Microsomal Glutathione S-Transferase

Beyond microtubules: The cellular environment at the endoplasmic reticulum attracts proteins to the nucleus, enabling nuclear transport

All proteins are translated in the cytoplasm, yet many, including transcription factors, play vital roles in the nucleus. While previous research has concentrated on molecular motors for the transport of these proteins to the nucleus, recent observations reveal perinuclear accumulation even in the absence of an energy source, hinting at alternative mechanisms. Here, we propose that structural properties of the cellular environment, specifically the endoplasmic reticulum (ER), can promote molecular transport to the perinucleus without requiring additional energy expenditure. Specifically, physical interaction between proteins and the ER impedes their diffusion and leads to their accumulation near the nucleus. This result explains why larger proteins, more frequently interacting with the ER membrane, tend to accumulate at the perinucleus. Interestingly, such diffusion in a heterogeneous environment follows Chapman's law rather than the popular Fick's law. Our findings suggest a novel protein transport mechanism arising solely from characteristics of the intracellular environment.

Identification and cross-species comparison of in vitro phase I brevetoxin (BTX-2) metabolites in northern Gulf of Mexico fish and human liver microsomes by UHPLC-HRMS(/MS)

Brevetoxins (BTX) are a group of marine neurotoxins produced by the harmful alga Karenia brevis. Numerous studies have shown that BTX are rapidly accumulated and metabolized in shellfish and mammals. However, there are only limited data on BTX metabolism in fish, despite growing evidence that fish serve as vectors for BTX transfer in marine food webs. In this study, we aimed to investigate the in vitro biotransformation of BTX-2, the major constituent of BTX profiles in K. brevis, in several species of northern Gulf of Mexico fish. Metabolism assays were performed using hepatic microsomes prepared in-house as well as commercially available human microsomes for comparison, focusing on phase I reactions mediated by cytochrome P450 monooxygenase (CYP) enzymes. Samples were analyzed by UHPLC-HRMS(/MS) to monitor BTX-2 depletion and characterize BTX metabolites based on MS/MS fragmentation pathways. Our results showed that both fish and human liver microsomes rapidly depleted BTX-2, resulting in a 72-99% reduction within 1 h of incubation. We observed the simultaneous production of 22 metabolites functionalized by reductions, oxidations, and other phase I reactions. We were able to identify the previously described congeners BTX-3 and BTX-B5, and tentatively identified BTX-9, 41,43-dihydro-BTX-2, several A-ring hydrolysis products, as well as several novel metabolites. Our results confirmed that fish are capable of similar BTX biotransformation reactions as reported for shellfish and mammals, but comparison of metabolite formation across the tested species suggested considerable interspecific variation in BTX-2 metabolism potentially leading to divergent BTX profiles. We additionally observed non-enzymatic formation of BTX-2 and BTX-3 glutathione conjugates. Collectively, these findings have important implications for determining the ecotoxicological fate of BTX in marine food webs.

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.

Caveolin-1β promotes the production of active human microsomal glutathione S-transferase in induced intracellular vesicles inSpodoptera frugiperda21 insect cells

The heterologous expression in Spodoptera frugiperda 21 (Sf21) insect cells of the β isoform of canine caveolin-1 (caveolin-1β), using a baculovirus-based vector, resulted in intracellular vesicles enriched in caveolin-1β. We investigated whether these vesicles could act as membrane reservoirs, and promote the production of an active membrane protein (MP) when co-expressed with caveolin-1β. We chose hMGST1 (human microsomal glutathione S-transferase 1) as the co-expressed MP. It belongs to the membrane-associated proteins in eicosanoid and glutathione metabolism (MAPEG) family of integral MPs, and, as a phase II detoxification enzyme, it catalyzes glutathione conjugation of lipophilic drugs present in the lipid membranes. In addition to its pharmaceutical interest, its GST activity can be conveniently measured. The expression of both MPs were followed by Western blots and membrane fractionation on density gradient, and their cell localization by immunolabeling and transmission electron microscopy. We showed that caveolin-1β kept its capacity to induce intracellular vesicles in the host when co-expressed with hMGST1, and that hMGST1 is in part addressed to these vesicles. Remarkably, a fourfold increase in the amount of active hMGST1 was found in the most enriched membrane fraction, along with an increase of its specific activity by 60% when it was co-expressed with caveolin-1β. Thus, heterologously expressed caveolin-1β was able to induce cytoplasmic vesicles in which a co-expressed exogenous MP is diverted and sequestered, providing a favorable environment for this cargo.

