Isolation of a mouse theta glutathione S-transferase active with methylene chloride

Development of a physiologically based pharmacokinetic (PBPK) model for methyl iodide in rats, rabbits, and humans

Methyl iodide (MeI) has been proposed as an alternative to methyl bromide as a pre-plant soil fumigant that does not deplete stratospheric ozone. In inhalation toxicity studies performed in animals as part of the registration process, three effects have been identified that warrant consideration in developing toxicity reference values for human risk assessment: nasal lesions (rat), acute neurotoxicity (rat), and fetal loss (rabbit). Uncertainties in the risk assessment can be reduced by using an internal measure of target tissue dose that is linked to the likely mode of action (MOA) for the toxicity of MeI, rather than the external exposure concentration. Physiologically based pharmacokinetic (PBPK) models have been developed for MeI and used to reduce uncertainties in the risk assessment extrapolations (e.g. interspecies, high to low dose, exposure scenario). PBPK model-derived human equivalent concentrations comparable to the animal study NOAELs (no observed adverse effect levels) for the endpoints of interest were developed for a 1-day, 24-hr exposure of bystanders or 8 hr/day exposure of workers. Variability analyses of the PBPK models support application of uncertainty factors (UF) of approximately 2 for intrahuman pharmacokinetic variability for the nasal effects and acute neurotoxicity.

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.

Induction of Hepatic CYP2E1 by a Subtoxic Dose of Acetaminophen in Rats: Increase in Dichloromethane Metabolism and Carboxyhemoglobin Elevation

Dichloromethane (DCM) is metabolically converted to carbon monoxide mostly by CYP2E1 in liver, resulting in elevation of blood carboxyhemoglobin (COHb) levels. We investigated the effects of a subtoxic dose of acetaminophen (APAP) on the metabolic elimination of DCM and COHb elevation in adult female rats. APAP, at 500 mg/kg i.p., was not hepatotoxic as measured by a lack of change in serum aspartate aminotransferase, alanine aminotransferase, and sorbitol dehydrogenase activities. In rats pretreated with APAP at this dose, the COHb elevation resulting from administration of DCM (3 mmol/kg i.p.) was enhanced significantly. Also blood DCM levels were reduced, and its disappearance from blood appeared to be increased. Hepatic CYP2E1-mediated activities measured with chlorzoxazone, p-nitrophenol, and p-nitroanisole as substrates were all induced markedly in microsomes of rats treated with APAP. Aminopyrine N-demethylase activity was also increased slightly, but significantly. Western blot analysis showed that APAP treatment induced the expression of CYP2E1 and CYP3A proteins. Neither hepatic glutathione contents nor glutathione S-transferase activity was changed by the dose of APAP used. The results indicate that, contrary to the well known hepatotoxic effects of this drug at large doses, a subtoxic dose of APAP may induce CYP2E1, and to a lesser degree, CYP3A expression. This is the first report that APAP can increase cytochrome P450 (P450)-mediated hepatic metabolism and the resulting toxicity of a xenobiotic in the whole animal. The pharmacological/toxicological significance of induction of P450s by a subtoxic dose of APAP is discussed.

Application of Physiologically Based Pharmacokinetic Modeling in Setting Acute Exposure Guideline Levels for Methylene Chloride

Acute exposure guideline levels (AEGLs) are derived to protect the human population from adverse health effects in case of single exposure due to an accidental release of chemicals into the atmosphere. AEGLs are set at three different levels of increasing toxicity for exposure durations ranging from 10 min to 8 h. In the AEGL setting for methylene chloride, specific additional topics had to be addressed. This included a change of relevant toxicity endpoint within the 10-min to 8-h exposure time range from central nervous system depression caused by the parent compound to formation of carboxyhemoglobin (COHb) via biotransformation to carbon monoxide. Additionally, the biotransformation of methylene chloride includes both a saturable step as well as genetic polymorphism of the glutathione transferase involved. Physiologically based pharmacokinetic modeling was considered to be the appropriate tool to address all these topics in an adequate way. Two available PBPK models were combined and extended with additional algorithms for the estimation of the maximum COHb levels. The model was validated and verified with data obtained from volunteer studies. It was concluded that all the mentioned topics could be adequately accounted for by the PBPK model. The AEGL values as calculated with the model were substantiated by experimental data with volunteers and are concluded to be practically applicable.

