Bioactivation of S-(2,2-dihalo-1,1-difluoroethyl)-L-cysteines and S-(trihalovinyl)-L-cysteines by cysteine S-conjugate beta-lyase: indications for formation of both thionoacylating species and thiiranes as reactive intermediates

Bioactivation of trichloroethylene to three regioisomeric glutathione conjugates by liver fractions and recombinant human glutathione transferases: Species differences and implications for human risk assessment

Enzymatic conjugation of glutathione (GSH) to trichloroethylene (TCE) followed by catabolism to the corresponding cysteine-conjugate, S-(dichlorovinyl)-L-cysteine (DCVC), and subsequent bioactivation by renal cysteine conjugate beta-lyases is considered to play an important role in the nephrotoxic effects observed in TCE-exposed rat and human. In this study, it is shown for the first time that three regioisomers of GSH-conjugates of TCE are formed by rat and human liver fractions, namely S-(1,2-trans-dichlorovinyl)-glutathione (1,2-trans-DCVG), S-(1,2-cis-dichlorovinyl)-glutathione (1,2-cis-DCVG) and S-(2,2-dichlorovinyl)-glutathione (2,2-DCVG). In incubations of TCE with rat liver fractions their amounts decreased in order of 1,2-cis-DCVG > 1,2-trans-DCVG > 2,2-DCVG. Human liver cytosol showed a more than 10-fold lower activity of GSH-conjugation, with amounts of regioisomers decreasing in order 2,2-DCVG > 1,2-trans-DCVG > 1,2-cis-DCVG. Incubations with recombinant human GSTs suggest that GSTA1-1 and GSTA2-2 play the most important role in human liver cytosol. GSTP1-1, which produces regioisomers in order 1,2-trans-DCVG > 2,2-cis-DCVG > 1,2-cis-DCVG, is likely to contribute to extrahepatic GSH-conjugation of TCE. Analysis of the products formed by a beta-lyase mimetic model showed that both 1,2-trans-DCVC and 1,2-cis-DCVC are converted to reactive products that form cross-links between the model nucleophile 4-(4-nitrobenzyl)-pyridine (NBP) and thiol-species. No NBP-alkylation was observed with 2,2-DCVC corresponding to its low cytotoxicity and mutagenicity. The lower activity of GSH-conjugation of TCE by human liver fractions, in combination with the lower fraction of potential nephrotoxic and mutagenic 1,2-DCVG-isomers, suggest that humans are at much lower risk for TCE-associated nephrotoxic effects than rats.

Chemical Toxicology of Reactive Intermediates Formed by the Glutathione-Dependent Bioactivation of Halogen-Containing Compounds

The concept that reactive intermediate formation during the biotransformation of drugs and chemicals is an important bioactivation mechanism was proposed in the 1970s and is now accepted as a major mechanism for xenobiotic-induced toxicity. The enzymology of reactive intermediate formation as well as the characterization of the formation and fate of reactive intermediates are now well-established. The mechanism by which reactive intermediates cause cell damage and death is, however, still poorly understood. Although most xenobiotic-metabolizing enzymes catalyze the bioactivation of chemicals, glutathione-dependent biotransformation has been largely associated with detoxication processes, particularly mercapturic acid formation. Abundant evidence now shows that glutathione-dependent biotransformation constitutes an important bioactivation mechanism for halogen-containing drugs and chemicals and has for many compounds been implicated in their organ-selective toxicity and in their mutagenic and carcinogenic potential. The glutathione-dependent biotransformation of haloalkenes is the first step in the cysteine S-conjugate beta-lyase pathway for the bioactivation of nephrotoxic haloalkenes. This pathway has been a rich source of reactive intermediates, including thioacyl halides, alpha-chloroalkenethiolates, 3-halo-alpha-thiolactones, 2,2,3-trihalothiiranes, halothioketenes, and vinylic sulfoxides. Glutathione-dependent bioactivation of gem-dihalomethanes and 1,2-, 1,3-, and 1,4-dihaloalkanes leads to the formation of alpha-chlorosulfides, thiiranium ions, sulfenate esters, and tetrahydrothiophenium ions, respectively, and these reactions lead to reactive intermediate formation.

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.

Formation and toxicity of anesthetic degradation products

Toxic degradation products are formed from a range of old and modern anesthetic agents. The common element in the formation of degradation products is the reaction of the anesthetic agent with the bases in the carbon dioxide absorbents in the anesthesia circuit. This reaction results in the conversion of trichloroethylene to dichloroacetylene, halothane to 2-bromo-2-chloro-1,1-difluoroethylene, sevoflurane to 2-(fluoromethoxy)-1,1,3,3,3-pentafluoro-1-propene (Compound A), and desflurane, isoflurane, and enflurane to carbon monoxide. Dichloroacetylene, 2-bromo-2-chloro-1,1-difluoroethylene, and Compound A form glutathione S-conjugates that undergo hydrolysis to cysteine S-conjugates and bioactivation of the cysteine S-conjugates by renal cysteine conjugate beta-lyase to give nephrotoxic metabolites. The elucidation of the mechanisms of formation and bioactivation of degradation products has allowed for the safe use of anesthetics that may undergo degradation in the anesthesia circuit.

