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

Proanthocyanidins mitigate bile acid-induced changes in GSTT2 levels in a panel of racially diverse patient-derived primary esophageal cell cultures

Persistent and symptomatic reflux of gastric and duodenal contents, known as gastroesophageal reflux disease (GERD), is the strongest risk factor for esophageal adenocarcinoma (EAC). Despite similar rates of GERD and other risk factors across racial groups, EAC progression disproportionately impacts Caucasians. We recently reported that elevated tissue levels of the detoxification enzyme GSTT2 in the esophagi of Blacks compared to Caucasians may contribute protection. Herein, we extend our research to investigate whether cranberry proanthocyanidins (C-PAC) mitigate bile acid-induced damage and GSTT2 levels utilizing a racially diverse panel of patient-derived primary esophageal cultures. We have shown that C-PACs mitigate reflux-induced DNA damage through GSTT2 upregulation in a rat esophageal reflux model, but whether effects are recapitulated in humans or differentially based on race remains unknown. We isolated normal primary esophageal cells from Black and Caucasian patients and assessed GSTT2 protein levels and cellular viability following exposure to a bile acid cocktail with and without C-PAC treatment. Constitutive GSTT2 levels were significantly elevated in Black (2.9-fold) compared to Caucasian patients, as were GSTT2 levels in Black patients with GERD. C-PAC treatment induced GSTT2 levels 1.6-fold in primary normal esophageal cells. GSTT2 induction by C-PAC was greatest in cells with constitutively low GSTT2 expression. Overall, C-PAC mitigated bile-induced reductions of GSTT2 and subsequent loss of cell viability regardless of basal GSTT2 expression or race. These data support that C-PAC may be a safe efficacious agent to promote epithelial fitness through GSTT2 induction and in turn protect against bile acid-induced esophageal injury.

A proteome-wide atlas of lysine-reactive chemistry

Recent advances in chemical proteomics have begun to characterize the reactivity and ligandability of lysines on a global scale. Yet, only a limited diversity of aminophilic electrophiles have been evaluated for interactions with the lysine proteome. Here, we report an in-depth profiling of >30 uncharted aminophilic chemotypes that greatly expands the content of ligandable lysines in human proteins. Aminophilic electrophiles showed disparate proteomic reactivities that range from selective interactions with a handful of lysines to, for a set of dicarboxaldehyde fragments, remarkably broad engagement of the covalent small-molecule-lysine interactions captured by the entire library. We used these latter 'scout' electrophiles to efficiently map ligandable lysines in primary human immune cells under stimulatory conditions. Finally, we show that aminophilic compounds perturb diverse biochemical functions through site-selective modification of lysines in proteins, including protein-RNA interactions implicated in innate immune responses. These findings support the broad potential of covalent chemistry for targeting functional lysines in the human proteome.

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.

Glutathione S-transferase M1 (GSTM1) and T1 (GSTT1) variants and breast cancer risk in Burkina Faso

Background and objective

Breast cancer remains the most common cause of cancer mortality in women. The aim of this study was to investigate associations between genetic variability in GSTM1 and GSTT1 and susceptibility to breast cancer.

Methods

Genomic DNA was extracted from blood samples for 80 cases of histologically diagnosed breast cancer and 100 control subjects. Genotyping analyses were performed by PCR-based methods. Associations between specific genotypes and the development of breast cancer were examined using logistic regression to calculate odds ratios [1] and 95% confidence intervals (95%CI).

Results

No correlation was found between GSTM1-null and breast cancer (OR = 1.83; 95%CI 0.90-3.71; p = 0.10), while GSTT1-null (OR = 2.42; 95%CI 1.17-5.02; p= 0.01) was associated with increased breast cancer risk. The GSTM1/GSTT1 double null was not associated with an increased risk of developing breast cancer (OR = 2.52; 95%CI 0.75-8.45; p = 0.20). Furthermore, analysis found no association between GSTM1-null (OR =1.12; 95%CI 0.08-15.50; p = 1.00) or GSTT1-null (OR = 1.71; 95%CI 0.13-22.51; p = 1.00) and the disease stage of familial breast cancer patients or sporadic breast cancer patients (GSTM1 (OR = 0.40; 95%CI 0.12-1.32; p = 0.20) and GSTT1 (OR = 1.41; 95%CI 0.39-5.12; p = 0.75)). Also, body mass index (BMI) was not associated with increased or decreased breast cancer risk in either GSTM1-null (OR = 0.60; 95%CI 0.21-1.68; p = 0.44) or GSTT1-null (OR = 0.60; 95%CI 0.21-1.68; p =0.45).

Conclusion

Our results suggest that only GSTT1-null is associated with increased susceptibility to breast cancer development.

