Mechanistic and structural insights into the in vitro inhibitory action of hypericin on glutathione reductase purified from baker's yeast

This work aims at studying the interaction between glutathione reductase (GR) and hypericin. The type of inhibition was determined by measuring changes in GR activity at increasing concentrations of hypericin as well as at varying concentrations of glutathione disulfide (GSSG) and nicotinamide adenine dinucleotide phosphate (NADPH), and the binding pose of hypericin was predicted by molecular docking. Accordingly, hypericin emerges as an effective inhibitor of GR. When the variable substrate is GSSG, the type of inhibition is competitive. When the variable substrate is NADPH, however, the type of inhibition appears to be linear mixed-type competitive. Our computational analyses suggest that hypericin binds in the large intermonomer cavity of GR, and that it may interfere with the normal positioning/functioning of the redox-active disulfide center at the enzyme's active site. Overall, besides its contributory role in promoting oxidative stress via the formation of reactive oxygen species in photodynamic therapy, hypericin can also weaken cancer cells through inhibiting GR.

A new thioredoxin reductase with additional glutathione reductase activity in Haemonchus contortus

We report, herein, the purification to homogeneity and the biochemical and kinetic characterization of HcTrxR3, a new isoform of thioredoxin reductase (TrxR) from Haemonchus contortus. HcTrxR3 was found to have a relative molecular weight of 134,000, while the corresponding value per subunit obtained under denaturing conditions, was of 67,000. By peptide mass spectrophotometric analysis, HcTrxR3 was determined to have 99% identity with the H. contortus HcTrxR1 although, and most importantly, they are different in their amino acid sequence in two amino acid positions: 48 (isoleucine instead of leucine) and 460 (leucine instead of proline). The enzyme catalyzes NADPH-dependent reduction of DTNB and, unexpectedly, it follows the pattern of glutathione reductases (GR) performing the reduction of oxidized glutathione (GSSG) to reduced glutathione using NADPH as the reducing cofactor. Hence, it is important to highlight this enzyme's new and unexpected condition that makes so special and one our main finding. Enzyme K_cat values for DTNB, GSSG and NADPH were 12, 3 and 8 s^-1, respectively. HcTrxR3 developed, into specific TrxR substrates: ebselen and sodium selenite, with activity at 0.5 and 0.068 (U/mg), respectively; and 0.044 (U/mg) for S-nitrosoglutathione through its GR activity. The enzyme was inhibited by gold compound auranofin (AU), a selective inhibitor of thiol-dependent flavoreductases. Although HcTrxR3 has both TrxR and GR activity as thioredoxin glutathione reductase (TGR) does, it is a TrxR because it has no glutaredoxin domain and it does not develop any hysteretic behavior as does TGR. The importance of this new enzyme is potential to further clarify the detoxification and haemostasis redox mechanism in H. contortus. Likewise, this enzyme could also be a protein model to recognize more differences between TrxR and GR.

In vitro effects of compounds isolated from Sideritis brevibracteata on bovine kidney cortex glutathione reductase

Glutathione reductase (GR, E.C 1.6.4.2) is a flavoprotein that catalyzes NADPH-dependent reduction of oxidized glutathione (GSSG) to reduced glutathione (GSH). The aim of this study was to investigate in vitro effects of phenolic compounds isolated from Sideritis brevibracteata on bovine kidney GR. The Sideritis species are widely found in nature and commonly used as medicinal plants. 7-O-glycosides of 8-OH-flavones (hypolaetin, isoscutellarein and 3'-hydroxy-4'-O-methylisoscutellarein) were isolated from aerial parts of Sideritis brevibracteata. These compounds inhibited bovine kidney cortex GR in a concentration-dependent manner. Kinetic characterization of the inhibition was also performed.

Purification and characterisation of rat kidney glutathione reductase

Glutathione reductase [GR, E.C.1.8.1.7] catalyses NADPH dependent reduction of glutathione disulfide (GSSG) to reduced glutathione (GSH). Thus, it is the crucial enzyme to maintain high [GSH]/[GSSG] ratio and physiological redox status in cells. Kidney and liver tissues were considered as a rich source of GR. In this study, rat kidney GR was purified and some of its properties were investigated. The enzyme was purified 2,356 fold with a yield of 16% by using heat-denaturation and Sephadex G25 gel filtration, 2',5'-ADP Agarose 4B, PBE94 column chromatographies. The purified enzyme had a specific activity (Vm) of 250 U/mg protein and the ratio of absorbances at wavelengths of A (273)/A (463,) A (280)/A (460), A (365)/A (460), and A (379)/A (463), were 7.1, 6.8, 1.2 and 1.0, respectively. Each mol of GR subunit bound 0.97 mol of FAD. NADH was used as a coenzyme by rat kidney GR but with a lower efficiency (32.7%) than NADPH. Its subunit molecular weight was estimated as 53 kDa. An optimum pH of 6.5 and optimum temperature of 65 degrees C were found for rat kidney GR. Its activation energy (Ea) and temperature coefficient (Q(10)) were calculated as 7.02 kcal/mol and 1.42, respectively. The Km((NADPH)) and kcat/Km ((NADPH)) values were found to be 15.3 +/- 1.4 microM and 1.68 x 10(7) M(-1) s(-1) for the concentration range of 10-200 microM NADPH and when GSSG is the variable substrate, the Km((GSSG)) and the kcat/Km((GSSG)) values of 53.1 +/- 3.4 microM and 4.85 x 10(6) M(-1) s(-1) were calculated for the concentration range of 20-1,200 microM GSSG.

Expression of a glutathione reductase from Brassica rapa subsp. pekinensis enhanced cellular redox homeostasis by modulating antioxidant proteins in Escherichia coli

Glutathione reductase (GR) is an enzyme that recycles a key cellular antioxidant molecule glutathione (GSH) from its oxidized form (GSSG) thus maintaining cellular redox homeostasis. A recombinant plasmid to overexpress a GR of Brassica rapa subsp. pekinensis (BrGR) in E. coli BL21 (DE3) was constructed using an expression vector pKM260. Expression of the introduced gene was confirmed by semiquantitative RT-PCR, immunoblotting and enzyme assays. Purification of the BrGR protein was performed by IMAC method and indicated that the BrGR was a dimmer. The BrGR required NADPH as a cofactor and specific activity was approximately 458 U. The BrGR-expressing E. coli cells showed increased GR activity and tolerance to H(2)O(2), menadione, and heavy metal (CdCl(2), ZnCl(2) and AlCl(2))-mediated growth inhibition. The ectopic expression of BrGR provoked the co-regulation of a variety of antioxidant enzymes including catalase, superoxide dismutase, glutathione peroxidase, and glucose-6-phosphate dehydrogenase. Consequently, the transformed cells showed decreased hydroperoxide levels when exposed to stressful conditions. A proteomic analysis demonstrated the higher level of induction of proteins involved in glycolysis, detoxification/oxidative stress response, protein folding, transport/binding proteins, cell envelope/porins, and protein translation and modification when exposed to H(2)O(2) stress. Taken together, these results indicate that the plant GR protein is functional in a cooperative way in the E. coli system to protect cells against oxidative stress.