Analysis of the Kinetic Mechanism of Haloalkane Conjugation by Mammalian θ-Class Glutathione Transferases

Glutathione (GSH) transferases (GSTs) catalyze the conjugation of small haloalkanes with GSH. In the case of dihalomethanes and vic-1,2-dihaloalkanes, the reaction leads to the formation of genotoxic GSH conjugates. A generally established feature of the reaction of the mammalian theta-class GSTs, which preferentially catalyze these reactions, is the lack of saturability of the rate with regard to the substrate concentration. However, the bacterial GST DM11 catalyzes the same reactions with a relatively low K(m). Recently, DM11 has been shown to exhibit burst kinetics, with a rate-determining k(off) rate for product (Stourman et al. (2003) Biochemistry 42, 11048-11056). We examined rat GST 5-5 and human GST T1-1 and did not detect any burst kinetics in the conjugation of C(2)H(5)Cl, CH(2)Br(2), or CH(2)Cl(2), distinguishing these enzymes from GST DM11. The kinetic results were fit to a minimal mechanism in which the rate-limiting step is halide displacement. The differences in the steady state kinetics of conjugations catalyzed by bacterial GST DM11 and the mammalian GSTs 5-5 and T1-1 are concluded to be the result of differences in the rate-limiting steps and not to inherent enzyme affinity for the haloalkanes. The results may be interpreted in the context of a model in which the halide order affects the rate of carbon-halogen bond cleavage of all such reactions catalyzed by the GSTs. With GST DM11, the halide order is manifested in the K(m) parameter but not k(cat). With mammalian GSTs, the high K(m) is difficult to estimate. With all of the GSTs, the halide order is seen in the enzyme efficiency, k(cat)/K(m), with C-Br cleavage approximately 10-fold faster than C-Cl cleavage. The ratio k(cat)/K(m) is the most relevant parameter for issues of risk assessment.

Genetic polymorphisms in assessing interindividual variability in delivered dose

Increasing sophistication in methods used to account for human variability in susceptibility to toxicants has been one of the success stories in the continuing evolution of risk assessment science. Genetic polymorphisms have been suggested as an important contributor to overall human variability. Recently, data on polymorphisms in metabolic enzymes have been integrated with physiologically based pharmacokinetic (PBPK) modeling as an approach to determining the resulting overall variability. We present an analysis of the potential contribution of polymorphisms in enzymes modulating the disposition of four diverse compounds: methylene chloride, warfarin, parathion, and dichloroacetic acid. Through these case studies, we identify key uncertainties likely to be encountered in the use of polymorphism data and highlight potential simplifying assumptions that might be required to test the hypothesis that genetic factors are a substantive source of human variability in susceptibility to environmental toxicants. These uncertainties include (1) the relative contribution of multiple enzyme systems, (2) the extent of induction/inhibition through coexposure, (3) allelic frequencies of major ethnic groups, (4) the absence of chemical-specific data on the kinetic parameters for the different allelic forms of key enzymes, (5) large numbers of low-frequency alleles, and (6) uncertainty regarding differences between in vitro and in vivo kinetic data. Our effort sets the stage for the acquisition of critical data and further integration of polymorphism data with PBPK modeling as a means to quantitate population variability.

