High throughput HPLC-ESI(-)-MS/MS methodology for mercapturic acid metabolites of 1,3-butadiene: Biomarkers of exposure and bioactivation

Association of Urinary N7-(1-hydroxyl-3-buten-1-yl) Guanine (EB-GII) Adducts and Butadiene-Mercapturic Acids with Lung Cancer Development in Cigarette Smokers

Approximately 10% of smokers will develop lung cancer. Sensitive predictive biomarkers are needed to identify susceptible individuals. 1,3-Butadiene (BD) is among the most abundant tobacco smoke carcinogens. BD is metabolically activated to 3,4-epoxy-1-butene (EB), which is detoxified via the glutathione conjugation/mercapturic acid pathway to form monohydroxybutenyl mercapturic acid (MHBMA) and dihydroxybutyl mercapturic acid (DHBMA). Alternatively, EB can react with guanine nucleobases of DNA to form N7-(1-hydroxyl-3-buten-1-yl) guanine (EB-GII) adducts. We employed isotope dilution LC/ESI-HRMS/MS methodologies to quantify MHBMA, DHBMA, and EB-GII in urine of smokers who developed lung cancer (N = 260) and matched smoking controls (N = 259) from the Southern Community Cohort (white and African American). The concentrations of all three biomarkers were significantly higher in smokers that subsequently developed lung cancer as compared to matched smoker controls after adjusting for age, sex, and race/ethnicity (p < 0.0001 for EB-GII, p < 0.0001 for MHBMA, and p = 0.0007 for DHBMA). The odds ratio (OR) for lung cancer development was 1.63 for MHBMA, 1.37 for DHBMA, and 1.97 for EB-GII, with a higher OR in African American subjects than in whites. The association of urinary EB-GII, MHBMA, and DHBMA with lung cancer status did not remain upon adjustment for total nicotine equivalents. These findings reveal that urinary MHBMA, DHBMA, and EB-GII are directly correlated with the BD dose delivered via smoking and are associated with lung cancer risk.

Use of Biomarker Data and Relative Potencies of Mutagenic Metabolites to Support Derivation of Cancer Unit Risk Values for 1,3-Butadiene from Rodent Tumor Data

Unit Risk (UR) values were derived for 1,3-butadiene (BD) based upon its ability to cause tumors in laboratory mice and rats. Metabolism has been established as the significant molecular initiating event of BD’s carcinogenicity. The large quantitative species differences in the metabolism of BD and potency of critical BD epoxide metabolites must be accounted for when rodent toxicity responses are extrapolated to humans. Previously published methods were extended and applied to cancer risk assessments to account for species differences in metabolism, as well as differences in mutagenic potency of BD metabolites within the context of data-derived adjustment factors (DDEFs). This approach made use of biomarker data (hemoglobin adducts) to quantify species differences in the internal doses of BD metabolites experienced in mice, rats, and humans. Using these methods, the dose–response relationships in mice and rats exhibit improved concordance, and result in upper bound UR values ranging from 2.1 × 10⁻⁵ to 1.2 × 10⁻³ ppm⁻¹ for BD. Confidence in these UR values was considered high based on high confidence in the key studies, medium-to-high confidence in the toxicity database, high confidence in the estimates of internal dose, and high confidence in the dose–response modeling.

1,3-Butadiene: a ubiquitous environmental mutagen and its associations with diseases

