Metabolites Of Bromobenzene Research Articles

Mechanisms of the formation and disposition of reactive metabolites that can cause acute liver injury.

Acetaminophen and pulegone are just two examples for many agents that can form reactive metabolites that can cause acute liver injury. Two other classic organic compounds that have been extensively studied are carbon tetrachloride (for a recent review see Ref. 159, and for other discussions see Refs. 8 and 9) and bromobenzene (for review see Ref. 160). Different kinds of protein adducts of reactive metabolites of bromobenzene have been partially characterized [161], and specific antibodies to these adducts are now being used to isolate and identify the proteins that are modified (162). In contrast, carbon tetrachloride and other agents, such as the herbicide diquat, may form radicals that bind to and/or oxidize lipids and proteins in causing liver injury (163, 164). Therefore, the recent development [165] of antibodies to detect oxidative damage to proteins will be important in the identification and characterization of macromolecules that do not form adducts with reactive metabolites but are damaged oxidatively. Thus, some major challenges in the coming years are to identify hepatocellular macromolecules that are modified by reactive metabolites, and then approach the more difficult task of integrating this information into a time course and sequence of events leading to lethal hepatocellular injury.

O- and S-Methylated Bromothiocatechols

Bromobenzene is metabolized by the Hartley guinea pig to two different bromothiocatechols, 4-bromo-2-hydroxythiophenol and 5-bromo-2-hydroxythiophenol. Both the thiol and phenol functional groups of thiocatechol undergo biological methylation. Methylation at the thiol group leads to the formation of (methylthio)bromophenol (S-methylated bromothiocatechol), while methylation of the phenol group leads to methoxybromothiophenol (O-methylated bromothiocatechol). This resulted in the urinary excretion of four O- and S-methylated bromothiocatechols. Bromothiocatechols could be formed by dehydrogenation of their corresponding bromodihydrobenzene thiolols. Both the 3-S- and 4-S-bromodihydrobenzene thiolols, as S-methylated products, were found as urinary metabolites of bromobenzene in the Hartley guinea pig. All four O- and S-methylated bromothiocatechols and two S-methylated bromodihydrobenzene thiolols were also found as urinary metabolites of bromobenzene in the golden Syrian hamster.

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.

Dihydroxylated mercapturic acid metabolites of bromobenzene.

Bromobenzene is metabolized to electrophilic epoxides and quinones which covalently bind to protein sulfur nucleophiles, yet no quinone-derived mercapturic acid metabolites of bromobenzene have been reported. To search for them, phenobarbital-induced Sprague-Dawley rats were dosed (0.5-1.5 mmol/kg, ip) with either bromobenzene, 2-, 3-, or 4-bromophenol (BP), 3- or 4-bromocatechol (BC), or 2-bromohydroquinone (BHQ). Urine (0-24 h) was alkaline permethylated (2 N NaOH/CH3I/80 degrees C), and the resulting thioanisole derivatives were analyzed by GC/MS. Three dimethoxythioanisoles and eight bromodimethoxythioanisoles were formed by alkaline permethylation of urine of rats treated with bromobenzene or 3-BP; alkaline permethylation of urine from rats in other treatment groups afforded characteristic subsets of these derivatives. The major thioanisole from all groups except 3-BC or 4-BC was 2,5-dimethoxythioanisole, which arises from (2,5-dihydroxyphenyl)mercapturic acid. The latter was isolated from rat urine and is the first debrominated metabolite of bromobenzene reported to date. Its formation from both 4-bromophenol and BHQ requires two parallel mechanisms for bromine loss: (1) nucleophilic addition to 1,4-benzoquinone formed by hydroxylative debromination of 4-bromophenol and (2) nucleophilic displacement of bromine from 2-bromo-1,4-benzoquinone by sulfur. The yields of quinone-derived mercapturic acids formed from bromobenzene are quite low (less than 1% of dose) compared to the high yields of epoxide-derived mercapturic acids (40% of dose). Potential reasons for this are discussed.

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.