[Catalytic properties of glutathione reductase from rats liver at norm and toxic hepatitis]

Homogeneous preparations of liver glutathione reductase (GR, EC 1.6.4.2.), obtained from control rats and animals with toxic hepatitis, catalytic properties of this enzyme have been investigated. A number of the enzyme properties did not change under conditions of hepatitis. These included electrophoretic mobility (R(f) = 0.23 +/- 0.01), molecular mass (104 +/- 5.20 kDa), pH-optimum (7.4 +/- 0.37). There was similarity in pK values of functional groups. At the same time, decrease in affinity of enzyme to a substrate and coenzyme and occurrence substrate-linked inhibition was observed under conditions of this pathology. There were some differencies in regulation of GR activity by metabolites of tricarboxylic acid cycle.

Effects of some antibiotics on glutathione reductase activities from human erythrocytesin vitroand from rat erythrocytesin vivo

The effects of streptomycin sulfate, gentamicin sulfate, thiamphenicol, penicillin G, teicoplanin, ampicillin, cefotaxime, and cefodizime on the enzyme activity of glutathione reductase (GR) were studied using human and rat erythrocyte GR enzymes in in vitro and in vivo studies, respectively. The enzyme was purified 5,342-fold from human erythrocytes in a yield of 29% with 50.75 U/mg. The purification procedure involved the preparation of hemolysate, ammonium sulfate precipitation, 2',5'-ADP Sepharose 4B affinity chromatography and Sephadex G-200 gel filtration chromatography. Purified enzyme was used in the in vitro studies, and rat erythrocyte hemolysate was used in the in vivo studies. In the in vitro studies, I50 and K(i) values were 12.179 mM and 6.5123 +/- 4.1139 mM for cefotaxime, and 1.682 mM and 0.7446 +/- 0.2216 mM for cefodizime, respectively, showing the inhibition effects on the purified enzyme. Inhibition types were noncompetitive for cefotaxime and competitive for cefodizime. In the in vivo studies, 300 mg/kg cefotaxime and 1000 mg/kg cefodizime when administered to rats inhibited enzyme activity during the first 2h (p < 0.01). Cefotaxime led to increased enzyme activity at 4h (p < 0.05), but neither cefotaxime nor cefodizime had any significant inhibition or activation effects over 6 h (p > 0.05).

Purification and characterization of glutathione reductase from rainbow trout (Oncorhynchus mykiss) liver and inhibition effects of metal ions on enzyme activity

Glutathione reductase (E C: 1.8.1.7; GR) was purified from rainbow trout (Oncorhynchus mykiss) liver, and some characteristics of the enzyme were investigated. The purification procedure consisted of four steps: preparation of homogenate, ammonium sulfate fractionation, affinity chromatography on 2',5'-ADP Sepharose-4B and gel filtration chromatography on Sephadex G-200. The enzyme, with a specific activity of 27.45 U/mg protein, was purified 1,654-fold with a yield of 41%. Optimal pH, stable pH, optimal temperature, optimum ionic strength, molecular mass, KM and Vmax values for GSSG and NADPH were also determined for the enzyme. In addition, Ki values and inhibition types were determined for GSH and NADP+. Additionally, inhibitory effects of metal ions (Cd+2, Cu+2, Pb+2, Hg+2, Fe+3 and Al+3) on glutathione reductase were investigated. Ki constants and IC50 values for metal ions were determined by Lineweaver-Burk graphs and plotting activity % vs. [I], respectively. IC50 values of Cd+2,Cu+2, Pb+2, Hg+2, Fe+3 and Al+3 were 0.0655, 0.082, 0.122, 0.509, 0.797 and 0.804 mM, and the Ki constants for Cd+2 and Cu+2 were 0.104+/-0.001, 0.117+/-0.001, respectively.

Purification and kinetic properties of glutathione reductase from bovine liver

Glutathione reductase (GR, NADPH: oxidized glutathione oxidoreductase, EC 1.6.4.2) catalyzes the reduction of oxidized glutathione (GSSG) to reduced glutathione (GSH) using NADPH as reducing cofactor. The aim of the present work was to purify and characterize GR from bovine liver. GR was purified using 2', 5' ADP-Sepharose 4B and DEAE-Sepharose Fast Flow columns. The enzyme has been purified 5456-fold and with a yield of 38.4%. The molecular and catalytic properties of bovine liver GR have been studied. Optimum temperature and pH was found to be 50 degrees C and 7, respectively. The activation energy of the reaction catalyzed by the enzyme was 9.065 kcal/mole. The molecular weight of the enzyme was found to be 55 kDa by SDS-PAGE. Kinetic characterization of bovine liver GR was also investigated, Km(NADPH) 0.063 +/- 0.008 mM and Km(GSSG) 0.154 +/- 0.015 mM were determined. It is accepted that parallel lines observed in these double reciprocal plots obeys Ping Pong mechanism and we have showed this in our steady state study. According to our results of statistical analysis, the Ping Pong mechanism is a suitable model since the loss function is less than the other mechanisms. However, competitive inhibition by a product could be accepted in sequential mechanisms but not in a Ping Pong mechanism. In this study, kinetic data are consistent with a branching reaction mechanism previously proposed for GR from other sources by other studies.

Purification and properties of glutathione reductase from liver of the anoxia-tolerant turtle, Trachemys scripta elegans

Glutathione reductase (GR) is a homodimeric flavoprotein that catalyzes the reduction of oxidized glutathione (GSSG) using NADPH as a cofactor. The enzyme is a major component of cellular defense mechanisms against oxidative injury. In this study, GR was purified from the liver of the anoxia-tolerant turtle, Trachemys scripta elegans. The overall fold purifications were 13.3- and 12.1-fold with final specific activities of 5.5 and 1.44 U/mg of protein for control and anoxic turtle GR, respectively. SDS-PAGE of purified turtle liver GR showed a single protein band at approximately 55 kDa. Reverse phase HPLC of turtle GR revealed a single peak that had the same retention time as yeast GR. No new isoform of GR was detected in liver of T. s. elegans during anoxia. The K (m) values of turtle GR for GSSG and NADPH was 44.6 and 6.82 microM, respectively, suggesting a substantially higher affinity of turtle GR toward GSSG than most other vertebrates. Unlike other human GR, NADP(+ )did not inhibit turtle GR activity. The activation energy of turtle GR, calculated from the slope of the Arrhenius plot, was 32.2 +/- 2.64 kJ/mol. Turtle GR had high activity under a broad pH range (having activity between pHs 4 and 10; optimal activity at pH 6.5) and the enzyme maintains activity under the pH drop that occurs under anoxic conditions. The high affinity of turtle GR suggests that turtles have high redox buffering capacity of tissues to protect against oxidative stress encountered during anoxia/reoxygenation.

