interestingly, mice were sensitive to BB acute toxicity at ZT6 while tolerant at ZT18. Moreover, in mice that were given a non-lethal dose of BB (540 mg (3.44 mmol)/kg), levels of alanine aminotransferase and aspartate aminotransferase, used as markers of hepatic injury, markedly increased in response to injection at ZT6, but did not increase significantly in response to injection at ZT18. In contrast, the markers of renal injury (creatinine and blood urea nitrogen), showed no significant difference in response to the two injection times. To further investigate this extreme circadian variation, we examined hepatic and renal lipid peroxidation levels, and conducted histopathological studies. Similar to our observation with alanine aminotransferase and creatinine, hepatic lipid peroxidation and histopathological changes were more pronounced than renal changes, and showed circadian variation. Our present investigation demonstrated that BB-induced mortality had clear circadian variation, and suggested that hepatic injury was one of the important factors for determination of this variation.

Production of cis-1,2-dihydrocatechols of high synthetic value by whole-cell fermentation using Escherichia coli JM109 (pDTG601): A detailed study

• Growing conditions for high cell density cultures of E. coli JM109 (pDTG601). • Biotransformation kinetics and effect of bacterial concentration on transformation productivities. • Biotransformation using bi-phasic procedures to decrease the toxic impact of the substrate. • Storage of active biocatalyst as a chemical reagent for dihydroxylation of organic substrates. Chiral cis -1,2-dihydrocatechols have been extensively used as starting materials for the synthesis of complex organic molecules. The preparation of these high added value compounds is carried out mainly by enzymatic dihydroxylation of arenes since no other efficient chemical method is known. In this paper we describe a detailed study of the biotechnological production at a preparative scale of cis -3-bromo-1,2-dihydrocatechol (BDC) from bromobenzene using Escherichia coli JM109 (pDTG601), a recombinant strain that harbors the toluene dioxygenase genes from Pseudomonas putida F1. High cell-density cultures of the microorganism (65 g/L cdw) were achieved in a 5 L bioreactor using fed-batch cultures in aerobic conditions. The influence of the biomass concentration in the volumetric and specific productivity of the system, as well as the kinetics of the biotransformation, was thoroughly studied. The use of liquid paraffin as a second phase to relieve bromobenzene's toxicity resulted in an increased specific productivity for the bi-phasic protocol, which resulted in higher final BDC concentrations. An improvement of 3.2-fold was obtained for BDC production compared to previous reports using this organism. Furthermore, storage strategies of the biocatalytic system as a reagent for organic synthesis are presented.

Integrated Metabonomic Analysis of Bromobenzene-Induced Hepatotoxicity: Novel Induction of 5-Oxoprolinosis

We present here a definitive metabonomic analysis in order to detect novel biomarker and metabolite information, implicating specific putative protein targets in the toxicological mechanism of bromobenzene-induced centrilobular hepatic necrosis. Male Han-Wistar rats were dosed with bromobenzene (1.5 g/kg, n = 25) and blood plasma, urine and liver samples were collected for NMR and magic angle spinning (MAS) NMR spectroscopy at various time-points postdose, with histopathology and clinical pathology performed in parallel. Liver samples were analyzed by 600 MHz 1H MAS NMR techniques and the resultant spectra were correlated to sequential 1H NMR measurements in urine and blood plasma using pattern recognition methods. 1D 1H NMR spectra were data-reduced and analyzed using principal components analysis (PCA) to show the time-dependent biochemical variations induced by bromobenzene toxicity. In addition to a holistic view of the effect of hepatic toxicity on the metabolome, a number of putative protein targets of bromobenzene and its metabolites were identified including those enzymes of the glutathione cycle, exemplified by the presence of a novel biomarker, 5-oxoproline, in liver tissue, blood plasma, and urine. As such, this work establishes the importance of metabonomics technology in resolving the mechanistic complexity of drug toxicity as well as the benefits of frontloading this approach in drug safety evaluation and biomarker discovery.

