Characterization of a novel glutathione S-transferase isoenzyme from mouse lung and liver having structural similarity to rat glutathione S-transferase 8-8

Assembly of ligands interaction models for glutathione-S-transferases from Plasmodium falciparum, human and mouse using enzyme kinetics and molecular docking

BACKGROUND

Glutathione-s-transferases (GSTs) are enzymes that principally catalyze the conjugation of electrophilic compounds to the endogenous nucleophilic glutathione substrate, besides, they have other non-catalytic functions. The Plasmodium falciparum genome encodes a single isoform of GST (PfGST) which is involved in buffering the toxic heme, thus considered a potential anti-malarial target. In mammals several classes of GSTs are available, each of various isoforms. The human (human GST Pi-1 or hGSTP1) and mouse (murine GST Mu-1 or mGSTM1) GST isoforms control cellular apoptosis by interaction with signaling proteins, thus considered as potential anti-cancer targets. In the course of GSTs inhibitors development, the models of ligands interactions with GSTs are used to guide rational molecular modification. In the absence of X-ray crystallographic data, enzyme kinetics and molecular docking experiments can aid in addressing ligands binding modes to the enzymes.

METHODS

Kinetic studies were used to investigate the interactions between the three GSTs and each of glutathione, 1-chloro-2,4-dinitrobenzene, cibacron blue, ethacrynic acid, S-hexyl glutathione, hemin and protoporphyrin IX. Since hemin displacement is intended for PfGST inhibitors, the interactions between hemin and other ligands at PfGST binding sites were studied kinetically. Computationally determined binding modes and energies were interlinked with the kinetic results to resolve enzymes-ligands interaction models at atomic level.

RESULTS

The results showed that hemin and cibacron blue have different binding modes in the three GSTs. Hemin has two binding sites (A and B) with two binding modes at site-A depending on presence of GSH. None of the ligands were able to compete hemin binding to PfGST except ethacrynic acid. Besides bind differently in GSTs, the isolated anthraquinone moiety of cibacron blue is not maintaining sufficient interactions with GSTs to be used as a lead. Similarly, the ethacrynic acid uses water bridges to mediate interactions with GSTs and at least the conjugated form of EA is the true hemin inhibitor, thus EA may not be a suitable lead.

CONCLUSIONS

Glutathione analogues with bulky substitution at thiol of cysteine moiety or at γ-amino group of γ-glutamine moiety may be the most suitable to provide GST inhibitors with hemin competition.

Evidence for the association of synaptotagmin with glutathione S-transferases: implications for a novel function in human breast cancer

OBJECTIVE

To analyze the pattern of changes in GSTs in cancerous and adjacent non-cancerous tissues obtained from breast cancer patients undergoing surgery.

DESIGN AND METHODS

Cytosolic GST purification, assay of GST, protein expression levels, and GST-synaptotagmin association were analyzed using standard biochemical techniques like GSH-affinity purification, spectrophotometry, SDS-PAGE, Western blots, and matrix-assisted laser desorption and ionization-time of flight (MALDI-TOF).

RESULTS

GST activity in cancerous tissues (0.26 U/mg protein) was significantly higher (P < 0.05) as compared to those from adjacent non-cancerous tissues (0.14 U/mg protein) of breast cancer patients. Further analysis of GST subunits on SDS-PAGE and Western blots using class-specific GST antibodies revealed significant elevation in GST-pi levels in cancer tissues with no appreciable changes in GST-alpha and GST-mu. Along with the elevation of GST-pi levels, high molecular weight proteins (approximately 70 kDa) cross reacting with GST antibodies were detected only in surgically resected tumor biopsies but not in the non-cancerous tissues adjacent to the tumor. Based on MALDI-TOF analysis, the high molecular weight band was identified as synaptotagmin V bound to GST-M1 with 47% sequence coverage after processing on an MS-FIT search engine.

CONCLUSIONS

Our results suggest a novel putative functional role for the GST-synaptotagmin complex in human breast cancers. As this association of GST M1-synaptotagmin was not seen in adjacent non-cancerous tissues, this can be used as a marker for breast cancers.

