Biotransformation of perchloroethene: dose-dependent excretion of trichloroacetic acid, dichloroacetic acid, and N-acetyl-S-(trichlorovinyl)-L-cysteine in rats and humans after inhalation

Reanalysis of Trichloroethylene and Tetrachloroethylene Metabolism to Glutathione Conjugates Using Human, Rat, and Mouse Liver in Vitro Models to Improve Precision in Risk Characterization

BACKGROUND

Both trichloroethylene (TCE) and tetrachloroethylene (PCE) are high-priority chemicals subject to numerous human health risk evaluations by a range of agencies. Metabolism of TCE and PCE determines their ultimate toxicity; important uncertainties exist in quantitative characterization of metabolism to genotoxic moieties through glutathione (GSH) conjugation and species differences therein.

OBJECTIVES

This study aimed to address these uncertainties using novel in vitro liver models, interspecies comparison, and a sensitive assay for quantification of GSH conjugates of TCE and PCE, S-(1,2-dichlorovinyl)glutathione (DCVG) and S-(1,2,2-trichlorovinyl) glutathione (TCVG), respectively.

METHODS

Liver in vitro models used herein were suspension, 2-D culture, and micropatterned coculture (MPCC) with primary human, rat, and mouse hepatocytes, as well as human induced pluripotent stem cell (iPSC)-derived hepatocytes (iHep).

RESULTS

We found that, although efficiency of metabolism varied among models, consistent with known differences in their metabolic capacity, formation rates of DCVG and TCVG generally followed the patterns human ≥ rat ≥ mouse , and primary hepatocytes > iHep . Data derived from MPCC were most consistent with estimates from physiologically based pharmacokinetic models calibrated to in vivo data.

DISCUSSION

For TCE, the new data provided additional empirical support for inclusion of GSH conjugation-mediated kidney effects as critical for the derivation of noncancer toxicity values. For PCE, the data reduced previous uncertainties regarding the extent of TCVG formation in humans; this information was used to update several candidate kidney-specific noncancer toxicity values. Overall, MPCC-derived data provided physiologically relevant estimates of GSH-mediated metabolism of TCE and PCE to reduce uncertainties in interspecies extrapolation that constrained previous risk evaluations, thereby increasing the precision of risk characterizations of these high-priority toxicants. https://doi.org/10.1289/EHP12006.

Toxicity assessments of selected trichloroethylene and perchloroethylene metabolites in three in vitro human placental models

Residential and occupational exposures to the industrial solvents perchloroethylene (PERC) and trichloroethylene (TCE) present public health concerns. In humans, maternal PERC and TCE exposures can be associated with adverse birth outcomes. Because PERC and TCE are biotransformed to toxic metabolites and placental dysfunction can contribute to adverse birth outcomes, the present study compared the toxicity of key PERC and TCE metabolites in three in vitro human placenta models. We measured cell viability and caspase 3 + 7 activity in the HTR-8/SVneo and BeWo cell lines, and caspase 3 + 7 activity in first trimester villous explant cultures. Cultures were exposed for 24 h to 5-100 µM S-(1,2-dichlorovinyl)-L-cysteine (DCVC) and S-(1,2,2-trichlorovinyl)-L-cysteine (TCVC), or 5-200 µM trichloroacetate (TCA) and dichloroacetate (DCA). DCVC significantly reduced cell viability and increased caspase 3 + 7 activity in HTR-8/SVneo cells at a lower concentration (20 µM) compared with concentrations toxic to BeWo cells and villous explants. Similarly, TCVC reduced cell viability and increased caspase 3 + 7 activity in HTR-8/SVneo cells but not in BeWo cells. TCA and DCA had only negligible effects on HTR-8/SVneo or BeWo cells. This study advances understanding of potential risks of PERC and TCE exposure during pregnancy by identifying metabolites toxic in placental cells and tissues.

Using Collaborative Cross Mouse Population to Fill Data Gaps in Risk Assessment: A Case Study of Population-Based Analysis of Toxicokinetics and Kidney Toxicodynamics of Tetrachloroethylene

BACKGROUND

Interindividual variability in susceptibility remains poorly characterized for environmental chemicals such as tetrachloroethylene (PERC). Development of population-based experimental models provide a potential approach to fill this critical need in human health risk assessment.

OBJECTIVES

In this study, we aimed to better characterize the contribution of glutathione (GSH) conjugation to kidney toxicity of PERC and the degree of associated interindividual toxicokinetic (TK) and toxicodynamic (TD) variability by using the Collaborative Cross (CC) mouse population.

METHODS

Male mice from 45 strains were intragastrically dosed with PERC ([Formula: see text]) or vehicle (5% Alkamuls EL-620 in saline), and time-course samples were collected for up to 24 h. Population variability in TK of S-(1,2,2-trichlorovinyl)GSH (TCVG), S-(1,2,2-trichlorovinyl)-L-cysteine (TCVC), and N-acetyl-S-(1,2,2-trichlorovinyl)-L-cysteine (NAcTCVC) was quantified in serum, liver, and kidney, and analyzed using a toxicokinetic model. Effects of PERC on kidney weight, fatty acid metabolism-associated genes [ Acot1 (Acyl-CoA thioesterase 1), Fabp1 (fatty acid-binding protein 1), and Ehhadh (enoyl-coenzyme A, hydratase/3-hydroxyacyl coenzyme A dehydrogenase)], and a marker of proximal tubular injury [KIM-1 (kidney injury molecule-1)/Hepatitis A virus cellular receptor 1 ( Havcr1)] were evaluated. Finally, quantitative data on interstrain variability in both formation of GSH conjugation metabolites of PERC and its kidney effects was used to calculate adjustment factors for the interindividual variability in both TK and TD.

RESULTS

Mice treated with PERC had significantly lower kidney weight, higher kidney-to-body weight (BW) ratio, and higher expression of fatty acid metabolism-associated genes ( Acot1, Fabp1, and Ehhadh) and a marker of proximal tubular injury (KIM-1/ Havcr1). Liver levels of TCVG were significantly correlated with KIM-1/ Havcr1 in kidney, consistent with kidney injury being associated with GSH conjugation. We found that the default uncertainty factor for human variability may be marginally adequate to protect 95%, but not more, of the population for kidney toxicity mediated by PERC.

DISCUSSION

Overall, this study demonstrates the utility of the CC mouse population in characterizing metabolism-toxicity interactions and quantifying interindividual variability. Further refinement of the characterization of interindividual variability can be accomplished by incorporating these data into in silico population models both for TK (such as a physiologically based pharmacokinetic model), as well as for toxicodynamic responses. https://doi.org/10.1289/EHP5105.

Comparative analysis of metabolism of trichloroethylene and tetrachloroethylene among mouse tissues and strains

Trichloroethylene (TCE) and tetrachloroethylene (PCE) are structurally similar chemicals that are metabolized through oxidation and glutathione conjugation pathways. Both chemicals have been shown to elicit liver and kidney toxicity in rodents and humans; however, TCE has been studied much more extensively in terms of both metabolism and toxicity. Despite their qualitative similarities, quantitative comparison of tissue- and strain-specific metabolism of TCE and PCE has not been performed. To fill this gap, we conducted a comparative toxicokinetic study where equimolar single oral doses of TCE (800 mg/kg) or PCE (1000 mg/kg) were administered to male mice of C57BL/6J, B6C3F1/J, and NZW/LacJ strains. Samples of liver, kidney, serum, brain, and lung were obtained for up to 36 h after dosing. For each tissue, concentrations of parent compounds, as well as their oxidative and glutathione conjugation metabolites were measured and concentration-time profiles constructed. A multi-compartment toxicokinetic model was developed to quantitatively compare TCE and PCE metabolism. As expected, the flux through oxidation metabolism pathway predominated over that through conjugation across all mouse strains examined, it is 1,200-3,800 fold higher for TCE and 26-34 fold higher for PCE. However, the flux through glutathione conjugation, albeit a minor metabolic pathway, was 21-fold higher for PCE as compared to TCE. The degree of inter-strain variability was greatest for oxidative metabolites in TCE-treated and for glutathione conjugation metabolites in PCE-treated mice. This study provides critical data for quantitative comparisons of TCE and PCE metabolism, and may explain the differences in organ-specific toxicity between these structurally similar chemicals.

Incorporation of the glutathione conjugation pathway in an updated physiologically-based pharmacokinetic model for perchloroethylene in mice

BACKGROUND

Perchloroethylene (perc) induced target organ toxicity has been associated with tissue-specific metabolic pathways. Previous physiologically-based pharmacokinetic (PBPK) modeling of perc accurately predicted oxidative metabolites but suggested the need to better characterize glutathione (GSH) conjugation as well as toxicokinetic uncertainty and variability.

OBJECTIVES

We updated the previously published "harmonized" perc PBPK model in mice to better characterize GSH conjugation metabolism as well as the uncertainty and variability of perc toxicokinetics.

METHODS

The updated PBPK model includes expanded models for perc and its oxidative metabolite trichloroacetic acid (TCA), and physiologically-based sub-models for conjugative metabolites. Previously compiled mouse kinetic data in B6C3F1 and Swiss-Webster mice were augmented to include data from a recent study in male C57BL/6J mice that measured perc and metabolites in serum and multiple tissues. Hierarchical Bayesian population analysis using Markov chain Monte Carlo was conducted to characterize uncertainty and inter-strain variability in perc metabolism.

