Molecular cloning and amino acid sequencing of rat liver class theta glutathione S-transferase Yrs-Yrs inactivating reactive sulfate esters of carcinogenic arylmethanols

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.

First molluscan theta-class Glutathione S-Transferase: identification, cloning, characterization and transcriptional analysis post immune challenges

Glutathione S-Transferases (GSTs) are multifunctional cytosolic isoenzymes, distinctly known as phase II detoxification enzymes. GSTs play a significant role in cellular defense against toxicity and have been identified in nearly all organisms studied to date, from bacteria to mammals. In this study, we have identified a full-length cDNA of the theta class GST from Ruditapes philippinarum (RpGSTθ), an important commercial edible molluscan species. RpGSTθ was cloned and the recombinant protein expressed, in order to study its biochemical characteristics and determine its physiological activities. The cDNA comprised an ORF of 693 bp, encoding 231 amino acids with a predicted molecular mass of 27 kDa and an isoelectric point of 8.2. Sequence analysis revealed that RpGSTθ possessed characteristic conserved domains of the GST_N family, Class Theta subfamily (PSSM: cd03050) and GST_C_family Super family (PSSM: cl02776). Phylogenetic analysis showed that RpGSTθ evolutionarily linked with other theta class homologues. The recombinant protein was expressed in Escherichia coli BL21(DE3) cells and the purified enzyme showed high activity with GST substrates like CDNB and 4-NBC. Glutathione dependent peroxidase activity of GST, investigated with cumene hydroperoxide as substrate affirmed the antioxidant property of rRpGSTθ. By quantitative PCR, RpGSTθ was found to be ubiquitously expressed in all tissues examined, with the highest levels occurring in gills, mantle, and hemocytes. Since GSTs may act as detoxification enzymes to mediate immune defense, the effects of pathogen associated molecular pattern, lipopolysaccharide and intact Vibrio tapetis bacteria challenge on RpGSTθ gene transcription were studied. Furthermore, the RpGSTθ expression changes induced by immune challenges were similar to those of the antioxidant defense enzyme manganese superoxide dismutase (RpMnSOD). To our knowledge, RpGSTθ is the first molluscan theta class GST reported, and its immune-related role in Manila clam may provide insights into potential therapeutic targets for protecting this important aquaculture species.

Mechanisms of induction of cytosolic and microsomal glutathione transferase (GST) genes by xenobiotics and pro-inflammatory agents

Glutathione transferase (GST) isoezymes are encoded by three separate families of genes (designated cytosolic, microsomal and mitochondrial transferases), with distinct evolutionary origins, that provide mammalian species with protection against electrophiles and oxidative stressors in the environment. Members of the cytosolic class Alpha, Mu, Pi and Theta GST, and also certain microsomal transferases (MGST2 and MGST3), are up-regulated by a diverse spectrum of foreign compounds typified by phenobarbital, 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene, pregnenolone-16α-carbonitrile, 3-methylcholanthrene, 2,3,7,8-tetrachloro-dibenzo-p-dioxin, β-naphthoflavone, butylated hydroxyanisole, ethoxyquin, oltipraz, fumaric acid, sulforaphane, coumarin, 1-[2-cyano-3,12-dioxooleana-1,9(11)-dien-28-oyl]imidazole, 12-O-tetradecanoylphorbol-13-acetate, dexamethasone and thiazolidinediones. Collectively, these compounds induce gene expression through the constitutive androstane receptor (CAR), the pregnane X receptor (PXR), the aryl hydrocarbon receptor (AhR), NF-E2-related factor 2 (Nrf2), peroxisome proliferator-activated receptor-γ (PPARγ) and CAATT/enhancer binding protein (C/EBP) β. The microsomal T family includes 5-lipoxygenase activating protein (FLAP), leukotriene C(4) synthase (LTC4S) and prostaglandin E(2) synthase (PGES-1), and these are up-regulated by tumour necrosis factor-α, lipopolysaccharide and transforming growth factor-β. Induction of genes encoding FLAP, LTC4S and PGES-1 is mediated by the transcription factors C/EBPα, C/EBPδ, C/EBPϵ, nuclear factor-κB and early growth response-1. In this article we have reviewed the literature describing the mechanisms by which cytosolic and microsomal GST are up-regulated by xenobiotics, drugs, cytokines and endotoxin. We discuss cross-talk between the different induction mechanisms, and have employed bioinformatics to identify cis-elements in the upstream regions of GST genes to which the various transcription factors mentioned above may be recruited.

Screening and characterization of variant Theta-class glutathione transferases catalyzing the activation of ethylene dibromide to a mutagen

Ethylene dibromide (EDB) is a widespread environmental pollutant and mutagen/carcinogen. Certain Theta-class glutathione transferases (GSTs), enzymes that catalyze the reaction of reduced glutathione (GSH) with electrophiles, activate EDB to a mutagen. Previous studies have shown that human GST T1-1, but not rat GST T2-2, activates EDB. We have constructed an E. coli lacZ reversion mutagenicity assay system in which expression of recombinant GST supports activation of EDB to a mutagen. Hexa-histidine N-terminal tagging of GST T1-1 results in greatly enhanced expression of the recombinant enzyme and gives a lacZ strain that shows a mutagenic response to EDB at extremely low levels (approximately 1 ng EDB per plate). The hexa-histidine-tagged enzyme was purified in one step by Ni(2+)-affinity chromatography. We applied the lacZ mutagenicity assay to the rapid screening of a library of variant GST Theta enzymes. Sequence variants with altered catalytic activities were identified, purified, and characterized.