Isomerization of Δ5-androstene-3,17-dione into Δ4-androstene-3,17-dione catalyzed by human glutathione transferase A3-3: a computational study identifies a dual role for glutathione

Glutathione transferases (GSTs) are important enzymes in the metabolism of electrophilic xenobiotic and endobiotic toxic compounds. In addition, human GST A3-3 also catalyzes the double bond isomerization of Δ5-androstene-3,17-dione (Δ(5)-AD) and Δ(5)-pregnene-3,20-dione (Δ(5)-PD), which are the immediate precursors of testosterone and progesterone. In fact, GST A3-3 is the most efficient human enzyme known to exist in the catalysis of these reactions. In this work, we have used density functional theory (DFT) calculations to propose a refined mechanism for the isomerization of Δ(5)-AD catalyzed by GST A3-3. In this mechanism the glutathione (GSH) thiol and Tyr9 catalyze the proton transfer from the Δ(5)-AD C4 atom to the Δ(5)-AD C6 atom, with a rate limiting activation energy of 15.8 kcal · mol(-1). GSH has a dual function, because it is also responsible for stabilizing the negative charge that is formed in the O3 atom of the enolate intermediate. The catalytic role of Tyr9 depends on significant conformational rearrangements of its side chain. Neither of these contributions to catalysis has been observed before. Residues Phe10, Leu111, Ala 208, and Ala 216 complete the list of the important catalytic residues. The mechanism detailed here is based on the GST A3-3:GSH:Δ(4)-AD crystal structure and is consistent with all available experimental data.

Glutathione S-transferase (GST) family in barley: identification of members, enzyme activity, and gene expression pattern

Barley (Hordeum vulgare) is one of the most important cereals in many developing countries where drought stress considerably diminishes agricultural production. Glutathione S-transferases (GSTs EC 2.5.1.18) are multifunctional enzymes which play a crucial role in cellular detoxification and oxidative stress tolerance. In this study, 84 GST genes were identified in barley by a comprehensive in silico approach. Sequence alignment and phylogenetic analysis grouped these HvGST proteins in eight classes. The largest numbers of the HvGST genes (50) were included in the Tau class followed by 21 genes in Phi, five in Zeta, two in DHAR, two in EF1G, two in Lambda, and one each in TCHQD and Theta classes. Phylogenetic analysis of the putative GSTs from Arabidopsis, rice, and barley indicated that major functional diversification within the GST family predated the monocot/dicot divergence. However, intra-specious duplication seems to be common. Expression patterns of five GST genes from Phi and Tau classes were investigated in three barley genotypes (Yusof [drought-tolerant], Moroc9-75 [drought-sensitive], and HS1 [wild ecotype]) under control and drought-stressed conditions, during the vegetative stage. All investigated genes were up-regulated significantly under drought stress and/or showed a higher level of transcripts in the tolerant cultivar. Additionally, GST enzyme activity was superior in Yusof and induced in the extreme-drought-treated leaves, while it was not changed in Moroc9-75 under drought conditions. Moreover, the lowest and highest levels of lipid peroxidation were observed in the Yusof and Moroc9-75 cultivars, respectively. Based on the achieved results, detoxification and antioxidant activity of GSTs might be considered an important factor in the drought tolerance of barley genotypes for further investigations.

Impact of polyphenol metabolites produced by colonic microbiota on expression of COX-2 and GSTT2 in human colon cells (LT97)

Polyphenols may play an important role in colon cancer prevention. After entering the colon, they are subjected to metabolism by the human gut microbiota. The objective of the present study was to analyze the impact of selected intestinal metabolites on modulation of enzymes involved in detoxification and inflammation in human adenoma cells LT97. LT97 cells were incubated with 3,4-dihydroxyphenylacetic acid (ES) and 3-(3,4-dihydroxyphenyl)-propionic acid (PS), metabolites of quercetin and chlorogenic acid/caffeic acid, respectively. The effect on cell number was analyzed using 4'- 6-diamino-2-phenylindole-dihydrochloride (DAPI)-staining. Modulation of glutathione S-transferase T2 (GSTT2) and cyclooxygenase-2 (COX-2) was measured by real-time PCR and Western blot. Comet assay was performed to assess the impact on DNA damage caused by the GSTT2 substrate cumene hydroperoxide (CumOOH). Polyphenol metabolites did not affect cell number but significantly upregulated GSTT2 expression and decreased COX-2. The latter was confirmed via Western blot. CumOOH-induced DNA damage was significantly reduced compared to the control. An upregulation of GSTT2 and downregulation of COX-2 could possibly contribute to the chemopreventive potential of polyphenols after degradation in the gut. Working with polyphenol metabolites is an important prerequisite to better understand the in vivo effects of pure polyphenols.

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.

Impact of apple polyphenols on GSTT2 gene expression, subsequent protection of DNA and modulation of proliferation using LT97 human colon adenoma cells

Apple extract (AE) enhances expression of glutathione S-transferases (e.g., GSTT2) in human colon cells (LT97). Therefore, aim of the present study was to identify functional consequences of GSTT2 induction by AE and to determine the relation of AE effects to isolated compounds. Polyphenol composition of AE was analyzed. LT97 cells were treated with AE or synthetic polyphenol mixture (SPM) under conditions that induced GSTT2, and challenged with GSTT2-2 substrate cumene hydroperoxide (CumOOH) to determine DNA damage using comet assay. Modulation of GSTT2 expression (real-time PCR) was reassessed, and the influence on cell proliferation and pro-oxidative potential of AE and SPM were assessed to understand additional mechanisms. Induction of GSTT2 by AE was accompanied by protection of LT97 cells from CumOOH-induced genotoxicity. Although SPM was unable to reflect AE-specific bioactivity related to GSTT2 modulation and anti-genotoxicity, inhibition of LT97 cell proliferation by SPM was comparable. Storage of AE caused changes in phenolic composition along with loss of activity regarding GSTT2 induction and amplified growth inhibition. At the applied concentrations, no H(2)O(2) formation was detectable with any of the substances. AE can protect against oxidatively induced DNA damage. Nevertheless, chemopreventive effects of AE strongly depend on the specific composition, which is modified by storage.