Effect of p-Xylene Inhalation on the Bioactivation of Bromobenzene in Rat Lung and Liver

It is unclear whether the pneumotoxicity observed with bromobenzene (BB) in phenobarbital-induced rats is related to BB bioactivation in lung, liver or both. To help differentiate pulmonary from hepatic bioactivation, BB was administered alone and in combination with/p-xylene, which inhibits pulmonary but induces hepatic cytochromes P450. Exposure to /vxylene alone (3400 ppm for 4 hr) produced no changes in bronchoalveolar lavage fluid (BALF) measurements (γ-glutamyl transpeptidase, lactate dehydrogenase, protein, white blood cell count) or serum sorbitol dehydrogenase. p-Xylene increased hepatic microsomal benzyloxy- (BROD), pentoxy- (PROD), and ethoxy- (EROD) resorufin O-dealkylase activities but decreased pulmonary microsomal BROD and PROD. Immunoblot analysis revealed an induction of hepatic but not pulmonary microsomal P450IIB apoprotein. When rats were exposed to p-xylene (2800 ppm) or room air for 4 hr, treated 12 hr later with BB (0.5 ml/kg, ip) or corn oil, and killed after 12 hr, p-xylene increased hepatic P450IIB (27-fold) concomittant with a similar increase in BROD activity. p-Xylene also increased hepatic P450IA apoprotein (3.4-fold) with a complimentary increase in EROD activity. p-Xylene potentiated BB-induced hepatotoxicity. In pulmonary micro-somes p-xylene and BB each produced similar decreases in both EROD and BROD activities. The combination of p-xylene and BB had an additive effect on pulmonary P450IA1 reduction. BALF analysis and histopathology revealed no pneumotoxicity with any treatment. p-Xylene potentiation of BB-induced hepatotoxicity without pneumotoxicity suggests that the liver does not produce metabolites of BB which are directly involved in pulmonary damage.

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.

Premercapturic acid metabolites of bromobenzene derived via its 2,3- and 3,4-oxide metabolites

1. A new premercapturic acid metabolite of bromobenzene was isolated from the urine of beta-naphthoflavone-induced rats; using 1H-n.m.r., FAB mass spectrometry and chemical degradation it was identified as S-(2-hydroxy-3-bromocyclohexa-3,5-dienyl)-N-acetylcysteine. 2. Two regioisomeric premercapturic acids apparently derived from bromobenzene-3,4-oxide were isolated as an inseparable 1:1 mixture from the urine of phenobarbital-induced rats and characterized by similar means. 3. Acid dehydration of bromobenzene 3,4- and 4,3-premercapturic acids (mixture) afforded only p-bromophenylmercapturic acid, whereas acid dehydration of 3,2-premercapturic acid gave both o- and m-bromophenylmercapturic acids. This implies a shift of sulphur in acid dehydration of the 3,4- and 3,2- but not the 4,3-premercapturic acids. 4. Base dehydration of the 3,4- and 4,3-premercapturic acid mixture gave a mixture of p- and m-bromophenylmercapturic acids, whereas base dehydration of the 3,2-premercapturic acid gave only m-bromophenylmercapturic acid. This indicates these premercapturic acids dehydrate by direct elimination without rearrangment. 5. The 3,2-premercapturic acid was detected only in the urine of BNF-induced animals, whereas the 3,4- and 4,3-premercapturic acids were detected in the urines of untreated as well as PB- and BNF-induced animals. 6. Together with earlier reports of the isolation of the 2,3-dihydrodiol, the isolation of the 3,2-premercapturic acid as a urinary metabolite of bromobenzene implies that bromobenzene-2,3-oxide is a discrete metabolite of bromobenzene and not merely a hypothetical intermediate.

ChemInform Abstract: Synthesis of Mercapturic Acid Derivatives of Putative Toxic Metabolites of Bromobenzene.

