Escherichia coli gamma-glutamylcysteine synthetase. Two active site metal ions affect substrate and inhibitor binding

Glutathione depletion results in S-nitrosylation of protein disulfide isomerase in neuroblastoma cells

AIMS

Protein disulfide isomerase (PDI) is an essential enzyme involved in oxidative protein folding. PDI is S-nitrosylated in the brains of Alzheimer's disease patients, and S-nitrosylated PDI is considered one of main causes of Alzheimer's disease. However, the mechanisms underlying PDI S-nitrosylation have not yet been elucidated. Because glutathione (GSH) depletion is a pathological feature of Alzheimer's disease, we investigated the effect of GSH depletion on the S-nitrosylation level of PDI.

MAIN METHODS

SH-SY5Y cells, which is a human derived neuroblastoma cells, were used in this study. Glutamate and buthionine sulfoximine (BSO) were used as GSH depletors. S-nitrosylated PDI was detected by biotin-switch assay.

KEY FINDINGS

GSH depletion by glutamate, a cystine/glutamate antiporter xCT inhibitor, increased S-nitrosylated PDI at C343 in SH-SY5Y cells, and induced IRE1α phosphorylation. BSO, a γ-glutamylcysteine synthetase inhibitor, also increased S-nitrosylated PDI and phosphorylated IRE1α upon GSH depletion. Because S-nitrosylated PDI at C343 is stable in neuro cells, S-nitrosylated PDI by GSH depletion progresses to neurodegeneration by the induction of endoplasmic reticulum stress via phosphorylated IRE1α signaling from the early to late stage. Furthermore, treatment with neohesperidin, but not N-acetylcysteine (NAC), improved PDI S-nitrosylation level in GSH-depleted SH-SY5Y cells because nitrosylated compound of NAC induces PDI S-nitrosylation.

SIGNIFICANCE

The results of our study provide a new insight into the mechanisms of neurodegeneration, and may be useful for the development of drugs for Alzheimer's diseases.

Characterization of γ-glutamyl cysteine ligases from Limosilactobacillus reuteri producing kokumi-active γ-glutamyl dipeptides

γ-Glutamyl cysteine ligases (Gcls) catalyze the first step of glutathione synthesis in prokaryotes and many eukaryotes. This study aimed to determine the biochemical properties of three different Gcls from strains of Limosilactobacillus reuteri that accumulate γ-glutamyl dipeptides. Gcl1, Gcl2, and Gcl3 were heterologously expressed in Escherichia coli and purified by affinity chromatography. Gcl1, Gcl2, and Gcl2 exhibited biochemical with respect to the requirement for metal ions, the optimum pH and temperature of activity, and the kinetic constants for the substrates cysteine and glutamate. The substrate specificities of the three Gcls to 14 amino acids were assessed by liquid chromatography-mass spectrometry. All three Gcls converted ala, met, glu, and gln into the corresponding γ-glutamyl dipeptides. None of the three were active with val, asp, and his. Gcl1 and Gcl3 but not Gcl2 formed γ-glu-leu, γ-glu-ile, and γ-glu-phe; Gcl3 exhibited stronger activity with gly, pro, and asp when compared to Gcl2. Phylogenetic analysis of Gcl and the Gcl-domain of GshAB in lactobacilli demonstrated that most of Gcls were present in heterofermentative lactobacilli, while GshAB was identified predominantly in homofermentative lactobacilli. This distribution suggests a different ecological role of the enzyme in homofermentative and heterofermentative lactobacilli. In conclusion, three Gcls exhibited similar biochemical properties but differed with respect to their substrate specificity and thus the synthesis of kokumi-active γ-glutamyl dipeptides. KEY POINTS: • Strains of Limosilactobacillus reuteri encode for up to 3 glutamyl cysteine ligases. • Gcl1, Gcl2, and Gcl3 of Lm. reuteri differ in their substrate specificity. • Gcl1 and Gcl3 produce kokumi-active dipeptides.

From Tea Leaves to Factories: A Review of Research Progress in l-Theanine Biosynthesis and Production

l-Theanine is the most popular nonprotein amino acid contained in tea leaves. It is one of the umami components of green tea, contributing to the unique flavor of tea. Because of its various health functions, l-theanine has been commercially developed as a valuable ingredient easily used for various applications in food and pharmaceutical industries. Nowadays, l-theanine is mass-produced by plant extraction, chemical synthesis, or enzymatic transformation in factories. This review embodies the available up to date information on the l-theanine synthesis metabolism in the tea plant as well as approaches to produce it, placing emphasis on the biotransformation of l-theanine. It also gives insight into the challenges of l-theanine production on a large scale, as well as directions for future research. This review comprehensively summarizes information on l-theanine to provide an approach for an in-depth study of l-theanine production.

Development of a highly efficient and specific L-theanine synthase

γ-Glutamylcysteine synthetase (γ-GCS) from Escherichia coli, which catalyzes the formation of L-glutamylcysteine from L-glutamic acid and L-cysteine, was engineered into an L-theanine synthase using L-glutamic acid and ethylamine as substrates. A high-throughput screening method using a 96-well plate was developed to evaluate the L-theanine synthesis reaction. Both site-saturation mutagenesis and random mutagenesis were applied. After three rounds of directed evolution, 13B6, the best-performing mutant enzyme, exhibited 14.6- and 17.0-fold improvements in L-theanine production and catalytic efficiency for ethylamine, respectively, compared with the wild-type enzyme. In addition, the specific activity of 13B6 for the original substrate, L-cysteine, decreased to approximately 14.6% of that of the wild-type enzyme. Thus, the γ-GCS enzyme was successfully switched to a specific L-theanine synthase by directed evolution. Furthermore, an ATP-regeneration system was introduced based on polyphosphate kinases catalyzing the transfer of phosphates from polyphosphate to ADP, thus lowering the level of ATP consumption and the cost of L-theanine synthesis. The final L-theanine production by mutant 13B6 reached 30.4 ± 0.3 g/L in 2 h, with a conversion rate of 87.1%, which has great potential for industrial applications.

First evidence of glutathione metabolism in Leptospira interrogans

BACKGROUND

Glutathione (GSH) plays a role as a main antioxidant metabolite in all eukaryotes and many prokaryotes. Most of the organisms synthesize GSH by a pathway involving two enzymatic reactions, each one consuming one molecule of ATP. In a first step mediated by glutamate-cysteine ligase (GCL), the carboxylate of l-glutamic acid reacts with l-cysteine to form the dipeptide γ-glutamylcysteine (γ-GC). The second step involves the addition of glycine to the C-terminal of γ-GC catalyzed by glutathione synthetase (GS). In many bacteria, such as in the pathogen Leptospira interrogans, the main intracellular thiol has not yet been identified and the presence of GSH is not clear.

METHODS

We performed the molecular cloning of the genes gshA and gshB from L. interrogans; which respectively code for GCL and GS. After heterologous expression of the cloned genes we recombinantly produced the respective proteins with high degree of purity. These enzymes were exhaustively characterized in their biochemical properties. In addition, we determined the contents of GSH and the activity of related enzymes (and proteins) in cell extracts of the bacterium.

RESULTS

We functionally characterized GCL and GS, the two enzymes putatively involved in GSH synthesis in L. interrogans serovar Copenhageni. LinGCL showed higher substrate promiscuity (was active in presence of l-glutamic acid, l-cysteine and ATP, and also with GTP, l-aspartic acid and l-serine in lower proportion) unlike LinGS (which was only active with γ-GC, l-glycine and ATP). LinGCL is significantly inhibited by γ-GC and GSH, the respective intermediate and final product of the synthetic pathway. GSH showed inhibitory effect over LinGS but with a lower potency than LinGCL. Going further, we detected the presence of GSH in L. interrogans cells grown under basal conditions and also determined enzymatic activity of several GSH-dependent/related proteins in cell extracts.

CONCLUSIONS

and General Significance. Our results contribute with novel insights concerning redox metabolism in L. interrogans, mainly supporting that GSH is part of the antioxidant defense in the bacterium.

