Synthesis and relative stereochemistry of the four mercapturic acids derived from styrene oxide and N-acetylcysteine

Impaired glutamate homeostasis in the nucleus accumbens in human cocaine addiction

Cocaine addiction is characterized by overwhelming craving for the substance, which drives its escalating use despite adverse consequences. Animal models suggest a disrupted glutamate homeostasis in the nucleus accumbens to underlie addiction-like behavior. After chronic administration of cocaine, rodents show decreased levels of accumbal glutamate, whereas drug-seeking reinstatement is associated with enhanced glutamatergic transmission. However, due to technical obstacles, the role of disturbed glutamate homeostasis for cocaine addiction in humans remains only partially understood, and accordingly, no approved pharmacotherapy exists. Here, we applied a tailored proton magnetic resonance spectroscopy protocol that allows glutamate quantification within the human nucleus accumbens. We found significantly reduced basal glutamate concentrations in the nucleus accumbens in cocaine-addicted (N = 26) compared with healthy individuals (N = 30), and increased glutamate levels during cue-induced craving in cocaine-addicted individuals compared with baseline. These glutamatergic alterations, however, could not be significantly modulated by a short-term challenge of N-acetylcysteine (2400 mg/day on 2 days). Taken together, our findings reveal a disturbed accumbal glutamate homeostasis as a key neurometabolic feature of cocaine addiction also in humans. Therefore, we suggest the glutamatergic system as a promising target for the development of novel pharmacotherapies, and in addition, as a potential biomarker for a personalized medicine approach in addiction.

Hydrolytic Cleavage Products of Globin Adducts in Urine as Possible Biomarkers of Cumulative Dose: Proof of Concept Using Styrene Oxide as a Model Adduct-Forming Compound

A new experimental model was designed to study the fate of globin adducts with styrene 7,8-oxide (SO), a metabolic intermediate of styrene and a model electrophilic compound. Rat erythrocytes were incubated with SO at 7 or 22 °C. Levels of specific amino acid adducts in globin were determined by LC/MS analysis of the globin hydrolysate, and erythrocytes with known adduct content were administered intravenously to recipient rats. The course of adduct elimination from the rat blood was measured over the following 50 days. In the erythrocytes incubated at 22 °C, a rapid decline in the adduct levels on the first day post-transfusion followed by a slow phase of elimination was observed. In contrast, the adduct elimination in erythrocytes incubated at 7 °C was nearly linear, copying elimination of intact erythrocytes. In the urine of recipient rats, regioisomeric SO adducts at cysteine, valine, lysine, and histidine in the form of amino acid adducts and/or their acetylated metabolites as well as SO-dipeptide adducts were identified by LC/MS supported by synthesized reference standards. S-(2-Hydroxy-1-phenylethyl)cysteine and S-(2-hydroxy-2-phenylethyl)cysteine, the most abundant globin adducts, were excreted predominantly in the form of the corresponding urinary mercapturic acids (HPEMAs). Massive elimination of HPEMAs via urine occurred within the first day from the erythrocytes incubated at both 7 and 22 °C. However, erythrocytes incubated at 7 °C also showed a slow second phase of elimination such that HPEMAs were detected in urine up to 50 days post-transfusion. These results indicate for the first time that globin adducts can be cleaved in vivo to modified amino acids and dipeptides. The cleavage products and/or their predictable metabolites are excreted in urine over the whole life span of erythrocytes. Some of the urinary adducts may represent a new type of noninvasive biomarker for exposure to adduct-forming chemicals.