Semisynthesis and characterization of mammalian thioredoxin reductase

Thioredoxin reductase and thioredoxin constitute the cellular thioredoxin system, which provides reducing equivalents to numerous intracellular target disulfides. Mammalian thioredoxin reductase contains the rare amino acid selenocysteine. Known as the "21st" amino acid, selenocysteine is inserted into proteins by recoding UGA stop codons. Some model eukaryotic organisms lack the ability to insert selenocysteine, and prokaryotes have a recoding apparatus different from that of eukaryotes, thus making heterologous expression of mammalian selenoproteins difficult. Here, we present a semisynthetic method for preparing mammalian thioredoxin reductase. This method produces the first 487 amino acids of mouse thioredoxin reductase-3 as an intein fusion protein in Escherichia coli cells. The missing C-terminal tripeptide containing selenocysteine is then ligated to the thioester-tagged protein by expressed protein ligation. The semisynthetic version of thioredoxin reductase that we produce in this manner has k(cat) values ranging from 1500 to 2220 min(-)(1) toward thioredoxin and has strong peroxidase activity, indicating a functional form of the enzyme. We produced the semisynthetic thioredoxin reductase with a total yield of 24 mg from 6 L of E. coli culture (4 mg/L). This method allows production of a fully functional, semisynthetic selenoenzyme that is amenable to structure-function studies. A second semisynthetic system is also reported that makes use of peptide complementation to produce a partially active enzyme. The results of our peptide complementation studies reveal that a tetrapeptide that cannot ligate to the enzyme (Ac-Gly-Cys-Sec-Gly) can form a noncovalent complex with the truncated enzyme to form a weak complex. This noncovalent peptide-enzyme complex has 350-500-fold lower activity than the semisynthetic enzyme produced by peptide ligation.

Glutathione reductase from pea leaves: response to abiotic stress and characterization of the peroxisomal isozyme

The glutathione reductase (GR; EC 1.6.4.2) isozyme present in peroxisomes has been purified for the first time, and its unequivocal localization in these organelles, by immunogold electron microscopy, is reported. The enzyme was purified c. 21-fold with a specific activity of 9523 units mg(-1) protein, and a yield of 44 microg protein kg(-1) leaves was obtained. The subunit size of the peroxisomal GR was 56 kDa and the isoelectric point was 5.4. The enzyme was recognized by a polyclonal antibody raised against total GR from pea (Pisum sativum) leaves. The localization of GR in peroxisomes adds to chloroplasts and mitochondria where GR isozymes are also present, and suggests a multiple targeting of this enzyme to distinct cell compartments depending on the metabolism of each organelle under the plant growth conditions. The expression level of GR in several organs of pea plants and under different stress conditions was investigated. The possible role of peroxisomal GR under abiotic stress conditions, such as cadmium toxicity, high light, darkness, high temperature, wounding and low temperature, is discussed.

Purification of glutathione reductase from chicken liver and investigation of kinetic properties

Glutathione reductase was purified from chicken liver and some characteristics of the enzyme were investigated. The purification procedure was composed of four steps: preparation of homogenate, ammonium sulfate precipitation, 2',5'-ADP Sepharose 4B affinity chromatography, and Sephadex G-200 gel filtration chromatography. Owing to the four consecutive procedures, the enzyme was purified 1714-fold, with a yield of 38%. Specific activity at the final step was 120 enzyme unit (EU)/mg of protein. The purified enzyme showed a single band on sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). The molecular weight of the enzyme was found to be 100 kDa by Sephadex G-200 gel filtration chromatography, and the subunit molecular weight was found to be 43 kDa by SDS-PAGE. Optimum pH, stable pH, optimum ionic strength, and optimum temperature were 7.0, 7.4, 0.75 M Tris-HCl buffer including 1 mM EDTA, and 50 degrees C, respectively. KM and Vmax values for NADPH and glutathione disulfide (GSSG) substrates were also determined for the enzyme.

Purification of human erythrocyte glucose 6-phosphate dehydrogenase and glutathione reductase enzymes using 2',5'-ADP Sepharose 4B affinity column material in single chromatographic step

The enzymes of glucose 6-phosphate dehydrogenase and glutathione reductase were purified from human erythrocytes in one chromatographic step consisting of the use of the commercially available resin 2',5'-ADP Sepharose 4B by using different washing buffers. Ammonium sulfate (30-70%) precipitation was performed on the hemolysate before applying to the affinity column. Using this procedure, G6PG, having the specific activity of 22.9 EU/mg proteins, was purified with a yield of 43% and 9150-fold; GR, having the specific activity of 20.7 EU/mg proteins, was purified with a yield of 26% and 8600-fold. The purity of the enzymes was checked on sodium dodecyl sulfate-polyacrylamide gel electrophoresis and each purified enzyme showed a single band on the gel. This procedure has advantages of preventing of enzyme denaturation, short experimental duration, and use of less chemical materials for purification of the enzymes.

Purification and characterization of glutathione reductase from bovine erythrocytes

Glutathione reductase (E.C.1.8.1.7; GR) was purified from bovine erythrocytes and some characteristics properties of the enzyme were investigated. The purification procedure was composed of preparation of the hemolysate, ammonium sulfate fractionation, affinity chromatography on 2',5'-ADP Sepharose 4B, and gel filtration chromatography on Sephadex G-200. As a result of four consecutive procedures, the enzyme was purified 31,250-fold with a yield of 11.39%. Specific activity at the final step was 62.5 U (mg proteins)(-1). For the enzyme, optimum pH, optimum temperature, optimum ionic strength, and stable pH were found to be 7.3, 55 degrees C, 435 mM, 7.3, respectively. The molecular weight of the enzyme was found to be 118 kDa by Sephadex G-200 gel filtration chromatography and the subunit molecular weight was found to be 58 kDa by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). In addition, Km and Vmax values were determined for glutathione disulfide (GSSG) and NADPH. Ki constants and inhibition types were established for glutathione (GSH) and NADP+. Also, effects of NADPH and GSSG were investigated on the enzyme activities.

Cell-free production of active E. coli thioredoxin reductase and glutathione reductase

Escherichia coli thioredoxin reductase (TR) and glutathione reductase (GR) are dimeric proteins that require a flavin adenine dinucleotide (FAD) cofactor for activity. A cell-free protein synthesis (CFPS) reaction supplemented with FAD was used to produce TR at 760 microg/ml with 89% of the protein being soluble. GR accumulated to 521 microg/ml in a cell-free reaction with 71% solubility. The TR produced was fully active with a specific activity of 1390 min(-1). The GR had a specific activity of 139 U/mg, which is significantly more active than reported for GR purified from cells. The specific activity for both TR and GR decreased without FAD supplementation. This research demonstrates that CFPS can be used to produce enzymes that are multimeric and require a cofactor.

Peroxisomes from pepper fruits (Capsicum annuum L.): purification, characterisation and antioxidant activity

Pepper is a vegetable of importance in human nutrition. Currently, one of the most interesting properties of natural products is their antioxidant content. In this work, the purification and characterisation of peroxisomes from fruits of a higher plant was carried out, and their antioxidative enzymatic and non-enzymatic content was investigated. Green and red pepper fruits (Capsicum annuum L., type Lamuyo) were used in this study. The analysis by electron microscopy showed that peroxisomes from both types of fruits contained crystalline cores which varied in shape and size, and the presence of chloroplasts and chromoplasts in green and red pepper fruits, respectively, was confirmed. Peroxisomes were purified by differential and sucrose density-gradient centrifugations. In the peroxisomal fractions, the activity of the photorespiration, beta-oxidation and glyoxylate cycle enzymes, and the ROS-related enzymes catalase, superoxide dismutase, xanthine oxidase, glutathione reductase and NADP(+)-dehydrogenases, was determined. Most enzymes studied had higher specific activity and protein content in green than in red fruits. By native PAGE and western blot analysis, the localisation of a Mn-SOD in fruit peroxisomes was demonstrated. The ascorbate and glutathione levels were also determined in crude extracts and in peroxisomes purified from both green and red peppers. The total ascorbate content (200-220 mg per 100 g FW) was similar in crude extracts from the two types of fruits, but higher in peroxisomes from red peppers. The glutathione concentration was 2-fold greater in green pepper crude extracts than in red fruits, whereas peroxisomes from both tissues showed similar values. The presence in pepper peroxisomes of different antioxidative enzymes and their corresponding metabolites implies that these organelles might be an important pool of antioxidants in fruit cells, where these enzymes could also act as modulators of signal molecules (O2*-, H202) during fruit maturation.