RESULTS

The updated model fit the data as well or better than the previously published "harmonized" PBPK model. Tissue dosimetry for both oxidative and conjugative metabolites was successfully predicted across the three strains of mice, with estimated residuals errors of 2-fold for majority of data. Inter-strain variability across three strains was evident for oxidative metabolism; GSH conjugation data were only available for one strain.

CONCLUSIONS

This updated PBPK model fills a critical data gap in quantitative risk assessment by predicting the internal dosimetry of perc and its oxidative and GSH conjugation metabolites and lays the groundwork for future studies to better characterize toxicokinetic variability.

Editor's Highlight: Comparative Dose-Response Analysis of Liver and Kidney Transcriptomic Effects of Trichloroethylene and Tetrachloroethylene in B6C3F1 Mouse

Trichloroethylene (TCE) and tetrachloroethylene (PCE) are ubiquitous environmental contaminants and occupational health hazards. Recent health assessments of these agents identified several critical data gaps, including lack of comparative analysis of their effects. This study examined liver and kidney effects of TCE and PCE in a dose-response study design. Equimolar doses of TCE (24, 80, 240, and 800 mg/kg) or PCE (30, 100, 300, and 1000 mg/kg) were administered by gavage in aqueous vehicle to male B6C3F1/J mice. Tissues were collected 24 h after exposure. Trichloroacetic acid (TCA), a major oxidative metabolite of both compounds, was measured and RNA sequencing was performed. PCE had a stronger effect on liver and kidney transcriptomes, as well as greater concentrations of TCA. Most dose-responsive pathways were common among chemicals/tissues, with the strongest effect on peroxisomal β-oxidation. Effects on liver and kidney mitochondria-related pathways were notably unique to PCE. We performed dose-response modeling of the transcriptomic data and compared the resulting points of departure (PODs) to those for apical endpoints derived from long-term studies with these chemicals in rats, mice, and humans, converting to human equivalent doses using tissue-specific dosimetry models. Tissue-specific acute transcriptional effects of TCE and PCE occurred at human equivalent doses comparable to those for apical effects. These data are relevant for human health assessments of TCE and PCE as they provide data for dose-response analysis of the toxicity mechanisms. Additionally, they provide further evidence that transcriptomic data can be useful surrogates for in vivo PODs, especially when toxicokinetic differences are taken into account.

Nonalcoholic Fatty Liver Disease Is a Susceptibility Factor for Perchloroethylene-Induced Liver Effects in Mice

Nonalcoholic fatty liver disease (NAFLD) is the most prevalent pathological liver condition in developed countries. NAFLD results in severe alterations in liver function, including xenobiotic metabolism. Perchloroethylene (PERC) is a ubiquitous environmental pollutant, a known hepatotoxicant in rodents, and a probable human carcinogen. It is known that PERC disposition and metabolism are affected by NAFLD in mice; here, we examined how NAFLD changes PERC-associated liver effects. Male C57Bl6/J mice were fed a low-fat diet (LFD), high-fat diet (HFD), or methionine/folate/choline-deficient diet (MCD) to model a healthy liver, or mild and severe forms of NAFLD, respectively. After 8 weeks on diets, mice were orally administered PERC (300 mg/kg/day) or vehicle (5% aqueous Alkamuls-EL620) for 5 days. PERC-induced liver effects were exacerbated in both NAFLD groups. PERC exposure was associated with up-regulation of genes involved in xenobiotic, lipid, and glutathione metabolism, and down-regulation of the complement and coagulation cascades, regardless of the diet. Interestingly, HFD-fed mice, not MCD-fed mice, were generally more sensitive to PERC-induced liver effects. This was indicated by histopathology and transcriptional responses, where induction of genes associated with cell cycle and inflammation were prominent. Liver effects positively correlated with diet-specific differences in liver concentrations of PERC. We conclude that NAFLD alters the toxicodynamics of PERC and that NAFLD is a susceptibility factor that should be considered in future risk management decisions for PERC and other chlorinated solvents.

Simultaneous detection of the tetrachloroethylene metabolites S-(1,2,2-trichlorovinyl) glutathione, S-(1,2,2-trichlorovinyl)-L-cysteine, and N-acetyl-S-(1,2,2-trichlorovinyl)-L-cysteine in multiple mouse tissues via ultra-high performance liquid chromatography electrospray ionization tandem mass spectrometry

Tetrachloroethylene (perchloroethylene; PERC) is a high-production volume chemical and ubiquitous environmental contaminant that is hazardous to human health. Toxicity attributed to PERC is mediated through oxidative and glutathione (GSH) conjugation metabolites. The conjugation of PERC by glutathione-s-transferase to generate S-(1,2,2-trichlorovinyl) glutathione (TCVG), which is subsequently metabolized to form S-(1,2,2-trichlorovinyl)-L-cysteine (TCVC) is of special importance to human health. Specifically, TCVC may be metabolized to N-acetyl-S-(1,2,2-trichlorovinyl)-L-cysteine (NAcTCVC) which is excreted through urine, or to electrophilic metabolites that are nephrotoxic and mutagenic. Little is known regarding toxicokinetics of TCVG, TCVC, and NAcTCVC as analytical methods for simultaneous determination of these metabolites in tissues have not yet been reported. Hence, an ultra-high-performance liquid chromatography electrospray ionization tandem mass spectrometry-based method was developed for analysis of TCVG, TCVC, and NAcTCVC in liver, kidneys, serum, and urine. The method is rapid, sensitive, robust, and selective for detection all three analytes in every tissue examined, with limits of detection (LOD) ranging from 1.8 to 68.2 femtomoles on column, depending on the analyte and tissue matrix. This method was applied to quantify levels of TCVG, TCVC, and NAcTCVC in tissues from mice treated with PERC (10 to 1000 mg/kg, orally) with limits of quantitation (LOQ) of 1-2.5 pmol/g in liver, 1-10 pmol/g in kidney, 1-2.5 pmol/ml in serum, and 2.5-5 pmol/ml in urine. This method is useful for further characterization of the GSH conjugative pathway of PERC in vivo and improved understanding of PERC toxicity.

Kidney and liver biomarkers in female dry-cleaning workers exposed to perchloroethylene

Blood and urine perchloroethylene and urine trichloroacetic acid, as markers of exposure, and serum AST, ALT, GGT and creatinine, urine total solutes and proteins, angiotensin converting enzyme, N-acetyl-ß-D-glucosaminidase and glutamine synthetase, as markers of effect, were measured in 40 dry-cleaning and 45 ironing-shop female workers. Average perchloroethylene air level in the dry-cleaning shops was 59.7 mg m(-3), i.e. three-fold lower than the current A.C.G.I.H. TLV-TWA (170 mg (m-3)). No statistically significant difference in the mean values of any of the effect markers was observed between the two groups, except for AST which was significantly higher in drycleaners. In addition, a statistically significant correlation was observed in dry-cleaners between environmental perchloroethylene and total urinary solutes (r = 0.308, p < 0.05) or urine glutamine synthetase (r= 0.469, p < 0 .01), between glutamine synthetase and blood perchloroethylene in post-shift (r= 0.406, p < 0.01) or urinary perchloroethylene in post(r= 0.571, p < 0.001) or pre-shift (r= 0.586, p < 0.001), and between urinary perchloroethylene in pre-shift and GGT (r= 0.407, p < 0.05). Interestingly, some statistically significant correlations between exposure and effect indices were found in ironing-shop workers alone, as in all subjects. Finally, transaminases, GGT and total urinary proteins were influenced by age and alcohol consumption which were significantly higher in dry-cleaners, thus providing an explanation for some of the correlations observed. In conclusion, our results show a dose-related increase of glutamine synthetase activity,a marker of damage of the pars recta of the kidney proximal tubule, in the urine of female subjects exposed to perchloroethylene concentrations in the work environment lower than current A.C.G.I.H. TLV-TWA.

Mutagenicity of the cysteine S-conjugate sulfoxides of trichloroethylene and tetrachloroethylene in the Ames test

The nephrotoxicity and nephrocarcinogenicity of trichloroethylene (TCE) and tetrachloroethylene (PCE) are believed to be mediated primarily through the cysteine S-conjugate β-lyase-dependent bioactivation of the corresponding cysteine S-conjugate metabolites S-(1,2-dichlorovinyl)-l-cysteine (DCVC) and S-(1,2,2-trichlorovinyl)-l-cysteine (TCVC), respectively. DCVC and TCVC have previously been demonstrated to be mutagenic by the Ames Salmonella mutagenicity assay, and reduction in mutagenicity was observed upon treatment with the β-lyase inhibitor aminooxyacetic acid (AOAA). Because DCVC and TCVC can also be bioactivated through sulfoxidation to yield the potent nephrotoxicants S-(1,2-dichlorovinyl)-l-cysteine sulfoxide (DCVCS) and S-(1,2,2-trichlorovinyl)-l-cysteine sulfoxide (TCVCS), respectively, the mutagenic potential of these two sulfoxides was investigated using the Ames Salmonella typhimurium TA100 mutagenicity assay. The results show both DCVCS and TCVCS were mutagenic, and TCVCS exhibited 3-fold higher mutagenicity than DCVCS. However, DCVCS and TCVCS mutagenic activity was approximately 700-fold and 30-fold lower than DCVC and TCVC, respectively. DCVC and DCVCS appeared to induce toxicity in TA100, as evidenced by increased microcolony formation and decreased mutant frequency above threshold concentrations. TCVC and TCVCS were not toxic in TA100. The toxic effects of DCVC limited the sensitivity of TA100 to DCVC mutagenic effects and rendered it difficult to investigate the effects of AOAA on DCVC mutagenic activity. Collectively, these results suggest that DCVCS and TCVCS exerted a definite but weak mutagenicity in the TA100 strain. Therefore, despite their potent nephrotoxicity, DCVCS and TCVCS are not likely to play a major role in DCVC or TCVC mutagenicity in this strain.