Combining combinatorial chemistry and affinity chromatography: highly selective inhibitors of human betaine: homocysteine S-methyltransferase

A new method to find novel protein targets for ligands of interest is proposed. The principle of this approach is based on affinity chromatography and combinatorial chemistry. The proteins within a crude rat liver homogenate were allowed to interact with a combinatorial library of phosphinic pseudopeptides immobilized on affinity columns. Betaine: homocysteine S-methyltransferase (BHMT) was one of the proteins that was retained and subsequently eluted from these supports. The phosphinic pseudopeptides, which served as immobilized ligands for the isolation of rat BHMT, were then tested for their ability to inhibit human recombinant BHMT in solution. The most potent inhibitor also behaved as a selective ligand for the affinity purification of BHMT from a complex media. Further optimization uncovered Val-Phe-psi[PO(2-)-CH(2)]-Leu-His-NH(2) as a potent BHMT inhibitor that has an IC(50) of about 1 microM.

An ensemble of theta class glutathione transferases with novel catalytic properties generated by stochastic recombination of fragments of two mammalian enzymes

The correlation between sequence diversity and enzymatic function was studied in a library of Theta class glutathione transferases (GSTs) obtained by stochastic recombination of fragments of cDNA encoding human GST T1-1 and rat GST T2-2. In all, 94 randomly picked clones were characterized with respect to sequence, expression level, and catalytic activity in the conjugation reactions between glutathione and six alternative electrophilic substrates. Out of these six different compounds, dichloromethane is a selective substrate for human GST T1-1, whereas 1-menaphthyl sulfate and 1-chloro-2,4-dinitrobenzene are substrates for rat GST T2-2. The other three substances serve as substrates for both enzymes. Through this broad characterization, we have identified enzyme variants that have acquired novel activity profiles that differ substantially from those of the original GSTs. In addition, the expression levels of many clones were improved in comparison to the parental enzyme. A library of mutants can thus display a distribution of properties from which highly divergent evolutionary pathways may emerge, resembling natural evolutionary processes. From the GST library, a clone was identified that, by the point mutation N49D in the rat GST T2-2 sequence, has a 1700% increased activity with 1-menaphthyl sulfate and a 60% decreased activity with 4-nitrophenethyl bromide. Through the N49D mutation, the ratio of these activities has thus been altered 40-fold. An extensive characterization of a population of stochastically mutated enzymes can accordingly be used to find variants with novel substrate-activity profiles and altered catalytic properties. Recursive recombination of selected sequences displaying optimized properties is a strategy for the engineering of proteins for medical and biochemical applications. Such sequential design is combinatorial protein chemistry based on remodeling of existing structural scaffolds and has similarities to evolutionary processes in nature.

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.

Identification of amine acceptor protein substrates of transglutaminase in liver extracts: use of 5-(biotinamido) pentylamine as a probe

Transglutaminase is a calcium-dependent enzyme which catalyzes amine incorporation and cross-linking of proteins. To isolate the amine acceptor protein substrates of transglutaminase in mammalian livers, a biotin-labeled primary amine substrate of transglutaminase, 5-(biotinamido) pentylamine, was used for biotin labeling of proteins in the liver extracts by endogenous transglutaminase activity. The biotin-labeled proteins were isolated and recovered by biotin-avidin-affinity chromatography. The obtained proteins were separated by SDS-PAGE. Proteins with molecular masses of 15, 24, 35, 40, 44, 93, and 134 kDa were the main components of labeled proteins in mouse liver extract. In rat and guinea pig liver extracts, 32-, 38-, 40-, 44-, and 134-kDa proteins and28-, 40-, 44-, 55-, 60-, 91-, and 134-kDa proteins were the main components of labeled proteins, respectively.Using amino-terminal amino acid sequence analyses and sequence homology searches, the 38-kDa protein from rat liver was identified as a subunit of glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.12), and the 28-kDa protein from guinea pig liver was identified as a subunit of glutathione S-transferase (class theta) (EC 2.5.1.18). Both the glyceraldehyde-3-phosphate dehydrogenase from rabbit muscle and glutathione S-transferase (class pi) from human placenta also could be amine acceptors in the amine incorporation catalyzed by guinea pig liver transglutaminase. These results suggest that these enzymes can be modified posttranslationally by cellular transglutaminase.