Detection of protein adduction derived from styrene oxide to cysteine residues by alkaline permethylation

Styrene oxide-cysteine adduction is predominantly involved in protein covalent modification after exposure in vivo to styrene or styrene oxide. In the present study, we developed an alkaline permethylation- and GC/MS-based approach to detect styrene oxide-derived protein adduction. Permethylation of the protein adducts produced two methylthiophenylethanols, namely 2-methylthio-2-phenyl-1-ethanol and 2-methylthio-1-phenyl-1-ethanol. To improve the permethylation efficiency, reaction conditions, including temperature, time, NaOH strength, and molar ratio of CH(3)I/NaOH, were explored. Under optimized conditions, the yields of the analyte formation resulting from permethylation of authentic standard alpha- and beta-mercapturic acids, representing alpha and beta isomers of cysteine adducts, were 35% and 28%, respectively. Permethylation of styrene oxide-modified bovine serum albumin released the two methylthiophenylethanols with an alpha-/beta-adduction ratio of 1.5. A concentration-dependent increase in both alpha- and beta-adduction was observed in mouse liver microsomes incubated with styrene at various concentrations. CD-1 mice were administered intraperitoneally with styrene at doses of 0, 50, and 400mg/kg daily for 5 days. The formation of protein adducts derived from styrene oxide in whole blood in 400mg/kg group was observed with an alpha/beta ratio of 4.8, suggesting that the reaction of styrene oxide with cysteine residues took place more likely at the alpha-carbon than the beta-carbon of styrene oxide.

Synthesis and characterization of styrene oxide adducts with cysteine, histidine, and lysine in human globin

Styrene 7,8-oxide (SO), a reactive metabolic intermediate of the industrial chemical styrene, binds covalently at nucleophilic amino acid residues of blood proteins in vivo and in vitro. In this study, SO adducts with cysteine, lysine, and histidine were synthesized, characterized, and then used as authentic standards to assign and quantitate the SO adducts in globin incubated with SO. S-(2-Hydroxy-1-phenylethyl)cysteine and S-(2-hydroxy-2-phenylethyl)cysteine were prepared by direct alkylation of cysteine with (R)-SO or (S)-SO. To prepare the SO adducts with lysine and histidine, Nalpha-Boc-protected amino acids were alkylated with (R)-SO or (S)-SO followed by deprotection of the Boc group to obtain Nepsilon-(2-hydroxy-1-phenylethyl)lysine and Nepsilon-(2-hydroxy-2-phenylethyl)lysine as well as Npi-(2-hydroxy-1-phenylethyl)histidine, Npi-(2-hydroxy-2-phenylethyl)histidine, Ntau-(2-hydroxy-1-phenylethyl)histidine, and Ntau-(2-hydroxy-2-phenylethyl)histidine. The individual regioisomers were isolated from their mixtures by semipreparative HPLC, and their structure was assigned using NMR techniques. The SO-modified globin, isolated from human hemoglobin incubated in vitro with racemic SO at a molar ratio SO/globin of 100:1 or 10:1, was digested with pronase and subjected to LC/MS and GC/MS analysis. All known regioisomers of the SO adducts were detected, with S-(2-hydroxy-1-phenylethyl)cysteine, Nepsilon-(2-hydroxy-1-phenylethyl)lysine, and Ntau-(2-hydroxy-2-phenylethyl)histidine being the most abundant in the modified globin. Deuterated analogues of the SO adducts were employed as internal standards. The SO-amino acid adducts described here appear to be suitable biomarkers for long-term exposures to styrene or SO.

Development of polyclonal antibodies for the detection of styrene oxide modified proteins

Styrene is widely used as one of the most important industrial materials for the production of synthetic rubbers, plastic, insulation, fiberglass, and automobile parts. Inhaled styrene has been reported to produce respiratory toxicity in humans and animals. Styrene oxide, a reactive metabolite of styrene formed via cytochrome P450 enzymes, has been reported to form covalent bonds with proteins, such as albumin and hemoglobin. Among all of the amino acids, cysteine is the most reactive amino acid to be modified by electrophilic species. The purpose of this study is to develop polyclonal antibodies for the detection of styrene oxide cysteinyl protein adducts. Two immunogens were designed, synthesized, and used to induce polyclonal antibodies in rabbits. Immune responses were observed from the raised antibodies by antiserum dilution tests. Competitive ELISA demonstrated that the resulting antibodies specifically recognized the styrene oxide-derived N-acetylcysteine adduct. Western blot results showed that the antibodies recognize styrene oxide-modified albumin. The binding was found to depend on the amount of protein adducts blotted and hapten loading in protein adducts. No cross reaction was observed from the native protein. Competitive Western blots further indicated that these antibodies specifically recognized styrene oxide cysteinyl-protein adducts. Immunoblots revealed the presence of several bands at a molecular weight ranging from 50 to 80 kDa in rat nasal mucosa treated with styrene. In conclusion, we successfully raised polyclonal antibodies to detect styrene oxide-derived protein/cysteine adducts.