Enhancing effects of trichloroethylene and tetrachloroethylene on type I allergic responses in mice

Trichloroethylene (TCE) and tetrachloroethylene (perchloroethylene; PCE) are commonly identified as environmental contaminants of groundwater. Previously, we investigated the enhancing effects of TCE and PCE on antigen-induced histamine release and inflammatory mediator production in rat mast cells. In this study, to examine the potential effect of TCE and PCE on antigen-induced histamine release from mouse mast cells, mouse bone marrow-derived mast cells (BMMC) were sensitized with anti-dinitrophenol (DNP) monoclonal IgE antibody and then stimulated with DNP-BSA containing with TCE or PCE. Both TCE and PCE significantly enhanced antigen-induced histamine release from BMMC. Next we investigated the effects of TCE and PCE on the passive cutaneous anaphylaxis (PCA) reaction in vivo using ICR mice. TCE and PCE significantly enhanced the PCA reaction in a dose-dependent manner. In addition, we examined the enhancing effects of ingesting small amount of TCE and PCE in drinking water on antigen-stimulated allergic responses. After the ICR mice had ingested TCE or PCE in their drinking water for 2 or 4 weeks, we performed the PCA reaction. Both TCE and PCE ingestion enhanced the PCA reaction in a dose-dependent manner for 4 weeks. These results suggest that exposure to TCE and PCE leads to the augmentation of type I allergic responses in many species.

Development and evaluation of a harmonized physiologically based pharmacokinetic (PBPK) model for perchloroethylene toxicokinetics in mice, rats, and humans

This article reports on the development of a "harmonized" PBPK model for the toxicokinetics of perchloroethylene (tetrachloroethylene or perc) in mice, rats, and humans that includes both oxidation and glutathione (GSH) conjugation of perc, the internal kinetics of the oxidative metabolite trichloroacetic acid (TCA), and the urinary excretion kinetics of the GSH conjugation metabolites N-Acetylated trichlorovinyl cysteine and dichloroacetic acid. The model utilizes a wider range of in vitro and in vivo data than any previous analysis alone, with in vitro data used for initial, or "baseline," parameter estimates, and in vivo datasets separated into those used for "calibration" and those used for "evaluation." Parameter calibration utilizes a limited Bayesian analysis involving flat priors and making inferences only using posterior modes obtained via Markov chain Monte Carlo (MCMC). As expected, the major route of elimination of absorbed perc is predicted to be exhalation as parent compound, with metabolism accounting for less than 20% of intake except in the case of mice exposed orally, in which metabolism is predicted to be slightly over 50% at lower exposures. In all three species, the concentration of perc in blood, the extent of perc oxidation, and the amount of TCA production is well-estimated, with residual uncertainties of ~2-fold. However, the resulting range of estimates for the amount of GSH conjugation is quite wide in humans (~3000-fold) and mice (~60-fold). While even high-end estimates of GSH conjugation in mice are lower than estimates of oxidation, in humans the estimated rates range from much lower to much higher than rates for perc oxidation. It is unclear to what extent this range reflects uncertainty, variability, or a combination. Importantly, by separating total perc metabolism into separate oxidative and conjugative pathways, an approach also recommended in a recent National Research Council review, this analysis reconciles the disparity between those previously published PBPK models that concluded low perc metabolism in humans and those that predicted high perc metabolism in humans. In essence, both conclusions are consistent with the data if augmented with some additional qualifications: in humans, oxidative metabolism is low, while GSH conjugation metabolism may be high or low, with uncertainty and/or interindividual variability spanning three orders of magnitude. More direct data on the internal kinetics of perc GSH conjugation, such as trichlorovinyl glutathione or tricholorvinyl cysteine in blood and/or tissues, would be needed to better characterize the uncertainty and variability in GSH conjugation in humans.

Evaluation of the impact of the exposure route on the human kinetic adjustment factor

The objective of this study was to assess the impact of the exposure route on the human kinetic adjustment factor (HKAF), for which a default value of 3.16 is used in non-cancer risk assessment. A multi-route PBPK model was modified from the literature and used for computing the internal dose metrics in adults, neonates, children, elderly and pregnant women following three route-specific scenarios to chloroform, bromoform, tri- or per-chloroethylene (TCE or PERC). These include 24-h inhalation exposure, body-weight adjusted oral exposure and 30 min dermal exposure to contaminated drinking water. Distributions for body weight (BW), height (BH) and hepatic cytochrome P450 2E1 (CYP2E1) content were obtained from the literature, whereas model parameters (flows, volumes) were calculated from BW and BH. Monte Carlo simulations were performed and the HKAF was calculated as the ratio of the 95th percentile value of internal dose metrics in subpopulation to the 50th percentile value in adults. On the basis of the area under the parent compound's arterial blood concentration vs time curve (AUC(pc)), highest HKAFs were obtained in neonates for every scenario considered, and were the highest for bromoform (range: 3.6-7.4). Exceedance of the default value based on AUC(PC) was also observed for an oral exposure to chloroform in neonates (4.9). In all other cases, HKAFs remained below the default value. Overall, this study has pointed out the dependency of the HKAF on the exposure route, dose metrics and subpopulation considered, as well as characteristics of the chemicals investigated.

Trichloroethylene risk assessment: a review and commentary

Trichloroethylene (TCE) is a widespread environmental contaminant that is carcinogenic when given in high, chronic doses to certain strains of mice and rats. The capacity of TCE to cause cancer in humans is less clear. The current maximum contaminant level (MCL) of 5 ppb (microg/L) is based on an US Environment Protection Agency (USEPA) policy decision rather than the underlying science. In view of major advances in understanding the etiology and mechanisms of chemically induced cancer, USEPA began in the late 1990s to revise its guidelines for cancer risk assessment. TCE was chosen as the pilot chemical. The USEPA (2005) final guidelines emphasized a "weight-of-evidence" approach with consideration of dose-response relationships, modes of action, and metabolic/toxicokinetic processes. Where adequate data are available to support reversible binding of the carcinogenic moiety to biological receptors as the initiating event (i.e., a threshold exists), a nonlinear approach is to be used. Otherwise, the default assumption of a linear (i.e., nonthreshold) dose-response is utilized. When validated physiologically based pharmacokinetic (PBPK) models are available, they are to be used to predict internal dosimetry as the basis for species and dose extrapolations. The present article reviews pertinent literature and discusses areas where research may resolve some outstanding issues and facilitate the reassessment process. Key research needs are proposed, including role of dichloroacetic acid (DCA) in TCE-induced liver tumorigenesis in humans; extension of current PBPK models to predict target organ deposition of trichloroacetic acid (TCA) and DCA in humans ingesting TCE in drinking water; use of human hepatocytes to ascertain metabolic rate constants for use in PBPK models that incorporate variability in metabolism of TCE by potentially sensitive subpopulations; measurement of the efficiency of first-pass elimination of trace levels of TCE in drinking water; and assessment of exogenous factors' (e.g., alcohol, drugs) ability to alter metabolic activation and risks at such low-level exposure.

Bayesian analysis of a physiologically based pharmacokinetic model for perchloroethylene in humans

Perchloroethylene (PCE) is a widely distributed pollutant in the environment, and is the primary chemical used in dry cleaning. PCE-induced liver cancer was observed in mice, and central nervous system (CNS) effects were reported in dry-cleaning workers. To support reconstruction of human PCE exposures, including the potential for CNS effects, an existing physiologically based pharmacokinetic (PBPK) model for PCE in the human (Covington et al., 2007) was modified by adding a brain compartment. A Bayesian approach, using Markov chain Monte Carlo (MCMC) analysis, was employed to re-estimate the parameters in the modified model by combining information from prior distributions for the model parameters and experimental data. Experimental data were obtained from five different human pharmacokinetic studies of PCE inhalation exposures ranging from 150 ppm to as low as 0.495 ppm. The data include alveolar or exhaled breath concentrations of PCE, blood concentrations of PCE and trichloroacetic acid (TCA), and urinary excretion of TCA. The PBPK model was used to predict target tissue dosimetry of PCE and its key metabolite, TCA, during and after the inhalation exposures. Posterior analysis was performed to see whether convergence criteria for each parameter were satisfied and whether the model with posterior distributions may be used to make accurate predictions of human kinetic data. With posteriors, the trend of percent of PCE metabolized in the liver at low concentrations was predicted under different exposure conditions. The 95th percentile for the fraction PCE metabolized at a concentration of 1 ppb was estimated to be 1.89%.

