Gamma-glutamylcysteine: a substrate for glutathione S-transferases

Assessment of Haloferax mediterranei Genome in Search of Copper-Molecular Machinery With Potential Applications for Bioremediation

Heavy metals are essential micronutrients at low concentrations, serving as cofactors for relevant microbial enzymes (i.e., respiratory nitrate and nitrite reductases NADH dehydrogenase-2, amine oxidase, etc.), but they become harmful cellular intoxicants at significant low concentrations compared to other chemical compounds. The increasing need to incorporate bioremediation in the removal of heavy metals and other contaminants from wastewaters has led extremophiles to the spotlight of research. The haloarchaeon Haloferax mediterranei has promising physiological characteristics regarding bioremediation. However, little is known about how haloarchaea manage to resist high concentrations of heavy metals in the environment. The aim of this work is to develop bioinformatics research as the first step for further omics-based studies to shed light on copper metabolism in haloarchaea by analyzing H. mediterranei genome (strain ATCC 33500). To reach this aim, genome and protein databases have been consulted, and copper-related genes have been identified. BLAST analysis has been carried out to find similarities between copper resistance genes described from other microorganisms and H. mediterranei genes. Plausible copper importer genes, genes coding for siderophores, and copper exporters belonging to P_1B-type ATPase group have been found apart from genes encoding copper chaperones, metal-responsive transcriptional regulators, and several proteins belonging to the cupredoxin superfamily: nitrite reductase, nitrous oxide reductases, cytochrome c oxidases, multicopper oxidases, and small blue copper proteins from the amicyanin/pseudoazurin families as halocyanins. As the presence of heavy metals causes oxidative stress, genes coding for proteins involved in antioxidant mechanisms have been also explored: thioredoxin, glutaredoxin, peroxiredoxin, catalase, and γ-glutamylcysteine as an analog of glutathione. Bioinformatic-based analysis of H. mediterranei genome has revealed a set of genes involved in copper metabolism that could be of interest for bioremediation purposes. The analysis of genes involved in antioxidative mechanisms against heavy metals makes it possible to infer the capability of H. mediterranei to synthesize inorganic polyphosphate granules against oxidative stress.

Structural Characterization of the Xi Class Glutathione Transferase From the Haloalkaliphilic Archaeon Natrialba magadii

Xi class glutathione transferases (GSTs) are a recently identified group, within this large superfamily of enzymes, specifically endowed with glutathione-dependent reductase activity on glutathionyl-hydroquinone. Enzymes belonging to this group are widely distributed in bacteria, fungi, and plants but not in higher eukaryotes. Xi class GSTs are also frequently found in archaea and here we focus on the enzyme produced by the extreme haloalkaliphilic archaeon Natrialba magadii (NmGHR). We investigated its function and stability and determined its 3D structure in the apo form by X-ray crystallography. NmGHR displays the same fold of its mesophilic counterparts, is enriched in negatively charged residues, which are evenly distributed along the surface of the protein, and is characterized by a peculiar distribution of hydrophobic residues. A distinctive feature of haloalkaliphilic archaea is their preference for γ-glutamyl-cysteine over glutathione as a reducing thiol. Indeed we found that the N. magadii genome lacks a gene coding for glutathione synthase. Analysis of NmGHR structure suggests that the thiol binding site (G-site) of the enzyme is well suited for hosting γ-glutamyl-cysteine.

Distribution of glutathione transferases in Gram-positive bacteria and Archaea

Glutathione transferases (GSTs) have been widely studied in Gram-negative bacteria and the structure and function of several representatives have been elucidated. Conversely, limited information is available about the occurrence, classification and functional features of GSTs both in Gram-positive bacteria and in Archaea. An analysis of 305 fully-sequenced Gram-positive genomes highlights the presence of 49 putative GST genes in the genera of both Firmicutes and Actinobacteria phyla. We also performed an analysis on 81 complete genomes of the Archaea domain. Eleven hits were found in the Halobacteriaceae family of the Euryarchaeota phylum and only one in the Crenarchaeota phylum. A comparison of the identified sequences with well-characterized GSTs belonging to both Gram-negative and eukaryotic GSTs sheds light on their putative function and the evolutionary relationships within the large GST superfamily. This analysis suggests that the identified sequences mainly cluster in the new Xi class, while Beta class GSTs, widely distributed in Gram-negative bacteria, are under-represented in Gram-positive bacteria and absent in Archaea.

