Formation and fate of nephrotoxic and cytotoxic glutathione S-conjugates: cysteine conjugate beta-lyase pathway

N-Deacetylation in Lincosamide Biosynthesis Is Catalyzed by a TldD/PmbA Family Protein

Lincosamides are clinically important antibiotics originally produced as microbial specialized metabolites. The complex biosynthesis of lincosamides is coupled to the metabolism of mycothiol as a sulfur donor. Here, we elucidated the N-deacetylation of the mycothiol-derived N-acetyl-l-cysteine residue of a lincosamide intermediate, which is comprised of an amino acid and an aminooctose connected via an amide bond. We purified this intermediate from the culture broth of a deletion mutant strain and tested it as a substrate of recombinant lincosamide biosynthetic proteins in the in vitro assays that were monitored via liquid chromatography-mass spectrometry. Our findings showed that the N-deacetylation reaction is catalyzed by CcbIH/CcbQ or LmbIH/LmbQ proteins in celesticetin and lincomycin biosynthesis, respectively. These are the first N-deacetylases from the TldD/PmbA protein family, from which otherwise only several proteases and peptidases were functionally characterized. Furthermore, we present a sequence similarity network of TldD/PmbA proteins, which suggests that the lincosamide N-deacetylases are unique among these widely distributed proteins.

The mercapturic acid pathway

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

Metabolism of Glutathione S-Conjugates: Multiple Pathways

Many potentially toxic electrophilic xenobiotics and some endogenous compounds are detoxified by conversion to the corresponding glutathione S-conjugate, which is metabolized to the N-acetylcysteine S-conjugate (mercapturate) and excreted. Some mercapturate pathway components, however, are toxic. Bioactivation (toxification) may occur when the glutathione S-conjugate (or mercapturate) is converted to a cysteine S-conjugate that undergoes a β-lyase reaction. If the sulfhydryl-containing fragment produced in this reaction is reactive, toxicity may ensue. Some drugs and halogenated workplace/environmental contaminants are bioactivated by this mechanism. On the other hand, cysteine S-conjugate β-lyases occur in nature as a means of generating some biologically useful sulfhydryl-containing compounds.

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.

Characterization of the chemical reactivity and nephrotoxicity of N-acetyl-S-(1,2-dichlorovinyl)-L-cysteine sulfoxide, a potential reactive metabolite of trichloroethylene

N-Acetyl-S-(1,2-dichlorovinyl)-L-cysteine (NA-DCVC) has been detected in the urine of humans exposed to trichloroethylene and its related sulfoxide, N-acetyl-S-(1,2-dichlorovinyl)-L-cysteine sulfoxide (NA-DCVCS), has been detected as hemoglobin adducts in blood of rats dosed with S-(1,2-dichlorovinyl)-L-cysteine (DCVC) or S-(1,2-dichlorovinyl)-L-cysteine sulfoxide (DCVCS). Because the in vivo nephrotoxicity of NA-DCVCS was unknown, in this study, male Sprague-Dawley rats were dosed (i.p.) with 230 μmol/kg b.w. NA-DCVCS or its potential precursors, DCVCS or NA-DCVC. At 24 h post treatment, rats given NA-DCVC or NA-DCVCS exhibited kidney lesions and effects on renal function distinct from those caused by DCVCS. NA-DCVC and NA-DCVCS primarily affected the cortico-medullary proximal tubules (S(2)-S(3) segments) while DCVCS primarily affected the outer cortical proximal tubules (S(1)-S(2) segments). When NA-DCVCS or DCVCS was incubated with GSH in phosphate buffer pH 7.4 at 37°C, the corresponding glutathione conjugates were detected, but NA-DCVC was not reactive with GSH. Because NA-DCVCS exhibited a longer half-life than DCVCS and addition of rat liver cytosol enhanced GSH conjugate formation, catalysis of GSH conjugate formation by the liver could explain the lower toxicity of NA-DCVCS in comparison with DCVCS. Collectively, these results provide clear evidence that NA-DCVCS formation could play a significant role in DCVC, NA-DCVC, and trichloroethylene nephrotoxicity. They also suggest a role for hepatic metabolism in the mechanism of NA-DCVC nephrotoxicity.

Inhibition of aminoacylase 3 protects rat brain cortex neuronal cells from the toxicity of 4-hydroxy-2-nonenal mercapturate and 4-hydroxy-2-nonenal

4-Hydroxy-2-nonenal (4HNE) and acrolein (ACR) are highly reactive neurotoxic products of lipid peroxidation that are implicated in the pathogenesis and progression of Alzheimer's and Parkinson's diseases. Conjugation with glutathione (GSH) initiates the 4HNE and ACR detoxification pathway, which generates the mercapturates of 4HNE and ACR that can be excreted. Prior work has shown that the efficiency of the GSH-dependent renal detoxification of haloalkene derived mercapturates is significantly decreased upon their deacetylation because of rapid transformation of the deacetylated products into toxic compounds mediated by β-lyase. The enzymes of the GSH-conjugation pathway and β-lyases are expressed in the brain, and we hypothesized that a similar toxicity mechanism may be initiated in the brain by the deacetylation of 4HNE- and ACR-mercapturate. The present study was performed to identify an enzyme(s) involved in 4HNE- and ACR-mercapturate deacetylation, characterize the brain expression of this enzyme and determine whether its inhibition decreases 4HNE and 4HNE-mercapturate neurotoxicity. We demonstrated that of two candidate deacetylases, aminoacylases 1 (AA1) and 3 (AA3), only AA3 efficiently deacetylates both 4HNE- and ACR-mercapturate. AA3 was further localized to neurons and blood vessels. Using a small molecule screen we generated high-affinity AA3 inhibitors. Two of them completely protected rat brain cortex neurons expressing AA3 from the toxicity of 4HNE-mercapturate. 4HNE-cysteine (4HNE-Cys) was also neurotoxic and its toxicity was mostly prevented by a β-lyase inhibitor, aminooxyacetate. The results suggest that the AA3 mediated deacetylation of 4HNE-mercapturate may be involved in the neurotoxicity of 4HNE.

Role of reactive metabolites in the circulation in extrahepatic toxicity

INTRODUCTION

Reactive metabolite-mediated toxicity is frequently limited to the organ where the electrophilic metabolites are generated. Some reactive metabolites, however, might have the ability to translocate from their site of formation. This suggests that for these reactive metabolites, investigations into the role of organs other than the one directly affected could be relevant to understanding the mechanism of toxicity.

AREAS COVERED

The authors discuss the physiological and biochemical factors that can enable reactive metabolites to cause toxicity in an organ distal from the site of generation. Furthermore, the authors present a case study which describes studies that demonstrate that S-(1,2-dichlorovinyl)-L-cysteine sulfoxide (DCVCS) and N-acetyl-S-(1,2-dichlorovinyl-L-cysteine sulfoxide (N-AcDCVCS), reactive metabolites of the known trichloroethylene metabolites S-(1,2-dichlorovinyl)-L-cysteine (DCVC), and N-acetyl-S-(1,2-dichlorovinyl)-L-cysteine (N-AcDCVC), are generated in the liver and translocate through the circulation to the kidney to cause nephrotoxicity.

EXPERT OPINION

The ability of reactive metabolites to translocate could be important to consider when investigating mechanisms of toxicity. A mechanistic approach, similar to the one described for DCVCS and N-AcDCVCS, could be useful in determining the role of circulating reactive metabolites in extrahepatic toxicity of drugs and other chemicals. If this is the case, intervention strategies that would not otherwise be feasible might be effective for reducing extrahepatic toxicity.