1,3-Butadiene (BD) is a petrochemical manufactured in high volumes. It is a human carcinogen and can induce lymphohematopoietic cancers, particularly leukemia, in occupationally-exposed workers. BD is an air pollutant with the major environmental sources being automobile exhaust and tobacco smoke. It is one of the major constituents and is considered the most carcinogenic compound in cigarette smoke. The BD concentrations in urban areas usually vary between 0.01 and 3.3 μg/m³ but can be significantly higher in some microenvironments. For BD exposure of the general population, microenvironments, particularly indoor microenvironments, are the primary determinant and environmental tobacco smoke is the main contributor. BD has high cancer risk and has been ranked the second or the third in the environmental pollutants monitored in most urban areas, with the cancer risks exceeding 10^-5. Mutagenicity/carcinogenicity of BD is mediated by its genotoxic metabolites but the specific metabolite(s) responsible for the effects in humans have not been determined. BD can be bioactivated to yield three mutagenic epoxide metabolites by cytochrome P450 enzymes, or potentially be biotransformed into a mutagenic chlorohydrin by myeloperoxidase, a peroxidase almost specifically present in neutrophils and monocytes. Several urinary BD biomarkers have been developed, among which N-acetyl-S-(4-hydroxy-2-buten-1-yl)-L-cysteine is the most sensitive and is suitable for biomonitoring BD exposure in the general population. Exposure to BD has been associated with leukemia, cardiovascular disease, and possibly reproductive effects, and may be associated with several cancers, autism, and asthma in children. Collectively, BD is a ubiquitous pollutant that has been associated with a range of adverse health effects and diseases with children being a subpopulation with potentially greater susceptibility. Its adverse effects on human health may have been underestimated and more studies are needed.

Intra- and Inter-Species Variability in Urinary N7-(1-Hydroxy-3-buten-2-yl)guanine Adducts Following Inhalation Exposure to 1,3-Butadiene

1,3-Butadiene is a known carcinogen primarily targeting lymphoid tissues, lung, and liver. Cytochrome P450 activates butadiene to epoxides which form covalent DNA adducts that are thought to be a key mechanistic event in cancer. Previous studies suggested that inter-species, -tissue, and -individual susceptibility to adverse health effects of butadiene exposure may be due to differences in metabolism and other mechanisms. In this study, we aimed to examine the extent of inter-individual and inter-species variability in the urinary N7-(1-hydroxy-3-buten-2-yl)guanine (EB-GII) DNA adduct, a well-known biomarker of exposure to butadiene. For a population variability study in mice, we used the collaborative cross model. Female and male mice from five strains were exposed to filtered air or butadiene (590 ppm, 6 h/day, 5 days/week for 2 weeks) by inhalation. Urine samples were collected, and the metabolic activation of butadiene by DNA-reactive species was quantified as urinary EB-GII adducts. We quantified the degree of EB-GII variation across mouse strains and sexes; then, we compared this variation with the data from rats (exposed to 62.5 or 200 ppm butadiene) and humans (0.004-2.2 ppm butadiene). We show that sex and strain are significant contributors to the variability in urinary EB-GII levels in mice. In addition, we find that the degree of variability in urinary EB-GII in collaborative cross mice, when expressed as an uncertainty factor for the inter-individual variability (UF_H), is relatively modest (≤threefold) possibly due to metabolic saturation. By contrast, the variability in urinary EB-GII (adjusted for exposure) observed in humans, while larger than the default value of 10-fold, is largely consistent with UF_H estimates for other chemicals based on human data for non-cancer endpoints. Overall, these data demonstrate that urinary EB-GII levels, particularly from human studies, may be useful for quantitative characterization of human variability in cancer risks to butadiene.

Quantitative NanoLC/NSI+-HRMS Method for 1,3-Butadiene Induced bis-N7-guanine DNA-DNA Cross-Links in Urine