Glutathione‐dependent Bioactivation of Haloalkanes and Haloalkenes

Haloalkanes and haloalkenes constitute an important group of widely used chemicals that have the potential to induce toxicity and cancer. The toxicity of haloalkanes and haloalkenes may be associated with cytochromes P450- or glutathione transferase-dependent bioactivation. This review is concerned with the glutathione- and glutathione transferase-dependent bioactivation of dihalomethanes, 1,2-dihaloalkanes, and haloalkenes. Dihalomethanes, e.g., dichloromethane, and 1,2-dihaloethanes, e.g., 1,2-dichloroethane and 1,2-dibromoethane, undergo glutathione transferase-catalyzed bioactivation to give S-(halomethyl)glutathione or glutathione episulfonium ions, respectively, as reactive intermediates. Haloalkenes, e.g., trichloroethene, hexachlorobutadiene, chlorotrifluoroethene, and tetrafluoroethene, undergo cysteine conjugate beta-lyase-dependent bioactivation to thioacylating intermediates, including thioacyl halides, thioketenes, and 2,2,3-trihalothiiranes. With all of these compounds, the formation of reactive intermediates is associated with their observed toxicity.

Reactive intermediate formation from the 2-(Fluoromethoxy)-1,1,3,3,3-pentafluoro-1-propene (compound A)-derived cysteine S-conjugate S-[2-(Fluoromethoxy)-1,1,3,3,3-pentafluoropropyl]-L-cysteine in pyridoxal model systems

2-(Fluoromethoxy)-1,1,3,3,3-pentafluoro-1-propene (compound A) is a degradation product of the anesthetic sevoflurane and undergoes cysteine conjugate beta-lyase-dependent bioactivation to nephrotoxic metabolites in rats. The present experiments were designed to identify reactive intermediates formed from S-[2-(fluoromethoxy)-1,1,3,3,3-pentafluoropropyl]-L-cysteine, a compound A-derived cysteine S-conjugate, in two pyridoxal model systems, namely Cu2+/pyridoxal and N-dodecylpyridoxal in cetyltrimethylammonium micelles. S-[2-(Fluoromethoxy)-1,1,3,3,3-pentafluoropropyl]-L-cysteine was incubated in the model systems with benzyl bromide, pentafluorobenzyl bromide, aniline, and o-phenylenediamine as trapping agents. The products were purified by TLC and identified by 19F and 1H NMR spectroscopy and by GC/MS. In the absence of trapping agents, 2-(fluoromethoxy)-3,3,3-trifluoropropanoic acid and 3,3,3-trifluorolactic acid, which have been identified previously in biotransformation studies, were formed. With the chemical models, 2-(fluoromethoxy)-1,1,3,3,3-pentafluoropropanethiolate, the expected first intermediate, was not trapped with benzyl bromide. Rather, the dehydrofluorination product 2-(fluoromethoxy)-1,3,3,3-tetrafluoro-1-propenylthiolate was trapped with benzyl bromide to give benzyl 2-(fluoromethoxy)-3,3,3-trifluoropropanethioate, which was formed in both chemical models. When pentafluorobenzyl bromide was used as a trapping agent, GC/MS analysis showed that the expected thiolate was trapped to give pentafluorobenzyl 2-(fluoromethoxy)-1,1,3,3,3-pentafluoropropyl sulfide in the N-dodecylpyridoxal model. In both chemical models, 2-(fluoromethoxy)-3,3,3-trifluorothioacyl fluoride was trapped with aniline to give N-phenyl 2-(fluoromethoxyl)-3,3,3-trifluoropropanethioamide, which cyclized to give 3-phenyl-4-thiono-5-(trifluoromethyl)-1,3-oxazolane. The results demonstrate that most of the reactive intermediates and products formed by the beta-lyase-catalyzed biotransformation of compound A-derived cysteine S-conjugates are also formed in the two chemical systems studied. Some products were, however, formed in chemical systems that have not been observed in previous in vivo and in vitro studies; it is not known whether these products are formed in biological systems and whether they contribute to the observed nephrotoxicity of cysteine S-conjugates.