Gene expression profiles in human peripheral blood mononuclear cells as biomarkers for nutritional in vitro and in vivo investigations

Identification of chemopreventive substances may be achieved by measuring biological endpoints in human cells in vitro. Since generally only tumour cells are available for such investigations, our aim was to test the applicability of peripheral blood mononuclear cells (PBMC) as an in vitro primary cell model since they mimic the human in vivo situation and are relatively easily available. Cell culture conditions were refined, and the basal variation of gene expression related to drug metabolism and stress response was determined. Results were compared with profiles of an established human colon cell line (HT29) as standard. For biomarker development of nutritional effects, PBMC and HT29 cells were treated with potentially chemopreventive substances (chrysin and butyrate), and gene expression was determined. Key results were that relevant stress response genes, such as glutathione S-transferase T2 (GSTT2) and GSTM2, were modulated by butyrate in PBMC as in HT29 cells, but the blood cells were less sensitive and responded with high individual differences. We conclude that these cells may serve as a surrogate tissue in dietary investigations and the identified differentially expressed genes have the potential to become marker genes for population studies on biological effects.

Residue 234 in glutathione transferase T1-1 plays a pivotal role in the catalytic activity and the selectivity against alternative substrates

GST (glutathione transferase) T1-1 plays an important role in the biotransformation of halogenated alkanes, which are used in large quantities as solvents and occur as environmental pollutants. Many reactions that are catalysed by GST T1-1 qualify as detoxification processes, but some reactions with dihalogenated alkanes lead to reactive products more toxic than the substrates. Murine GST T1-1 is particularly active with dichloromethane, which may explain the high carcinogenicity of dichloromethane in the mouse. Human GST T1-1 activity is considerably lower with halogenated hydrocarbons and some related substrates. Human GST T1-1 is polymorphic with a frequent null phenotype, suggesting that it is advantageous, under some circumstances, to lack the functional enzyme, which catalyses GSH conjugations that may cause bioactivation. The present study shows that amino acid residue 234 is a determinant of the differences in catalytic efficiency between the human and the rodent enzymes. The replacement of Trp234 in human GST T1-1 by arginine, found in the rodent enzyme, enhanced the alkyltransferase activity by an order of magnitude with a series of homologous iodoalkanes and some typical GST substrates. The specific activity of the alternative mutant Trp234-->Lys was lower than for the parental human GST T1-1 with many substrates, showing that a positive charge is not sufficient for increased activity. The enhanced activity of Trp234-->Arg with alkylating agents was dependent on the substrate tested, whereas no increase of the peroxidase activity with cumene hydroperoxide was noted. Residue 234 therefore is also involved in the control of the substrate selectivity of GST T1-1.

Investigation of glutathione S-transferase M1 and T1 deletions in lung cancer

Glutathione S-transferases (GSTs) M1 and T1 are known to be polymorphic in humans. Both polymorphisms are due to gene deletions which are responsible for the existence of null genotypes. Previous studies have suggested that GST genotypes may play a role in determining susceptibility to a number of unrelated cancers, including lung cancer. The GSTM1 and GSTT1 polymorphisms were determined by PCR-based analysis in 75 lung cancer patients and 55 controls. The unconditional logistic regression analysis was used to calculate ORs and 95% CI. The frequencies of GSTM1 and GSTT1 null genotypes were 37.3 and 22.7% in lung cancer patients and 27.3 and 16.4% in controls, respectively. When analyzed by histology the GSTM1 null genotype was more prevalent in squamous-cell carcinoma and adenocarcinoma patients. Whereas, GSTT1 null genotype frequency was lower in small-cell lung cancer patients than controls. But these differences were not statistically significant. According to smoking status, null genotype for both gene are associated with an increase in risk for lung cancer. Our results suggest that GSTM1 and GSTT1 polymorphisms may play a role in the development of lung cancer for some histological subtypes and modifies the risk of smoking-related lung cancer.

GSTT2 promoter polymorphisms and colorectal cancer risk

Background

Glutathione S-transferases are a group of enzymes that participate in detoxification and defense mechanisms against toxic carcinogens and other compounds. These enzymes play an important role in human carcinogenesis. In the present study, we sought to determine whether GSTT2 promoter single nucleotide polymorphisms (SNPs) are associated with colorectal cancer risk.

