The distribution of microsomal glutathione transferase among different organelles, different organs, and different organisms

Comprehensive analysis of biological landscape of oxidative stress-related genes in diabetic erectile dysfunction

Oxidative stress plays a pivotal role in the pathogenesis of diabetic erectile dysfunction, while specific mechanisms have not been illuminated. The study aims to reveal the genetic expression patterns of oxidative stress in diabetic erectile dysfunction. Transcriptome data of diabetic erectile dysfunction and oxidative stress-related genes (OSRGs) in the Gene Expression Omnibus database were downloaded and analyzed based on differential expression. Functional enrichment analyses were conducted to clarify the biological functions. A protein interaction framework was established, and significant gene profiles were validated in the cavernous endothelial cells, clinical patients, and rat models. A miRNA-OSRGs network was predicted and validated. The results were analyzed using Student's t-test. The analysis screened 203 differentially expressed OSRGs (p < 0.05), which had a close association with oxidoreductase activities, glutathione metabolism, and autophagy. A four-gene signature comprised of EPS8L2 (p = 0.044), GSTA3 (p = 0.015), LOX (p < 0.001) and MGST1 (p = 0.002) was well validated and regarded as the hub OSRGs. Compared with the control group, notable increases and decreases were observed in the expressions of GSTA3 (3.683 ± 0.636 vs. 0.416 ± 0.507) and LOX (2.104 ± 1.895 vs. 18.804 ± 2.751) in the validated diabetic erectile dysfunction group. The hub OSRGs-related miRNAs participated in smooth muscle cell proliferation. Besides, miR-125a-3p (p = 0.034) and miR-138-2-3p (p = 0.012) were validated as promising oxidative stress-related miRNA biomarkers. Our findings revealed the genetic alternations of oxidative stress in diabetic erectile dysfunction. These results will be instructive to explore the molecular landscape and the potential treatment for diabetic erectile dysfunction.

Kinetic Behavior of Glutathione Transferases: Understanding Cellular Protection from Reactive Intermediates

Glutathione transferases (GSTs) are the primary catalysts protecting from reactive electrophile attack. In this review, the quantitative levels and distribution of glutathione transferases in relation to physiological function are discussed. The catalytic properties (random sequential) tell us that these enzymes have evolved to intercept reactive intermediates. High concentrations of enzymes (up to several hundred micromolar) ensure efficient protection. Individual enzyme molecules, however, turn over only rarely (estimated as low as once daily). The protection of intracellular protein and DNA targets is linearly proportional to enzyme levels. Any lowering of enzyme concentration, or inhibition, would thus result in diminished protection. It is well established that GSTs also function as binding proteins, potentially resulting in enzyme inhibition. Here the relevance of ligand inhibition and catalytic mechanisms, such as negative co-operativity, is discussed. There is a lack of knowledge pertaining to relevant ligand levels in vivo, be they exogenous or endogenous (e.g., bile acids and bilirubin). The stoichiometry of active sites in GSTs is well established, cytosolic enzyme dimers have two sites. It is puzzling that a third of the site's reactivity is observed in trimeric microsomal glutathione transferases (MGSTs). From a physiological point of view, such sub-stoichiometric behavior would appear to be wasteful. Over the years, a substantial amount of detailed knowledge on the structure, distribution, and mechanism of purified GSTs has been gathered. We still lack knowledge on exact cell type distribution and levels in vivo however, especially in relation to ligand levels, which need to be determined. Such knowledge must be gathered in order to allow mathematical modeling to be employed in the future, to generate a holistic understanding of reactive intermediate protection.

Microsomal glutathione transferase 1 in cancer and the regulation of ferroptosis

Microsomal glutathione transferase 1 (MGST1) is a member of the MAPEG family (membrane associated proteins in eicosanoid and glutathione metabolism), defined according to enzymatic activities, sequence motifs, and structural properties. MGST1 is a homotrimer which can bind three molecules of glutathione (GSH), with one modified to a thiolate anion displaying one-third-of-sites-reactivity. MGST1 has both glutathione transferase and peroxidase activities. Each is based on stabilizing the GSH thiolate in the same active site. MGST1 is abundant in the liver and displays a broad subcellular distribution with high levels in endoplasmic reticulum and mitochondrial membranes, consistent with a physiological role in protection from reactive electrophilic intermediates and oxidative stress. In this review paper, we particularly focus on recent advances made in understanding MGST1 activation, induction, broad subcellular distribution, and the role of MGST1 in apoptosis, ferroptosis, cancer progression, and therapeutic responses.

Enzymology of reactive intermediate protection: kinetic analysis and temperature dependence of the mesophilic membrane protein catalyst MGST1

Glutathione transferases (GSTs) are a class of phase II detoxifying enzymes catalysing the conjugation of glutathione (GSH) to endogenous and exogenous electrophilic molecules, with microsomal glutathione transferase 1 (MGST1) being one of its key members. MGST1 forms a homotrimer displaying third-of-the-sites-reactivity and up to 30-fold activation through modification of its Cys-49 residue. It has been shown that the steady-state behaviour of the enzyme at 5 °C can be accounted for by its pre-steady-state behaviour if the presence of a natively activated subpopulation (~ 10%) is assumed. Low temperature was used as the ligand-free enzyme is unstable at higher temperatures. Here, we overcame enzyme lability through stop-flow limited turnover analysis, whereby kinetic parameters at 30 °C were obtained. The acquired data are more physiologically relevant and enable confirmation of the previously established enzyme mechanism (at 5 °C), yielding parameters relevant for in vivo modelling. Interestingly, the kinetic parameter defining toxicant metabolism, k_cat /K_M , is strongly dependent on substrate reactivity (Hammett value 4.2), underscoring that glutathione transferases function as efficient and responsive interception catalysts. The temperature behaviour of the enzyme was also analysed. Both the K_M and K_D values decreased with increasing temperature, while the chemical step k₃ displayed modest temperature dependence (Q₁₀ : 1.1-1.2), mirrored in that of the nonenzymatic reaction (Q₁₀ : 1.1-1.7). Unusually high Q₁₀ values for GSH thiolate anion formation (k₂ : 3.9), k_cat (2.7-5.6) and k_cat /K_M (3.4-5.9) support that large structural transitions govern GSH binding and deprotonation, which limits steady-state catalysis.

Integral Membrane Enzymes in Eicosanoid Metabolism: Structures, Mechanisms and Inhibitor Design

Eicosanoids are potent lipid mediators involved in central physiological processes such as hemostasis, renal function and parturition. When formed in excess, eicosanoids become critical players in a range of pathological conditions, in particular pain, fever, arthritis, asthma, cardiovascular disease and cancer. Eicosanoids are generated via oxidative metabolism of arachidonic acid along the cyclooxygenase (COX) and lipoxygenase (LOX) pathways. Specific lipid species are formed downstream of COX and LOX by specialized synthases, some of which reside on the nuclear and endoplasmic reticulum, including mPGES-1, FLAP, LTC4 synthase, and MGST2. These integral membrane proteins are members of the family "membrane-associated proteins in eicosanoid and glutathione metabolism" (MAPEG). Here we focus on this enzyme family, which encompasses six human members typically catalyzing glutathione dependent transformations of lipophilic substrates. Enzymes of this family have evolved to combat the topographical challenge and unfavorable energetics of bringing together two chemically different substrates, from cytosol and lipid bilayer, for catalysis within a membrane environment. Thus, structural understanding of these enzymes are of utmost importance to unravel their molecular mechanisms, mode of substrate entry and product release, in order to facilitate novel drug design against severe human diseases.