Guinea pig liver Mu-class glutathione S-transferase M1-2 cross-reacts with antibodies to both rat Mu- and theta-class glutathione S-transferases

Two novel major heterodimeric Mu-class glutathione (GSH) S-transferases (GSTs), designated M1-2 and M1-3*, were isolated from guinea pig (gp) liver cytosol and purified to homogeneity together with a known major homodimeric Mu-class gpGSTM1-1 (reported as GST b by R. Oshino, K. Kamei, M. Nishioka, and M. Shin, 1990, J. Biochem. 107, 105-110). These three gpGSTs were quantitatively retained on an S-hexyl-GSH affinity column and separated as homogeneous proteins by chromatofocusing. Subunits of the heterodimers were inseparable on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, but could be completely separated by reverse-phase partition high-performance liquid chromatography. A molecular cloning study demonstrated that the gpGST subunit M2 consisted of 217 amino acid residues with a calculated molecular mass of 25,562 and shared 84% identity in overall amino acid sequence with gpGSTM1-1. N-terminal amino acid sequences of peptides from the gpGST subunit M3* with a blocked N-terminus strongly suggested that it should belong to the Mu class. Western blot analysis using antisera raised against purified rat (r) GSTsA1-2 (Alpha), M1-1, P1-1 (Pi), and T2-2 (Theta) indicated that gpGSTsM1-1 and M1-3* cross-reacted only with anti-rGSTM1 antibody. However, gpGSTM1-2 cross-reacted intensely to almost the same extent with antibodies to both rGSTsM1-1 and T2-2. A homodimeric gpGSTM2-2, artificially constructed from native gpGSTM1-2 by treatment with guanidine hydrochloride followed by dialysis, intensely cross-reacted with antibodies to both the rat Mu- and Theta-class GSTs. Thus, the gpGST subunit M2 provided the first evidence for the double immuno-cross-reaction of a GST with polyclonal antibodies to two different classes of GSTs.

Purification and characterization of a novel glutathione transferase from Ochrobactrum anthropi

Glutathione transferase was purified from Ochrobactrum anthropi and its N-terminal sequence was determined to be MKLYYKVGACSLAPHIILSEAGLPY. The apparent molecular mass of the protein (24 kDa) was determined by SDS-polyacrylamide gel electrophoresis analysis. The amino acid sequence obtained showed similarities with known bacterial glutathione transferases in the range of 72-64%. Immunoblotting experiments performed with antisera raised against glutathione transferase from O. anthropi did not show cross-reactivity with two bacterial glutathione transferases belonging to Serratia marcescens and Proteus mirabilis.

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).

Molecular phylogeny of glutathione-S-transferases

The glutathione-S-transferase (GST) protein superfamily is currently composed of nearly 100 sequences. This study documents a greater phylogenetic diversity of GSTs than previously realized. Parsimony and distance phylogenetic methods of GST amino acid sequences yielded virtually the same results. There appear to be at least 25 groups (families) of GST-like proteins, as different from one another as are the currently recognized classes. This diversity will require the design of a new nomenclature for this large protein superfamily. There is one well-supported large clade containing the mammalian mu, pi, and alpha classes as well as GSTs from molluscs, helminths, nematodes, and arthropods.

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.

Mass spectrometric analysis of rat ovary and testis cytosolic glutathione S-transferases (GSTs): identification of a novel class-alpha GST, rGSTA6*, in rat testis

Cytosolic glutathione S-transferases (GSTs) from rat ovaries and testis were purified by a combination of GSH and S-hexylglutathione affinity chromatography. The isolated GSTs were subjected to reverse-phase HPLC, electrospray MS and N-terminal peptide sequencing analysis. The major GST isoenzymes expressed in ovaries are subunits A3, A4, M1, M2 and P1. Other isoenzymes detected are subunits A1, M3 and M6*. In rat testis, the major GST isoenzymes expressed are subunits A3, M1, M2, M3, M5* and M6*. Subunits A1, A4 and P1 are expressed in lesser amounts. We could not detect post-translational modifications of any GSTs with known cDNA sequence. The molecular masses of subunits M5* and M6*, two class-Mu GSTs that have not been cloned, were determined to be 25495 and 26538 Da respectively. An N-terminally modified protein from rat testis with molecular mass 25737 Da was isolated from the S-hexylglutathione column. Results from internal peptide sequencing analysis indicate that this is a novel class-Alpha GST that has not been previously reported. We designate this protein rGSTA6*.

Analysis of a Rhizobium leguminosarum gene encoding a protein homologous to glutathione S-transferases

A novel Rhizobium leguminosarum gene, gstA, the sequence of which indicated that it was a member of the gene family of glutathione S-transferases (GSTs), was identified. The homology was greatest to the GST enzymes of higher plants. The Rhizobium gstA gene was normally expressed at a very low level. The product of gstA was over-expressed and purified from Escherichia coli. It was shown to bind to the affinity matrix glutathione-Sepharose, but no enzymic GST activity with 1-chloro-2,4-dinitrobenzene as substrate was detected. gstA encoded a polypeptide of 203 amino acid residues with a calculated molecular mass of 21990 Da. Transcribed divergently from gstA is another gene, gstR, which was similar in sequence to the LysR family of bacterial transcriptional regulators. A mutation in gstR had no effect on the transcription of itself or gstA under the growth conditions used here. Mutations in gstA and gstR caused no obvious phenotypic defect and the biological functions of these genes remain to be determined.

Overexpression of glutathione S-transferase pi messenger RNA and its relationship to gene amplification in head and neck squamous cell carcinoma

Human glutathione S-transferase pi has been known to be a good marker for several tumor types because of the high frequency with which it is overexpressed. In order to determine whether GST pi is useful as an indicator for head and neck cancers, expression of GST pi was investigated by Northern analysis. Overexpression of mRNA was detected in 9 of 36 primary head and neck squamous cell carcinomas. To examine the relationship between overexpression and amplification of GST pi gene, Southern analysis was performed on all samples. Only 3 of the 36 tumors showed amplification GST pi genes, indicating that gene amplification may not play a key role in GST pi mRNA overexpression in these cancers.

