Human Mu-class glutathione S-transferases present in liver, skeletal muscle and testicular tissue

Glutathione and glutathione-dependent enzymes: From biochemistry to gerontology and successful aging

The tripeptide glutathione (GSH), namely γ-L-glutamyl-L-cysteinyl-glycine, is an ubiquitous low-molecular weight thiol nucleophile and reductant of utmost importance, representing the central redox agent of most aerobic organisms. GSH has vital functions involving also antioxidant protection, detoxification, redox homeostasis, cell signaling, iron metabolism/homeostasis, DNA synthesis, gene expression, cysteine/protein metabolism, and cell proliferation/differentiation or death including apoptosis and ferroptosis. Various functions of GSH are exerted in concert with GSH-dependent enzymes. Indeed, although GSH has direct scavenging antioxidant effects, its antioxidant function is substantially accomplished by glutathione peroxidase-catalyzed reactions with reductive removal of H2O2, organic peroxides such as lipid hydroperoxides, and peroxynitrite; to this antioxidant activity also contribute peroxiredoxins, enzymes further involved in redox signaling and chaperone activity. Moreover, the detoxifying function of GSH is basically exerted in conjunction with glutathione transferases, which have also antioxidant properties. GSH is synthesized in the cytosol by the ATP-dependent enzymes glutamate cysteine ligase (GCL), which catalyzes ligation of cysteine and glutamate forming γ-glutamylcysteine (γ-GC), and glutathione synthase, which adds glycine to γ-GC resulting in GSH formation; GCL is rate-limiting for GSH synthesis, as is the precursor amino acid cysteine, which may be supplemented as N-acetylcysteine (NAC), a therapeutically available compound. After its cell export, GSH is degraded extracellularly by the membrane-anchored ectoenzyme γ-glutamyl transferase, a process occurring, as GSH synthesis and export, in the γ-glutamyl cycle. GSH degradation occurs also intracellularly by the cytoplasmic enzymatic ChaC family of γ-glutamyl cyclotransferase. Synthesis and degradation of GSH, together with its export, translocation to cell organelles, utilization for multiple essential functions, and regeneration from glutathione disulfide by glutathione reductase, are relevant to GSH homeostasis and metabolism. Notably, GSH levels decline during aging, an alteration generally related to impaired GSH biosynthesis and leading to cell dysfunction. However, there is evidence of enhanced GSH levels in elderly subjects with excellent physical and mental health status, suggesting that heightened GSH may be a marker and even a causative factor of increased healthspan and lifespan. Such aspects, and much more including GSH-boosting substances administrable to humans, are considered in this state-of-the-art review, which deals with GSH and GSH-dependent enzymes from biochemistry to gerontology, focusing attention also on lifespan/healthspan extension and successful aging; the significance of GSH levels in aging is considered also in relation to therapeutic possibilities and supplementation strategies, based on the use of various compounds including NAC-glycine, aimed at increasing GSH and related defenses to improve health status and counteract aging processes in humans.

Alpha-class glutathione S-transferases involved in the detoxification of aflatoxin B1 in ducklings

The objective of this study was to identify the key glutathione S-transferase (GST) isozymes involved in the detoxification of Aflatoxin B₁ (AFB₁) in ducks' primary hepatocytes. The full-length cDNA encoding the 10 GST isozymes (GST, GST3, GSTM3, MGST1, MGST2, MGST3, GSTK1, GSTT1, GSTO1 and GSTZ1) were isolated/synthesized from ducks' liver and cloned into the pcDNA3.1(+) vector. The results showed that pcDNA3.1(+)-GSTs plasmids were successfully transferred into the ducks' primary hepatocytes and the mRNA of the 10 GST isozymes were overexpressed by 1.9-3274.7 times. Compared to the control, 75 μg/L (IC₃₀) or 150 μg/L (IC₅₀) AFB₁ treatment reduced the cell viability by 30.0-50.0% and increased the LDH activity by 19.8-58.2% in the ducks' primary hepatocytes. Notably, the AFB₁-induced changes in cell viability and LDH activity were mitigated by overexpression of GST and GST3. Compared to the cells treated with AFB₁, exo-AFB₁-8,9-epoxide (AFBO)-GSH, as the major detoxified product of AFB₁, was increased in the cells overexpression of GST and GST3. Moreover, the sequences, phylogenetic and domain analysis revealed that the GST and GST3 were orthologous to Meleagris gallopavo GSTA3 and GSTA4. In conclusion, this study found that the ducks' GST and GST3 were orthologous to Meleagris gallopavo GSTA3 and GSTA4, which were involved in the detoxification of AFB₁ in ducks' primary hepatocytes.