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

The effects of methanol extract and gallic acid (3,4,5-trihydroxybenzoic acid) of Orostachys japonicus A. Berger on hepatic drug metabolizing enzymes and lipid peroxidation were investigated in rats treated with bromobenzene. The methanol extract of Orostachys japonicus reduced the activities of phase I enzymes, aminopyrine N-demethylase and aniline hydroxylase, that had been increased by i.p. injection of bromobenzene. Gallic acid isolated from Orostachys japonicus also reduced the aniline hydroxylase activity, while it did not affect the aminopyrine N-demethylase activity. The methanol extract and gallic acid restored the activity of epoxide hydrolase which had been decreased by bromobenzene. Hepatic glutathione content was lowered, along with increase in hepatic lipid peroxide, by bromobenzene administration. The hepatic lipid peroxidation induced by bromobenzene was prevented with the methanol extract and gallic acid of Orostachys japonicus. However, the decrease in glutathione was not altered by gallic acid. The present results suggest that the methanol extract and gallic acid of Orostachys japonicus may protect liver from bromobenzene toxicity through, at least in part, inhibiting the cytochrome P450-dependent monooxygenase activities and enhancing the activity of epoxide hydrolase. Antioxidant effect also may contribute to the protection of Orostachys japonicus against the bromobenzene-induced hepatotoxicity.

Toxicogenomics of bromobenzene hepatotoxicity: a combined transcriptomics and proteomics approach

Toxicogenomics is a novel approach integrating the expression analysis of thousands of genes (transcriptomics) or proteins (proteomics) with classical methods in toxicology. Effects at the molecular level are related to pathophysiological changes of the organisms, enabling detailed comparison of mechanisms and early detection and prediction of toxicity. This report addresses the value of the combined use of transcriptomics and proteomics technologies in toxicology. Acute hepatotoxicity was induced in rats by bromobenzene administration resulting in depleted glutathione levels and reduced average body weights, 24 hr after dosage. These physiological symptoms coincided with many changes of hepatic mRNA and protein content. Gene induction confirmed involvement of glutathione- S-transferase isozymes and epoxide hydrolase in bromobenzene metabolism and identified many genes possibly relevant in bromobenzene toxicity. Observed glutathione depletion coincided with induction of the key enzyme in glutathione biosynthesis, γ-glutamylcysteine synthetase. Oxidative stress was apparent from strong upregulation of heme oxygenase, peroxiredoxin 1 and other genes. Bromobenzene-induced protein degradation was suggested from two-dimensional gel electrophoresis, upregulated mRNA levels for proteasome subunits and lysosomal cathepsin L, whereas also genes were upregulated with a role in protein synthesis. Both protein and gene expression profiles from treated rats were clearly distinct from controls as shown by principal component analysis, and several proteins found to significantly change upon bromobenzene treatment were identified by mass spectrometry. A modest overlap in results from proteomics and transcriptomics was found. This work indicates that transcriptomics and proteomics technologies are complementary to each other and provide new possibilities in molecular toxicology.

Inhibition of Glutathione Synthesis with Propargylglycine Enhances N-Acetylmethionine Protection and Methylation in Bromobenzene-Treated Syrian Hamsters