Differential responses in the biotransformation systems of the oyster Crassostrea gigas (Thunberg, 1789) elicited by pyrene and fluorene: Molecular, biochemical and histological approach - Part II

Pyrene (PYR) and fluorene (FLU) are among the sixteen priority Polycyclic Aromatic Hydrocarbons (PAH) of the United States Environmental Protection Agency and are both frequently detected in contaminated sites. Due to the importance of bivalve mollusks in biomonitoring programs and the scarce information on the biotransformation system in these organisms, the aim of this study was to investigate the effect of PYR and FLU at the transcriptional level and the enzymatic activities of some biotransformation systems in the Pacific oyster Crassostrea gigas, and to evaluate the histological effects in their soft tissues. Oysters C. gigas were exposed for 24 h and 96 h to PYR (0.25 and 0.5 μM) and FLU (0.6 and 1.2 μM). After exposure, transcript levels of cytochrome P450 coding genes (CYP1-like, CYP2-like, CYP2AU2, CYP356A1, CYP17α-like), glutathione S tranferase genes (omega GSTO-like and microsomal, MGST-like) and sulfotransferase gene (SULT-like), and the activity of ethoxyresorufin O-deethylase (EROD), Glutathione S-transferase (GST) and microssomal GST (MGST) were evaluated in gills. Histologic changes were also evaluated after the exposure period. PYR and FLU bioconcentrated in oyster soft tissues. The half-life time of PYR in water was lower than fluorene, which is in accordance to the higher lipophilicity and bioconcentration of the former. EROD activity was below the limit of detection in all oysters exposed for 96 h to PYR and FLU. The reproductive stage of the oysters exposed to PYR was post-spawn. Exposure to PYR caused tubular atrophy in digestive diverticula, but had no effect on transcript levels of biotransformation genes. However, the organisms exposed for 96 h to PYR 0.5 μM showed higher MGST activity, suggesting a protective role against oxidative stress in gills of oysters under higher levels of PYR in the tissues. Increased number of mucous cells in mantle were observed in oysters exposed to the higher FLU concentration, suggesting a defense mechanisms. Oysters exposed for 24 h to FLU 1.2 μM were in the ripe stage of gonadal development and showed higher transcript levels of CYP2AU2, GSTO-like and SULT-like genes, suggesting a role in the FLU biotransformation. In addition, after 96 h of exposure to FLU there was a significant increase of mucous cells in the mantle of oysters but no effect was observed on the EROD, total GST and MGST activities. These results suggest that PAH have different effects on transcript levels of biotransformation genes and enzyme activities, however these differences could also be related to the reproductive stage.

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.

Role of AccMGST1 in oxidative stress resistance in Apis cerana cerana

As detoxification enzymes, proteins in the glutathione S-transferase (GST) superfamily are reported to participate in oxidative stress resistance. Nevertheless, microsomal GSTs (MGSTs), a unique subclass of the GST superfamily associated with membranes, are rarely studied in insects. Here, we isolated an MGST gene in Apis cerana cerana (AccMGST1) and verified its role in oxidative stress response. We found higher expression of AccMGST1 in protective or defensive tissue, that is, the epidermis, which indicated its role in stress resistance. Real-time quantitative PCR (qRT-PCR) analysis indicated that AccMGST1 was upregulated by oxidative stresses at the transcriptional level. In contrast, AccMGST1 expression was inhibited when the antioxidant vitamin C (VC) was fed to experimental bees. Through western blotting, we found that the protein level of AccMGST1 under oxidative stress corresponded to the transcript level. Disc diffusion and mixed-function oxidation (MFO) assays suggested that AccMGST1 can protect not only cells but also DNA against oxidative damage. Furthermore, we discovered that the expression patterns of known antioxidant genes were changed in A. cerana cerana after AccMGST1 was silenced by RNA interference (RNAi). Thus, we concluded that the gene AccMGST1 exerts a significant role in the antioxidant mechanism.

Application of Bioactive Thermal Proteome Profiling to Decipher the Mechanism of Action of the Lipid Lowering 132-Hydroxy-pheophytin Isolated from a Marine Cyanobacteria

The acceleration of the process of understanding the pharmacological application of new marine bioactive compounds requires identifying the compound protein targets leading the molecular mechanisms in a living cell. The thermal proteome profiling (TPP) methodology does not fulfill the requirements for its application to any bioactive compound lacking chemical and functional characterization. Here, we present a modified method that we called bTPP for bioactive thermal proteome profiling that guarantees target specificity from a soluble subproteome. We showed that the precipitation of the microsomal fraction before the thermal shift assay is crucial to accurately calculate the melting points of the protein targets. As a probe of concept, the protein targets of 13²-hydroxy-pheophytin, a compound previously isolated from a marine cyanobacteria for its lipid reducing activity, were analyzed on the hepatic cell line HepG2. Our improved method identified 9 protein targets out of 2500 proteins, including 3 targets (isocitrate dehydrogenase, aldehyde dehydrogenase, phosphoserine aminotransferase) that could be related to obesity and diabetes, as they are involved in the regulation of insulin sensitivity and energy metabolism. This study demonstrated that the bTPP method can accelerate the field of biodiscovery, revealing protein targets involved in mechanisms of action (MOA) connected with future applications of bioactive compounds.