Methods

A total of 436 colorectal cancer patients and 568 healthy controls were genotyped for three GSTT2 promoter SNPs (-537G>A, -277T>C and -158G>A), using real-time TaqMan assay and direct sequencing. An electrophoretic mobility shift assay (EMSA) was performed to determine the effects of polymorphisms on protein binding to the GSTT2 promoter.

Results

The -537A allele (-537G/A or A/A) was significantly associated with colorectal cancer risk (OR = 1.373, p = 0.025), while the -158A allele (-158G/A or A/A) was involved in protection against colorectal cancer (OR = 0.539, p = 0.032). Haplotype 2 (-537A, -277T, -158G) was significantly associated with colorectal cancer risk (OR = 1.386, p = 0.021), while haplotype 4 (-537G, -277C, -158A) protected against colorectal cancer (OR = 0.539, p = 0.032). EMSA data revealed lower promoter binding activity in the -537A allele than its -537G counterpart.

Conclusion

Our results collectively suggest that SNPs and haplotypes of the GSTT2 promoter region are associated with colorectal cancer risk in the Korean population.

Polymorphism of human mu class glutathione transferases

OBJECTIVES AND METHODS

A combined database mining approach was used to detect polymorphisms in the mu class glutathione-S-transferase (GST) genes. Although a large number of potential polymorphisms were detected in the five genes that comprise the Mu class GSTs using sequence alignment programs and by searching single nucleotide polymorphism databases, the majority were not validated or detected in three major ethnic populations (African, Southern Chinese and Australian European).

RESULTS

Two new polymorphisms were detected and characterized in the GSTM3 gene. A rare pG147W substitution was detected only in the Southern Chinese subjects. A more common pV224I substitution was found in each of the ethnic groups studied, and significant differences in allele frequencies were observed between each group. These two polymorphisms can combine to form four distinct haplotypes (GSTM3A [p.G147;V224], GSTM3C [p.G147;I224], GSTM3D [p.W147;V224], GSTM3E [p.W147;I224]). The four isoforms were expressed in Escherichia coli and characterized enzymatically with several substrates including 1-chloro-2,4-dinitrobenzene (CDNB), cumene hydroperoxide and t-nonenal. GSTM3-3 containing the variant p.W147 residue tended to show diminished specific activity and catalytic efficiency with CDNB. In contrast, GSTM3-3 containing the variant p.I224 residue tended to show increased specific activity and catalytic efficiency with CDNB. Interactions between the different p.147 and p.224 residues were also observed, with the GSTM3C isoform exhibiting the greatest activity with each substrate, and GSTM3E the lowest.

CONCLUSION

These functional polymorphisms may play a significant role in modulating the ability of GSTM3-3 to metabolize substrates such as the chemotherapeutic agent 1,3-bis(2-chloroethyl)-1-nitrosourea.

Detection and characterization of a novel functional polymorphism in the GSTT1 gene

A novel functional polymorphism in the GSTT1 gene associated with the non-conjugator phenotype has been identified. Sequencing of GSTT1 cDNA revealed a single nucleotide substitution, 310A>C, that altered the amino acid residue 104 from threonine to proline (T104P). Modelling studies of GSTT1 have suggested that residue 104 is located in the middle of alpha-helix 4. Introduction of an alpha-helix-disrupting proline most likely distorts the conformation of the protein. Individuals that lacked GSTT1 activity and carried the variant allele, tentatively denoted GSTT1*B, had no detectable GSTT1 immunoreactive protein. An allele-specific polymerase chain reaction method was developed to determine the frequency of the GSTT1*B allele. In 497 ethnic Swedes, the frequency of the active GSTT1*A allele was 0.65 [95% confidence interval (CI) 0.62-0.68] whereas the frequencies of the non-functional alleles GSTT1*O and the novel GSTT1*B allele were 0.34 (CI 0.31-0.37) and 0.01 (CI 0.01-0.02), respectively. In 100 Swedish Saamis, the GSTT1*B allele appeared to be slightly more common with a frequency of 0.03 (CI 0.01-0.07). The GSTT1 enzyme activity was measured in erythrocytes using methyl chloride as substrate. Individuals with the GSTT1*A/*A genotype had a two-fold higher GSTT1 activity compared to individuals with the GSTT1*A/*B genotype and subjects with the GSTT1*O/*B genotype totally lacked GSTT1 activity, indicating a strict gene-dose effect. By combining the analyses for the novel single nucleotide polymorphism with analyses for the deletion polymorphism, the accuracy in predicting all three GSTT1 conjugator phenotypes was improved from 96% to 99%.

Identification and characterization of GSTT3, a third murine Theta class glutathione transferase

A novel Theta class glutathione transferase (GST) isoenzyme from mouse termed mGSTT3 has been identified by analysis of the expressed sequence tag database. The gene encoding mGSTT3 is clustered with the mGSTT1 and mGSTT2 genes on chromosome 10 and has an exon/intron structure that is similar to that of the other Theta class genes. mGSTT3 is expressed strongly in the liver and to a decreasing extent in the kidney and testis. Recombinant mGSTT3-3 expressed in Escherichia coli had a substrate-specificity profile that differed significantly from that of GSTT1-1 and GSTT2-2 isoenzymes. A molecular model of mGSTT3 suggested that, in comparison with GSTT2, a decrease in volume of the hydrophobic substrate-binding site and the loss of the sulphate-binding pocket prevents its use of the GSTT2 substrate 1-menaphthyl sulphate.