The mercapturic acid pathway

The mercapturic acid pathway is a major route for the biotransformation of xenobiotic and endobiotic electrophilic compounds and their metabolites. Mercapturic acids (N-acetyl-l-cysteine S-conjugates) are formed by the sequential action of the glutathione transferases, γ-glutamyltransferases, dipeptidases, and cysteine S-conjugate N-acetyltransferase to yield glutathione S-conjugates, l-cysteinylglycine S-conjugates, l-cysteine S-conjugates, and mercapturic acids; these metabolites constitute a "mercapturomic" profile. Aminoacylases catalyze the hydrolysis of mercapturic acids to form cysteine S-conjugates. Several renal transport systems facilitate the urinary elimination of mercapturic acids; urinary mercapturic acids may serve as biomarkers for exposure to chemicals. Although mercapturic acid formation and elimination is a detoxication reaction, l-cysteine S-conjugates may undergo bioactivation by cysteine S-conjugate β-lyase. Moreover, some l-cysteine S-conjugates, particularly l-cysteinyl-leukotrienes, exert significant pathophysiological effects. Finally, some enzymes of the mercapturic acid pathway are described as the so-called "moonlighting proteins," catalytic proteins that exert multiple biochemical or biophysical functions apart from catalysis.

Microsomal glutathione transferase 1 attenuated ROS-induced lipid peroxidation in Apostichopus japonicus

Microsomal glutathione transferase (mGST) is a membrane bound glutathione transferase in multifunctional detoxification isoenzymes family and also plays crucial roles in innate immunity. In the present study, a novel microsomal GST homology was identified from Apostichopus japonicus (designated as AjmGST1) by RACE approaches. The full-length cDNA of AjmGST1 was of 1296 bp encoded a protein of 169 amino acids residues. Multiple sequence alignment and phylogenetic analysis together supported that AjmGST1 belonged to a new member in invertebrates mGST family. Spatial expression analysis revealed that AjmGST1was ubiquitously expressed in all examined tissues with the larger magnitude in tentacle. Time-course expression of AjmGST1 mRNA in coelomocytes was up-regulated after Vibrio splendidus challenge from 6 h until 72 h with the peak expression in 24 h, compared with that in the control group. Similarly, the induced expression of AjmGST1 expression was also detected in lipopolysaccharide (LPS) exposed primary coelomocytes. The purified recombinant protein of AjmGST1 showed high activity with GST substrate at pH of 7.0 and temperature of 35 °C. Meantime, the recombinant AjmGST1 depressed H₂O₂-induced MDA production both in vivo and in vitro. All of these results indicated that AjmGST1 was an important regulator in elimination of lipid peroxidation under immune response.

Microsomal and Cytosolic Scaling Factors in Dog and Human Kidney Cortex and Application for In Vitro-In Vivo Extrapolation of Renal Metabolic Clearance

In vitro-in vivo extrapolation of drug metabolism data obtained in enriched preparations of subcellular fractions rely on robust estimates of physiologically relevant scaling factors for the prediction of clearance in vivo. The purpose of the current study was to measure the microsomal and cytosolic protein per gram of kidney (MPPGK and CPPGK) in dog and human kidney cortex using appropriate protein recovery marker and evaluate functional activity of human cortex microsomes. Cytochrome P450 (CYP) content and glucose-6-phosphatase (G6Pase) activity were used as microsomal protein markers, whereas glutathione-S-transferase activity was a cytosolic marker. Functional activity of human microsomal samples was assessed by measuring mycophenolic acid glucuronidation. MPPGK was 33.9 and 44.0 mg/g in dog kidney cortex, and 41.1 and 63.6 mg/g in dog liver (n = 17), using P450 content and G6Pase activity, respectively. No trends were noted between kidney, liver, and intestinal scalars from the same animals. Species differences were evident, as human MPPGK and CPPGK were 26.2 and 53.3 mg/g in kidney cortex (n = 38), respectively. MPPGK was 2-fold greater than the commonly used in vitro-in vivo extrapolation scalar; this difference was attributed mainly to tissue source (mixed kidney regions versus cortex). Robust human MPPGK and CPPGK scalars were measured for the first time. The work emphasized the importance of regional differences (cortex versus whole kidney–specific MPPGK, tissue weight, and blood flow) and a need to account for these to improve assessment of renal metabolic clearance and its extrapolation to in vivo.

Lactococcus lactis is an Efficient Expression System for Mammalian Membrane Proteins Involved in Liver Detoxification, CYP3A4, and MGST1

Despite the great importance of human membrane proteins involved in detoxification mechanisms, their wide use for biochemical approaches is still hampered by several technical difficulties considering eukaryotic protein expression in order to obtain the large amounts of protein required for functional and/or structural studies. Lactococcus lactis has emerged recently as an alternative heterologous expression system to Escherichia coli for proteins that are difficult to express. The aim of this work was to check its ability to express mammalian membrane proteins involved in liver detoxification, i.e., CYP3A4 and two isoforms of MGST1 (rat and human). Genes were cloned using two different strategies, i.e., classical or Gateway-compatible cloning, and we checked the possible influence of two affinity tags (6×-His-tag and Strep-tag II). Interestingly, all proteins could be successfully expressed in L. lactis at higher yields than those previously obtained for these proteins with classical expression systems (E. coli, Saccharomyces cerevisiae) or those of other eukaryotic membrane proteins expressed in L. lactis. In addition, rMGST1 was fairly active after expression in L. lactis. This study highlights L. lactis as an attractive system for efficient expression of mammalian detoxification membrane proteins at levels compatible with further functional and structural studies.

Chemical Reactivity Window Determines Prodrug Efficiency toward Glutathione Transferase Overexpressing Cancer Cells

Glutathione transferases (GSTs) are often overexpressed in tumors and frequently correlated to bad prognosis and resistance against a number of different anticancer drugs. To selectively target these cells and to overcome this resistance we previously have developed prodrugs that are derivatives of existing anticancer drugs (e.g., doxorubicin) incorporating a sulfonamide moiety. When cleaved by GSTs, the prodrug releases the cytostatic moiety predominantly in GST overexpressing cells, thus sparing normal cells with moderate enzyme levels. By modifying the sulfonamide it is possible to control the rate of drug release and specifically target different GSTs. Here we show that the newly synthesized compounds, 4-acetyl-2-nitro-benzenesulfonyl etoposide (ANS-etoposide) and 4-acetyl-2-nitro-benzenesulfonyl doxorubicin (ANS-DOX), function as prodrugs for GSTA1 and MGST1 overexpressing cell lines. ANS-DOX, in particular, showed a desirable cytotoxic profile by inducing toxicity and DNA damage in a GST-dependent manner compared to control cells. Its moderate conversion of 500 nmol/min/mg, as catalyzed by GSTA1, seems hereby essential since the more reactive 2,4-dinitrobenzenesulfonyl doxorubicin (DNS-DOX) (14000 nmol/min/mg) did not display a preference for GSTA1 overexpressing cells. DNS-DOX, however, effectively killed GSTP1 (20 nmol/min/mg) and MGST1 (450 nmol/min/mg) overexpressing cells as did the less reactive 4-mononitrobenzenesulfonyl doxorubicin (MNS-DOX) in a MGST1-dependent manner (1.5 nmol/min/mg) as shown previously. Furthermore, we show that the mechanism of these prodrugs involves a reduction in GSH levels as well as inhibition of the redox regulatory enzyme thioredoxin reductase 1 (TrxR1) by virtue of their electrophilic sulfonamide moiety. TrxR1 is upregulated in many tumors and associated with resistance to chemotherapy and poor patient prognosis. Additionally, the prodrugs potentially acted as a general shuttle system for DOX, by overcoming resistance mechanisms in cells. Here we propose that GST-dependent prodrugs require a conversion rate "window" in order to selectively target GST overexpressing cells, while limiting their effects on normal cells. Prodrugs are furthermore a suitable system to specifically target GSTs and to overcome various drug resistance mechanisms that apply to the parental drug.