Isolation and characterization of glutathione reductase from Physarum polycephalum and stage-specific expression of the enzyme in life-cycle stages with different oxidation-reduction levels

Physarum polycephalum has a life cycle with several distinct phases that have different oxidation-reduction requirements. To investigate the relationship between the life cycle and the oxidation-reduction state, we isolated glutathione reductase (GR; EC 1.6.4.2) from Physarum microplasmodia. The enzyme was found to be a homodimer with a subunit M(r) of 49,000, and K(m) values for oxidized glutathione and NADPH of 40 and 28.6 microM, respectively. We then constructed a cDNA library from microplasmodium mRNA and cloned GR cDNA from the library. The isolated cDNA consisted of 1,475 bp encoding a polypeptide of 452 amino acids. The amino acid sequence similarity was about 50% with GRs of other organisms, and several conserved sequence motifs thought to be necessary for activity are evident in the Physarum enzyme. Escherichia coli transformed with an expression vector containing the cDNA synthesized the active GR. Genomic Southern blot analysis indicated that the GR gene is present as a single copy in the Physarum genome. Immunoblot analysis and RT-PCR analysis detected GR mRNA expression in the microplasmodium, plasmodium, and sclerotium, but not in the spore or flagellate. GR activity was low in the spore and flagellate. These results suggest that the glutathione oxidation-reduction system relates to the Physarum life cycle.

Glutathione reductase in wheat grain. 1. Isolation and characterization

Durum wheat (Triticum durum, Desf.) endosperm of mature kernels contained a single form of glutathione reductase (GR); it appeared about the 18th day after anthesis while another isoform, present at the early stages of grain development, disappeared between the 20th and 30th days after flowering. The form that was present at grain maturity was isolated and characterized. It was composed of two monomers, each one having an apparent molecular mass of about 60 kDa. The K(m) values for NADPH and for GSSG were 3.7 and 9.1 microM, respectively, and the V(m) values for NADPH and for GSSG were 594 and 575 microkat.mg(-)(1) protein, respectively. The pH(i) of the enzyme was situated between pH 4.4 and 4.5. At a constant temperature of 25 degrees C, the optimum GR activity was found to be between pH 7.5 and 8.0. It was relatively resistant to high temperatures and was very resistant to very low temperatures.

Characterisation of pea cytosolic glutathione reductase expressed in transgenic tobacco

Expression in transgenic tobacco (Nicotiana tabacum L.) of a pea (Pisum sativum L.) GOR2 cDNA, encoding an isoform of glutathione reductase (GOR2), resulted in a 3- to 7-fold elevation of total foliar glutathione reductase (GR) activity. The enzyme encoded by GOR2 was confirmed to be extraplastidial in organelle fractionation studies and was considered most likely to be localised in the cytosol. A partial purification of GOR2 was achieved but a standard affinity chromatography step, using adenosine-2',5'-diphosphate-Sepharose and often employed in the purification of GR from diverse sources, was unsuccessful with this isoform. Preparative isoelectric focussing was employed as part of the purification procedure of GOR2 and a complete separation from plastidial/mitochondrial glutathione reductase (GOR1) was achieved. The isoform GOR2 was shown to have a slower migration on non-denaturing polyacrylamide gels compared with GOR1 and properties typical of GR enzymes from plant sources.

Deletion of the parasite-specific insertions and mutation of the catalytic triad in glutathione reductase from chloroquine-sensitive Plasmodium falciparum 3D7

The flavoenzyme glutathione reductase (GR; NADPH+glutathione disulphide+H(+)-->NADP(+)+2 glutathione-SH) of Plasmodium falciparum is a promising drug target against tropical malaria. As P. falciparum genes are assumed to be highly polymorphic we have cloned and expressed the GR cDNA of the chloroquine-sensitive strain 3D7. In comparison to the known GR of the chloroquine-resistant K1 strain there are three base exchanges all of them leading to amino acid substitutions (residues 281, 285 and 335). The catalytic efficiency k(cat)/K(m) of the 3D7 enzyme is 5-fold lower than for the K1 enzyme. In contrast, vis-à-vis the drugs carmustine, methylene blue and fluorophenyliso-alloxazine the two enzyme species exhibited identical inhibition kinetics. Two structural motifs which are specific for P. falciparum GR were studied by mutational deletion analysis of 3D7 GR. Loop 126-138 appears to be important for folding and stability of the enzyme, whereas the subdomain 318-350 was found to be involved in FAD-binding. The subdomain has no major influence on the known functions of the catalytic triad Cys-40, Cys-45 and His-485'. Flavin absorption spectroscopy of inactive point mutants showed that Cys-45 forms a thiolate charge transfer complex and Cys-40 is the interchange thiol, which reduces glutathione disulphide. The mutant His-485-->Gln had a normal K(m) for glutathione disulphide reduction but only 0.8% residual catalytic activity when compared with wild-type GR, which confirms its function as an acid/base catalyst. The parasite-specific domains in combination with the reactive catalytic residues appear to be a suitable target matrix for inhibiting GR in vivo.

Purification of glutathione reductase from bovine brain, generation of an antiserum, and immunocytochemical localization of the enzyme in neural cells

Glutathione reductase (GR) is an essential enzyme for the glutathione-mediated detoxification of peroxides because it catalyzes the reduction of glutathione disulfide. GR was purified from bovine brain 5,000-fold with a specific activity of 145 U/mg of protein. The homogeneity of the enzyme was proven by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and silver staining of the gel. The purified GR from bovine brain is a dimer of two subunits that have an apparent molecular mass of 55 kDa. The purified GR was used to generate a rabbit antiserum with the intention to localize GR in brain cells. The antiserum was useful for the detection of GR by double-labeling immunocytochemical staining in astroglia-rich and neuron-rich primary cultures from rat brain. In homogenates of these cultures, no significant difference in the specific activities of GR was determined. However, not all cell types present in these cultures showed identical staining intensity for GR. In astroglia-rich primary cultures, strong GR immunoreactivity was found in cells positive for the cellular markers galactocerebroside and C3b (antibody Ox42), indicating that oligodendroglial and microglial cells, respectively, contain GR. In contrast, only weak immunoreactivity for GR was found in cells positive for glial fibrillary acidic protein. In neuron-rich primary cultures, GAP43-positive cells stained with the antiserum against GR. These data demonstrate that, in cultures of neural cells, neurons, oligodendroglial cells, and microglial cells express high levels of GR.