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.

Expression of glutathione S-transferase alpha, P1-1 and T1-1 in the human gastrointestinal tract

Glutathione S-transferases (GSTs) form a family of enzymes, which play an important role in the prevention of cancer by detoxifying numerous potentially carcinogenic compounds. GSTs catalyze the conjugation of glutathione to such harmful molecules, and enable their secretion. Human GSTs can be divided into five main classes. The theta class of isoenzymes was only recently identified and limited (immunohistochemical) data on these enzymes are available. In the present study, paraffin-embedded sections of different gastrointestinal tissues were analyzed immunohistochemically for GSTalpha, GSTP1-1 and GSTT1-1 expression using specific antibodies. GSTalpha, GSTP1-1 and GSTT1-1 were highly expressed in all gastrointestinal tissues examined, with a unique cellular distribution. GSTT1-1 is the first GST isoenzyme demonstrated in duodenal Paneth cells and glands of Brunner. The common expression of GSTalpha, GSTT1-1 and GSTP1-1 in many cell types along the human gastrointestinal tract suggests an important role in the protection against carcinogens and other xenobiotics.

Mutagenic analysis of conserved arginine residues in and around the novel sulfate binding pocket of the human Theta class glutathione transferase T2-2

The human Theta class glutathione transferase GSTT2-2 has a novel sulfatase activity that is not dependent on the presence of a conserved hydrogen bond donor in the active site. Initial homology modeling and the crystallographic studies have identified three conserved Arg residues that contribute to the formation of (Arg107 and Arg239), and entry to (Arg242), a sulfate binding pocket. These residues have been individually mutated to Ala to investigate their potential role in substrate binding and catalysis. The mutation of Arg107 had a significant detrimental effect on the sulfatase reaction, while the Arg242 mutation caused only a small reduction in sulfatase activity. Surprisingly, the Arg239 had an increased activity resulting from a reduction in stability. Thus, Arg239 appears to play a role in maintaining the architecture of the active site. Electrostatic calculations performed on the wild-type and mutant forms of the enzyme are in good agreement with the experimental results. These findings, along with docking studies, suggest that prior to conjugation, the location of 1-menaphthyl sulfate, a model substrate for the sulfatase reaction, is approximately midway between the position ultimately occupied by the naphthalene ring of 1-menaphthylglutathione and the free sulfate. It is further proposed that the Arg residues in and around the sulfate binding pocket have a role in electrostatic substrate recognition.

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.

Glutathione S-transferases of the yeast Yarrowia lipolytica have unusually large molecular mass

Two similar glutathione S-transferases (GSTs), which do not bind to glutathione- or S-hexylglutathione-agarose affinity resins, have been purified from the yeast Yarrowia lipolytica. An approx. 400-fold purification was obtained by a combination of DEAE-Sephadex, phenyl-Sepharose, hydroxyapatite and Mono-Q anion-exchange chromatography. The native molecular mass of both proteins was estimated as approx. 110 kDa by both Superose-12 gel-filtration chromatography and non-denaturing electrophoresis. SDS/PAGE indicated a subunit mass of 50 kDa. Reverse-phase HPLC of purified proteins gave a single, well-resolved, peak, suggesting that the proteins are homodimers. Identical behaviour on HPLC, native electrophoresis and SDS/PAGE, N-terminal sequencing, sensitivity to a panel of inhibitors and identical specific activities with 1-chloro-2,4-dinitrobenzene as substrate suggest that the two isoenzymes are very similar. The enzymes do not immunoblot with antisera to any of the main GST classes, and N-terminal sequencing suggests no clear relationship with previously characterized enzymes, such as that of the fungus, Phanerochaete chrysosporium [Dowd, Buckley and Sheehan (1997) Biochem. J. 324, 243-248]. It is possible that the two isoenzymes arise as a result of post-translational modification of a single GST isoenzyme.

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.

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.

Effect of glutathione S-transferase M1 polymorphisms on biomarkers of exposure and effects