The finding that liver necrosis caused by the environmental glutathione (GSH)-depleting chemical, bromobenzene (BB) is associated with marked impairment in O- and S-methylation of BB metabolites in Syrian hamsters raises questions concerning the role of methyl deficiency in BB toxicity. N-Acetylmethionine (NAM) has proven to be an effective antidote against BB toxicity when given after liver GSH has been depleted extensively. The mechanism of protection by NAM may occur via a replacement of methyl donor and/or via an increase of GSH synthesis. If replacement of the methyl donor is an important process, then blocking the resynthesis of GSH in the methyl-repleted hamsters should not decrease NAM protection. This hypothesis was examined in this study. Propargylglycine (PPG), an irreversible inhibitor of cystathionase, was used to inhibit the utilization of NAM for GSH resynthesis. Two groups of hamsters were pretreated with an intraperitoneal (ip) dose of PPG (30 mg/kg) or saline 24 h before BB administration (800 mg/kg, ip). At 5 h after BB treatment, an ip dose of NAM (1200 mg/kg) was given. Light microscopic examinations of liver sections obtained 24 h after BB treatment indicated that NAM provided better protection (P < 0.05) in the PPG + BB + NAM group than in the BB + NAM group. Liver GSH content, however, was lower in the PPG + BB + NAM group than in the BB + NAM group. The Syrian hamster has a limited capability to N-deacetylated NAM. The substitution of NAM with methionine (Met; 450 mg/kg) resulted in a higher level of GSH in the BB + Met group than in the BB + NAM group (P < 0.05). The enhanced protection by PPG in the PPG + BB + NAM group was accompanied by higher (P < 0.05) urinary excretions of specificO- and S-methylated bromothiocatechols than in the BB + NAM group. The results suggest that NAM protection occurs primarily via a replacement of the methyl donor and that methyl deficiency occurring in response to GSH repletion plays a potential role in BB toxicity.

Treatment with aged garlic extract protects against bromobenzene toxicity to precision cut rat liver slices

Precision-cut liver slices from phenobarbital-induced rats were incubated for 6 h with the model hepatotoxin bromobenzene (BB) at a final concentration of 1 mM. Severe toxicity was indicated by a decreased K +, adenosine triphosphate and glutathione (GSH) content of the slices, increased release of alanine aminotransferase and lactate dehydrogenase into the medium, and increased formation of thiobarbituric acid reacting substances. Pretreatment of animals for 7 days with aged garlic extract (AGE) (Kyolic®) at doses of 2 and 10 ml/kg/day dramatically reduced the toxicity of BB in a dose-dependent manner. The GSH content of liver slices from rats treated with AGE at 2 or 10 ml/kg/day increased by 50 and 80%, respectively. The BB-induced decrease in GSH content was less in slices derived from AGE-treated rats compared with slices from control rats. Pretreatment with AGE did not affect cytochrome P450 when assayed as 7-ethoxycoumarin O-deethylase and 7-pentoxyresorufin O-depentylase activities in hepatic microsomes. Thus, the mechanism by which pretreatment with AGE protects against BB hepatotoxicity involves both an elevation of hepatic GSH content, and a GSH sparing effect, possibly due to conjugation of organosulphur compounds in AGE with toxic BB metabolites. Only this GSH sparing effect was seen in our earlier study on the in vitro hepatoprotective effect of AGE [Wang et al., 1998. Toxicology 126, 213–222].

Molecular structure and biochemical toxicity of four halogeno-benzenes on the unicellular green alga Selenastrum capricornutum

Comparative toxicity of hexachlorobenzene, 1,2,3-trichlorobenzene, bromobenzene, and chlorobenzene on the growth, protein concentration, ATP content and relevant enzyme activities of the unicellular green alga Selenastrum capricornutum was studied. Five typical molecular descriptive parameters: E homo, 1 χ v, K ow, E s, π x of these organic chemicals were selected to study the structure and bio-activities of the four halogeno-benzenes. The results indicated that the protein concentration, activities of G6PDH and AP, and ATP content were more sensitive to halogeno-benzenes than dry weight. Among the four halogeno-benzenes, the order of toxicity on the alga was: hexachlorobenzene>1,2,3-trichlorobenzene>bromobenzene>chlorobenzene. There was a good correlation between the molecular structure of halogeno-benzenes and their biochemical toxicity.