ChemInformVolume 21, Issue 41 Isocyclic Compounds ChemInform Abstract: Synthesis of Mercapturic Acid Derivatives of Putative Toxic Metabolites of Bromobenzene. R. P. HANZLIK, R. P. HANZLIK Dep. Med. Chem., Univ. Kans., Lawrence, KS 66045, USASearch for more papers by this authorP. E. WELLER, P. E. WELLER Dep. Med. Chem., Univ. Kans., Lawrence, KS 66045, USASearch for more papers by this authorJ. DESAI, J. DESAI Dep. Med. Chem., Univ. Kans., Lawrence, KS 66045, USASearch for more papers by this authorJ. ZHENG, J. ZHENG Dep. Med. Chem., Univ. Kans., Lawrence, KS 66045, USASearch for more papers by this authorL. R. HALL, L. R. HALL Dep. Med. Chem., Univ. Kans., Lawrence, KS 66045, USASearch for more papers by this authorD. E. SLAUGHTER, D. E. SLAUGHTER Dep. Med. Chem., Univ. Kans., Lawrence, KS 66045, USASearch for more papers by this author R. P. HANZLIK, R. P. HANZLIK Dep. Med. Chem., Univ. Kans., Lawrence, KS 66045, USASearch for more papers by this authorP. E. WELLER, P. E. WELLER Dep. Med. Chem., Univ. Kans., Lawrence, KS 66045, USASearch for more papers by this authorJ. DESAI, J. DESAI Dep. Med. Chem., Univ. Kans., Lawrence, KS 66045, USASearch for more papers by this authorJ. ZHENG, J. ZHENG Dep. Med. Chem., Univ. Kans., Lawrence, KS 66045, USASearch for more papers by this authorL. R. HALL, L. R. HALL Dep. Med. Chem., Univ. Kans., Lawrence, KS 66045, USASearch for more papers by this authorD. E. SLAUGHTER, D. E. SLAUGHTER Dep. Med. Chem., Univ. Kans., Lawrence, KS 66045, USASearch for more papers by this author First published: October 9, 1990 https://doi.org/10.1002/chin.199041150AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume21, Issue41October 9, 1990 RelatedInformation

Early mitochondrial disfunction in bromobenzene treated mice: A possible factor of liver injury

The membrane potential of liver mitochondria isolated from bromobenzene treated mice was studied. Specifically, the efficiency of the energy-transducing mitochondrial membrane was measured during the phase between the occurrence of a massive loss of hepatic GSH, after 2–3 hr of bromobenzene intoxication, and the appearance of lipid peroxidation and cell death (12–15 hr after treatment). Partial uncoupling of oxidative phosphorylation was observed in mitochondria during the early period of intoxication (3–9 hr). These anomalies in oxidative metabolism did not result in irreversible damage to the mitochondrial inner membrane. The possibility that phenolic metabolites of bromobenzene are responsible for the uncoupling effects was examined. Orto- and especially para-bromphenol reproduced the alterations of mitochondrial function when added to normal mitochondira at concentrations comparable to those found in the livers of the intoxicated animals. Since the concentration of the bromophenols (especially p-bromophenol) largely increases after the intoxidation times as tested here, mitochondrial uncoupling may represent a mechanism of liver damage acting synergistically with or even independently of other factors such as oxidative stress and lipid peroxidation.

Synthesis of mercapturic acid derivatives of putative toxic metabolites of bromobenzene

The synthesis and characterization of nine isomerically defined S-arylmercapturic acids of interest in connection with the metabolism of the model hepatotoxin bromobenzene is described. In addition, several related compounds with two or no bromine atoms are described. These syntheses depend on two basic methods, 1,4-addition of various arene thiols to acetamidoacrylic acid or the 1,4-addition of N-acetyl-L-cysteine to various benzoquinone derivatives

Specific determination of bromobenzene metabolites by high-performance liquid chromatography.

A chromatographic method for the quantitative and simultaneous determination of phenolic and mercapturic acid type metabolites of bromobenzene is presented. Acid hydrolysis (1.5 N HCl, 100 degrees C, 10 min) yielded unconjugated bromophenols. After extraction (ethyl acetate, pH 2), evaporation to dryness and dissolution in methanol, samples were analysed by high-performance liquid chromatography (HPLC) with a c18 reverse-phase column and a phosphate buffer (0.1 M,pH 9 + 5 mM tetrabutylammonium dihydrogen phosphate) - acetonitrile gradient with ultraviolet detection (225 nm). Benzylmercapturic acid and o-chlorophenol were used as internal standards. Using this method, it was possible to determine simultaneously o, m- and p-bromophenols together with o-, m- and p-bromophenylmercapturic acids. Additional validation data were obtained by analysis of urine samples from rats treated with bromobenzene. This new method was compared to existing procedures.