Membrane association of glutathione S-transferase mGSTA4-4, an enzyme that metabolizes lipid peroxidation products

Lipid peroxidation products have signaling functions and at higher concentrations are toxic and may trigger cell death. The compounds are metabolized predominantly by glutathione S-transferases exemplified by mGSTA4-4, an enzyme highly efficient in glutathione conjugation of 4-hydroxyalkenals, and possessing glutathione peroxidase activity toward phospholipid hydroperoxides. mGSTA4-4 belongs to the predominant group of "canonical" glutathione S-transferases that are soluble and generally localized in the cytoplasm. The intracellular localization of mGSTA4-4 was examined in hepatocytes of normal mouse liver and in transfected HepG2 cells by fluorescence microscopy and digital deconvolution. mGSTA4-4 was found to be predominantly localized at or near the plasma membrane in transfected HepG2 cells, as well as in hepatocytes endogenously expressing the protein. In vitro, mGSTA4-4 associated with liposomes, and this interaction was potentiated when the liposomes contained negatively charged phospholipids. Mutating lysine 115 to glutamic acid resulted in a loss of the plasma membrane targeting of mGSTA4-4 as well as in a significant reduction of its binding to liposomes in vitro. These data suggest preferential targeting of mGSTA4-4 to the plasma membrane that may contain the major substrate(s) for this enzyme. Lysine 115 is critically important for the membrane association of mGSTA4-4, most likely by entering into an electrostatic interaction with negatively charged phospholipid headgroups.

(S)-preferential detoxification of 4-hydroxy-2(E)-nonenal enantiomers by hepatic glutathione S-transferase isoforms in guinea-pigs and rats

In guinea-pig liver cytosol, racemic 4-hydroxy-2(E)-nonenal (HNE), a reactive and highly toxic product released from biomembranes by lipid peroxidation, was detoxified (S)-preferentially by GSH conjugation mediated by glutathione S-transferases (GSTs) and (R)-preferentially by NAD(+)-dependent oxidation mediated by aldehyde dehydrogenase (ALDH). The GST-mediated detoxification of the HNE enantiomers proceeded at much higher rates than that mediated by ALDH in guinea-pig liver cytosol. All the major guinea-pig GSTs, A1-1, M1-1, M1-2 and M1-3*, isolated from guinea-pig liver cytosol also catalysed the (S)-preferential conjugation of the HNE enantiomers. The liver and other major tissues of guinea-pigs had no immunologically detectable level of a putative GSTA4-4 orthologue, which exists as a minor GST protein in rat, mouse and human livers and exhibits extremely high catalytic activity towards HNE. All the hepatic rat GSTs, A1-1(2), A1-3, A4-4, M1-1, M1-2 and M2-2, also catalysed the (S)-preferential conjugation of HNE enantiomers.

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.

4-Hydroxy-2(E)-nonenal enantiomers: (S)-selective inactivation of glyceraldehyde-3-phosphate dehydrogenase and detoxification by rat glutathione S-transferase A4-4

The glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase was irreversibly and (S)-selectively inactivated by the enantiomers of racemic 4-hydroxy-2(E)-nonenal (HNE), a reactive product released from biomembranes by lipid peroxidation in cells. Rates of the enzyme inactivations were 1.7, 3.0, and 6.0 M(-1).s(-1) for (R)-, racemic and (S)-HNEs respectively. In rat liver cytosol the HNE was detoxified 2.5-fold more (S)-selectively by GSH conjugation and 2. 4-fold more (R)-selectively by NADH-dependent reduction mediated by alcohol dehydrogenase (ADH) than the opposite enantiomers. However, in the cytosol the GSH conjugation of (R)-HNE proceeded at a much higher rate than did its ADH-mediated reduction. The minor glutathione S-transferase (GST) isoform, A4-4, in the rat (r) liver had a major role in the cytosolic (S)-selective GSH conjugation. The catalytic efficiency, k(cat)/K(m), of purified rGSTA4-4 was 4-fold higher for (S)-HNE than for (R)-HNE; the K(m) was 3-fold higher for (R)-HNE than for (S)-HNE. (S)-HNE was preferentially detoxified to (R)-HNE by rGSTA4-4 when racemic HNE was used as a substrate.