Moonlighting in drug metabolism

Drug metabolizing enzymes catalyze the biotransformation of many of drugs and chemicals. The drug metabolizing enzymes are distributed among several evolutionary families and catalyze a range of detoxication reactions, including oxidation/reduction, conjugative, and hydrolytic reactions that serve to detoxify potentially toxic compounds. This detoxication function requires that drug metabolizing enzymes exhibit substrate promiscuity. In addition to their catalytic functions, many drug metabolizing enzymes possess functions unrelated to or in addition to catalysis. Such proteins are termed 'moonlighting proteins' and are defined as proteins with multiple biochemical or biophysical functions that reside in a single protein. This review discusses the diverse moonlighting functions of drug metabolizing enzymes and the roles they play in physiological functions relating to reproduction, vision, cell signaling, cancer, and transport. Further research will likely reveal new examples of moonlighting functions of drug metabolizing enzymes.

Genotype and Phenotype of Glutathione-S-Transferase in Patients with Head and Neck Carcinoma

OBJECTIVE: Association of glutathione-S-transferase M1 (GSTM1) polymorphisms and cancer has been demonstrated. Possible underlying mechanisms and genotype-phenotype correlations are not adequately investigated. The aim of this study was to investigate the influence of the GSTM1-null-genotype on the level of GSTM enzyme concentration and on the enzyme activity of GST in patients with head and neck cancer (HNC).

METHODS: We investigated in 83 patients and 91 healthy controls the GSTM1 polymorphisms, GSTM1 protein concentration, GSTM1 protein in tumor tissues, and total GST enzyme activity.

RESULTS: Total GST enzyme activity was significantly lower in patients with HNC (208 ± 9 μmol/min1) than in controls (264 ± 11 μmol/min1, P < 0.0001) but did not depend on GSTM1-genotype ( P = 0.1). GSTM protein concentration in null-genotype patients (3.6 ± 2.5 μg/mL, mean ± SE) was significantly lower than in GSTM1 allele carriers (26.7 ± 9.6 μg/ml, P < 0.0001); GSTM protein expression did not depend on GSTM1-genotype ( P > 0.5).

CONCLUSION: GST enzyme activity in patients with HNC is suppressed, indicating impaired detoxification capacity of tobacco-smoke-related carcinogens. This suppression is not correlated with the GSTM1-genotype. (Otolaryngol Head Neck Surg 2004;130:718-25.)

Identification and characterization of seven glutathione S-transferase genes from citrus red mite, Panonychus citri (McGregor)

The citrus red mite, Panonychus citri (McGregor), is a global citrus pest, and has developed severe resistance to several types of acaricides. However, the molecular mechanisms of resistance in this mite remain unknown. In this study, seven full-length cDNAs encoding glutathione S-transferases (GSTs) genes were identified and characterized in P. citri. The effects of pyridaben and fenpropathrin exposure on the expression of these genes were also investigated. Phylogenetic analysis revealed that the seven GSTs genes in P. citri cloned in this study belong to three different cytosolic classes, including four in mu, two in delta and one in zeta. Among these seven GSTs genes, the relative expression level of PcGSTm1 was significantly higher in adult than in the other life stages (egg, larvae and nymph). Compared with the control, the mRNA levels of the seven GST genes did not change significantly following exposure to pyridaben at LC10. However, RT-qPCR results showed that, when exposed to LC10 of fenpropathrin, six GSTs gene (PcGSTm1, PcGSTm3, PcGSTm4, PcGSTd1, PcGSTd2 and PcGSTz1) transcripts increased in a time-dependent manner. This is the first insight into the molecular characteristics of GSTs gene cDNAs in P. citri. The elevated GSTs gene transcripts following exposure to fenpropathrin might be one of the mechanisms involved in detoxification of this acaricide.