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.

Heterologous expression, purification and characterization of rat class theta glutathione transferase T2-2

Rat glutathione transferase (GST) T2-2 of class Theta (rGST T2-2), previously known as GST 12-12 and GST Yrs-Yrs, has been heterologously expressed in Escherichia coli XLI-Blue. The corresponding cDNA was isolated from a rat hepatoma cDNA library, ligated into and expressed from the plasmid pKK-D. The sequence is the same as that of the previously reported cDNA of GST Yrs-Yrs. The enzyme was purified using ion-exchange chromatography followed by affinity chromatography with immobilized ferric ions, and the yield was approx. 200 mg from a 1 litre bacterial culture. The availability of a stable recombinant rGST T2-2 has paved the way for a more accurate characterization of the enzyme. The functional properties of the recombinant rGST T2-2 differ significantly from those reported earlier for the enzyme isolated from rat tissues. These differences probably reflect the difficulties in obtaining fully active enzyme from sources where it occurs in relatively low concentrations, which has been the case in previous studies. 1-Chloro-2,4-dinitrobenzene, a substrate often used with GSTs of classes Alpha, Mu and Pi, is a substrate also for rGST T2-2, but the specific activity is relatively low. The Km value for glutathione was determined with four different electrophiles and was found to be in the range 0.3 mM-0.8 mM. The Km values for some electrophilic substrates were found to be in the micromolar range, which is low compared with those determined for GSTs of other classes. The highest catalytic efficiency was obtained with menaphthyl sulphate, which gave a Kcat/Km value of 2.3 x 10(6) s-1.M-1 and a rate enhancement over the uncatalysed reaction of 3 x 10(10).

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

A glutathione S-transferase metabolizing methylene chloride has been isolated from mouse liver using a variety of chromatographic methods. N-terminal and internal amino acid sequences show that the enzyme, designated GST T1-1*, is closely related to the rat Theta-class GST 5-5. The mouse enzyme, molecular mass 25000 Da, has been isolated to homogeneity in active form with an approximate yield of 2% of the cytosolic activity towards methylene chloride. GST T1-1* has a specific activity of about 5.5 micromol/min per mg of protein whereas the rat GST 5-5 is reported to have a specific activity of about 11 micromol/min per mg of protein [Meyer, Coles, Pemble, Gilmore, Fraser and Ketterer (1991) Biochem. J. 274, 409-414], demonstrating that both the rat and mouse enzymes have similar activity with this substrate. Limited evidence was obtained for a second enzyme, with a similar molecular mass (25400 Da), which had an N-terminal sequence identical to that of rat GST 12-12. This protein, which was sequenced from a band on a gel, was extremely labile and could not be isolated to homogeneity. The partially purified enzyme was not active with methylene chloride.

Amino acid sequencing, molecular cloning and modelling of the chick liver class-theta glutathione S-transferase CL1

Glutathione S-transferase CL1-2 heterodimers purified from 1-day-old chick livers were digested with Achromobacter proteinase I. The resulting fragments were separated for amino acid sequence analysis. Oligonucleotide probes were constructed based on sequence similarity to class-Theta glutathione S-transferases for PCR using a chicken liver cDNA library as template. A full-length clone (1725 bp) encoding a polypeptide comprising 261 amino acids was isolated. Including conservative substitutions, this protein has 70-73% sequence similarity with other mammalian class-Theta glutathione S-transferases. Based on known X-ray crystal structures of class-Alpha, -Mu and -Pi glutathione S-transferases, a model is constructed for the N-terminal 232 residues of CL1.

Potential contribution of the glutathione S-transferase supergene family to resistance to oxidative stress

The glutathione S-transferase (GST) supergene family comprises gene families that encode isoenzymes that are widely expressed in mammalian tissue cytosols and membranes. Both cytosolic (particularly the isoenzymes encoded by the alpha, mu and theta gene families) and microsomal GST catalyse the conjugation of reduced glutathione (GSH) with a wide variety of electrophiles which include known carcinogens as well as various compounds that are products of oxidative stress including oxidised DNA and lipid. Indeed, several lines of evidence suggest certain of these isoenzymes play a pivotal role in protecting cells from the consequences of such stress. An assessment of the importance of these GST in humans is presently difficult however, because the number of alpha and theta class genes is not known and, the catalytic preferences of even identified isoforms is not always clear.

Molecular cloning of a cDNA and chromosomal localization of a human theta-class glutathione S-transferase gene (GSTT2) to chromosome 22

Until recently the Theta-class glutathione S-transferases (GSTs) were largely overlooked due to their low activity with the model substrate 1-chloro-2,4-dinitrobenzene (CDNB) and their failure to bind to immobilized glutathione affinity matrices. Little is known about the number of genes in this class. Recently, Pemble et al. (Biochem J. 300: 271-276, 1994) reported the cDNA cloning of a human Theta-class GST, termed GSTT1. In this study, we describe the molecular cloning of a cDNA encoding a second human Theta-class GST (GSTT2) from a lambda gt11 human liver 5'-stretch cDNA library. The encoded protein contains 244 amino acids and has 78.3% sequence identity with the rat subunit 12 and only 55.0% identity with human GSTT1. GSTT2 has been mapped to chromosome 22 by somatic cell hybrid analysis. The precise position of the gene was localized to subband 22q11.2 by in situ hybridization. The absence of other regions of hybridization suggests that there are no closely related sequences (e.g., reverse transcribed pseudogenes) scattered throughout the genome and that if there are closely related genes, they must be clustered near GSTT2. Southern blot analysis of human DNA digested with BamHI shows that the size of the GSTT2 gene is relatively small, as the coding sequence falls within a 3.6-kb BamHI fragment.