Marked expression of glutathione S-transferase A4-4 detoxifying 4-hydroxy-2(E)-nonenal in the skin of rats irradiated by ultraviolet B-band light (UVB)

Enzyme, Western blot, and immunohistochemical analyses indicated that rat skin cytosol contained no detectable level of the homodimeric, alpha-class glutathione S-transferase (rGST) A4-4 which catalyzes the GSH conjugation of the toxic product, 4-hydroxy-2(E)-nonenal (HNE), nonenzymatically formed from n-6 polyunsaturated fatty acid residues of lipids by lipid peroxidation. Rats irradiated by single doses (4000-24,000 mJ/cm(2)) of ultraviolet B-band light (UVB, 200 mJ/cm(2)/min) markedly expressed rGSTA4-4 in the skin at a level one-fifth that of the liver in apparent specific activity toward HNE at a single dose of 24,000 mJ/cm(2). Skin rGSTA4-4 was isolated, purified to homogeneity, and identified with hepatic rGSTA4-4 by reverse-phase partition HPLC and by amino acid sequence analysis of its CNBr fission peptides. Immunohistochemistry with polyclonal antibody raised against rGSTA4-4 demonstrated the selective expression of rGSTA4-4 in epidermis and sebaceous glands localized in dermis after UVB irradiation.

Human glutathione transferase A4-4 crystal structures and mutagenesis reveal the basis of high catalytic efficiency with toxic lipid peroxidation products

The oxidation of lipids and cell membranes generates cytotoxic compounds implicated in the etiology of aging, cancer, atherosclerosis, neurodegenerative diseases, and other illnesses. Glutathione transferase (GST) A4-4 is a key component in the defense against the products of this oxidative stress because, unlike other Alpha class GSTs, GST A4-4 shows high catalytic activity with lipid peroxidation products such as 4-hydroxynon-2-enal (HNE). The crystal structure of human apo GST A4-4 unexpectedly possesses an ordered C-terminal alpha-helix, despite the absence of any ligand. The structure of human GST A4-4 in complex with the inhibitor S-(2-iodobenzyl) glutathione reveals key features of the electrophilic substrate-binding pocket which confer specificity toward HNE. Three structural modules form the binding site for electrophilic substrates and thereby govern substrate selectivity: the beta1-alpha1 loop, the end of the alpha4 helix, and the C-terminal alpha9 helix. A few residue changes in GST A4-4 result in alpha9 taking over a predominant role in ligand specificity from the N-terminal loop region important for GST A1-1. Thus, the C-terminal helix alpha9 in GST A4-4 provides pre-existing ligand complementarity rather than acting as a flexible cap as observed in other GST structures. Hydrophobic residues in the alpha9 helix, differing from those in the closely related GST A1-1, delineate a hydrophobic specificity canyon for the binding of lipid peroxidation products. The role of residue Tyr212 as a key catalytic residue, suggested by the crystal structure of the inhibitor complex, is confirmed by mutagenesis results. Tyr212 is positioned to interact with the aldehyde group of the substrate and polarize it for reaction. Tyr212 also coopts part of the binding cleft ordinarily formed by the N-terminal substrate recognition region in the homologous enzyme GST A1-1 to reveal an evolutionary swapping of function between different recognition elements. A structural model of catalysis is presented based on these results.