Multiple pathways for the formation of the γ-glutamyl peptides γ-glutamyl-valine and γ- glutamyl-valyl-glycine in Saccharomyces cerevisiae

The role of glutathione (GSH) in eukaryotic cells is well known. The biosynthesis of this γ-glutamine tripeptide is well studied. However, other γ-glutamyl peptides were found in various sources, and the pathways of their formation were not always clear. The aim of the present study was to determine whether Saccharomyces cerevisiae can produce γ-glutamyl tripeptides other than GSH and to identify the pathways associated with the formation of these peptides. The tripeptide γ-Glu-Val-Gly (γ-EVG) was used as a model. Wild-type yeast cells were shown to produce this peptide during cultivation in minimal synthetic medium. Two different biosynthetic pathways for this peptide were identified. The first pathway consisted of two steps. In the first step, γ-Glu-Val (γ-EV) was produced from glutamate and valine by the glutamate-cysteine ligase (GCL) Gsh1p or by the transfer of the γ-glutamyl group from GSH to valine by the γ-glutamyltransferase (GGT) Ecm38p or by the (Dug2p-Dug3p)2 complex. In the next step, γ-EV was combined with glycine by the glutathione synthetase (GS) Gsh2p. The second pathway consisted of transfer of the γ-glutamyl residue from GSH to the dipeptide Val-Gly (VG). This reaction was carried out mainly by the (Dug2p-Dug3p)2 complex, whereas the GGT Ecm38p did not participate in this reaction. The contribution of each of these two pathways to the intracellular pool of γ-EVG was dependent on cultivation conditions. In this work, we also found that Dug1p, previously identified as a Cys-Gly dipeptidase, played an essential role in the hydrolysis of the dipeptide VG in yeast cells. It was also demonstrated that γ-EV and γ-EVG could be effectively imported from the medium and that γ-EVG was imported by Opt1p, known to be a GSH importer. Our results demonstrated that γ-glutamyl peptides, particularly γ-EVG, are produced in yeast as products of several physiologically important reactions and are therefore natural components of yeast cells.

The glutathione cycle: Glutathione metabolism beyond the γ-glutamyl cycle

Glutathione was discovered in 1888, over 125 years ago. Since then, our understanding of various functions and metabolism of this important molecule has grown over these years. But it is only now, in the last decade, that a somewhat complete picture of its metabolism has emerged. Glutathione metabolism has till now been largely depicted and understood by the γ-glutamyl cycle that was proposed in 1970. However, new findings and knowledge particularly on the transport and degradation of glutathione have revealed that many aspects of the γ-glutamyl cycle are incorrect. Despite this, an integrated critical analysis of the cycle has never been undertaken and this has led to the cycle and its errors perpetuating in the literature. This review takes a careful look at the γ-glutamyl cycle and its shortcomings and presents a "glutathione cycle" that captures the current understanding of glutathione metabolism. © 2018 IUBMB Life, 70(7):585-592, 2018.

Glutathione Degradation

SIGNIFICANCE

Glutathione degradation has for long been thought to occur only on noncytosolic pools. This is because there has been only one enzyme known to degrade glutathione (γ-glutamyl transpeptidase) and this localizes to either the plasma membrane (mammals, bacteria) or the vacuolar membrane (yeast, plants) and acts on extracellular or vacuolar pools. The last few years have seen the discovery of several new enzymes of glutathione degradation that function in the cytosol, throwing new light on glutathione degradation. Recent Advances: The new enzymes that have been identified in the last few years that can initiate glutathione degradation include the Dug enzyme found in yeast and fungi, the ChaC1 enzyme found among higher eukaryotes, the ChaC2 enzyme found from bacteria to man, and the RipAY enzyme found in some bacteria. These enzymes play roles ranging from housekeeping functions to stress responses and are involved in processes such as embryonic neural development and pathogenesis.

CRITICAL ISSUES

In addition to delineating the pathways of glutathione degradation in detail, a critical issue is to find how these new enzymes impact cellular physiology and homeostasis.

FUTURE DIRECTIONS

Glutathione degradation plays a far greater role in cellular physiology than previously envisaged. The differential regulation and differential specificities of various enzymes, each acting on distinct pools, can lead to different consequences to the cell. It is likely that the coming years will see these downstream effects being unraveled in greater detail and will lead to a better understanding and appreciation of glutathione degradation. Antioxid. Redox Signal. 27, 1200-1216.

Ophthalmic acid accumulation in an Escherichia coli mutant lacking the conserved pyridoxal 5'-phosphate-binding protein YggS

Escherichia coli YggS is a highly conserved pyridoxal 5'-phosphate (PLP)-binding protein whose biochemical function is currently unknown. A previous study with a yggS-deficient E. coli strain (ΔyggS) demonstrated that YggS controls l-Ile- and l-Val-metabolism by modulating 2-ketobutyrate (2-KB), l-2-aminobutyrate (l-2-AB), and/or coenzyme A (CoA) availability in a PLP-dependent fashion. In this study, we found that ΔyggS accumulates an unknown metabolite as judged by amino acid analyses. LC/MS and MS/MS analyses of the compound with propyl chloroformate derivatization, and co-chromatography analysis identified this compound as γ-l-glutamyl-l-2-aminobutyryl-glycine (ophthalmic acid), a glutathione (GSH) analogue in which the l-Cys moiety is replaced by l-2-AB. We also determine the metabolic consequence of the yggS mutation. Absence of YggS initially increases l-2-AB availability, and then causes ophthalmic acid accumulation and CoA limitation in the cell. The expression of a γ-glutamylcysteine synthetase and a glutathione synthetase in a ΔyggS background causes high-level accumulation of ophthalmic acid in the cells (∼1.2 nmol/mg cells) in a minimal synthetic medium. This opens the possibility of a first fermentative production of ophthalmic acid.

Characterization of bifunctional L-glutathione synthetases from Actinobacillus pleuropneumoniae and Actinobacillus succinogenes for efficient glutathione biosynthesis

Glutathione (GSH), an important bioactive substance, is widely applied in pharmaceutical and food industries. In this work, two bifunctional L-glutathione synthetases (GshF) from Actinobacillus pleuropneumoniae (GshFAp) and Actinobacillus succinogenes (GshFAs) were successfully expressed in Escherichia coli BL-21(DE3). Similar to the GshF from Streptococcus thermophilus (GshFSt), GshFAp and GshFAs can be applied for high titer GSH production because they are less sensitive to end-product inhibition (Ki values 33 and 43 mM, respectively). The active catalytic forms of GshFAs and GshFAp are dimers, consistent with those of GshFPm (GshF from Pasteurella multocida) and GshFSa (GshF from Streptococcus agalactiae), but are different from GshFSt (GshF from S. thermophilus) which is an active monomer. The analysis of the protein sequences and three dimensional structures of GshFs suggested that the binding sites of GshFs for substrates, L-cysteine, L-glutamate, γ-glutamylcysteine, adenosine-triphosphate, and glycine are highly conserved with only very few differences. With sufficient supply of the precursors, the recombinant strains BL-21(DE3)/pET28a-gshFas and BL-21(DE3)/pET28a-gshFap were able to produce 36.6 and 34.1 mM GSH, with the molar yield of 0.92 and 0.85 mol/mol, respectively, based on the added L-cysteine. The results showed that GshFAp and GshFAs are potentially good candidates for industrial GSH production.

Role and Regulation of Glutathione Metabolism in Plasmodium falciparum

Malaria in humans is caused by one of five species of obligate intracellular protozoan parasites of the genus Plasmodium. P. falciparum causes the most severe disease and is responsible for 600,000 deaths annually, primarily in Sub-Saharan Africa. It has long been suggested that during their development, malaria parasites are exposed to environmental and metabolic stresses. One strategy to drug discovery was to increase these stresses by interfering with the parasites’ antioxidant and redox systems, which may be a valuable approach to disease intervention. Plasmodium possesses two redox systems—the thioredoxin and the glutathione system—with overlapping but also distinct functions. Glutathione is the most abundant low molecular weight redox active thiol in the parasites existing primarily in its reduced form representing an excellent thiol redox buffer. This allows for an efficient maintenance of the intracellular reducing environment of the parasite cytoplasm and its organelles. This review will highlight the mechanisms that are responsible for sustaining an adequate concentration of glutathione and maintaining its redox state in Plasmodium. It will provide a summary of the functions of the tripeptide and will discuss the potential of glutathione metabolism for drug discovery against human malaria parasites.

