Mercapturic acids revisited as biomarkers of exposure to reactive chemicals in occupational toxicology: a minireview

A novel method for the accurate quantification of two isomeric mercapturic acids of 1,3-dichlorobenzene in human urine using isotope dilution online-SPE-LC-MS/MS

1,3-dichlorobenzene (1,3-DCB) is an aromatic solvent that might be formed during thermal decomposition of bis(2,4-dichlorobenzoyl)peroxide used as initiator in silicone rubber production with many workers exposed worldwide. During metabolism of 1,3-DCB, two isomeric mercapturic acids can be formed from ring oxidation of 1,3-DCB in the liver, namely 2,4-dichlorophenylmercapturic acid (24CPhMA) and 3,5-dichlorophenylmercapturic acid (35CPhMA). These urinary mercapturic acids might serve as biomarkers of the toxicologically relevant absorbed dose of 1,3-DCB and have not been determined so far. Thus, we were aimed to develop an analytical method for quantification of these biomarkers. Authentic standards of both mercapturic acids as well as deuterium-labelled analogues were self-synthesized. A method for the quantification of both CPhMAs in human urine using online-SPE LC/MS/MS was developed and validated with an LOQ of 0.1 ng mL-1 for both CPhMAs. The analytes were extracted from urine by online-SPE on a restricted access material phase, transferred to the analytical column and quantified by tandem mass spectrometry. Interday (n = 6) and Intraday (n = 10) precision for both CPhMAs ranged from 1.7 to 4.3 % with accuracies between 99.4 and 109.9 % at concentrations of 0.6 and 3 ng mL-1. We applied the method on post-shift urine samples of 16 workers of the silicone rubber industry with occupational exposure to 1,3-DCB. Both CPhMAs were above LOQ in 15 of 16 urine samples with median levels (range) for 24CPhMA and 35CPhMA of 1.64 ng mL-1 (<0.1 - 8.2 ng mL-1) and 3.98 ng mL-1 (0.36 - 24.1 ng mL-1), respectively. This is the first report on specific urinary mercapturic acids of 1,3-DCB in humans. Our results show that ring oxidation of 1,3-DCB is considered to be a toxicologically relevant metabolic pathway in humans. This might improve risk assessment of 1,3-DCB-emissions in silicone rubber industry.

Alkenylbenzenes in Foods: Aspects Impeding the Evaluation of Adverse Health Effects

Alkenylbenzenes are naturally occurring secondary plant metabolites, primarily present in different herbs and spices, such as basil or fennel seeds. Thus, alkenylbenzenes, such as safrole, methyleugenol, and estragole, can be found in different foods, whenever these herbs and spices (or extracts thereof) are used for food production. In particular, essential oils or other food products derived from the aforementioned herbs and spices, such as basil-containing pesto or plant food supplements, are often characterized by a high content of alkenylbenzenes. While safrole or methyleugenol are known to be genotoxic and carcinogenic, the toxicological relevance of other alkenylbenzenes (e.g., apiol) regarding human health remains widely unclear. In this review, we will briefly summarize and discuss the current knowledge and the uncertainties impeding a conclusive evaluation of adverse effects to human health possibly resulting from consumption of foods containing alkenylbenzenes, especially focusing on the genotoxic compounds, safrole, methyleugenol, and estragole.

Biotransformation Capacity of Zebrafish (Danio rerio) Early Life Stages: Functionality of the Mercapturic Acid Pathway

Zebrafish (Danio rerio) early life stages offer a versatile model system to study the efficacy and safety of drugs or other chemicals with regard to human and environmental health. This is because, aside from the well-characterized genome of zebrafish and the availability of a broad range of experimental and computational research tools, they are exceptionally well suited for high-throughput approaches. Yet, one important pharmacokinetic aspect is thus far only poorly understood in zebrafish embryo and early larvae: their biotransformation capacity. Especially, biotransformation of electrophilic compounds is a critical pathway because they easily react with nucleophile molecules, such as DNA or proteins, potentially inducing adverse health effects. To combat such adverse effects, conjugation reactions with glutathione and further processing within the mercapturic acid pathway have evolved. We here explore the functionality of this pathway in zebrafish early life stages using a reference substrate (1-chloro-2,4-dinitrobenzene, CDNB). With this work, we show that zebrafish embryos can biotransform CDNB to the respective glutathione conjugate as early as 4 h postfertilization. At all examined life stages, the glutathione conjugate is further biotransformed to the last metabolite of the mercapturic acid pathway, the mercapturate, which is slowly excreted. Being able to biotransform electrophiles within the mercapturic acid pathway shows that zebrafish early life stages possess the potential to process xenobiotic compounds through glutathione conjugation and the formation of mercapturates. The presence of this chemical biotransformation and clearance route in zebrafish early life stages supports the application of this model in toxicology and chemical hazard assessment.