Degradation of glutathione S-conjugates in Physcomitrella patens is initiated by cleavage of glycine

Glutathione-dependent detoxification is a key pathway that allows plants to efficiently remove toxic compounds like heavy metals or electrophilic xenobiotics. Under persistent exposure to toxins plants need to respond to continuous demand with efficient synthesis of glutathione (GSH) and ideally fast and efficient removal of potentially toxic glutathione S-conjugates. With the aim of studying the respective degradation pathway in Physcomitrella patens we initially characterized fluorescence labeling of protonema cultures with GSH-specific xenobiotic monochlorobimane (MCB). Incubation of protonema with 200 μM MCB for 24 h resulted in a steady increase of total bimane label, which was not confined to glutathione S-bimane (GS-B), but predominantly present in γ-glutamylcysteine S-bimane (γ-EC-B) and cysteine S-bimane (Cys-B). Pulse-chase experiments identified γ-EC-B and Cys-B as degradation products of GS-B, suggesting initial cleavage of the C-terminal glycine, followed by cleavage of the γ-glutamyl bond. The amount of GS-B formed, increased linearly at 90 nmol GSH g fw⁻¹ h⁻¹ for 24 h and after ∼1.5 h already surpassed the amount of GSH present in control protonema. This demand-driven biosynthesis of GSH depends on sufficient supply of sulfate in the incubation medium.

Reversed-phase chromatographic method for specific determination of glutathione in cultured malignant cells

A chromatographic method for the specific determination of glutathione in malignant cell lines is described. The method is based on the ability of glutathione-S-transferase to specifically and quantitatively conjugate glutathione to 1-chloro-2,4-dinitrobenzene and chromatographic quantitation of the resultant conjugate, dinitrophenyl-S-glutathione, by reversed-phase liquid chromatography. The assay can be performed on 20,000 g supernatants of cell homogenates without acid extraction. 2-Mercaptoethanol, a sulfhydryl compound often used as a thiol-protective agent to preserve enzymatic activities of a number of enzymes, did not interfere with glutathione determination by this method. The dinitrophenyl-S-glutathione isolated from either standard glutathione samples or from cell homogenates was shown to be identical to authentic dinitrophenyl-S-glutathione using mass spectrometry. Recovery of glutathione in standard samples by the current method was identical to that determined using 5,5'-dithiobis(2-nitrobenzoic acid). Exogenous glutathione added to supernatants of cell homogenate in the presence or absence of 2-mercaptoethanol was also completely recovered.

Glutathione metabolism and its role in hepatotoxicity

Glutathione (GSH) fulfills several essential functions: Detoxification of free radicals and toxic oxygen radicals, thiol-disulfide exchange and storage and transfer of cysteine. GSH is present in all mammalian cells, but may be especially important for organs with intense exposure to exogenous toxins such as the liver, kidney, lung and intestine. Within the cell mitochondrial GSH is the main defense against physiological oxidant stress generated by cellular respiration and may be a critical target for toxic oxygen and electrophilic metabolites. Glutathione homeostasis is a highly complex process, which is predominantly regulated by the liver, lung and kidney.

The three-dimensional structure of class pi glutathione S-transferase in complex with glutathione sulfonate at 2.3 A resolution

The three-dimensional structure of class pi glutathione S-transferase from pig lung, a homodimeric enzyme, has been solved by multiple isomorphous replacement at 3 A resolution and preliminarily refined at 2.3 A resolution (R = 0.24). Each subunit (207 residues) is folded into two domains of different structure. Domain I (residues 1-74) consists of a central four-stranded beta-sheet flanked on one side by two alpha-helices and on the other side, facing the solvent, by a bent, irregular helix structure. The topological pattern resembles the bacteriophage T4 thioredoxin fold, in spite of their dissimilar sequences. Domain II (residues 81-207) contains five alpha-helices. The dimeric molecule is globular with dimensions of about 55 A x 52 A x 45 A. Between the subunits and along the local diad, is a large cavity which could possibly be involved in the transport of nonsubstrate ligands. The binding site of the competitive inhibitor, glutathione sulfonate, is located on domain I, and is part of a cleft formed between intrasubunit domains. Glutathione sulfonate is bound in an extended conformation through multiple interactions. Only three contact residues, namely Tyr7, Gln62 and Asp96 are conserved within the family of cytosolic glutathione S-transferases. The exact location of the binding site(s) of the electrophilic substrate is not clear. Catalytic models are discussed on the basis of the molecular structure.