Traceless synthesis of protein thioesters using enzyme-mediated hydrazinolysis and subsequent self-editing of the cysteinyl prolyl sequence

A traceless thioester-producing protocol featuring carboxypeptidase Y-mediated hydrazinolysis of cysteinyl prolyl leucine-tagged peptides has been developed. The hydrazinolysis followed by thioesterification affords cysteinyl prolyl thioesters. Self-editing of the tag and subsequent trans-thioesterification yields peptide thioesters. The developed protocol was successfully applied to the conversion of recombinant proteins to thioesters.

Activation versus inhibition of microsomal glutathione S-transferase activity by acrolein. Dependence on the concentration and time of acrolein exposure

The toxicity of acrolein, an α,β-unsaturated aldehyde, is due to its soft electrophilic nature and primarily involves the adduction of protein thiols. The thiol glutathione (GSH) forms the first line of defense against acrolein. The present study confirms that acrolein added to isolated rat liver microsomes can increase microsomal GSH transferase (MGST) activity 2-3 fold, which can be seen as a direct adaptive increase in the protection against acrolein. At a relatively high exposure level, acrolein appeared to inhibit MGST. The activation is due to adduction of thiol groups, and the inactivation probably involves adduction of amino groups in the enzyme by acrolein. The preference of acrolein to react with thiol groups over amino groups can explain why the enzyme is activated at a low exposure level and inhibited at a high exposure level of acrolein. These opposite forms of direct adaptation on the level of enzyme activity further narrow the thin line between survival and promotion of cell death, governed by the level of exposure.

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.

Global Kinetic Mechanism of Microsomal Glutathione Transferase 1 and Insights into Dynamic Enzyme Activation

Microsomal glutathione transferase 1 (MGST1) has a unique ability to be activated, ≤30-fold, by modification with sulfhydryl reagents. MGST1 exhibits one-third-of-the-sites reactivity toward glutathione and hence heterogeneous binding to different active sites in the homotrimer. Limited turnover stopped-flow kinetic measurements of the activated enzyme allowed us to more accurately determine the K_D for the "third" low-affinity GSH binding site (1.4 ± 0.3 mM). The rate of thiolate formation, k₂ (0.77 ± 0.06 s^-1), relevant to turnover, could also be determined. By deriving the steady-state rate equation for a random sequential mechanism for MGST1, we can predict K_M, k_cat, and k_cat/K_M values from these and previously determined pre-steady-state rate constants (all determined at 5 °C). To assess whether the pre-steady-state behavior can account for the steady-state kinetic behavior, we have determined experimental values for kinetic parameters at 5 °C. For reactive substrates and the activated enzyme, data for the microscopic steps account for the global mechanism of MGST1. For the unactivated enzyme and more reactive electrophilic substrates, pre-steady-state and steady-state data can be reconciled only if a more active subpopulation of MGST1 is assumed. We suggest that unactivated MGST1 can be partially activated in its unmodified form. The existence of an activated subpopulation (approximately 10%) could be demonstrated in limited turnover experiments. We therefore suggest that MSGT1 displays a preexisting dynamic equilibrium between high- and low-activity forms.

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.

A single amino acid substitution in isozyme GST mu in Triclabendazole resistant Fasciola hepatica (Sligo strain) can substantially influence the manifestation of anthelmintic resistance

The helminth parasite Fasciola hepatica causes fascioliasis in human and domestic ruminants. Economic losses due to this infection are estimated in U$S 2000-3000 million yearly. The most common method of control is the use of anthelmintic drugs. However, there is an increased concern about the growing appearance of F. hepatica resistance to Triclabendazole (TCBZ), an anthelmintic with activity over adult and young flukes. F. hepatica has eight Glutathione S-Transferase (GST) isozymes, which are enzymes involved in the detoxification of a wide range of substrates through chemical conjugation with glutathione. In the present work we identified and characterized the GST mu gene isolated from the TCBZ-susceptible and TCBZ-resistant F. hepatica strains. Total RNA was transcribed into cDNA by reverse transcription and a 657 bp amplicon corresponding to the GST mu gene was obtained. The comparative genetic analysis of the GST mu gene of the TCBZ susceptible strain (Cullompton) and TCBZ resistant strain (Sligo) showed three nucleotide changes and one amino acid change at position 143 in the GST mu isozyme of the TCBZ-resistant strain. These results have potential relevance as they contribute better understand the mechanisms that generate resistance to anthelmintics.

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.