Restricting glutamylcysteine synthetase activity to the cytosol or glutathione biosynthesis to the plastid is sufficient for normal plant development and stress tolerance

The tripeptide glutathione (GSH) is an important metabolite with a broad spectrum of functions, and its homeostasis is essential to maintain cellular redox poise and effective responses to stress in plants. In Arabidopsis GSH is synthesised in two successive enzymatic steps by γ-glutamylcysteine synthetase (GSH1), localised exclusively in plastids, forming the pathway intermediate γ-glutamylcysteine (γ-EC), and then by glutathione synthetase (GSH2), which is located in both plastids and cytosol. This suggests a mechanism for γ-EC export from the plastids and, because the majority of GSH2 transcripts (90%) encode the cytosolic isoform, it is speculated that the cytosol may be the main compartment for GSH biosynthesis. With the availability of knockout lethal mutants of GSH1 and GSH2 in Arabidopsis, we were able to manipulate the GSH biosynthetic pathway within cells through transgenic techniques. We successfully complemented the gsh1 and gsh2 null mutants with a cytosol-targeted bacterial EcGSHA and plastid-targeted Arabidopsis GSH2 protein, respectively, to wild-type phenotypes. These transgenics were little affected under heavy metal (cadmium) or oxidative stress (H2 O2 ) when compared to the wild type. Collectively, our data show that redirecting GSH1 activity exclusively to the cytosol or restricting GSH biosynthesis to the plastids has no significant impact on development or stress resistance, suggesting efficient exchange of γ-EC and GSH between the plastid and cytosol compartments within cells.

Probing the origins of glutathione biosynthesis through biochemical analysis of glutamate-cysteine ligase and glutathione synthetase from a model photosynthetic prokaryote

Glutathione biosynthesis catalysed by GCL (glutamate-cysteine ligase) and GS (glutathione synthetase) is essential for maintaining redox homoeostasis and protection against oxidative damage in diverse eukaroytes and bacteria. This biosynthetic pathway probably evolved in cyanobacteria with the advent of oxygenic photosynthesis, but the biochemical characteristics of progenitor GCLs and GSs in these organisms are largely unexplored. In the present study we examined SynGCL and SynGS from Synechocystis sp. PCC 6803 using steady-state kinetics. Although SynGCL shares ~15% sequence identity with the enzyme from plants and α-proteobacteria, sequence comparison suggests that these enzymes share similar active site residues. Biochemically, SynGCL lacks the redox regulation associated with the plant enzymes and functions as a monomeric protein, indicating that evolution of redox regulation occurred later in the green lineage. Site-directed mutagenesis of SynGCL establishes this enzyme as part of the plant-like GCL family and identifies a catalytically essential arginine residue, which is structurally conserved across all forms of GCLs, including those from non-plant eukaryotes and γ-proteobacteria. A reaction mechanism for the synthesis of γ-glutamylcysteine by GCLs is proposed. Biochemical and kinetic analysis of SynGS reveals that this enzyme shares properties with other prokaryotic GSs. Initial velocity and product inhibition studies used to examine the kinetic mechanism of SynGS suggest that it and other prokaryotic GSs uses a random ter-reactant mechanism for the synthesis of glutathione. The present study provides new insight on the molecular mechanisms and evolution of glutathione biosynthesis; a key process required for enhancing bioenergy production in photosynthetic organisms.

Redox-dependent stability of the γ-glutamylcysteine synthetase enzyme of Escherichia coli: a novel means of redox regulation

Glutathione is a thiol-containing tripeptide that plays important roles in redox-related processes. The first step in glutathione biosynthesis is catalysed by γ-GCS (γ-glutamylcysteine synthetase). The crystal structure of Escherichia coli γ-GCS has revealed the presence of a disulfide bond. As the disulfide-bonding cysteine residues Cys372 and Cys395 are not well conserved among γ-GCS enzymes in this lineage, we have initiated a biochemical genetic strategy to investigate the functional importance of these and other cysteine residues. In a cysteine-free γ-GCS that was non-functional, suppressor analysis yielded combinations of cysteine and aromatic residues at the position of the disulfide bond, and one mutant that lacked any cysteine residues. Kinetic analysis of the wild-type and mutant enzymes revealed that the disulfide bond was not involved in determining the affinity of the enzyme towards its substrate, but had an important role in determining the stability of the protein, and its catalytic efficiency. We show that in vivo the γ-GCS enzyme can also exist in a reduced form and that the mutants lacking the disulfide bond show a decreased half-life. These results demonstrate a novel means of regulation of γ-GCS by the redox environment that works by an alteration in its stability.

Characterization of a cold-adapted glutathione synthetase from the psychrophile Pseudoalteromonas haloplanktis

Glutathione (GSH) biosynthesis occurs through two ATP-dependent reactions, usually involving distinct enzymes; in the second step of this process, catalysed by glutathione synthetase (GshB), GSH is formed from γ-glutamylcysteine and glycine. A recombinant form of GshB from the cold-adapted source Pseudoalteromonas haloplanktis (rPhGshB) was purified and characterised. The enzyme formed a disulfide adduct with β-mercaptoethanol, when purified in the presence of this reducing agent. The homotetrameric form of rPhGshB observed at high protein concentration disassembled into two homodimers at low concentration. A new method for directly determining the rPhGshB activity was developed, based on [γ-(32)P]ATP hydrolysis coupled to the GSH synthesis. The ATPase activity required the presence of both γ-glutamylcysteine and glycine and its optimum was reached in the 7.4-8.6 pH range; a divalent cation was absolutely required for the activity, whereas monovalent cations were dispensable. rPhGshB was active at low temperatures and had a similar affinity for ATP (K(m) 0.26 mM) and γ-glutamylcysteine (K(m) 0.25 mM); a lower affinity was measured for glycine (K(m) 0.75 mM). The oxidised form of glutathione (GSSG) acted as an irreversible inhibitor of rPhGshB (K(i) 10.7 mM) and formed disulfide adducts with the enzyme. rPhGshB displayed a great temperature-dependent increase in its activity with an unusually high value of energy of activation (75 kJ mol(-1)) for a psychrophilic enzyme. The enzyme was moderately thermostable, its half inactivation temperature being 50.5 °C after 10 min exposure. The energy of activation of the heat inactivation process was 208 kJ mol(-1). To our knowledge, this is the first contribution to the characterization of a GshB from cold-adapted sources.

Dissecting the role of glutathione biosynthesis in Plasmodium falciparum

Glutathione (γ-glutamylcysteinyl-glycine, GSH) has vital functions as thiol redox buffer and cofactor of antioxidant and detoxification enzymes. Plasmodium falciparum possesses a functional GSH biosynthesis pathway and contains mM concentrations of the tripeptide. It was impossible to delete in P. falciparum the genes encoding γ-glutamylcysteine synthetase (γGCS) or glutathione synthetase (GS), the two enzymes synthesizing GSH, although both gene loci were not refractory to recombination. Our data show that the parasites cannot compensate for the loss of GSH biosynthesis via GSH uptake. This suggests an important if not essential function of GSH biosynthesis pathway for the parasites. Treatment with the irreversible inhibitor of γGCS L-buthionine sulfoximine (BSO) reduced intracellular GSH levels in P. falciparum and was lethal for their intra-erythrocytic development, corroborating the suggestion that GSH biosynthesis is important for parasite survival. Episomal expression of γgcs in P. falciparum increased tolerance to BSO attributable to increased levels of γGCS. Concomitantly expression of glutathione reductase was reduced leading to an increased GSH efflux. Together these data indicate that GSH levels are tightly regulated by a functional GSH biosynthesis and the reduction of GSSG.