TLRR (lrrc67) interacts with PP1 and is associated with a cytoskeletal complex in the testis

BACKGROUND INFORMATION

Spermatozoa are formed via a complex series of cellular transformations, including acrosome and flagellum formation, nuclear condensation and elongation and removal of residual cytoplasm. Nuclear elongation is accompanied by the formation of a unique cytoskeletal structure, the manchette. We have previously identified a leucine-rich repeat protein that we have named TLRR (testis leucine-rich repeat), associated with the manchette that contains a PP1 (protein phosphatase-1)-binding site. Leucine-rich repeat proteins often mediate protein-protein interactions; therefore, we hypothesize that TLRR acts as a scaffold to link signalling molecules, including PP1, to the manchette near potential substrate proteins important for spermatogenesis.

RESULTS

TLRR and PP1 interact with one another as demonstrated by co-immunoprecipitation and the yeast two-hybrid assay. TLRR binds more strongly to PP1 gamma 2 than it does to PP1 alpha. Anti-phosphoserine antibodies immunoprecipitate TLRR from testis lysate, indicating that TLRR is a phosphoprotein. TLRR is part of a complex in testis that includes cytoskeletal proteins and constituents of the ubiquitin-proteasome pathway. The TLRR complex purified from 3T3 cells contains similar proteins, co-localizes with microtubules and is enriched at the microtubule-organizing centre. TLRR is also detected near the centrosome of elongated, but not mid-stage, spermatids.

CONCLUSION

We demonstrate here that TLRR interacts with PP1, particularly the testis-specific isoform, PP1 gamma 2. Immunoaffinity purification confirms that TLRR is associated with the spermatid cytoskeleton. In addition, proteins involved in protein stability are part of the TLRR complex. These results support our hypothesis that TLRR links signalling molecules to the spermatid cytoskeleton in order to regulate important substrates involved in spermatid transformation. The translocation of TLRR from the manchette to the centrosome region suggests a possible role for this protein in tail formation. Our finding that TLRR is associated with microtubules in cultured cells suggests that TLRR may play a common role in modulating the cytoskeleton in other cell types besides male germ cells.

The Mu class glutathione transferase is abundant in striated muscle and is an isoform-specific regulator of ryanodine receptor calcium channels

Members of the glutathione transferase (GST) structural family are novel regulators of cardiac ryanodine receptor (RyR) calcium channels. We present the first detailed report of the effect of endogenous muscle GST on skeletal and cardiac RyRs. An Mu class glutathione transferase is specifically expressed in human muscle. An hGSTM2-2-like protein was isolated from rabbit skeletal muscle and sheep heart, at concentrations of approximately 17-93 microM. When added to the cytoplasmic side of RyRs, hGSTM2-2 and GST isolated from skeletal or cardiac muscle, modified channel activity in an RyR isoform-specific manner. High activity skeletal RyR1 channels were inactivated at positive potentials or activated at negative potentials by hGSTM2-2 (8-30 microM). Inactivation became faster as the positive voltage was increased. Channels recovered from inactivation when the voltage was reversed, but recovery times were significantly slowed in the presence of hGSTM2-2 and muscle GSTs. Low activity RyR1 channels were activated at both potentials. In contrast, hGSTM2-2 and GSTs isolated from muscle (1-30 microM) in the cytoplasmic solution, caused a voltage-independent inhibition of cardiac RyR2 channels. The results suggest that the major GST isoform expressed in muscle regulates Ca2+ signalling in skeletal and cardiac muscle and conserves Ca2+ stores in the sarcoplasmic reticulum.

Comparative proteomic analysis of neoplastic and non-neoplastic germ cell tissue

A comparative proteomic analysis of neoplastic versus non-neoplastic seminoma identified glutathione S-transferase M3 as a differentially expressed protein. This expression difference could also be observed at the mRNA level, implying neoplasm-associated alterations in transcriptional or post-transcriptional mechanisms.