Role of active-site residues 107 and 108 of glutathione S-transferase mGSTA4-4 in determining the catalytic properties of the enzyme for 4-hydroxynonenal

The murine alpha-class glutathione S-transferase mGSTA4-4 displays a high catalytic activity with 4-hydroxynonenal (4-HNE), a cytotoxic product of lipid peroxidation. The X-ray crystal structure of mGSTA4-4 was used to design mutations targeting the 4-HNE binding site, with the goal of defining the structural elements of the mGSTA4-4 protein necessary for the high conjugative activity with 4-HNE. Two candidate positions, 107 and 108, were investigated. Of these, residue 108 appears to be significant in codetermining the catalytic properties of mGSTA4-4 toward 4-HNE. Systematic mutagenesis of amino acid 108 indicated that high activity toward 4-HNE is contingent on the presence of an aliphatic, hydrophobic side chain in this position. In particular, replacement of the wild-type V108 with leucine led to a more than fivefold increase in both absolute activity of the enzyme for 4-HNE and its selectivity for 4-HNE over the model substrate 1-chloro-2,4-dinitrobenzene, due to a selective increase of the turnover number for 4-HNE with no change in the affinity of the protein for this substrate and no changes in the kinetic parameters for 1-chloro-2,4-dinitrobenzene. In contrast, the A107L mutation decreased activity of the enzyme for both 4-HNE and CDNB and partially reversed the positive effect of the V108L mutation in a double mutant.

Circadian variation of hepatic glutathione S-transferase activities in the mouse

1. The circadian variation in glutathione S-transferase (GST) activity was studied in the hepatic cytosolic fraction of the male and female mouse. A circadian variation in GST activity towards 1-chloro-2,4-dinitrobenzene (CDNB) was observed in the male, the activity being higher in the light phase (07:00-19:00 h) than in the dark phase (19:00-07:00 h) during a day under normal lighting conditions. 2. The circadian variation was only existed from June to October. The difference between the lowest activity (at 01:00 h) and the highest activity (at 13:00 h) was maximum in August. 3. In both the normal and reversed light/dark cycle (lights on 07:00 and 19:00 h, respectively), reduced glutathione (GSH) content was lowest in the middle of the light period and highest in the middle of the dark period and GST activity toward 1,2-epoxy-3-(p-nitrophenoxy)propane (EPNP) exhibited opposite peaks and troughs. GST activities toward CDNB and 1,2-dichloro-4-nitrobenzene (DCNB) during the normal lighting schedule was higher at 13:00 h than at 01:00 h, but no differences were observed under reversed lighting conditions. 4. A circadian variation in GST activity for CDNB and DCNB was also observed in the female in a similar manner to the male, but the variation in the activity for EPNP was not observed in the female. 5. Thus, the circadian variation of hepatic GST activities in mouse were dependent on the enzyme substrates used, and seemed to be reflected by the difference in each isozyme levels. The daily change in the hepatic GSH levels is also thought involved, at least in part, in the regulation of GST activity.

Crystal structure of a murine alpha-class glutathione S-transferase involved in cellular defense against oxidative stress

Glutathione S-transferases (GSTs) are ubiquitous multifunctional enzymes which play a key role in cellular detoxification. The enzymes protect the cells against toxicants by conjugating them to glutathione. Recently, a novel subgroup of alpha-class GSTs has been identified with altered substrate specificity which is particularly important for cellular defense against oxidative stress. Here, we report the crystal structure of murine GSTA4-4, which is the first structure of a prototypical member of this subgroup. The structure was solved by molecular replacement and refined to 2.9 A resolution. It resembles the structure of other members of the GST superfamily, but reveals a distinct substrate binding site.

Glutathione S-transferases of female A/J mouse lung and their induction by anticarcinogenic organosulfides from garlic