Cloning of a novel glutathione S-transferase 3 (GST3) gene and expressionanalysis in pearl oyster, Pinctada martensii

Microsomal glutathione S-transferase (MGST) functions in cellular defense against xenobiotics and provides protection against the action of lipid hydroperoxides produced as a consequence of oxidative stress. In this study, a full-length cDNA encoding MGST3 (referred to as PmMGST3) was identified from the pearl oyster, Pinctada martensii by a combination of expressed sequence tag (EST) analysis and rapid amplification of cDNA ends (RACE). The full-length cDNA of PmMGST3 is 971 bp and contains a 5' UTR of 39 bp, a 3' UTR of 491 bp with a canonical polyadenylation signal sequence (AATAAA), and an open reading frame (ORF) of 447 bp encoding a polypeptide of 146 residues. The deduced polypeptide contains a conserved motif (FNCx(1)QRx(2)H) characteristic of the MGST3 subfamily. The PmMGST3 transcript could be detected in all tissues tested, with highest transcript level seen in hepatopancreas. Cadmium treatment significantly increased PmMGST3 mRNA levels in gill and hepatopancreas, while bacterial challenge initially depressed mRNA levels and then increased its level in haemocytes, gill and hepatopancreas in a time-dependent manner. In an assay using cumene hydroperoxide as a substrate, we demonstrated that PmMGST3 possesses glutathione-dependent peroxidase activity. These results suggest that PmMGST3 plays an important role in cellular defense against oxidative stress caused by cadmium and bacteria.

Synthesis and characterization of a series of highly fluorogenic substrates for glutathione transferases, a general strategy

Glutathione transferases (GSTs) are used in biotechnology applications as fusion partners for facile purification and are also overexpressed in certain tumors. Consequently, there is a need for sensitive detection of the enzymes. Here we describe a general strategy for the synthesis and characterization of novel fluorogenic substrates for GSTs. The substrates were synthesized by introducing an electrophilic sulfonamide linkage to fluorescent molecules containing an amino group [e.g., 2,4-dinitrobenzenesulfonamide (DNs) derivatives of coumarin, cresyl violet, and rhodamine]. The derivatives were essentially nonfluorescent, and upon GST catalyzed cleavage of the dinitrobenzenesulfonamide, free fluorophore is released (and 1-glutathionyl-2,4-dinitrobenzene + SO(2)). All the coumarin-, cresyl violet- and rhodamine-based fluorogenic probes turned out to be good substrates for most GSTs, especially for GSTA(1-1), in terms of strong fluorescence increases (71-1200-fold), high k(cat)/K(m) values (10(4)-10(7) M(-1) s(-1)) and significant rate enhancements (10(6)-10(9)-fold). The substrates were successfully applied to quantitate very low levels of GST activity in cell extracts and DNs-cresyl violet was also successfully applied to the imaging of microsomal MGST(1) activity in living cells. The cresyl violet stained cells retained their fluorescence after fixation, which is a very useful property. In summary, we describe a general and versatile strategy to generate fluorogenic GST substrates, some of them providing the most sensitive assays so far described for GSTs.

Mechanisms of induction of cytosolic and microsomal glutathione transferase (GST) genes by xenobiotics and pro-inflammatory agents

Glutathione transferase (GST) isoezymes are encoded by three separate families of genes (designated cytosolic, microsomal and mitochondrial transferases), with distinct evolutionary origins, that provide mammalian species with protection against electrophiles and oxidative stressors in the environment. Members of the cytosolic class Alpha, Mu, Pi and Theta GST, and also certain microsomal transferases (MGST2 and MGST3), are up-regulated by a diverse spectrum of foreign compounds typified by phenobarbital, 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene, pregnenolone-16α-carbonitrile, 3-methylcholanthrene, 2,3,7,8-tetrachloro-dibenzo-p-dioxin, β-naphthoflavone, butylated hydroxyanisole, ethoxyquin, oltipraz, fumaric acid, sulforaphane, coumarin, 1-[2-cyano-3,12-dioxooleana-1,9(11)-dien-28-oyl]imidazole, 12-O-tetradecanoylphorbol-13-acetate, dexamethasone and thiazolidinediones. Collectively, these compounds induce gene expression through the constitutive androstane receptor (CAR), the pregnane X receptor (PXR), the aryl hydrocarbon receptor (AhR), NF-E2-related factor 2 (Nrf2), peroxisome proliferator-activated receptor-γ (PPARγ) and CAATT/enhancer binding protein (C/EBP) β. The microsomal T family includes 5-lipoxygenase activating protein (FLAP), leukotriene C(4) synthase (LTC4S) and prostaglandin E(2) synthase (PGES-1), and these are up-regulated by tumour necrosis factor-α, lipopolysaccharide and transforming growth factor-β. Induction of genes encoding FLAP, LTC4S and PGES-1 is mediated by the transcription factors C/EBPα, C/EBPδ, C/EBPϵ, nuclear factor-κB and early growth response-1. In this article we have reviewed the literature describing the mechanisms by which cytosolic and microsomal GST are up-regulated by xenobiotics, drugs, cytokines and endotoxin. We discuss cross-talk between the different induction mechanisms, and have employed bioinformatics to identify cis-elements in the upstream regions of GST genes to which the various transcription factors mentioned above may be recruited.