Protective effects of aged garlic extract against bromobenzene toxicity to precision cut rat liver slices

Precision-cut liver slices from phenobarbital-treated rats were incubated for up to 8 h with the industrial solvent and hepatotoxin bromobenzene at a final concentration of 1 mM. Phenobarbital pretreatment potentiates bromobenzene hepatotoxicity by inducing those P450 isoforms responsible for the formation of the active hepatotoxin, namely bromobenzene-3,4-oxide. A reduction in cell viability was indicated by a decrease in the K +, ATP and glutathione content of the slices and the increased release of the intracellular enzymes, lactate dehydrogenase and alanine aminotransferase, into the medium. Furthermore, levels of lipid peroxidation as judged by the formation of thiobarbituric acid reactive substances, were increased approximately 5-fold. Aged garlic extract (AGE) at concentrations of 1–5% (v/v) reduced the toxicity of bromobenzene in a concentration-dependent manner as judged by all of the parameters of viability studied, with the exception of lipid peroxidation which was reduced to control levels even at the lowest concentration of garlic extract used. AGE was found to cause partial inhibition of cytochrome P450 when assayed as both 7-ethoxycoumarin O-deethylase and 7-pentoxyresorufin O-depentylase activities, but even the highest concentration used inhibited both activities by less than 50%. It is suggested that the hepatoprotective effects of AGE are due primarily to the reduced glutathione-sparing properties of its constituents, most probably its organosulphur compounds.

Glutathione Depletion-Induced Thymidylate Insufficiency for DNA Repair Synthesis

Dietary methionine (Met) deficiency is known to divert folate away from de novo biosyntheses of purines and the pyrimidine, thymidylate, to the resynthesis of Met resulting in deoxynucleoside triphosphate imbalance. We have recently shown that Met can easily be depleted and methylation can be impaired by exposure to a model glutathione (GSH)-depleting agent, bromobenzene (BB). GSH depletion-induced Met depletion, therefore, could cause thymidylate insufficiency for DNA repair synthesis. The administration of thymidine (Thy) should repair this impairment. When this hypothesis was examined in the present study, several interesting results were found. The administration of Thy labeled with [2-14C]Thy to BB-treated Syrian hamsters at either 1, 5, 7 or 9 h after BB resulted in an attenuation of liver toxicity. Intrahepatic hemorrhage, which is a typical characteristic of BB toxicity in the Syrian hamster, was decreased in the BB + Thy groups. The attenuation of liver toxicity was accompanied by a progressive increase of Thy incorporation into liver genomic DNA at 24 h after BB. With respect to the time points chosen for Thy administration, Thy incorporation found in the BB + Thy groups were 2-, 2-, 3- and 4-fold of the controls that received only Thy. The results provide evidence that BB causes a progressive increase of thymidylate insufficiency in liver cells. Thymidylate insufficiency is due to Met depletion, a depletion that occurs as a result of GSH depletion.

Protective effects of calcium channel blockers on acute bromobenzene toxicity to isolated rat hepatocytes. Inhibition of phenylephrine-induced calcium oscillations.

Protective effects of verapamil, nifedipine, diltiazem, and ethylene glycol tetraacetic acid (EGTA) on acute bromobenzene (BB) toxicity to rat hepatocytes were evaluated, and cytosolic [Ca2+]i was monitored in single BB-exposed rat hepatocytes. Additionally, the effect of nifedipine on phenylephrine-stimulated calcium oscillations was investigated. BB at 0.8-2.4 mM increased the lactate dehydrogenase (LDH) leakage rate dose-dependently. Pretreatment with verapamil (25-35 microM), nifedipine (35-45 microM), diltiazem (25 microM), or EGTA (1.5-5 mM) markedly attenuated the BB-induced (1.6 mM) LDH leakage rate during 2 h of incubations. BB did not cause any detectable acute change in [Ca2+]i. BB interfered with phenylephrine-stimulated calcium oscillations, by blocking the oscillations in 58% of the cells and reducing the oscillation frequency in the rest. Nifedipine (100 and 200 microM) blocked the phenylephrine-induced calcium oscillations completely in 55% and 88% of the cells, respectively. The findings demonstrate that verapamil, nifedipine, diltiazem, and EGTA significantly protect rat hepatocytes against BB toxicity. BB interferes with phenylephrine-stimulated calcium oscillations. Nifedipine inhibits the oscillations at doses higher than those exerting a protective effect.