1,3-Butadiene (BD) is a common environmental and industrial chemical widely used in plastic and rubber manufacturing and also present in cigarette smoke and automobile exhaust. BD is classified as a known human carcinogen based on evidence of carcinogenicity in laboratory animals treated with BD by inhalation and epidemiological studies revealing an increased risk of leukemia and lymphohematopoietic cancers in workers occupationally exposed to BD. Upon exposure via inhalation, BD is bioactivated to several toxic epoxides including 3,4-epoxy-1-butene (EB), 3,4-epoxy-1,2-butanediol (EBD), and 1,2,3,4-diepoxybutane (DEB); these are conjugated with glutathione and excreted as 2-(N-acetyl-L-cystein-S-yl)-1-hydroxybut-3-ene/1-(N-acetyl-L-cystein-S-yl)-2-hydroxybut-3-ene (MHBMA), 4-(N-acetyl-L-cystein-S-yl)-1,2-dihydroxybutane (DHBMA), and 1,4-bis-(N-acetyl-L-cystein-S-yl)butane-2,3-diol (bis-BDMA). Exposure to DEB generates monoalkylated DNA adducts, DNA-DNA crosslinks, and DNA-protein crosslinks, which can cause base substitutions, genomic rearrangements, and large genomic deletions. In this study, we developed a quantitative nanoLC/NSI+-HRMS methodology for 1,4-bis-(gua-7-yl)-2,3-butanediol (bis-N7G-BD) adducts in urine (LOD: 0.1 fmol/mL urine, LOQ: 1.0 fmol/mL urine). This novel method was used to quantify bis-N7G-BD in urine of mice treated with 590 ± 150 ppm BD for 2 weeks (6 h/day, 5 days/week). Bis-N7G-BD was detected in urine of male and female BD-exposed mice (574.6 ± 206.0 and 571.1 ± 163.4 pg/mg of creatinine, respectively). In addition, major urinary metabolites of BD, bis-BDMA, MHBMA and DHBMA, were measured in the same samples. Urinary bis-N7G-BD adduct levels correlated with DEB-derived metabolite bis-BDMA (r = 0.80, Pearson correlation), but not with the EB-derived DNA adducts (EB-GII) or EB-derived metabolites MHBMA and DHBMA (r = 0.24, r = 0.14, r = 0.18, respectively, Pearson correlations). Urinary bis-N7G-BD could be employed as a novel non-invasive biomarker of exposure to BD and bioactivation to its most mutagenic metabolite, DEB. This method will be useful for future studies of 1,3-butadiene exposure and metabolism.

Ethnic differences in excretion of butadiene-DNA adducts by current smokers

1,3-Butadiene (BD) is a known human carcinogen used in the synthetic polymer industry and also found in cigarette smoke, automobile exhaust and wood burning smoke. BD is metabolically activated by cytochrome P450 monooxygenases (CYP) 2E1 and 2A6 to 3,4-epoxy-1-butene (EB), which can be detoxified by GST-catalyzed glutathione conjugation or hydrolysis. We have previously observed ethnic differences in urinary levels of EB-mercapturic acids in white, Japanese American and Native Hawaiian smokers. In the present study, similar analyses were extended to urinary BD-DNA adducts. BD-induced N7-(1-hydroxy-3-buten-2-yl) guanine (EB-GII) adducts were quantified in urine samples obtained from smokers and non-smokers belonging to three racial/ethnic groups: white, Japanese American and Native Hawaiian. After adjusting for sex, age, nicotine equivalents, body mass index and batch, we found that Japanese American smokers excreted significantly higher amounts of urinary EB-GII than whites [1.45 (95% confidence interval: 1.12-1.87) versus 0.68 (95% confidence interval: 0.52-0.85) fmol/ml urine, P = 4 × 10-5]. Levels of urinary EB-GII in Native Hawaiian smokers were not different from those in whites [0.67 (95% confidence interval: 0.51-0.84) fmol/ml urine, P = 0.938]. There were no racial/ethnic differences in urinary EB-GII adduct levels in non-smokers. Racial/ethnic differences in urinary EB-GII adduct levels in smokers could not be explained by GSTT1 gene deletion or CYP2A6 enzymatic activity. Urinary EB-GII adduct levels in smokers were significantly associated with concentrations of BD metabolite dihyroxybutyl mercapturic acid. Overall, our results reveal that urinary EB-GII adducts in smokers differ across racial/ethnic groups. Future studies are required to understand genetic and epigenetic factors that may be responsible for these differences.