Absence of MGST1 mRNA and protein expression in human neuroblastoma cell lines and primary tissue

A recent study identified a haplotype on a small region of chromosome 12, between markers D12S1725 and D12S1596, shared by all patients with familial neuroblastoma (NB). We previously localized the human MGST1 gene, whose gene product protects against oxidative stress, to this very same chromosomal region (12p112.1-p13.33). Owing to the chromosomal location of MGST1; its roles in tumorigenesis, drug resistance, and oxidative stress; and the known sensitivity of NB cell lines to oxidative stress, we considered a role for MGST1 in NB development. Surprisingly there was no detectable MGST1 mRNA or protein in either NB cell lines or NB primary tumor tissue, although all other human tissues, cell lines, and primary tumor tissue examined to date express MGST1 at high levels. The mechanism behind the failure of NB cells and tissue to express MGST1 mRNA is unknown and involves the failure of MGST1 pre-mRNA expression, but does not involve chromosomal rearrangement or nucleotide variation in the promoter, exons, or 3' untranslated region of MGST1. MGST1 provides significant protection against oxidative stress and constitutes 4 to 6% of all protein in the outer membrane of the mitochondria. As NB cells are extremely sensitive to oxidative stress, and often used as a model system to investigate mitochondrial response to endogenous and exogenous stress, these findings may be due to the lack of expression MGST1 protein in NB. The significance of this finding to the development of neuroblastoma (familial or otherwise), however, is unknown and may even be incidental. Although our studies provide a molecular basis for previous work on the sensitivity of NB cells to oxidative stress, and possibly marked variations in NB mitochondrial homeostasis, they also imply that the results of these earlier studies using NB cells are not transferable to other tumor and cell types that express MGST1 at high concentrations.

Microsomal glutathione transferase 1 protects against toxicity induced by silica nanoparticles but not by zinc oxide nanoparticles

Microsomal glutathione transferase 1 (MGST1) is an antioxidant enzyme located predominantly in the mitochondrial outer membrane and endoplasmic reticulum and has been shown to protect cells from lipid peroxidation induced by a variety of cytostatic drugs and pro-oxidant stimuli. We hypothesized that MGST1 may also protect against nanomaterial-induced cytotoxicity through a specific effect on lipid peroxidation. We evaluated the induction of cytotoxicity and oxidative stress by TiO(2), CeO(2), SiO(2), and ZnO in the human MCF-7 cell line with or without overexpression of MGST1. SiO(2) and ZnO nanoparticles caused dose- and time-dependent toxicity, whereas no obvious cytotoxic effects were induced by nanoparticles of TiO(2) and CeO(2). We also noted pronounced cytotoxicity for three out of four additional SiO(2) nanoparticles tested. Overexpression of MGST1 reversed the cytotoxicity of the main SiO(2) nanoparticles tested and for one of the supplementary SiO(2) nanoparticles but did not protect cells against ZnO-induced cytotoxic effects. The data point toward a role of lipid peroxidation in SiO(2) nanoparticle-induced cell death. For ZnO nanoparticles, rapid dissolution was observed, and the subsequent interaction of Zn(2+) with cellular targets is likely to contribute to the cytotoxic effects. A direct inhibition of MGST1 by Zn(2+) could provide a possible explanation for the lack of protection against ZnO nanoparticles in this model. Our data also showed that SiO(2) nanoparticle-induced cytotoxicity is mitigated in the presence of serum, potentially through masking of reactive surface groups by serum proteins, whereas ZnO nanoparticles were cytotoxic both in the presence and in the absence of serum.

Characterization of new potential anticancer drugs designed to overcome glutathione transferase mediated resistance

Resistance against anticancer drugs remains a serious obstacle in cancer treatment. Here we used novel strategies to target microsomal glutathione transferase 1 (MGST1) and glutathione transferase pi (GSTP) that are often overexpressed in tumors and confer resistance against a number of cytostatic drugs, including cisplatin and doxorubicin (DOX). By synthetically combining cisplatin with a GST inhibitor, ethacrynic acid, to form ethacraplatin, it was previously shown that cytosolic GST inhibition was improved and that cells became more sensitive to cisplatin. Here we show that ethacraplatin is easily taken up by the cells and can reverse cisplatin resistance in MGST1 overexpressing MCF7 cells. A second and novel strategy to overcome GST mediated resistance involves using GST releasable cytostatic drugs. Here we synthesized two derivatives of DOX, 2,4-dinitrobenzenesulfonyl doxorubicin (DNS-DOX) and 4-mononitrobenzenesulfonyl doxorubicin (MNS-DOX) and showed that they are substrates for MGST1 and GSTP (releasing DOX). MGST1 overexpressing cells are resistant to DOX. The resistance is partially reversed by DNS-DOX. Interestingly, the less reactive MNS-DOX was more cytotoxic to cells overexpressing MGST1 than control cells. It would appear that, by controlling the reactivity of the prodrug, and thereby the DOX release rate, selective toxicity to MGST1 overexpressing cells can be achieved. In the case of V79 cells, DOX resistance proportional to GSTP expression levels was noted. In this case, not only was drug resistance eliminated by DNS-DOX but a striking GSTP-dependent increase in toxicity was observed in the clonogenic assay. In summary, MGST1 and GSTP resistance to cytostatic drugs can be overcome and cytotoxicity can be enhanced in GST overexpressing cells.

Microsomal glutathione transferase 1: mechanism and functional roles

Microsomal glutathione transferase 1 (MGST1) belongs to a superfamily named MAPEG (membrane-associated proteins in eicosanoid and glutathione metabolism). This family is represented in all life forms, except archae. Of the six human members, three are specialized in the synthesis of leukotrienes and prostaglandin E, whereas the others (MGST1-3) have potential roles in drug metabolism. MGST1 has a well-established role in the conjugation of electrophiles and oxidative stress protection, whereas MGST2 and 3 have been less studied. Here, we review the recent advances regarding the structure, mechanism, and functional roles of MGST1. Emerging data show that the enzyme is overexpressed in certain tumors and support a role for the enzyme in protecting cells from cytostatic drugs.

Role of MGST1 in reactive intermediate-induced injury

Microsomal glutathione transferase (MGST1, EC 2.5.1.18) is a membrane bound glutathione transferase extensively studied for its ability to detoxify reactive intermediates, including metabolic electrophile intermediates and lipophilic hydroperoxides through its glutathione dependent transferase and peroxidase activities. It is expressed in high amounts in the liver, located both in the endoplasmic reticulum and the inner and outer mitochondrial membranes. This enzyme is activated by oxidative stress. Binding of GSH and modification of cysteine 49 (the oxidative stress sensor) has been shown to increase activation and induce conformational changes in the enzyme. These changes have either been shown to enhance the protective effect ascribed to this enzyme or have been shown to contribute to cell death through mitochondrial permeability transition pore formation. The purpose of this review is to elucidate how one enzyme found in two places in the cell subjected to the same conditions of oxidative stress could both help protect against and contribute to reactive oxygen species-induced liver injury.