N-biotinyl-S-(1,2-dichlorovinyl)-L-cysteine sulfoxide as a potential model for S-(1,2-dichlorovinyl)-L-cysteine sulfoxide: characterization of stability and reactivity with glutathione and kidney proteins in vitro

S-(1,2-Dichlorovinyl)-L-cysteine sulfoxide (DCVCS) is a reactive and potent nephrotoxic metabolite of the human trichloroethylene metabolite S-(1,2-dichlorovinyl)-L-cysteine (DCVC). Because DCVCS covalent binding to kidney proteins likely plays a role in its nephrotoxicity, in this study biotin-tagged DCVCS, N-biotinyl-DCVCS (NB-DCVCS), was synthesized, and its stability in buffer alone and in the presence of rat blood or plasma was characterized in vitro. In addition, reactivity toward GSH and covalent binding to selected model enzymes and isolated kidney proteins were characterized. The half-lives of NB-DCVCS (39.6 min) and the DCVCS (diastereomer 1, 14.4 min; diastereomer 2, 6 min) in the presence of GSH were comparable. Incubating the model enzymes glutathione reductase and malate dehydrogenase with 10 μM NB-DCVCS for 3 h at 37 °C followed by immunoblotting using antibiotin antibodies demonstrated that glutathione reductase and malate dehydrogenase were extensively modified by NB-DCVCS. When rat kidney cytosol (6 μg/μL) was incubated with NB-DCVCS (312.5 nM to 5 μM) for 3 h at 37 °C followed by immunoblotting, a concentration-dependent increase in signal with multiple proteins with different molecular weights was observed, suggesting that NB-DCVCS binds to multiple kidney proteins with different selectivity. Incubating rat kidney cytosol with DCVCS (10-100 μM) prior to the addition of NB-DCVCS (2.5 μM) reduced the immunoblotting signal, suggesting that NB-DCVCS and DCVCS compete for the same binding sites. A comparison of the stability of NB-DCVCS and DCVCS in rat blood and plasma was determined in vitro, and NB-DCVCS exhibited higher stability than DCVCS in both media. Collectively, these results suggest that NB-DCVCS shows sufficient stability, reactivity, and selectivity to warrant further investigations into its possible use as a tool for future characterization of the role of covalent modification of renal proteins by DCVCS in nephrotoxicity.

Measurement of cysteine S-conjugate β-lyase activity

Cysteine S-conjugate β-lyases are pyridoxal 5'-phosphate (PLP)-containing enzymes that catalyze the conversion of cysteine S-conjugates [RSCH(2)CH(NH(3) (+))CO(2) (-)] and selenium Se-conjugates [RSeCH(2)CH(NH(3) (+))CO(2) (-)] that contain a leaving group in the β position to pyruvate, ammonium and a sulfur-containing fragment (RSH) or selenium-containing fragment (RSeH), respectively. In mammals, at least ten PLP enzymes catalyze β-elimination reactions with such cysteine S-conjugates. All are enzymes involved in amino acid metabolism that do not normally catalyze a β-lyase reaction, but catalyze a non-physiological β-lyase side-reaction that depends on the electron-withdrawing properties of the -SR or -SeR moiety. In the case of cysteine S-conjugates, if the eliminated RSH is stable, the compound may be S-thiomethylated and excreted (thiomethyl shunt) or S-glucuronidated and harmlessly excreted. However, if RSH is chemically reactive, the cysteine S-conjugate may be toxic as a result of the β-lyase reaction. The cysteine S-conjugate β-lyase pathway is of particular interest to toxicologists because it is involved in the bioactivation (toxification) of halogenated alkenes and certain drugs. This unit provides protocols for the analysis of cysteine S-conjugate β-lyase activity.

Structures of aminoacylase 3 in complex with acetylated substrates

Trichloroethylene (TCE) is one of the most widespread environmental contaminants, which is metabolized to N-acetyl-S-1,2-dichlorovinyl-L-cysteine (NA-DCVC) before being excreted in the urine. Alternatively, NA-DCVC can be deacetylated by aminoacylase 3 (AA3), an enzyme that is highly expressed in the kidney, liver, and brain. NA-DCVC deacetylation initiates the transformation into toxic products that ultimately causes acute renal failure. AA3 inhibition is therefore a target of interest to prevent TCE induced nephrotoxicity. Here we report the crystal structure of recombinant mouse AA3 (mAA3) in the presence of its acetate byproduct and two substrates: N(α)-acetyl-L-tyrosine and NA-DCVC. These structures, in conjunction with biochemical data, indicated that AA3 mediates substrate specificity through van der Waals interactions providing a dynamic interaction interface, which facilitates a diverse range of substrates.

Cysteine S-conjugate β-lyases: important roles in the metabolism of naturally occurring sulfur and selenium-containing compounds, xenobiotics and anticancer agents

Cysteine S-conjugate β-lyases are pyridoxal 5'-phosphate-containing enzymes that catalyze β-elimination reactions with cysteine S-conjugates that possess a good leaving group in the β-position. The end products are aminoacrylate and a sulfur-containing fragment. The aminoacrylate tautomerizes and hydrolyzes to pyruvate and ammonia. The mammalian cysteine S-conjugate β-lyases thus far identified are enzymes involved in amino acid metabolism that catalyze β-lyase reactions as non-physiological side reactions. Most are aminotransferases. In some cases the lyase is inactivated by reaction products. The cysteine S-conjugate β-lyases are of much interest to toxicologists because they play an important key role in the bioactivation (toxication) of halogenated alkenes, some of which are produced on an industrial scale and are environmental contaminants. The cysteine S-conjugate β-lyases have been reviewed in this journal previously (Cooper and Pinto in Amino Acids 30:1-15, 2006). Here, we focus on more recent findings regarding: (1) the identification of enzymes associated with high-M(r) cysteine S-conjugate β-lyases in the cytosolic and mitochondrial fractions of rat liver and kidney; (2) the mechanism of syncatalytic inactivation of rat liver mitochondrial aspartate aminotransferase by the nephrotoxic β-lyase substrate S-(1,1,2,2-tetrafluoroethyl)-L-cysteine (the cysteine S-conjugate of tetrafluoroethylene); (3) toxicant channeling of reactive fragments from the active site of mitochondrial aspartate aminotransferase to susceptible proteins in the mitochondria; (4) the involvement of cysteine S-conjugate β-lyases in the metabolism/bioactivation of drugs and natural products; and (5) the role of cysteine S-conjugate β-lyases in the metabolism of selenocysteine Se-conjugates. This review emphasizes the fact that the cysteine S-conjugate β-lyases are biologically more important than hitherto appreciated.

Structure, expression, and function of kynurenine aminotransferases in human and rodent brains

Kynurenine aminotransferases (KATs) catalyze the synthesis of kynurenic acid (KYNA), an endogenous antagonist of N-methyl-D: -aspartate and alpha 7-nicotinic acetylcholine receptors. Abnormal KYNA levels in human brains are implicated in the pathophysiology of schizophrenia, Alzheimer's disease, and other neurological disorders. Four KATs have been reported in mammalian brains, KAT I/glutamine transaminase K/cysteine conjugate beta-lyase 1, KAT II/aminoadipate aminotransferase, KAT III/cysteine conjugate beta-lyase 2, and KAT IV/glutamic-oxaloacetic transaminase 2/mitochondrial aspartate aminotransferase. KAT II has a striking tertiary structure in N-terminal part and forms a new subgroup in fold type I aminotransferases, which has been classified as subgroup Iepsilon. Knowledge regarding KATs is vast and complex; therefore, this review is focused on recent important progress of their gene characterization, physiological and biochemical function, and structural properties. The biochemical differences of four KATs, specific enzyme activity assays, and the structural insights into the mechanism of catalysis and inhibition of these enzymes are discussed.