Stereochemistry of styrene biotransformation

Metabolism of styrene, an important industrial monomer, is reviewed. Attention is focused on the stereoselectivity of its oxidation to 7,8-styrene oxide as well as on further stereoselective biotransformation by hydrolytic and mercapturic acid pathway. Toxic effects such as mutagenicity, genotoxicity, hepatotoxicity, and pneumotoxicity may be related to the ratio of styrene oxide enantiomers at the target site. In rats formation of the less mutagenic (S)-styrene oxide and a faster detoxication of the (R)-enantiomer is favored. In mice metabolic activation of styrene favors the formation of (R)-styrene oxide but this more toxic enantiomer is detoxified faster, so that a nearly racemic styrene oxide results. Stereochemistry of biotransformation can contribute to the species differences in toxicity but can hardly be interpreted as a crucial factor. Due to lack of relevant data the stereochemistry of human metabolism cannot be interpreted in relation to the toxic effects.

Biotransformation of styrene in mice. Stereochemical aspects

Biotransformation of styrene and its toxic metabolite, phenyloxirane (1), in mice in vivo was studied. Mice were treated with single intraperitoneal doses of styrene (400 mg/kg of body weight), and with (R)-, (S)-, or racemic styrene oxide (150 mg/kg of body weight). Profiles of neutral and acidic metabolites were determined by GC/MS. Mandelic acid (3) and two mercapturic acids, N-acetyl-S-(2-hydroxy-2-phenylethyl)cysteine (5) and N-acetyl-S-(2-hydroxy-1-phenylethyl)cysteine (6), were found to be major urinary metabolites of both styrene and phenyloxirane. 1-Phenylethane-1,2-diol (2) was the main neutral metabolite. The rate of excretion of this metabolite, as determined by GC, was 5-10 times lower than that of mandelic acid. Several minor acidic metabolites were also identified. Among them, novel phenolic metabolites, namely, 2-(4-hydroxyphenyl)ethanol (7), (4-hydroxyphenyl)acetic acid (11), and two isomeric hydroxymandelic acids (12), are of toxicological significance. Main stereogenic metabolites were isolated as methyl esters from extracts of pooled acidified urine treated with diazomethane. The mandelic acid that was obtained was converted to diastereomeric Mosher's derivatives prior to analysis by NMR. Mercapturic acids were analyzed directly by (13)C NMR. Pure enantiomers of 1 were metabolized predominantly but not exclusively to corresponding enantiomers of 3. Styrene yielded predominantly (S)-mandelic acid. Fractions of mercapturic acids 5 and 6 isolated from urine amounted to 12-15% of the dose for all compounds that were administered. Conversion to mercapturic acids was highly regio- and stereoselective, yielding predominantly regioisomer 5. Styrene, as compared to racemic phenyloxirane, yielded slightly more diastereomers arising from (S)-1 than from (R)-1. These data can be explained by formation of a moderate excess of the less mutagenic (S)-1 in the metabolic activation of styrene in mice in vivo.