Bioactivation of chemopreventive selenocysteine Se-conjugates and related amino acids by amino acid oxidases novel route of metabolism of selenoamino acids

Several selenocysteine Se-conjugates have been shown to possess potent chemopreventive activity in animal models for chemical carcinogenesis. As a mechanism of action, beta-elimination reactions to form chemopreventive selenols, ammonia, and pyruvate has been proposed. The enzymes involved in these beta-elimination reactions, however, have been partially elucidated. Next to cysteine conjugate beta-lyases, as yet unidentified non-pyridoxal-5'-phosphate-dependent enzymes also appear to be involved in cytosolic beta-elimination reactions. In the present study, it was investigated whether amino acid oxidases contribute to the bioactivation of selenocysteine Se-conjugates. Using purified L-amino acid oxidase from Crotalus adamanteus as a model enzyme, significant beta-elimination activities were indeed observed upon incubation with Se-methylselenocysteine (K(m), 195 microM; k(cat), 48 min(-1)), Se-allylselenocysteine (K(m), 608 microM; k(cat), 34 min(-1)), Se-phenylselenocysteine (K(m), 107 microM; k(cat), 57 min(-1)) and Se-benzylselenocysteine (K(m), 59 microM; k(cat), 13 min(-1)). For all selenocysteine Se-conjugates tested, the rate of pyruvate formation was comparable to that of hydrogen peroxide, one of the products of oxidative deamination. The fact that addition of catalase did not alter pyruvate formation indicated that the beta-elimination reaction observed was not mediated by selenoxidation/syn-elimination due to the hydrogen peroxide formed via the oxidative deamination pathway. Using D-amino acid oxidase from porcine kidney and D-SeCys conjugates similar results were obtained. To delineate whether mammalian L-amino acid oxidases are also able to catalyze beta-elimination of selenocysteine Se-conjugates, rat renal cytosol was fractionated and screened for beta-elimination and oxidative deamination activities. One of the fractions isolated displayed oxidative deamination activity with several amino acids and cysteine S-conjugates. With selenocysteine Se-conjugates as substrates, however, this fraction displayed both oxidative deamination and beta-elimination activities, when incubated in the presence of aminoxyacetic acid to block contribution of pyridoxal-5'-phosphate-dependent enzymes. The potential significance of this novel bioactivation route for the chemopreventive activity of selenocysteine Se-conjugates is discussed.

Toxicity of methylmercury conjugated with L-cysteine on rat thymocytes and human leukemia K562 cells in comparison with that of methylmercury chloride

In order to reveal the implication of use of methylmercury chloride (MeHgCl) in in vitro study, the effects of 10 µM MeHgCl on rat thymocytes and human leukemia K562 cells were compared with those of methylmercury conjugated with L-cysteine (10 µM MeHg-Cys) using a flow cytometer and fluorescent probes to monitor cellular physiological and pathological parameters. MeHgCl hyperpolarized membranes of thymocytes, followed by depolarization within a few minutes after the application, while MeHg-Cys persistently hyperpolarized them. MeHgCl increased intracellular concentration of Ca(2+), decreased cellular content of glutathione and increased generation of superoxide anion in the cells. The effects of MeHg-Cys were much less than those of MeHgCl. MeHgCl greatly increased both numbers of the cells undergoing apoptosis and dead cells in cell suspension containing thymocytes, while this was not the case for MeHg-Cys. MeHgCl reduced the cell viability of human leukemia K562 cells and completely inhibited the cell growth. The effects of MeHg-Cys on K562 cells were less than those of MeHgCl. It can be concluded that the effects of MeHgCl on rat thymocytes and K562 cells are different from those of MeHg-Cys. The results obtained from the in vitro studies using MeHgCl may be less implicit to elucidate the mechanism of MeHg intoxication in humans and experimental animals because MeHg are present in forms of MeHg-Cys and/or MeHg-S conjugate under the in vivo conditions.

Bioactivation mechanisms of haloalkene cysteine S-conjugates modeled by gas-phase, ion-molecule reactions

Glutathione conjugate formation plays important roles in the detoxification and bioactivation of xenobiotics. A range of nephrotoxic haloalkenes undergo bioactivation that involves glutathione and cysteine S-conjugate formation. The cysteine S-conjugates thus formed may undergo cysteine conjugate beta-lyase-catalyzed biotransformation to form cytotoxic thiolates or thiiranes. In the studies presented here, cysteine conjugate beta-lyase-catalyzed biotransformations were modeled by anion-induced elimination reactions of S-(2-bromo-1,1, 2-trifluoroethyl)-N-acetyl-L-cysteine methyl ester, S-(2-chloro-1,1, 2-trifluoroethyl)-N-acetyl-L-cysteine methyl ester, and S-(2-fluoro-1,1,2-trifluoroethyl)-N-acetyl-L-cysteine methyl ester in the gas phase. Examination of these processes in the gas phase allowed direct observation of the formation of cysteine S-conjugate-derived thiolates and thiiranes, whose formation is inferred from condensed-phase results. The cysteine S-conjugates of these haloethenes exhibit distinctive patterns of mutagenicity that are thought to be correlated with the nature of the products formed by their cysteine conjugate beta-lyase-catalyzed biotransformation. In particular, S-(2-bromo-1,1,2-trifluoroethyl)-L-cysteine is mutagenic, whereas the chloro and fluoro analogues are not. It has been proposed that the mutagenicity of S-(2-bromo-1,1, 2-trifluoroethyl)-L-cysteine is correlated with the greater propensity of the bromine-containing cysteine S-conjugate to form a thiirane compared with those of the chlorine- or fluorine-containing conjugates. The ease of thiirane formation is consistent with the gas-phase results presented here, which show that the bromine-containing conjugate has a greater propensity to form a thiirane on anionic base-induced elimination than the chloro- or fluoro-substituted analogues. The blocked cysteine S-conjugates were deprotonated by gas-phase ion-molecule reactions with hydroxide, methoxide, and ethoxide ions and then allowed to decompose. The mechanisms for these decompositions are discussed as well as the insights into the bioactivation of these cysteine S-conjugates provided by the further decompositions of thiolate intermediates.