Exploiting plants for glutathione (GSH) production: Uncoupling GSH synthesis from cellular controls results in unprecedented GSH accumulation

Glutathione (GSH) is a key factor for cellular redox homeostasis and tolerance against abiotic and biotic stress (May et al., 1998; Noctor et al., 1998a). Previous attempts to increase GSH content in plants have met with moderate success (Rennenberg et al., 2007), largely because of tight and multilevel control of its biosynthesis (Rausch et al., 2007). Here, we report the in planta expression of the bifunctional gamma-glutamylcysteine ligase-glutathione synthetase enzyme from Streptococcus thermophilus (StGCL-GS), which is shown to be neither redox-regulated nor sensitive to feedback inhibition by GSH. Transgenic tobacco plants expressing StGCL-GS under control of a constitutive promoter reveal an extreme accumulation of GSH in their leaves (up to 12 micromol GSH/gFW, depending on the developmental stage), which is more than 20- to 30-fold above the levels observed in wild-type (wt) plants and which can be even further increased by additional sulphate fertilization. Surprisingly, this dramatically increased GSH production has no impact on plant growth while enhancing plant tolerance to abiotic stress. Furthermore, StGCL-GS-expressing plants are a novel, cost-saving source for GSH production, being competitive with current yeast-based systems (Li et al., 2004).

Systematic engineering of phytochelatin synthesis and arsenic transport for enhanced arsenic accumulation in E. coli

Phytochelatin (PC) is a naturally occurring peptide with high affinity towards arsenic (As). In this article, we demonstrated the systematic engineering of PC-producing E. coli for As accumulation by addressing different bottlenecks in PC synthesis as well as As transport. Phytochelatin synthase from Schizosaccharomyces pombe (SpPCS) was expressed in E. coli resulting in 18 times higher As accumulation. PC production was further increased by co-expressing a feedback desensitized gamma-glutamylcysteine synthetase (GshI*), resulting in 30-fold higher PC levels and additional 2-fold higher As accumulation. The significantly increased PC levels were exploited further by co-expressing an arsenic transporter GlpF, leading to an additional 1.5-fold higher As accumulation. These engineering steps were finally combined in an arsenic efflux deletion E. coli strain to achieve an arsenic accumulation level of 16.8 micromol/g DCW, a 80-fold improvement when compared to a control strain not producing phytochelatins.

Early onset senescence occurs when fibroblasts lack the glutamate-cysteine ligase modifier subunit

Cellular senescence is the irreversible entry of cells into growth arrest. Senescence of primary cells in culture has long been used as an in vitro model for aging. Glutamate-cysteine ligase (GCL) controls the synthetic rate of the important cellular antioxidant glutathione (GSH). The catalytic subunit of GCL, GCLC, is catalytically active and essential for life. By contrast the modifier subunit of GCL, GCLM, is dispensable in mice. Although it is recognized that GCLM increases the rate of GSH synthesis, its physiological role is unclear. Herein, we show that loss of Gclm leads to premature senescence of primary murine fibroblasts as characterized by: (a) diminished growth rate, (b) cell morphology consistent with senescence, (c) increases in senescence-associated beta-galactosidase activity, and (d) cell cycle arrest at the G(1)/S and G(2)/M boundaries. These changes are accompanied by increased intracellular ROS, accumulation of DNA damage, and induction of p53 and p21 proteins. We also found that N-acetylcysteine increases intracellular GSH and prevents premature senescence in Gclm(-/-) cells. These results suggest that the control of GCLM, which in turn controls aspects of the cellular redox environment via GSH, is important in determining the replicative capacity of the cell.

Spirulina platensis protects against gentamicin-induced nephrotoxicity in rats

The present study aimed to investigate the protective effect of Spirulina platensis (SP) on gentamicin sulphate (GS)-induced changes in the levels of lipid peroxidation and endogenous antioxidants in the kidney of rats. Sprague-Dawley rats were treated in separate groups as follows for 7 consecutive days: control (C), gentamicin sulphate (100 mg/kg i.p.) (GS), Spirulina platensis (1000 mg/kg orally) (SP) and Spirulina platensis (1000 mg/kg orally) plus gentamicin sulphate (100 mg/kg i.p.) (SP + GS). The degree of protection was evaluated by determining the effects of Spirulina platensis on malondialdehyde (MDA), glutathione (GSH), superoxide dismutase (SOD), glutathione peroxidase (GPX) and nitric oxide (NO), and plasma creatinine and urea levels were estimated in kidney homogenates to evaluate antioxidant activity, and the kidney was histologically examined as well. Spirulina platensis elicited significant nephroprotective activity by decreasing lipid peroxidation (MDA) and elevated the levels of GSH, SOD, GPX, NO, creatinine and urea. Furthermore, these biochemical observations were supplemented by histological examination of the rat kidneys. In conclusion, the present study indicates a very important role of reactive oxygen species (ROS) and the relation to renal dysfunction and point to the therapeutic potential of Spirulina platensis in gentamicin sulphate induced nephrotoxicity.

Age-associated perturbations in glutathione synthesis in mouse liver

The nature of the mechanisms underlying the age-related decline in glutathione (GSH) synthetic capacity is at present unclear. Steady-state kinetic parameters of mouse liver GCL (glutamate-cysteine ligase), the rate-limiting enzyme in GSH synthesis, and levels of hepatic GSH synthesis precursors from the trans-sulfuration pathway, such as homocysteine, cystathionine and cysteine, were compared between young and old C57BL/6 mice (6- and 24-month-old respectively). There were no agerelated differences in GCL V(max), but the apparent K(m) for its substrates, cysteine and glutamate, was higher in the old mice compared with the young mice (approximately 800 compared with approximately 300 microM, and approximately 710 compared with 450 microM, P<0.05 for cysteine and glutamate in young and old mice respectively). Amounts of cysteine, cystathionine and Cys-Gly increased with age by 91, 24 and 28% respectively. Glutathione (GSH) levels remained unchanged with age, whereas GSSG content showed an 84% increase, suggesting a significant pro-oxidizing shift in the 2GSH/GSSG ratio. The amount of the toxic trans-sulfuration/glutathione biosynthetic pathway intermediate, homocysteine, was 154% higher (P<0.005) in the liver of old mice compared with young mice. The conversion of homocysteine into cystathionine, a rate-limiting step in trans-sulfuration catalysed by cystathionine beta-synthase, was comparatively less efficient in the old mice, as indicated by cystathionine/homocysteine ratios. Incubation of tissue homogenates with physiological concentrations of homocysteine caused an up to 4.4-fold increase in the apparent K(m) of GCL for its glutamate substrate, but had no effect on V(max). The results suggest that perturbation of the catalytic efficiency of GCL and accumulation of homocysteine from the trans-sulfuration pathway may adversely affect de novo GSH synthesis during aging.

Bacteria metabolically engineered for enhanced phytochelatin production and cadmium accumulation

Phytochelatins (PCs) with good binding affinities for a wide range of heavy metals were exploited to develop microbial sorbents for cadmium removal. PC synthase from Schizosaccharomyces pombe (SpPCS) was overexpressed in Escherichia coli, resulting in PC synthesis and 7.5-times-higher Cd accumulation. The coexpression of a variant gamma-glutamylcysteine synthetase desensitized to feedback inhibition (GshI) increased the supply of the PC precursor glutathione, resulting in further increases of 10- and 2-fold in PC production and Cd accumulation, respectively. A Cd transporter, MntA, was expressed with SpPCS and GshI to improve Cd uptake, resulting in a further 1.5-fold increase in Cd accumulation. The level of Cd accumulation in this recombinant E. coli strain (31.6 micromol/g [dry weight] of cells) was more than 25-fold higher than that in the control strain.