Harmonization of acronyms for volatile organic compound metabolites using a standardized naming system

Increased interest in volatile organic compound (VOC) exposure has led to an increased need for consistent, systematic, and informative naming of VOC metabolites. As analytical methods have expanded to include many metabolites in a single assay, the number of acronyms in use for a single metabolite has expanded in an unplanned and inconsistent manner due to a lack of guidance or group consensus. Even though the measurement of VOC metabolites is a well-established means to investigate exposure to VOCs, a formal attempt to harmonize acronyms amongst investigators has not been published. The aim of this work is to establish a system of acronym naming that provides consistency in current acronym usage and a foundation for creating acronyms for future VOC metabolites.

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.

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.

Selective and Efficient Cysteine Conjugation by Maleimides in the Presence of Phosphine Reductants

Sulfhydryl functions of thiol-containing amino acids are prime attachment sites for conjugation of labels, ligands, or drugs to proteinaceous compounds. Usually the thiol is offered a xenobiotic electrophilic moiety from the molecule to be attached such as a maleimido function. As sulfhydryls tend to oxidize into disulfides they must be reduced before conjugation. A popular thiol reduction reagent in biosciences is the substituted phosphine tris(2-carboxyethyl)phosphine (TCEP). Yet, phosphines are nucleophilic, too, and thus potentially compete with thiols for the electron-poor alkene moiety of maleimide resulting in complex product mixtures. To overcome this shortcoming we developed a method to eliminate excess reducing agent in the reaction mixture by selective oxidation of the phosphine with azidobenzoic acid before coupling. This results in a selective and efficient labeling of cysteines by maleimides.

Gills as a glutathione-dependent metabolic barrier in Pacific oysters Crassostrea gigas: Absorption, metabolism and excretion of a model electrophile

The mercapturic acid pathway (MAP) is a major phase II detoxification route, comprising the conjugation of electrophilic substances to glutathione (GSH) in a reaction catalyzed by glutathione S-transferase (GST) enzymes. In mammals, GSH-conjugates are exported from cells, and the GSH-constituent amino acids (Glu/Gly) are subsequently removed by ectopeptidases. The resulting Cys-conjugates are reabsorbed and, finally, a mercapturic acid is generated through N-acetylation. This pathway, though very well characterized in mammals, is poorly studied in non-mammalian biological models, such as bivalve mollusks, which are key organisms in aquatic ecosystems, aquaculture activities and environmental studies. In the present work, the compound 1-chloro-2,4-dinitrobenzene (CDNB) was used as a model electrophile to study the MAP in Pacific oysters Crassostrea gigas. Animals were exposed to 10μM CDNB and MAP metabolites were followed over 24h in the seawater and in oyster tissues (gills, digestive gland and hemolymph). A rapid decay was detected for CDNB in the seawater (half-life 1.7h), and MAP metabolites peaked in oyster tissues as soon as 15min for the GSH-conjugate, 1h for the Cys-conjugate, and 4h for the final metabolite (mercapturic acid). Biokinetic modeling of the MAP supports the fast CDNB uptake and metabolism, and indicated that while gills are a key organ for absorption, initial biotransformation, and likely metabolite excretion, hemolymph is a possible milieu for metabolite transport along different tissues. CDNB-induced GSH depletion (4h) was followed by increased GST activity (24h) in the gills, but not in the digestive gland. Furthermore, the transcript levels of glutamate-cysteine ligase, coding for the rate limiting enzyme in GSH synthesis, and two phase II biotransformation genes (GSTpi and GSTo), presented a fast (4h) and robust (∼6-70 fold) increase in the gills. Waterborne exposure to electrophilic compounds affected gills, but not digestive gland, while intramuscular exposure was able to modulate biochemical parameters in both tissues. This study is the first evidence of a fully functional and interorgan MAP pathway in bivalves. Hemolymph was shown to be responsible for the metabolic interplay among tissues, and gills, acting as a powerful GSH-dependent metabolic barrier against waterborne electrophilic substances, possibly also participating in metabolite excretion into the sea water. Altogether, experimental and modeled data fully agree with the existence of a classical mechanism for phase II xenobiotic metabolism and excretion in bivalves.