Applying Tobacco, Environmental, and Dietary-Related Biomarkers to Understand Cancer Etiology and Evaluate Prevention Strategies

Many human cancers are caused by environmental and lifestyle factors. Biomarkers of exposure and risk developed by our team have provided critical data on internal exposure to toxic and genotoxic chemicals and their connection to cancer in humans. This review highlights our research using biomarkers to identify key factors influencing cancer risk as well as their application to assess the effectiveness of exposure intervention and chemoprevention protocols. The use of these biomarkers to understand individual susceptibility to the harmful effects of tobacco products is a powerful example of the value of this type of research and has provided key data confirming the link between tobacco smoke exposure and cancer risk. Furthermore, this information has led to policy changes that have reduced tobacco use and consequently, the tobacco-related cancer burden. Recent technological advances in mass spectrometry led to the ability to detect DNA damage in human tissues as well as the development of adductomic approaches. These new methods allowed for the detection of DNA adducts in tissues from patients with cancer, providing key evidence that exposure to carcinogens leads to DNA damage in the target tissue. These advances will provide valuable insights into the etiologic causes of cancer that are not tobacco-related.See all articles in this CEBP Focus section, "Environmental Carcinogenesis: Pathways to Prevention."

A review of the analysis of biomarkers of exposure to tobacco and vaping products

Quantification of exposure to different chemicals from both combustible cigarettes and vaping products is important in providing information on the potential health risks of these products. To assess the exposure to tobacco products, biomarkers of exposure (BOEs) are measured in a variety of biological matrices. In this review paper, current knowledge on analytical methods applied to the analysis of biomarkers of exposure to tobacco products is discussed. Numerous sample preparation techniques are available for the extraction and sample clean up for the analysis of BOEs to tobacco and nicotine delivery products. Many tobacco products-related exposure biomarkers have been analyzed using different instrumental techniques, the most common techniques being gas and liquid chromatography coupled with mass spectrometry (GC-MS, GC-MS/MS and LC-MS/MS). To assess exposure to emerging tobacco products and study exposure in dual tobacco users, the list of biomarkers analyzed in urine samples has been expanded. Therefore, the current state of the literature can be used in preparing a preferred list of biomarkers based on the aim of each study. The information summarized in this review is expected to be a handy tool for researchers involved in studying exposures to tobacco products, as well as in risk assessment of biomarkers of exposure to vaping products.

Interindividual Differences in DNA Adduct Formation and Detoxification of 1,3-Butadiene-Derived Epoxide in Human HapMap Cell Lines

Smoking-induced lung cancer is a major cause of cancer mortality in the US and worldwide. While 11-24% of smokers will develop lung cancer, risk varies among individuals and ethnic/racial groups. Specifically, African American and Native Hawaiian cigarette smokers are more likely to get lung cancer as compared to Caucasians, Japanese Americans, and Latinos. It is important to identify smokers who are at the greatest risk of developing lung cancer as they should be candidates for smoking cessation and chemopreventive intervention programs. Among 60+ tobacco smoke carcinogens, 1,3-butadiene (BD) is one of the most potent and abundant (20-75 μg per cigarette in mainstream smoke and 205-361 μg per cigarette in side stream smoke). BD is metabolically activated to 3,4-epoxy-1-butene (EB), which can be detoxified by glutathione S-transferase theta 1 (GSTT1)-mediated conjugation with glutathione, or can react with DNA to form 7-(1-hydroxy-3-buten-2-yl)guanine (EB-GII) adducts. In the present study, we employed EBV-transformed human lymphoblastoid cell lines (HapMap cells) with known GSTT1 genotypes to examine the influence of the GSTT1 gene on interindividual variability in butadiene metabolism, DNA adduct formation/repair, and biological outcomes (apoptosis). We found that GSTT1- HapMap cells treated with EB in culture produced lower levels of glutathione conjugates and were more susceptible to apoptosis but had similar numbers of EB-GII adducts as GSTT1+ cells. Our results suggest that GSTT1 can influence an individual's susceptibility to butadiene-derived epoxides.