Fifty hertz extremely low-frequency electromagnetic field causes changes in redox and differentiative status in neuroblastoma cells

The current study was designed to establish whether extremely low-frequency electromagnetic fields might affect neuronal homeostasis through redox-sensitive mechanisms. To this end, intracellular reactive oxygen species production, antioxidant and glutathione-based detoxifying capability and genomic integrity after extremely low-frequency electromagnetic fields exposure were investigated. Moreover, we also studied potential extremely low-frequency electromagnetic fields-dependent changes in the proliferative and differentiative cellular status. Results seem to support redox-mediated extremely low-frequency electromagnetic fields effects on biological models as, although no major oxidative damage was detected, after exposure we observed a positive modulation of antioxidant enzymatic expression, as well as a significant increase in reduced glutathione level, indicating a shift of cellular environment towards a more reduced state. In addition, extremely low-frequency electromagnetic fields treatment induced a more differentiated phenotype as well as an increased expression in peroxisome proliferators-activated receptor isotype beta, a class of transcription factors related to neuronal differentiation and cellular stress response. As second point, to deepen how extremely low-frequency electromagnetic fields treatment could affect neuroblastoma cell antioxidant capacity, we examined the extremely low-frequency electromagnetic fields-dependent modifications of cell susceptibility to pro-oxidants. Results clearly showed that 50 Hz extremely low-frequency electromagnetic fields exposure reduces cell tolerance towards oxidative attacks.

Overexpression of caveolin-1 in a human melanoma cell line results in dispersion of ganglioside GD3 from lipid rafts and alteration of leading edges, leading to attenuation of malignant properties

Caveolin-1 is a component of lipid rafts, and is considered to be a tumor suppressor molecule. However, the mechanisms by which caveolin-1 functions in cancer cells are not well understood. We generated caveolin-1 transfectant cells (Cav-1(+) cells) using a human melanoma cell line (SK-MEL-28) and investigated the effects of caveolin-1 overexpression on the GD3-mediated malignant properties of melanomas. Cav-1(+) cells had decreased cell growth and motility, and reduced phosphorylation levels of p130Cas and paxillin relative to controls. In floatation analysis, although GD3 was mainly localized in glycolipid-enriched microdomain (GEM)/rafts in control cells, it was dispersed from GEM/rafts in Cav-1(+) cells. Correspondingly, GD3 in Cav-1(+) cells stained uniformly throughout the membrane, whereas control cells showed partial staining of the membrane, probably at the leading edge. p130Cas and paxillin were stained in the leading edges and colocalized with GD3 in the control cells. In contrast, these molecules were diffusely stained and no definite leading edges were detected in Cav-1(+) cells. These results suggest that caveolin-1 regulates GD3-mediated malignant signals by altering GD3 distribution and leading edge formation. These results reveal one of the mechanisms by which caveolin-1 curtails the malignant properties of tumor cells.

Mode of action of natural and synthetic drugs against Trypanosoma cruzi and their interaction with the mammalian host

Current knowledge of the biochemistry of Trypanosoma cruzi has led to the development of new drugs and the understanding of their mode of action. Some trypanocidal drugs such as nifurtimox and benznidazole act through free radical generation during their metabolism. T. cruzi is very susceptible to the cell damage induced by these metabolites because enzymes scavenging free radicals are absent or have very low activities in the parasite. Another potential target is the biosynthetic pathway of glutathione and trypanothione, the low molecular weight thiol found exclusively in trypanosomatids. These thiols scavenge free radicals and participate in the conjugation and detoxication of numerous drugs. Inhibition of this key pathway could render the parasite much more susceptible to the toxic action of drugs such as nifurtimox and benznidazole without affecting the host significantly. Other drugs such as allopurinol and purine analogs inhibit purine transport in T. cruzi, which cannot synthesize purines de novo. Nitroimidazole derivatives such as itraconazole inhibit sterol metabolism. The parasite's respiratory chain is another potential therapeutic target because of its many differences with the host enzyme complexes. The pharmacological modulation of the host's immune response against T. cruzi infection as a possible chemotherapeutic target is discussed. A large set of chemicals of plant origin and a few animal metabolites active against T. cruzi are enumerated and their likely modes of action are briefly discussed.

Novel substrate specificity of glutathione synthesis enzymes from Streptococcus agalactiae and Clostridium acetobutylicum

Glutathione (GSH) is synthesized by gamma-glutamylcysteine synthetase (gamma-GCS) and glutathione synthetase (GS) in living organisms. Recently, bifunctional fusion protein, termed gamma-GCS-GS catalyzing both gamma-GCS and GS reactions from gram-positive firmicutes Streptococcus agalactiae, has been reported. We revealed that in the gamma-GCS activity, S. agalactiae gamma-GCS-GS had different substrate specificities from those of Escherichia coli gamma-GCS. Furthermore, S. agalactiae gamma-GCS-GS synthesized several kinds of gamma-glutamyltripeptide, gamma-Glu-X(aa)-Gly, from free three amino acids. In Clostridium acetobutylicum, the genes encoding gamma-GCS and putative GS were found to be immediately adjacent by BLAST search, and had amino acid sequence homology with S. agalactiae gamma-GCS-GS, respectively. We confirmed that the proteins expressed from each gene showed gamma-GCS and GS activity, respectively. C. acetobutylicum GS had broad substrate specificities and synthesized several kinds of gamma-glutamyltripeptide, gamma-Glu-Cys-X(aa). Whereas the substrate specificities of gamma-GCS domain protein and GS domain protein of S. agalactiae gamma-GCS-GS were the same as those of S. agalactiae gamma-GCS-GS.

Structural Basis for the Redox Control of Plant Glutamate Cysteine Ligase

Glutathione (GSH) plays a crucial role in plant metabolism and stress response. The rate-limiting step in the biosynthesis of GSH is catalyzed by glutamate cysteine ligase (GCL) the activity of which is tightly regulated. The regulation of plant GCLs is poorly understood. The crystal structure of substrate-bound GCL from Brassica juncea at 2.1-A resolution reveals a plant-unique regulatory mechanism based on two intramolecular redox-sensitive disulfide bonds. Reduction of one disulfide bond allows a beta-hairpin motif to shield the active site of B. juncea GCL, thereby preventing the access of substrates. Reduction of the second disulfide bond reversibly controls dimer to monomer transition of B. juncea GCL that is associated with a significant inactivation of the enzyme. These regulatory events provide a molecular link between high GSH levels in the plant cell and associated down-regulation of its biosynthesis. Furthermore, known mutations in the Arabidopsis GCL gene affect residues in the close proximity of the active site and thus explain the decreased GSH levels in mutant plants. In particular, the mutation in rax1-1 plants causes impaired binding of cysteine.

Enzymic systems proposed to be involved in the dissimilatory reduction of selenite in the purple non-sulfur bacteria Rhodospirillum rubrum and Rhodobacter capsulatus

Various enzymic systems, such as nitrite reductase, sulfite reductase and glutathione reductase, have been proposed for, or suspected to be involved in, the reduction of selenite in bacteria. As alphaproteobacteria have been shown to be highly tolerant to transition metal oxyanions, it seemed interesting to investigate the hypothetical involvement of these different enzymes in the reduction of selenite in the purple non-sulfur bacteria Rhodospirillum rubrum and Rhodobacter capsulatus. The hypothetical involvement of nitrite reductase and sulfite reductase in the reduction of selenite in these bacteria was investigated by analysing the effects of nitrite and sulfite amendments on the growth and kinetics of selenite reduction. The reduction of selenite was not concomitant with that of either sulfite or nitrite in Rs. rubrum, suggesting that the reduction pathways operate independently. In Rb. capsulatus, strong interactions were observed between the nitrite reduction and selenite reduction pathways. However, in both organisms, selenite reduction took place during both the growth phase and the stationary phase, indicating that selenite metabolism is constitutively expressed. In contrast, neither nitrite nor sulfite was transformed during stationary phase, suggesting that the metabolism of both ions is induced, which implies that identical reduction pathways for selenite and nitrite or selenite and sulfite are excluded. Buthionine sulfoximine (BSO, S-n-butyl homocysteine sulfoximine), a specific inhibitor of glutathione synthesis, was used to depress the intracellular glutathione level. In stationary-phase cultures of both Rs. rubrum and Rb. capsulatus amended with BSO, the rate of reduction of selenite was slowed, indicating that glutathione may be involved in the dissimilatory reduction of selenite in these organisms. The analysis of the headspace gases of the cultures indicated that the synthesis of methylated selenium compounds was prevented in the presence of 3.0 mM BSO in both organisms, implying that glutathione is also involved in the transformation of selenite to volatile selenium compounds.