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.

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.

Expression profiles of seven glutathione S-transferase (GST) genes in cadmium-exposed river pufferfish (Takifugu obscurus)

Glutathione S-transferase (GST; EC 2.5.1.18) plays a critical role in detoxification pathways. In this study, we report cloning and expression of seven genes of the GST family of the pufferfish Takifugu obscurus together with mRNA tissue distribution pattern and time-course of expression in response to exposure to cadmium. At basal levels of tissue expression, GST-Mu is highly expressed in liver compared with other tissues. When fish were exposed to cadmium (5 mg/L for 96 h), expression of GST-MAPEG, GST-Mu, GST-Omega, and GST-Zeta was greatly increased, whereas GST-Alpha and GST-Kappa genes showed no significant response. These findings suggest that gene expression of a number of GST isoforms in T. obscurus is modulated in response to exposure to cadmium. We propose GST-Mu, GST-Theta, and GST-Zeta as candidate biomarkers for heavy metal exposure in this fish.

Microsomal glutathione transferase 1 exhibits one-third-of-the-sites-reactivity towards glutathione

The trimeric membrane protein microsomal glutathione transferase 1 (MGST1) possesses glutathione transferase and peroxidase activity. Previous data indicated one active site/trimer whereas structural data suggests three GSH-binding sites. Here we have determined ligand interactions of MGST1 by several techniques. Nanoelectrospray mass spectrometry of native MGST1 revealed binding of three GSH molecules/trimer and equilibrium dialysis showed three product molecules/trimer (K(d)=320+/-50 microM). All three product molecules could be competed out with GSH. Reinvestigation of GSH-binding showed one high affinity site per trimer, consistent with earlier data. Using single turnover stopped flow kinetic measurements, K(d) could be determined for a low affinity GSH-binding site (2.5+/-0.5 mM). Thus we can reconcile previous observations and show here that MGST1 contains three active sites with different affinities for GSH and that only the high affinity site is catalytically competent.

Molecular cloning, phylogenetic analysis and expression of a MAPEG superfamily gene from the pufferfish Takifugu obscurus

The microsomal glutathione S-transferases (MGSTs) of membrane-associated proteins in eicosanoid and glutathione metabolism (MAPEG) superfamily play an important role in xenobiotics detoxification. Compared to mammals, there is limited information on MAPEGS from fish. We cloned a full length of cDNA sequence of a MGST gene from the river pufferfish (Takifugu obscurus), studied its phylogenetic relationship, and measured its expression in different tissues and in liver of fish exposed to cadmium. Phylogenetic analysis revealed that the identified gene encoded for MGST3. Liver showed the highest expression of MGST3 transcripts. When MSGT expression was compared with the expression of other GSTs (GST-Alpha, GST-Mu and GST-Theta), a similar pattern of highest expression was observed in the liver. Upon Cd exposure (5 ppm) for 96 h, the highest expression of MGST was observed at 24 h. GST-Mu also showed highest expression at 24 h. These findings indicate that MGSTs may be playing a role in detoxification of xenobiotics or free radicals generated by Cd-induced oxidative stress in fish.

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.

Marine glutathione S-transferases

The aquatic environment is generally affected by the presence of environmental xenobiotic compounds. One of the major xenobiotic detoxifying enzymes is glutathione S-transferase (GST), which belongs to a family of multifunctional enzymes involved in catalyzing nucleophilic attack of the sulfur atom of glutathione (gamma-glutamyl-cysteinylglycine) to an electrophilic group on metabolic products or xenobiotic compounds. Because of the unique nature of the aquatic environment and the possible pollution therein, the biochemical evolution in terms of the nature of GSTs could by uniquely expressed. The full complement of GSTs has not been studied in marine organisms, as very few aquatic GSTs have been fully characterized. The focus of this article is to present an overview of the GST superfamily and their critical role in the survival of organisms in the marine environment, emphasizing the critical roles of GSTs in the detoxification of marine organisms and the unique characteristics of their GSTs compared to those from non-marine organisms.

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.