Genotypes responsible for interindividual differences in ability to activate or detoxify genotoxic agents are recognized as biomarkers of susceptibility. Among the most studied genotypes are human glutathione transferases. The relationship of genetic susceptibility to biomarkers of exposure and effects was studied especially in relation to the genetic polymorphism of glutathione S-transferase M1 (GSTM1). For this review papers reporting the effect of GSTM1 genotype on DNA adducts, protein adducts, urine mutagenicity, Comet assay parameters, chromosomal aberrations, sister chromatid exchanges (SCE), micronuclei, and hypoxanthine-guanine phosphoribosyl transferase mutations were assessed. Subjects in groups occupationally exposed to polycyclic aromatic hydrocarbons, benzidine, pesticides, and 1,3-butadiene were included. As environmentally exposed populations, autopsy donors, coal tar-treated patients, smokers, nonsmokers, mothers, postal workers, and firefighters were followed. From all biomarkers the effect of GSTM1 and N-acetyl transferase 2 was seen in coke oven workers on mutagenicity of urine and of glutathione S-transferase T1 on the chromosomal aberrations in subjects from 1,3-butadiene monomer production units. Effects of genotypes on DNA adducts were found from lung tissue of autopsy donors and from placentas of mothers living in an air-polluted region. The GSTM1 genotype affected mutagenicity of urine in smokers and subjects from polluted regions, protein adducts in smokers, SCE in smokers and nonsmokers, and Comet assay parameters in postal workers. A review of all studies on GSTM1 polymorphisms suggests that research probably has not reached the stage where results can be interpreted to formulate preventive measures. The relationship between genotypes and biomarkers of exposure and effects may provide an important guide to the risk assessment of human exposure to mutagens and carcinogens.

Zeta, a novel class of glutathione transferases in a range of species from plants to humans

Sequence alignment and phylogenetic analysis has identified a new subgroup of glutathione S-transferase (GST)-like proteins from a range of species extending from plants to humans. This group has been termed the Zeta class. An atomic model of the N-terminal domain suggests that the members of the Zeta class have a similar structure to that of other GSTs, binding glutathione in a similar orientation in the G site. Recombinant human GSTZ1-1 has been expressed in Escherichia coli and characterized. The protein is a dimer composed of 24.2 kDa subunits and has minimal glutathione-conjugating activity with ethacrynic acid and 7-chloro-4-nitrobenz-2-oxa-1, 3-diazole. Although low in comparison with other GSTs, GSTZ1-1 has glutathione peroxidase activity with t-butyl and cumene hydroperoxides. The members of the Zeta class have been conserved over a long evolutionary period, suggesting that they might have a role in the metabolism of a compound that is common in many living cells.

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.

The role of human glutathione transferases and epoxide hydrolases in the metabolism of xenobiotics

Human glutathione transferases (GSTs) are a multigene family of enzymes that are involved in the metabolism of a wide range of electrophilic compounds of both exogenous and endogenous origin. GSTs are generally recognized as detoxifying enzymes by catalyzing the conjugation of these compounds with glutathione, but they may also be involved in activation of some carcinogens. The memmalian GSTs can be differentiated into four classes of cytosolic enzymes and two membrane bound enzymes. Human epoxide hydrolases (EHs) catalyze the addition of water to epoxides to form the corresponding dihydrodiol. The enzymatic hydration is essentially irreversible and produces mainly metabolites of lower reactivity that can be conjugated and excreted. The reaction of EHs is therefore generally regarded as detoxifying. The mammalian EHs can be distinguished by their physical and enzymatic properties. Microsomal EH (mEH) exhibits a broad substrate specificity, while the soluble EH (sEH) is an enzyme with a "complementary" substrate specificity to mEH. Cholesterol EH and leukotriene A4 hydrolase are two EHs with very limited substrate specificity. The activities of either GSTs or EHs expressed in vivo exhibit a relatively large interindividual variation, which might be explained by induction, inhibition, or genetic factors. These variations in levels or activities of individual isoenzymes are of importance with respect to an individual's susceptibility to genotoxic effects. This article gives a general overview of GSTs and EHs, discussing the modulation of activities, determination of these enzymes ex vivo, and the polymorphic expression of some isoenzymes.

Glutathione S-transferases from the white-rot fungus, Phanerochaete chrysosporium

A glutathione S-transferase (GST) was purified to homogeneity from the white-rot fungus, Phanerochaete chrysosporium, by affinity chromatography on glutathione-agarose followed by Mono-Q ion-exchange FPLC. This protein immunoblotted with antisera to rat Theta class GST 5-5 and also showed N-terminal sequence similarity to the Theta class, including the presence of a conserved serine residue that has been specifically implicated in catalysis in this class [Wilce, Board, Feil and Parker (1995) EMBO J. 14, 2133-2143] and other residues conserved in plant sequences. Catalytic activity was found to be highly labile in the purified protein, although preliminary evidence for activity (approx. 120 m-units/mg) with 1,2-epoxy-3-(p-nitrophenoxy)propane was obtained in some preparations. The enzyme seems to be a dimer with a subunit molecular mass of 25 kDa by SDS/PAGE. The native molecular masses estimated by non-denaturing electrophoresis and by Superose-12 gel filtration were 58 and 45 kDa respectively. A second protein purified in this study also gave low level of activity with 1,2-epoxy-3-(p-nitrophenoxy)propane and had a subunit molecular mass of 28 kDa (native size 62-63 kDa), but did not immunoblot with any GST class and seemed to be N-terminally blocked.

Homology model for the human GSTT2 Theta class glutathione transferase

A tertiary model of the human GSTT2 Theta class glutathione transferase is presented based on the recently solved crystal structure of a related thetalike isoenzyme from Lucilia cuprina. Although the N-terminal domains are quite homologous, the C-terminal domains share less than about 20% identity. The model is used to consolidate the role of Ser 11 in the active site of the enzyme as well as to identify other residues and mechanisms of likely catalytic importance. The T2 subfamily of theta class enzymes have been shown to inactivate reactive sulfate esters arising from arylmethanols. A possible reaction pathway involving the conjugation of glutathione with one such sulfate ester, 1-menaphthyl-sulfate, is described. It is also proposed that the C-terminal region of the enzyme plays an important role in allowing substrate access to the active site.