Characterization of the Bifunctional γ-Glutamate-cysteine Ligase/Glutathione Synthetase (GshF) of Pasteurella multocida

Glutamate-cysteine ligase (gamma-ECL) and glutathione synthetase (GS) are the two unrelated ligases that constitute the glutathione biosynthesis pathway in most eukaryotes, purple bacteria, and cyanobacteria. gamma-ECL is a member of the glutamine synthetase family, whereas GS enzymes group together with highly diverse carboxyl-to-amine/thiol ligases, all characterized by the so-called two-domain ATP-grasp fold. This generalized scheme toward the formation of glutathione, however, is incomplete, as functional steady-state levels of intracellular glutathione may also accumulate solely by import, as has been reported for the Pasteurellaceae member Haemophilus influenzae, as well as for certain Gram-positive enterococci and streptococci, or by the action of a bifunctional fusion protein (termed GshF), as has been reported recently for the Gram-positive firmicutes Streptococcus agalactiae and Listeria monocytogenes. Here, we show that yet another member of the Pasteurellaceae family, Pasteurella multocida, acquires glutathione both by import and GshF-driven biosynthesis. Domain architecture analysis shows that this P. multocida GshF bifunctional ligase contains an N-terminal gamma-proteobacterial gamma-ECL-like domain followed by a typical ATP-grasp domain, which most closely resembles that of cyanophycin synthetases, although it has no significant homology with known GS ligases. Recombinant P. multocida GshF overexpresses as an approximately 85-kDa protein, which, on the basis of gel-sizing chromatography, forms dimers in solution. The gamma-ECL activity of GshF is regulated by an allosteric type of glutathione feedback inhibition (K(i) = 13.6 mM). Furthermore, steady-state kinetics, on the basis of which we present a novel variant of half-of-the-sites reactivity, indicate intimate domain-domain interactions, which may explain the bifunctionality of GshF proteins.

The influence of cell growth media on the stability and antitumour activity of methionine enkephalin

Studies with cultured tumour cell lines are widely used in vitro to evaluate peptide-induced cytotoxicity as well as molecular and biochemical interactions. The objectives of this study were to investigate the influence of the cell culture medium on peptide metabolic stability and in vitro antitumour activity. The degradation kinetics of the model peptide methionine enkephalin (Met-E, Tyr-Gly-Gly-Phe-Met), demonstrated recently to play an important role in the rate of proliferation of tumour cells in vitro and in vivo, were investigated in cell culture systems containing different amounts of fetal bovine serum (FBS). The influence of enzyme inhibitors (bestatin, captopril, thiorphan) on the Met-E degradation was also investigated. The results obtained in the Dulbecco's modified Eagle medium containing 10% FBS indicated a rapid degradation of Met-E (t(1/2) = 2.8 h). Preincubation of the medium with a mixture of peptidase inhibitors reduced the hydrolysis of Met-E, as shown by the increased half-life to 10 h. The in vitro activity of Met-E against poorly differentiated cells from lymph node metastasis of colon carcinoma (SW620) and human larynx carcinoma (HEp-2) cells was determined. Tumour cells were grown for 3 weeks prior to the experiment in a medium supplemented with 10%, 5% or 2% FBS. Statistically significant to mild or no suppression of cell proliferation was observed in all cultures. In both cell lines, a significant suppression of cell growth by a combination of peptidase inhibitors and Met-E, compared with cells exposed to the peptide alone and cells grown in the absence of Met-E, was observed. This study indicated that caution must be exercised in interpreting the antiproliferative effects of peptide compounds in conventional drug-response assays.

Sulfur assimilation and glutathione metabolism under cadmium stress in yeast, protists and plants

Glutathione (gamma-glu-cys-gly; GSH) is usually present at high concentrations in most living cells, being the major reservoir of non-protein reduced sulfur. Because of its unique redox and nucleophilic properties, GSH serves in bio-reductive reactions as an important line of defense against reactive oxygen species, xenobiotics and heavy metals. GSH is synthesized from its constituent amino acids by two ATP-dependent reactions catalyzed by gamma-glutamylcysteine synthetase and glutathione synthetase. In yeast, these enzymes are found in the cytosol, whereas in plants they are located in the cytosol and chloroplast. In protists, their location is not well established. In turn, the sulfur assimilation pathway, which leads to cysteine biosynthesis, involves high and low affinity sulfate transporters, and the enzymes ATP sulfurylase, APS kinase, PAPS reductase or APS reductase, sulfite reductase, serine acetyl transferase, O-acetylserine/O-acetylhomoserine sulfhydrylase and, in some organisms, also cystathionine beta-synthase and cystathionine gamma-lyase. The biochemical and genetic regulation of these pathways is affected by oxidative stress, sulfur deficiency and heavy metal exposure. Cells cope with heavy metal stress using different mechanisms, such as complexation and compartmentation. One of these mechanisms in some yeast, plants and protists is the enhanced synthesis of the heavy metal-chelating molecules GSH and phytochelatins, which are formed from GSH by phytochelatin synthase (PCS) in a heavy metal-dependent reaction; Cd(2+) is the most potent activator of PCS. In this work, we review the biochemical and genetic mechanisms involved in the regulation of sulfate assimilation-reduction and GSH metabolism when yeast, plants and protists are challenged by Cd(2+).

Is insulin resistance caused by defects in insulin's target cells or by a stressed mind?

The importance of understanding insulin action is emphasized by the increasing prevalence of insulin resistance in various populations and by the fact that it plays an important pathophysiological role in many common disorders, for example, diabetes, obesity, hypertension and dyslipidemia. The primary factors responsible for the development of insulin resistance are so far unknown, although both genetic and environmental factors are involved. The genetic defects responsible for the common forms of insulin resistance, for example, in type 2 diabetes, are largely unidentified. Some studies from our group as well as by other investigators suggest that cellular insulin resistance is reversible and that it may be secondary to factors in the in vivo environment. These may include insulin-antagonistic action of hormones like catecholamines, glucocorticoids, sex steroids and adipokines as well as dysregulation of autonomic nervous activity and they could contribute to the early development of insulin resistance. Some of these factors can directly impair glucose uptake capacity and this might be due to alterations in key proteins involved in insulin's intracellular signaling pathways. This article briefly summarizes proposed mechanisms behind cellular and whole-body insulin resistance. In particular, we question the role of intrinsic defects in insulin's target cells as primary mechanisms in the development of insulin resistance in type 2 diabetes and we suggest that metabolic and neurohormonal factors instead are the main culprits.

Enzyme inhibitors as chemical tools to study enzyme catalysis: rational design, synthesis, and applications

Carefully designed molecules that are intimately related to the reaction mechanism of enzymes are often highly selective and potent inhibitors that serve as extremely useful chemical probes for understanding the reaction mechanism and structure of enzymes. This article describes the design, synthesis, and applications of specific inhibitors of two mechanistically distinct groups of enzymes, ATP-dependent amide ligases and Ser- and Thr-hydrolases. Our strategy is based on the premise that stable analogues of the transition state (transition-state analogues) are highly potent inhibitors that serve as good mechanistic probes, and that a key structure of a good inhibitor of one enzyme is also utilized for the inhibitors of other enzymes that share the same chemistry in their catalyzed reactions, irrespective of the degree of structural similarity and evolutionary link between the enzymes. According to these principles, we designed and synthesized a series of phosphinate- and sulfoximine-based transition-state analogue inhibitors of glutathione synthetase, gamma-glutamylcysteine synthetase and asparagine synthetase. For the second group of enzymes, we synthesized a gamma-monofluorophosphono glutamate analogue for mechanism-based affinity labeling of gamma-glutamyltranspeptidase and fluorescent phosphonic acid esters for the active-site titration of lipase. These inhibitors were used successfully as ligands for detailed kinetic analyses, X-ray crystallography, and mass analysis of the enzymes to identify the key amino acid residues responsible for catalysis and substrate recognition in the transition state.