Lowering of lipid peroxidation and acute toxicity of bromobenzene by a polymeric form of zinc-metallothionein

Plasma and liver contents of malonic aldehyde are studied one day after administration of bromobenzene to mice pretreated with a polymeric form of zinc-metallothionein from rat liver. It is found that zinc-metallothionein injected in a dose of 1–4 mg/kg 5–10 min prior to injection of bromobenzene (2 g/kg, about 56% of LD50) markedly lowers the malonic dialdehyde level and active toxicity of this xenobiotic. Administration of a mixture modeling Zn-metallothionein (albumin, cysteine, and zinc) in a dose of 4 mg/kg has no appreciable effect on the malonic dialdehyde level raised after bromobenzene injection, and does not change its LD50. It is concluded that the protective effect of exogenous zinc-metallothionein is due to its antioxidant activity, which allows for normalization of lipid peroxidation.

Influence of a series of natural flavonoids on free radical generating systems and oxidative stress.

1. A series of flavonoids isolated from Indian medicinal plants: kaempferol-3-O-galactoside, hispidulin, nepetin, scutellarein, scutellarein-7-O-glucuronide, hibifolin and morelloflavone were studied for their activity as inhibitors of microsomal lipid peroxidation and scavengers of oxygen free radicals in vitro as well as in a model of xenobiotic toxicity in mouse. 2. All compounds inhibited lipid peroxidation in vitro. The most potent compounds were nepetin (non-enzymic lipid peroxidation) and morelloflavone (enzymic lipid peroxidation) with IC50's in the micromolar range. Some of the compounds behaved as scavengers of hydroxyl radical in the deoxyribose degradation assay, with a calculated rate constant for kaempferol-3-O-galactoside of 1.55 x 10(10) M-1 s-1. 3. Scutellarein and nepetin were found to be inhibitors of xanthine oxidase activity, whereas morelloflavone acted as a scavenger of superoxide generated by hypoxanthine/xanthine oxidase. 4. Treatment of mice with scutellarein, hispidulin, nepetin and kaempferol-3-O-galactoside after bromobenzene intoxication decreased serum glumate-pyruvate transaminase activity, although only the last flavonoid was able to significantly reduce hepatic lipid peroxidation products and to increase the reduced glutathione level. In contrast, morelloflavone increased bromobenzene toxicity.

Bromobenzene detoxification in the human liver-derived HepG2 cell line.

1. The applicability of the human hepatoma cell line, HepG2, as a cell culture model for studying xenobiotic liver toxicity has been investigated using the well-characterized hepatotoxic chemical, bromobenzene. 2. Bromobenzene caused a concentration- (0-10 mM) and time-dependent (0-180 min) decrease in HepG2 cell viability. The degree of toxicity was dependent upon the culture medium composition and the state of cell growth. Toxicity in Modified Earle's and Williams' E Media was maximal at 7 days growth compared with 3 and 10 days, and was greater in Williams' than in Earle's medium. Toxicity in Dulbecco's medium was apparent only at 10 days growth and was less than the maximum toxicity in the other media. 3. Bromobenzene was detoxified by epoxide hydrase. The question of metabolic activation by P450 remained unresolved, but any involvement of P450 was by forms not inhibited by ketoconazole. 4. The mechanism of bromobenzene toxicity did not appear to involve lipid peroxidation, depletion of reduced glutathione, calcium-mediated proteolysis or metabolic activation by prostaglandin synthetase, but may have involved direct solvent-induced cell damage. 5. This study demonstrates the potential usefulness of HepG2 cells in toxicity testing and highlights the importance of standardizing culture conditions.