Globin monoadducts and cross-links provide evidence for the presence of S-(1,2-dichlorovinyl)-L-cysteine sulfoxide, chlorothioketene, and 2-chlorothionoacetyl chloride in the circulation in rats administered S-(1,2-dichlorovinyl)-L-cysteine

S-(1,2-Dichlorovinyl)-L-cysteine (DCVC), a mutagenic and nephrotoxic metabolite of trichloroethylene, is bioactivated to S-(1,2-dichlorovinyl)-L-cysteine sulfoxide (DCVCS) and chlorothioketene and/or 2-chlorothionoacetyl chloride by cysteine conjugate S-oxidase (S-oxidase) and cysteine conjugate beta-lyase (beta-lyase), respectively. Previously, we identified DCVCS-globin monoadducts and cross-links upon treating rats with DCVCS or incubating erythrocytes with DCVCS. In this study, the formation of DCVC-derived reactive intermediates was investigated after rats were given a single (230 or 460 micromol/kg, i.p.) or multiple (3 or 30 micromol/kg daily for 5 days) DCVC doses. LC/ESI/MS of trypsin-digested globin peptides revealed both S-oxidase and beta-lyase-derived globin monoadducts and cross-links consistent with in vivo DCVC bioactivation by both pathways. MS/MS analyses of trypsin-digested fractions of globin from one of the rats treated with multiple 30 micromol/kg DCVC doses led to identification of beta-lyase-derived monoadducts on both Cys93 and Cys125 of the beta-chains. While rats dosed with the 230 micromol/kg DCVC dose exhibited beta-lyase-dependent monoadducts and cross-links only (four out of four rats), rats given the 460 micromol/kg DCVC dose (two out of four) and rats administered the multiple DCVC doses (two out of four) exhibited both beta-lyase- and S-oxidase-derived monoadducts and cross-links. Because previous incubations of erythrocytes with DCVC did not result in detection of DCVCS-derived monoadducts or cross-links and had only resulted in detection of beta-lyase-derived monoadducts and cross-links, the DCVCS-globin monoadducts and cross-links detected in this study are likely the result of DCVC bioactivation outside the circulation and subsequent translocation of DCVCS and N-acetylated DCVCS into the erythrocytes.

Evaluation of Physiologically Based Pharmacokinetic Models in Risk Assessment: An Example with Perchloroethylene

One of the more problematic aspects of the application of physiologically based pharmacokinetic (PBPK) models in risk assessment is the question of whether the model has been adequately validated to provide confidence in the dose metrics calculated with it. A number of PBPK models have been developed for perchloroethylene (PCE), differing primarily in the parameters estimated for metabolism. All of the models provide reasonably accurate simulations of selected kinetic data for PCE in mice and humans and could thus be considered to be "validated" to some extent. However, quantitative estimates of PCE cancer risk are critically dependent on the prediction of the rate of metabolism at low environmental exposures. Recent data on the urinary excretion of trichloroacetic acid (TCA), the major metabolite of PCE, for human subjects exposed to lower concentrations than those used in previous studies, make it possible to compare the high- to low-dose extrapolation capability of the various published human models. The model of Gearhart et al., which is the only model to include a description of TCA kinetics, provided the closest predictions of the urinary excretion observed in these low-concentration exposures. Other models overestimated metabolite excretion in this study by 5- to 15-fold. A systematic discrepancy between model predictions and experimental data for the time course of the urinary excretion of TCA suggested a contribution from TCA formed by metabolism of PCE in the kidney and excreted directly into the urine. A modification of the model of Gearhart et al. to include metabolism of PCE to TCA in the kidney at 10% of the capacity of the liver, with direct excretion of the TCA formed in the kidney into the urine, markedly improved agreement with the experimental time-course data, without altering predictions of liver metabolism. This case study with PCE demonstrates the danger of relying on parent chemical kinetic data to validate a model that will be used for the prediction of metabolism.

Analyses of representative biomarkers of exposure and effect by chromatographic, mass spectrometric, and nuclear magnetic resonance techniques: method development and application in life sciences

Biomarkers are essential tools in monitoring studies, which include environmental monitoring, biological monitoring, biological effect monitoring, and health surveillance, as well as drug development processes. Their discovery, validation, and analysis require highly sensitive and selective analytical technologies. In this regard, gas and liquid chromatography, mass spectrometry, and nuclear magnetic resonance spectroscopy have facilitated great achievements in all these areas. In addition and closely related to biomarkers, the ongoing developments in these techniques promise a better understanding of the nature and mechanisms of toxic effects originating from various chemical, biological, or physical sources. This Review compiles studies performed on selected biomarkers with respect to both method development and application. Section 1 summarizes the concept of biomarkers; their application in various industrial/occupational, agricultural, drug developmental, and medical/clinical platforms. This section also focuses on biotransformation studies in close relation to biomarker discovery and validation, and on major techniques utilized in this area. In Section 2, biotransformation of volatile anesthetics in humans with a focus on mercapturic acid derivatives as potential biomarkers of effect is reviewed. The use of GC-ECD, GC/MS, and 19F-NMR in these studies is described. Section 3 focuses on the analysis of aldehydic lipid peroxidation degradation products by GC-ECD in mammalian cells in which oxidative stress induced chemically, and in humans after various challenges; anesthetic exposure, ischemia-reperfusion, and controlled endurance exercise. In Section 4, method development for protein and DNA oxidation products by LC-tandem MS and its application in mammalian cells and in humans are summarized. Possibilities, limitations, and future perspectives are discussed in Section 5.

S-(1,2,2-Trichlorovinyl)-l-cysteine Sulfoxide, a Reactive Metabolite ofS-(1,2,2-Trichlorovinyl)-l-cysteine Formed in Rat Liver and Kidney Microsomes, Is a Potent Nephrotoxicant

Previously, we have provided evidence that cytochromes P450 (P450s) and flavin-containing monooxygenases (FMOs) are involved in the oxidation of S-(1,2,2-trichlorovinyl)-L-cysteine (TCVC) in rabbit liver microsomes to yield the reactive metabolite TCVC sulfoxide (TCVCS). Because TCVC is a known nephrotoxic metabolite of tetrachloroethylene, the nephrotoxic potential of TCVCS in rats and TCVCS formation in rat liver and kidney microsomes were investigated. At 5 mM TCVC, rat liver microsomes formed TCVCS at a rate nearly 5 times higher than the rate measured with rat kidney microsomes, whereas at 1 mM TCVC only the liver activity was detectable. TCVCS formation in liver and kidney microsomes was dependent upon the presence of NADPH and was inhibited by the addition of methimazole or 1-benzylimidazole, but not superoxide dismutase, catalase, KCN, or deferoxamine, consistent with the involvement of both FMOs and P450s. Rats given TCVCS at 230 micromol/kg i.p. exhibited acute tubular necrosis at 2 and 24 h after treatment, and they had elevated blood urea nitrogen levels at 24 h, whereas TCVC was a much less potent nephrotoxicant than TCVCS. Furthermore, pretreatment with aminooxyacetic acid enhanced TCVC toxicity. In addition, reduced nonprotein thiol concentrations in the kidney were decreased by nearly 50% 2 h after TCVCS treatment compared with saline-treated rats, whereas the equimolar dose of TCVC had no effect on kidney nonprotein thiol status. No significant lesions or changes in nonprotein thiol status were observed in liver with either TCVC or TCVCS. Collectively, the results suggest that TCVCS may play a role in TCVC-induced nephrotoxicity.

Impact of repeated exposure on toxicity of perchloroethylene in Swiss Webster mice

The aim was to study the subchronic toxicity of perchloroethylene (Perc) by measuring injury and repair in liver and kidney in relation to disposition of Perc and its major metabolites. Male SW mice (25-29g) were given three dose levels of Perc (150, 500, and 1000 mg/kg day) via aqueous gavage for 30 days. Tissue injury was measured during the dosing regimen (0, 1, 7, 14, and 30 days) and over a time course of 24-96h after the last dose (30 days). Perc produced significant liver injury (ALT) after single day exposure to all three doses. Liver injury was mild to moderate and regressed following repeated exposure for 30 days. Subchronic Perc exposure induced neither kidney injury nor dysfunction during the entire time course as evidenced by normal renal histology and BUN. TCA was the major metabolite detected in blood, liver, and kidney. Traces of DCA were also detected in blood at initial time points after single day exposure. With single day exposure, metabolism of Perc to TCA was saturated with all three doses. AUC/dose ratio for TCA was significantly decreased with a concomitant increase in AUC/dose of Perc levels in liver and kidney after 30 days as compared to 1 day exposures, indicating inhibition of metabolism upon repeated exposure to Perc. Hepatic CYP2E1 expression and activity were unchanged indicating that CYP2E1 is not the critical enzyme inhibited. Hepatic CYP4A expression, measured as a marker of peroxisome proliferation was increased transiently only on day 7 with the high dose, but was unchanged at later time points. Liver tissue repair peaked at 7 days, with all three doses and was sustained after medium and high dose exposure for 14 days. These data indicate that subchronic Perc exposure via aqueous gavage does not induce nephrotoxicity and sustained hepatotoxicity suggesting adaptive hepatic repair mechanisms. Enzymes other than CYP2E1, involved in the metabolism of Perc may play a critical role in the metabolism of Perc upon subchronic exposure in SW mice. Liver injury decreased during repeated exposure due to inhibition of metabolism and possibly due to adaptive tissue repair mechanisms.