Structural Basis for Detoxification and Oxidative Stress Protection in Membranes

Synthesis of mediators of fever, pain and inflammation as well as protection against reactive molecules and oxidative stress is a hallmark of the MAPEG superfamily (membrane associated proteins in eicosanoid and glutathione metabolism). The structure of a MAPEG member, rat microsomal glutathione transferase 1, at 3.2 A resolution, solved here in complex with glutathione by electron crystallography, defines the active site location and a cytosolic domain involved in enzyme activation. The glutathione binding site is found to be different from that of the canonical soluble glutathione transferases. The architecture of the homotrimer supports a catalytic mechanism involving subunit interactions and reveals both cytosolic and membraneous substrate entry sites, providing a rationale for the membrane location of the enzyme.

Differential expression of glutathione-S-transferase isoenzymes in various types of anemia in Taiwan

BACKGROUND

Published reports concerning the expression of GST in various anemias including aplastic, hemolytic, iron deficiency and thalassemia anemia has been insufficient. We improved the conventional GST assay by incorporating a chloroform treatment to remove the interference of hemoglobin and evaluated the altered expression of GSTs in various anemias in Taiwan.

METHODS

We incorporated a chloroform treatment to eliminate the interference of hemoglobin. Erythrocyte total GST and isoenzymes activities from 35 control subjects and 125 subjects of various anemias, including aplastic, hemolytic, iron deficiency and thalassemia anemias were measured spectrophotometrically.

RESULTS

Chloroform treatment did not significantly affect GST activities in erythrocytes of control subjects while the activities of erythrocyte total GST and alpha-GST were significantly increased in all anemic patients (P<0.001). The expression of mu-GST was significantly decreased, although at a less extent, in cases of aplastic, iron deficiency and thalassemia anemia (P<0.05), but pi-GST was not physiologically different in various types of anemia.

CONCLUSION

The determination of changes in erythrocyte GST activity is a promising indicator of oxidative stress conditions that occur in various types of anemia. Measurement of GST activity might be useful for the evaluation of prophylactic treatment in trials of antioxidant strategies.

Glutathione transferases in the genomics era: new insights and perspectives

In the last decade the tumultuous development of "omics" greatly improved our ability to understand protein structure, function and evolution, and to define their roles and networks in complex biological processes. This fast accumulating knowledge holds great potential for biotechnological applications, from the development of biomolecules with novel properties of industrial and medical importance, to the creation of transgenic organisms with new, favorable characteristics. This review focuses on glutathione transferases (GSTs), an ancient protein superfamily with multiple roles in all eukaryotic organisms, and attempts to give an overview of the new insights and perspectives provided by omics into the biology of these proteins. Among the aspects considered are the redefinition of GST subfamilies, their evolution in connection with structurally related families, present and future biotechnological outcomes.

Reactive nitrogen species derived activation of rat liver microsomal glutathione S-transferase

The effect of reactive nitrogen species on rat liver microsomal glutathione S-transferase (MGST1) was investigated using microsomes and purified MGST1. When microsomes or the purified enzyme were incubated with peroxynitrite (ONOO(-)), the GST activity was increased to 2.5-6.5 fold in concentration-dependent manner and a small amount of the MGST1 dimer was detected. MGST1 activity was increased by ONOO(-) in the presence of high amounts of reducing agents including glutathione (GSH) and the activities increased by ONOO(-) or ONOO(-) plus GSH treatment were decreased by 30-40% by further incubation with dithiothreitol (DTT, reducing disulfide) or by sodium arsenite (reducing sulfenic acid). Furthermore, GSH was detected by HPLC from the MGST1 which was incubated with ONOO(-) plus GSH or S-nitrosoglutathione followed by DTT treatment. In addition, the MGST1 activity increased by nitric oxide (NO) donors such as S-nitrosoglutathione, S-nitrosocysteine or the non-thiol NO donor 1-hydroxy-2-oxo-3 (3-aminopropyl)-3-isopropyl was restored by the DTT treatment. Since DTT can reduce S-nitrosothiol and disulfide bond to thiol, S-nitrosylation and a mixed disulfide bond formation of MGST1 were suggested. Thus, it was demonstrated that MGST1 is activated by reactive nitrogen species through a forming dimeric protein, mixed disulfide bond, nitrosylation and sulfenic acid.

Insights into the membrane proteome of rat liver peroxisomes: microsomal glutathione-S-transferase is shared by both subcellular compartments