Role of glutathione in metabolic degradation of dichloromethane in rats

Dichloromethane (DCM) elimination and carboxyhemoglobin (COHb) generation were examined in adult female SD rats pretreated with a glutathione (GSH) depletor(s). Rats were treated with either buthionine sulfoximine (BSO; 2 mmol/kg, i.p.), diethylmaleate (DEM; 3 mmol/kg, i.p.), phorone (PHO; 1 mmol/kg, i.p.) or BSO plus PHO (BSO; 2 mmol/kg +PHO; 0.5 mmol/kg, i.p.). The hepatic GSH concentration was significantly reduced by each treatment. Decrease in hepatic GSH was maintained at least for 10 h after BSO treatment but recovered rapidly in rats treated with DEM or PHO. The hepatic p-nitrophenol hydroxylase activity was not affected by the GSH depletors at the dose used in this study. Rats were treated with an i.p. injection of DCM (3 mmol/kg) and the concentrations of DCM and the COHb levels in blood were monitored. In rats pretreated with a GSH depletor, the peak COHb level was significantly greater than that of rats treated with DCM only. The peak COHb level attained in each group of rats appeared to be inversely related to the magnitude of reduction in hepatic GSH levels. The half-life of DCM in blood was also increased in rats pretreated with the GSH depletor(s). The results indicate that the GSH-dependent metabolic reaction has an important role in the overall elimination of DCM as well as in the metabolic generation of carbon monoxide (CO) from this solvent.

Direct comparison of the nature of mouse and human GST T1-1 and the implications on dichloromethane carcinogenicity

Dichloromethane (DCM) is a hepatic and pulmonary carcinogen in mice exposed to high doses by inhalation. It has been shown previously that the incidence of liver and lung tumors does not increase in rats or hamsters exposed to the dihaloalkane under conditions similar to those that produced tumors in mice. The biological consequences of DCM exposure to humans is therefore uncertain. The carcinogenic effects of DCM in the mouse are caused by the interaction with DNA of a glutathione (GSH) conjugate that is produced by the class theta glutathione S-transferase T1-1 (GST T1-1). The species specificity is thought to be due to the greater amount of transferase activity in mouse target organs and specific nuclear localization of GST T1-1 in target cells. This paper directly compares the relative capacity and locality of DCM activation in mouse and human tissues. The results show that mouse GST T1-1 is more efficient in catalyzing the conjugation of DCM with GSH than the orthologous human enzyme. In addition, the mouse expresses higher levels of the transferase than humans in hepatic tissue. Histochemical analysis confirmed the presence of GST T1-1 in the nucleus of mouse liver cells. However, in human liver GST T1-1 was detected in bile duct epithelial cells and hepatocyte nuclei but was also present in the cytoplasm. Taking this information into account, it is unlikely that humans have a sufficiently high capacity to activate DCM for this compound to be considered to represent a carcinogenic risk.

Dichloromethane mediated in vivo selection and functional characterization of rat glutathione S-transferase theta 1-1 variants

Methylobacterium dichloromethanicum DM4 is able to grow with dichloromethane as the sole carbon and energy source by using a dichloromethane dehalogenase/glutathione S-transferase (GST) for the conversion of dichloromethane to formaldehyde. Mammalian homologs of this bacterial enzyme are also known to catalyze this reaction. However, the dehalogenation of dichloromethane by GST T1-1 from rat was highly mutagenic and toxic to methylotrophic bacteria. Plasmid-driven expression of rat GST T1-1 in strain DM4-2cr, a mutant of strain DM4 lacking dichloromethane dehalogenase, reduced cell viability 10(5)-fold in the presence of dichloromethane. This effect was exploited to select dichloromethane-resistant transconjugants of strain DM4-2cr carrying a plasmid-encoded rGSTT1 gene. Transconjugants that still expressed the GST T1 protein after dichloromethane treatment included rGSTT1 mutants encoding protein variants with sequence changes from the wild-type ranging from single residue exchanges to large insertions and deletions. A structural model of rat GST T1-1 suggested that sequence variation was clustered around the glutathione activation site and at the protein C-terminus believed to cap the active site. The enzymatic activity of purified His-tagged GST T1-1 variants expressed in Escherichia coli was markedly reduced with both dichloromethane and the alternative substrate 1,2-epoxy-3-(4'-nitrophenoxy)propane. These results provide the first experimental evidence for the involvement of Gln102 and Arg107 in catalysis, and illustrate the potential of in vivo approaches to identify catalytic residues in GSTs whose activity leads to toxic effects.