The use of Markov chain Monte Carlo uncertainty analysis to support a Public Health Goal for perchloroethylene

The current Public Health Goal (PHG) for perchloroethylene (PCE) was derived using upper-bound estimates of fractional PCE metabolism in humans. These estimates were in part obtained from a published evaluation of the uncertainty and variability in human PCE metabolism conducted using a physiologically-based pharmacokinetic (PBPK) model in a Markov chain Monte Carlo (MCMC) analysis; however, the data used in that analysis were limited to post-exposure PCE blood and exhaled air concentrations from a single study. A more recent study [Volkel, W., Friedewald, M., Lederer, E., Pahler, A., Parker, J., Dekant, W., 1998. Biotransformation of perchloroethene: dose-dependent excretion of trichloroacetic acid, dichloroacetic acid, and N-acetyl-S-(trichlorovinyl)-l-cysteine in rats and humans after inhalation. Toxicol. Appl. Pharmacol. 153(1), 20-27.] provides data on blood concentrations of PCE and its major metabolite, trichloroacetic acid (TCA), and urinary excretion of TCA following exposure of human subjects to lower concentrations of PCE (10-40ppm) than in previous studies. In the present effort, a new MCMC analysis was performed that focused on data from this study along with two others [Fernandez, J., Guberan, E., Caperos, J., 1976. Experimental human exposures to tetrachloroethylene vapor and elimination in breath after inhalation. Am. Ind. Hyg. Assoc. J. 37, 143-150; Monster, A., Boersma, G., Steenweg, H., 1979. Kinetics of tetrachloroethylene in volunteers; influence of exposure concentration and work load. Int. Arch. Occup. Environ. Health 42, 303-309.] providing data on PCE blood concentrations and urinary excretion of TCA. To provide an accurate prediction of TCA kinetics, the PBPK model used here includes a description of the metabolism of PCE to TCA in both the liver and kidney. The resulting upper 95th percentile estimates of fraction of PCE metabolized by inhalation and oral routes were 2.1 and 5.2%, respectively, compared to 58 and 79% used in the derivation of the PHG.

Novel aerobic perchloroethylene degradation by the white-rot fungus Trametes versicolor

Perchloroethylene (PCE) is one of the most important groundwater pollutants around the world. It is a suspected carcinogen and is believed to be recalcitrant to microbial degradation. We report here, for the first time, aerobic degradation of PCE by the white rot fungus, Trametes versicolor, to less hazardous products. Aerobic degradation rate of PCE was 0.20 and 0.28 nmol h(-1) mg(-1) dry weight of fungal biomass. Trichloroacetic acid (TCA) was identified as the main intermediate using [2-13C]-PCE as the substrate. Chloride released and TCA produced were stoichiometric with PCE degradation. Our studies using 1 -aminobenzotriazole (ABT), an inhibitor of cytochrome P-450, suggested that a cytochrome P-450 system may be involved in PCE degradation by T. versicolor. These results are of particular interest because TCA production from PCE has not been reported to date in bacteria or fungi.

Toxicokinetics of Inhaled Trichloroethylene and Tetrachloroethylene in Humans at 1 ppm: Empirical Results and Comparisons with Previous Studies

Trichloroethylene (TRI) and tetrachloroethylene (TETRA) are solvents that have been widely used in a variety of industries, and both are widespread environmental contaminants. In order to provide a better basis for understanding their toxicokinetics at environmental exposures, seven human volunteers were exposed by inhalation to 1 ppm of TRI or TETRA for 6 h, with biological samples collected for analysis during exposure and up to 6-days postexposure. Concentrations of TRI, TETRA, free trichloroethanol (TCOH), total TCOH (free TCOH plus glucuronidated TCOH), and trichloroacetic acid (TCA) were determined in blood and urine; TRI and TETRA concentrations were measured in alveolar breath. Toxicokinetic time courses and empirical analyses of classical toxicokinetic parameters were compared with those reported in previous human volunteer studies, most of which involved exposures that were at least 10-fold higher. Qualitatively, TRI and TETRA toxicokinetics were consistent with previous human studies. Quantitatively, alveolar retention and clearance by exhalation were similar to those found previously but blood and urine data suggest a number of possible toxicokinetic differences. For TRI, data from the current study support lower apparent blood-air partition coefficients, greater apparent metabolic clearance, less TCA production, and greater glucuronidation of TCOH as compared to previous studies. For TETRA, the current data suggest TCA formation that is similar or slightly lower than that of previous studies. Variability and uncertainty in empirical estimates of total TETRA metabolism are substantial, with confidence intervals among different studies substantially overlapping. Relative contributions to observed differences from concentration-dependent toxicokinetics and interindividual and interoccasion variability remain to be determined.

Quantitation of mercapturic acids from acrylamide and glycidamide in human urine using a column switching tool with two trap columns and electrospray tandem mass spectrometry

A sensitive and specific electrospray tandem mass spectrometry method using a column switching unit with two trap columns was established to quantify the mercapturates (MAs) of acrylamide (AA) and glycidamide (GA) in human urine. A specially endcapped material was applied for trapping the hydrophilic MAs and a pre-trap column was used to remove lipophilic compounds from the directly injected urine to protect the trap column. The limits of quantitation for AA-MA and GA-MA in urine were 0.5 microg/L and 1 microg/L, respectively. Urine was spiked with deuterated internal standards and injected directly into LC-MS/MS. Urine of smokers (n=13) revealed the highest concentrations of AA-MA and GA-MA in the range of 61-706 microg/L and 5-54 microg/L, respectively. Lower levels for AA-MA (14-102 microg/L) and GA-MA (1-11 microg/L) were detected in non-smokers (n=13).

Dose-response relationships in human experimental exposure to solvents

Previous studies carried out in the field of experimental toxicology have shown evidence of biphasic dose-response relationships for different experimental models, endpoints and chemicals tested. As these studies excluded humans as the experimental model, we have examined the literature of the last three decades in order to verify data concerning human experimental exposure with the aim of highlighting possible biphasic dose-response relationships. The substances used for experimental exposures included hydrocarbons, esters, alcohols, ketones, ethers, glycoethers, halogenated hydrocarbons, and carbon sulphide; the absorption route was inhalation. We did not detect any biphasic dose-response relationship and, in the studies reviewed, our examination revealed major methodological limitations that prevented us making a more detailed examination of experimental data. We concluded that the experimental data available did not allow us to support evidence of biphasic dose-response relationships in human experimental exposure to the above-mentioned chemical substances.

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.

Green chemistry in urinalysis for trichloroethanol and trichloroacetic acid as markers of exposure to chlorinated hydrocarbon solvents

The aim of the present study was to develop a method of urinalysis for trichloroacetic acid (TCA) and trichloroethanol (TCE), and therefore total trichloro-compounds (TTC) as the sum, with least use of hazardous chemicals, being green in that sense. After acid hydrolysis followed by dilution with an ethanol (EtOH)-methanol (MeOH)-water mixture, capillary gas-choromatography with an electron-capture detector can quantify TCA and TCE in the diluted hydrolyzate. Comparison studies showed that the results were identical among three methods, i.e., 1. the method developed in the present study, 2. a head-space GC with acid hydrolysis of conjugated TCE and methyl-esterification of TCA, and 3. traditional colorimetry with Fujiwara reaction. When applied to exposure-excretion analysis, the three methods gave results reproducible to each other. Over-all evaluation therefore was such that the method developed in the present study is as equally reliable as previously developed methods. It should be further noted that the procedures are very simple, with minimum use of occupationally or environmentally hazardous chemicals. In case the determination of only TCA is requested, it is possible to skip the hydrolysis step so that the treatment prior to the GC analysis is even simpler, i.e., just a 60-fold dilution of the urine sample with the EtOH-MeOH-water mixture. It was also demonstrated that correction of urinary analyte levels for urine density in terms of creatinine or specific gravity did not improve the correlation with the intensity of TRI exposure.

Rapid detection and identification of N-acetyl-L-cysteine thioethers using constant neutral loss and theoretical multiple reaction monitoring combined with enhanced product-ion scans on a linear ion trap mass spectrometer

A sensitive and specific liquid chromatography-mass spectrometry (LC-MS) method based on the combination of constant neutral loss scans (CNL) with product ion scans was developed on a linear ion trap. The method is applicable for the detection and identification of analytes with identical chemical substructures (such as conjugates of xenobiotics formed in biological systems) which give common CNLs. A specific CNL was observed for thioethers of N-acetyl-L-cysteine (mercapturic acids, MA) by LC-MS/MS. MS and HPLC parameters were optimized with 16 MAs available as reference compounds. All of these provided a CNL of 129 Da in the negative-ion mode. To assess sensitivity, a multiple reaction monitoring (MRM) mode with 251 theoretical transitions using the CNL of 129 Da combined with a product ion scan (IDA thMRM) was compared with CNL combined with a product ion scan (IDA CNL). An information-dependent acquisition (IDA) uses a survey scan such as MRM (multiple reaction monitoring) to generate "informations" and starting a second acquisition experiment such as a product ion scan using these "informations." Th-MRM means calculated transitions and not transitions generated from an available standard in the tuning mode. The product ion spectra provide additional information on the chemical structure of the unknown analytes. All MA standards were spiked in low concentrations to rat urines and were detected with both methods with LODs ranging from 60 pmol/mL to 1.63 nmol/mL with IDA thMRM. The expected product ion spectra were observed in urine. Application of this screening method to biological samples indicated the presence of a number of MAs in urine of unexposed rats, and resulted in the identification of 1,4-dihydroxynonene mercapturic acid as one of these MAs by negative and positive product ion spectra. These results show that the developed methods have a high potential to serve as both a prescreen to detect unknown MAs and to identify these analytes in complex matrix.