Glutathione Synthesis in Streptococcus agalactiae

Gamma-glutamylcysteine synthetase (gamma-GCS) and glutathione synthetase (GS), distinct enzymes that together account for glutathione (GSH) synthesis, have been isolated and characterized from several Gram-negative prokaryotes and from numerous eukaryotes including mammals, amphibians, plants, yeast, and protozoa. Glutathione synthesis is relatively uncommon among the Gram-positive bacteria, and, to date, neither the genes nor the proteins involved have been identified. In the present report, we show that crude extracts of Streptococcus agalactiae catalyze the gamma-GCS and GS reactions and can synthesize GSH from its constituent amino acids. The putative gene for S. agalactiae gamma-GCS was identified and cloned, and the corresponding protein was expressed and purified. Surprisingly, it was found that the isolated enzyme catalyzes both the ATP-dependent synthesis of L-gamma-glutamyl-L-cysteine from L-glutamate and L-cysteine and the ATP-dependent synthesis of GSH from L-gamma-glutamyl-L-cysteine and glycine. This novel bifunctional enzyme, referred to as gamma-GCS-GS, has been characterized in terms of catalytic activity, substrate specificity, and inhibition by GSH, cystamine, and transition state analog sulfoximines. The N-terminal 518 amino acids of gamma-GCS-GS (total M(r) 85,000) show 32% identity and 43% similarity with E. coli gamma-GCS (M(r) 58,000), but the C-terminal putative GS domain (remaining 202 amino acids) of gamma-GCS-GS shows no significant homology with known GS sequences. The C terminus (360 amino acids) is, however, homologous to D-Ala, D-Ala ligase (24% identity; 38% similarity), an enzyme having the same protein fold as known GS proteins. These results are discussed in terms of the evolution of GSH synthesis and the possible occurrence of a similar bifunctional GSH synthesis enzyme in other bacterial species.

Kinetic characteristics of native gamma-glutamylcysteine ligase in the aging housefly, Musca domestica L

The catalytic activity of gamma-glutamylcysteine ligase (gamma-GCL; EC 6.3.2.2) was compared between relatively young (4-day-old) and old (19-day-old) houseflies (Musca domestica) in order to understand the mechanism of putative deterioration of glutathione homeostasis during the aging process. Hanes-Woolf analyses ([S]/v vs [S]) indicated that gamma-GCL had significantly higher affinities for its substrates in the young than in the old flies. The K(m) values in the young and old flies were, respectively, for glutamate 0.6 and 5.5 mM; for cysteine 0.3 and 4.6 mM; and for ATP 1.2 and 2.9 mM. Furthermore, young but not old flies exhibited substrate-dependent inhibition of gamma-GCL activity at >5 mM cysteine indicating a loss of metabolic regulation during aging. The age-associated differences in the affinity of native gamma-GCL towards its substrates suggest that de novo synthesis of glutathione would be relatively less efficient in the old houseflies.

Function and therapeutic potential of host defence peptides

Cationic host defence (antimicrobial) peptides are an important component of the innate immune systems of a wide variety of plants, animals, and bacteria. Although most of these compounds have direct antimicrobial activities under specific conditions, a greater appreciation for the diversity of functions of these molecules is beginning to develop in the field. In addition to their directly antimicrobial activities, they also have a broad spectrum of activity on the host immune system, with both pro-inflammatory and anti-inflammatory effects being invoked. Increasingly sophisticated approaches to understand the role of host defence peptides in modulating innate immunity are already serving to guide the development of novel therapeutics.

Crystal structure of gamma-glutamylcysteine synthetase: insights into the mechanism of catalysis by a key enzyme for glutathione homeostasis

Gamma-glutamylcysteine synthetase (gammaGCS), a rate-limiting enzyme in glutathione biosynthesis, plays a central role in glutathione homeostasis and is a target for development of potential therapeutic agents against parasites and cancer. We have determined the crystal structures of Escherichia coli gammaGCS unliganded and complexed with a sulfoximine-based transition-state analog inhibitor at resolutions of 2.5 and 2.1 A, respectively. In the crystal structure of the complex, the bound inhibitor is phosphorylated at the sulfoximido nitrogen and is coordinated to three Mg2+ ions. The cysteine-binding site was identified; it is formed inductively at the transition state. In the unliganded structure, an open space exists around the representative cysteine-binding site and is probably responsible for the competitive binding of glutathione. Upon inhibitor binding, the side chains of Tyr-241 and Tyr-300 turn, forming a hydrogen-bonding triad with the carboxyl group of the inhibitor's cysteine moiety, allowing this moiety to fit tightly into the cysteine-binding site with concomitant accommodation of its side chain into a shallow pocket. This movement is caused by a conformational change of a switch loop (residues 240-249). Based on this crystal structure, the cysteine-binding sites of mammalian and parasitic gammaGCSs were predicted by multiple sequence alignment, although no significant sequence identity exists between the E. coli gammaGCS and its eukaryotic homologues. The identification of this cysteine-binding site provides important information for the rational design of novel gammaGCS inhibitors.

Bacterial acetone carboxylase is a manganese-dependent metalloenzyme

Bacterial acetone carboxylase catalyzes the ATP-dependent carboxylation of acetone to acetoacetate with the concomitant production of AMP and two inorganic phosphates. The importance of manganese in Rhodobacter capsulatus acetone carboxylase has been established through a combination of physiological, biochemical, and spectroscopic studies. Depletion of manganese from the R. capsulatus growth medium resulted in inhibition of acetone-dependent but not malate-dependent cell growth. Under normal growth conditions (0.5 microm Mn2+ in medium), growth with acetone as the carbon source resulted in a 4-fold increase in intracellular protein-bound manganese over malate-grown cells and the appearance of a Mn2+ EPR signal centered at g = 2 that was absent in malate-grown cells. Acetone carboxylase purified from cells grown with 50 microm Mn2+ had a 1.6-fold higher specific activity and 1.9-fold higher manganese content than cells grown with 0.5 microm Mn2+, consistently yielding a stoichiometry of 1.9 manganese/alpha2beta2gamma2 multimer, or 0.95 manganese/alphabetagamma protomer. Manganese in acetone carboxylase was tightly bound and not removed upon dialysis against various metal ion chelators. The addition of acetone to malate-grown cells grown in medium depleted of manganese resulted in the high level synthesis of acetone carboxylase (15-20% soluble protein), which, upon purification, exhibited 7% of the activity and 6% of the manganese content of the enzyme purified from acetone-grown cells. EPR analysis of purified acetone carboxylase indicates the presence of a mononuclear Mn2+ center, with possible spin coupling of two mononuclear sites. The addition of Mg.ATP or Mg.AMP resulted in EPR spectral changes, whereas the addition of acetone, CO2, inorganic phosphate, and acetoacetate did not perturb the EPR. These studies demonstrate that manganese is essential for acetone carboxylation and suggest a role for manganese in nucleotide binding and activation.

Arabidopsis thaliana glutamate-cysteine ligase: functional properties, kinetic mechanism, and regulation of activity

In plants, glutathione accumulates in response to different stress stimuli as a protective mechanism, but only limited biochemical information is available on the plant enzymes that synthesize glutathione. Glutamatecysteine ligase (GCL) catalyzes the first step in glutathione biosynthesis and plays an important role in regulating the intracellular redox environment. Because the putative Arabidopsis thaliana GCL (AtGCL) displays no significant homology to the GCL from bacteria and other eukaryotes, the identity of this protein as a GCL has been debated. We have purified AtGCL from an Escherichia coli expression system and demonstrated that the recombinant enzyme catalyzes the ATP-dependent formation of gamma-glutamylcysteine from glutamate (Km = 9.1 mm) and cysteine (Km = 2.7 mm). Glutathione feedback inhibits AtGCL (Ki approximately 1.0 mm). As with other GCL, buthionine sulfoximine and cystamine inactivate the Arabidopsis enzyme but with inactivation rates much slower than those of the mammalian, bacterial, and nematode enzymes. The slower inactivation rates observed with AtGCL suggest that the active site differs structurally from that of other GCL. Global fitting analysis of initial velocity data indicates that a random terreactant mechanism with a preferred binding order best describes the kinetic mechanism of AtGCL. Unlike the mammalian GCL, which consists of a catalytic subunit and a regulatory subunit, AtGCL functions and is regulated as a monomeric protein. In response to redox environment, AtGCL undergoes a reversible conformational change that modulates the enzymatic activity of the monomer. These results explain the reported posttranslational change in AtGCL activity in response to oxidative stress.