Mitochondrial glutathione transferases involving a new function for membrane permeability transition pore regulation

The mitochondria in mammalian cells are a predominant resource of reactive oxygen species (ROS), which are produced during respiration-coupled oxidative metabolism or various chemical stresses. End-products from membrane-lipid peroxidation caused by ROS are highly toxic, thereby their elimination/scavenging are protective of mitochondria and cells against oxidative damages. In mitochondria, soluble (kappa, alpha, mu, pi, zeta) and membrane-bound glutathione transferases (GSTs) (MGST1) are distributed. Mitochondrial GSTs display both glutathione transferase and peroxidase activities that detoxify such harmful products through glutathione (GSH) conjugation or GSH-mediated peroxide reduction. Some GST isoenzymes are induced by oxidative stress, an adaptation mechanism for the protection of cells from oxidative stress. Membrane-bound MGST1 is activated through the thiol modification in oxidative conditions. Protective action of MGST1 against oxidative stress has been confirmed using MCF7 cells highly expressed of MGST1. In recent years, mitochondria have been recognized as a regulator of cell death via both apoptosis and necrosis, where oxidative stress-induced alteration of the membrane permeability is an important step. Recent studies have shown that MGST1 in the inner mitochondrial membrane could interact with the mitochondrial permeability transition (MPT) regulator proteins, such as adenine nucleotide translocator (ANT) and/or cyclophilin D, and could contribute to oxidant-induced MPT pores. Interaction of GST alpha with ANT has also been shown. In this review, functions of the mitochondrial GSTs, including a new role for mitochondria-mediated cell death, are described.

Glutathione transferase zeta: discovery, polymorphic variants, catalysis, inactivation, and properties of Gstz1-/- mice

Glutathione transferase zeta (GSTZ1) is a member of the GST superfamily of proteins that catalyze the reaction of glutathione with endo- and xenobiotics. GSTZ1-1 was discovered by a bioinformatics strategy that searched the human-expressed sequence-tag database with a sequence that matched a putative plant GST. A sequence that was found was expressed and termed GSTZ1-1. In common with other GSTs, GSTZ1-1 showed some peroxidase activity, but lacked activity with most known GST substrates. GSTZ1-1 was also found to be identical with maleylacetoacetate isomerase, which catalyzes the penultimate step in the tyrosine-degradation pathway. Further studies showed that dichloroacetate (DCA) and a range of α-haloalkanoates and α,α-dihaloalkanoates were substrates. A subsequent search of the human-expressed sequence-tag database showed the presence of four polymorphic alleles: 1a, 1b, 1c, and 1d; GSTZ1c was the most common and was designated as the wild-type gene. DCA was shown to be a k(cat) inactivator of human, rat, and mouse GSTZ1-1; human GSTZ1-1 was more resistant to inactivation than mouse or rat GSTZ1-1. Proteomic analysis showed that hGSTZ1-1 was inactivated when Cys-16 was modified by glutathione and the carbon skeleton of DCA. The polymorphic variants of hGSTZ1-1 differ in their susceptibility to inactivation, with 1a-1a being more resistant to inactivation than the other variants. The targeted deletion of GSTZ1 yielded mice that were not phenotypically distinctive. Phenylalanine proved, however, to be toxic to Gstz1(-/-) mice, and these mice showed evidence of organ damage and leucopenia.

Multiple roles of microsomal glutathione transferase 1 in cellular protection: a mechanistic study

The aim of this study was to investigate the involvement of membrane-bound microsomal glutathione transferase 1 (MGST1) in cellular resistance against oxidative stress as well as its mechanism of protection. MGST1 is ubiquitously expressed and predominantly located in the endoplasmic reticulum and outer mitochondrial membrane. Utilizing MCF7 cells overexpressing MGST1 we show significant protection against agents that are known to induce lipid peroxidation (e.g., cumene hydroperoxide and tert-butylhydroperoxide) and an end-product of lipid peroxidation (e.g., 4-hydroxy-2-nonenal). Furthermore, our results demonstrate that MGST1 protection can be enhanced by vitamin E when toxicity depends on oxidative stress, but not when direct alkylation is the dominant mechanism. Mitochondria in MGST1-overexpressing cells were shown to be protected from oxidative insult as measured by calcium loading capacity and respiration. MGST1 induces cellular resistance against cisplatin. Here we used vitamin E to elucidate whether oxidative stress caused by cisplatin is significant for cell toxicity. The results indicate that oxidative stress and induction of lipid peroxidation are not the most prominent toxic mechanism of cisplatin in our cell system. We thus conclude that MGST1 protects cells (and mitochondria) by both conjugation and glutathione peroxidase functions. A new protective mechanism against cisplatin is also indicated.

Role of microsomal glutathione transferase 1 in the mechanism-based biotransformation of glyceryl trinitrate in LLC-PK1 cells

Although glyceryl trinitrate (GTN) has been used in the treatment of angina for many years, details of its conversion to the proximal activator (presumed to be NO or an NO congener) of soluble guanylyl cyclase (sGC) are still unclear. We reported previously that purified microsomal glutathione transferase 1 (MGST1) mediates the denitration of GTN. In the current study, we investigated in intact cells whether this enzyme also converts GTN to species that activate sGC (mechanism-based biotransformation). We utilized LLC-PK1 cells, a cell line with an intact NO/sGC/cGMP system, and generated a stable cell line that overexpressed MGST1. MGST1 in the stably transfected cells was localized to the endoplasmic reticulum, and microsomes from these cells exhibited markedly increased GST activity. Although incubation of these cells with GTN resulted in a 3-4-fold increase in GTN biotransformation, attributed primarily to an increase in formation of the 1,3-glyceryl dinitrate metabolite, GTN-induced cGMP accumulation in cells overexpressing MGST1 was not different than that observed in wild type cells or in cells stably transfected with empty vector. To determine whether overexpression of NADPH cytochrome P450 reductase might act in concert with MGST1 to generate activators of sGC, we assessed GTN-induced cGMP accumulation in MGST1-overexpressing cells that had been transiently transfected with CPR. In this case, GTN-induced cGMP accumulation was also not different than that observed in wild type cells. We conclude that although MGST1 mediates the biotransformation of GTN in intact cells, this biotransformation does not contribute to the formation of activators of sGC.

The role of biotransformation and bioactivation in toxicity

Biotransformation is essential to convert lipophilic chemicals to water-soluble and readily excretable metabolites. Formally, biotransformation reactions are classified into phase I and phase II reactions. Phase I reactions represent the introduction of functional groups, whereas phase II reactions are conjugations of such functional groups with endogenous, polar products. Biotransformation also plays an essential role in the toxicity of many chemicals due to the metabolic formation of toxic metabolites. These may be classified as stable but toxic products, reactive electrophiles, radicals, and reactive oxygen metabolites. The interaction of toxic products formed by biotransformation reactions with cellular macromolecules initiates the sequences resulting in cellular damage, cell death and toxicity.

The Endoplasmic Reticulum in Xenobiotic Toxicity

The endoplasmic reticulum (ER) is involved in an array of cellular functions that play important roles in xenobiotic toxicity. The ER contains the majority of cytochrome P450 enzymes involved in xenobiotic metabolism, as well as a number of conjugating enzymes. In addition to its role in drug bioactivation and detoxification, the ER can be a target for damage by reactive intermediates leading to cell death or immune-mediated toxicity. The ER contains a set of luminal proteins referred to as ER stress proteins (including GRP78, GRP94, protein disulfide isomerase, and calreticulin). These proteins help regulate protein processing and folding of membrane and secretory proteins in the ER, calcium homeostasis, and ER-associated apoptotic pathways. They are induced in response to ER stress. This review discusses the importance of the ER in molecular events leading to cell death following xenobiotic exposure. Data showing that the ER is important in both renal and hepatic toxicity will be discussed.