Resolution and Quantitation of Mercapturic Acids Derived from Crotonaldehyde, Methacrolein, and Methyl Vinyl Ketone in the Urine of Smokers and Nonsmokers

Using improved HPLC analysis conditions, we report the separation of three isomers of mercapturic acid conjugates previously assigned in the literature only to 3-hydroxy-1-methylpropylmercapturic acid (HMPMA-1), a human urinary metabolite of crotonaldehyde. The new conditions, employing a biphenyl column cooled to 5 °C and eluted with a gradient of formic acid, acetonitrile, and methanol, allow the analysis of human urinary mercapturic acids derived not only from crotonaldehyde but also from its isomers methacrolein (3-hydroxy-2-methylpropyl mercapturic acid, HMPMA-2) and methyl vinyl ketone (3-hydroxy-3-methylpropyl mercapturic acid, HMPMA-3). The mercapturic acids were detected and quantified by LC-ESI-MS/MS using the corresponding stable isotope labeled mercapturic acids as internal standards. The analysis was validated for accuracy and precision and applied to urine samples collected from cigarette smokers and nonsmokers. Smokers had significantly higher levels of all three mercapturic acids than did nonsmokers. The results demonstrated that HMPMA-3 from methyl vinyl ketone comprised the major portion of the peaks previously ascribed in multiple studies to HMPMA-1. HMPMA-1 had concentrations intermediate between those of HMPMA-2 and HMPMA-3 in both smokers and nonsmokers. This study reports the first quantitation of HMPMA-2 and HMPMA-3 in human urine. The observation of higher levels of HMPMA-3 than in the other two mercapturic acids suggests a previously unrecognized potential significance of methyl vinyl ketone as a toxicant in smokers and nonsmokers.

Urinary N7-(1-hydroxy-3-buten-2-yl) guanine adducts in humans: temporal stability and association with smoking

1,3-Butadiene (BD) is a known human carcinogen found in cigarette smoke, automobile exhaust, and urban air. Workers occupationally exposed to BD in the workplace have an increased incidence of leukemia and lymphoma. BD undergoes cytochrome P450-mediated metabolic activation to 3,4-epoxy-1-butene (EB), 1,2,3,4-diepoxybutane (DEB) and 1,2-dihydroxy-3,4-epoxybutane (EBD), which form covalent adducts with DNA. We have previously reported a quantitative nanoLC/ESI+-HRMS3 method for urinary N7-(1-hydroxy-3-buten-2-yl) guanine (EB-GII) adducts as a mechanism-based biomarker of BD exposure. In the present study, the method was updated to include high throughput 96-well solid phase extraction (SPE) and employed to establish urinary EB-GII biomarker stability and association with smoking. Urinary EB-GII levels were measured bimonthly for 1 year in 19 smokers to determine whether single adduct measurement provides reliable levels of EB-GII in an individual smoker. In addition, association of EB-GII with smoking was studied in 17 individuals participating in a smoking cessation program. EB-GII levels decreased 34% upon smoking cessation, indicating that it is associated with smoking status, but may also originate from sources other than exposure to cigarette smoke.

The mercapturic acid pathway

The mercapturic acid pathway is a major route for the biotransformation of xenobiotic and endobiotic electrophilic compounds and their metabolites. Mercapturic acids (N-acetyl-l-cysteine S-conjugates) are formed by the sequential action of the glutathione transferases, γ-glutamyltransferases, dipeptidases, and cysteine S-conjugate N-acetyltransferase to yield glutathione S-conjugates, l-cysteinylglycine S-conjugates, l-cysteine S-conjugates, and mercapturic acids; these metabolites constitute a "mercapturomic" profile. Aminoacylases catalyze the hydrolysis of mercapturic acids to form cysteine S-conjugates. Several renal transport systems facilitate the urinary elimination of mercapturic acids; urinary mercapturic acids may serve as biomarkers for exposure to chemicals. Although mercapturic acid formation and elimination is a detoxication reaction, l-cysteine S-conjugates may undergo bioactivation by cysteine S-conjugate β-lyase. Moreover, some l-cysteine S-conjugates, particularly l-cysteinyl-leukotrienes, exert significant pathophysiological effects. Finally, some enzymes of the mercapturic acid pathway are described as the so-called "moonlighting proteins," catalytic proteins that exert multiple biochemical or biophysical functions apart from catalysis.