Glutathione S-transferases (GSTs) of female A/J mouse lung have been purified and characterized for their (a) structural interrelationships, (b) substrate specificities toward the ultimate carcinogenic metabolite of benzo(a)pyrene (BP), (+)-anti-7 beta, 8 alpha-dihydroxy-9 alpha, 10 alpha-oxy-7,8,9,10-tetrahydrobenzo(a)pyrene [(+)-anti-BPDE], and (c) induction by three naturally occurring organosulfides (OSCs)-from garlic [diallyl sulfide (DAS), diallyl trisulfide (DATS) and dipropyl sulfide (DPS)], which significantly differ in their efficacy against BP-induced lung cancer in mice. The GST activity in the lung was due to two alpha class (pI 9.4 and 6.0), two mu class (pI 8.7 and 8.6), and one pi class (pI 8.9) isoenzyme. The GST isoenzyme profile of the lung was different from that of the A/J mouse forestomach, which also is a target organ for BP-induced cancer in mice. Noticeably, an alpha class heterodimeric isoenzyme (pI 9.5) present in the forestomach of A/J mouse, which is exceptionally efficient in the glutathione (GSH) conjugation of (+)-anti-BPDE [X. Hu, S.K. Srivastava, H. Xia, Y. C. Awasthi, and S. V. Singh (1996) J. Biol. Chem. 271, 32684-32688], could not be detected in the lung. The specific activities of the lung GSTs in the GSH conjugation of (+)-anti-BPDE were in the order of GST 8.9 > GST 8.7 > GST 9.4 > GST 6.0. While DPS treatment did not increase the levels of any pulmonary GST isoenzyme, the expression of pi class GST 8.9 was significantly increased in response to both DAS and DATS administrations. Interestingly, DATS, an OSC which lacks activity against BP-induced lung cancer in mice, was a relatively more potent inducer of pi class GST isoenzyme than DAS, which is a potent inhibitor of BP-induced lung tumorigenesis. The results of the present study suggest that a mechanism(s) other than GST induction is likely to be responsible for the differential effects of DAS and DATS on BP-induced lung cancer in mice. Our results also suggest that relatively lower efficacies of the OSCs against BP-induced lung cancer than against forestomach neoplasia may be attributed to (a) a lack of expression in the lung of an isoenzyme corresponding to forestomach GST 9.5 and (b) a comparatively lower level of induction of pi type GST in the lung than in the forestomach by these OSCs.

Purification and characterization of glutathione S-transferases of rat uterus

Glutathione S-transferases (GSTs) provide protection to cells against electrophilic xenobiotics as well as lipid hydroperoxides and 4-hydroxynonenal generated during lipid peroxidation. The catalytic properties of the alpha class GSTs are well suited for detoxification of electrophilic products of lipid peroxidation. An immunologically distinct subgroup of the alpha class GST isozymes having high activity towards 4-hydroxynonenal has been recently identified in several mammalian tissues [Zimniak et al. (1994) J. Biol. Chem. 269, 992-1000]. Since oxidative stress can exert deleterious effects during gestation, the present studies were performed to determine whether the rat homolog of this group of GSTs, rGST 8-8, is expressed in gravid rat uterus, where it may function as a defense mechanism against oxidative stress. GSTs were purified by GSH-affinity chromatography. Individual GST isozymes were separated by column isoelectric focusing and their immunologic identities were established using highly specific polyclonal antibodies in Western blot analysis. Their expression was quantified and kinetic properties were characterized. Rat uterus contained an alpha class GST (pI 9.8), a pi class GST (pI 8.1), two mu class GSTs (pI 6.7 and 6.2) and rGST 8-8. This result indicated that rGST subunits 1, 2, 3, 4, 7 and 8 are present in rat uterus. The relative abundance of rGST 8-8 in the gravid rat uterus was found to be about three-fold higher as compared with that previously seen in rat liver. Higher relative abundance of rGST8-8 in gravid rat uterus suggests that it may play a protective role against the deleterious effects of lipid peroxidation during gestation.

Time-dependent and tissue-specific variations of glutathione transferase activity during gestation in the mouse

Glutathione transferases (GSTs; EC. 2.1.5.18) activity was measured in maternal liver and conceptal tissues during gestation. In maternal liver, maximum activity was found at gestational day (GD) 9 after which it slowly decreased up to the end of gestation. The placental GSTs activity at GD18 was three times lower than that found at GD14. Conversely, fetal liver GSTs at GD14 was about 75% that at GD18. It was also observed that GSTs activity at GD9 and GD10 was higher in visceral yolk sac than in embryo proper. Substrate specificity measurements, SDS PAGE analysis and HPLC runs, carried out on GSH-affinity purified fractions, revealed that with the progress of gestation in maternal liver an increase in pi class GSTs subunit occurs, with a concomitant decrease in alpha class GSTs. With respect to the time of gestation, a significant change in alpha, mu and pi class GSTs expression also occurred in fetal liver and in chorioallantoic placenta. It was concluded that during gestation the GSTs system is subjected to a time-dependent and tissue-specific modulation which may play a protective role against developmental toxicants.