The glutathione S-transferase supergene family: regulation of GST and the contribution of the isoenzymes to cancer chemoprotection and drug resistance

The glutathione S-transferases (GST) represent a major group of detoxification enzymes. All eukaryotic species possess multiple cytosolic and membrane-bound GST isoenzymes, each of which displays distinct catalytic as well as noncatalytic binding properties: the cytosolic enzymes are encoded by at least five distantly related gene families (designated class alpha, mu, pi, sigma, and theta GST), whereas the membrane-bound enzymes, microsomal GST and leukotriene C4 synthetase, are encoded by single genes and both have arisen separately from the soluble GST. Evidence suggests that the level of expression of GST is a crucial factor in determining the sensitivity of cells to a broad spectrum of toxic chemicals. In this article the biochemical functions of GST are described to show how individual isoenzymes contribute to resistance to carcinogens, antitumor drugs, environmental pollutants, and products of oxidative stress. A description of the mechanisms of transcriptional and posttranscriptional regulation of GST isoenzymes is provided to allow identification of factors that may modulate resistance to specific noxious chemicals. The most abundant mammalian GST are the class alpha, mu, and pi enzymes and their regulation has been studied in detail. The biological control of these families is complex as they exhibit sex-, age-, tissue-, species-, and tumor-specific patterns of expression. In addition, GST are regulated by a structurally diverse range of xenobiotics and, to date, at least 100 chemicals have been identified that induce GST; a significant number of these chemical inducers occur naturally and, as they are found as nonnutrient components in vegetables and citrus fruits, it is apparent that humans are likely to be exposed regularly to such compounds. Many inducers, but not all, effect transcriptional activation of GST genes through either the antioxidant-responsive element (ARE), the xenobiotic-responsive element (XRE), the GST P enhancer 1(GPE), or the glucocorticoid-responsive element (GRE). Barbiturates may transcriptionally activate GST through a Barbie box element. The involvement of the Ah-receptor, Maf, Nrl, Jun, Fos, and NF-kappa B in GST induction is discussed. Many of the compounds that induce GST are themselves substrates for these enzymes, or are metabolized (by cytochrome P-450 monooxygenases) to compounds that can serve as GST substrates, suggesting that GST induction represents part of an adaptive response mechanism to chemical stress caused by electrophiles. It also appears probable that GST are regulated in vivo by reactive oxygen species (ROS), because not only are some of the most potent inducers capable of generating free radicals by redox-cycling, but H2O2 has been shown to induce GST in plant and mammalian cells: induction of GST by ROS would appear to represent an adaptive response as these enzymes detoxify some of the toxic carbonyl-, peroxide-, and epoxide-containing metabolites produced within the cell by oxidative stress. Class alpha, mu, and pi GST isoenzymes are overexpressed in rat hepatic preneoplastic nodules and the increased levels of these enzymes are believed to contribute to the multidrug-resistant phenotype observed in these lesions. The majority of human tumors and human tumor cell lines express significant amounts of class pi GST. Cell lines selected in vitro for resistance to anticancer drugs frequently overexpress class pi GST, although overexpression of class alpha and mu isoenzymes is also often observed. The mechanisms responsible for overexpression of GST include transcriptional activation, stabilization of either mRNA or protein, and gene amplification. In humans, marked interindividual differences exist in the expression of class alpha, mu, and theta GST. The molecular basis for the variation in class alpha GST is not known. (ABSTRACT TRUNCATED)

Microbes, enzymes and genes involved in dichloromethane utilization

Dichloromethane (DCM) is efficiently utilized as a carbon and energy source by aerobic, Gram-negative, facultative methylotrophic bacteria. It also serves as a sole carbon and energy source for a nitrate-respiring Hyphomicrobium sp. and for a strictly anaerobic co-culture of a DCM-fermenting bacterium and an acetogen. The first step of DCM utilization by methylotrophs is catalyzed by DCM dehalogenase which, in a glutathione-dependent substitution reaction, forms inorganic chloride and S-chloromethyl glutathione. This unstable intermediate decomposes to glutathione, inorganic chloride and formaldehyde, a central metabolite of methylotrophic growth. Genetic studies on DCM utilization are beginning to shed some light on questions pertaining to the evolution of DCM dehalogenases and on the regulation of DCM dehalogenase expression. DCM dehalogenase belongs to the glutathione S-transferase supergene family. Analysis of the amino acid sequences of two bacterial DCM dehalogenases reveals 56% identity, and comparison of these sequences to those of glutathione S-transferases indicates a closer relationship to class Theta eukaryotic glutathione S-transferases than to a number of bacterial glutathione S-transferases whose sequences have recently become available. dcmA, the structural gene of the highly substrate-inducible DCM dehalogenase, is carried in most DCM utilizing methylotrophs on large plasmids. In Methylobacterium sp. DM4 its expression is governed by dcmR, a regulatory gene located upstream of dcmA, dcmR encodes a trans-acting factor which negatively controls DCM dehalogenase formation at the transcriptional level. Our working model thus assumes that the dcmR product is a repressor which, in the absence of DCM, binds to the promoter region of dcmA and thereby inhibits initiation of transcription.