Glutathione Transferase Zeta-Catalyzed Bioactivation of Dichloroacetic Acid: Reaction of Glyoxylate with Amino Acid Nucleophiles

Dichloroacetic acid (DCA) is a drinking water contaminant, a therapeutic agent, and a rodent carcinogen. Glutathione transferase zeta (GSTZ1-1) catalyzes the biotransformation of a range of alpha-haloalkanoates and the cis-trans isomerization of maleylacetoacetate. GSTZ1-1 catalyzes the bioactivation of fluorine-lacking dihaloacetates to S-(alpha-halocarboxymethyl)glutathione, a reactive intermediate that covalently modifies and inactivates the enzyme or is hydrolyzed to glyoxylate. The purpose of this study was to examine the GSTZ1-1-catalyzed bioactivation of DCA, including the reaction of DCA-derived glyoxylate with amino acid nucleophiles and the characterization of the structures and kinetics of adduct formation by LC/MS. The binding of [1-(14)C]DCA-derived label to bovine serum albumin required both GSTZ1-1 and GSH, whereas the binding to dialyzed rat liver cytosolic protein was increased in the presence of GSH. Studies with model peptides (antiflammin-2 and IL-8 inhibitor) indicated that glyoxylate, rather than S-(alpha-chlorocarboxymethyl)glutathione, was the reactive species that modified amino acid nucleophiles. Both addition (+74 Da) and addition-elimination (+56 Da) adducts of glyoxylic acid were observed. Addition adducts (+74 Da) could not be characterized completely by mass spectrometry, whereas addition-elimination adducts (+56 Da) were characterized as 2-carboxy-4-imidazolidinones. 2-Carboxy-4-imidazolidinones were formed by the rapid equilibrium reaction of glyoxylate with the N-terminal amino group of antiflammin-2 to give an intermediate carbinolamine (K(eq) = 0.63 mM(-1)), which slowly eliminated water to give an intermediate imine (k(2) = 0.067 hour(-1)), which rapidly cyclized to give the 2-carboxy-4-imidazolidinone. Glucose 6-phosphate dehydrogenase was inactivated partially by glyoxylate when reactants were reduced with sodium borodeuteride, which may indicate that glyoxylate reacts with selective lysine epsilon-amino groups. The results of the present study demonstrate that GSTZ1-1 catalyzes the bioactivation of DCA to the reactive metabolite glyoxylate. The reaction of glyoxylate with cellular macromolecules may be associated with the multiorgan toxicity of DCA.

Plasma binding of trichloroacetic acid in mice, rats, and humans under cancer bioassay and environmental exposure conditions

Trichloroacetic acid (TCA), a mouse liver carcinogen, is a drinking water contaminant and a metabolite of solvents such as trichloroethylene and perchloroethylene. Because acidic drugs are often bound more strongly to human than to rodent plasma proteins, a study was undertaken to determine whether this was the case for TCA and to clarify the mechanistic bases for species differences. Equilibrium dialysis was used to measure in vitro binding of a range of TCA concentrations to plasma of humans, rats, and mice. Plots of observed data for free versus bound TCA concentrations were compared with simulations from each of three binding models: a single saturable site model; a saturable plus nonsaturable site model; and a two-saturable site model. Dissociation values (Kd) did not differ significantly from one species to another, but N (number of binding sites/molecule) ranged from 2.97 for humans to 0.17 for mice. Binding capacities (Bmax) for humans, rats, and mice were 709, 283, and 29 microM, respectively. The greater plasma protein binding of TCA in humans would be expected to not only increase the residence time of the compound in the bloodstream, but to substantially reduce the proportion of TCA that is available for uptake by the liver and other tissues. Species differences in the bound fraction diminished at very low, environmentally relevant TCA concentrations, but the percentage bound increased markedly. These findings suggest that the practice of using total blood levels of TCA as a dose metric in interspecies extrapolation of cancer risks needs to be re-examined.

Effect of perchloroethylene, smoking, and race on oxidative DNA damage in female dry cleaners

Perchloroethylene (PERC) is used widely as an industrial dry cleaning solvent and metal degreaser. PERC is an animal carcinogen that produces increased incidence of renal adenomas, adenocarcinomas, mononuclear cell leukemia, and hepatocellular tumors. Oxidative DNA damage and lipid peroxidation were assessed in 38 women with (dry cleaners) or without (launderers) occupational exposure to PERC. PERC exposure was assessed by collecting breathing zone samples on two consecutive days of a typical work week. PERC levels were measured in blood drawn on the morning of the second day of breathing zone sample collection in dry cleaners and before a typical workday in launderers. Blood PERC levels were two orders of magnitude higher in dry cleaners compared to launderers. A significant correlation was noted between time weighted average (TWA) PERC and blood PERC in dry cleaners (r=0.7355, P<0.002). 8-Hydroxydeoxyguanosine (8-OHdG), ng/mg deoxyguanosine (dG) in leukocyte nuclear DNA was used as an index of steady-state oxidative DNA damage. Urinary 8-OHdG, microg/g creatinine was used as an index of oxidative DNA damage repair. Urinary 8-epi-prostaglandin F(2alpha) (8-epi-PGF), ng/g creatinine was used as an index of lipid peroxidation. The mean+/-S.D. leukocyte 8-OHdG in launderers was 16.0+/-7.3 and was significantly greater than the 8.1+/-3.6 value for dry cleaners. Urinary 8-OHdG and 8-epi-PGF were not significantly different between dry cleaners and launderers. Unadjusted Pearson correlation analysis of log transformed PERC exposure indices and biomarkers of oxidative stress indicated a significant association in launderers between blood PERC and day 1 urinary 8-OHdG (r=0.4661, P<0.044). No significant associations between exposure indices and biomarkers were evident in linear models adjusted for age, body mass index, race, smoking (urinary cotinine, mg/g creatinine) and blood levels of the antioxidants Vitamin E and beta-carotene. The mean+/-S.D. leukocyte 8-OHdG value in control white women was 17.8+/-7.4 and was significantly greater than the 11.8+/-5.9 in control black women. No significant differences by race were evident for the other biomarkers. Smoking status was not significantly associated with any of the oxidative damage indices. Results indicate a reduction in oxidative DNA damage in PERC exposed dry cleaners relative to launderers, but PERC could not clearly be defined as the source of the effect.

Mass spectral characterization of dichloroacetic acid-modified human glutathione transferase zeta

Glutathione transferase zeta (GSTZ1-1) is widely expressed in eukaryotic species, and four human allelic variants of hGSTZ1-1 have been described. GSTZ1-1 catalyzes the cis-trans isomerization of maleylacetoacetate to fumarylacetoacetate and the biotransformation of a range of alpha-haloalkanoic acids. GSTZ1-1-catalyzed biotransformation of fluorine-lacking alpha,alpha-dihaloalkanoic acids, including dichloroacetic acid (DCA), results in the mechanism-based inactivation and covalent modification of the enzyme. The objective of this study was to investigate further the DCA-induced inactivation of hGSTZ1c-1c and to explore the mechanism of inactivation by characterization of the sites and types of DCA-induced covalent modifications. The partition ratio for the DCA-induced, mechanism-based inactivation of hGSTZ1c-1c was (5.7 +/- 0.5) x 10(2), and the k(cat) for the biotransformation of DCA was 39 min(-)(1). Inactivation of hGSTZ1c-1c in vitro was limited at high enzyme concentrations and was inhibited by glyoxylate. The stoichiometry of DCA binding to hGSTZ1c-1c was approximately 0.5 mol of DCA/mol of enzyme monomer. A single DCA-derived adduct was observed and was assigned to cysteine-16 by a combination of matrix-assisted laser-desorption-ionization time-of-flight and electrospray-ionization quadrupole ion-trap mass spectrometry and by analysis of [1-(14)C]DCA binding to C16A hGSTZ1c-1c. The DCA-derived adduct contained both glutathione and the carbon skeleton of DCA, presumably in a dithioacetal linkage. Also, cysteine-16 formed a mixed disulfide bond with glutathione. These data support a mechanism of inactivation whereby glutathione displaces a chlorine atom from DCA, and cysteine-16 in the enzyme active site displaces the second chlorine atom to result in a covalently modified and inactivated enzyme. These findings explain the DCA-induced inactivation of GSTZ1-1 observed in humans and rats.

Tetrachloroethylene oxide: hydrolytic products and reactions with phosphate and lysine

Tetrachloroethylene, or perchloroethylene (PCE), has considerable industrial use and is of toxicological interest because of a variety of effects. Most of the existing literature presents PCE oxide as a critical intermediate in the oxidative metabolism of PCE to Cl(3)CCO(2)H, oxalic acid, and products covalently bound to proteins, including trichloroacetyl derivatives of lysine. PCE oxide was synthesized by photochemical oxidation of PCE and characterized. Decomposition at neutral pH (t(1/2) = 7.9 min at 0 degrees C, 5.8 min at 23 degrees C, 2.6 min at 37 degrees C) yielded only trace ( approximately 1%) Cl(3)CCO(2)H; the major products identified were CO (73% yield) and CO(2) (63% yield). In phosphate buffer (0.10 M) a major product was identified as oxalyl phosphate. Oxalyl chloride also reacted to form CO and CO(2) in aqueous solution and to form oxalyl phosphate in neutral phosphate buffer. Oxalyl phosphate decomposed to oxalic acid (t(1/2) = 53 min at 37 degrees C) but did not react with lysine. Reaction of PCE oxide with free lysine yielded the oxalic acid amide derivatives of lysine plus lysine dimers in which cross-linking of the amino groups involved oxalo linkage. The reaction of PCE oxide with albumin yielded mainly N(6)-oxalolysine and some (<5%) N(6)-trichloroacetyllysine. We propose a reaction pathway for PCE oxide based on our previous studies with trichloroethylene oxide, in which C-C bond scission is a major product of reaction in aqueous buffer and yields CO and CO(2). Oxalyl species are proposed as intermediates and prominent acylating species formed in the reactions of the epoxide. The formation of Cl(3)CCO(2)H in cytochrome P450 reactions is postulated to result from intramolecular migration within an enzyme intermediate.