Adrenocorticotrophic-hormone-dependent regulation of a mu-class glutathione transferase in mouse adrenocortical cells

Three different forms of glutathione transferase (GST) have been resolved in the two mouse adrenal tumour cell lines Y1 and Kin 8. Two of these belong to the mu and pi classes respectively. The third form is so far unidentified. In the Y1 cells, the levels of the mu form (mGTmu1) and the unidentified form, are both down-regulated in the presence of adrenocorticotrophic hormone (ACTH) while the pi form is unaffected. The Kin 8 cell line is derived from Y1 cells and harbours a defect in the cyclic AMP (cAMP)-dependent protein kinase, making it refractory to cAMP-dependent regulation of several enzymes. The GST levels in this cell line were unaffected by ACTH. Also, the steady-state levels of mGTmu1 mRNA were much lower in Y1 cells treated with forskolin (which activates adenylate cyclase) compared with control cells, but there was no difference in mGTmu1 mRNA levels between control and forskolin-treated Kin 8 cells. This indicates that the ACTH-dependent regulation of the mu class GST is pre-translational and that a functional cAMP-dependent protein kinase is required for the regulation. We have further shown that the difference in mRNA steady-state levels between control and forskolin-treated Y1 cells is abolished when transcription is inhibited by actinomycin D. In light of the stability of mGTmu1 mRNA, it would appear most likely that actinomycin D inhibits the transcription of short-lived factors which regulate the turn-over of mGTmu1 transcripts in response to changes in intracellular cAMP levels.

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)

Human glutathione S-transferases

1. Multiple forms of glutathione S-transferase (GST) isoenzymes present in human tissues are dimers of subunits belonging to three distinct gene families namely alpha, mu and pi. Only the subunits within each class hybridize to give active dimers. 2. These subunits are differentially expressed in a tissue-specific manner and the composition of glutathione S-transferases in various tissues differs significantly. 3. Minor GST subunits not belonging to these three classes are also present in some tissues. 4. An ortholog of rat GST 8-8 and mouse mGSTA4-4 is selectively expressed in some human tissues including bladder, brain, heart, liver, and pancreas. This isoenzyme designated as GST 5.8 expresses several fold higher activity towards 4-hydroxy-2,3-trans-nonenal as compared to the routinely used substrate 1-chloro-2,4-dinitrobenzene.

A novel glutathione S-transferase isozyme similar to GST 8-8 of rat and mGSTA4-4 (GST 5.7) of mouse is selectively expressed in human tissues

A mouse glutathione S-transferase (GST) isozyme designated as GST 5.7 or mGSTA4-4 belongs to a distinct subclass of the alpha-class isozymes of GST. It is characterized by kinetic properties intermediate between the alpha- and pi-classes of GSTs. We have recently cloned and expressed this isozyme (rec-mGSTA4-4) in E. coli and have reported its complete primary sequence (Zimniak, P., et al. (1992) FEBS Lett., 313, 173-176). Using antibodies raised against the homogenous rec-mGSTA4-4 expressed in E. coli, we now demonstrate that an ortholog of this isozyme was selectively expressed in various human tissues. The human ortholog of mGST A4-4 purified from liver had a pI value of 5.8 and constituted approx. 1.7% of total GST protein of human liver. Similar to other alpha-class GSTs, the N-terminus of this isozyme (GST 5.8) was also blocked. CNBr digestion of the enzyme yielded two major fragments with M(r) values of 12 kDa and 6 kDa. The sequences of these two fragments showed identities in 16 out of 20 residues and 17 out of 20 residues with the corresponding sequences of its mouse ortholog (mGSTA4-4), and showed significant homologies with the rat and chicken orthologs, GST 8-8 and GST CL3. Human liver GST 5.8 showed more than an order of magnitude higher activity towards t-4-hydroxy-2-nonenal as compared to 1-chloro-2,4-dinitrobenzene. This isozyme also expressed glutathione-peroxidase activity towards fatty acid, as well as phospholipid hydroperoxides suggesting its role in protection mechanisms against the toxicants generated during lipid peroxidation. Western blot analysis of human tissues revealed that this GST isozyme was selectively expressed in human liver, pancreas, heart, brain and bladder tissues, but absent in lung, skeletal muscle, spleen and colon.