Protection of cells from oxidative stress by microsomal glutathione transferase 1

Rat liver microsomal glutathione transferase 1 (MGST1) is a membrane-bound enzyme that displays both glutathione transferase and glutathione peroxidase activities. We hypothesized that physiologically relevant levels of MGST1 is able to protect cells from oxidative damage by lowering intracellular hydroperoxide levels. Such a role of MGST1 was studied in human MCF7 cell line transfected with rat liver mgst1 (sense cell) and with antisense mgst1 (antisense cell). Cytotoxicities of two hydroperoxides (cumene hydroperoxide (CuOOH) and hydrogen peroxide) were determined in both cell types using short-term and long-term cytotoxicity assays. MGST1 significantly protected against CuOOH and against hydrogen peroxide (although less pronounced and only in short-term tests). These results demonstrate that MGST1 can protect cells from both lipophilic and hydrophilic hydroperoxides, of which only the former is a substrate. After CuOOH exposure MGST1 significantly lowered intracellular ROS as determined by FACS analysis.

Tissue and species distribution of the glutathione pathway transcriptome

The goal of this study was to compare and contrast the basal gene expression levels of the various enzymes involved in glutathione metabolism among tissues and genders of the rat, mouse and canine. The approach taken was to use Affymetrix GeneChip microarray data for rat, mouse and canine tissues, comparing intensity levels for individual probes between tissues and genders. As was hypothesized, the relative expression in liver, lung, heart, kidney and testis varied from gene to gene, with differences of expression between tissues sometimes greater than a 1000-fold. The pattern of differential expression was usually similar between male and female animals, but varied greatly between the three species. Gstp1 appears to be expressed at high levels in male mouse liver, reasonable levels in canine liver, but very low levels in male rat liver. In all species examined, Gstp1 expression was below detectable levels in testis. Gsta3/Yc2 expression appeared high in rodent liver and female canine liver, but not male canine liver. Finally, Mgst1 and Gpx3 expression appeared to be lower in canine heart and testis than seen in rodents. Given the critical role of the glutathione pathway in the detoxification of many drugs and xenobiotics, the observed differences in basal tissue distribution among mouse, rat and canine has far-reaching implications in comparing responses of these species in safety testing.

Further evidence that rat liver microsomal glutathione transferase 1 is not a cellular protein target for S-nitrosylation

By adopting biotin switch method, we recently reported that liver microsomal glutathione transferase 1 (MGST1) might not be a protein target for S-nitrosylation in rat microsomes or in vivo. However, alternative analytic methods are needed to confirm this observation, as a single biotin switch method in judging specific protein S-nitrosylation in biological samples is increasingly recognized as insufficient, or even unreliable. Besides, only MGST1 localized on endoplasmic reticulum (ER), but not mitochondria which favors protein S-nitrosylation was examined in the previous report. Present study was therefore carried out to address these issues. Primary cultured hepatocytes were used. A physiological existing nitric oxide (NO) donor S-nitrosoglutathione (GSNO) was adopted to trigger protein S-nitrosylation. MGST1 was immunoprecipitated and its S-nitrosothiol content was measured by the NO probe 2,3-diaminonaphthalene. In parallel, S-nitrosylated proteins were immunoprecipitated by a monoclonal anti-S-nitrosocysteine antibody and probed with an anti-MGST1 antibody. In hepatocytes, neither ER nor mitochondria were found to contain S-nitrosylated MGST1 after GSNO treatment, showing that differently distributed MGST1 was consistently un-nitrosylable in the cellular environment. But under broken cell conditions, when samples were incubated directly with GSNO, MGST1 S-nitrosylation was indeed detectable in both the microsomal and mitochondrial proteins, indicating that previous failure in detecting MGST1 S-nitrosylation in microsomes is due to the limitations of biotin switch method. These results clearly, if not definitely, demonstrate that MGST1 is not a ready candidate for S-nitrosylation in the cellular content, despite its susceptibility to S-nitrosylation under broken cell conditions.

Integration of hepatic drug transporters and phase II metabolizing enzymes: Mechanisms of hepatic excretion of sulfate, glucuronide, and glutathione metabolites

The liver is the primary site of drug metabolism in the body. Typically, metabolic conversion of a drug results in inactivation, detoxification, and enhanced likelihood for excretion in urine or feces. Sulfation, glucuronidation, and glutathione conjugation represent the three most prevalent classes of phase II metabolism, which may occur directly on the parent compounds that contain appropriate structural motifs, or, as is usually the case, on functional groups added or exposed by phase I oxidation. These three conjugation reactions increase the molecular weight and water solubility of the compound, in addition to adding a negative charge to the molecule. As a result of these changes in the physicochemical properties, phase II conjugates tend to have very poor membrane permeability, and necessitate carrier-mediated transport for biliary or hepatic basolateral excretion into sinusoidal blood for eventual excretion into urine. This review summarizes sulfation, glucuronidation, and glutathione conjugation reactions, as well as recent progress in elucidating the hepatic transport mechanisms responsible for the excretion of these conjugates from the liver. The discussion focuses on alterations of metabolism and transport by chemical modulators, and disease states, as well as pharmacodynamic and toxicological implications of hepatic metabolism and/or transport modulation for certain active phase II conjugates. A brief discussion of issues that must be considered in the design and interpretation of phase II metabolite transport studies follows.

Metabolism of melphalan by rat liver microsomal glutathione S-transferase

One of the major problems in the treatment of human cancer is the phenomenon of drug resistance. Increased glutathione (gamma-glutamylcysteinylglycine, GSH) conjugation (inactivation) due to elevated level of cytosolic glutathione S-transferase (GST) is believed to be an important mechanism in tumor cell resistance. However, the potential involvement of microsomal GST in the establishment of acquired drug resistance (ADR) remains uncertain. In our experiments, a combination of liquid chromatography/electrospray ionization/mass spectrometry (LC/ESI/MS) was employed for structural characterization of the resulting conjugates between GSH and melphalan, one of the alkylating agents. The spontaneous reaction of 1mM melphalan with 5mM GSH at 37 degrees C in aqueous phosphate buffer for 1h gave primarily the monoglutathionyl and diglutathionyl melphalan derivatives, with small amounts of mono- and dihydroxy melphalan derivatives. We demonstrated that rat liver microsomal GST presented a strong catalytic effect on the reaction as determined by the increase of monoglutathionyl and diglutathionyl melphalan derivatives and the decrease of melphalan. We showed that microsomal GST was activated by melphalan in a concentration- and time-dependent manner. Microsomal GST which was stimulated approximately 1.5-fold with melphalan had a stronger catalytic effect. Thus microsomal GST may play a potential role in the metabolism of melphalan in biological membranes, and in the development of ADR.