Mammalian class theta GST and differential susceptibility to carcinogens: a review

Glutathione S-transferases (GSTs) are an important part of the cellular detoxification system and, perhaps, evolved to protect cells against reactive oxygen metabolites. Theta is considered the most ancient among the GSTs and theta-like GSTs are found in mammals, fish, insects, plants, unicellular algae, and bacteria. It is thought that an ancestral theta-gene underwent an early duplication before the divergence of fungi and animals and further duplications generated the variety of the other classes of GSTs (alpha, mu, phi, etc.). The comparison of the aminoacidic homologies among mammals suggests that a duplication of an ancient GST theta occurred before the speciation of mammals and resulted in the subunits GSTT1 and GSTT2. The ancestral GST theta has a dehalogenase activity towards several halogenated compounds, such as the dichloromethane. In fact, some aerobic and anaerobic methylotrophic bacteria can use these molecules as the sole carbon and energy source. The mammalian GST theta cannot sustain the growth of bacteria but still retains the dehalogenating activity. Therefore, although mammalian GST theta behaves as a scavenger towards electrophiles, such as epoxides, it acts also as metabolic activator for halogenated compounds, producing a variety of intermediates potentially dangerous for DNA and cells. For example, mice exposed to dichloromethane show a dose-dependent incidence of cancer via the GSTT1-1 pathway. Because GSTT1-1 is polymorphic in humans, with about 20% of Caucasians and 80% of Asians lacking the enzyme, the relationship between the phenotype and the incidence of cancer has been investigated extensively in order to detect GSTT1-1-associated differential susceptibility towards endogenous or exogenous carcinogens. The lack of the enzyme is related to a slightly increased risk of cancer of the bladder, gastro-intestinal tract, and for tobacco-related tumors (lung or oral cavity). More pronounced risks were found in males with the GSTT1-null genotype for brain diseases and skin basal cell carcinomas not related to sunlight exposures. Moreover, there was an increased risk of kidney and liver tumors in humans with the GSTT1-1 positive genotype following exposures to halogenated solvents. Interestingly, the liver and kidney are two organs that express the highest level of GST theta in the human body. Thus, the GSTT1-1 genotype is suspected to confer decreased or increased risk of cancer in relation to the source of exposure; in vitro studies, mostly conducted on metabolites of butadiene, confirm the protective action of GSTT1-1, whereas, thus far, experimental studies prove that the increasing risk is limited.

Comparison of cytogenetic effects of 3,4-epoxy-1-butene and 1,2:3, 4-diepoxybutane in mouse, rat and human lymphocytes following in vitro G0 exposures

To understand better the species differences in carcinogenicity caused by 1,3-butadiene (BD), we exposed G0 lymphocytes (either splenic or peripheral blood) from rats, mice and humans to 3, 4-epoxy-1-butene (EB) (20 to 931 microM) or 1,2:3,4-diepoxybutane (DEB) (2.5 to 320 uM), two of the suspected active metabolites of BD. Short EB exposures induced little measurable cytogenetic damage in either rat, mouse, or human G0 lymphocytes as measured by either sister chromatid exchange (SCE) or chromosome aberration (CA) analyses. However, DEB was a potent inducer of both SCEs and CAs in G0 splenic and peripheral blood lymphocytes. A comparison of the responses among species showed that the rat and mouse were approximately equisensitive to the cytogenetic damaging effects of DEB, but the situation for the human subjects was more complex. The presence of the GSTT1-1 gene (expressed in the erythrocytes) reduced the relative sensitivity of the lymphocytes to the SCE-inducing effects of DEB. However, additional factors also appear to influence the genotoxic response of humans to DEB. This study is the first direct comparison of the genotoxicity of EB and DEB in the cells from all three species.