Comparison of styrene-7,8-oxide adducts formed via reaction with cysteine, N-terminal valine and carboxylic acid residues in human, mouse and rat hemoglobin

The reactive metabolite of styrene, styrene-7,8-oxide (SO), reacts with a variety of nucleophilic sites in hemoglobin (Hb) to form SO-Hb adducts. Following the in vitro incubation of SO with blood from humans, NMRI mice and Sprague-Dawley rats, the second-order reaction rate constants were determined for the reaction of SO with cysteine (through both the alpha- and beta-carbons of SO), N-terminal valine (through the beta-carbon of SO), and carboxylic acid (presumably through both the alpha- and beta-carbons of SO) residues in Hb. The rate constants for cysteine adducts vary dramatically between species [2.04, 10.7, 133 L (mol Hb)-1 h-1 (alpha binding) for humans, mice and rats, respectively] and [0.078, 2.16, 20.4 L (mol Hb)-1 h-1 (beta binding), respectively]. The considerably higher rate of reaction with cysteine in rat Hb probably reflects the presence of an additional cysteine residue at position beta 125. Although the rate constants for valine adducts (1.82, 0.80, 0.29 L (mol Hb)-1 h-1, respectively) and COOH adducts (3.55, 1.94, 2.37 L (mol Hb)-1 h-1, respectively) are much more consistent, the inter-species differences are statistically significant for the reaction of SO with the N-terminal valine of Hb. Following the i.p. administration of styrene to mice and styrene and SO to rats, the levels of adducts at each of these sites were used in conjunction with the calculated rate constants to predict the integrated blood doses of SO. While the SO doses predicted from cysteine and valine adducts were very similar, that based upon COOH-binding was significantly different, presumably due to the instability of SO-COOH adducts. This research affirms the use of both cysteine and valine adducts, but not carboxylic acid adducts, as biomarkers of exposure to styrene and SO.

Excretion of N-acetyl-S-(1-phenyl-2-hydroxyethyl)-cysteine and N-acetyl-S-(2-phenyl-2-hydroxyethyl)-cysteine in workers exposed to styrene

Styrene (S) has been shown to be responsible for neurotoxic effects, including behavioural changes and neuroendocrine disturbances. The initial step of S metabolism is conversion to styrene 7,8-epoxide (SO), which is present in two enantiomeric forms [(R)(+)-SO and (S)(-)-SO]; this electrophilic intermediate is considered to be directly responsible for most toxic effects of S. The major urinary metabolites derived from the biotransformation of SO in man are mandelic acid (MA) and phenylglyoxylic acid (PGA). In rats an alternative pathway has been demonstrated, which involves the conjugation of SO to glutathione (GSH), leading to the excretion of two specific mercapturic acids, N-acetyl-S-(-(1-phenyl-2-hydroxyethyl)-cysteine [M1] and N-acetyl-S-(2-phenyl-2-hydroxy-ethyl)-cysteine [M2]; a close relationship has been found between exposure to S and urinary excretion of M1 and M2 in rats. As a consequence of the chiral nature of SO, both M1 and M2 consist of two diastereoisomers (M1-'R', M1-'S', M2-'R' and M2-'S'). Early reports have shown that the conversion of S to mercapturic acids is much lower in man (below 1% of the absorbed dose) than in rats (about 10%). We propose an analytical method for the determination of urinary M1 and M2 in man, which involves a urine clean-up by a chromatographic technique with a short reversed-phase pre-column; purified samples are then deacetylated with porcine acylase and deproteinized by centrifugal ultrafiltration. A derivatization is then performed with o-phthaldialdehyde and 2-mercaptoethanol and the fluorescent derivatives are separated on a reversed-phase analytical column. The mobile phase consists of acetate buffer and methanol mixed at variable proportions, the fluorescence detector is set at 330 nm (exc.) and 440 nm (em.). M1-'S' and M1-'R' are separated (retention times = 52.8 and 73.7 min, respectively) while the diastereoisomers of M2 coelute as a single peak at 70.5 min. The detection limit is about 7 micrograms/l, the coefficients of variation are below 7% and the error percentages are less than 6%. The method was applied to 25 urine samples from workers exposed to S: significant correlations were found between mercapturic acids and MA and PGA, the best correlation being between M2 and PGA (r = 0.79). Urine samples form unexposed subjects showed no detectable amounts of the analytes. A high stereoselectivity is shown by the enzymes involved in the metabolism of S to mercapturic acids: M1-'S', which derives from (S)-SO, is excreted in much higher amounts than M1-'R', which derives from (R)-SO.