Isolation and characterization of the Methylophilus sp. strain DM11 gene encoding dichloromethane dehalogenase/glutathione S-transferase

The restricted facultative methylotroph Methylophilus sp. strain DM11 utilizes dichloromethane as the sole carbon and energy source. It differs from other dichloromethane-utilizing methylotrophs by faster growth on this substrate and by possession of a group B dichloromethane dehalogenase catalyzing dechlorination at a fivefold-higher rate than the group A enzymes of slow-growing strains. We isolated dcmA, the structural gene of the strain DM11 dichloromethane dehalogenase, to elucidate its relationship to the previously characterized dcmA gene of Methylobacterium sp. strain DM4, which encodes a group A enzyme. Nucleotide sequence determination of dcmA from strain DM11 predicts a protein of 267 amino acids, corresponding to a molecular mass of 31,197 Da. The 5' terminus of in vivo dcmA transcripts was determined by primer extension to be 70 bp upstream of the translation initiation codon. It was preceded by a putative promoter sequence with high resemblance to the Escherichia coli sigma 70 consensus promoter sequence. dcmA and 130 bp of its upstream sequence were brought under control of the tac promoter and expressed in E. coli to approximately 20% of the total cellular protein by induction with isopropylthiogalactopyranoside (IPTG) and growth at 25 degrees C. Expression at 37 degrees C led to massive formation of inclusion bodies. Comparison of the strain DM11 and strain DM4 dichloromethane dehalogenase sequences revealed 59% identity at the DNA level and 56% identity at the protein level, thus indicating an ancient divergence of the two enzymes. Both dehalogenases are more closely related to eukaryotic class theta glutathione S-transferases than to a number of bacterial glutathione S-transferases.

Molecular cloning and functions of rat liver hydroxysteroid sulfotransferases catalysing covalent binding of carcinogenic polycyclic arylmethanols to DNA

Three sulfotransferases (STs) catalysing the metabolic activation of potent carcinogenic polycyclic arylmethanols were purified from female Sprague-Dawley (SD) rat liver cytosol without loss of their enzyme activities in the presence of Tween 20 used for preventing the enzymes from aggregation during purification and identified as hydroxysteroid sulfotransferases (HSTs). All the purified HSTs, STa, STb, and STc, with different electric charges had an apparently equal size of subunit (30.5 kDa) and cross-reacted with polyclonal antibody raised against STa. Our study on molecular cloning of cDNA libraries from two female SD rat livers indicated that both contained cDNA inserts coding for 5 different HST subunits, consisting of 284-285 amino acid residues (M(r), 33,084-33,535) and sharing strong amino acid sequence identity (> 83%). Of the 5 HST subunits, two had an identical amino acid sequence except for only one amino acid residue, and the other two contained only 6 amino acid substitutions in their sequences.

Human glutathione S-transferase theta (GSTT1): cDNA cloning and the characterization of a genetic polymorphism

In humans, glutathione-dependent conjugation of halomethanes is polymorphic, with 60% of the population classed as conjugators and 40% as non-conjugators. We report the characterization of the genetic polymorphism causing the phenotypic difference. We have isolated a cDNA that encodes a human class Theta GST (GSTT1) and which shares 82% sequence identity with rat class Theta GST5-5. From PCR and Southern blot analyses, it is shown that the GSTT1 gene is absent from 38% of the population. The presence or absence of the GSTT1 gene is coincident with the conjugator (GSST1+) and non-conjugator (GSTT1-) phenotypes respectively. The GSTT1+ phenotype can catalyse the glutathione conjugation of dichloromethane, a metabolic pathway which has been shown to be mutagenic in Salmonella typhimurium mutagenicity tester strains and is believed to be responsible for carcinogenicity of dichloromethane in the mouse. In humans, the enzyme is found in the erythrocyte and this may act as a detoxification sink. Characterization of the GSTT1 polymorphism will thus enable a more accurate assessment of human health risk from synthetic halomethanes and other industrial chemicals.

Purification, molecular cloning and heterologous expression of a glutathione S-transferase from the Australian sheep blowfly (Lucilia cuprina)

Three glutathione S-transferases from Lucilia cuprina (Australian sheep blowfly) pupae were purified by affinity chromatography and anion-exchange chromatography. One isoenzyme was composed of M(r)-24,800 subunits, and two isoenzymes had subunits of M(r) 23,900. The M(r)-23,900 subunits showed immunological identity and were immunologically distinct from the M(r)-24,800 subunits. All three enzymes were active with the substrate 1-chloro-2,4-dinitrobenzene and had low activity with 1,2-dichloro-4-nitrobenzene. A cDNA clone encoding a M(r)-23,900 subunit (LuGST1) was isolated and sequenced. The sequence has close similarities (> 81%) to that of GSTs from the fruitfly Drosophila melanogaster and Musca domestica (housefly). The deduced amino acid sequence of the Lu GST1 subunit showed no significant similarity to that of the mammalian GSTs to the Alpha, Mu and Pi classes, but shows some similarity (33%) over the first 100 residues with the rat subunit 12 Theta-class GST. Southern blots of genomic DNA hybridized with the LuGST1 cDNA identified many hybridizing fragments. Taken together, these data indicated that the L. cuprina genome contains multiple glutathione S-transferase genes.