Taurine protects acetaminophen-induced oxidative damage in mice kidney through APAP urinary excretion and CYP2E1 inactivation

Acute exposure of acetaminophen (APAP), a widely used analgesic and antipyretic drug, causes severe renal damage and no specific agent has been reported so far that plays any beneficial role in this organ pathophysiology. In the present study, the protective role of taurine on APAP-induced nephrotoxicity was investigated in mice. In order to induce acute nephrotoxicity, APAP was administered at a single dose of 2g/kg body weight orally to male adult albino mice of Swiss strain. APAP exposure for 24h significantly increased plasma level of blood urea nitrogen (BUN), creatinine, uric acid, TNF-alpha, NO production, urinary gamma-glutamyl transpeptidase (gamma-GT) activity, total urinary protein and urinary glucose level accompanied by a decrease in Na(+)-K(+)-ATPase activity. Moreover, APAP administration significantly increased MDA, protein carbonylation, GSSG level, intracellular ROS production and cytochrome P450 enzyme (CYPP450) activity. The same exposure decreased GSH level, ferric reducing/antioxidant power (FRAP) as well as the activities of antioxidant enzymes indicating that APAP-induced renal damage was mediated through oxidative stress. Besides, APAP exposure significantly reduced mitochondrial membrane potential and induced up-regulation of CYP2E1 in renal tissues although JNK did not play any significant role in this APAP-induced renal pathophysiology. Caspase 9/3 immunoblot and DNA fragmentation analyses showed that APAP-induced renal cell damage was mostly necrotic in nature, although some apoptosis also occurred simultaneously. Taurine treatment both pre and post (150 mg/kg body weight for 3 days, orally) to APAP exposure, however, significantly reduced APAP-induced nephrotoxicity through its antioxidant properties, urinary excretion of APAP and suppression of CYP2E1. Results suggest that taurine might be a potential therapeutic candidate against APAP-induced acute nephrotoxicity.

Transport of N-acetyl-S-(1,2-dichlorovinyl)-L-cysteine, a metabolite of trichloroethylene, by mouse multidrug resistance associated protein 2 (Mrp2)

N-acetyl-S-(1,2-dichlorovinyl)-l-cysteine (Ac-DCVC) and S-(1,2-dichlorovinyl)-l-cysteine (DCVC) are the glutathione conjugation pathway metabolites of a common industrial contaminant and potent nephrotoxicant trichloroethylene (TCE). Ac-DCVC and DCVC are accumulated in the renal proximal tubule where they may be secreted into the urine by an unknown apical transporter(s). In this study, we explored the hypothesis that the apical transport of Ac-DCVC and/or DCVC may be mediated by the multidrug resistance associated protein 2 (Mrp2, ABCC2), which is known to mediate proximal tubular apical ATP-dependent transport of glutathione and numerous xenobiotics and endogenous substances conjugated with glutathione. Transport experiments using membrane vesicles prepared from mouse proximal tubule derived cells expressing mouse Mrp2 utilizing ATPase assay and direct measurements of Ac-DCVC/DCVC using liquid chromatography/tandem mass-spectrometry (LC/MS/MS) demonstrated that mouse Mrp2 mediates ATP-dependent transport of Ac-DCVC. Expression of mouse Mrp2 antisense mRNA significantly inhibited the vectorial basolateral to apical transport of Ac-DCVC but not DCVC in mouse proximal tubule derived cells endogenously expressing mouse Mrp2. The results suggest that Mrp2 may be involved in the renal secretion of Ac-DCVC.

Enzymes Involved in Processing Glutathione Conjugates

Many potentially toxic electrophiles react with glutathione to form glutathione S-conjugates in reactions catalyzed or enhanced by glutathione S-transferases. The glutathione S-conjugate is sequentially converted to the cysteinylglycine-, cysteine- and N-acetyl-cysteine S-conjugate (mercapturate). The mercapturate is generally more polar and water soluble than the parent electrophile and is readily excreted. Excretion of the mercapturate represents a detoxication mechanism. Some endogenous compounds, such as leukotrienes, prostaglandin (PG) A2, 15-deoxy-Δ^12,14-PGJ₂, and hydroxynonenal can also be metabolized to mercapturates and excreted. On occasion, however, formation of glutathione S- and cysteine S-conjugates are bioactivation events as the metabolites are mutagenic and/or cytotoxic. When the cysteine S-conjugate contains a strong electron-withdrawing group attached at the sulfur, it may be converted by cysteine S-conjugate β-lyases to pyruvate, ammonium and the original electrophile modified to contain an –SH group. If this modified electrophile is highly reactive then the enzymes of the mercapturate pathway together with the cysteine S-conjugate β-lyases constitute a bioactivation pathway. Some endogenous halogenated environmental contaminants and drugs are bioactivated by this mechanism. Recent studies suggest that coupling of enzymes of the mercapturate pathway to cysteine S-conjugate β-lyases may be more common in nature and more widespread in the metabolism of electrophilic xenobiotics than previously realized.

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.

Mechanistic study of BNP7787-mediated cisplatin nephroprotection: modulation of gamma-glutamyl transpeptidase

PURPOSE

The mechanisms for cisplatin-induced renal cell injury have been the focus of intense investigation for many years with a view to provide a more effective and convenient form of nephroprotection. BNP7787 (disodium 2,2'-dithio-bis ethane sulfonate; dimesna, Tavocept), is a water-soluble disulfide investigational new drug that is undergoing clinical development for the prevention and mitigation of clinically important chemotherapy-induced toxicities associated with platinum-type chemotherapeutic agents. We hypothesized that part of BNP7787's mechanism of action (MOA) pertaining to the potential prevention of cisplatin-induced nephrotoxicity involves the inhibition of gamma-glutamyl transpeptidase (GGT) activity, mediated by BNP7787-derived mesna-disulfide heteroconjugates that contain a terminal gamma-glutamate moiety [e.g., mesna-glutathione (MSSGlutathione) and mesna-cysteinyl-glutamate (MSSCE)].

METHODS

Inhibition studies were conducted on human and porcine GGT to determine the effect of mesna-disulfide heteroconjugates on the enzyme's activity in vitro. These studies utilized a fluorimetric assay that monitored the hydrolysis of L-gamma-glutamyl-7-amino-4-trifluoromethylcoumarin (GG-AFC) to AFC.

RESULTS

Mesna-disulfide heteroconjugates that contained gamma-glutamyl moieties were potent inhibitors of human and porcine GGT. An in situ-generated mesna-cisplatin conjugate was not a substrate for GGT.

CONCLUSIONS

The GGT xenobiotic metabolism pathway is postulated to be a major toxification pathway for cisplatin nephrotoxicity, and BNP7787 may play a novel and critical therapeutic role in the modulation of GGT activity. We further postulate that there are two general mechanisms for BNP7787-mediated nephroprotection against cisplatin-induced nephrotoxicity involving this pathway. First, the active BNP7787 pharmacophore, mesna, produces an inactive mesna-cisplatin conjugate that is not a substrate for the GGT toxification pathway (GGT xenobiotic metabolism pathway) and, second, BNP7787-derived mesna-disulfide heteroconjugates may serve as selective, potent inhibitors of GGT, possibly resulting in nephroprotection by a novel means.

Cysteine conjugate beta-lyase activity of rat erythrocytes and formation of beta-lyase-derived globin monoadducts and cross-links after in vitro exposure of erythrocytes to S-(1,2-dichlorovinyl)-L-cysteine

S-(1,2-Dichlorovinyl)-L-cysteine (DCVC), a mutagenic and nephrotoxic metabolite of trichloroethylene, can be bioactivated to reactive metabolites, S-(1,2-dichlorovinyl)-L-cysteine sulfoxide (DCVCS) or chlorothioketene and/or 2-chlorothionoacetyl chloride, by cysteine conjugate S-oxidase (S-oxidase) and cysteine conjugate beta-lyase (beta-lyase), respectively. Previously, we characterized the reactivity of DCVCS with Hb upon incubation of erythrocytes with DCVCS and provided evidence for the formation of distinct DCVCS-Hb monoadducts and cross-links in both isolated erythrocytes and rats given DCVCS. In the present study, we investigated DCVC bioactivation and Hb adduct formation in isolated rat erythrocytes incubated with DCVC (9 and 450 microM) at 37 degrees C and pH 7.4. The results suggested that no DCVCS monoadducts or cross-links were formed; however, LC/electrospray ionization/MS and matrix-assisted laser desorption/ionization/MS of trypsin-digested globin peptides revealed the presence of beta-lyase-derived globin monoadducts and cross-links. Adducts and cross-links in which the sulfur atom of the reactive sulfur intermediates were replaced by oxygen have also been detected. Use of SDS-PAGE provided additional evidence for globin cross-link formation in the presence of DCVC. Interestingly, the MS results suggest that the observed peptide selectivity of the beta-lyase-derived reactive sulfur/oxygen-containing species was different than that previously observed with DCVCS. While these results suggested that erythrocytes have beta-lyase but not S-oxidase activity, further support for this hypothesis was obtained using S-(2-benzothiazolyl)-L-cysteine, an alternative substrate for beta-lyases. Collectively, the results demonstrate the utility of Hb adducts and cross-links to characterize the metabolic pathway responsible for DCVC bioactivation in erythrocytes and to provide distinct biomarkers for each reactive metabolite.