A note on individual differences in the urinary excretion of optical enantiomers of styrene metabolites and of styrene-derived mercapturic acids in humans

Urine samples from 20 male workers in the polyester industry exposed by inhalation to styrene concentrations ranging from 29 to 41 ppm were investigated. Excretion products of styrene metabolism, mandelic acid and mercapturic acids, were purified from the urine over an extraction column packed with Porapak Q, with subsequent ether elution. The optical enantiomers R- and S-mandelic acid were then determined by thin layer chromatography (TLC) using chiral plate material and selective staining with vanadium pentoxide. Quantitative analysis of these compounds was performed using commercial reference substances. Styrene-specific mercapturic acids were analyzed by a modified TLC method, using synthesized reference substances. The concentration of racemic mandelic acid in the individual urine samples ranged from 80 to 1610 mg/l, and the ratio of the R- and S-enantiomers ranged from 0.7 to 2.2. These individual variations are not explained by differences in individual styrene exposure levels, or by differences in the concentration of the urine samples (in relation to creatinine excretion). Styrene-specific mercapturic acids were detected in the urine of only 1 of the 20 workers, at a concentration much lower than expected from previous investigations by others in humans and laboratory animals, in which less specific analytical methods had been used. The results point to marked interindividual differences in metabolism of styrene, probably related to enzyme polymorphisms.

Analysis of mercapturic acid conjugates of xenobiotic compounds using negative ionization and tandem mass spectrometry

Mass spectra of mercapturic acid conjugates of two xenobiotic products of lipid peroxidation (trans-4-hydroxy-2-hexenal and trans-4-hydroxy-2-nonenal) as well as conjugates of 1,3-dichloropropene, styrene oxide, 1,2-naphthalene oxide and alpha-chlorotoluene were obtained using fast atom bombardment or negative chemical ionization. Fragmentation pathways were investigated using linked scan and mass-analyzed ion kinetic energy spectrometric techniques. Characteristics of the spectra obtained using different ionization and sample introduction techniques are compared. Deprotonated molecular ions of mercapturic acids gave simple daughter ion spectra, with the dominant mode of decomposition involving cleavage of C-S bonds giving a characteristic neutral loss of 129 Da. Screening for mercapturates in urine samples was performed using neutral loss scanning and yielded limits of detection in the low nanogram per milliliter range. Quantitative analysis of the S-benzyl mercapturic acid at 1 p.p.b. in urine has been demonstrated using combined gas chromatography/electron capture mass spectrometry with d3-S-benzyl mercapturic acid as internal standard.

Biotransformation of diethenylbenzenes. III: Identification of metabolites of 1,3-diethenylbenzene in rat

1. Biotransformation of 1,3-diethenylbenzene (1) in rat gave four major metabolites, namely, 3-ethenylphenylglyoxylic acid (2), 3-ethenylmandelic acid (3), N-acetyl-S-[2-(3-ethenylphenyl)-2-hydroxyethyl]-L-cysteine (4) and N-acetyl-S-[1-(3-ethenylphenyl)-2-hydroxyethyl]-L-cysteine (5) were isolated from urine and identified by n.m.r. and mass spectrometry. 2. Four minor metabolites, 3-ethenylbenzoic acid (6), 3-ethenylphenylacetic acid (7), 3-ethenylbenzoylglycine (8) and 2-(3-ethenylphenyl)ethanol (9) were identified by g.l.c.-mass spectrometric analysis of urine extract derivatized in two different ways. 3. All identified metabolites are derived from 3-ethenylphenyloxirane (10), a reactive metabolic intermediate. No product of any metabolic transformation of second ethenyl group has been identified. However, several minor unidentified metabolites were detected by g.l.c.-mass spectrometry. 4. Total thioether excretion in 24 h urine after a single i.p. dose of 1 amounted to 28.3 +/- 3.5 dose (mean +/- SD). No significant differences in the thioether fraction were observed in the dose range 100-300 mg/kg. 5. Thioether metabolites consisted mainly of mercapturic acids 4 and 5. The ratio of metabolites 5 to 4 was 62:38. Each mercapturic acid consisted of two diastereomers. Their ratio, as determined by quantitative 13C-n.m.r. measurement was 95:5 and 79:21 for mercapturic acids 4 and 5, respectively.

