Glutathione transferase isoenzymes from human testis

Identifying and characterizing binding sites on the irreversible inhibition of human glutathione S-transferase P1-1 by S-thiocarbamoylation

Human glutathione S-transferase P1-1 (hGST P1-1) is involved in cell detoxification processes through the conjugation of its natural substrate, reduced glutathione (GSH), with xenobiotics. GSTs are known to be overexpressed in tumors, and naturally occurring isothiocyanates, such as benzyl isothiocyanate (BITC), are effective cancer chemopreventive compounds. To identify and characterize the potential inhibitory mechanisms of GST P1-1 induced by isothiocyanate conjugates, we studied the binding of GST P1-1 and some cysteine mutants to the BITC-SG conjugate as well as to the synthetic S-(N-benzylcarbamoylmethyl)glutathione conjugate (BC-SG). We report here the inactivation of GST P1-1 through the covalent modification of two Cys47 residues per dimer and one Cys101. The evidence has been compiled by isothermal titration calorimetry (ITC) and electrospray ionization mass spectrometry (ESI-MS). ITC experiments suggest that the BITC-SG conjugate generates adducts with Cys47 and Cys101 at physiological temperatures through a corresponding kinetic process, in which the BITC moiety is covalently bound to these enzyme cysteines through an S-thiocarbamoylation reaction. ESI-MS analysis of the BITC-SG incubated enzymes indicates that although the Cys47 in each subunit is covalently attached to the BITC ligand moiety, only one of the Cys101 residues in the dimer is so attached. A plausible mechanism is given for the emergence of inactivation through the kinetic processes with both cysteines. Likewise, our molecular docking simulations suggest that steric hindrance is the reason why only one Cys101 per dimer is covalently modified by BITC-SG. No covalent inactivation of GST P1-1 with the BC-SG inhibitor has been observed. The affinities and inhibitory potencies for both conjugates are high and very similar, but slightly lower for BC-SG. Thus, we conclude that the presence of the sulfur atom from the isothiocyanate moiety in BITC-SG is crucial for its irreversible inhibition of GST P1-1.

Relationship between GST Yp Induction and Hepatocyte Proliferation in Rats Treated with Phase II Drug Metabolizing Enzyme Inducers

Butylated hydroxyanisole (BHA) and 1,2-bis(2-pyridyl)ethylene (2PY-e) are phase II drug metabolizing enzyme inducers which cause hepatomegaly without hepatocyte hypertrophy and induce glutathione S-transferase Yp (GST Yp, pi-class GST), which is known as a tumor marker. To evaluate the relationship between GST Yp induction and hepatocyte proliferation, male F344/DuCrj rats were treated with BHA, 2PY-e, or phenobarbital (PB) for three or seven days. All three chemicals caused increases in liver weight after three and seven days. Immunohistochemical examinations revealed that BHA and 2PY-e induced GST Yp in the hepatocytes of the periportal and centrilobular areas at three and seven days, respectively, whereas PB did not. Significant increases in the BrdU labeling indices were found in the livers of rats in each of the three-day treatment groups, but the labeling index of rat livers treated with BHA was decreased to the control level at seven days, although the high labeling indices of 2PY-e and PB persisted at seven days. Double immunostaining confirmed that BrdU-positive nuclei corresponded to GST Yp-positive hepatocytes in both BHA and 2PY-e treated rats. These results suggest that the GST Yp induction caused by BHA or 2PY-e has some kind of relationship with hepatocyte proliferation.

Glutathione S-transferase Pi has at least three distinguishable xenobiotic substrate sites close to its glutathione-binding site