The glutathione-binding site in glutathione S-transferases. Investigation of the cysteinyl, glycyl and gamma-glutamyl domains

The GSH-binding site of glutathione S-transferase (GST) isoenzymes was studied by investigating their substrate-specificity for three series of GSH analogues; further, a model of the interactions of GSH with the G-site is proposed. Twelve glycyl-modified GSH analogues, four ester derivatives of GSH and three cysteinyl-modified GSH analogues were synthesized and tested with purified forms of rat liver GST (1-1, 2-2, 3-3 and 4-4). The glycyl analogues exhibited spontaneous chemical reaction rates with 1-chloro-2,4-dinitrobenzene comparable with the GSH rate. In contrast, the enzymic rates (Vmax.) differed greatly, from less than 1 up to 140 mumol/min per mg; apparently, a reaction mechanism is followed that is very sensitive to substitutions at the glycyl domain. No correlation exists between the chemical rates and Vmax. values for the analogues. Analogues of GSH in which L-cysteine was replaced by D-cysteine, L-homocysteine or L-penicillamine showed little or no capacity to replace GSH as co-substrate for the GSTs. GSH monomethyl and monoethyl esters showed Vmax. values greater than the Vmax. measured with GSH: the Vmax. for the monoethyl ester of GSH and GST 3-3 was 5-fold that for GSH. The data obtained in this and previous studies [Adang, Brussee, Meyer, Coles, Ketterer, van der Gen & Mulder (1988) Biochem. J. 255, 721-724; Adang, Meyer, Brussee, van der Gen, Ketterer & Mulder (1989) Biochem. J. 264, 759-764] allow a model of the interactions of GSH in the G-site in GSTs to be postulated. The gamma-glutamyl site is the main binding determinant: the alpha-carboxylate group is obligatory, whereas shifting of the amino group and shortening of the peptide backbone only decreased kcat./Km. Furthermore, the GSTs appear to be very critical with respect to a correct orientation of the thiol group of the GSH analogue. The glycyl site is the least restrictive domain in the G-site of GSTs: amino acid analogues all showed Km values between 0.2 and 0.6 mM (that for GSH is 0.2-0.3 mM), but large differences in Vmax. exist. The glycyl carboxylate group is not essential for substrate recognition, since decarboxy analogues and ester derivatives showed high activities. The possible mechanisms for an increased Vmax. in some analogues are briefly discussed.

Influence of ethanol on glutathione-S-transferase activity and glutathione content in the isolated perfused rabbit lung

The induction of pulmonary glutathione-S-transferase (GST) by ethanol was investigated using the isolated perfused rabbit lung (IPRL) preparation with particular attention paid to the duration and route of ethanol administration. For perfusion with buffer containing 0.2% ethanol or acute ethanol treatment (4 g/kg by gastric intubation) 4 h before the IPRL preparation, there were no differences in the rate of glutathione (GSH) conjugation with 1-chloro-2,4-dinitrobenzene (CDNB) at low substrate concentrations (100-400 microM) but a decrease was observed in the rate at high substrate concentrations (500-1000 microM). Lungs from rabbits treated acutely showed the lowest maximal rate of GSH conjugation in the IPRL. Prolonged treatment with ethanol (10% in drinking water for 3 weeks) increased GSH conjugation (CDNB concentration of 300-750 microM). None of these ethanol treatments altered GSH conjugation with 1,2-epoxy(p-nitrophenoxy)propane (ENP). Upon termination of perfusion, there were no differences in pulmonary GSH concentration between control and ethanol-treated groups. Therefore, the effect of altered GSH level as a co-substrate on GST activity in lung might be excluded as an explanation for the effects of ethanol. These data suggest that ethanol has differential effects on GST activity depending upon the substrate and duration of ethanol administration.

Glutathione conjugation in the isolated perfused rabbit lung: the effects of carbon monoxide, buthionine sulfoximine and cibacron blue

The effect of carbon monoxide (CO) on pulmonary glutathione-S-transferase (GST) activity was investigated using the isolated perfused rabbit lung (IPRL) preparation. CO at a concentration of 7.5% did not alter the conjugation of glutathione (GSH) with 1,2-epoxy-(p-nitrophenoxy)propane (ENP) in the IPRL. The rate of GSH conjugate formation with ENP was not altered when the lung was exposed to a selective inhibitor of gamma-glutamylcysteine synthetase, buthionine sulfoximine (0.1 mM), but was significantly diminished (P less than 0.01) by co-administration of GST inhibitor cibacron blue (1 microM) with ENP. These results indicate that the rabbit lung actively conjugates GSH with ENP and that this reaction may be decreased by specific inhibitors of GST. However, the rate of GSH conjugation is not affected by CO ventilation in the IPRL.