Structure-nephrotoxicity relationships of glutathione pathway intermediates derived from organic solvents

The nephrotoxicity of glutathione (GSH) pathway metabolites derived from toluene (TOL), styrene (STYR), bromobenzene (BB), acrylonitrile (ACLN) and 2-chloroacrylonitrile (CACLN) were compared with that of dichlorovinylcysteine (DCVC), using renal brush border and basal-lateral uptake parameters as indices. Cysteine conjugates and mercapturates of ACLN did not alter p-aminohippurate (PAH) uptake by renal tubule suspensions in contrast to its chlorinated homologue. O-, m- and p-conjugates of BB inhibited PAH uptake by 43-82%, the mercapturates showing more potency than corresponding cysteine conjugates. The TOL derivatives N-acetylbenzylcysteine curtailed PAH uptake but benzylcysteine was more effective. The GSH conjugate and mercapturate synthesized from STYR oxide were also active inhibitors but not its cysteine conjugate. Among all GSH pathway metabolites studied, only DCVC and phenylhydroxyethylglutathione, derived from STYR oxide, impeded the renal basal-lateral uptake of [14C]tetraethylammonium (TEA) while DCVC was the sole inhibitor of brush border transport events such as the uptakes of [3H]glutamate and [14C]alpha-methyl-D-glucoside. These data indicate that GSH conjugation represents a non-nephrotoxic detoxication pathway for ACLN. In contrast, GSH conjugation with 2-chloroacrylonitrile and with aromatic solvents like TOL, STYR, BB gives rise to nephrotoxic mercapturates which may be less potent but show more specificity for the organic anion transport system than DCVC.

Binding of styrene oxide to amino acids, human serum proteins and hemoglobin

The covalent binding of 3H-styrene oxide to amino acids, and whole human blood was investigated in vitro. After reaction, serum, and red cells were separated, and proteins were digested into amino acids; styrene oxide derivatives were isolated by HPLC. The order of binding to free amino acids was cysteine much greater than histidine greater than lysine greater than serine. In serum proteins and hemoglobin cysteine-derivatives predominated. When styrene oxide was reacted with free cysteine and with proteins two isomers were observed. These were likely to present binding through the alpha and beta carbon of styrene oxide, and their abundance was about 2:1.

Mercapturic acid metabolites from 2-, 3-, and 4-ethenyl-methylbenzenes in the rat

N-Acetyl-L-cysteine was reacted with 2-(2-, 3-, or 4-methylphenyl)-oxiranes to give mixtures of the two possible regio isomers N-acetyl-S-[1-(2-, 3-, or 4-methylphenyl)-2-hydroxyethyl]-L-cysteine and N-acetyl-S-[2-(2-, 3-, or 4-methylphenyl)-2-hydroxyethyl]-L-cysteine, respectively. These were isolated in pure form by h.p.l.c.. The diastereomers were characterized by n.m.r. and mass spectrometry. The 2-, 3- and 4-ethenyl-methylbenzenes and the 2-(2-, 3-, and 4-methylphenyl)-oxiranes were injected i.p. into rats. G.l.c.-mass spectrometry showed similar patterns of acidic metabolites in the urine. Comparison with authentic mass spectra showed that the N-acetyl-S-[1-(methylphenyl)-2-hydroxyethyl]-L-cysteines accounted for over 80% of the mercapturic acids.