Gene structure, expression and chromosomal localization of murine theta class glutathione transferase mGSTT1-1

We have isolated and characterized a cDNA and partial gene encoding a murine subfamily 1 Theta class glutathione transferase (GST). The cDNA derived from mouse GSTT1 has an open reading frame of 720 bp encoding a peptide of 240 amino acids with a calculated molecular mass of 27356 Da. The encoded protein shares only 51% deduced amino acid sequence identity with mouse GSTT2, but greater than 80% deduced amino acid sequence identity with rat GSTT1 and human GSTT1. Mouse GSTT1-1 was expressed in Escherichia coli as an N-terminal 6x histidine-tagged protein and purified using immobilized-metal affinity chromatography on nickel-agarose. The yield of the purified recombinant protein from E. coli cultures was approx. 14 mg/l. Recombinant mouse GSTT1-1 was catalytically active towards 1, 2-epoxy-3-(p-nitrophenoxy)propane, 4-nitrobenzyl chloride and dichloromethane. Low activity towards 1-menaphthyl sulphate and 1-chloro-2,4-dinitrobenzene was detected, whereas mouse GSTT1-1 was inactive towards ethacrynic acid. Recombinant mouse GSTT1-1 exhibited glutathione peroxidase activity towards cumene hydroperoxide and t-butyl hydroperoxide, but was inactive towards a range of secondary lipid-peroxidation products, such as the trans-alk-2-enals and trans,trans-alka-2,4-dienals. Mouse GSTT1 mRNA is most abundant in mouse liver and kidney, with some expression in intestinal mucosa. Mouse GSTT1 mRNA is induced in liver by phenobarbital, but not by butylated hydroxyanisole, beta-napthoflavone or isosafrole. The structure of mouse GSTT1 is conserved with that of the subfamily 2 Theta class GST genes mouse GSTT2 and rat GSTT2, comprising five exons interrupted by four introns. The mouse GSTT1 gene was found, by in situ hybridization, to be clustered with mouse GSTT2 on chromosome 10 at bands B5-C1. This region is syntenic with the location of the human Theta class GSTs clustered on chromosome 22q11.2. Similarity searches of a mouse-expressed sequence tag database suggest that there may be two additional members of the Theta class that share 70% and 88% protein sequence identity with mouse GSTT1, but less than 55% sequence identity with mouse GSTT2.

Structure and organization of the human theta-class glutathione S-transferase and D-dopachrome tautomerase gene complex

The structure and organization of the human Theta-class glutathione S-transferase (GST) genes have been determined. GSTT1 and GSTT2 are separated by approx. 50 kb. They have a similar structure, being composed of five exons with identical exon/intron boundaries. GSTT1 is 8.1 kb in length, while GSTT2 is only 3.7 kb. The GSTT2 gene lies head-to-head with a gene encoding d-dopachrome tautomerase (DDCT), which extends over 8.5 kb and contains four exons. The sequence between GSTT2 and DDCT may contain a bidirectional promoter. The GSTT2 and DDCT genes have been duplicated in an inverted repeat. Sequence analysis of the duplicated GSTT2 gene has identified an exon 2/intron 2 splice site abnormality and a premature translation stop signal at codon 196. These changes suggest that the duplicate gene is a pseudogene, and it has been named GSTT2P.