Structural insight into the inhibition of human kynurenine aminotransferase I/glutamine transaminase K

Human kynurenine aminotransferase I (hKAT I) catalyzes the formation of kynurenic acid, a neuroactive compound. Here, we report three high-resolution crystal structures (1.50-1.55 A) of hKAT I that are in complex with glycerol and each of two inhibitors of hKAT I: indole-3-acetic acid (IAC) and Tris. Because Tris is able to occupy the substrate binding position, we speculate that this may be the basis for hKAT I inhibition. Furthermore, the hKAT/IAC complex structure reveals that the binding moieties of the inhibitor are its indole ring and a carboxyl group. Six chemicals with both binding moieties were tested for their ability to inhibit hKAT I activity; 3-indolepropionic acid and DL-indole-3-lactic acid demonstrated the highest level of inhibition, and as they cannot be considered as substrates of the enzyme, these two inhibitors are promising candidates for future study. Perhaps even more significantly, we report the discovery of two different ligands located simultaneously in the hKAT I active center for the first time.

Mouse aminoacylase 3: a metalloenzyme activated by cobalt and nickel

Aminoacylase 3 (AA3) deacetylates N-acetyl-aromatic amino acids and mercapturic acids including N-acetyl-1,2-dichlorovinyl-L-cysteine (Ac-DCVC), a metabolite of a xenobiotic trichloroethylene. Previous studies did not demonstrate metal-dependence of AA3 despite a high homology with a Zn(2+)-metalloenzyme aminoacylase 2 (AA2). A 3D model of mouse AA3 was created based on homology with AA2. The model showed a putative metal binding site formed by His21, Glu24 and His116, and Arg63, Asp68, Asn70, Arg71, Glu177 and Tyr287 potentially involved in catalysis/substrate binding. The mutation of each of these residues to alanine inactivated AA3 except Asn70 and Arg71, therefore the corrected 3D model of mouse AA3 was created. Wild type (wt) mouse AA3 expressed in E. coli contained approximately 0.35 zinc atoms per monomer. Incubation with Co(2+) and Ni(2+) activated wt-AA3. In the cobalt-activated AA3 zinc was replaced with cobalt. Metal removal completely inactivated wt-AA3, whereas addition of Zn(2+), Mn(2+) or Fe(2+) restored initial activity. Co(2+) and to a lesser extent Ni(2+) increased activity several times in comparison with intact wt-AA3. Co(2+) drastically increased the rate of deacetylation of Ac-DCVC and significantly increased the toxicity of Ac-DCVC in the HEK293T cells expressing wt-AA3. The results indicate that AA3 is a metalloenzyme significantly activated by Co(2+) and Ni(2+).

Role of glutathione S-transferase Pi in cisplatin-induced nephrotoxicity

One of the dose-limiting toxicities of cisplatin is nephrotoxicity. Renal toxicity is localized to quiescent proximal tubule cells, where the formation of DNA-adducts cannot account for the dose-limiting toxicity. Our earlier results have shown that a glutathione conjugate of cisplatin is metabolized to a nephrotoxicant via gamma-glutamyl transpeptidase (GGT) and a cysteine S-conjugate beta-lyase. The present study was designed to evaluate the potential role of glutathione S-transferase Pi (GSTP) in the initial steps of the bioactivation of cisplatin. Wild-type mice and mice deficient in both murine GSTP genes (GstP1/P2) were treated with cisplatin. Toxicity in both male and female mice was evaluated 5 days after treatment and renal damage was most severe in wild-type male mice. Wild-type males have approximately 10-fold higher levels of GSTP expression in the liver than females, suggesting that hepatic GSTP in the wild-type males contributed to the formation of the nephrotoxic platinum-glutathione conjugate. In GstP1/P2 null mice the gender difference in toxicity was eliminated. Our data show that GSTP expression is a determinant in cisplatin-induced nephrotoxicity and its levels contribute to sex-dependent differences.

Detection of multiple globin monoadducts and cross-links after in vitro exposure of rat erythrocytes to S-(1,2-dichlorovinyl)-L-cysteine sulfoxide and after in vivo treatment of rats with S-(1,2-dichlorovinyl)-L-cysteine sulfoxide

S-(1,2-dichlorovinyl)- L-cysteine sulfoxide (DCVCS), a Michael acceptor produced by an FMO3-mediated oxidation of the trichloroethylene metabolite S-(1,2-dichlorovinyl)- L-cysteine (DCVC), is a more potent nephrotoxicant than DCVC. Because DCVCS incubations with N-acetyl- L-cysteine at pH 7.4, 37 degrees C resulted in the formation of three diastereomeric monoadducts and one diadduct, globin monoadducts and cross-links formed after in vitro incubations of rat erythrocytes with DCVCS (0.9-450 microM) for 2 h and those present at 30 min after in vivo treatment of rats with DCVCS (23 and 230 micromol/kg) were characterized. ESI/MS of intact globin chains revealed adduction of 1 DCVCS moiety on the beta2 chain at the three lowest DCVCS concentrations and on the beta1 chain after the in vivo treatment with 230 micromol/kg DCVCS. Interestingly, intact globin dimers and trimers were detectable by ESI/MS with all DCVCS concentrations in vitro (also by SDS-PAGE) and in vivo. LC/MS and MALDI/FTICR of trypsin digested peptides from globin samples obtained after in vitro (450 microM DCVCS) or in vivo exposure to DCVCS (230 micromol/kg) suggested the formation of DCVCS monoadducts not only with Cys93 and Cys125 of the beta chains but also with Cys13 of the alpha chains, whereas no monoadducted peptides were detected at lower DCVCS concentrations in vitro or in vivo. However, LC/MS and MALDI-TOF/TOF suggested the presence of several DCVCS-derived peptide cross-links both in vivo and in vitro at all DCVCS exposure levels. Collectively, the results indicate at least 4 out of the 5 cysteine moieties of the rat hemoglobin heterodimer may be alkylated by DCVCS, in reactions that could also lead to the formation of multiple cross-links. DCVCS- and N-acetyl-DCVCS (NA-DCVCS)-derived globin cross-links containing GSH and Cys were also detected by mass spectrometry, providing strong evidence for the reactivity and/or cross-linking ability of DCVCS, NA-DCVCS, and their GSH or Cys conjugates in both the in vitro and the in vivo. Thus, hemoglobin adducts and cross-links may be useful biomarkers to investigate the possible presence of DCVCS in circulation after DCVC or trichloroethylene exposure.

Substrate specificity of human glutamine transaminase K as an aminotransferase and as a cysteine S-conjugate beta-lyase

Rat kidney glutamine transaminase K (GTK) exhibits broad specificity both as an aminotransferase and as a cysteine S-conjugate beta-lyase. The beta-lyase reaction products are pyruvate, ammonium and a sulfhydryl-containing fragment. We show here that recombinant human GTK (rhGTK) also exhibits broad specificity both as an aminotransferase and as a cysteine S-conjugate beta-lyase. S-(1,1,2,2-Tetrafluoroethyl)-l-cysteine is an excellent aminotransferase and beta-lyase substrate of rhGTK. Moderate aminotransferase and beta-lyase activities occur with the chemopreventive agent Se-methyl-l-selenocysteine. l-3-(2-Naphthyl)alanine, l-3-(1-naphthyl)alanine, 5-S-l-cysteinyldopamine and 5-S-l-cysteinyl-l-DOPA are measurable aminotransferase substrates, indicating that the active site can accommodate large aromatic amino acids. The alpha-keto acids generated by transamination/l-amino acid oxidase activity of the two catechol cysteine S-conjugates are unstable. A slow rhGTK-catalyzed beta-elimination reaction, as measured by pyruvate formation, was demonstrated with 5-S-l-cysteinyldopamine, but not with 5-S-l-cysteinyl-l-DOPA. The importance of transamination, oxidation and beta-elimination reactions involving 5-S-l-cysteinyldopamine, 5-S-l-cysteinyl-l-DOPA and Se-methyl-l-selenocysteine in human tissues and their biological relevance are discussed.