On the mechanism of covalent binding of butylated hydroxytoluene to microsomal protein

The structures of cysteine conjugates of 3,5-di-tert-butyl-4-hydroxytoluene (BHT) and the binding sites of BHT metabolites on microsomal protein were investigated by 13C nuclear magnetic resonance (13C-NMR) and gas-liquid chromatography/mass spectrometry. The cysteine conjugates of 2,6-di-tert-butyl-4-hydroxymethylphenol (BHT-alcohol) and 2,6-di-tert-butyl-4-methylene-2,5-cyclohexadienone (quinone methide), which are metabolites of BHT found in rat liver and specifically reacts with thiol compounds, were prepared as alcoholic aqueous solutions. The molecular structure of the cysteine conjugate of BHT-alcohol agreed completely with that of quinone methide in 13C-NMR spectra or mass spectra. These spectra of both conjugates further showed that the conjugates are due to thioether binding between the 4-methyl group of metabolites and the sulfhydryl group of cysteine. When [14C]BHT-bound microsomes prepared in vitro were enzymatically hydrolyzed with Pronase E, the major radioactive material that eluted with methanol from a column of Amberlite XAD-2 and gave a positive ninhydrin reaction was identified as a cysteine conjugate of BHT by comparing its Rf values on TLC and mass spectrum. On the basis of the results, it was apparent that the binding site of activated substituents of BHT on protein was mainly the sulfhydryl group of cysteine residue.

Advances in methods and techniques for the identification of xenobiotic conjugates

This review discusses recent advances in methods and instrumentation that are potentially useful for the characterization of polar xenobiotic conjugates. Topics that are discussed include: fast atom bombardment, secondary ion, 252Cf-plasma desorption, field desorption and direct chemical ionization mass spectrometry and, proton and carbon-13 nuclear magnetic resonance spectrometry.

Stereoselective formation of bromobenzene glutathione conjugates

Two bromobenzene-glutathione conjugates have been detected as both in vivo and in vitro metabolites of bromobenzene. Separation and purification by high pressure liquid chromatography (HPLC) and analysis by 13C and 1H-NMR spectroscopy indicated that the metabolites are trans-3-bromo-6-(glutathion-S-yl)-cyclohexa-2,4-dien-1-ol and trans-4-bromo-6-(glutathion-S-yl)-cyclohexa-2,4-dien-1-ol. The two conjugates are formed in unequal amounts; over a dose range of 25-500 mg/kg the ratio of the two conjugates excreted into bile in 6 h was 1.6 +/- 0.1 (mean +/- S.E.). Pretreatment of rats with either phenobarbital or 3-methyl-cholanthrene did not significantly alter the ratio of the two conjugates excreted into bile. When bromobenzene was incubated with rat liver microsomes and glutathione, the same two conjugates were formed in the presence but not in the absence of 100 000 x g supernatant. Furthermore, in the presence of 100 000 x g supernatant from control animals, microsomes from rats treated with phenobarbital formed both conjugates 6 times more rapidly than did microsomes from control rats, whereas microsomes from rats treated with 3-methylcholanthrene formed both conjugates less rapidly than did those from control rats. Thus, the data suggest that both conjugates are formed via bromobenzene 3,4-oxide and that their formation requires in liver cytosol.

Mutagenicity of (R) and (S) styrene 7,8-oxide and the intermediary mercapturic acid metabolites formed from styrene 7,8-oxide

We have tested the two enantiomers of styrene 7,8-oxide and various thioether metabolites of racemic styrene 7,8-oxide for their direct mutagenicity in Salmonella typhimurium TA100. The mutagenicity data suggests that the (R) enantiomer is more mutagenic than the (S) enantiomer, with the racemic mixture intermediate between the two. The thioether metabolites were not mutagenic. The difference in the mutagenicities of enantiomers probably resulted from a stereoselective process in the Salmonella tester strain. At the present time it is not clear whether the rate-limiting reaction is the interaction of the enantiomers with DNA or some other cellular component.