Effects of 2-Phenethyl Isothiocyanate on Metabolism of 1,3-Butadiene in Smokers

2-Phenethyl isothiocyanate (PEITC) is a natural product found as a conjugate in cruciferous vegetables. It has been reported to have preventative properties against lung cancer and to inhibit metabolic activation of tobacco carcinogens. In this study, we evaluated the ability of PEITC to influence the metabolism of the human carcinogen 1,3-butadiene in current smokers in a phase II clinical trial with a crossover design. Urinary mercapturic acids of 1,3-butadiene were quantified at baseline and during PEITC treatment. Seventy-nine smokers were randomly assigned to one of two arms: PEITC followed by placebo or placebo followed by PEITC. During the 1-week treatment period, each subject took PEITC (10 mg in 1 mL of olive oil, 4 times per day). There was a 1-week washout period between the PEITC and placebo periods. Oral ingestion of PEITC increased urinary levels of BD-mercapturic acids (MHBMA and DHBMA) by 11.1% and 3.7%, respectively, but these increases were not statistically significant (P = 0.17 and 0.64, respectively). A much stronger effect was observed among subjects with the null genotype of both GSTM1 and GSTT1: in these individuals, PEITC increased urinary levels of MHBMA by 58.7% (P = 0.004) and 90.0% (P = 0.001), respectively, but did not have a significant effect on urinary DHBMA. These results reveal a potentially protective effect of PEITC treatment with respect to the detoxification of 1,3-butadiene in cigarette smokers, specifically in those null for GSTT1, and provide further evidence in support of stronger chemopreventive effects from consumption of dietary isothiocyanates in these individuals.

Genetic Determinants of 1,3-Butadiene Metabolism and Detoxification in Three Populations of Smokers with Different Risks of Lung Cancer

Background: 1,3-Butadiene (BD) is an important carcinogen in tobacco smoke that undergoes metabolic activation to DNA-reactive epoxides. These species can be detoxified via glutathione conjugation and excreted in urine as the corresponding N-acetylcysteine conjugates. We hypothesize that single nucleotide polymorphisms (SNPs) in BD-metabolizing genes may change the balance of BD bioactivation and detoxification in White, Japanese American, and African American smokers, potentially contributing to ethnic differences in lung cancer risk.Methods: We measured the levels of BD metabolites, 1- and 2-(N-acetyl-L-cysteine-S-yl)-1-hydroxybut-3-ene (MHBMA) and N-acetyl-S-(3,4-dihydroxybutyl)-L-cysteine (DHBMA), in urine samples from a total of 1,072 White, Japanese American, and African American smokers and adjusted these values for body mass index, age, batch, and total nicotine equivalents. We also conducted a genome-wide association study to identify genetic determinants of BD metabolism.Results: We found that mean urinary MHBMA concentrations differed significantly by ethnicity (P = 4.0 × 10-25). African Americans excreted the highest levels of MHBMA followed by Whites and Japanese Americans. MHBMA levels were affected by GSTT1 gene copy number (P < 0.0001); conditional on GSTT1, no other polymorphisms showed a significant association. Urinary DHBMA levels also differed between ethnic groups (P = 3.3 × 10-4), but were not affected by GSTT1 copy number (P = 0.226).Conclusions:GSTT1 gene deletion has a strong effect on urinary MHBMA levels, and therefore BD metabolism, in smokers.Impact: Our results show that the order of MHBMA levels among ethnic groups is consistent with their respective lung cancer risk and can be partially explained by GSTT1 genotype. Cancer Epidemiol Biomarkers Prev; 26(7); 1034-42. ©2017 AACR.