Measurement of aminoacylases

The unit describes the assays for aminoacylase activity, with specific emphasis on aminoacylase III. The methods are based on the determination of deacetylated products of the aminoacylase-catalyzed reactions in fluorescence and absorbance assays. The unit also provides methods for isolation of the aminoacylase III from tissues and bacterial or mammalian cells and subsequent purification and analysis.

Formation of three N-acetyl-L-cysteine monoadducts and one diadduct by the reaction of S-(1,2-dichlorovinyl)-L-cysteine sulfoxide with N-acetyl-L-cysteine at physiological conditions: chemical mechanisms and toxicological implications

Previously, our laboratory has shown that S-(1,2-dichlorovinyl)-L-cysteine sulfoxide (DCVCS), a Michael acceptor produced by a flavin-containing monooxygenase 3 (FMO3)-mediated oxidation of S-(1,2-dichlorovinyl)-L-cysteine (DCVC), is a more potent nephrotoxicant than DCVC. In the present study, we characterized reactions of DCVCS with nucleophilic amino acids. DCVCS incubations with N-acetyl-L-cysteine (NAC) at pH 7.4 and 37 degrees C for 1 h resulted in the formation of three monoadducts and one diadduct characterized by LC/MS, 1H NMR, and 1H-detected heteronuclear single quantum correlation. The formation of all adducts (with relative ratios of 29, 31, 24, and 12%, respectively) was rapid and time-dependent; the half-lives of the two DCVCS diastereomers in the presence of NAC were 13.8 (diastereomer I) and 9.4 min (diastereomer II). Adducts 1 and 2 were determined to be diastereomers of S-[1-chloro-2-(N-acetyl-L-cystein- S-yl)vinyl]-L-cysteine sulfoxide formed by Michael addition of NAC to the terminal vinylic carbon of DCVCS followed by loss of HCl. Adduct 4 was determined to be S-[2-chloro-2-(N-acetyl-L-cystein- S-yl)vinyl]-L-cysteine sulfoxide formed from the initial Michael addition product followed by a less favorable loss of HCl and/or by a rearrangement of adduct 2 through the formation of a cyclic chloronium ion. The addition of another molecule of NAC to monoadducts 1, 2, or 4 resulted in the formation of the novel diadduct, S-[2,2-( N-acetyl-L-cystein-S-yl)vinyl]-L-cysteine sulfoxide (adduct 3), whose detection in relatively large amount suggests that DCVCS could act as a cross-linking agent. DCVCS was not reactive with N-acetyl-L-lysine or L-valinamide at similar incubation conditions. Collectively, the results suggest selective reactivity of DCVCS toward protein sulfhydryl groups. Furthermore, the cross-linking properties of DCVCS may in part explain its high nephrotoxic potency.

Drug metabolite profiling and elucidation of drug-induced hepatotoxicity

Drug metabolism studies, together with pathologic and histologic evaluation, provide critical data sets to help understand mechanisms underlying drug-related hepatotoxicity. A common practice is to trace morphologic changes resulting from liver injury back to perturbation of biochemical processes and to identify drug metabolites that affect those processes as possible culprits. This strategy can be illustrated in efforts of elucidating the cause of acetaminophen-, troglitazone- and valproic acid-induced hepatic necrosis, microvesicular steatosis and cholestasis with the aid of information from qualitative and quantitative analysis of metabolites. From a pharmaceutical research perspective, metabolite profiling represents an important function because a structure-activity relationship is essential to rational drug design. In addition, drugs are known to induce idiosyncratic hepatotoxicity, which usually escapes the detection by preclinical safety assessment and clinical trials. This issue is addressed, at present, by eliminating those molecules that are prone to metabolic bioactivation, based on the concept that formation of electrophilic metabolites triggers covalent protein modification and subsequent organ toxicity. Although pragmatic, such an approach has its limitations as a linear correlation does not exist between toxicity and the extent of bioactivation. It may be possible in the future that the advance of proteomics, metabonomics and genomics would pave the way leading to personalized medication in which beneficial effect of a drug is maximized, whereas toxicity risk is minimized.

Structural characterization of dimeric murine aminoacylase III

Aminoacylase III (AAIII) plays an important role in deacetylation of acetylated amino acids and N-acetylated S-cysteine conjugates of halogenated alkenes and alkanes. AAIII, recently cloned from mouse kidney and partially characterized, is a mixture of tetramers and dimers. In the present work, AAIII dimers were purified and shown to be enzymatically active. Limited trypsinolysis showed two domains of approximately 9 and 25 kDa. The three-dimensional structure of the dimer was studied by electron microscopy of negative stained samples and by single-particle reconstruction. A 16A resolution model of the AAIII dimer was created. It has an unusual, cage-like, structure. A realistic AAIII tetramer model was built from two dimers.

Specificity of Aminoacylase III-Mediated Deacetylation of Mercapturic Acids

Trichloroethylene (TCE) and other halogenated alkenes are known environmental contaminants with cytotoxic and nephrotoxic effects, and are potential carcinogens. Their metabolism via the mercapturate metabolic pathway was shown to lead to their detoxification. The final products of this pathway, mercapturic acids or N-acetyl-l-cysteine S-conjugates, are secreted into the lumen in the renal proximal tubule. The proximal tubule may also deacetylate mercapturic acids, and the resulting cysteine S-conjugates are transformed by cysteine S-conjugate beta-lyases to nephrotoxic reactive thiols. The specificity and rate of mercapturic acid deacetylation may determine the toxicity of certain mercapturic acids; however, the exact enzymologic processes involved are not known in detail. In the present study we characterized the kinetics of the recently cloned mouse aminoacylase III (AAIII) toward a wide spectrum of halogenated mercapturic acids and N-acetylated amino acids. In general, the V(max) value of AAIII was significantly larger with chlorinated and brominated mercapturic acids, whereas fluorination significantly decreased it. The enzyme deacetylated mercapturic acids derived from the TCE metabolism including N-acetyl-S-(1,2-dichlorovinyl)-l-cysteine (NA-1,2-DCVC) and N-acetyl-S-(2,2-dichlorovinyl)-l-cysteine (NA-2,2-DCVC). Both mercapturic acids induced cytotoxicity in mouse proximal tubule mPCT cells expressing AAIII, which was decreased by an inhibitor of beta-lyase, aminooxyacetate. The toxic effect of NA-2,2-DCVC was smaller than that of NA-1,2-DCVC, indicating that factors other than the intracellular activity of AAIII mediate the cytotoxicity of these mercapturic acids. Our results indicate that in proximal tubule cells, AAIII plays an important role in deacetylating several halogenated mercapturic acids, and this process may be involved in their cyto- and nephrotoxicity.