Isozyme selective arylation of cytosolic glutathione S-transferase by [ 14C]bromobenzene metabolites

[ 14C]Bromobenzene was incubated with NADPH-fortified liver homogenates from phenobarbital-treated rats, after which the glutathione S-transferases were isolated from the incubation mixture. Glutathione S-transferase activity, with 1-chloro-2,4-dinitrobenzene as the substrate, in the homogenate was unchanged after incubation with bromobenzene. Radioactivity derived from the [ 14C]bromobenzene remained associated with the cytosolic glutathione S-transferases after DE52 and Sephadex G-100 chromatography. Further purification of the cytosolic glutathione S-transferase by CM52 and hydroxylapatite chromatography showed that bromobenzene metabolites were bound to fractions containing glutathione S-transferase subunits 4, 5, and 1. The primary site of arylation appeared to be subunit 1, as indicated by autoradiography and hydroxylapatite chromatography. [ 14C]Bromobenzene metabolites were not bound to microsomal glutathione S-transferases, These data show that hepatic cytosolic glutathione S-transferases, especially glutathione S-transferases 4-4/5-5, 3-4, and 1-1 may act as trapping or scavenger proteins for reactive metabolites and that this effect is not associated with a loss of catalytic activity.

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

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.

Lipid peroxidation, protein thiols and calcium homeostasis in bromobenzene-induced liver damage.

Previous studies from our laboratory1 have shown that bromobenzene or iodobenzene administration to mice results in the development of lipid peroxidation when the hepatic glutathione (GSH) depletion reaches critical values. Liver necrosis behaves similarly and strictly correlates with the extent of lipid peroxidation. The treatment of the intoxicated animals with Trolox C (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid), a lower homolog of vitamin E, completely prevented both lipid peroxidation and necrosis, while not changing at all the extent of the covalent binding of bromobenzene metabolites to liver protein. This would suggest that lipid peroxidation is an important factor in the pathogenesis of the bromobenzene-induced liver cell death.

Effects of chemical and enzymic probes on microsomal covalent binding of bromobenzene and derivatives. Evidence for quinones as reactive metabolites.

1. The chemical reactivity of bromobenzene metabolite(s) responsible for its protein covalent binding was investigated by determining the effects of many chemical and enzymic probes on the metabolism and covalent binding of [3,5-3H]bromobenzene with rat liver microsomes in vitro. 2. Classical cytochrome P-450 enzyme inhibitors decreased both metabolism and binding in parallel, whereas scavenging agents for reactive oxygen species and free radicals exhibited little or no effect. Sulphur nucleophiles were extremely efficient in decreasing binding with little or no effect on metabolism. Reducing agents such as ascorbate and diaphorase decreased binding slightly more than metabolism. 3. UDP-Glucuronic acid inhibited neither metabolism nor binding, but all three mono-bromophenols decreased binding more than metabolism. Trichloropropene oxide was unique in decreasing metabolism more than binding. 4. The effects of ascorbate, glutathione, bisulphite and butylated hydroxytoluene (BHT) on metabolism and binding of five ortho-substituted bromobenzene derivatives (o-BrC6H4X; X = OCH3, CH3, Br, CF3, and CN) were similar to their effects on the metabolism and binding of bromobenzene. 5. Collectively these results support a major role for quinones as the reactive metabolites responsible for the majority of the protein covalent binding of bromobenzene and its ortho-substituted derivatives in microsomal systems in vitro.

Mitochondrial toxicity of 2-bromohydroquinone in rabbit renal proximal tubules

2-Bromohydroquinone (BHQ) is a nephrotoxic metabolite of bromobenzene and a model toxic hydroquinone. The primary goal of these studies was to determine whether BHQ produces toxicity in rabbit renal proximal tubules by inhibiting mitochondrial function. BHQ induces a specific sequence of cellular events. Initially there was a decrease in tubular glutathione content followed by a decrease in nystatin-stimulated ouabain-sensitive respiration. A decrease in cell viability, as measured by a decrease in lactate dehydrogenase retention, was a late event. Associated with the decrease in respiration was a decrease in intracellular ATP content. Probing of mitochondrial function in the tubule revealed that BHQ did inhibit mitochondrial function in a somewhat selective manner. State 3 respiration was inhibited prior to changes in the rate of electron flow through cytochrome c-cytochrome oxidase. It is postulated that BHQ may initially inhibit state 3 respiration by inhibiting the adenine nucleotide transloca. and/or the F 1-ATPase.