Differential nitros(yl)ation of blood and tissue constituents during glyceryl trinitrate biotransformation in vivo

Nitric oxide (NO)-derived products may modify tissue constituents, forming S- and N-nitroso adducts and metal nitrosyls implicated in NO signaling. Nitrovasodilator drugs have been in widespread use for more than a century, yet their biotransformation pathways to NO and their effects as NO donors across tissues remain ill defined. By using a metabonomics approach (termed "NObonomics") for detailing the global NO-related metabolism of the cornerstone nitrovasodilator, glyceryl trinitrate (GTN; 0.1-100 mg/kg), in the rat in vivo, we find that GTN biotransformation elicits extensive tissue nitros(yl)ation throughout all major organ systems. The corresponding reaction products remained detectable hours after administration, and vascular tissue was not a major nitros(yl)ation site. Extensive heart and liver modifications involved both S- and N-nitrosation, and RBC S-nitrosothiol formation emerged as a sensitive indicator of organic nitrate metabolism. The dynamics of GTN-derived oxidative NO metabolites in blood did not reflect the nitros(yl)ation patterns in the circulation or in tissues, casting doubt on the usefulness of plasma nitrite/nitrate as an index of NO/NO-donor biodynamics. Target-tissue NO metabolites varied in amount and type with GTN dose, suggesting a dose-sensitive shift in the prevailing routes of GTN biotransformation ("metabolic shunting") from thiol nitrosation to heme nitrosylation. We further demonstrate that GTN-induced nitros(yl)ation is modulated by a complex, tissue-selective interplay of enzyme-catalyzed pathways. These findings provide insight into the global in vivo metabolism of GTN at pharmacologically relevant doses and offer an additional experimental paradigm for the NObonomic analysis of NO-donor metabolism and signaling.

Metabolism of chlorambucil by rat liver microsomal glutathione S-transferase

Clinical efficacy of alkylating anticancer drugs, such as chlorambucil (4-[p-[bis [2-chloroethyl] amino] phenyl]-butanoic acid; CHB), is often limited by the emergence of drug resistant tumor cells. Increased glutathione (gamma-glutamylcysteinylglycine; GSH) conjugation (inactivation) of alkylating anticancer drugs due to overexpression of cytosolic glutathione S-transferase (GST) is believed to be an important mechanism in tumor cell resistance to alkylating agents. However, the potential involvement of microsomal GST in the establishment of acquired drug resistance (ADR) to CHB remains uncertain. In our experiments, a combination of lipid chromatography/electrospray ionization mass spectrometry (LC/ESI/MS) was employed for structural characterization of the resulting conjugates between CHB and GSH. The spontaneous reaction of 1mM CHB with 5 mM GSH at 37 degrees C in aqueous phosphate buffer for 1 h gave primarily the monoglutathionyl derivative, 4-[p-[N-2-chloroethyl, N-2-S-glutathionylethyl] amino]phenyl]-butanoic acid (CHBSG) and the diglutathionyl derivative, 4-[p-[2-S-glutathionylethyl] amino]phenyl]-butanoic acid (CHBSG2) with small amounts of the hydroxy-derivative, 4-[p-[N-2-S-glutathionylethyl, N-2-hydroxyethyl] amino]phenyl]-butanoic acid (CHBSGOH), 4-[p-[bis[2-hydroxyethyl] amino]phenyl]-butanoic acid (CHBOH2), 4-[p-[N-2-chloroethyl, N-2-S-hydroxyethyl]amino]phenyl]-butanoic acid (CHBOH). We demonstrated that rat liver microsomal GST presented a strong catalytic effect on these reactions as determined by the increase of CHBSG2, CHBSGOH and CHBSG and the decrease of CHB. We showed that microsomal GST was activated by CHB in a concentration and time dependent manner. Microsomal GST which was stimulated approximately two-fold with CHB had a stronger catalytic effect. Thus, microsomal GST may play a potential role in the metabolism of CHB in biological membranes, and in the development of ADR.

IDENTIFICATION AND METABOLISM OF A NOVEL DIHYDROHYDROXY-S-GLUTATHIONYL CONJUGATE OF A PEROXISOME PROLIFERATOR-ACTIVATED RECEPTOR AGONIST, MK-0767 [(±)-5-[(2,4-DIOXOTHIAZOLIDIN-5-YL)METHYL]-2-METHOXY-N-[[(4-TRIFLUOROMETHYL) PHENYL]METHYL]BENZAMIDE], IN RATS

MK-0767 [(+/-)-5-[(2,4-dioxothiazolidin-5-yl)methyl]-2-methoxy-N-[[(4-trifluoromethyl)phenyl]methyl]benzamide] is a novel thiazolidinedione-containing peroxisome proliferator-activated receptor alpha/gamma agonist. In rats dosed orally with [14C]MK-0767, a dihydrohydroxy-S-glutathionyl conjugate of the parent compound was identified in the bile using liquid chromatography-mass spectometry and 1H NMR techniques. The formation of the conjugate likely proceeded via an arene oxide intermediate. The corresponding cysteinylglycine and cysteinyl conjugates likely formed from the further metabolism of the dihydrohydroxy-S-glutathionyl conjugate also were detected in rat bile. The dihydrohydroxy-S-glutathionyl conjugate was formed in vitro following the incubation of MK-0767 and glutathione with rat, dog, or monkey liver microsomes, and its formation was NADPH-dependent; however, this conjugate was not detected in human liver microsomal incubations. When incubated with rat intestinal contents, the dihydrohydroxy-S-glutathionyl conjugate was reduced to the parent compound (MK-0767), suggesting the involvement of intestinal microflora in its metabolism. There was no reduction of the conjugate by rat intestinal cytosol.

Structural organization of the murine microsomal glutathione S-transferase gene (MGST1) from the 129/SvJ strain: identification of the promoter region and a comprehensive examination of tissue expression

The structure and regulation of the murine microsomal glutathione transferase gene (MGST1) from the 129/SvJ strain is described and demonstrates considerable difference in nucleotide sequence and consequently in restriction enzyme sites as compared to other mouse strains. A comparison of the amino acid sequence for MGST1 revealed one difference in exon 2 between the 129/SvJ strain (arginine at position 5) and the sequence previously reported for the Balb/c strain (lysine). The promoter region immediately upstream of the dominant first exon is functional, transcriptionally responds to oxidative stress, and is highly homologous to the human region. Oxidative stress also induced the production of endogenous MGST1 mRNA. The tissue-specific expression of MGST1 mRNA was studied, and as anticipated, was indeed highest in liver. There was, however, marked mRNA expression in several tissues not previously studied including smooth muscle, epidymus, ovaries, and endocrine glands in which the expression of various peroxidases is also very high (salivary and thyroid). Overall, there was a good agreement between the mRNA content detected and previous reports of MGST1 activity with the exception of brain tissue.

Biochemical and genetic characterization of a murine class Kappa glutathione S-transferase

The class Kappa family of glutathione S-transferases (GSTs) currently comprises a single rat subunit (rGSTK1), originally isolated from the matrix of liver mitochondria [Harris, Meyer, Coles and Ketterer (1991) Biochem. J. 278, 137-141; Pemble, Wardle and Taylor (1996) Biochem. J. 319, 749-754]. In the present study, an expressed sequence tag (EST) clone has been identified which encodes a mouse class Kappa GST (designated mGSTK1). The EST clone contains an open reading frame of 678 bp, encoding a protein composed of 226 amino acid residues with 86% sequence identity with the rGSTK1 polypeptide. The mGSTK1 and rGSTK1 proteins have been heterologously expressed in Escherichia coli and purified by affinity chromatography. Both mouse and rat transferases were found to exhibit GSH-conjugating and GSH-peroxidase activities towards model substrates. Analysis of expression levels in a range of mouse and rat tissues revealed that the mRNA encoding these enzymes is expressed predominantly in heart, kidney, liver and skeletal muscle. Although other soluble GST isoenzymes are believed to reside primarily within the cytosol, subcellular fractionation of mouse liver demonstrates that this novel murine class Kappa GST is associated with mitochondrial fractions. Through the use of bioinformatics, the genes encoding the mouse and rat class Kappa GSTs have been identified. Both genes comprise eight exons, the protein coding region of which spans approx. 4.3 kb and 4.1 kb of DNA for mGSTK1 and rGSTK1 respectively. This conservation in primary structure, catalytic properties, tissue-specific expression, subcellular localization and gene structure between mouse and rat class Kappa GSTs indicates that they perform similar physiological functions. Furthermore, the association of these enzymes with mitochondrial fractions is consistent with them performing a specific conserved biological role within this organelle.