Isotope Dilution nanoLC/ESI+-HRMS3 Quantitation of Urinary N7-(1-Hydroxy-3-buten-2-yl) Guanine Adducts in Humans and Their Use as Biomarkers of Exposure to 1,3-Butadiene

1,3-Butadiene (BD) is an important industrial and environmental chemical classified as a known human carcinogen. Occupational exposure to BD in the polymer and monomer industries is associated with an increased incidence of lymphoma. BD is present in automobile exhaust, cigarette smoke, and forest fires, raising concern about potential exposure of the general population to this carcinogen. Following inhalation exposure, BD is bioactivated to 3,4-epoxy-1-butene (EB). If not detoxified, EB is capable of modifying guanine and adenine bases of DNA to form nucleobase adducts, which interfere with accurate DNA replication and cause cancer-initiating mutations. We have developed a nanoLC/ESI+-HRMS3 methodology for N7-(1-hydroxy-3-buten-2-yl) guanine (EB-GII) adducts in human urine (limit of detection: 0.25 fmol/mL urine; limit of quantitation: 1.0 fmol/mL urine). This new method was successfully used to quantify EB-GII in urine of F344 rats treated with 0-200 ppm of BD, occupationally exposed workers, and smokers belonging to two different ethnic groups. EB-GII amounts increased in a dose-dependent manner in urine of laboratory rats exposed to 0, 62.5, or 200 ppm of BD. Urinary EB-GII levels were significantly increased in workers occupationally exposed to 0.1-2.2 ppm of BD (1.25 ± 0.51 pg/mg of creatinine) as compared to administrative controls exposed to <0.01 ppm of BD (0.22 ± 0.08 and pg/mg of creatinine) (p = 0.0024), validating the use of EB-GII as a biomarker of human exposure to BD. EB-GII was also detected in smokers' urine with European American smokers excreting significantly higher amounts of EB-GII than African American smokers (0.48 ± 0.09 vs 0.12 ± 0.02 pg/mg of creatinine, p = 3.1 × 10-7). Interestingly, small amounts of EB-GII were observed in animals and humans with no known exposure to BD, providing preliminary evidence for its endogenous formation. Urinary EB-GII adduct levels and urinary mercapturic acids of BD (MHBMA, DHBMA) were compared in a genotyped multiethnic smoker cohort.

In vitro cleavage of diisocyanate-glutathione conjugates by human gamma-glutamyl transpeptidase-1

Isocyanates differ from many other xenobiotics in their ability to form S-linked conjugates with glutathione (GSH) through direct nucleophilic addition reactions (e.g. without enzymatic "preactivation" and/or transferase activity), potentially predisposing them to metabolism via the mercapturic acid pathway. In vivo, mono-isocyanates are metabolized via the mercapturic acid pathway and excreted as N-acetylated cysteine conjugates, however, the metabolism of di-isocyanates remains unclear. We assessed the ability of purified human gamma-glutamyl transpeptidase-1 (GGT-1), a primary enzyme of the mercapturic acid pathway, to cleave S-linked GSH conjugates of 4,4'-methylene diphenyl diisocyanate (MDI) and 1,6-hexamethylene diisocyanate (HDI), two widely used industrial chemicals. A combination of liquid chromatography (LC), tandem mass spectrometry (MS/MS) and hydrogen-deuterium exchange studies confirmed GGT-1 mediated formation of the 607.2 and 525.2 m/z (M + H)(+) ions corresponding to bis(cys-gly)-MDI and bis(cys-gly)-HDI, respectively, the cleavage products expected from the corresponding bis(GSH)-diisocyanate conjugates. Additional intermediate metabolites and mono(cys-gly)-conjugates with partially hydrolyzed diisocyanate were also observed. Consistent with GGT enzyme kinetics, metabolism proceeded more rapidly under conditions that favored transpeptidation versus hydrolytic mechanisms of cleavage. Together the data demonstrate the capacity of human GGT-1 to cleave GSH conjugates of both aromatic and aliphatic diisocyanates, suggesting a potential role in their metabolism.