Metabolomics reveals effects of maternal smoking on endogenous metabolites from lipid metabolism in cord blood of newborns

INTRODUCTION

A general detrimental effect of smoking during pregnancy on the health of newborn children is well-documented, but the detailed mechanisms remain elusive.

OBJECTIVES

Beside the specific influence of environmental tobacco smoke derived toxicants on developmental regulation the impact on the metabolism of newborn children is of particular interest, first as a general marker of foetal development and second due to its potential predictive value for the later occurrence of metabolic diseases.

METHODS

Tobacco smoke exposure information from a questionnaire was confirmed by measuring the smoking related metabolites S-Phenyl mercapturic acid, S-Benzyl mercapturic acid and cotinine in maternal urine by LC-MS/MS. The impact of smoking on maternal endogenous serum metabolome and children's cord blood metabolome was assessed in a targeted analysis of 163 metabolites by an LC-MS/MS based assay. The anti-oxidative status of maternal serum samples was analysed by chemoluminiscence based method.

RESULTS

Here we present for the first time results of a metabolomic assessment of the cordblood of 40 children and their mothers. Several analytes from the group of phosphatidylcholines, namely PCaaC28:1, PCaaC32:3, PCaeC30:1, PCaeC32:2, PCaeC40:1, and sphingomyelin SM C26:0, differed significantly in mothers and children's sera depending on smoking status. In serum of smoking mothers the antioxidative capacity of water soluble compounds was not significantly changed while there was a significant decrease in the lipid fraction.

CONCLUSION

Our data give evidence that smoking during pregnancy alters both the maternal and children's metabolome. Whether the different pattern found in adults compared to newborn children could be related to different disease outcomes should be in the focus of future studies.

Rapid determination of four short-chain alkyl mercapturic acids in human urine by column-switching liquid chromatography-tandem mass spectrometry

We developed and validated an analytical method for the simultaneous determination of methyl mercapturic acid (MeMA), ethyl mercapturic acid (EtMA), n-propyl mercapturic acid (PrMA) and iso-propyl mercapturic acid (iPrMA) in human urine. These alkyl mercapturic acids are known or presumed biomarkers of exposure to several alkylating agents including methyl bromide, dimethyl sulfate, ethyl bromide, 1-bromopropane and 2-bromopropane. The method involves a column switching arrangement for online solid phase extraction of the analytes with subsequent analytical separation and detection using liquid chromatography and tandem mass spectrometry. Within day and day-to-day imprecision was determined to range from 4.5 to 12.2%. The analytical method is distinguished by its wide linear working range of up to 2,500 μg/L with detection limits ranging from 2.0 μg/L (for PrMA) to 5.1 μg/L (for MeMA) that render possible the application in various biomonitoring studies regarding exposure to alkylating agents. The results of a pilot study on urine samples of 30 individuals occupationally non-exposed to alkylating agents using the new procedure confirmed the background excretion of MeMA (<5.1-35.6 μg/L) and PrMA (<2.0-95.7 μg/L).

Biomarkers of toluene exposure in rats: mercapturic acids versus traditional indicators (urinary hippuric acid and o-cresol and blood toluene)

1. Toluene (TOL) is a neurotoxic, ototoxic and reprotoxic solvent which is metabolized via the glutathione pathway, producing benzylmercapturic, o-, m- and p-toluylmercapturic acids (MAs). These metabolites could be useful as biomarkers of TOL exposure. 2. The aims of this study were (1) to provide data on MAs excretion in rat urine following TOL exposure by inhalation, (2) to compare them to data from traditional TOL biomarkers, i.e. TOL in blood (Tol-B), and urinary hippuric acid (HA) and o-cresol (oCre) and (3) to establish a relationship between these different indicators and the airborne TOL concentration (Tol-A). 3. Sprague-Dawley rats were exposed to a range of TOL concentrations. Blood and urine were collected and analyzed to determine biomarker levels. 4. Levels of the four MAs correlate strongly with Tol-A (comparable to the correlation with Tol-B). 5. MAs are thus clearly superior to oCre and HA as potential markers of exposure to TOL.