Effect of vitamin E on glutathione-dependent enzymes

Reactive oxygen species and various electrophiles are involved in the etiology of diseases varying from cancer to cardiovascular and pulmonary disorders. The human body is protected against damaging effects of these compounds by a wide variety of systems. An important line of defense is formed by antioxidants. Vitamin E (consisting of various forms of tocopherols and tocotrienols) is an important fat-soluble, chain-breaking antioxidant. Besides working as an antioxidant, this compound possesses other functions with possible physiological relevance. The glutathione-dependent enzymes form another line of defense. Two important enzymes in this class are the free radical reductase and glutathione S-transferases (GSTs). The GSTs are a family of phase II detoxification enzymes. They can catalyze glutathione conjugation with various electrophiles. In most cases the electrophiles are detoxified by this conjugation, but in some cases the electrophiles are activated. Antioxidants do not act in isolation but form an intricate network. It is, for instance, known that vitamin E, together with glutathione (GSH) and a membrane-bound heat labile GSH-dependent factor, presumably an enzyme, can prevent damaging effects of reactive oxygen species on polyunsaturated fatty acids in biomembranes (lipid peroxidation). This manuscript reviews the interaction between the two defense systems, vitamin E and glutathione-dependent enzymes. On the simplest level, antioxidants such as vitamin E have protective effects on glutathione-dependent enzymes; however, we will see that reality is somewhat more complicated.

Expression of microsomal glutathione S-transferase type 3 mRNA in the rat nervous system

Microsomal glutathione S-transferase type 3 (MGST3) is a recently identified member of a large superfamily of enzymes involved in biotransformation of xenobiotics and biosynthesis of eicosanoids, including prostaglandins and leukotrienes. Using in situ hybridization histochemistry and reverse transcription polymerase chain reaction, we characterized the expression of MGST3 mRNA in the rat nervous system based on the cloned rat MGST3 gene, under normal conditions and after systemic administration of lipopolysaccharide (LPS). The MGST3 mRNA seemed to be confined to neurons. The broad distribution in the brain was characterized by a strong signal in the hippocampal formation and in the nuclei of the cranial nerves. A moderate signal was found in the cortex, thalamus, amygdala and substantia nigra and a weak signal in the hypothalamus. Motoneurons in the spinal cord and sensory neurons in dorsal root ganglia displayed strong MGST3 mRNA signal. No significant changes in the level of expression of MGST3 mRNA in the brain were found 1, 3 or 6 h after LPS administration. The pattern of distribution of MGST3 mRNA in the rat nervous system and the lack of response to LPS do not support a role for MGST3 in the biosynthesis of proinflammatory eicosanoids but rather suggest other functions, perhaps in metabolic detoxication and neuroprotection.

Assessing the health risks following environmental exposure to hexachlorobutadiene

Hexachloro-1,3-butadiene (HCBD) has been reported to be toxic to the rat kidney in a 2 year study at doses higher than 0.2 mg/kg/day. The toxicity is known to be a consequence of the metabolism of HCBD by glutathione conjugation and the renal beta-lyase pathway. Neither toxicity data, nor data on the metabolism of HCBD, are available in humans. In the current work, the potential of HCBD to cause kidney damage in humans environmentally exposed to this chemical has been assessed quantitatively by comparing the key metabolic steps in rats and humans. To that end, the hepatic conjugation of HCBD with glutathione, the metabolism of the cysteine conjugate by renal beta-lyases and N-acetyltransferases, and the metabolism of the N-acetylcysteine conjugate by renal acylases has been compared in vitro in rat and human tissues. Rates for each metabolic step were lower in humans than in rats; 5-fold for glutathione conjugation, 3-fold for beta-lyase and 3.5-fold for N-acetyltransferase. Acylase activity could not be detected in human kidney cytosol. Use of these data in a physiologically based toxicokinetic model to quantify metabolism by the beta-lyase pathway demonstrated that metabolism in humans was an order of magnitude lower than that in rats. At the no effect level for kidney toxicity in the rat the concentration of beta-lyase metabolites was calculated by the model to be 137.7 mg/l. In humans the same concentration would be achieved following exposure to 1.41 ppm HCBD. This is in contrast to the figure of 0.6 ppb which is obtained when it is assumed that the risk is associated with the internal dose of HCBD itself rather than beta-lyase metabolites.

Activation of microsomal glutathione s-transferase by peroxynitrite

Peroxynitrite (ONOO-) toxicity is associated with protein oxidation and/or tyrosine nitration, usually resulting in inhibition of enzyme activity. We examined the effect of ONOO- on the activity of purified rat liver microsomal glutathione S-transferase (GST) and found that the activity of reduced glutathione (GSH)-free enzyme was increased 4- to 5-fold by 2 mM ONOO-; only 15% of this increased activity was reversed by dithiothreitol. Exposure of the microsomal GST to ONOO- resulted in concentration-dependent oxidation of protein sulfhydryl groups, dimer and trimer formation, protein fragmentation, and tyrosine nitration. With the exception of sulfhydryl oxidation, these modifications of the enzyme correlated well with the increase in enzyme activity. Nitration or acetylation of tyrosine residues of the enzyme using tetranitromethane and N-acetylimidazole, respectively, also resulted in increased enzyme activity, providing additional evidence that modification of tyrosine residues can alter catalytic activity. Addition of ONOO--treated microsomal GST to microsomal membrane preparations caused a marked reduction in iron-induced lipid peroxidation, which raises the possibility that this enzyme may act to lessen the degree of membrane damage that would otherwise occur under pathophysiological conditions of increased ONOO- formation.

Toxic, halogenated cysteine S-conjugates and targeting of mitochondrial enzymes of energy metabolism

Several haloalkenes are metabolized in part to nephrotoxic cysteine S-conjugates; for example, trichloroethylene and tetrafluoroethylene are converted to S-(1,2-dichlorovinyl)-L-cysteine (DCVC) and S-(1,1,2,2-tetrafluoroethyl)-L-cysteine (TFEC), respectively. Although DCVC-induced toxicity has been investigated since the 1950s, the toxicity of TFEC and other haloalkene-derived cysteine S-conjugates has been studied more recently. Some segments of the US population are exposed to haloalkenes either through drinking water or in the workplace. Therefore, it is important to define the toxicological consequences of such exposures. Most halogenated cysteine S-conjugates are metabolized by cysteine S-conjugate beta-lyases to pyruvate, ammonia, and an alpha-chloroenethiolate (with DCVC) or an alpha-difluoroalkylthiolate (with TFEC) that may eliminate halide to give a thioacyl halide, which reacts with epsilon-amino groups of lysine residues in proteins. Nine mammalian pyridoxal 5'-phosphate (PLP)-containing enzymes catalyze cysteine S-conjugate beta-lyase reactions, including mitochondrial aspartate aminotransferase (mitAspAT), and mitochondrial branched-chain amino acid aminotransferase (BCAT(m)). Most of the cysteine S-conjugate beta-lyases are syncatalytically inactivated. TFEC-induced toxicity is associated with covalent modification of several mitochondrial enzymes of energy metabolism. Interestingly, the alpha-ketoglutarate- and branched-chain alpha-keto acid dehydrogenase complexes (KGDHC and BCDHC), but not the pyruvate dehydrogenase complex (PDHC), are susceptible to inactivation. mitAspAT and BCAT(m) may form metabolons with KGDHC and BCDHC, respectively, but no PLP enzyme is known to associate with PDHC. Consequently, we hypothesize that not only do these metabolons facilitate substrate channeling, but they also facilitate toxicant channeling, thereby promoting the inactivation of proximate mitochondrial enzymes and the induction of mitochondrial dysfunction.