Glutathione-dependent bioactivation of haloalkenes

Several halogenated alkenes are nephrotoxic in rodents. A mechanism for the organ-specific toxicity of these compounds to the kidney has been elucidated. The mechanism involves hepatic glutathione conjugation to dihaloalkenyl or 1,1-difluoroalkyl glutathione S-conjugates, which are cleaved by gamma-glutamyltransferase and dipeptidases to cysteine S-conjugates. Haloalkene-derived cysteine S-conjugates may have four fates in the organism: (a) They may be substrates for renal cysteine conjugate beta-lyases, which cleave them to form reactive intermediates identified as thioketenes (chloroalkene-derived S-conjugates), thionoacyl halides (fluoroalkene-derived S-conjugates not containing bromide), thiiranes, and thiolactones (fluoroalkene-derived S-conjugates containing bromine); (b) cysteine S-conjugates may be N-acetylated to excretable mercapturic acids; (c) they may undergo transamination or oxidation to the corresponding 3-mercaptopyruvic acid S-conjugate; (d) finally, oxidation of the sulfur atom in halovinyl cysteine S-conjugates and corresponding mercapturic acids forms Michael acceptors and may also represent a bioactivation reaction. The formation of reactive intermediates by cysteine conjugate beta-lyase may play a role in the target-organ toxicity and in the possible renal tumorigenicity of several chlorinated olefins widely used in many chemical processes.

Theoretical evaluation of two plausible routes for bioactivation of S-(1,1-difluoro-2,2-dihaloethyl)-L-cysteine conjugates: thiirane vs thionoacyl fluoride pathway

The selective nephrotoxicity of halogenated alkenes has been attributed to a glutathione (GSH) S-conjugate pathway involving enzymatic hydrolysis to the cysteine S-conjugate and beta-lyase bioactivation to thiolates, which are presumed to give rise to the ultimate mutagenic or cytotoxic reactive species. Studies have shown that the brominated S-(2,2-dihalo-1,1-difluoroethyl)-L-cysteine conjugates are mutagenic in the Ames test, whereas the nonbrominated analogues are nonmutagenic. While careful experimentation has contributed much to current understanding, the ultimate reactive species responsible for the differing mutagenic effects remain unknown. Computational methods were applied to the investigation of two proposed metabolic pathways leading from the thiolate to either a thiirane or thionoacyl fluoride intermediate, both electrophilic species presumed capable of binding to proteins or DNA. Studied were six F-, Cl-, and Br-substituted 2,2-dihalo-1,1-difluoroethane-1-thiolates (2,2-dihalo-DFETs). Pathway preference was determined for each thiolate by comparison of reaction energy profiles and activation energies. At all but the lowest level of ab initio theory, a thionoacyl fluoride pathway was predicted for 2,2-difluoro-DFET, while a thiirane pathway was energetically preferred for the brominated 2,2-dihalo-DFETs. These results offer a clear mechanism-based rationale for distinguishing 2,2-difluoro-DFET from the brominated 2,2-dihalo-DFETs, while the results are less clear for the 2,2-dichloro and 2-chloro-2-fluoro-DFETs, which at the highest level of ab initio treatment had a relatively small energy preference (2.4 kcal/mol) for the thiirane pathway. The predicted clear preference for a thiirane pathway for the brominated 2,2-dihalo-DFETs is not consistent with a recently proposed pathway involving alpha-thiolactone formation through a thionoacyl fluoride intermediate [Finkelstein, M. B., et al. (1995) J. Am. Chem. Soc. 117, 9590-9591], but is supported by results of a recent study providing experimental evidence for thiirane formation from the brominated 2,2-dihalo-DFETs [Finkelstein, M. B., et al. (1996) Chem. Res. Toxicol. 9, 227-231].