Cisplatin-induced toxicity is associated with platinum deposition in mouse kidney mitochondria in vivo and with selective inactivation of the alpha-ketoglutarate dehydrogenase complex in LLC-PK1 cells

The anticancer drug cisplatin is nephrotoxic and neurotoxic. Previous data support the hypothesis that cisplatin is bioactivated to a nephrotoxicant. The final step in the proposed bioactivation is the formation of a platinum-cysteine S-conjugate followed by a pyridoxal 5'-phosphate (PLP)-dependent cysteine S-conjugate beta-lyase reaction. This reaction would generate pyruvate, ammonium, and a highly reactive platinum (Pt)-thiol compound in vivo that would bind to proteins. In this work, the cellular location and identity of the PLP-dependent cysteine S-conjugate beta-lyase were investigated. Pt was shown to bind to proteins in kidneys of cisplatin-treated mice. The concentration of Pt-bound proteins was higher in the mitochondrial fraction than in the cytosolic fraction. Treatment of the mice with aminooxyacetic acid (AOAA, a PLP enzyme inhibitor), which had previously been shown to block the nephrotoxicity of cisplatin, decreased the binding of Pt to mitochondrial proteins but had no effect on the amount of Pt bound to proteins in the cytosolic fraction. These data indicate that a mitochondrial enzyme catalyzes the PLP-dependent cysteine S-conjugate beta-lyase reaction. PLP-dependent mitochondrial aspartate aminotransferase (mitAspAT) is a mitochondrial enzyme that catalyzes beta-elimination reactions with cysteine S-conjugates of halogenated alkenes. We reasoned that the enzyme might also catalyze a beta-lyase reaction with the cisplatin-cysteine S-conjugate. In this study, mitAspAT was stably overexpressed in LLC-PK(1) cells. Cisplatin was significantly more toxic in confluent monolayers of LLC-PK(1) cells that overexpressed mitAspAT than in control cells containing vector alone. AOAA completely blocked the cisplatin toxicity in confluent mitAspAT-transfected cells. The Pt-thiol compound could rapidly bind proteins and inactivate enzymes in close proximity of the PLP-dependent cysteine S-conjugate beta-lyase. Treatment with 50 or 100 microM cisplatin for 3 h, followed by removal of cisplatin from the medium for 24 h, resulted in a pronounced loss of alpha-ketoglutarate dehydrogenase complex (KGDHC) activity in both mitAspAT-transfected cells and control cells. Exposure to 100 microM cisplatin resulted in a significantly greater loss of KGDHC activity in the cells overexpressing mitAspAT than in control cells. Aconitase activity was diminished in both cell types, but only at the higher level of exposure to cisplatin. AspAT activity was also significantly decreased by cisplatin treatment. By contrast, several other enzymes (both cytosolic and mitochondrial) involved in energy/amino acid metabolism were not significantly affected by cisplatin treatment in the LLC-PK(1) cells, whether or not mitAspAT was overexpressed. The susceptibility of KGDHC and aconitase to inactivation in kidney cells exposed to cisplatin metabolites may be due to the proximity of mitAspAT to KGDHC and aconitase in mitochondria. These findings support the hypothesis that a mitochondrial cysteine S-conjugate beta-lyase converts the cisplatin-cysteine S-conjugate to a toxicant, and the data are consistent with the hypothesis that mitAspAT plays a role in the bioactivation of cisplatin.

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.

Contribution of acetaminophen-cysteine to acetaminophen nephrotoxicity in CD-1 mice: I. Enhancement of acetaminophen nephrotoxicity by acetaminophen-cysteine

Acetaminophen (APAP) nephrotoxicity has been observed both in humans and research animals. Recent studies suggest a contributory role for glutathione (GSH)-derived conjugates of APAP in the development of nephrotoxicity. Inhibitors of either gamma-glutamyl transpeptidase (gamma-GT) or the probenecid-sensitive organic anion transporter ameliorate APAP-induced nephrotoxicity but not hepatotoxicity in mice and inhibition of gamma-GT similarly protected rats from APAP nephrotoxicity. Protection against APAP nephrotoxicity by disruption of these GSH conjugate transport and metabolism pathways suggests that GSH conjugates are involved. APAP-induced renal injury may involve the acetaminophen-glutathione (APAP-GSH) conjugate or a metabolite derived from APAP-GSH. Acetaminophen-cysteine (APAP-CYS) is a likely candidate for involvement in APAP nephrotoxicity because it is both a product of the gamma-GT pathway and a probable substrate for the organic anion transporter. The present experiments demonstrated that APAP-CYS treatment alone depleted renal but not hepatic glutathione (GSH) in a dose-responsive manner. This depletion of renal GSH may predispose the kidney to APAP nephrotoxicity by diminishing GSH-mediated detoxification mechanisms. Indeed, pretreatment of male CD-1 mice with APAP-CYS before challenge with a threshold toxic dose of APAP resulted in significant enhancement of APAP-induced nephrotoxicity. This was evidenced by histopathology and plasma blood urea nitrogen (BUN) levels at 24 h after APAP challenge. APAP alone was minimally nephrotoxic and APAP-CYS alone produced no detectable injury. By contrast, APAP-CYS pretreatment did not alter the liver injury induced by APAP challenge. These data are consistent with there being a selective, contributory role for APAP-GSH-derived metabolites in APAP-induced renal injury that may involve renal-selective GSH depletion.

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.

The role of glutamine transaminase K (GTK) in sulfur and alpha-keto acid metabolism in the brain, and in the possible bioactivation of neurotoxicants

Glutamine transaminase K (GTK), which is a freely reversible glutamine (methionine) aromatic amino acid aminotransferase, is present in most mammalian tissues, including brain. Quantitatively, the most important amine donor in vivo is glutamine. The product of glutamine transamination (i.e., alpha-ketoglutaramate; alphaKGM) is rapidly removed by cyclization and/or conversion to alpha-ketoglutarate. Transamination is therefore "pulled" in the direction of glutamine utilization. Major biological roles of GTK are to maintain low levels of phenylpyruvate and to close the methionine salvage pathway. GTK also catalyzes the transamination of cystathionine, lanthionine, and thialysine to the corresponding alpha-keto acids, which cyclize to ketimines. The cyclic ketimines and several metabolites derived therefrom are found in brain. It is not clear whether these compounds have a biological function or are metabolic dead-ends. However, high-affinity binding of lanthionine ketimine (LK) to brain membranes has been reported. Mammalian tissues possess several enzymes capable of catalyzing transamination of kynurenine in vitro. Two of these kynurenine aminotransferases (KATs), namely KAT I and KAT II, are present in brain and have been extensively studied. KAT I and KAT II are identical to GTK and alpha-aminoadipate aminotransferase, respectively. GTK/KAT I is largely cytosolic in kidney, but mostly mitochondrial in brain. The same gene codes for both forms, but alternative splicing dictates whether a 32-amino acid mitochondrial-targeting sequence is present in the expressed protein. The activity of KAT I is altered by a missense mutation (E61G) in the spontaneously hypertensive rat. The symptoms may be due in part to alteration of kynurenine transamination. However, owing to strong competition from other amino acid substrates, the turnover of kynurenine to kynurenate by GTK/KAT I in nervous tissue must be slow unless kynurenine and GTK are sequestered in a compartment distinct from the major amino acid pools. The possibility is discussed that the spontaneous hypertension in rats carrying the GTK/KAT I mutation may be due in part to disruption of glutamine transamination. GTK is one of several pyridoxal 5'-phosphate (PLP)-containing enzymes that can catalyze non-physiological beta-elimination reactions with cysteine S-conjugates containing a good leaving group attached at the sulfur. These elimination reactions may contribute to the bioactivation of certain electrophiles, resulting in toxicity to kidney, liver, brain, and possibly other organs. On the other hand, the beta-lyase reaction catalyzed by GTK may be useful in the conversion of some cysteine S-conjugate prodrugs to active components in vivo. The roles of GTK in (a) brain nitrogen, sulfur, and aromatic amino acid/kynurenine metabolism, (b) brain alpha-keto acid metabolism, (c) bioactivation of certain electrophiles in brain, (d) prodrug targeting, and (e) maintenance of normal blood pressure deserve further study.

Structural characterization, tissue distribution, and functional expression of murine aminoacylase III

Many xenobiotics are detoxified through the mercapturate metabolic pathway. The final product of the pathway, mercapturic acids ( N-acetylcysteine S-conjugates), are secreted predominantly by renal proximal tubules. Mercapturic acids may undergo a transformation mediated by aminoacylases and cysteine S-conjugate β-lyases that leads to nephrotoxic reactive thiol formation. The deacetylation of cysteine S-conjugates of N-acyl aromatic amino acids is thought to be mediated by an aminoacylase whose molecular identity has not been determined. In the present study, we cloned aminoacylase III, which likely mediates this process in vivo, and characterized its function and structure. The enzyme consists of 318 amino acids and has a molecular mass (determined by SDS-PAGE) of ∼35 kDa. Under nondenaturing conditions, the molecular mass of the enzyme is ∼140 kDa as determined by size-exclusion chromatography, which suggests that it is a tetramer. In agreement with this hypothesis, transmission electron microscopy and image analysis of aminoacylase III showed that the monomers of the enzyme are arranged with a fourfold rotational symmetry. Northern analysis demonstrated an ∼1.4-kb transcript that was expressed predominantly in kidney and showed less expression in liver, heart, small intestine, brain, lung, testis, and stomach. In kidney, aminoacylase III was immunolocalized predominantly to the apical domain of S1 proximal tubules and the cytoplasm of S2 and S3 proximal tubules. The data suggest that in kidney proximal tubules, aminoacylase III plays an important role in deacetylating mercapturic acids. The predominant cytoplasmic localization of aminoacylase III may explain the greater sensitivity of the proximal straight tubule to the nephrotoxicity of mercapturic acids.