Biotransformation of L-cysteine S-conjugates and N-acetyl-L-cysteine S-conjugates of the sevoflurane degradation product fluoromethyl-2,2-difluoro-1-(trifluoromethyl)vinyl ether (compound A) in human kidney in vitro: interindividual variability in N-acetylation, N-deacetylation, and beta-lyase-catalyzed metabolism

Fluoromethyl-2,2-difluoro-1-(trifluoromethyl)vinyl ether (FDVE; 1) is a fluoroalkene formed by the base-catalyzed degradation of the anesthetic sevoflurane. FDVE is nephrotoxic in rats. In both rats and humans, FDVE undergoes glutathione-dependent conjugation, cleavage to cysteine S-conjugates, and renal beta-lyase-catalyzed metabolism to reactive intermediates, which may cause nephrotoxicity. Interindividual variability in renal metabolism of FDVE is unknown. Therefore, this investigation quantified beta-lyase-catalyzed bioactivation and N-acetyltransferase-catalyzed inactivation of FDVE cysteine S-conjugates and reactivation of mercapturates by N-deacetylase in cytosol and microsomes from 20 human kidneys. In cytosol, N-acetylation ranged from 0.008 to 0.045 (0.024 +/- 0.01) nmol of mercapturate/mg/min and 0.001 to 0.07 (0.024 +/- 0.02) nmol of mercapturate/mg/min for alkane and alkene cysteine S-conjugates, respectively. Similar results for microsomal N-acetylation were obtained; N-acetylation ranged from 0.005 to 0.055 (0.025 +/- 0.02) nmol of mercapturate/mg/min and 0.001 to 0.06 (0.030 +/- 0.02) nmol of mercapturate/mg/min for alkane and alkene cysteine S-conjugates, respectively. Beta-lyase-catalyzed metabolism to pyruvate varied from 0.004 to 0.14 (0.051 +/- 0.04) nmol/mg/min and from 0.10 to 0.40 (0.26 +/- 0.08) nmol/mg/min for alkane and alkene cysteine-S-conjugates, respectively. N-deacetylation of mercapturates ranged from 0.8 to 2.5 (1.25 +/- 0.57) nmol of cysteine S-conjugate formed/mg/min and 0.05 to 0.37 (0.17 +/- 0.10) nmol of cysteine S-conjugate formed/mg/min for alkane and alkene FDVE mercapturates. Cytosolic cysteine S-conjugates metabolism by renal beta-lyase predominated over N-acetylation (ratio of activities was 0.2-6 and 3-146 for the alkane and alkene cysteine S-conjugates). N-deacetylation predominated over N-acetylation (ratio of activities was 20-205 and 2-54 for alkane and alkene S-conjugates). There was considerable (up to 50-fold) interindividual variability in rates of FDVE toxication (beta-lyase metabolism and N-deacetylation) and detoxication. This interindividual variability may effect individual susceptibility to the nephrotoxicity of FDVE and other haloalkenes.

Glutathione S-conjugation of the sevoflurane degradation product, fluoromethyl-2,2-difluoro-1-(trifluoromethyl)vinyl ether (compound A) in human liver, kidney, and blood in vitro

Fluoromethyl-2,2-difluoro-1-(trifluoromethyl)vinyl ether (FDVE) is a fluorinated alkene formed by degradation of the volatile anesthetic sevoflurane in anesthesia machines. FDVE is nephrotoxic in rats and undergoes glutathione-dependent conjugation to form two alkane (G1, G2) and two alkene glutathione S-conjugates (G3, G4), cleavage to cysteine S-conjugates, and beta-lyase-catalyzed metabolism to reactive thionoacyl fluorides, which may react with cellular macromolecules to cause nephrotoxicity. Although similar metabolites have been identified in human urine in vivo, little is known about sites and mechanisms of GSH conjugation in humans. This investigation quantified FDVE-GSH conjugates formed by human hepatic and renal microsomal and cytosolic fractions and blood in vitro. LC-MS/MS analysis identified all four GSH conjugates (G1-G4) formed in all human subcellular fractions. Quantitative analysis indicated that the relative order of formation was G2 > G1 > G4 > G3 with human liver and kidney subfractions. In blood, the order was G1 > G4 > G2 > G3. These results demostrate that FDVE undergoes GSH-dependent conjugation in human liver and kidney microsomes and cytosol as well as blood, which may account for the detection of corresponding mercapturic acids in the urine of patients exposed to FDVE.

Long-duration low-flow sevoflurane and isoflurane effects on postoperative renal and hepatic function

Sevoflurane degradation by carbon dioxide absorbents during low-flow anesthesia forms the haloalkene Compound A, which causes nephrotoxicity in rats. Numerous studies have shown no effects of Compound A formation on postoperative renal function after moderate-duration (3-4 h) low-flow sevoflurane; however, effects of longer exposures remain unresolved. We compared renal function after long-duration low-flow (<1 L/min) sevoflurane and isoflurane anesthesia in consenting surgical patients with normal renal function. To maximize degradant exposure, Baralyme was used, and anesthetic concentrations were maximized (no nitrous oxide and minimal opioids). Inspired and expired Compound A concentrations were quantified. Blood and urine were obtained for laboratory evaluation. Sevoflurane (n = 28) and isoflurane (n = 27) groups were similar with respect to age, sex, weight, ASA status, and anesthetic duration (9.1 +/- 3.0 and 8.2 +/- 3.0 h, mean +/- SD) and exposure (9.2 +/- 3.6 and 9.1 +/- 3.7 minimum alveolar anesthetic concentration hours). Maximum inspired Compound A was 25 +/- 9 ppm (range, 6-49 ppm), and exposure (area under the concentration-time curve) was 165 +/- 95 (35-428) ppm. h. There was no significant difference between anesthetic groups in 24- or 72-h serum creatinine, blood urea nitrogen, creatinine clearance, or 0- to 24-h or 48- to 72-h urinary protein or glucose excretion. Proteinuria and glucosuria were common in both groups. There was no correlation between Compound A exposure and any renal function measure. There was no difference between anesthetic groups in 24- or 72-h aspartate aminotransferase or alanine aminotransferase. These results show that the renal and hepatic effects of long-duration low-flow sevoflurane and isoflurane were similar. No evidence for low-flow sevoflurane nephrotoxicity was observed, even at high Compound A exposures as long as 17 h. Proteinuria and glucosuria were common and nonspecific postoperative findings. Long-duration low-flow sevoflurane seems as safe as long-duration low-flow isoflurane anesthesia.

IMPLICATIONS

Postoperative renal function after long-duration low-flow sevoflurane (with Compound A exposures greater than those typically reported) and isoflurane anesthesia were not different, as assessed by serum creatinine, blood urea nitrogen, and urinary excretion of protein and glucose. This suggests that low-flow sevoflurane is as safe as low-flow isoflurane, even at long exposures.

The application of physiologically based pharmacokinetic modelling in the analysis of occupational exposure to perchloroethylene

A physiologically based pharmacokinetic model for perchloroethylene was parameterized, calibrated and validated using anatomic, physiologic, biochemical and physicochemical data obtained from the literature. The model was used to analyse human exposure data obtained under controlled conditions and from dry cleaning establishments in the Padua area of northern Italy. Whilst the model satisfactorily simulated the urinary excretion of trichloroacetic acid, following experimental inhalation exposure to 10, 20 and 40 ppm perchloroethylene under controlled conditions the opposite was true for the occupational exposure data. However, further model refinement to incorporate inter-individual variability of anatomical, physiological and biochemical parameters which have an impact on model output, would further improve the predictive capabilities of the model. The possibility of perchloroethylene and trichloroethylene co-exposure in the occupational setting was indicated by the model.

The role of CYP forms in the metabolism and metabolic activation of HCFCs and other halocarbons

The use of hydrochlorofluorocarbons (HCFCs) such as HCFC-123 (2,2-dichloro-1,1,1-trifluoroethane) and HCFC-141b (1,1-dichloro-1-fluoroethane) is becoming widespread as replacements for the ozone depleting chlorofluorocarbons. Hepatic activation of HCFC-123 or the unsaturated perchloroethylene through oxidative pathways leads to the formation of the electrophiles trifluoroacetyl chloride or trichloroacetyl chloride, respectively. These can react with epsilon-NH(2) functions of lysine in proteins and give rise to neoantigens. In the case of HCFC-123, this reaction is catalysed primarily by CYP2E1 and to a much lesser extent by the constitutive CYP2C19, CYP2B6 and CYP2C8. For perchloroethylene, the extent of activation is less and the reaction is catalysed primarily by the CYP2B family. While acute hepatotoxicity has been seen in humans exposed to HCFC-123 or halothane, little short- or long-term toxicity in rodents is observed. No immunological related toxicity of perchloroethylene has been reported in exposed humans. Long-term exposure of rats can lead to renal tubule carcinomas and in mice, hepatocellular carcinomas. These toxic reactions do not appear to be directly related to the formation of the putative trichloroacetyl chloride intermediate.