A Subclass of Mu Glutathione S‐Transferases Selectively Expressed in Testis and Brain

A subclass of glutathione S-transferases (GSTs), exemplified by the human hGSTM3 and rodent GSTM5 subunits, has properties that distinguish it from other Mu class GSTs. Thus, they originate from single copy genes that are in an inverted order and, apart from the coding regions, share little sequence homology relative to the others in the Mu cluster. The genes for this M3/M5 subgroup encode for proteins that are in many ways unique, including their extended lengths with key amino acid substitutions. The M3/M5 subclass is selectively expressed in testis and brain and could function differently from the other GSTs.

Glutathione S-transferases and aromatic DNA adducts in smokers' bronchoalveolar macrophages

Interindividual differences in the expression of carcinogen-metabolizing enzymes in the lung may modify the effective dose of tobacco carcinogens in this organ. We investigated the role of detoxifying glutathione S-transferases (GST) in the formation of aromatic DNA adducts in bronchoalveolar macrophages (BAM) of active smokers. The effect of GSTs on aromatic DNA adducts was studied separately and in combination with the PAH-metabolizing cytochrome P450 enzyme, CYP3A. GSTA, GSTM3, GSTP, and CYP3A protein levels were analyzed by Western blotting, GSTM1 and GSTP1 genotypes were determined by polymerase chain reaction (PCR) based methods, and numbers of aromatic DNA adducts were measured by nuclease P1 enhanced 32P-postlabeling method in BAM of 31 active smokers. No correlation was observed between GSTA or GSTP proteins or GSTM1 or GSTP1 genotypes and the level of aromatic DNA adducts. A high or medium expression level of GSTM3 was associated with a lower level of aromatic DNA adducts in the smokers who smoked less than 20 cigarettes per day, when the effect of GSTM3 was analyzed in combination with CYP3A (regression analysis; F(6,24)=6.3, P<0.001). No protection by GSTM3 was observed in heavy smokers. High CYP3A levels, on the other hand, increased the number of DNA adducts regardless of the amount of smoking.

Mercapturate metabolism of 4-hydroxy-2-nonenal in rat and human cerebrum

4-Hydroxy-2-nonenal (HNE), a potent toxin formed in the brain from oxidation of polyunsaturated fatty acids, is increased in Alzheimer disease (AD), where it is a proposed effector of amyloid beta peptide-mediated neurotoxicity. Detoxification of HNE via the mercapturic acid pathway (MAP) is the primary means by which other organs, such as liver, limit its toxic effects. Here we examined the distribution and activity of MAP detoxification for HNE in cerebrum. Our results showed that rat cerebral cortex and especially synaptosomes were less well equipped to detoxify HNE via the MAP than liver. Glutathione transferases (GSTs) catalyze the committed step in the MAP; GST-mu and GST-pi, but not OST-alpha, were detected in neurons and astrocytes in cerebrum from AD patients and controls. MAP activity in frontal cortex of AD patients was modestly but significantly increased compared to controls. These data suggest that lipid peroxidation may present a greater toxic burden to cerebrum than to other organs, and that a component of response to injury in late stage AD is a slight increase in MAP activity.

Ultrastructural localization of 28 kDa glutathione S-transferase in adult Clonorchis sinensis

Glutathione S-transferase (28GST) with molecular mass of 28 kDa is an antioxidant enzyme abundant in Clonorchis sinensis. In adult C. sinensis, 28GST was localized in tegumental syncytium, cytons, parenchyma, and sperm tails examined by immunoelectron microscopy. C. sinensis 28GST was earlier found to neutralize bioreactive compounds and to be rich in eggs. Accordingly, it is suggested that 28GST plays important roles in phase II defense system and physiological roles in worm fecundity of C. sinensis.