The contributions of excitotoxicity, glutathione depletion and DNA repair in chemically induced injury to neurones: exemplified with toxic effects on cerebellar granule cells

Six chemicals, 2-halopropionic acids, thiophene, methylhalides, methylmercury, methylazoxymethanol (MAM) and trichlorfon (Fig. 1), that cause selective necrosis to the cerebellum, in particular to cerebellar granule cells, have been reviewed. The basis for the selective toxicity to these neurones is not fully understood, but mechanisms known to contribute to the neuronal cell death are discussed. All six compounds decrease cerebral glutathione (GSH), due to conjugation with the xenobiotic, thereby reducing cellular antioxidant status and making the cells more vulnerable to reactive oxygen species. 2-Halopropionic acids and methylmercury appear to also act via an excitotoxic mechanism leading to elevated intracellular Ca2+, increased reactive oxygen species and ultimately impaired mitochondrial function. In contrast, the methylhalides, trichlorfon and MAM all methylate DNA and inhibit O6-guanine-DNA methyltransferase (OGMT), an important DNA repair enzyme. We propose that a combination of reduced antioxidant status plus excitotoxicity or DNA damage is required to cause cerebellar neuronal cell death with these chemicals. The small size of cerebellar granule cells, the unique subunit composition of their N-methyl-d-aspartate (NMDA) receptors, their low DNA repair ability, low levels of calcium-binding proteins and vulnerability during postnatal brain development and distribution of glutathione and its conjugating and metabolizing enzymes are all important factors in determining the sensitivity of cerebellar granule cells to toxic compounds.

Mutagenicity of the glutathione and cysteine S-conjugates of the haloalkenes 1,1,2-trichloro-3,3,3-trifluoro-1-propene and trichlorofluoroethene in the Ames test in comparison with the tetrachloroethene-analogues

The nephrotoxic and nephrocarcinogenic potential of the haloalkenes is associated with the conjugation of the chemicals to L-glutathione. Subsequent processing of the haloalkene glutathione S-conjugates via the cysteine conjugate beta-lyase pathway in the mammalian kidney yields nephrotoxic and mutagenic species. To investigate whether S-conjugates of the model chlorofluoroalkenes 1,1,2-trichloro-3,3,3-trifluoro-1-propene (CAS # 431-52-7) and trichlorofluoroethene (CAS # 359-29-5) show comparable effects, we have synthesised the respective cysteine and glutathione S-conjugates and subjected them to the Ames test. The cysteine and glutathione S-conjugates of tetrachloroethene (CAS # 127-18-4), S-(1,2,2-trichlorovinyl)-L-cysteine (TCVC) and S-(1,2,2-trichlorovinyl)glutathione (TCVG) were used as positive controls and reference substances. S-(1,2-dichloro-3,3,3-trifluoro-1-propenyl)-L-cysteine (DCTFPC) and S-(2,2-dichloro-1-fluorovinyl)-L-cysteine (DCFVC) showed clear dose-dependent mutagenic effects with the Salmonella typhimurium tester strains TA100 and TA98. Using TCVC as a reference substance the following ranking in mutagenic response was established: TCVC>DCTFPC>DCFVC. S-(1,2-dichloro-3,3,3-trifluoro-1-propenyl)glutathione (DCTFPG) and S-(2,2-dichloro-1-fluorovinyl)glutathione (DCFVG) showed potent dose-dependent mutagenic effects with the S. typhimurium tester strain TA100 in the presence of a rat kidney S9-protein fraction; tests carried out in the absence of the bioactivation system resulted only in background rates of revertants. Using TCVG as a reference substance the following ranking in mutagenic response was established: TCVG=DCTFPG>DCFVG. The data obtained provide a basis for further studies on the mutagenic and presumable carcinogenic potential of the substances.

High pressure liquid chromatography and mass spectrometry characterization of the nephrotoxic biotransformation products of Cisplatin

Previous studies have shown that cisplatin requires metabolic activation to become nephrotoxic. The activation is proposed to be via the metabolism of a glutathione-platinum conjugate to a cysteinyl-glycine-platinum conjugate, which is further processed to a cysteine conjugate. Preincubating cisplatin with glutathione (GSH), cysteinyl-glycine, or N-acetylcysteine (NAC) results in a transient increase in the toxicity of cisplatin toward renal proximal tubular cells. In this study, the preincubation solutions were analyzed by high pressure liquid chromatography (HPLC), atomic absorption spectrometry, and mass spectrometry (MS) to characterize the formation and structure of the platinum conjugates. HPLC analysis of the cisplatin-GSH, cisplatin-cysteinyl-glycine, and cisplatin-NAC preincubation solutions revealed two new platinum-containing peaks in each of the solutions. MS-MS analysis of the peaks revealed a diplatinum- and a monoplatinum conjugate in each of the solutions. Analysis of the composition and toxicity of the solutions with time showed that the transient increase in toxicity correlated with the formation of the monoplatinum conjugate whereas prolonged preincubation decreased toxicity and correlated with the formation of the diplatinum conjugate. The monoplatinum-monoglutathione conjugate is a substrate for gamma-glutamyl transpeptidase, an enzyme that is essential for the nephrotoxicity of cisplatin. The monoplatinum-mono-NAC conjugate can be deacetylated to a cysteine conjugate, which is a substrate for pyroxidol phosphate (PLP)-dependent cysteine S-conjugate beta-lyase. This PLP-dependent enzyme is proposed to catalyze the final step in the metabolic activation of cisplatin. Identification of the structure and toxicity of these conjugates further elucidates the metabolism of cisplatin to a nephrotoxin.

Pharmacokinetics and metabolism of a COX-2 inhibitor, valdecoxib, in mice

The pharmacokinetics and metabolism of valdecoxib, a potent cyclooxygenase-2 selective inhibitor, were investigated in mice. Valdecoxib was extensively metabolized after a single 5 mg/kg oral administration of [(14)C]valdecoxib and elimination of unchanged drug was minor (less than 1%) in male and female mice. The total mean percentage of administered radioactive dose recovered was 99.8% in the male mice and 94.7% in the female mice. Sixteen metabolites were identified in mouse plasma, red blood cells, urine, and feces. The main phase I metabolic pathway of valdecoxib in mice involved the oxidation of the 5-methyl group to form the active hydroxymethyl metabolite M1. M1 was further oxidized to the carboxylic acid metabolite M4, which underwent opening of the isoxazole ring to form M6 and M13. Phase II metabolism included glucuronide, glucoside, and methyl sulfone conjugations. M1 was also conjugated with glucuronic acid and glucose to yield M-G and M1-glucose, respectively. Three novel methylsulfone conjugates M20, M21, and M21-G were detected in blood or urine. Valdecoxib and M1 were the major radioactive components in plasma and red blood cells. The plasma area under the curve from zero to infinity (AUC(0-infinity)) values for valdecoxib and M1 were 3.58 and 0.850 microg. h/ml in males and 2.08 and 1.63 microg. h/ml in females, respectively. The RBC AUC(0-infinity) values for valdecoxib and M1 were 12.1 and 22.6 microg. h/g in males and 6.42 and 35.2 microg. h/g in females, respectively.