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.

Regulation of microsomal and cytosolic glutathione S-transferase activities by S-nitrosylation

There is increasing evidence that S-nitrosylation is a mechanism for the regulation of protein function via the modification of critical sulfhydryl groups. The activity of rat liver microsomal glutathione S-transferase (GST) is increased after treatment with N-ethylmaleimide (NEM), a sulfhydryl alkylating reagent, and is also increased under conditions of oxidative stress. In the present study, preincubation of purified rat liver microsomal GST with S-nitrosoglutathione (GSNO) or the nitric oxide (NO) donor, 1,1-diethyl-2-hydroxy-2-nitrosohydrazine (DEA/NO), resulted in a 2-fold increase in enzyme activity. This increase in activity was reversed by dithiothreitol. The initial treatment of microsomal GST with either GSNO or DEA/NO was associated with an 85% loss of free sulfhydryl groups. After removal of the nitrosylating agents over a 6-hr period, approximately 50% of the enzyme was still nitrosylated, as determined by redox chemiluminescence. Furthermore, preincubation of either purified enzyme or hepatic microsomes with GSNO or DEA/NO prevented further enzyme activation by NEM, suggesting that NEM and the NO donors interact with a common population of sulfhydryl groups in the enzyme. In contrast, both NEM and NO donors partially inhibited the activity of cytosolic GST isoforms. The inhibitory activity of NEM and NO donors was much more evident when the GST pi isoform was used instead of a mixture of GST isoforms. These data suggest that there may be differential regulation of microsomal and cytosolic GST activities under conditions of nitrosative stress.

The evolution of glutathione metabolism in phototrophic microorganisms

Of the many roles ascribed to glutathione (GSH) the one most clearly established is its role in the protection of higher eucaryotes against oxygen toxicity through destruction of thiol-reactive oxygen byproducts. If this is the primary function of GSH then GSH metabolism should have evolved during or after the evolution of oxygenic photosynthesis. That many bacteria do not produce GSH is consistent with this view. In the present study we have examined the low-molecular-weight thiol composition of a variety of phototrophic microorganisms to ascertain how evolution of GSH production is related to evolution of oxygenic photosynthesis. Cells were extracted in the presence of monobromobimane (mBBr) to convert thiols to fluorescent derivatives, which were analyzed by high-pressure liquid chromatography. Significant levels of GSH were not found in the green bacteria (Chlorobium thiosulfatophilum and Chloroflexus aurantiacus). Substantial levels of GSH were present in the purple bacteria (Chromatium vinosum, Rhodospirillum rubrum, Rhodobacter sphaeroides, and Rhodocyclus gelatinosa), the cyanobacteria [Anacystis nidulans, Microcoleus chthonoplastes S.G., Nostoc muscorum, Oscillatoria amphigranulata, Oscillatoria limnetica, Oscillatoria sp. (Stinky Spring, Utah), Oscillatoria terebriformis, Plectonema boryanum, and Synechococcus lividus], and eucaryotic algae (Chlorella pyrenoidsa, Chlorella vulgaris, Euglena gracilis, Scenedesmus obliquus, and Chlamydomonas reinhardtii). Other thiols measured included cysteine, gamma-glutamylcysteine, thiosulfate, coenzyme A, and sulfide; several unidentified thiols were also detected. Many of the organisms examined also exhibited a marked ability to reduce mBBr to syn-(methyl,methyl)bimane, an ability that was quenched by treatment with 2-pyridyl disulfide or 5,5'-bisdithio-(2-nitrobenzoic acid) prior to reaction with mBBr. These observations indicate the presence of a reducing system capable of electron transfer to mBBr and reduction of reactive disulfides. The distribution of GSH in phototrophic eubacteria indicates that GSH synthesis evolved at or around the time that oxygenic photosynthesis evolved.

Microsomal GST-I: genomic organization, expression, and alternative splicing of the human gene

In this paper we report the genomic organization of the human microsomal GST-I gene. This gene spans 18 kb, and contains seven exons. Sequences that encode the 155 amino acid open reading frame are present in Exons II, III, IV, the 5'-untranslated region is present in Exons Ia, Ib, Ic, Id, and II, and the 3'-untranslated region is present in Exon IV. Exons Ia, Ib, Ic, Id, and III are alternatively spliced to generate at least six different mGST-I transcripts. The results of EST and PCR analysis show that most mGST-I transcripts terminate within Exon Ib, and primer extension analysis shows these transcripts initiate at three major sites located at 79, 81, and 88 nucleotides upstream of the ATG initiation codon. Sequences surrounding the putative initiation sites are G-C rich, and several Sp1 consensus binding sites were identified. Northern analysis shows that the human GST-I gene is preferentially expressed as a 1.0 kb transcript in liver, and in several other tissues. Finally, a comparison of the mGST-I and PIG12 sequences with those of FLAP, LTC4 synthase, mGST-II, and mGST-III suggests that these proteins are the related products of a dispersed microsomal GST gene superfamily.

The role of human glutathione S-transferases hGSTA1-1 and hGSTA2-2 in protection against oxidative stress

In order to elucidate the protective role of glutathione S-transferases (GSTs) against oxidative stress, we have investigated the kinetic properties of the human alpha-class GSTs, hGSTA1-1 and hGSTA2-2, toward physiologically relevant hydroperoxides and have studied the role of these enzymes in glutathione (GSH)-dependent reduction of these hydroperoxides in human liver. We have cloned hGSTA1-1 and hGSTA2-2 from a human lung cDNA library and expressed both in Escherichia coli. Both isozymes had remarkably high peroxidase activity toward fatty acid hydroperoxides, phospholipid hydroperoxides, and cumene hydroperoxide. In general, the activity of hGSTA2-2 was higher than that of hGSTA1-1 toward these substrates. For example, the catalytic efficiency (kcat/Km) of hGSTA1-1 for phosphatidylcholine (PC) hydroperoxide and phosphatidylethanolamine (PE) hydroperoxide was found to be 181.3 and 199.6 s-1 mM-1, respectively, while the catalytic efficiency of hGSTA2-2 for PC-hydroperoxide and PE-hydroperoxide was 317.5 and 353 s-1 mM-1, respectively. Immunotitration studies with human liver extracts showed that the antibodies against human alpha-class GSTs immunoprecipitated about 55 and 75% of glutathione peroxidase (GPx) activity of human liver toward PC-hydroperoxide and cumene hydroperoxide, respectively. GPx activity was not immunoprecipitated by the same antibodies from human erythrocyte hemolysates. These results show that the alpha-class GSTs contribute a major portion of GPx activity toward lipid hydroperoxides in human liver. Our results also suggest that GSTs may be involved in the reduction of 5-hydroperoxyeicosatetraenoic acid, an important intermediate in the 5-lipoxygenase pathway.