Mutagenesis of the active site of the human Theta-class glutathione transferase GSTT2-2: catalysis with different substrates involves different residues

The role of serine-11 in the catalytic mechanism of recombinant human GSTT2-2 was examined by site-directed mutagenesis. Amino acid sequence comparison of the Theta-class isoenzymes has identified a conserved serine residue in the N-terminal domain [Wilce, Board, Feil and Parker (1995) EMBO J. 14, 2133-2143]. This conserved serine has been implicated in the activation of the enzyme-bound glutathione [Board, Coggan and Parker (1995) Biochem. J. 311, 247-250]. Mutating the equivalent serine (residue 11) of GSTT2-2 to Ala, Thr or Tyr abolished the catalytic properties of GSTT2-2 with cumene hydroperoxide and ethacrynic acid as second substrate. However, with l-menaphthyl sulphate (MSu) as the second substrate, the specific activity of the S11A mutant was doubled, while the S11T mutant retained half the wild-type activity and the S11Y mutant was inactive. The role of Ser-11 in catalysis seems to vary with different second substrates. In the substitution reaction with MSu, GSTT2-2 activity appears to depend on the size of the Ser-11 replacement rather than the presence of a side-chain hydroxy group. In addition, the reaction rate appears to be a function of pH, and there is no non-enzymic reaction even at high pH. We demonstrated that a reaction between MSu and an alternative thiol such as L-cysteine or 2-mercaptoethanol can take place in the presence of S-methylglutathione and GSTT2-2. We propose that the catalytic activity of GSTT2-2 with MSu is preceded by a conformational or charge modification to the enzyme upon the binding of glutathione or S-methylglutathione. This is followed by the binding of MSu and the subsequent removal of the sulphate group, giving rise to the carbonium ion of l-methylnaphthelene as the electrophile that reacts with the nucleophilic species. The reaction mechanism of GSTT2-2 with MSu may represent a novel function of GSTT2-2 as a glutathione-dependent sulphatase.

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.

Purification and characterization of a new glutathione S-transferase, class theta, from human erythrocytes

A new polymorphic form of glutathione S-transferase (GST), metabolising monohalogenated methanes, ethylene oxide and dichloromethane, has been purified from human erythrocytes and characterized. Several characteristics, such as similar elution patterns on different chromatographic matrices, KM-values and activity towards antibodies, confirm a previous assumption that this novel GST is a class theta enzyme. Although the presence or absence of the enzyme activity in human red blood cells is parallel with the polymorphism of the human GST T1 gene, the new GST theta in red blood cells may differ from the known GST T1-1 enzyme from other tissues in terms of substrate specificity, since established GST T1-1 substrates [1,2-epoxy-3-(p-nitro-phenoxy)propane and p-nitro-benzyl chloride] are not metabolized. The substrate specificity of the new enzyme in erythrocytes resembles more closely that of GST T2-2, most likely due to a common N-terminal modification which modifies substrate binding. The new polymorphic GST-isoform in human red blood cells therefore may be considered to represent an N-terminally modified isoform of GST T1-1.

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).

Purification and characterization of a recombinant human Theta-class glutathione transferase (GSTT2-2)

A cDNA encoding the human Theta-class glutathione transferase GSTT2-2 was expressed in Escherichia coli as a ubiquitin fusion protein. The co-translational removal of the ubiquitin by a cloned ubiquitin-specific protease, Ubp1, generates enzymically active GSTT2-2 without any additional N-terminal residues. The recombinant isoenzyme was purified to apparent homogeneity by DEAE anion-exchange, gel filtration, dye ligand chromatography and high resolution anion-exchange chromatography on Mono Q FPLC. The recombinant enzyme had significant activity with a range of substrates, including cumene hydroperoxide and 1-menapthyl sulphate. The activity of GSTT2-2 with a range of secondary lipid peroxidation products such as the trans,trans-alka-2,4-dienals and trans-alk-2-enals, as well as its glutathione peroxidase activity with organic hydroperoxides, suggest that it may play a significant role in protection against the products of lipid peroxidation.

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.

Production and characterization of monoclonal antibodies against class theta glutathione S-transferase T1-1

The recently discovered human class theta glutathione S-transferase T1-1 (GSTT1-1) is responsible for the GSH-dependent detoxification of naturally occurring monohalomethanes. The detoxifying role of GSTT1-1 has not been investigated in cancer susceptibility and the polymorphism of the protein is unknown in different populations. The purpose of our work was to produce a panel of mouse monoclonal antibodies (MAbs) that could bind to different regions of the GSTT1-1 protein and would help us select suitable MAbs for Western blot analyses and immunohistochemistry, and develop an ELISA assay for detection of GSTT1-1 in whole blood. Six highly specific MAbs were generated against GSTT1-1. Out of six MAbs, one was able to recognize only the native form of the enzyme and possesses two binding sites on the dimeric GSTT1-1 molecule. The other five MAbs bind to both native and denatured GSTT1-1 enzyme in direct and antigen capture ELISA or Western blot. The antibodies recognize at least four different epitopes on the GSTT1-1 molecule. Using MAbs 4G1 and 2D8, a sensitive ELISA assay for determination of GSTT1-1 in whole blood was developed.