Neoantigen formation and clastogenic action of HCFC-123 and perchloroethylene in human MCL-5 cells

In this study, the metabolic activation of 2,2-dichloro-1,1,1-trifluoroethane (hydrochlorofluorocarbons-123, HCFC-123), halothane or 1,1-dichloro-1-fluoroethane (HCFC-141b) was compared to that of perchloroethylene, using lymphoblastoma derived cell lines expressing human CYP1A1, CYP1A2, CYP2E1, CYP2A6 and CYP3A4 (MCL-5 cells). A dose dependent increase in micronucleus formation was detected over a nominal concentration range of 0.05-2 mM for HCFC-123 and halothane, but this was not seen with HCFC-141b. No dose response for HCFC-123 was seen in a control cHo1 cell line not expressing this cytochrome P450's. Cell lines expressing individual human cytochrome P-450 (CYP) forms were also used to define the enzymes responsible for the clastogenic events and to investigate the formation of immunoreactive protein by microsomal fractions. It was shown that CYP2E1 or CYP2B6 catalysed the clastogenic response, but CYP2D6, CYP3A4, CYP1A2 or CYP1A1 all appeared to be inactive. The formation of neoantigenic trifluoroacetylated protein adducts by microsomal mixtures incubated with HCFC-123 and NADPH was catalysed primarily by CYP2E1 and to a lesser extent by CYP2C19, whereas, only trace levels of immunoreactive protein were seen with microsomes expressing CYP2B6 or CYP2C8. With perchloroethylene as a substrate, the extent of activation was low in comparison with HCFC-123, as judged by the absence of micronuclei formation in the MCL-5 cell line and the weak immunoreactivity of proteins following Western blotting. CYP1A2, CYP2B6 and CYP2C8 appeared to be responsible for perchloroethylene immunoreactivity and in contrast to the findings with the HCFC's, no activation of perchloroethylene by CYP2E1 could be detected. These results show that even though both saturated and unsaturated halocarbons can result in neoantigen formation, there is a marked difference in the specificity of the CYP enzymes involved in their metabolic activation.

Chemical-induced nephrotoxicity mediated by glutathione S-conjugate formation

Glutathione conjugation has been identified as an important detoxication reaction. However, several glutathione-dependent bioactivation reactions have been identified. Current knowledge on the mechanisms and the possible biological importance of these reactions is discussed in this article. Vicinal dihaloalkanes are transformed by glutathione S-transferase-catalyzed reactions to mutagenic and nephrotoxic S-(2-haloethyl) glutathione S-conjugates. Electrophilic episulphonium ions are the ultimate reactive intermediates formed and interact with nucleic acids. Several polychlorinated alkenes are bioactivated in a complex, glutathione-dependent pathway. The first step is hepatic glutathione S-conjugate formation followed by cleavage to the corresponding cysteine S-conjugates, and, after translocation to the kidney, metabolism by renal cystein conjugate beta-lyase. Beta-Lyase-dependent metabolism of halovinyl cysteine S-conjugates yields electrophilic thioketenes, whose covalent binding to cellular macromolecules is likely to be responsible for the observed nephrotoxicity of the parent compounds. Finally, hepatic glutathione conjugate formation with hydroquinones and aminophenols yields conjugates that are directed to gamma-glutamyltransferase-rich tissues, such as the kidney, where they cause alkylation or redox cycling reactions, or both, that cause organ-selective damage.

Inactivation of glutathione transferase zeta by dichloroacetic acid and other fluorine-lacking alpha-haloalkanoic acids

Dichloroacetic acid (DCA) is a contaminant of chlorinated drinking water supplies, is carcinogenic in rats and mice, and is a therapeutic agent used for the treatment of congenital lactic acidosis. The biotransformation of DCA to glyoxylic acid is catalyzed by glutathione transferase zeta (GSTZ). Treatment of rats and human subjects with DCA increases its plasma elimination half-life and reduces the extent of DCA biotransformation in rat hepatic cytosol. In the investigation presented here, the kinetics of the DCA-induced inactivation of GSTZ, the turnover of GSTZ, and the susceptibility of GSTZ to inactivation by a panel of alpha-haloacids were studied. DCA rapidly inactivated GSTZ in both rat hepatic cytosol and intact Fischer 344 rats. The time course of inactivation in vivo was mirrored by a concomitant loss of immunoreactive GSTZ protein. The turnover of GSTZ in rat liver was 0.21 day(-1), which corresponded to a half-life of 3.3 days. The degree of GSTZ inactivation after daily administration of DCA could be predicted from the amount of inactivation after a single treatment. Other fluorine-lacking dihaloacetic acids also inactivated GSTZ, whereas alpha-monohaloacids and fluorine-containing dihaloacetic acids failed to inactivate GSTZ. These data show that the observed DCA-induced decrease in the level of DCA metabolism is caused by the inactivation of GSTZ.

Dose-dependent metabolism of fluoromethyl-2,2-difluoro-1-(trifluoromethyl)vinyl ether (compound A), an anesthetic degradation product, to mercapturic acids and 3,3,3-trifluoro-2-(fluoromethoxy)propanoic acid in rats

The volatile anesthetic sevoflurane is degraded in anesthesia machines to fluoromethyl-2,2-difluoro-1-(trifluoromethyl)vinyl ether (FDVE), to which humans are exposed. FDVE is metabolized in rats and humans to two alkane and two alkene glutathione S-conjugates that are hydrolyzed to the corresponding cysteine S-conjugates. The latter are N-acetylated to mercapturic acids, or bioactivated by renal cysteine conjugate beta-lyase to metabolites which may react with cellular macromolecules or hydrolyze to 3,3,3-trifluoro-2-(fluoromethoxy)propanoic acid. FDVE causes nephrotoxicity in rats, which evidence suggests is mediated by renal uptake of FDVE S-conjugates and metabolism by beta-lyase. Although pathways of FDVE metabolism have been described qualitatively, the purpose of this investigation was to quantify FDVE metabolism via mercapturic acid and beta-lyase pathways. Fischer 344 rats underwent 3-h nose-only exposure to FDVE (0 +/- 0, 46 +/- 19, 98 +/- 7, 150 +/- 29, and 220 +/- 40 ppm), and urine was collected for 24 h. Urine concentrations of the mercapturates, N-acetyl-S-(1,1,3,3, 3-pentafluoro-2-fluoromethoxypropyl)-L-cysteine and N-acetyl-S-(1-fluoro-2-fluoromethoxy-2-(trifluoromethyl)vinyl)-L- cysteine, the beta-lyase-dependent metabolite 3,3, 3-trifluoro-2-(fluoromethoxy)propanoic acid, and its degradation product trifluorolactic acid, were determined by GC/MS. There was dose-dependent urinary excretion of the alkane mercapturate N-acetyl-S-(1,1,3,3,3-pentafluoro-2-fluoromethoxypropyl)-L- cysteine and 3,3,3-trifluoro-2-(fluoromethoxy)propanoic acid, while excretion of the alkene mercapturate N-acetyl-S-(1-fluoro-2-fluoromethoxy-2-(trifluoromethyl)vinyl)-L- cysteine plateaued at higher FDVE exposures. The alkane:alkene mercapturic acid excretion ratio was between 2:1 and 4:1. Trifluorolactic acid was only rarely observed. Urine excretion of the beta-lyase-dependent metabolite 3,3, 3-trifluoro-2-(fluoromethoxy)propanoic acid was 10-fold greater than that of the combined mercapturates. Results show that FDVE cysteine S-conjugates undergo facile metabolism via renal beta-lyase, particularly in comparison with detoxication by mercapturic acid formation. The quantitative assay developed herein may provide a biomarker for FDVE exposure and relative metabolism via toxification and detoxifying pathways, applicable to animal and human investigations.

Gas chromatography-negative ion chemical ionization mass spectrometry as a powerful tool for the detection of mercapturic acids and DNA and protein adducts as biomarkers of exposure to halogenated olefins

The studies on metabolism of halogenated olefins presented here outline the advantages of modern mass spectrometry. The perchloroethene (PER) metabolite N-acetyl-S-(trichlorovinyl)-L-cysteine (N-ac-TCVC) is an important biomarker for the glutathione dependent biotransformation of PER. In urine of rats and humans exposed to PER, N-ac-TCVC was quantified as methyl ester after BF3-MeOH derivatization by gas chromatography with chemical ionization and negative ion detection mass spectrometry (GC-NCI-MS). The detection limit was 10 fmol/microliter injected solution using [2H3]N-ac-TCVC methyl ester as the stable isotope internal standard. Cleavage of S-(trichlorovinyl)-L-cysteine by beta-lyase enzymes results in an electrophilic and highly reactive thioketene which reacts with nucleophilic groups in DNA and proteins. Protein adduct formation was shown in kidney mitochondria by identification of dichloroacetylated lysine after derivatization with 1,1,3,3-tetrafluoro-1,3-dichloroacetone by GC-NCI-MS. In addition, chlorothioketene was generated in organic solvents and reacted with cytosine to give N4-chlorothioacetyl cytosine. After derivatization with pentafluorobenzyl bromide this compound exhibited good gas chromatographic properties and was detectable with a limit of detection of 50 fmol/injected volume. The detection of chemically induced protein modifications in the target organ of toxic metabolite formation and the study of DNA modifications with chemically generated metabolites provide important information on organ toxicity and possible tumorigenicity of halogenated olefins.