Purification of a glutathione S-transferase and a glutathione conjugate-specific dehydrogenase involved in isoprene metabolism in Rhodococcus sp. strain AD45

A glutathione S-transferase (GST) with activity toward 1, 2-epoxy-2-methyl-3-butene (isoprene monoxide) and cis-1, 2-dichloroepoxyethane was purified from the isoprene-utilizing bacterium Rhodococcus sp. strain AD45. The homodimeric enzyme (two subunits of 27 kDa each) catalyzed the glutathione (GSH)-dependent ring opening of various epoxides. At 5 mM GSH, the enzyme followed Michaelis-Menten kinetics for isoprene monoxide and cis-1, 2-dichloroepoxyethane, with Vmax values of 66 and 2.4 micromol min-1 mg of protein-1 and Km values of 0.3 and 0.1 mM for isoprene monoxide and cis-1,2-dichloroepoxyethane, respectively. Activities increased linearly with the GSH concentration up to 25 mM. 1H nuclear magnetic resonance spectroscopy showed that the product of GSH conjugation to isoprene monoxide was 1-hydroxy-2-glutathionyl-2-methyl-3-butene (HGMB). Thus, nucleophilic attack of GSH occurred on the tertiary carbon atom of the epoxide ring. HGMB was further converted by an NAD+-dependent dehydrogenase, and this enzyme was also purified from isoprene-grown cells. The homodimeric enzyme (two subunits of 25 kDa each) showed a high activity for HGMB, whereas simple primary and secondary alcohols were not oxidized. The enzyme catalyzed the sequential oxidation of the alcohol function to the corresponding aldehyde and carboxylic acid and followed Michaelis-Menten kinetics with respect to NAD+ and HGMB. The results suggest that the initial steps in isoprene metabolism are a monooxygenase-catalyzed conversion to isoprene monoxide, a GST-catalyzed conjugation to HGMB, and a dehydrogenase-catalyzed two-step oxidation to 2-glutathionyl-2-methyl-3-butenoic acid.

Microinjected glutathione reductase crystals as indicators of the redox status in living cells

The flavoenzyme glutathione reductase catalyses electron transfer reactions between two major intracellular redox buffers, namely the NADPH/NADP+ couple and the 2 glutathione/glutathione disulfide couple. On this account, microcrystals of the enzyme were tested as redox probes of intracellular compartments. For introducing protein crystals into human fibroblasts, different methods (microinjection, particle bombardment and optical tweezers) were explored and compared. When glutathione reductase crystals are present in a cytosolic environment, the transition of the yellow Eox form to the orange-red 2-electron reduced charge transfer form, EH2, is observed. Taking into account the midpoint potential of the Eox/EH2 couple, the redox potential of the cytosol was found to be < -270 mV at pH 7.4 and 37 degrees C. As a general conclusion, competent proteins in crystalline--that is signal-amplifying--form are promising probes for studying intracellular events.

Optimized heterologous expression of glutathione reductase from Cyanobacterium anabaena PCC 7120 and characterization of the recombinant protein

Glutathione reductase (GR) from the cyanobacterium Anabaena PCC 7120 was heterologously expressed in Escherichia coli SG5. Silent random mutations were introduced in the 5' region of DNA encoding the enzyme in order to generate a high-level expression clone. To maximize protein expression, the culture conditions were also optimized. In the high-level expression clones selected, E. coli-preferred codons were selectively used at certain positions. Under the optimal expression conditions, a yield of 17 mg recombinant protein per liter was obtained, which is about 10-fold higher than that of the wild-type enzyme. A hexahistidine tag was added at the C-terminal of the protein in order to allow IMAC affinity purification. This strategy simplified the purification process and provided a homogeneous enzyme for functional characterization. Anabaena GR uses NADPH as a coenzyme, like most of the GRs from other sources, but the KM values for NADPH and GSSG are higher than those of enzymes previously studied. The Anabaena enzyme also shows significant activity when NADH is used as a reductant.

Evidence for the co-existence of glutathione reductase and trypanothione reductase in the non-trypanosomatid Euglenozoa: Euglena gracilis Z

Two NADPH-dependent disulfide reductases, glutathione reductase and trypanothione reductase, were shown to be present in Euglena gracilis, purified to homogeneity and characterized. The glutathione reductase (Mr 50 kDa) displays a high specificity towards glutathione disulfide with a KM of 54 microM. The amino acid sequences of two peptides derived from the trypanothione reductase (Mr 54 kDa) show a high level of identity (81% and 64%) with sequences of trypanothione reductases from trypanosomatids. The trypanothione reductase is able to efficiently reduce trypanothione disulfide (KM 30.5 microM) and glutathionylspermidine disulfide (KM 90.6 microM) but not glutathione disulfide, nor Escherichia coli thioredoxin disulfide, nor 5,5'-dithiobis(2-nitrobenzoate) (DTNB). These results demonstrate for the first time (i) the existence of trypanothione reductase in a non-trypanosomatid organism and (ii) the coexistence of trypanothione reductase and glutathione reductase in E. gracilis.

Enterococcus faecalis glutathione reductase: purification, characterization and expression under normal and hyperbaric O2 conditions

Glutathione reductase is found ubiquitously in eukaryotes and Gram-negative bacteria, and plays a significant role in bacterial defense against oxidative stress. Glutathione reductase from the Gram-positive bacterium Enterococcus faecalis was purified to homogeneity using anion exchange, hydrophobic interaction, and affinity chromatography. The homogeneous 49-kDa enzyme contained 1 mol bound FAD per subunit. The determined N-terminal amino acid sequence of the E. faecalis enzyme displays significant identity with glutathione reductases from other Gram-negative and Gram-positive bacteria, as well as yeast and human erythrocyte reductases. The kinetic mechanism is ping-pong, and the determined kinetic parameters exhibited by the E. faecalis glutathione reductase are similar to those found for glutathione reductases from yeast, Escherichia coli, and human erythrocyte. A two-fold increased expression of glutathione reductase activity and a three-fold induction of glutathione peroxidase activity were observed under hyperbaric O2 growth conditions without a corresponding change in the total glutathione and soluble thiol content. The difference in the expression of the enzyme, and its cognate substrate's intracellular concentration, under these conditions suggest that the gene encoding glutathione reductase is responsive to oxygen concentration, but that the genes encoding the glutathione synthesizing enzymes are not linked to an oxygen-sensitive promoter.

Recombinant Plasmodium falciparum glutathione reductase is inhibited by the antimalarial dye methylene blue

Plasmodium falciparum glutathione reductase (PfGR) has emerged as a drug target against tropical malaria. Here we report the expression of PfGR in Escherichia coli SG5(DE3) and isolation procedures for this protein. Recombinant PfGR does not differ from the authentic enzyme in its enzymic properties, the turnover number being 9900 min(-1). The dimeric flavoenzyme exhibits redox-dependent absorption spectra; the single tryptophan residue (per 57.2 kDa subunit) is strongly fluorescent. PfGR can be inhibited by the antimalarial drug methylene blue at therapeutic concentrations; the Ki for non-competitive inhibition is 6.4 microM. The sensitivity to methylene blue is observed also at high ionic strength so that, by analogy to human GR, analysis of crystalline enzyme-drug complexes can be envisaged.