Human theta class glutathione transferase: the crystal structure reveals a sulfate-binding pocket within a buried active site

BACKGROUND

Glutathione S-transferases (GSTs) comprise a multifunctional group of enzymes that play a critical role in the cellular detoxification process. These enzymes reduce the reactivity of toxic compounds by catalyzing their conjugation with glutathione. As a result of their role in detoxification, GSTs have been implicated in the development of cellular resistance to antibiotics, herbicides and clinical drugs and their study is therefore of much interest. In mammals, the cytosolic GSTs can be divided into five distinct classes termed alpha, mu, pi, sigma and theta. The human theta class GST, hGST T2-2, possesses several distinctive features compared to GSTs of other classes, including a long C-terminal extension and a specific sulfatase activity. It was hoped that the determination of the structure of hGST T2-2 may help us to understand more about this unusual class of enzymes.

RESULTS

Here we present the crystal structures of hGST T2-2 in the apo form and in complex with the substrates glutathione and 1-menaphthyl sulfate. The enzyme adopts the canonical GST fold with a 40-residue C-terminal extension comprising two helices connected by a long loop. The extension completely buries the substrate-binding pocket and occludes most of the glutathione-binding site. The enzyme has a purpose-built novel sulfate-binding site. The crystals were shown to be catalytically active: soaks with 1-menaphthyl sulfate result in the production of the glutathione conjugate and cleavage of the sulfate group.

CONCLUSIONS

hGST T2-2 shares less than 15% sequence identity with other GST classes, yet adopts a similar three-dimensional fold. The C-terminal extension that blocks the active site is not disordered in either the apo or complexed forms of the enzyme, but nevertheless catalysis occurs in the crystalline state. A narrow tunnel leading from the active site to the surface may provide a pathway for the entry of substrates and the release of products. The results suggest a molecular basis for the unique sulfatase activity of this GST.

Kinetic characterization of recombinant human glutathione transferase T1-1, a polymorphic detoxication enzyme

Recombinant human theta class glutathione transferase T1-1 has been heterologously expressed in Escherichia coli and a simple purification method involving immobilized ferric ion affinity chromatography and Orange A dye chromatography is described. The catalytic properties of the enzyme differ significantly from those of other glutathione transferases, also within the theta class, with respect to both substrate selectivity and kinetic parameters. In addition to 1,2-epoxy-3-(4-nitrophenoxy)propane, the substrate used previously to monitor the enzyme, human glutathione transferase T1-1 has activity with the naturally occurring phenethylisothiocyanate and also displays glutathione peroxidase activity with cumene hydroperoxide. Further, the enzyme is active with 4-nitrobenzyl chloride and 4-nitrophenethyl bromide, but shows no detectable activity with the more chemically reactive 1-chloro-2,4-dinitrobenzene. The Michaelis constant for glutathione, K(m)GSH, with 1,2-epoxy-3-(4-nitrophenoxy)propane as second substrate, is high at low pH values but decreases at higher pH values. This is mirrored in kcat/K(m)GSH which increases with an apparent pKa value of 9.0, reflecting the ionization of the thiol group of glutathione in solution. The same results are obtained with 4-nitrophenethyl bromide as electrophilic substrate, although the K(m)GSH value (0.72 mM at pH 7.5), as well as the pKa (8.1) derived from the pH dependence of kcat/K(m)GSH, are lower with this substrate. In contrast, kcat and kcat/K(m)electrophile display either a maximum or a plateau at pH 7.0-7.5, and an apparent pKa value of 5.7 was determined for the pH dependence of kcat with both 4-nitrophenethyl bromide and 1,2-epoxy-3-(4-nitrophenoxy)propane as electrophilic substrates. This pKa value reflects an ionization of enzyme-bound GSH, most probably involving the sulfhydryl group, whose pKa value thus is lowered by the enzyme. Three differences in the cDNA as compared to the sequence previously published were found. One of these differences causes a change in the deduced amino acid sequence and involves the nucleotide triplet encoding amino acid 126, which was determined as GAG (Glu), instead of the published GGG (Gly).