Metabolism of Cisplatin to a nephrotoxin in proximal tubule cells

Cisplatin, a commonly used chemotherapeutic agent, is nephrotoxic. The mechanism by which cisplatin selectively kills the proximal tubule cells was heretofore unknown. Recent studies in mice and rats have shown that the nephrotoxicity of cisplatin can be blocked by acivicin or (aminooxy)acetic acid, the same enzyme inhibitors that block the metabolic activation of a series of nephrotoxic halogenated alkenes. In this study, it was hypothesized that cisplatin is activated in the kidney to a toxic metabolite through the same pathway that has been shown to activate the halogenated alkenes. This activation begins with the formation of a glutathione-conjugate that is metabolized to a cysteinyl-glycine-conjugate, to a cysteine-conjugate, and finally to a reactive thiol. In this study, a protocol was developed in which confluent monolayers of LLC-PK(1) cells were exposed to clinically relevant concentrations of cisplatin or cisplatin-conjugate for 3 h. Cell viability was assayed at 72 h. The role of gamma-glutamyl transpeptidase (GGT) and cysteine-S-conjugate beta-lyase in the metabolism of each of the cisplatin-conjugates was investigated. Pre-incubation of cisplatin with glutathione, cysteinyl-glycine, or N-acetyl-cysteine to allow for the spontaneous formation of cisplatin-conjugates increased the toxicity of cisplatin toward LLC-PK(1) cells. Inhibition of GGT activity showed that GGT was necessary only for the toxicity of the cisplatin-glutathione-conjugate. Inhibition of cysteine-S-conjugate beta-lyase reduced the toxicity of each of the cisplatin-conjugates. These data demonstrate that metabolism of cisplatin in proximal tubule cells is required for its nephrotoxicity. The elucidation of this pathway provides new targets for the inhibition of cisplatin nephrotoxicity.

Mitochondrial aspartate aminotransferase catalyses cysteine S-conjugate beta-lyase reactions

Rat liver mitochondrial aspartate aminotransferase (a homodimer) was shown to catalyse a beta-lyase reaction with three nephrotoxic halogenated cysteine S-conjugates [ S -(1,1,2,2-tetrafluoroethyl)-L-cysteine, S -(1,2-dichlorovinyl)-L-cysteine and S -(2-chloro-1,1,2-trifluoroethyl)-L-cysteine], and less effectively so with a non-toxic cysteine S-conjugate [benzothiazolyl-L-cysteine]. Transamination competes with the beta-lyase reaction, but is not favourable. The ratio of beta elimination to transamination in the presence of S -(1,1,2,2-tetrafluoroethyl)-L-cysteine and 2-oxoglutarate is >100. Syncatalytic inactivation by the halogenated cysteine S-conjugates is also observed. The enzyme turns over approx. 2700 molecules of halogenated cysteine S-conjugate on average for every monomer inactivated. Kidney mitochondria are known to be especially sensitive to toxic halogenated cysteine S-conjugates. Evidence is presented that 15-20% of the cysteine S-conjugate beta-lyase activity towards S -(1,1,2,2-tetrafluoroethyl)-L-cysteine in crude kidney mitochondrial homogenates is due to mitochondrial aspartate aminotransferase. The possible involvement of mitochondrial aspartate aminotransferase in the toxicity of halogenated cysteine S-conjugates is also discussed.

Toxic, halogenated cysteine S-conjugates and targeting of mitochondrial enzymes of energy metabolism

Several haloalkenes are metabolized in part to nephrotoxic cysteine S-conjugates; for example, trichloroethylene and tetrafluoroethylene are converted to S-(1,2-dichlorovinyl)-L-cysteine (DCVC) and S-(1,1,2,2-tetrafluoroethyl)-L-cysteine (TFEC), respectively. Although DCVC-induced toxicity has been investigated since the 1950s, the toxicity of TFEC and other haloalkene-derived cysteine S-conjugates has been studied more recently. Some segments of the US population are exposed to haloalkenes either through drinking water or in the workplace. Therefore, it is important to define the toxicological consequences of such exposures. Most halogenated cysteine S-conjugates are metabolized by cysteine S-conjugate beta-lyases to pyruvate, ammonia, and an alpha-chloroenethiolate (with DCVC) or an alpha-difluoroalkylthiolate (with TFEC) that may eliminate halide to give a thioacyl halide, which reacts with epsilon-amino groups of lysine residues in proteins. Nine mammalian pyridoxal 5'-phosphate (PLP)-containing enzymes catalyze cysteine S-conjugate beta-lyase reactions, including mitochondrial aspartate aminotransferase (mitAspAT), and mitochondrial branched-chain amino acid aminotransferase (BCAT(m)). Most of the cysteine S-conjugate beta-lyases are syncatalytically inactivated. TFEC-induced toxicity is associated with covalent modification of several mitochondrial enzymes of energy metabolism. Interestingly, the alpha-ketoglutarate- and branched-chain alpha-keto acid dehydrogenase complexes (KGDHC and BCDHC), but not the pyruvate dehydrogenase complex (PDHC), are susceptible to inactivation. mitAspAT and BCAT(m) may form metabolons with KGDHC and BCDHC, respectively, but no PLP enzyme is known to associate with PDHC. Consequently, we hypothesize that not only do these metabolons facilitate substrate channeling, but they also facilitate toxicant channeling, thereby promoting the inactivation of proximate mitochondrial enzymes and the induction of mitochondrial dysfunction.

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 conjugation as a bioactivation reaction

In general, glutathione conjugation is regarded as a detoxication reaction. However, depending on the properties of the substrate, bioactivation is also possible. Four types of activation reaction have been recognized: direct-acting compounds, conjugates that are activated through cysteine conjugate beta-lyase, conjugates that are activated through redox cycling and lastly conjugates that release the original reactive parent compound. The glutathione S-transferases have three connections with the formation of biactivated conjugates: they catalyze their formation in a number of cases, they are the earliest available target for covalent binding by these conjugates and lastly, the parent alkylating agents are regularly involved in the induction of the enzymes. Individual susceptibility for each of these agents is determined by individual transferase subunit composition and methods are becoming available to assess this susceptibility.

Ascorbic acid promotes recovery of cellular functions following toxicant-induced injury

We have shown that renal proximal tubular cells (RPTC) recover cellular functions following sublethal injury induced by the oxidant t-butylhydroperoxide but not by the nephrotoxic cysteine conjugate dichlorovinyl-L-cysteine (DCVC). This study investigated whether L-ascorbic acid phosphate (AscP) promotes recovery of RPTC functions following DCVC-induced injury. DCVC exposure (200 microM; 100 min) resulted in 60% RPTC death and loss from the monolayer at 24 h independent of physiological (50 microM) or pharmacological (500 microM) AscP concentrations. Likewise, the DCVC-induced decrease in mitochondrial function (54%), active Na(+) transport (66%), and Na(+)-K(+)-ATPase activity (77%) was independent of the AscP concentration. Analysis of Na(+)-K(+)-ATPase protein expression and distribution in the plasma membrane using immunocytochemistry and confocal laser scanning microscopy revealed the loss of Na(+)-K(+)-ATPase protein from the basolateral membrane of RPTC treated with DCVC. DCVC-injured RPTC cultured in the presence of 50 microM AscP did not proliferate nor recover their physiological functions over time. In contrast, RPTC cultured in the presence of 500 microM AscP proliferated, recovered all examined physiological functions, and the basolateral membrane expression of Na(+)-K(+)-ATPase by day 4 following DCVC injury. These results demonstrate that pharmacological concentrations of AscP do not prevent toxicant-induced cell injury and death but promote complete recovery of mitochondrial function, active Na(+) transport, and proliferation following toxicant-induced injury.

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.

Purification and characterisation of a novel cysteine conjugate beta-lyase from the tapeworm Moniezia expansa

The paper presents the first report of the purification of an invertebrate cysteine conjugate beta-lyase (CCBL). CCBL activity was shown to predominate within the cytosolic fraction of tissue from the tapeworm Moniezia expansa. The monomeric cytosolic enzyme was isolated with a M(r) of 72 kDa and co-purified with transaminase activity towards L-aspartate. The substrate profile for M. expansa CCBL is different from that of mammalian CCBLs. Exploiting the differences in mammalian and parasite substrate profiles will facilitate the development of helminth targeted conjugates which will not be activated by host (mammalian) CCBLs.