Prevention of Bromobenzene Toxicity by N-Acetylmethionine: Correlation between Toxicity and the Impairment in O-Methylation and S-Methylation of Bromothiocatechols

Bromobenzene (800 mg/kg, ip) caused severe liver necrosis with massive hemorrhage in the golden Syrian hamster within the first 24 hr. Kidney injury was also observed. Treatment with N-acetylmethionine (NAM) at an ip dose of 1200 mg/kg at 5 hr after bromobenzene administration significantly protected the liver and kidney against injuries. Plasma glutamate pyruvate transaminase and blood urea nitrogen levels were substantially decreased in the NAM-treated animals. Histological evaluations confirmed these results. When the urinary neutral and phenolic metabolites of bromobenzene from NAM-treated and untreated hamsters were isolated and compared by GC and GC/ MS, a striking result was observed in terms of 0- and S-methylated thiol-containing metabolite formation. The NAM-treated animals showed approximately a 8- to 14-fold increase in the excretion of the four isomeric O and S-methylated bromothiocatechols. These thiocatechols, which are now known to be the 3,4- series metabolites of bromobenzene, can undergo methylation at either the thiol or the hydroxyl functional group. The excretion of 3-S- and 4-S-methylated bromodihydrobenzene thiolols was also increased significantly in the NAM-treated hamster, but other neutral and phenolic metabolites were relatively unchanged. These results suggest that bromobenzene toxicity in the Syrian hamster may be associated with impaired methylation capabilities, an impairment that could be due to methionine and glutathione depletion.

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 [ 3H/ 14C] 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.

Bromobenzene-glutathione excretion into bile reflects toxic activation of bromobenzene in rats

This investigation was designed to determine whether biliary excretion of bromobenzene(BB)-glutathione(GSH) conjugate can be used as an index of in vivo activation of BB. In order to test this hypothesis, the effect of chemicals known to alter the toxicity and biotransformation of BB (i.e., cytochrome P-450 inducers and inhibitors) on the biliary excretion of BB-GSH was studied in rats. BB-GSH was the major BB metabolite in bile. A linear relationship was observed between the dosage of BB administered and BB-GSH excreted into bile, up to a dosage of 250 μmol kg of BB. Of the inducers tested, phenobarbital, which is known to increase the toxicity of BB, dramatically increased (700%) the rate of biliary excretion of BB-GSH over that in control animals. In contrast, 3-methylcholanthrene, which is known to decrease the hepatotoxicity of BB, decreased the biliary excretion of BB-GSH (56%). Inhibitors of P-450, such as SKF 525-A and piperonyl butoxide which are known to decrease the activation and hepatotoxicity of BB, also decreased the biliary excretion of BB-GSH. These findings are in agreement with the hypothesis that the biliary excretion of BB-GSH reflects the formation of the reactive BB metabolite in liver and the rate of biliary excretion can be used to determine factors that are important in determining the toxicity of BB.

Identification of epoxide- and quinone-derived bromobenzene adducts to protein sulfur nucleophiles