MIF protein are theta-class glutathione S-transferase homologs

MIF proteins are mammalian polypeptides of approximately 13,000 molecular weight. This class includes human macrophage migration inhibitory factor (MIF), a rat liver protein that has glutathione S-transferase (GST) activity (TRANSMIF), and the mouse delayed early response gene 6 (DER6) protein. MIF proteins were previously linked to GSTs by demonstrating transferase activity and observing N-terminal sequence homology with a mu-class GST (Blocki, F.A., Schlievert, P.M., & Wackett, L.P., 1992, Nature 360, 269-270). In this study, MIF proteins are shown to be structurally related to the theta class of GSTs. This is established in three ways. First, unique primary sequence patterns are developed for each of the GST gene classes. The patterns identify the three MIF proteins as theta-like transferase homologs. Second, pattern analysis indicates that GST members of the theta class contain a serine residue in place of the N-terminal tyrosine that is implicated in glutathione deprotonation and activation in GSTs of known structure (Liu, S., et al., 1992, J. Biol. Chem. 267, 4296-4299). The MIF proteins contain a threonine at this position. Third, polyclonal antibodies raised against recombinant human MIF cross-react on Western blots with rat theta GST but not with alpha and mu GSTs. That MIF proteins have glutathione-binding ability may provide a common structural key toward understanding the varied functions of this widely distributed emerging gene family. Because theta is thought to be the most ancient evolutionary GST class, MIF proteins may have diverged early in evolution but retained a glutathione-binding domain.

Significance of an unusually low Km for glutathione in glutathione transferases of the alpha, mu and pi classes

1. Interactions of glutathione transferases (GST) of the alpha, mu and pi classes with glutathione (GSH) and glutathione conjugates (GS-X) are in contrast with those of a GST of the theta class (GST5-5). 2. GST 5-5 has a Km for GSH of approx. 5 mM. Thus Km/ambient [GSH] is approx. 1, within the range of Km/ambient [s] of glycolytic enzymes. GSTs of the alpha, mu and pi classes yield much lower values of Km for GSH (approx. 0.1 mM) hence Km/ambient [s] is significantly lower than those of most (non-GST) enzymes (p < 0.025). 3. GSTs of the alpha, mu and pi classes are sensitive to inhibition by GS-X (i.e. product) and GS-X analogues. GST 5-5 is not. 4. Rate enhancements up to 10(10), similar to an average enzyme (10(8)-10(12)), are seen in catalysis by GST 5-5, but not in catalysis by GSTs of alpha, mu and pi classes (> 10(7)). 5. Comparisons of primary structure indicate that theta class GSTs may have a decreased binding of the glu-alpha-amino- and gly-COO(-)-groups of GSH compared with GSTs of the other classes. 6. It is concluded that GSTs of alpha, mu and pi classes have evolved towards increased product binding at the expense of catalytic efficiency. Thus GSH is uniquely utilized both as a nucleophile and a 'tag' which can be used to bind and sequester product particularly during GSH-depletion. This interpretation unifies the catalytic and binding properties of these GSTs and alters their perceived role in detoxication.

Chemical modification of GSH transferase P1-1 confirms the presence of Arg-13, Lys-44 and one carboxylate group in the GSH-binding domain of the active site

GSH transferase P1-1 (GSTP1-1) was modified with group-specific reagents. Kinetic experiments demonstrated that inactivation of GSTP1-1 occurred upon reaction of one arginine residue per subunit with diacetyl, one lysine residue per subunit with 2,4,6-trinitrobenzene sulphonate, or one carboxylate group per subunit with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide. All three inactivation reactions were inhibited by compounds known to bind at the GSH site of the enzyme but were unaffected by the electrophile 1-chloro-2,4-dinitrobenzene. N-terminal sequence analysis showed that Arg-13 was modified by diacetyl and that this modification was inhibited by GSH. Arg-11 was not modified. The lysine residue modified by 2,4,6-trinitrobenzene sulphonate and protected by S-octylglutathione was identified as Lys-44 by sequencing of tryptic peptides. The findings are in agreement with the involvement of Arg-13 and Lys-44 in binding of GSH, as determined from the crystal structure [Reinemer, Dirr, Ladenstein, Huber, Lo Bello, Frederici and Parker (1992) J. Mol. Biol. 227, 214-226]. The present data also implicate a single carboxylate in GSH binding, consistent with the involvement of Asp-98 of subunit B determined from the crystallographic study. The GSH-binding determinants of GSTP1-1 are compared using sequence similarity with those of GSTs of Alpha, Mu and Theta classes.