Methods for purification of glutathione peroxidase and related enzymes

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

Oxidative stress response in yeast: purification and characterization of glutathione reductase from Hansenula mrakii

Glutathione reductase was purified from a yeast. Hansenula mrakii IFO 0895, to approximately 3500-fold with 59% activity yield. The enzyme was homogeneous on polyacrylamide gel electrophoresis. The molecular weight of the enzyme was estimated to be 56 kDa by SDS-polyacrylamide gel electrophoresis, and 123 kDa by gel filtration using a calibrated Sephadex G-150 column. The Km values for glutathione disulfide and NADPH were 21.3 microM and 14.3 microM, respectively. The enzyme was most active at pH 7.5, 55 degrees C. The enzyme was stable up to 40 degrees C, and between pHs 4 and 10. The enzyme was inhibited by p-chloromercuribenzoate and metal ions such as Fe3+, Cd2+, Cu2+, and Zn2+.

Glutathionylspermidine metabolism in Escherichia coli

Intracellular levels of glutathione and glutathionylspermidine conjugates have been measured throughout the growth phases of Escherichia coli. Glutathionylspermidine was present in mid-log-phase cells, and under stationary and anaerobic growth conditions accounted for 80% of the total glutathione content. N1,N8-bis(glutathionyl)spermidine (trypanothione) was undetectable under all growth conditions. The catalytic constant kcat/Km of recombinant E. coli glutathione reductase for glutathionylspermidine disulphide was approx. 11,000-fold lower than that for glutathione disulphide. The much higher catalytic constant for the mixed disulphide of glutathione and glutathionylspermidine (11% that of GSSG), suggests a possible explanation for the low turnover of trypanothione disulphide by E. coli glutathione reductase, given the apparent lack of a specific glutathionylspermidine disulphide reductase in E. coli.

Denaturing behavior of glutathione reductase from cyanobacterium Spirulina maxima in guanidine hydrochloride

The influence of guanidine hydrochloride (Gdn-HCl) on glutathione reductase from Spirulina maxima has been studied by measuring the changes in enzymatic activity, protein fluorescence, circular dichroism, thiol groups accessibility, and gel filtration chromatography. It was found that the denaturation process involves several intermediate states. At low, Gdn-HCl concentrations (Cm = 0.4 M), reductase activity was fully lost. However, below 3 M Gdn-HCl, this inhibition was freely reversible upon removal of the denaturing agent. Gel filtration experiments revealed that this reversible inhibition was not due to dissociation of the tetrameric enzyme. Structural studies strongly suggest that the conformation of this intermediate state is similar to that of native enzyme. A model in which a local region of the polypeptide chain assumes an extended conformation (D. T. Haynie, and E. Freire, Proteins 16,115-140) is proposed for the reversibly inactivated enzyme. Between 3 and 4 M Gdn-HCl (Cm = 3.5), the enzyme activity was irreversibly lost, this inhibition being concomitant with the loss of ellipticity, changes in both wavelength and intensity at the maximum of fluorescence emission, and dissociation of the enzyme into unfolded monomers; these results reveal that gross changes in the protein conformation occur under these conditions. At 4 M Gdn-HCl an equilibrium exists between the denatured forms of dimer and monomer, which is completely shifted toward the unfolded monomers at 5 M Gdn-HCl. Irreversibility in the Gdn-HCl-induced denaturation of S. maxima glutathione reductase was not due to aggregation of the unfolded enzyme.

The purification and properties of glutathione reductase from the cestode Moniezia expansa

Glutathione reductase has a central role in glutathione metabolism and as such is a potential target for chemotherapy. The aim of the work was to purify and characterise glutathione reductase from the cestode Moniezia expansa and to compare the properties of the helminth enzyme with its mammalian counterpart. The enzyme was purified by a combination of anion exchange and affinity chromatography and further characterized by chromatofocusing and gel electrophoresis. Analysis revealed a single isoenzyme of glutathione reductase in Moniezia expansa, with a pI of 5.8. The enzyme was a homodimer of native molecular weight 114 kDa, subunit weight 63 kDa. Enzyme activity was affected by buffer concentration and the presence of monovalent sodium salts. The pH optimum was 7.4 with NADPH as cofactor and 5 with NADH. The Kma for oxidized glutathione was 76 microM and for NADPH and NADH, 21 and 350 microM respectively. In addition to oxidized glutathione only the mixed disulphide between CoA and glutathione (CoASSG) showed any significant activity as substrate. The cestode enzyme was inhibited by a variety of compounds including arsonic derivatives, 2,4,6 trinitrobenzene sulfonate 1,3-bis (2-chlorethyl)-1-nitrosourea and oxidized glutathione. In conclusion the glutathione reductase of M. expansa resembles the mammalian enzyme in its general physical properties and its substrate and inhibitor profile. However, the parasite enzyme shows an unusually high activity with the mixed disulphide of coenzyme A and glutathione (CoASSG) and appears to be more sensitive to inhibition by sodium ions.

[A comparative study of the interaction between glutathione reductase and 6-phosphogluconate dehydrogenase of rat liver with certain affinity sorbents]

A comparative study of the conditions of interaction of some group-specific ligands with rat hepatic 6-phosphogluconate dehydrogenase and glutathione reductase was accomplished. The influence of ionic strength and coenzyme concentration in buffer solution on the extent of enzymes binding with immobilized 2',5'-ADP and procion red HE-3B was studied according to the residual enzymatic activity after the interaction of enzyme and adsorbent. The optimal conditions have been found for enzymes binding and for their elution resulted in the effective purification of the enzymes mentioned above. Enzymatic preparations were free from the contaminating activity of each other. Dye-ligand chromatography step on red sepharose was introduced into the three step scheme of the purification of 6-phosphogluconate dehydrogenase. The final preparation of 6-phosphogluconate dehydrogenase obtained by gradient salt elution of the enzyme from the red sepharose column (0-0.15 M KCl) has the specific activity of 30-34 E/mg and shows electrophoretical homogeneity. The contribution of ionic strength and biospecific interaction of the nucleotide binding site of the enzymes during chromatography on these adsorbents is discussed.

Purification and characterization of glutathione reductase isozymes specific for the state of cold hardiness of red spruce

Isozymes of glutathione reductase (GR) have been purified from red spruce (Picea rubens Sarg.) needles. Two isozymes could be separated by anion-exchange chromatography from both nonhardened or cold-hardened tissue. Based on chromatographic elution profiles, the isozymes were designated GR-1NH and GR-2NH in preparations from nonhardened needles, and GR-1H and GR-2H in preparations from hardened needles. N-terminal sequencing and immunological data with antisera obtained against GR-1H and GR-2H established that the isozymes from hardened needles are different gene products and show significant structural differences from each other. Chromatographic, electrophoretic, and immunological data revealed only minor differences between GR-2NH and GR-2H, and it is concluded that these isozymes are very similar or identical. Anion-exchange chromatography and native polyacrylamide gel electrophoresis also established that GR-1NH and GR-1H are different proteins. From these data we conclude that GR-1H is a distinct gene product, present only in hardened needles. Therefore, GR-1H can be considered to be a cold-hardiness-specific GR isozyme, and GR-1NH can be considered to be specific for nonhardened needles. It is proposed that GR-1H is a cold-acclimation protein.