Three-dimensional structure of glutathione S-transferase from Arabidopsis thaliana at 2.2 A resolution: structural characterization of herbicide-conjugating plant glutathione S-transferases and a novel active site architecture

Glutathione S-transferases (GST) are a family of multifunctional enzymes involved in the metabolization of a broad variety of xenobiotics and reactive endogenous compounds. The interest in plant glutathione S-transferases may be attributed to their agronomic value, since it has been demonstrated that glutathione conjugation for a variety of herbicides is the major resistance and selectivity factor in plants. The three-dimensional structure of glutathione S-transferase from the plant Arabidopsis thaliana has been solved by multiple isomorphous replacement and multiwavelength anomalous dispersion techniques at 3 A resolution and refined to a final crystallographic R-factor of 17.5% using data from 8 to 2.2 A resolution. The enzyme forms a dimer of two identical subunits each consisting of 211 residues. Each subunit is characterized by the GST-typical modular structure with two spatially distinct domains. Domain I consists of a central four-stranded beta-sheet flanked on one side by two alpha-helices and on the other side by an irregular segment containing three short 3(10)-helices, while domain II is entirely helical. The dimeric molecule is globular with a prominent large cavity formed between the two subunits. The active site is located in a cleft situated between domains I and II and each subunit binds two molecules of a competitive inhibitor S-hexylglutathione. Both hexyl moieties are oriented parallel and fill the H-subsite of the enzyme's active site. The glutathione peptide of one inhibitor, termed productive binding, occupies the G-subsite with multiple interactions similar to those observed for other glutathione S-transferases, while the glutathione backbone of the second inhibitor, termed unproductive binding, exhibits only weak interactions mediated by two polar contacts. A most striking difference from the mammalian glutathione S-transferases, which share a conserved catalytic tyrosine residue, is the lack of this tyrosine in the active site of the plant glutathione S-transferase.

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.

Evidence for an essential serine residue in the active site of the Theta class glutathione transferases

A consistent feature of the Alpha-, Mu- and Pi-class glutathione transferases (GSTs) is the presence near the N-terminus of a tyrosine residue that contributes to the activation of glutathione. While this residue appears to be conserved in many Theta-class GSTs, its absence in some suggested that the Theta-class GSTs may have a significantly different structure or catalytic mechanism. The elucidation of the crystal structure of the Theta-class GST from the Australian sheep blowfly, Lucilia cuprina, has indicated that a serine residue rather than a tyrosine residue can form a hydrogen bond with the glutathionyl sulphur atom. The present studies show that mutation of Ser-9 to alanine substantially inactivates the L. cuprina GST, confirming its importance in the reaction mechanism. As this serine is conserved in all Theta-class enzymes reported so far, it seems that an active-site serine is a significant factor that distinguishes the Theta-class GSTs from members of the Alpha-, Mu- and Pi-class isoenzymes.

Polymorphism in the glutathione conjugation activity of human erythrocytes towards ethylene dibromide and 1,2-epoxy-3-(p-nitrophenoxy)-propane

In this study a polymorphism in the conjugating activity of human erythrocyte cytosol towards the dihaloethane, ethylene dibromide (EDB; 1,2-dibromoethane) was found. Two out of 12 human erythrocyte cytosols did not catalyze the formation of glutathione (GSH) conjugates of [1,2-14C]EDB. Ten cytosols formed the S,S'-ethylenebis(GSH) conjugate at a rate ranging from 0.5 to 3.2 (mean 1.76 +/- 0.95) pmol min-1 (mg protein)-1. The activity of the cytosols towards EDB was compared with the activity towards 1,2-epoxy-3-(p-nitrophenoxy)-propane (EPNP) and 1-chloro-2,4-dinitrobenzene (CDNB). The GSH conjugates formed from EDB, EPNP and CDNB were all quantified by HPLC. Every cytosol was active with the classical GST substrate CDNB (2.04 +/- 0.74 nmol min-1 (mg protein)-1). The two samples not showing any detectable activity towards EDB were also inactive towards EPNP: The activity towards EDB correlated significantly with EPNP (rs = 0.90, P < 0.005; Spearman's rank correlation), but not with CDNB (rs = 0.36, P > 0.10). In the incubations with EPNP, the alpha-, mu-, and pi- class glutathione S-transferase (GST) inhibitor S-hexyl(GSH) was included, indicating that the class-theta GST is the principal GST class conjugating EDB in erythrocyte cytosol. The apparent polymorphism of GST-theta which has recently been recognized to be crucial for several mono- and dihalomethanes, will thus also have considerable implications for the risk assessment of EDB.