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

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

Relaxed thiol substrate specificity of glutathione transferase effected by a non-substrate glutathione derivative

Rat glutathione transferase 4-4 catalyzed the conjugation of 2-mercaptoethanol with 1-chloro-2,4-dinitrobenzene in the presence of S-methyl-glutathione. The reaction was linearly dependent on enzyme concentration and saturation was seen with respect to both 2-mercaptoethanol and S-methyl-glutathione concentration. High concentrations of S-methyl-glutathione were inhibitory. The results suggest that the natural substrate glutathione has two distinct functions in the normal catalytic reaction, (i) induction of a catalytically competent conformation of the enzyme and (ii) provision of the substrate sulfhydryl group in the reaction catalyzed.

Glutathione transferases--structure and catalytic activity

The glutathione transferases are recognized as important catalysts in the biotransformation of xenobiotics, including drugs as well as environmental pollutants. Multiple forms exist, and numerous transferases from mammalian tissues, insects, and plants have been isolated and characterized. Enzymatic properties, reactions with antibodies, and structural characteristics have been used for classification of the glutathione transferases. The cytosolic mammalian enzymes could be grouped into three distinct classes--Alpha, Mu, and Pi; the microsomal glutathione transferase differs greatly from all the cytosolic enzymes. Members of each enzyme class have been identified in human, rat, and mouse tissues. Comparison of known primary structures of representatives of each class suggests a divergent evolution of the enzyme proteins from a common precursor. Products of oxidative metabolism such as organic hydroperoxides, epoxides, quinones, and activated alkenes are possible "natural" substrates for the glutathione transferases. Particularly noteworthy are 4-hydroxyalkenals, which are among the best substrates found. Homologous series of substrates give information about the properties of the corresponding binding site. The catalytic mechanism and the active-site topology have been probed also by use of chiral substrates. Steady-state kinetics have provided evidence for a "sequential" mechanism.

Structure-activity relationships of 4-hydroxyalkenals in the conjugation catalysed by mammalian glutathione transferases

The substrate specificities of 15 cytosolic glutathione transferases from rat, mouse and man have been explored by use of a homologous series of 4-hydroxyalkenals, extending from 4-hydroxypentenal to 4-hydroxypentadecenal. Rat glutathione transferase 8-8 is exceptionally active with the whole range of 4-hydroxyalkenals, from C5 to C15. Rat transferase 1-1, although more than 10-fold less efficient than transferase 8-8, is the second most active transferase with the longest chain length substrates. Other enzyme forms showing high activities with these substrates are rat transferase 4-4 and human transferase mu. The specificity constants, kcat./Km, for the various enzymes have been determined with the 4-hydroxyalkenals. From these constants the incremental Gibbs free energy of binding to the enzyme has been calculated for the homologous substrates. The enzymes responded differently to changes in the length of the hydrocarbon side chain and could be divided into three groups. All glutathione transferases displayed increased binding energy in response to increased hydrophobicity of the substrate. For some of the enzymes, steric limitations of the active site appear to counteract the increase in binding strength afforded by increased chain length of the substrate. Comparison of the activities with 4-hydroxyalkenals and other activated alkenes provides information about the active-site properties of certain glutathione transferases. The results show that the ensemble of glutathione transferases in a given species may serve an important physiological role in the conjugation of the whole range of 4-hydroxyalkenals. In view of its high catalytic efficiency with all the homologues, rat glutathione transferase 8-8 appears to have evolved specifically to serve in the detoxication of these reactive compounds of oxidative metabolism.

Enzyme-catalyzed detoxication reactions: mechanisms and stereochemistry

Enzyme catalyzed detoxication reactions are one of the primary defenses organisms have against chemical insult. This article reviews current chemical approaches to understanding the cooperative role of enzymes in the metabolism of foreign compounds. Emphasis is placed on chemical and stereochemical studies which help elucidate the mechanism of action and active-site topologies of the detoxication enzymes. The stereoselectivity of the cytochromes P-450 and flavin containing monooxygenases as well as the role of hemoglobin and lipid peroxidation in the primary metabolism of xenobiotics is discussed. Current knowledge of the mechanism and stereoselectivity of epoxide hydrolase is also presented. Three enzymes involved in secondary metabolism of xenobiotics, UDP-glucuronosyltransferase, sulfotransferase and glutathione S-transferase are discussed with particular emphasis on active site topology and cooperative participation with the enzymes of primary metabolism.