Folding of the four domains and dimerization are impaired by the Gly446-->Glu exchange in human glutathione reductase. Implications for the design of antiparasitic drugs

Glutathione reductase (NADPH+GSSG+H+-->NADP(+) + 2GSH) is a homodimeric flavoenzyme of known geometry. Each subunit contains four well-defined domains and contributes essential residues to the active sites; consequently, the monomer is expected to be inactive. As part of our program to develop dimerization inhibitors of human glutathione reductase (hGR) as antimalarial agents, we mutagenized the residues 446 and 447 which, together with their counterparts on the other subunit, represent the tightest contact between the subunits [Karplus, P. A., & Schulz, G. E. (1987) J. Mol. Biol. 195, 701-729]. Wild-type human glutathione reductase and mutants of this protein were produced in plasmid-transformed Escherichia coli SG5 cells. Active enzyme species, namely, wild-type hGR, N-terminally truncated delta(1-15)hGR, and the point mutant F447P-hGR, were purified by 2',5'-ADP-Sepharose chromatography and crystallization. Inactive mutants such as G446E-hGR or the double mutants G446E/F447P-hGR and G446P/F447P-hGR were isolated by immunoadsorption chromatography. G446E/F447P-hGR was studied in detail. This mutant behaved like a poorly folded monomeric protein, as indicated by the following properties: absence of the intersubunit disulfide bridge, Cys90-Cys90'; failure to bind FAD; failure to bind NADPH and analogues thereof; a short half-life (< 4 min) in E. coli cells; and high susceptibility to trypsin in vitro. The results suggest that the sequence around G446 can control dimerization as well as domain folding. This is unexpected since the FAD-binding domain and the NADPH-binding domain occur in many different enzymes and have been regarded as autonomous folding units.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification and properties of bovine thioredoxin system

Using a variety of chromatographic techniques, a crude extract from bovine liver was fractionated to obtain pure preparations of thioredoxin reductase, thioredoxin, glutaredoxin and glutathione reductase with good yields. The turbidimetric assay of thioredoxin with insulin as the disulfide substrate was optimized; by incorporation of the lag time (tau) into the calculations, linearity was maintained for a wider range of thioredoxin concentrations, and a distinction could be made between reduced and non-reduced forms. Subunit composition and molecular mass, absorption spectrum and kinetic parameters of thioredoxin reductase were similar to those of other mammalian thioredoxin reductases. By chromatofocusing, two peaks of activity were detected at pH 5.5 and 5.8. Structural changes undergone by the thioredoxin molecule upon oxido-reduction were detected by isoelectric focusing, with a shift of 0.1 pH unit of its pI, and by analytical anion exchange chromatography, with a conspicuous shift of its retention time. These two methods also revealed the presence of a form of thioredoxin not undergoing the above mentioned redox-mediated structural shifts that accounted for > 75% of the total activity.

Horse-liver glutathione reductase: purification and characterization

1. Purification of horse-liver glutathione reductase was obtained by affinity chromatography on N6-(6-aminohexyl)-adenosine-1'5'-bisphosphate Sepharose (N6-2'5'-ADP-Sepharose) and Reactive Red-120-Agarose, and chromatography on DEAE-Sephadex and Sephacryl S-300. 2. The final preparation had 248 U/mg specific activity after 11,174-fold purification with 47% final recovery, and was homogeneous by SDS-electrophoresis. It showed charge heterogeneity in non-denaturing electrophoresis and chromatofocusing, with several peaks of pI between 5.7 and 6.7. 3. The enzyme was homodimeric (107,000 native MW), with S20w = 6.31 S, and 41.22 A of hydrodynamic radius. It showed absorption peaks at 270, 370 and 462 nm, a characteristic of flavoproteins. 4. When NADPH was substituted by deamino-NADPH or NADH the enzyme showed 69 and 8.5% activity, respectively, while with glutathione-CoA mixed disulfide the enzyme had 23% of the activity shown with GSSG. Apparent Km values of 8.8, 680, 59, and 560 microM were measured for NADPH, NADH, GSSG and ferricyanide, respectively.

Purification and characterization of glutathione reductase from Rhodospirillum rubrum

Glutathione reductase (NAD(P)H:GSSG oxidoreductase EC 1.6.4.2.) was purified 1160-fold to homogeneity from the nonsulfurous purple bacteria Rhodospirillum rubrum (wild type). Specific activity of the pure preparation was 102 U/mg. The enzyme displayed a typical flavoprotein absorption spectrum with maxima at 274,365, and 459 nm and an absorbance ratio A280/A459 of 7.6. The amino acid analysis revealed an unusually high content of glycine and arginine residues. Titration of the enzyme with 5,5'-dithiobis(2-nitrobenzoic acid) showed a total of two free thiol groups per subunit, one of which is made accessible only under denaturing conditions. An isoelectric point of 5.2 was found for the native enzyme. Km values, determined at pH 7.5, were 6.1 and 90 microM for NADPH and GSSG, respectively. NADH was about 2% as active as NADPH as an electron donor. The enzyme's second choice in disulfide substrate was the mixed disulfide of coenzyme A and glutathione, for which the specific activity and Km values were 5.1 U/mg and 3.4 mM, respectively. A native molecular weight of 118,000 was found, while denaturing electrophoresis gave a value of 54,400 per subunit, thus suggesting that R. rubrum glutathione reductase exists as a dimeric protein. Other physicochemical constants of the enzyme, such as Stokes radius (4.2 nm) and sedimentation coefficient (5.71 S), were also consistent with a particle of 110,000.

Engineering surface charge. 1. A method for detecting subunit exchange in Escherichia coli glutathione reductase

The gene gor encoding Escherichia coli glutathione reductase was mutated to create a positively charged N-terminal extension consisting of five arginine residues followed by a factor Xa cleavage site to the enzyme polypeptide chain. The modified protein assembled in vivo to yield a dimeric enzyme with kinetic parameters indistinguishable from those of wild-type glutathione reductase. The N-terminal extension could not be released by treatment with factor Xa but could be removed by exposure to trypsin, again without effect on the enzyme activity. The modified enzyme was readily separated from the wild-type enzyme by means of ion-exchange chromatography or nondenaturing polyacrylamide gel electrophoresis. Incubation of the modified and wild-type enzymes, separately or as a mixture, with NADH led to their partial inactivation, and activity was restored by exposure to 1 mM reduced glutathione. No hybrid dimer was formed in the mixture of modified and wild-type enzymes, as judged by polyacrylamide gel electrophoresis, strongly suggesting that the inactivation induced by NADH was not due to dissociation of the parental dimers. The addition of otherwise benign positively or negatively charged extensions to the N- or C-terminal regions of the constituent polypeptide chains of oligomeric enzymes offers a simple route to detecting hybrid formation and the causative subunit dissociation and exchange.