Glutathione S-transferase alpha expressed in porcine Sertoli cells is under the control of follicle-stimulating hormone and testosterone

Glutathione S-transferases (GSTs) are a family of detoxification isoenzymes present in different tissues including the testis and that conjugate many toxic substrates to glutathione. Among these substrates are carcinogens, mutagens and products of oxidative processes. In the present report we show that GSTalpha is expressed in somatic testicular Leydig cells and Sertoli cells. GSTalpha expression in Sertoli cells is under the hormonal control of FSH, testosterone, and estradiol. In Leydig cells, immunoreactive GSTalpha was present at the neonatal, pubertal, and adult periods. In Sertoli cells, GSTalpha was predominant in pubertal and adult testes (but not in neonatal testes), suggesting that its expression is controlled by gonadotropins. The regulatory action and the mechanisms of action of FSH and testosterone on GSTalpha mRNA and protein levels were studied by using a model of primary cultures of porcine testicular Sertoli cells. FSH increased GSTalpha mRNA levels in a dose-dependent manner (ED50 = 18.5 nm/ml) with a maximal effect observed after 48 h of exposure (a 3-fold increase; P < 0.001). In addition, FSH increased GSTalpha protein, which was detected as a doublet of 28 kDa. Treatment with testosterone enhanced GSTalpha mRNA levels in a dose-dependent (ED50 = 1.4 ng/ml) and time-dependent manner with a maximal effect delayed at 8 h of exposure (a 2-fold increase; P < 0.001). Similarly, Sertoli cell treatment with testosterone metabolites, dihydrotestosterone (DHT) and estradiol, led to an increase in GSTalpha mRNA levels. Because stimulatory effects of FSH and androgens were also observed on GSTalpha protein, we therefore had to determine whether the different hormones were affecting GSTalpha gene transcriptional activity, or GSTalpha mRNA stability, or both. FSH and 8-Br-cAMP (but not testosterone) increased the stability of GSTalpha mRNA. The effects of FSH and testosterone on GSTalpha protein were additive, confirming that both hormones act through distinct mechanisms on the expression of the enzyme. Taken together, the present observations indicate that Sertoli cell GSTalpha is targeted by FSH, testosterone, and its metabolites, and they reinforce the concept that Sertoli cells exert a protective role and are under endocrine control to ward against toxic agents in the context of Sertoli-germ cell interactions during spermatogenesis.

Immunohistochemical localization and activity of glutathione transferase zeta (GSTZ1-1) in rat tissues

Glutathione transferase zeta (GSTZ1-1) catalyzes the biotransformation of a range of alpha-haloacids, including dichloroacetic acid (DCA), and the penultimate step in the tyrosine degradation pathway. DCA is a rodent carcinogen and a common drinking water contaminant. DCA also causes multiorgan toxicity in rodents and dogs. The objective of this study was to determine the expression and activities of GSTZ1-1 in rat tissues with maleylacetone and chlorofluoroacetic acid as substrates. GSTZ1-1 protein was detected in most tissues by immunoblot analysis after immunoprecipitation of GSTZ1-1 and by immunohistochemical analysis; intense staining was observed in the liver, testis, and prostate; moderate staining was observed in the brain, heart, pancreatic islets, adrenal medulla, and the epithelial lining of the gastrointestinal tract, airways, and bladder; and sparse staining was observed in the renal juxtaglomerular regions, skeletal muscle, and peripheral nerve tissue. These patterns of expression corresponded to GSTZ1-1 activities in the different tissues with maleylacetone and chlorofluoroacetic acid as substrates. Specific activities ranged from 258 +/- 17 (liver) to 1.1 +/- 0.4 (muscle) nmol/min/mg of protein with maleylacetone as substrate and from 4.6 +/- 0.89 (liver) to 0.09 +/- 0.01 (kidney) nmol/min/mg of protein with chlorofluoroacetic acid as substrate. Rats given DCA had reduced amounts of immunoreactive GSTZ1-1 protein and activities of GSTZ1-1 in most tissues, especially in the liver. These findings indicate that the DCA-induced inactivation of GSTZ1-1 in different tissues may result in multiorgan disorders that may be associated with perturbed tyrosine metabolism.