Carcinogenic 3-nitrobenzanthrone but not 2-nitrobenzanthrone is metabolised to an unusual mercapturic acid in rats

3-Nitrobenzanthrone (3-NBA) is an extremely potent mutagen and suspect human carcinogen found in diesel exhaust. Its isomer 2-nitrobenzanthrone (2-NBA) has also been found in ambient air. These isomers differ in mutagenicity in Salmonella by 2-3 orders of magnitude. To identify their urinary metabolites and also to assess the assumed differences in their excretion, rats were dosed orally with 2mg/kg b.w. of either 2-NBA or 3-NBA. Their urine was collected for two consecutive days after dosage. Both LC-ESI-MS and GC-MS confirmed formation of the corresponding aminobenzanthrones (ABA). Excretion of these metabolites within the first day after dosing with 2- and 3-ABA amounted to 0.32±0.06 and 0.83±0.40% of the doses, respectively, while the excretion within the second day was by one order of magnitude lower. A novel mercapturic acid metabolite of 3-NBA was identified in urine by LC-ESI-MS as N-acetyl-S-(3-aminobenzanthron-2-yl)cysteine (3-ABA-MA) by comparison with the authentic standard. Its excretion amounted to 0.49±0.15 and 0.02±0.01% of dose within the first and second day after dosing, respectively. In contrast, no mercapturic acid was detected in the urine of rats dosed with 2-NBA. Observed difference in the mercapturic acid formation between 2- and 3-NBA is a new distinctive feature reflecting differences in the critical step of their metabolism, i.e., benzanthronylnitrenium ion formation that is intrinsically associated with biological activities of these two isomers. Moreover, 3-ABA-MA is a promising candidate biomarker of exposure to the carcinogenic 3-NBA.

Glutathione pathway in ethylbenzene metabolism: novel biomarkers of exposure in the rat

Glutathione pathway was specifically studied in rats exposed by inhalation to a range of ethylbenzene vapours (5-2000 ppm). Urines were collected during exposure (6h) and over the 18 h following the exposure. The potential metabolites coming from either side-chain or ring oxidation were synthesized: 1-, 2-phenylethylmercapturic acids (1-, and 2-PEMA) and 2-, 3- and 4-ethylphenylmercapturic acids (2-, 3-, and 4-EPMA). Their synthesis was fully described and the molecules characterized. Urine samples were analysed using a selective HPLC-fluorescence method. Among the five metabolites, 2-PEMA was never observed in any urine sample. By contrast, 1-PEMA was discovered in its two diastereomeric forms, and it was shown that one of them was mainly present. 2-EPMA, 3-EPMA and 4-EPMA (in the ratio 1:2:6) were also found, and their combined excretion levels were similar to that of 1-PEMA. The atmospheric concentrations and urinary excretions yielded very close correlations which allow us to consider these mercapturic acids as novel ethylbenzene exposure biomarkers.

Analysis of endogenous glutathione-adducts and their metabolites

The ability to conduct validated analyses of glutathione (GSH)-adducts and their metabolites is critically important in order to establish whether they play a role in cellular biochemical or pathophysiological processes. The use of stable isotope dilution (SID) methodology in combination with liquid chromatography-tandem mass spectrometry (LC-MS/MS) provides the highest bioanalytical specificity possible for such analyses. Quantitative studies normally require the high sensitivity that can be obtained by the use of multiple reaction monitoring (MRM)/MS rather than the much less sensitive but more specific full scanning methodology. The method employs a parent ion corresponding to the intact molecule together with a prominent product ion that obtained by collision induced dissociation. Using SID LC-MRM/MS, analytes must have the same relative LC retention time to the heavy isotope internal standard established during the validation procedure, the correct parent ion and the correct product ion. This level of specificity cannot be attained with any other bioanalytical technique employed for biomarker analysis. This review will describe the application of SID LC-MR/MS methodology for the analysis of GSH-adducts and their metabolites. It will also discuss potential future directions for the use of this methodology for rigorous determination of their utility as disease and exposure biomarkers.

Stable-isotope dilution LC–MS for quantitative biomarker analysis

The ability to conduct validated analyses of biomarkers is critically important in order to establish the sensitivity and selectivity of the biomarker in identifying a particular disease. The use of stable-isotope dilution (SID) methodology in combination with LC–MS/MS provides the highest possible analytical specificity for quantitative determinations. This methodology is now widely used in the discovery and validation of putative exposure and disease biomarkers. This review will describe the application of SID LC–MS methodology for the analysis of small-molecule and protein biomarkers. It will also discuss potential future directions for the use of this methodology for rigorous biomarker analysis.