Metabolism of bromobenzene. Analytical chemical and structural problems associated with studies of the metabolism of a model aromatic compound.

Our studies of bromobenzene metabolism have shown that the 3,4-oxide is metabolized to 3- and 4-bromophenol through an extended glutathione pathway. The mechanism of sulfur elimination from a dihydrobromobenzene metabolite is not known, although it is known that the aromatization reaction will occur in a 9000-g supernatant fraction of rat liver. The hepatotoxic and nephrotoxic metabolites of bromobenzene are most likely bromthiocatechols and a bromothiopyrogallol, respectively.

Effect of a cysteine prodrug (L‐2‐oxothiazolidine‐4‐carboxylic acid) on the metabolism and toxicity of bromobenzene: A repeated exposure study

The relationship between dose, toxicity, and metabolism of bromobenzene and the use of urinary metabolite excretion as an index of internal exposure to the reactive electrophilic intermediate bromobenzene 3,4-epoxide after repeated treatments with bromobenzene in presence or absence of L-2-oxothiazolidine-4-carboxylic acid (OTCA) were evaluated in mice. Repeated treatments with bromobenzene doses of 0.5, 0.75, and 1.0 mmol/kg ip twice a day for 18 d produced a marked reduction in 24-h urinary excretion of bromophenylmercapturic acid and p-bromophenol; this was accompanied by increases in plasma transaminases. Treatment with OTCA (1.0-3.0 mmol/kg) prevented toxicity and enhanced the 24-h urinary excretion of various bromobenzene metabolites by approximately 30-75, 104-145, and 164-269% for bromobenzene doses of 0.5, 0.75, and 1.0 mmol/kg, respectively. The effect of OTCA was further characterized by investigating the metabolism of bromobenzene given as a challenge dose of 4.0 mmol/kg to mice pretreated with bromobenzene and OTCA for 18 d. Pretreatment with bromobenzene reduced the 0- to 6-h urinary excretion of all metabolites after the challenge dose; this effect was virtually reversed by OTCA. It is concluded that repeated bromobenzene administration reduces its own detoxification to mercapturic acid and phenolic metabolites and elicits toxicity. This phenomenon is reversed after OTCA administration. This study further provides evidence that internal exposure to the reactive electrophilic intermediate bromobenzene 3,4-epoxide could be assessed more accurately by summing the urinary excretion of bromophenylmercapturic acid and p-bromophenol after OTCA treatment.

Effect of a cysteine prodrug, L-2-oxothiazolidine-4-carboxylic acid, on the metabolism and toxicity of bromobenzene: an acute study.

The effect of a cysteine prodrug, L-2-oxothiazolidine-4-carboxylic acid (OTCA), on certain aspects of the metabolism and toxicity of bromobenzene administered acutely to mice was investigated by (i) characterizing the influence of OTCA on the metabolic profile of low and high bromobenzene dose at 0-6, 6-12, and 12-24 h, (ii) determining the effective doses range and administration time for OTCA, as well as the optimum period for urine sampling; and (iii) measuring the efficacy of OTCA for protection against bromobenzene induced toxicity. Coadministration of OTCA and bromobenzene enhanced the urinary excretion of mercapturic acid and phenolic metabolites, during 6-12 h, by approximately 152 and 193%, respectively. Maximum efficacy was observed when OTCA (16.0 mmol/kg) was administered concomitantly with bromobenzene (4.0 mmol/kg). Finally, OTCA administration was found to afford substantial protection against elevation of plasma transaminases used as indices of bromobenzene-induced hepatotoxicity. N-acetylcysteine, another cysteine prodrug, had essentially similar effects on the metabolism and toxicity of bromobenzene. Thus, administration of cysteine prodrugs enhances the urinary excretion of several metabolites of bromobenzene and affords protection against bromobenzene-induced hepatotoxicity.