Genetic polymorphisms of MnSOD, GSTM1, GSTT1, and OGG1 in coal workers' pneumoconiosis

Little is known about the genetic susceptibility to coal workers' pneumoconiosis (CWP). We investigated the association between genetic polymorphisms of MnSOD, GSTM1, GSTT1, or OGG1 and susceptibility to CWP. The study population was composed of 259 Chinese retired coal miners who had similar dust exposure histories. Of these, there were 99 cases with International Labor Organization chest radiologic criteria for CWP and 160 controls (with no radiologic criteria for CWP). Individual dust exposure variables were estimated from work histories, and smoking information was obtained from interviews. Polymerase chain reaction-based techniques evaluated the genotypes of all study subjects. There were no differences in genotype frequency of MnSOD, GSTM1, GSTT1, and OGG1 between miners with CWP and miners without CWP, by logistic regression analysis. Cumulative dust exposures, but not genetic polymorphisms, were associated significantly with the presence of CWP. This study illustrates the complexity of factors that may contribute to the development of CWP.

The ability of mineral dusts and fibres to initiate lipid peroxidation. Part II: relationship to different particle-induced pathological effects

Exposure to pathogenic mineral dusts and fibres is associated with pulmonary changes including fibrosis and cancer. Investigations into aetiological mechanisms of these diseases have identified modifications in specific macromolecules as well as changes in certain early processes, which have preceded fibrosis and cancer. Peroxidation of lipids is one such modification, which is observed following exposure to mineral dusts and fibres. Their ability to initiate lipid peroxidation and the parameters that determine this ability have recently been reviewed. Part II of this review examines the relationship between the capacity of mineral dusts and fibres to initiate lipid peroxidation and a number of pathological changes they produce. The oxidative modification of polyunsaturated fatty acids is a major contributor to membrane damage in cells and has been implicated in a great variety of pathological processes. In most pathological conditions where an induction of lipid peroxidation is observed it is assumed to be the consequence of disease, without further establishing if the induction of lipid peroxidation may have preceded or accompanied the disease. In the great majority of instances, however, despite the difficulty in proving this association, a causal relationship between lipid peroxidation and disease cannot be ruled out.

Effects of phenethylisothiocyanate on the expression of glutathione S-transferases and hepatotoxicity induced by acetaminophen

1. The effect of PEITC on the expression of hepatic glutathione S-transferases (GST) and the glutathione (GSH) conjugation has been investigated in the Sprague-Dawley rat, and it has been determined whether hepatotoxicity of acetaminophen (AA) could be inhibited through the induction of GST expression in mouse. 2. The hepatic GST activity and protein levels of alpha class (Ya, Yc) and mu class (Yb1, Yb2) of GST were elevated in a dose-dependent manner after treatment with PEITC (0, 3.16, 10, 31.6, 100 and 200 mg/kg, p.o., 3 days). The mRNA levels of GST Ya and GST Yb1 were also markedly increased 1 day after treatment with PEITC at dosages ranging from 31.6 to 200 mg/kg. The hepatic GSH content was significantly increased to 200% of control at dose of 200 mg/kg PEITC. 3. Pretreatment with 100 mg/kg PEITC significantly enhanced the biliary excretion of glutathione conjugate of AA 2-fold, whereas treatment with 200 mg/kg did not affect it. 4. In mouse, PEITC (100 and 200 mg/kg, 3 days) decreased the lethality and hepatotoxicity caused by AA. 5. These results indicate that (1) the induction of GST by PEITC is presumably under transcriptional regulation, and (2) PEITC may have a protective function against AA-induced hepatotoxicity by induction effect on GST, in combination of enhancement of hepatic GSH.