Simultaneous determination of six mercapturic acid metabolites of volatile organic compounds in human urine

The widespread exposure to potentially harmful volatile organic compounds (VOCs) merits the development of practical and accurate exposure assessment methods. Measuring the urinary concentrations of VOC mercapturic acid (MA) metabolites provides noninvasive and selective information about recent exposure to certain VOCs. We developed a liquid chromatography-tandem mass spectrometry method for quantifying urinary levels of six MAs: N-acetyl-S-(2-carboxyethyl)-L-cysteine (CEMA), N-acetyl-S-(3-hydroxypropyl)-L-cysteine (HPMA), N-acetyl-S-(2-hydroxy-3-butenyl)-L-cysteine (MHBMA), N-acetyl-S-(3,4-dihydroxybutyl)-L-cysteine (DHBMA), N-acetyl-S-(2-hydroxyethyl)-L-cysteine (HEMA), and N-acetyl-S-(phenyl)-L-cysteine (PMA). The method provides good accuracy (102% mean accuracy) and high precision (3.5% mean precision). The sensitivity (limits of detection of 0.01-0.20 microg/L) and wide dynamic detection range (0.025-500 microg/L) make this method suitable for assessing VOC exposure of minimally exposed populations and those with significant exposures, such as cigarette smokers. We used this method to quantify MA levels in urine collected from smokers and nonsmokers. Median levels of creatinine-corrected CEMA, HPMA, MHBMA, DHBMA, HEMA, and PMA among nonsmokers (n = 59) were 38.1, 24.3, 21.3, 104.7, 0.9, and 0.5 microg/g creatinine, respectively. Among smokers (n = 61), median levels of CEMA, HPMA, MHBMA, DHBMA, HEMA, and PMA were 214.4, 839.7, 10.2, 509.7, 2.2, and 0.9 microg/g creatinine, respectively. All VOC MAs measured were higher among smokers than among nonsmokers, with the exception of MHBMA.

Toward an "omic" physiopathology of reactive chemicals: thirty years of mass spectrometric study of the protein adducts with endogenous and xenobiotic compounds

Cancer and degenerative diseases are major causes of morbidity and death, derived from the permanent modification of key biopolymers such as DNA and regulatory proteins by usually smaller, reactive molecules, present in the environment or generated from endogenous and xenobiotic components by the body's own biochemical mechanisms (molecular adducts). In particular, protein adducts with organic electrophiles have been studied for more than 30 [see, e.g., Calleman et al., 1978] years essentially for three purposes: (a) as passive monitors of the mean level of individual exposure to specific chemicals, either endogenously present in the human body or to which the subject is exposed through food or environmental contamination; (b) as quantitative indicators of the mean extent of the individual metabolic processing which converts a non-reactive chemical substance into its toxic products able to damage DNA (en route to cancer induction through genotoxic mechanisms) or key proteins (as in the case of several drugs, pesticides or otherwise biologically active substances); (c) to relate the extent of protein modification to that of biological function impairment (such as enzyme inhibition) finally causing the specific health damage. This review describes the role that contemporary mass spectrometry-based approaches employed in the qualitative and quantitative study of protein-electrophile adducts play in the discovery of the (bio)chemical mechanisms of toxic substances and highlights the future directions of research in this field. A particular emphasis is given to the measurement of often high levels of the protein adducts of several industrial and environmental pollutants in unexposed human populations, a phenomenon which highlights the possibility that a number of small organic molecules are generated in the human organism through minor metabolic processes, the imbalance of which may be the cause of "spontaneous" cases of cancer and of other degenerative diseases of still uncharacterized etiology. With all this in mind, it is foreseen that a holistic description of cellular functions will take advantage of new analytical methods based on time-integrated metabolomic measurements of a new biological compartment, the "adductome," aimed at better understanding integrated organism response to environmental and endogenous stressors.