Comparative studies on the effect of butylated hydroxyanisole on glutathione and glutathione S-transferases in the tissues of male and female CD-1 mice

1. Male CD-1 mice had about 1.6-fold higher glutathione (GSH), 2-fold higher glutathione S-transferase (GST) activity and 2.8-fold higher GST protein in their livers as compared to the female mice. 2. When mice were fed a diet containing 0.75% BHA for 2 weeks, a 1.8-fold increase was observed in GSH levels of female mice liver as opposed to only 1.2-fold increase in male mice. BHA caused 10-fold increase in GST activity and protein in livers of female mice as compared to only about 3-4-fold increase in livers of males. Differential induction of GSH and GST in males and females was also observed in other tissue besides liver but was not as remarkable. 3. Sex-related differences were also observed in the induction of the alpha- and mu- and pi-classes of GSTs by BHA; most noticeable being GST pi, which was induced to about 10-fold in female liver as opposed to only 3.4-fold in male liver.

A subgroup of class alpha glutathione S-transferases. Cloning of cDNA for mouse lung glutathione S-transferase GST 5.7

A full-length cDNA clone encoding the previously purified mouse glutathione S-transferase GST 5.7 [(1991), Biochem. J. 278, 793-799] has been isolated from a mouse lung cDNA library in lambda gt11. Sequencing of the clone revealed the presence of microheterogeneity in GST 5.7. Comparison of the deduced protein sequence with other glutathione S-transferases, together with previous information available on GST 5.7, indicates that the enzyme belongs to a novel subgroup within the alpha class of glutathione S-transferases. Members of the subgroup, which also include the rat GST 8-8 and perhaps chicken GST CL3, show high sequence homology with each other, but only moderate similarity to other alpha class enzymes. They share a substrate specificity profile that resembles pi-class enzymes, and are active in the conjugation of lipid peroxidation products.

Glutathione S-transferases of mouse liver: sex-related differences in the expression of various isozymes

Sex-related differences in the expression of glutathione S-transferase (GST) isozymes of mouse liver have been described. There were no apparent qualitative differences in the isoelectric focusing profiles of the GST isozymes from male and female mouse liver. Both male and female mice have at least four GST isozymes in their liver with pI values of 9.8, 8.7, 6.4 and 5.7. Kinetic, immunological, and structural properties including the N-terminal region amino acid sequences of these isozymes have been determined and they have been classified into alpha, mu, and pi classes. The most cationic isozyme (pI 9.8) belongs to the alpha class and is comparatively more abundant in female liver. The isozyme having pI 8.7 belongs to the pi class and is more abundant in male liver. The mu class GST pI 6.4 as well as the isozyme having pI 5.7 which corresponded to the a class isozyme GST 8-8 of rat liver were more abundant (about 1.5-fold) in male mouse liver as compared to the female. Interestingly, present studies reveal sex-related differences in the heat stabilities of the alpha and pi class GSTs of mouse liver. The alpha class GST pI (9.8) isolated from female mouse liver was more thermostable as compared to the corresponding enzyme from male mouse liver. On the contrary, the pi class GST (pI 8.7) from male mouse was more thermostable as compared to the corresponding enzyme from the female mouse.