Identification of mu-class glutathione transferases M2-2 and M3-3 as cytosolic prostaglandin E synthases in the human brain

Cytosolic prostaglandin (PG) E synthase was purified from human brain cortex. The N-terminal amino acid sequence, PMTLGYXNIRGL, was identical to that of the human mu-class glutathione transferase (GST) M2 subunit. Complementary DNAs for human GSTM2, GSTM3, and GSTM4 subunits were cloned, and recombinant proteins were expressed as homodimers in Escherichia coli. The recombinant GSTM2-2 and 3-3 catalyzed the conversion of PGH2 to PGE2 at the rates of 282 and 923 nmol/min/mg of protein, respectively, at the optimal pH of 8, whereas GSTM4-4 was inactive; although all three enzymes showed GST activity. The PGE synthase activity depended on thiols, such as glutathione, dithiothreitol, 2-mercaptoethanol, or L-cysteine. Michaelis-Menten constants and turnover numbers for PGH2 were 141 microM and 10.8 min(-1) for GSTM2-2 and 1.5 mM and 130 min(-1) for GSTM3-3, respectively. GSTM2-2 and 3-3 may play crucial roles in temperature regulation, nociception, and sleep-wake regulation by producing PGE2 in the brain.

Determination of glutathione S-transferase mu and theta polymorphisms in neurological disease

1. Correlations between deletions in two glutathione S-transferase (GST) genes, GSTM1 and GSTT1 and susceptibility to Alzheimer's disease (AD), motor neuron disease (MND) and Parkinson's disease (PD) have been investigated by PCR, using primers specific for both genes. 2. It was found that males with a deletion of the GSTM1 gene were more susceptible to PD and males with a deletion of the GSTT1 gene more susceptible to MND and PD, possibly implying that environmental factors which specifically target men may be involved. Furthermore, subjects with a deletion of the GSTT1 gene were more susceptible to AD.

Increased bioactivation of dihaloalkanes in rat liver due to induction of class theta glutathione S-transferase T1-1

A characteristic feature of the class Theta glutathione S-transferase (GST) T1-1 is its ability to activate dichloromethane and dibromoethane by catalysing the formation of mutagenic conjugates. The level of the GSTT1 subunit within tissues is an important determinant of susceptibility to the carcinogenic effects of these dihaloalkanes. In the present study it is demonstrated that hepatic GST activity towards these compounds can be elevated significantly in female and male Fischer-344 rats by feeding these animals on diets supplemented with cancer chemopreventive agents. Immunoblotting experiments showed that increased activity towards the dihaloalkanes is associated with elevated levels of the GSTT1 subunit in rat liver. Sex-specific effects were observed in the induction of GSTT1 protein. Amongst the chemopreventive agents tested, indole-3-carbinol proved to be the most potent inducer of hepatic GSTT1 in male rats (6.2-fold), whereas coumarin was the most potent inducer of this subunit in the livers of female rats (3. 5-fold). Phenobarbital showed significant induction of GSTT1 only in male rat liver and had little effect in female rat liver. Western blotting showed that class Alpha, Mu and Pi GST subunits are not co-ordinately induced with GSTT1, indicating that the expression of GSTT1 is determined, at least in part, by mechanisms distinct from those that regulate levels of other transferases. The increase in amount of hepatic GSTT1 protein was also reflected by an increase in the steady-state level of mRNA in response to treatment with chemopreventive agents and model inducers. Immunohistochemical detection of GSTT1 in rat liver supported the Western blotting data, but showed, in addition to cytoplasmic staining, significant nuclear localization of the enzyme in hepatocytes from some treated animals, including those fed on an oltipraz-containing diet. Significantly, the hepatic level of cytochrome P-450 2E1, an enzyme which offers a detoxification pathway for dihaloalkanes, was unchanged by the various inducing agents studied. It is concluded that the induction of GSTT1 by dietary components and its localization within cells are important factors that should be considered when assessing the risk dihaloalkanes pose to human health.