Engineering surface charge. 2. A method for purifying heterodimers of Escherichia coli glutathione reductase

Two gor genes encoding different mutants of Escherichia coli glutathione reductase have been expressed in the same E. coli cell, leading to the creation of a hybrid form of the enzyme dimer. One of the gor genes carried, in addition to various directed mutations, a 5' extension that encodes a benign penta-arginine "arm" added to the N-terminus of the glutathione reductase polypeptide chain [Deonarain, M.P., Scrutton, N.S., & Perham, R.N. (1992) Biochemistry (preceding paper in this issue)]. This made possible, by means of ion-exchange chromatography or nondenaturing polyacrylamide gel electrophoresis, the facile separation of the hybrid enzyme from the two parental forms. Moreover, the two subunits in the hybrid enzyme could be made to carry different mutations. In this way, glutathione reductases with only one active site per dimer were generated: the effects of replacing tyrosine-177 with glycine in the NADPH-binding site, which greatly diminishes the Km for glutathione and switches the kinetic mechanism from ping-pong to ordered sequential, and of replacing His-439 with glutamine in the glutathione-binding site, which greatly diminishes the Km for NADPH, were both found to be restricted to the one active site carrying the mutations. This system of generating separable enzyme hybrids is generally applicable and should make it possible now to undertake a more systematic study of catalytic mechanism and assembly for the many enzymes with quaternary structure.

High-performance affinity chromatography of NADP+ dehydrogenases from cell-free extracts using a nucleotide analogue as general ligand

An epoxy-activated silica column (50 cm x 0.45 cm I.D.) was derivatized with 8-[6-aminohexyl)amino]-2'-phosphoadenosine-5'-diphosphoribose; the bound ligand concentration was 11.4 mumol/g of dry silica, and the useful loading capacity was 2.3 mg of glutathione reductase. The new high-performance liquid chromatographic column specifically retained NADP(+)-dependent enzymes, which were quantitatively eluted specifically by NADP+ or, with better resolution, by potassium chloride. The new high-performance liquid chromatographic support was applied to the purification of glutathione reductase and glucose-6-phosphate dehydrogenase from cell-free extracts of baker's yeast, fish liver and rabbit hemolysates, with high recoveries and excellent purification factors.

High-yield extraction and purification of glutathione reductase from baker's yeast

Glutathione reductase was extracted from toluene-treated baker's yeast cells by a two-stage buffer autolysis method. The yeast cells were treated with toluene for 1 h at 40 degrees C. After removal of the toluene, the cells were then allowed to autolysis in buffer for 72 h at 4 degrees C. The cells were collected and resuspended in buffer. A second stage autolysis was carried out for another 96 h at 4 degrees C. The enzyme was purified to 786-fold from the second stage cell autolysate by using two steps of affinity chromatography with triazine dyes (Yellow H-E4G and Yellow H-E6G) coupled to Sepharose CL-4B. By using this simplified method, 1.44 mg (165 units/mg) of glutathione reductase was obtained from 65 g (wet weight) of yeast cells, equivalent to 80% enzyme recovery.

The three-dimensional structure of glutathione reductase from Escherichia coli at 3.0 A resolution

The structure of glutathione reductase from Escherichia coli has been solved at 3 A resolution using multiple isomorphous replacement, solvent flattening, and molecular replacement on the basis of the homologous (53% identical residues) and structurally well-established human enzyme. The structures of both enzyme species agree with each other in a global way; there is no domain rearrangement. In detail, clear structural differences can be observed. The structure analysis of the E. coli enzyme was tackled in order to understand site-directed mutants, the most spectacular of which changed the cofactor specificity of this enzyme from NADP to NAD (Scrutton et al., 1990, Nature 343:38-43).

Human jejunal glutathione reductase: purification and evaluation of the NADPH- and glutathione-induced changes in redox state

Human proximal jejunal glutathione reductase (EC 1.6.4.2) was purified to homogeneity by affinity chromatography on 2', 5'-ADP-Sepharose 4B. In most of its molecular and kinetic properties, the enzyme resembled glutathione reductase from other sources: The subunit mass was 56 kDa; the isoelectric point and pH optimum were 6.75 and 7.25, respectively; Michaelis constants, determined at pH 7.4, 37 degrees C, fell within the range of previously reported values [Km(NADPH) = 20 microM, Km(GSSG) = 80 microM]. The response of the enzyme to reducing conditions, on the other hand, had unique features: Preincubation with 1 mM NADPH resulted in 90% loss of activity which could be partially reversed by 2 mM GSSG, but not GSH. (Treatment with GSSG regenerated 68% of the original activity.) Reduction by GSH also caused inactivation which potentially amounted to greater than 80%. This inactivation could not be reversed by GSSG. The protective effect of GSSG against inactivation by GSH was studied. Except where [GSSG] far exceeded [GSH], the presence of GSSG in the preincubation medium decreased the extent of inhibition without affecting the rate constant for approach to equilibrium activity. At [GSSG] greater than [GSH] a decrease in the rate constant for inactivation was also observed. The results were interpreted in terms of a three-step mechanism: (1) preequilibrium reduction of Eox to Ered; (2) rate-limiting change in conformation from Ered to E'red, and (3) irreversible conversion to catalytically inferior products.

Inhibition of glutathione reductase by oncomodulin

Evidence for a specific interaction between oncomodulin and glutathione reductase is presented. Glutathione reductase (EC 1.6.4.2) isolated from either the bovine intestinal mucosa or the rat liver was bound in a Ca2(+)-dependent manner to oncomodulin which was covalently attached to Sepharose. In addition, glutathione reductase was able to catalyze the reduction of the disulfide-linked dimer of oncomodulin. The interaction of these proteins could also be indirectly demonstrated by monitoring glutathione reductase activity since oncomodulin was shown to inhibit the enzyme in a dose-dependent manner with an apparent IC50 of approximately 5 microM. The kinetic analysis of the oncomodulin-dependent effects on glutathione reductase activity indicates that oncomodulin interacts at a site other than the active site as the oncomodulin-induced inhibition was of the noncompetitive type. The in vivo inhibition of glutathione reductase appears to be an oncomodulin-specific effect as closely related members of the troponin C superfamily such as rabbit (pI 5.5) or carp (pI 4.25) parvalbumins, as well as calmodulin, failed to affect the activity of this enzyme. The present in vitro study indicating that oncomodulin can regulate the activity of glutathione reductase could be very significant with respect to the elucidation of a physiological role for oncomodulin.

Sheep brain glutathione reductase: purification and general properties

Sheep brain glutathione reductase was purified about 11,000-fold with an overall yield of 40%. The method included ammonium sulphate fractionation, heat denaturation, 2',5'-ADP Sepharose 4B and Sephadex G-200 chromatography steps. Specific activity at the final step was 193 IU/mg. The Mr of the enzyme was found to be 116,000 by gel filtration chromatography. On SDS-PAGE, two identical subunits of Mr 64,000 were obtained. From the spectral data, about 2 mol FAD per mol of enzyme were calculated.

[Isolation of highly-purified NADP(H)-dependent enzymes from the rat liver using NADP-hydrazidoadipoyl oxypropyl sepharose]

NADP-hydrazidoadipoyl oxypropyl sepharose was synthesized from epoxyactivated sepharose through a hydrazid derivative and used for isolation of NADP(H)-dependent enzymes such as glutathione reductase, isocitrate dehydrogenase and malate dehydrogenase. The isolation technique involves fractionation with ammonium sulphate, affinity chromatography on NADP-hydrazidoadipoyl oxypropyl sepharose and chromatography on hydroxylapatite. The proposed technique enabled the authors to obtain malate dehydrogenase isocitrate dehydrogenase and glutathione reductase preparations homogeneous according to SDS-electrophoresis in polyacrylamide gel.