Methyl mercapturate synthesis: an efficient, convenient and simple method

A safe and simple method for methyl S-arylmercapturate synthesis is described. Thirteen such compounds, to be used afterwards in metabolism studies, have been obtained with yields ranging from 71 to 99.6%. These compounds were obtained using a sulfa-Michael addition and synthesized by adding the corresponding thiophenols to a mixture composed of methyl 2-acetamidoacrylate (MAA), potassium carbonate and a phase transfer catalyst, Aliquat 336. MAA, the initial synthon, was itself isolated in quasi quantitative yield following a fully described synthesis.

Simultaneous determination of mercapturic acids derived from ethylene oxide (HEMA), propylene oxide (2-HPMA), acrolein (3-HPMA), acrylamide (AAMA) and N,N-dimethylformamide (AMCC) in human urine using liquid chromatography/tandem mass spectrometry

Mercapturic acids are highly important and specific biomarkers of exposure to carcinogenic substances in occupational and environmental medicine. We have developed and validated a reliable, specific and very sensitive method for the simultaneous determination of five mercapturic acids derived from several high-production chemicals used in industry, namely ethylene oxide, propylene oxide, acrylamide, acrolein and N,N-dimethylformamide. Analytes are enriched and cleaned up from urinary matrix by offline solid-phase extraction. The mercapturic acids are subsequently separated by means of high-performance liquid chromatography on a Luna C8 (2) column and specifically quantified by tandem mass spectrometric detection using isotopically labelled analytes as internal standards. The limits of detection (LODs) for N-acetyl-S-2-carbamoylethylcysteine (AAMA) and N-acetyl-S-2-hydroxyethylcysteine (HEMA) were 2.5 microg/L and 0.5 microg/L urine, while for N-acetyl-S-3-hydroxypropylcysteine (3-HPMA), N-acetyl-S-2-hydroxypropylcysteine (2-HPMA) and N-acetyl-S-(N-methylcarbamoyl)cysteine (AMCC) it was 5 microg/L. These LODs were sufficient to detect the background exposure of the general population. We applied the method on spot urine samples of 28 subjects of the general population with no known occupational exposure to these substances. Median levels for AAMA, HEMA, 3-HPMA, 2-HPMA and AMCC in non-smokers (n = 14) were 52.6, 2.0, 155, 7.1 and 113.6 microg/L, respectively. In smokers (n = 14), median levels for AAMA, HEMA, 3-HPMA, 2-HPMA and AMCC were 243, 5.3, 1681, 41.7 and 822 microg/L, respectively. Due to the simultaneous quantification of these mercapturic acids, our method is well suited for the screening of workers with multiple chemical exposures as well as the determination of the background excretion of the general population.

Directing Role of Organic Anion Transporters in the Excretion of Mercapturic Acids of Alkylated Polycyclic Aromatic Hydrocarbons

Excretion of mercapturic acids of a xenobiotic is a good indicator for the formation of electrophilic intermediates. However, the route of excretion, urine or feces, is important for usage of a given mercapturic acid as a biomarker in humans. In the present study we investigated the excretion routes of 1-methylpyrenyl mercapturic acid (MPMA) and 1,8-dimethylpyrenyl mercapturic acid (DMPMA) formed from the corresponding benzylic alcohols in rats. Whereas MPMA was primarily excreted in urine (72% of the total urinary and fecal level), DMPMA clearly preferred the fecal route (88%). We then examined interactions of these mercapturic acids with renal basolateral organic anion transporters (OATs) using HEK293 cells stably expressing human OAT1 and OAT3. The uptake rates of MPMA by OAT1- and OAT3-expressing cells were 2.8- and 1.7-fold, respectively, higher than that by control cells. MPMA was a competitive inhibitor of p-aminohippurate uptake by OAT1 and estrone sulfate uptake by OAT3 with K(i) values of 14.5 microM and 1.5 microM, respectively. In contrast, DMPMA was not transported by OAT1 and only modestly transported by OAT3 (1.25-fold over control). Thus, we suspect that the substrate specificities, alone or together with other factors, played a directing role in the excretion of MPMA and DMPMA. Although the mechanistic link requires verification, our results clearly show that a minute structural difference (the presence or absence of an additional methyl group in an alkylated four-ring polycyclic hydrocarbon) can strongly affect the interaction with transporter proteins and direct the excretion route of mercapturic acids.