Studies on glutathione S-transferases important for sperm function: evidence of catalytic activity-independent functions

Our earlier studies reported the identification of a rat testicular protein of 24 kDa with significant similarity at the N-terminus with Mu class glutathione S-transferases (GSTs). Treatment of goat sperm with antisera against this protein identified immunoreactive sites on the spermatozoa and inhibited in vitro fertilization of goat oocytes by the antibody-treated sperm. The above observations indicated the presence of GST-like molecule(s) important for fertility related events on goat spermatozoa. In this study, we report the purification of goat sperm GSTs (GSP1) which were purified by glutathione affinity chromatography and were enzymically active towards 1-chloro-2,4,-dinitrobenzene, a general GST substrate, and ethacrynic acid, a substrate for Pi class GSTs. GSP1 resolved into three major components on reverse-phase HPLC: peaks 1 and 2 with molecular masses of 26.5 kDa and peak 3 with a molecular mass of 25.5 kDa, as determined by SDS/PAGE. Multiple attempts to obtain N-terminal sequences of the first two peaks failed, indicating N-terminal block; however, they reacted to specific anti-Mu-GST antisera on Western blots and ELISA, and not to anti-Pi-GST antisera, which provides evidence for the presence of Mu-GST-reactive sites on peaks 1 and 2. The third component showed 80% N-terminal similarity with human and rat GSTP1-1 over an overlap of 15 amino acids, and reacted to anti-Pi-specific antisera in ELISA. Sperm labelled with antibodies against a 10-mer and an 11-mer peptide, designed from the N-terminal sequences of Mu and Pi class GSTs respectively, showed the presence of both Mu- and Pi-GST on goat sperm surface at distinct cellular domains. Selective inhibition of Pi class GST by the Pi-specific antisera, either at 0 h or at 3 h after initiation of sperm capacitation, leads to a reduction in fertilization rates. In contrast, the inhibition of Mu class GST by specific antisera at 0 h does not inhibit fertilization, although such treatment at 3 h after the initiation of capacitation reduces fertilization rates. The results indicate that both Pi- and Mu-GSTs are involved in fertilization, but the Mu-GST sites essential for fertilization are exposed only after 3 h of capacitation. The enzymic activity of GSP1 or live spermatozoa is not inhibited by the two antisera. The inability of the antibodies to cause such inhibition indicates that the reduction in fertilization rates and acrosome reaction caused by the antibodies is through a mechanism which does not interfere with the catalytic activity of the molecule. Therefore we established the presence of Pi and Mu class GST on goat sperm, their localization and their possible function in fertility-related events.

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.

Enzymic denitrosation of 1,3-dimethyl-2-cyano-1-nitrosoguanidine in rat liver cytosol and the fate of the immediate product S-nitrosoglutathione

The tumorigenicity of certain N-nitrosoguanidinium compounds is limited, in rodents, by the propensity of these agents to be detoxified by denitrosation. Previous studies have revealed that rodent glutathione transferase isoenzymes are capable of catalyzing this process, generating exclusively the denitrosated guanidinium compound and S-nitrosoglutathione (GSNO). Experiments considering the denitrosation of 1,3-dimethyl-2-cyano-1-nitrosoguanidine (CyanoDMNG) in rat liver cytosol incubates are reported, with emphasis on the fate of GSNO. Incubates composed with equimolar CyanoDMNG and reduced glutathione (GSH) effected 100% denitrosation; the GSNO yield was less than expected as was the quantity of GSH consumed. When the anticipated 100% yield concentration of GSNO was applied to cytosol incubates, 20-40% of it rapidly disappeared. Nitrosated protein thiols accounted for 35% of the NO moiety released, nitrite ion 30%, and nitric oxide production was detectable. Concomitant with GSNO loss, GSH and oxidized glutathione (GSSG) were generated in yields similar to those detected in the CyanoDMNG/GSH incubates. Thus, the fate of GSNO in cytosol determines the yields of glutathione-based products, and the stoichiometry of the glutathione transferase reaction is demonstrated. In incubates composed with equimolar CyanoDMNG, GSH, and NADPH, denitrosation was again 100%, but GSNO yields were very low and residual GSH increased. Inclusion of NADPH in incubates containing the anticipated 100% yield concentration of GSNO resulted in rapid GSNO degradation, producing GSH and a detected but unidentified product; S-nitrosated protein, nitrite, and nitrate yields were minimal, nitric oxide production was abolished, and incubate response to a mercuric chloride/azo dye assay approached zero. The fate of the NO moiety consequent to this GSNO catabolism is presently unknown.

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)