Peroxisomes are ubiquitous "multipurpose" organelles of eukaryotic cells. Their matrix enzymes catalyze mainly catabolic and anabolic reactions of lipid metabolism, thus contributing to the regulation of lipid homeostasis. Since most metabolites must be actively transported across the peroxisomal membrane and since individual proteins and protein complexes play functional roles in such transport processes, we analyzed the peroxisomal membrane proteome. Benzyldimethyl-n-hexadecylammoniumchloride (16-BAC)/SDS-2-D-PAGE and mass spectrometry were used to characterize the proteomes of highly purified "light" and "heavy" peroxisomes of rat liver obtained by density gradient centrifugation. In both populations, the major integral membrane proteins could be detected in high concentrations, verifying 16-BAC/SDS-2-D-PAGE as a suitable tool for the preparation of membrane proteomes destined for mass spectrometric analysis. Both reliable and reproducible detection of a distinct set of microsomal (ER) membrane proteins, including microsomal glutathione-S-transferase (mGST), in light and heavy peroxisomal fractions was also possible. Compared with the abundance of most microsomal membrane proteins, we found mGST to be specifically enriched in peroxisomal membrane fractions. Furthermore, C terminus epitope-tagged mGST versions were localized at least in part to peroxisomes in different mammalian cell lines. Taken together, these data suggest that the peroxisomal GST is not a mere ER-contaminant, but a bona fide protein comprising the membrane proteome of both intracellular compartments. In addition, we could detect several mitochondrial proteins in light peroxisome fractions. This finding may likely indicate a physical association of light peroxisomes with mitochondria, since the organelles could be partly separated by mechanical stress. Whether this association is of functional importance awaits further investigation.

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.

Bioinformatic and enzymatic characterization of the MAPEG superfamily

The membrane associated proteins in eicosanoid and glutathione metabolism (MAPEG) superfamily includes structurally related membrane proteins with diverse functions of widespread origin. A total of 136 proteins belonging to the MAPEG superfamily were found in database and genome screenings. The members were found in prokaryotes and eukaryotes, but not in any archaeal organism. Multiple sequence alignments and calculations of evolutionary trees revealed a clear subdivision of the eukaryotic MAPEG members, corresponding to the six families of microsomal glutathione transferases (MGST) 1, 2 and 3, leukotriene C4 synthase (LTC4), 5-lipoxygenase activating protein (FLAP), and prostaglandin E synthase. Prokaryotes contain at least two distinct potential ancestral subfamilies, of which one is unique, whereas the other most closely resembles enzymes that belong to the MGST2/FLAP/LTC4 synthase families. The insect members are most similar to MGST1/prostaglandin E synthase. With the new data available, we observe that fish enzymes are present in all six families, showing an early origin for MAPEG family differentiation. Thus, the evolutionary origins and relationships of the MAPEG superfamily can be defined, including distinct sequence patterns characteristic for each of the subfamilies. We have further investigated and functionally characterized representative gene products from Escherichia coli, Synechocystis sp., Arabidopsis thaliana and Drosophila melanogaster, and the fish liver enzyme, purified from pike (Esox lucius). Protein overexpression and enzyme activity analysis demonstrated that all proteins catalyzed the conjugation of 1-chloro-2,4-dinitrobenzene with reduced glutathione. The E. coli protein displayed glutathione transferase activity of 0.11 micromol.min(-1).mg(-1) in the membrane fraction from bacteria overexpressing the protein. Partial purification of the Synechocystis sp. protein yielded an enzyme of the expected molecular mass and an N-terminal amino acid sequence that was at least 50% pure, with a specific activity towards 1-chloro-2,4-dinitrobenzene of 11 micromol.min(-1).mg(-1). Yeast microsomes expressing the Arabidopsis enzyme showed an activity of 0.02 micromol.min(-1).mg(-1), whereas the Drosophila enzyme expressed in E. coli was highly active at 3.6 micromol.min(-1).mg(-1). The purified pike enzyme is the most active MGST described so far with a specific activity of 285 micromol.min(-1).mg(-1). Drosophila and pike enzymes also displayed glutathione peroxidase activity towards cumene hydroperoxide (0.4 and 2.2 micromol.min(-1).mg(-1), respectively). Glutathione transferase activity can thus be regarded as a common denominator for a majority of MAPEG members throughout the kingdoms of life whereas glutathione peroxidase activity occurs in representatives from the MGST1, 2 and 3 and PGES subfamilies.

Two‐Dimensional Crystallization and Electron Crystallography of MAPEG Proteins

Members of the membrane-associated proteins in the eicosanoid and glutathione metabolism (MAPEG) superfamily have been subjected to two-dimensional crystallization experiments. A common denominator for successful attempts has been the use of a low lipid/protein ratio in the range of 1-9 (mol/mol). Electron crystallography demonstrated either hexagonal or orthorhombic packing of trimeric protein units. Three-dimensional structure analysis of the MAPEG member microsomal glutathione transferase 1 has shown that the monomer for this protein contains a left-handed bundle of four transmembrane helices. It is likely that this is a common structural motif for MAPEG proteins, because projection maps of all structurally characterized members are very similar.

Human microsomal prostaglandin E synthase 1: a member of the MAPEG protein superfamily

In this chapter, we briefly review the MAPEG superfamily (membrane associated proteins in eicosanoid and glutathione metabolism), a family of proteins in which all human members except one possess glutathione conjugating capacity. Recent findings regarding the biological functions of MAPEG proteins are highlighted. More extensively, the characterization of human microsomal prostaglandin E synthase 1 is presented, including results and applied methodology.