Cloning and characterization of the major hepatic glutathione S-transferase from a marine teleost flatfish, the plaice (Pleuronectes platessa), with structural similarities to plant, insect and mammalian Theta class isoenzymes

A cDNA clone (PLGSTA) of 896 bp, containing an open reading frame encoding a 225-amino-acid polypeptide of M(r) 25,723, was isolated from a cDNA library constructed in lambda gt11 from the liver of a marine teleost flatfish, the plaice (Pleuronectes platessa). The identification of this cDNA as that coding for the subunit of the major cytosolic glutathione S-transferase of plaice liver, GST-A, was supported by its heterologous expression in and purification of its protein product from Escherichia coli. The recombinant-derived protein exhibited identical M(r) and immunoreactivity and a similar substrate specificity to GST-A previously isolated from plaice liver. Comparison of the deduced amino acid sequence of the plaice GST-A polypeptide with the primary structures of GSTs from other Phyla revealed that it showed the greatest similarity to plant, insect and mammalian Theta class GSTs. Southern blot analysis of plaice DNA hybridized to the PLGSTA cDNA showed a banding pattern indicative of the presence of a single gene. Northern blot analysis of a variety of plaice tissues showed hybridizing bands of approx. 1100 nucleotides in all tissues tested, with the highest relative amounts in liver and intestinal mucosa. A marked increase in hybridization intensity was observed in hepatic RNA samples from plaice treated with trans-stilbene oxide, suggesting that GST-A is induced by epoxides in this species.

The amino acid sequence of glutathione transferase from Proteus mirabilis, a prototype of a new class of enzymes

The complete amino acid sequence of glutathione transferase from Proteus mirabilis was determined. The sequence was reconstructed by analysis of peptides obtained after cleavage by trypsin, Glu-C and Asp-N endoproteinases. The enzyme subunit is composed of 203 amino acid residues corresponding to a molecular mass of 22856 Da. Comparison of this sequence with other known primary structures of the corresponding enzyme from different sources shows a low level of identity (17-26%) with only seven conserved residues in all the sequences considered. This novel glutathione transferase could represent the prototype of a new class, possibly including other bacterial enzymes.

An evolutionary perspective on glutathione transferases inferred from class-theta glutathione transferase cDNA sequences

We report the cDNA sequence for rat glutathione transferase (GST) subunit 5, which is one of at least three class Theta subunits in this species. This sequence, when compared with that of subunit 12 recently published by Ogura, Nishiyama, Okada, Kajita, Narihata, Watabe, Hiratsuka & Watabe [(1991) Biochem. Biophys. Res. Commun. 181, 1294-1300] proves that Theta is a separate multigene class of GST with little amino acid sequence identity with Mu-, Alpha- or Pi-class enzymes. The amino acid sequence identity of class-Theta subunits is highly conserved in rat, the fruitfly Drosophila, maize (Zea mays) and Methylobacterium, which suggests that this family is representative of the ancient progenitor GST gene and originates from the endosymbioses of a purple bacterium leading to the mitochondrion. The high conservation of class Theta brings into prominence that Alpha-, Mu- and Pi-class enzymes, which are not present in plants, derive from a Theta-class gene duplication before the divergence of fungi and animals and, given the binding properties of the Alpha-, Mu- and Pi-classes, suggests a role for these in the evolution of fungi and animals.

Glutathione S-transferase activity and isoenzyme composition in benign ovarian tumours, untreated malignant ovarian tumours, and malignant ovarian tumours after platinum/cyclophosphamide chemotherapy

Glutathione S-transferase (GST) isoenzyme composition, isoenzyme quantities and enzymatic activity were investigated in benign (n = 4) ovarian tumours and malignant ovarian tumours, before (n = 20) and after (n = 16) chemotherapy. Enzymatic activity of GST in cytosols was measured by determining 1-chloro-2,4-dinitrobenzene conjugation with glutathione, cytosolic GST subunits were determined by wide pore reversed phase HPLC, using a S-hexylglutathione-agarose affinity column, and isoelectric focussing. Both GST activity and GST pi amount were not related to histopathologic type, differentiation grade, or tumour volume index in untreated malignant tumours. GST isoenzyme patterns were identical in benign tumours and malignant tumours before and after platinum/cyclophosphamide chemotherapy, while GST pi was the predominant transferase. Mean GST activity and GST pi amount were decreased (P < 0.05) in malignant ovarian tumours after platinum/cyclophosphamide chemotherapy compared to untreated ovarian malignant tumours. No relation was found in untreated ovarian tumours between GST pi amount and response to platinum/cyclophosphamide chemotherapy. Thus, within the limitations of the current study no arguments were found for a role of GST in in vivo drug resistance of malignant ovarian tumours to platinum/cyclophosphamide chemotherapy.

Characterization of a human class-Theta glutathione S-transferase with activity towards 1-menaphthyl sulphate

A purification scheme is described for a glutathione S-transferase (GST) from human liver that catalyses the conjugation of 1-menaphthyl sulphate (MS) with GSH; the method devised results in an approx. 500-fold increase in specific activity towards MS. The human enzyme which metabolizes MS is a homodimer comprising subunits of M(r) 25,100, and immunochemical experiments have shown it to be a member of the class-Theta GSTs. Automated Edman degradation of this enzyme has confirmed that it is a Theta-class GST bu the amino acid sequence obtained differs from that of GST theta described previously [Meyer, Coles, Pemble, Gilmore, Fraser & Ketterer (1991) Biochem. J. 274, 409-414]. We have therefore designated the enzyme that catalyses the conjugation of MS with GSH GST T2-2* (in the absence of complete amino acid sequence data, the T1 and T2 subunits are provisionally designated T1* and T2*); the evidence which indicates that GST theta (which should possibly now be called GST T1-1*) and GST T2-2* represent distinct isoenzymes is discussed.