Rat liver theta-class glutathione S-transferases T1-1 and T2-2: their chromatographic, electrophoretic, immunochemical, and functional properties

A method was established for simultaneously isolating Theta-class glutathione (GSH) S-transferases (GSTs) T1-1 and T2-2 as homogeneous proteins from rat (r) liver cytosol. The established method of using an 8-aminooctyl Sepharose 4B column to separate rGSTT1-1 from rGSTT2-2 at the final stage of their purification was a modification of the method previously reported for the isolation of rGSTT2-2 (Hiratsuka et al., J. Biol. Chem., 265, 11973-11981, 1990). Specific substrates used for purification of the Theta-class rGSTs were dichloromethane for T1-1 and 5-sulfoxymethylchrysene for T2-2. rGSTsT1-1 and T2-2 existed at a ratio of 1:7 at a total concentration of 0.5% of that of the cytosolic protein. Purified rGSTsT1-1 and T2-2 were separated as single bands at 28 and 26.5 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and as single peaks at retention times of 36 and 34 min, respectively, by reverse-phase partition high-performance liquid chromatography on a microBondasphere column eluted with a linear gradient of acetonitrile in water containing trifluoroacetic acid. Western blot analysis indicated that rabbit antisera raised against rGSTsT1-1 and T2-2 intensely reacted with the corresponding antigens, but showed no detectable reactivity with the different isoforms of Theta-class rGSTs as well as with representative hepatic rGSTs of other classes. The Theta-class rGSTs showed higher GSH peroxidase activity than rGSTA1-2 toward hydroperoxides of cumene, arachidonic acid, and linoleic acid. Cumene hydroperoxide was a better substrate for rGST T1-1 than for rGST T2-2, while the fatty acid hydroperoxides were the better substrates for rGST T2-2 than for rGST T1-1.

Evidence that human class Theta glutathione S-transferase T1-1 can catalyse the activation of dichloromethane, a liver and lung carcinogen in the mouse. Comparison of the tissue distribution of GST T1-1 with that of classes Alpha, Mu and Pi GST in human

The cDNA encoding human glutathione S-transferase (GST) T1 has been expressed as two recombinant forms in Escherichia coli that could be purified by affinity chromatography on either IgG-Sepharose or nickel-agarose; one form of the transferase was synthesized from the pALP 1 expression vector as a Staphylococcus aureus protein A fusion, whereas the other form was synthesized from the pET-20b expression vector as a C-terminal polyhistidine-tagged recombinant. The yields of the two purified recombinant proteins from E. coli cultures were approx. 15 mg/l for the protein A fusion and 25 mg/l for the C-terminal polyhistidine-tagged GST T1-1. The purified recombinant proteins were catalytically active, although the protein A fusion was typically only 5-30% as active as the histidine-tagged GST. Both recombinant forms could catalyse the conjugation of glutathione with the model substrates 1,2-epoxy-3-(4'-nitrophenoxy)propane,4-nitrobenzyl chloride and 4-nitrophenethyl bromide but were inactive towards 1-chloro-2,4-dinitrobenzene, ethacrynic acid and 1-menaphthyl sulphate. Recombinant human GST T1-1 was found to exhibit glutathione peroxidase activity and could catalyse the reduction of cumene hydroperoxide. In addition, recombinant human GST T1-1 was found to conjugate glutathione with dichloromethane, a pulmonary and hepatic carcinogen in the mouse. Immunoblotting with antibodies raised against different transferase isoenzymes showed that GST T1-1 is expressed in a large number of human organs in a tissue-specific fashion that differs from the pattern of expression of classes Alpha, Mu and Pi GST. Most significantly, GST T1-1 was found in only low levels in human pulmonary soluble extract of cells, suggesting that in man the lung has little capacity to activate the volatile dichloromethane.