Benzyl isothiocyanate (BITC), present in cruciferous vegetables, is an efficient substrate of human glutathione S-transferase P1-1 (hGST P1-1). BITC also acts as an affinity label of hGST P1-1 in the absence of glutathione, yielding an enzyme inactive toward BITC as substrate. As monitored by using BITC as substrate, the dependence of k of inactivation (K(I)) of hGST P1-1 on [BITC] is hyperbolic, with K(I) = 66 +/- 7 microM. The enzyme incorporates 2 mol of BITC/mol of enzyme subunit upon complete inactivation. S-Methylglutathione and 8-anilino-1-naphthalene sulfonate (ANS) each yield partial protection against inactivation and decrease reagent incorporation, whereas S-(N-benzylthiocarbamoyl)glutathione or S-methylglutathione + ANS protects completely. Mapping of proteolytic digests of modified enzyme by using mass spectrometry reveals that Tyr(103) and Cys(47) are modified equally. S-Methylglutathione reduces modification of Cys(47), indicating this residue is at/near the glutathione binding region, whereas ANS decreases modification of Tyr(103), suggesting this residue is at/near the BITC substrate site, which is also near the binding site of ANS. The Y103F and Y103S mutant enzymes were generated, expressed, and purified. Both mutants handle substrate 1-chloro-2,4-dinitrobenzene normally; however, Y103S exhibits a 30-fold increase in K(m) for BITC and binds ANS poorly, whereas Y103F has a normal K(m) for BITC and K(d) for ANS. These results indicate that an aromatic residue at position 103 is essential for the binding of BITC and ANS. This study provides evidence for the existence of a novel xenobiotic substrate site in hGST P1-1, which can be occupied by benzyl isothiocyanate and is distinct from that of monobromobimane and 1-chloro-2,4 dinitrobenzene.

Monobromobimane occupies a distinct xenobiotic substrate site in glutathione S-transferase pi

Monobromobimane (mBBr), functions as a substrate of porcine glutathione S-transferase pi (GST pi): The enzyme catalyzes the reaction of mBBr with glutathione. S-(Hydroxyethyl)bimane, a nonreactive analog of monobromobimane, acts as a competitive inhibitor with respect to mBBr as substrate but does not affect the reaction of GST pi with another substrate, 1-chloro-2,4-dinitrobenzene (CDNB). In the absence of glutathione, monobromobimane inactivates GST pi at pH 7.0 and 25 degrees C as assayed using mBBr as substrate, with a lesser effect on the enzyme's use of CDNB as substrate. These results indicate that the sites occupied by CDNB and mBBr are not identical. Inactivation is proportional to the incorporation of 2 moles of bimane/mole of subunit. Modification of GST pi with mBBr does not interfere with its binding of 8-anilino-1-naphthalene sulfonate, indicating that this hydrophobic site is not the target of monobromobimane. S-Methylglutathione and S-(hydroxyethyl)bimane each yield partial protection against inactivation and decrease reagent incorporation, while glutathionyl-bimane protects completely against inactivation. Peptide analysis after trypsin digestion indicates that mBBr modifies Cys45 and Cys99 equally. Modification of Cys45 is reduced in the presence of S-methylglutathione, indicating that this residue is at or near the glutathione binding region. In contrast, modification of Cys99 is reduced in the presence of S-(hydroxyethyl)bimane, suggesting that this residue is at or near the mBBr xenobiotic substrate binding site. Modification of Cys99 can best be understood by reaction with monobromobimane while it is bound to its xenobiotic substrate site in an alternate orientation. These results support the concept that glutathione S-transferase accomplishes its ability to react with a diversity of substrates in part by harboring distinct xenobiotic substrate sites.

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.

Methods for purification of glutathione peroxidase and related enzymes

The different preparative techniques and related analytical methods used for purification of glutathione peroxidase, glutathione transferase and glutathione reductase, described in papers published in the last ten years, have been reviewed in this article. Among the different purification techniques, chromatography has played a relevant role, being reported in all the papers reviewed, whereas other preparative techniques such as electrophoresis and isoelectric focusing were less employed and have been reported in only ca. 3% of cases. Frequently, several different chromatographic modes and several rechromatography steps have been employed. The use of at least three different chromatographic modes has been reported in 53% of total reviewed papers, whereas 41% of them employed two differents modes and in only 6% a single preparative chromatographic step was used. To evaluate losses and improve recovery, analytical methods for quantitation of protein and assay of enzymatic activity must be used in each purification step. Among these analytical techniques, gel electrophoresis, under denaturing conditions, has been widely used to assess purity of enzyme preparation. A discussion of the different activity assay methods used for these three enzymes is also presented in this article.