Bromobenzene (BB) hepatotoxicity is widely attributed to the alkylation of cellular proteins by chemically reactive metabolites, particularly BB-3,4-oxide. This laboratory recently reported the first conclusive evidence that BB epoxides actually do alkylate proteins; i.e., acid hydrolysates of hepatic proteins from phenobarbital-(PB-) induced BB-treated rats contain S-(o-, S-(m-, and S-(p-bromophenyl)cysteine [Weller, P.E., and Hanzlik, R.P. (1991) Chem. Res. Toxicol. 4, 17-20]. However, these three compounds account for less than 0.5% of total protein covalent binding. Bromoquinone metabolites of BB are also suspected of alkylating proteins. To search for such adducts to protein cysteinyl or methionyl residues, we heated hepatic proteins from PB-induced BB-treated rats with a two-phase mixture of 16 N KOH and CH3I ("alkaline permethylation"). Under these conditions S-alkylated residues are cleaved via elimination and the phenoxide and thiophenoxide groups on the fragments are methylated. Product analysis by 14C HPLC and GC/MS revealed o-, m-, and p-bromothioanisoles in amounts comparable to the content of S-(bromophenyl)cysteines found by acid hydrolysis (para much greater than meta, ortho). This, too, clearly implicates protein-SH alkylation by BB-2,3- and 3,4-oxides. In addition, 2,3-dimethoxy-5-bromothioanisole and another unidentified isomer were observed. However, by far the major adduct (5-6% of total covalent binding) was 2,5-dimethoxythioanisole (i.e., a debrominated adduct). When BB-d5 was administered, the latter contained mostly 3 deuterium atoms/mol. These latter results clearly show that alkylation of protein sulfur nucleophiles in vivo by quinone metabolites is 10-15 times more extensive than their alkylation by BB epoxides. After BB-d5 was administered, the bromothioanisoles and dimethoxybromothioanisoles contained 4 and 2 deuterium atoms/mol, respectively. A weighted average calculation of deuterium retention across the six major sulfur adducts agreed well with 3H/14C retention ratios determined earlier for total liver protein covalent binding of dual-labeled [3H/14C]BB, indicating that the overall pattern of BB metabolite binding to all protein nucleophiles may closely parallel that seen here specifically for protein sulfhydryl groups. The identification of a variety of specific BB-derived adducts to protein now affords the opportunity to investigate their relative contributions to the toxicity of bromobenzene.

Activation and toxicity of bromobenzene in nasal tissues in mice

Autoradiography of mice injected i.v. with bromobenzene-14C (BB; 25 mumol/kg body wt) revealed a high concentration of non-volatile metabolites in the olfactory mucosa and in the glands around the maxillary sinuses. As determined with solvent-extracted tissue sections, there was a high level of irreversibly bound metabolites in the Bowman's glands in the olfactory mucosa, while the level of bound metabolites was low in the glands around the maxillary sinuses. Histopathological examination of the nose region of mice given a single i.p. dose of unlabelled BB (greater than or equal to 4.8 mmol/kg body wt) revealed degeneration and necrosis of the glands of Bowman. Degenerative changes in the olfactory epithelium were also observed. Moreover, focal degeneration and necrosis were found in the lateral nasal glands (greater than or equal to 4.8 mmol/kg body wt). Cyst-like dilatation of acini was observed in the lateral nasal glands and in the maxillary glands located around the maxillary sinus. Incubation of BB (26 microM) with a homogenate prepared from the olfactory mucosa revealed an irreversible binding, which clearly exceeded that of the liver. It is suggested that BB is activated in situ to a cytotoxic metabolite that reacts with the glands of Bowman. The specific toxicity observed in the lateral nasal glands correlated with a high concentration of non-volatile but extractable metabolites in these glands.

The contribution of bromobenzene to our current understanding of chemically-induced toxicities

The metabolism and toxicity of bromobenzene has been investigated for well over one hundred years. The urinary excretion of mercapturic acids was first reported in 1879, in animals treated with bromobenzene. Bromobenzene has since proven to be a valuable tool in efforts to unravel the complexities involved in chemical- induced toxicities. For example, the importance of metabolic activation via the cytochrome(s) P-450; the role of glutathione in the detoxification of reactive metabolites; and the toxicological significance of covalent binding, enzyme inactivation and lipid peroxidation have all been illustrated in studies with bromobenzene. Thus, many of the principles involved in chemical-induced toxicity have been exemplified in studies with bromobenzene. These studies have provided substantial insight into the role of chemically reactive metabolites in the genesis of xenobiotic-mediated cytotoxicity.