Metabolic pathway engineering to enhance aerobic degradation of chlorinated ethenes and to reduce their toxicity by cloning a novel glutathione S-transferase, an evolved toluene o-monooxygenase, and gamma-glutamylcysteine synthetase

Aerobic, co-metabolic bioremediation of trichloroethylene (TCE), cis-1,2-dichloroethylene (cis-DCE) and other chlorinated ethenes with monooxygenase-expressing microorganisms is limited by the toxic epoxides produced as intermediates. A recombinant Escherichia coli strain less sensitive to the toxic effects of cis-DCE, TCE and trans-1,2-dichloroethylene (trans-DCE) degradation has been created by engineering a novel pathway consisting of eight genes including a DNA-shuffled toluene ortho-monooxygenase from Burkholderia cepacia G4 (TOM-Green), a newly discovered glutathione S-transferase (GST) from RhodococcusAD45 (IsoILR1), found to have activity towards epoxypropane and cis-DCE epoxide, and an overexpressed E. coli mutant gamma-glutamylcysteine synthetase (GSHI*). Along with IsoILR1, another new RhodococcusAD45 GST, IsoILR2, was cloned that lacks activity towards cis-DCE epoxide and differs from IsoILR1 by nine amino acids. The recombinant strain in which TOM-Green and IsoILR1 were co-expressed on separate plasmids degraded 1.9-fold more cis-DCE compared with a strain that lacked IsoILR1. In the presence of IsoILR1 and TOM-Green, the addition of GSH1* resulted in a sevenfold increase in the intracellular GSH concentration and a 3.5-fold improvement in the cis-DCE degradation rate based on chloride released (2.1 +/- 0.1 versus 0.6 +/- 0.1 nmol min(-1) mg(-1) protein at 540 microM), a 1.8-fold improvement in the trans-DCE degradation rate (1.29 +/- 0.03 versus 0.71 +/- 0.04 nmol x min(-1) mg(-1) protein at 345 microM) and a 1.7-fold improvement in the TCE degradation rate (6.8 +/- 0.24 versus 4.1 +/- 0.16 nmol x min(-1) mg(-1) protein at 339 microM). For cis-DCE degradation with TOM-Green (based on substrate depletion), V(max) was 27 nmol x min(-1) mg(-1) protein with both IsoILR1 and GSHI* expressed compared with V(max) = 10 nmol x min(-1) mg(-1) protein for the GST(-)GSHI*(-) strain. In addition, cells expressing IsoILR1 and GSHI* grew 78% faster in rich medium than a strain lacking these two heterologous genes.

Determination of enzyme/substrate specificity constants using a multiple substrate ESI-MS assay

The traditional method used to investigate the reaction specificity of an enzyme with different substrates is to perform individual kinetic measurements. In this case, a series of varied concentrations are required to study each substrate and a non-regression analysis program is used several times to obtain all the specificity constants for comparison. To avoid the large amount of experimental materials, long analysis time, and redundant data processing procedures involved in the traditional method, we have developed a novel strategy for rapid determination of enzyme substrate specificity using one reaction system containing multiple competing substrates. In this multiplex assay method, the electrospray ionization mass spectrometry (ESI-MS) technique was used for simultaneous quantification of multiple products and a steady-state kinetics model was established for efficient specificity constant calculation. The system investigated was the bacterial sulfotransferase NodH (NodST), which is a host specific nod gene product that catalyzes the sulfate group transfer from 3'-phosphoadenosine 5'-phosphosulfate (PAPS) to natural Nod factors or synthetic chitooligosaccharides. Herein, the reaction specificity of NodST for four chitooligosaccharide acceptor substrates of different chain length (chitobiose, chitotriose, chitotetraose, and chitopentaose) was determined by both individual kinetic measurements and the new multiplex ESI-MS assay. The results obtained from the two methods were compared and found to be consistent. The multiplex ESI-MS assay is an accurate and valid method for substrate specificity evaluation, in which multiple substrates can be evaluated in one assay.

Scavenging system efficiency is crucial for cell resistance to ROS-mediated methylglyoxal injury

Methylglyoxal is a reactive dicarbonyl compound endogenously produced mainly from glycolytic intermediates. Recent research indicates that methylglyoxal is a potent growth inhibitor and genotoxic agent. The antiproliferative activity of methylglyoxal has been investigated for pharmacological application in cancer chemotherapy. However, various cells are not equally sensitive to methylglyoxal toxicity. Therefore, it would be important to establish the cellular factors responsible for the different cell-type specific response to methylglyoxal injury, in order to avoid the risk of failure of a therapy based on increasing the intracellular level of methylglyoxal. To this purpose, we comparatively evaluated the signaling transduction pathway elicited by methylglyoxal in human glioblastoma (ADF) and neuroblastoma (SH-SY 5Y) cells. Results show that methylglyoxal causes early and extensive reactive oxygen species generation in both cell lines. However, SH-SY 5Y cells show higher sensitivity to methylglyoxal challenge due to a defective antioxidant and detoxifying ability that, preventing these cells from an efficient scavenging action, elicits extensive caspase-9 dependent apoptosis. These data emphasize the pivotal role of antioxidant and detoxifying systems in determining the grade of sensitivity of cells to methylglyoxal.

Antioxidant effect of hydroxytyrosol (DPE) and Mn2+ in liver of cadmium-intoxicated rats

Liver TBARS formation in cadmium-intoxicated rats was completely reduced by administering a low amount of MnCl(2) (2 mg/kg b.w.) 1 h before intoxication. A similar antioxidant effect was first shown by hydroxytyrosol (2-(3,4-dihydroxyphenyl)ethanol, (DPE), a phenolic compound present in olive oil, given twice to rats (9 mg/kg b.w.) after cadmium administration. The antioxidant properties shown in vivo by both Mn(2+) and DPE were also active in vitro when rat liver microsomes were subjected to lipid peroxidation by cadmium or other prooxidant systems. The increase in liver glutathione concentrations occurring in cadmium-intoxicated rats, was also found, for the first time, 24 h after MnCl(2) administration. Unlike cadmium intoxication, which caused a higher formation of both glutathione and TBARS, Mn(2+) induced glutathione synthesis without any TBARS formation. The same situation was also observed when cadmium plus Mn(2+) or cadmium plus DPE was given to rats. Our data show that: (a). both DPE and low Mn(2+) concentrations may have an antioxidant effect in the livers of cadmium-intoxicated rats and (b). Mn(2+), like cadmium, induces liver glutathione synthesis and this effect is probably independent of TBARS formation.

Determination of glutamate-cysteine ligase (gamma-glutamylcysteine synthetase) activity by high-performance liquid chromatography and electrochemical detection

The tripeptide glutathione (gamma-glutamylcysteinylglycine; GSH) is the predominant low molecular mass thiol in cells. The function of GSH is of considerable interest, with the molecule being implicated in numerous cellular processes in addition to being a major cellular antioxidant. The enzyme glutamate-cysteine ligase (GCL) is the rate-limiting step in GSH synthesis. The GCL assay described here is based on high-performance liquid chromatography and exploits the electrochemically active nature of gamma-glutamylcysteine (gamma-GC), the product of GCL activity. This method allows for the direct detection of gamma-GC rather than relying on derivatization of the molecule or linked assays. The sensitivity of the assay is sufficient to allow for the measurement of GCL activity in cultured cells. The specific activity of GCL in rat primary culture astrocytes was 9.7 +/- 1.7 nmol gamma-GC synthesized/min/mg protein.