Microsomal Glutathione Transferase 1

Microsomal glutathione transferase 1 (MGST1) is an abundant membrane-bound glutathione transferase and peroxidase constituting 3% of the endoplasmic reticulum protein in rat liver (and 5% of the outer mitochondrial membrane). The enzyme is most well studied in mammals and belongs to a large and widely distributed superfamily. Cellular and organelle protection versus oxidative stress has been demonstrated. The enzyme displays activity to a multitude of reactive substrates ranging from products of lipid peroxidation to cytostatic drugs. The methods developed for the study of MGST1 by necessity differs from that of cytosolic glutathione transferases, because detergents or lipids are included. Here, purification, assay, and preparation procedures that maintain the enzyme in its native functional state during isolation and characterization are described. Microsomal glutathione transferase 1 is activated by sulfhydryl reagents (and proteolysis), and procedures for activation and study of the activated enzyme are described. In new developments, the enzyme is studied by pre-steady state methods, as well as mass spectrometry involving direct observation of the native enzyme.

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.

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.

NADPH dependent activation of microsomal glutathione transferase 1

Microsomal glutathione transferase 1 (MGST1) can become activated up to 30-fold by several mechanisms in vitro (e.g. covalent modification by reactive electrophiles such as N-ethylmaleimide (NEM)). Activation has also been observed in vivo during oxidative stress. It has been noted that an NADPH generating system (g.s.) can activate MGST1 (up to 2-fold) in microsomal incubations, but the mechanism was unclear. We show here that NADPH g.s treatment impaired N-ethylmaleimide activation, indicating a shared target (identified as cysteine-49 in the latter case). Furthermore, NADPH activation was prevented by sulfhydryl compounds (glutathione and dithiothreitol). A well established candidate for activation would be oxidative stress, however we could exclude that oxidation mediated by cytochrome P450 2E1 (or flavine monooxygenase) was responsible for activation under a defined set of experimental conditions since superoxide or hydrogen peroxide alone did not activate the enzyme (in microsomes prepared by our routine procedure). Actually, the ability of MGST1 to become activated by hydrogen peroxide is critically dependent on the microsome preparation method (which influences hydrogen peroxide decomposition rate as shown here), explaining variable results in the literature. NADPH g.s. dependent activation of MGST1 could instead be explained, at least partly, by a direct effect observed also with purified enzyme (up to 1.4-fold activation). This activation was inhibited by sulfhydryl compounds and thus displays the same characteristics as that of the microsomal system. Whereas NADPH, and also ATP, activated purified MGST1, several nucleotide analogues did not, demonstrating specificity. It is thus an intriguing possibility that MGST1 function could be modulated by ligands (as well as reactive oxygen species) during oxidative stress when sulfhydryls are depleted.

Observation of an Intact Noncovalent Homotrimer of Detergent-solubilized Rat Microsomal Glutathione Transferase-1 by Electrospray Mass Spectrometry

Microsomal glutathione transferase-1 (MGST1) is a membrane-bound enzyme involved in the detoxification of xenobiotics and the protection of cells against oxidative stress. The proposed active form of the enzyme is a noncovalently associated homotrimer that binds one substrate glutathione molecule/trimer. In this study, this complex has been directly observed by electrospray mass spectrometry analysis of active rat liver MGST1 reconstituted in a minimum amount of detergent. The measured mass of the homotrimer is 53 kDa, allowing for the mass of three MGST molecules in complex with one glutathione molecule. Collision-induced dissociation of the trimer complex resulted in the formation of monomer and homodimer ion species. Two distinct species of homodimer were observed, one unliganded and one identified as a homodimer.glutathione complex. Activation of the enzyme by N-ethylmaleimide through modification of Cys(49) (Svensson, R., Rinaldi, R., Swedmark, S., and Morgenstern, R. (2000) Biochemistry 39, 15144-15149) was monitored by the observation of an appropriate increase in mass in both the denatured monomeric and native trimeric forms of MGST1. Together, the data correspond well with the proposed functional organization of MGST1. These results also represent the first example of direct electrospray mass spectrometry analysis of a detergent-solubilized multimeric membrane protein complex in its native state.

The diversity of globin-coupled sensors

The recently discovered globin-coupled sensors (GCSs) are heme-containing two-domain transducers distinct from the PAS domain superfamily. We have identified an additional 22 GCSs with varying multi-domain C-terminal transmitters through a search of the complete and incomplete microbial genome datasets. The GCS superfamily is composed of two major subfamilies: the aerotactic and gene regulators. We postulate the existence of protoglobin in Archaea as the predecessor to the chimeric GCS.