Exposure to ethylene oxide in hospitals: biological monitoring and influence of glutathione S-transferase and epoxide hydrolase polymorphisms

Ethylene oxide is considered as a human carcinogen. A biomarker of exposure would be a useful instrument to assess the risk in occupationally exposed workers. This cross-sectional study aimed at examining (a) whether the urinary excretion of a metabolite of ethylene oxide, 2-hydroxyethyl mercapturic acid (HEMA), could be used for monitoring occupational exposure and (b) whether glutathione S-transferase (GST) and epoxide hydrolase genotypes influenced biological monitoring. Exposure to ethylene oxide was measured by personal sampling in 80 hospital workers (95% of those eligible). HEMA concentrations were determined in three urine samples (baseline, end of shift, and next morning) by liquid chromatography with tandem mass spectrometry. GSTs (GSTT1, GSTM1, and GSTP1) and epoxide hydrolase (EPHX1) were also genotyped. The influence of exposure, genotypes, and several other factors was examined in multiple regression analyses. Exposure was always <1 parts per million. On a group basis, exposure and a non-null GSTT1 genotype increased the HEMA concentrations in the urine sample collected at the end of the shift and these factors remained statistically significant after considering possible confounding or modifying factors.

Quantitation of N-acetyl-S-(9,10-dihydro-9-hydroxy-10-phenanthryl)-L-cysteine in human urine: comparison with glutathione-S-transferase genotypes in smokers

There are major interindividual differences in carcinogenic polycyclic aromatic hydrocarbon (PAH) metabolism in humans, and it has been hypothesized that these differences may be related to cancer risk in smokers and other exposed people. One important pathway of PAH metabolism involves the detoxification of the epoxide and diol epoxide metabolites by reaction with glutathione, catalyzed by glutathione-S-transferases (GSTs). Interindividual differences in these pathways have been examined by genotyping methods, investigating polymorphisms in GSTM1 and GSTP1. We are developing a phenotyping approach to assessing individual differences in PAH metabolism by quantifying human urinary metabolites of the ubiquitous PAH phenanthrene (1). In this study, we developed a method for quantitation of a mercapturic acid, N-acetyl-S-(9,10-dihydro-9-hydroxy-10-phenanthryl)-l-cysteine (PheO-NAC, 12), the end product of the reaction of phenanthrene-9,10-epoxide (11) with glutathione. [D(10)]PheO-NAC was added to the urine as internal standard, and the PheO-NAC fraction was enriched by solid-phase extraction. PheO-NAC was quantified by liquid chromatography electrospray ionization tandem mass spectrometry with selected reaction monitoring. The detection limit was approximately 4 fmol/mL of urine. PheO-NAC was detected in the urine of 46 of 104 smokers, mean (S.D.) 57.9 +/- 144 fmol/mL. PheO-NAC was detected significantly more frequently (P < 0.0001) in subjects who were GSTM1 positive than in those who were GSTM1 null, and the levels of PheO-NAC were significantly higher in the GSTM1 positive subjects, consistent with a role for GSTM1 in the detoxification of phenanthrene-9,10-epoxide. There were no significant relationships between PheO-NAC levels and the occurrence of two GSTP1 polymorphisms. The results of this study provide the first evidence for a PAH-derived mercapturic acid in human urine and should be useful in the development of a phenotyping approach to assess individual differences in PAH metabolism.

Metabonomics and Biomarker Discovery: LC−MS Metabolic Profiling and Constant Neutral Loss Scanning Combined with Multivariate Data Analysis for Mercapturic Acid Analysis

In the field of metabonomics, 1H NMR and full scan mass spectrometry methods have usually been combined with principal component analysis (PCA) and partial least squares discriminant analysis (PLS-DA) to detect patterns in biofluids that correspond to specific effects, usually a toxic site effect of a compound. Confounders together with great interindividual variation complicate such analysis in humans, and therefore, metabonomic data are almost restricted to animals. In our study, a constant neutral loss (CNL) scan on a linear ion trap demonstrated increased sensitivity and specificity compared to a full scan approach and was performed to detect mercapturic acids (MA), a class of effect markers. The method was applied to human volunteers administered 50 and 500 mg of acetaminophen (AAP), a model compound known to form MAs. Using a new algorithm to prepare the CNL data for chemometrics, discrimination of control and postdose samples could be performed using PCA and PLS-DA. The loadings plots clearly revealed AAP-MA as a marker, even at low-dose levels. Orthogonal signal correction (OSC) was carried out to investigate background information that is not due to exposure. Surprisingly, the OSC data provided a classification of male and female subjects showing the performance of the new approach.