Knowledge-based modeling of a bacterial dichloromethane dehalogenase

A three-dimensional structural model of the dichloromethane dehalogenase (DCMD) from Methylophilus sp. DM11 is constructed based on sequence similarities to the glutathione S-transferases (GSTs). To maximize sequence identity and minimize gaps in the alignment, a hybrid approach is used that takes advantage of the increased homology found between DM11 and domain I of the sheep blowfly theta class GST (residues 1-79) and domain II of the human alpha class GST (residues 81-222). The resulting structure has C alpha root mean square deviations of 1.16 A in domain I and 1.83 A in domain II from the template GSTs, which compare well to those seen in other GST inter-class comparisons. The model is further applied to explore the structural basis for substrate binding and catalysis. A conserved network of hydrogen bonds is described that binds glutathione to the G site, placing the thiol group in a suitable location for nucleophilic attack of dichloromethane. A mechanism is proposed that involves activation through a hydrogen bond interaction between Ser12 and glutathione, similar to that found in the theta-GSTs. The model also demonstrates how aromatic residues in the hydrophobic site (H site) could play a role in promoting catalysis: His116 and Trp117 are ideally situated to accept a growing negative charge on a chlorine of dichloromethane, stabilizing displacement. This scheme is consistent with experimental results of single-point mutations and comparisons with other GST structures and mechanisms.

Methylene chloride induced mouse liver and lung tumours: an overview of the role of mechanistic studies in human safety assessment

B6C3F1 mice exposed to high dose levels of methylene chloride by inhalation for 2 years had an elevated incidence of liver and lung tumours. These tumours were not increased in rats or hamsters exposed under the same or similar conditions. This paper gives an overview of research conducted over the last 10 years into the mechanism of action of methylene chloride as a mouse carcinogen and into the relevance of the mouse data to humans exposed to this chemical. Data are presented on the comparative metabolism and pharmacokinetics of methylene chloride in mice, rats, hamsters and humans, on the toxicity of methylene chloride to the target organs in the mouse, and on the genotoxicity of methylene chloride in vitro and in vivo. The enzyme which activates methylene chloride to its carcinogenic form has been isolated, sequenced, and cloned, and its distribution studied within cells, organs and between species. Evidence has been obtained to show the methylene chloride caused cancer in mice as a result of interactions between metabolites of the glutathione S-transferase pathway and DNA. Damage to mouse lung Clara cells and increased cell division are believed to have influenced the development of the lung tumours. The species specificity was a direct consequence of the very high activity and specific cellular and nuclear localisation of a theta class glutathione S-transferase enzyme which was unique to the mouse. Consequently, DNA damage was not detectable in rats in vivo, or in hamster and human hepatocytes exposed to cytotoxic dose levels of methylene chloride in vitro. These results provide evidence that the mouse is unique in its response to methylene chloride and that it is an inappropriate model for human health assessment.

The distribution of theta-class glutathione S-transferases in the liver and lung of mouse, rat and human

Two murine Theta-class glutathione S-transferases (GSTs), mGSTT1 and mGSTT2, have been cloned and sequenced. The murine cDNAs, together with the published sequences of the rat and human enzymes, were used to design oligonucleotide probes in order to determine the distribution of mRNA for these enzymes in the liver and lung of rat, mouse and human. The mRNA distribution was compared with that of enzyme protein determined with an antibody to rat GSTT2-2. Both the antibody and the oligonucleotide probes gave the same distribution patterns. Both enzymes were present at significantly higher concentrations in mouse tissues than in rat or human tissues. In mouse liver, both enzymes were localized in specific cell types and in nuclei. Although the distribution of GSTT2-2 in rat liver was similar to that seen in the mouse, GSTT1-1 was not localized in a specific cell type or in the nuclei of either rat or human liver. In the lungs, very high concentrations of the Theta enzymes were present in mouse-lung Clara cells and ciliated cells, with much lower levels in the Clara cells only of rat lung. Low levels of human transferase GSTT1-1 were detected in a small number of Clara cells and ciliated cells at the alveolar/ bronchiolar junction. The relative activities between species, and the cellular and sub-cellular distribution within the liver and lungs of each species, provides an explanation for the species-specificity of methylene chloride, a mouse-specific carcinogen activated by glutathione S-transferase GSTT1-1.