Identification of a unique mu-class glutathione S-transferase in mouse spermatogenic cells

The fibrous sheath is a major cytoskeletal structure in the principal piece of the mammalian sperm flagellum. Two peptide sequences obtained from a tryptic digest of mouse fibrous sheath proteins exhibited high homology with mu-class glutathione S-transferases (GSTs). Using a DNA probe amplified from degenerate polymerase chain reaction (PCR) primers predicted from these two peptide sequences, a approximately 1.1 kb cDNA clone for fibrous sheath component 2 (Fsc2) was isolated which had 84% nucleic acid and 89% amino acid sequence identity with a previously reported mu-class human GST gene (hGSTM3; Campbell et al., 1990: J Biol Chem 265:4188-9193). Sequences corresponding to those of the two fibrous sheath peptides were present in the protein encoded by the Fsc2 cDNA. Northern analysis with the full length Fsc2 cDNA detected a approximately 1.1 kb mRNA in 12 of 15 somatic tissues examined, as well as in testis and isolated spermatogenic cells. However, 5'(nt--96 to 12) or 3'(nt 637 to 808) Fsc2 probes, containing mostly noncoding sequences, detected a approximately 1.1 kb mRNA abundant in testis and isolated spermatogenic cells, but absent or present at low levels in somatic tissues. Northern analysis with RNA from testes of mice of different postnatal ages and purified spermatogenic cell populations indicated that this transcript is first present during the meiotic phase of germ cell development. These results suggest that a previously unreported mu-class GST gene (mGSTM5.) is expressed at a specific time during the development of spermatogenic cells in the mouse. Immunoblot analysis indicated that a mu-class GST protein is associated with the fibrous sheath, suggesting that it becomes an integral part of the mouse sperm cytoskeleton.

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)

Rapid method for the determination of glutathione transferase isoenzymes in crude extracts

The analysis of glutathione transferase (GST) isoenzyme patterns is of interest in many fields as hepatic glutathione transferase activity is increased by exposure to a variety of xenobiotics and its isoenzymatic forms are induced differentially. A high-performance liquid chromatography method has been developed for the rapid determination of individual isoenzyme levels in crude extracts using an anion-exchange column connected to an on-line system to automatically detect GST activity with 1-chloro-2,4-dinitrobenzene as the substrate. When 50-200 microliters of a cytosolic fraction of fish liver containing up to 15 mg/ml of protein and less than 2 units of GST were injected, a high resolution and highly reproducible chromatogram was obtained. The activity profile determined automatically showed eight to twelve peaks (depending on the sample) that were quantified and could be classified into three groups. Starting from intact tissue, a complete isoenzyme pattern could be obtained in less than 3 h. The method has been applied to ecotoxicological studies with fish samples.

Glutathione S-transferases in human testicular germ cell tumors: changes of expression and activity

Glutathione S-transferases are involved in the detoxification of carcinogens and xenobiotics and are potentially associated with the development of drug-resistance. Forty-six testicular germ cell tumors and 33 adjacent normal testicular tissue specimens were analyzed at the RNA level for the expression of glutathione S-transferase alpha and pi. Glutathione S-transferase alpha was expressed in 31 of the 33 normal testicular tissues (94%) but in only three of the 46 germ cell tumors (7%). Glutathione S-transferase pi mRNA was detected in all normal and malignant testicular tissue samples. Thirteen testicular germ cell tumors and eight normal testicular tissue samples were analyzed at the protein level. The mean specific activity of total cytosolic glutathione S-transferase in tumor tissue was decreased by about 80% as compared to normal testicular tissue. Protein analysis of the glutathione S-transferase subunits of normal testicular tissue demonstrated the presence of the glutathione S-transferase classes alpha, mu and pi, with a predominance of the mu class. In testicular germ cell tumors the glutathione S-transferase subunit pattern showed a predominance of glutathione S-transferase pi representing 88% +/- 3% of total glutathione S-transferase. Since all three glutathione S-transferase isoenzyme classes contribute to the resistance to antineoplastic drugs, the altered glutathione S-transferase isoenzyme pattern and the decrease of glutathione S-transferase activity may play a role in the high inherent drug sensitivity of human testicular germ cell tumors.

Glutathione transferases and cancer

The glutathione transferases, a family of multifunctional proteins, catalyze the glutathione conjugation reaction with electrophilic compounds biotransformed from xenobiotics, including carcinogens. In preneoplastic cells as well as neoplastic cells, specific molecular forms of glutathione transferase are known to be expressed and have been known to participate in the mechanisms of their resistance to drugs. In this article, following a brief description of recently identified molecular forms, we review new findings regarding the respective molecular forms involved in carcinogenesis and anticancer drug resistance, with particular emphasis on Pi class forms in preneoplastic tissues. The rat Pi class form, GST-P (GST 7-7), is strongly expressed not only in hepatic foci and hepatomas, but also in initiated cells that occur at the very early stages of chemical hepatocarcinogenesis, and is regarded as one of the most reliable markers for preneoplastic lesions in the rat liver. 12-O-Tetradecanoylphorbol-13-acetate (TPA)-responsive element-like sequences have been identified in upstream regions of the GST-P gene, and oncogene products c-jun and c-fos are suggested to activate the gene. The Pi-class forms possess unique enzymatic properties, including broad substrate specificity, glutathione peroxidase activity toward lipid hydroperoxides, low sensitivity to organic anion inhibitors, and high sensitivity to active oxygen species. The possible functions of Pi class glutathione transferases in neoplastic tissues and drug-resistant cells are discussed.

Partial purification and isoelectric focusing patterns of the buffalo (Bubalus bubalis) and the Kedah-Kelantan cattle (Bos indicus) glutathione transferases

1. Glutathione transferases from the liver, lung and kidney tissues of the buffalo (Bubalus bubalis) and the Kedah-Kelantan cattle (Bos indicus) were partially purified by ammonium sulphate precipitation and Sephadex G-75 gel filtration. 2. Liver tissue contains the highest enzyme activity when compared to the lung and kidney tissues. 3. The activity in cattle is higher than that in the buffalo. 4. Isoelectric focusing separates the activities into the acidic, near neutral and basic fractions. 5. The focused patterns are different for each of the tissues and in each of the species investigated.

Glutathione transferase isoenzymes from human prostate

By using affinity-chromatography and isoelectric-focusing techniques, several forms of glutathione transferase (GSTs) were resolved from human prostate cytosol. All the three major classes of GST, i.e. Alpha, Mu and Pi, are present in human prostate. However, large inter-individual variation in the qualitative and quantitative expression of different isoenzymes resulted in the samples investigated. The most abundant group of prostate isoenzymes showed acid (pI 4.3-4.7) behaviour and were classified as Pi class GSTs on the basis of their immunological and structural properties. Immunohistochemical staining of Pi class GSTs was prevalently distributed in the epithelial cells surrounding the alveolar lumen. Class Mu GSTs are also expressed, although in small amounts and in a limited number of samples, by human prostate. The major cationic isoenzyme purified from prostate, GST-9.6; (pI 9.6; apparent subunit molecular mass of 28 kDa), appears to be different from the cationic GST alpha-epsilon forms isolated from human liver and kidney as evidenced by its structural, kinetical and immunological properties. This enzyme, which accounts for about 20-30% (on protein basis) of total amount of GSTs, is expressed by only 40% of samples. GST-9.6 has the ability to cross-react in immunoblotting analysis with antisera raised against rat liver GST 2-2, rather than with antisera raised against members of human Alpha, Mu and Pi class GSTs. Although prostate GST-9.6 shows close relationship with the human skin GST pI 9.9, it does not correspond to any other known human GST.

Genetic heterogeneity of the human glutathione transferases: a complex of gene families

The glutathione transferases (GSTs) are involved in the metabolism of a wide range of compounds of both exogenous and endogenous origin. There is evidence that deficiency of GST may increase sensitivity to certain environmentally derived carcinogens. In contrast, elevated expression has been implicated in resistance to therapeutic drugs. The GSTs are the products of several gene families. This review summarizes the present knowledge of the genetic interrelationships between the various isoenzymes, their deficiencies and the physical locations of their genes.