Purification and characterization of gamma-glutamylcysteine synthetase from Ascaris suum

(Homo)glutathione deficiency impairs root-knot nematode development in Medicago truncatula

Root-knot nematodes (RKN) are obligatory plant parasitic worms that establish and maintain an intimate relationship with their host plants. During a compatible interaction, RKN induce the redifferentiation of root cells into multinucleate and hypertrophied giant cells essential for nematode growth and reproduction. These metabolically active feeding cells constitute the exclusive source of nutrients for the nematode. Detailed analysis of glutathione (GSH) and homoglutathione (hGSH) metabolism demonstrated the importance of these compounds for the success of nematode infection in Medicago truncatula. We reported quantification of GSH and hGSH and gene expression analysis showing that (h)GSH metabolism in neoformed gall organs differs from that in uninfected roots. Depletion of (h)GSH content impaired nematode egg mass formation and modified the sex ratio. In addition, gene expression and metabolomic analyses showed a substantial modification of starch and γ-aminobutyrate metabolism and of malate and glucose content in (h)GSH-depleted galls. Interestingly, these modifications did not occur in (h)GSH-depleted roots. These various results suggest that (h)GSH have a key role in the regulation of giant cell metabolism. The discovery of these specific plant regulatory elements could lead to the development of new pest management strategies against nematodes.

General redox environment and carotid body chemoreceptor function

Carotid body (CB) chemoreceptor cells detect physiological levels of hypoxia and generate a hyperventilation, homeostatic in nature, aimed to minimize the deleterious effects of hypoxia. Intimate mechanisms involved in oxygen sensing in chemoreceptor cells remain largely unknown, but reactive oxygen species (ROS) had been proposed as mediators of this process. We have determined glutathione levels and calculated glutathione redox potential (E(GSH); indicator of the general redox environment of cells) in rat diaphragms incubated in the presence of oxidizing agents of two types: nonpermeating and permeating through cell membranes; in the latter group, unspecific oxidants and inhibitors of ROS-disposing enzymes were used. Selected concentrations of oxidizing agents were tested for their ability to modify the normoxic and hypoxic activity of chemoreceptor cells measured in vitro as their rate of release of neurotransmitters. Results evidence variable relationships between E(GSH) and the activity of chemoreceptor cells. The independence of chemoreceptor cell activity from the E(GSH) would imply that the ability of the CB to play its homeostatic role is largely preserved in any pathological or toxicological contingency causing oxidative stress. Consistent with this suggestion, it was also found that CB-mediated hypoxic hyperventilation was not altered by treatment of intact animals with agents that markedly decreased the E(GSH) in all tissues assayed.

Gamma-glutamylcysteine synthetase-glutathione synthetase: domain structure and identification of residues important in substrate and glutathione binding

In most organisms, glutathione (GSH) is synthesized by the sequential action of distinct enzymes, gamma-glutamylcysteine synthetase (gamma-GCS) and GSH synthetase (GS). In Streptococcus agalactiae, GSH synthesis is catalyzed by a single enzyme, gamma-glutamylcysteine synthetase-glutathione synthetase (gamma-GCS-GS). The N-terminal sequence of gamma-GCS-GS is similar to Escherichia coli gamma-GCS, but the C-terminal sequence is an ATP-grasp domain more similar to d-Ala, d-Ala ligase than to any known GS. In the present studies, C-terminally and N-terminally truncated constructs were characterized in order to define the limits of the gamma-GCS and GS domains, respectively. Although WT gamma-GCS-GS is nearly uninhibited by GSH (K(i) approximately 140 mM), shorter gamma-GCS domain constructs were unexpectedly found to be strongly inhibited (K(i) approximately 15 mM), reproducing a physiologically important regulation seen in monofunctional gamma-GCS enzymes. Because studies with E. coli gamma-GCS implicate a flexible loop region in GSH binding, chimeras of S. agalactiae gamma-GCS-GS were made containing gamma-GCS domain flexible loop sequences from Enterococcus faecalis and Pasteurella multocida gamma-GCS-GS, isoforms that are inhibited by GSH. Inhibition remained S. agalactiae-like (i.e., very weak). C-Terminal constructs of gamma-GCS-GS have GS activity (0.01-0.04% of WT), but proper folding and significant GS activity required a covalently linked gamma-GCS domain. In addition, site-directed mutants in the middle region of the gamma-GCS-GS sequence established that GS activity depends on residues in a region that is also part of the gamma-GCS domain. Our results provide new insights into the structure of gamma-GCS-GS and suggest gamma-GCS-GS evolved from a monomeric gamma-GCS that became C-terminally fused to a multimeric ATP-grasp protein.

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.

Glutathione, Altruistic Metabolite in Fungi

Glutathione (GSH; gamma-L-glutamyl-L-cysteinyl-glycine), a non-protein thiol with a very low redox potential (E'0 = 240 mV for thiol-disulfide exchange), is present in high concentration up to 10 mM in yeasts and filamentous fungi. GSH is concerned with basic cellular functions as well as the maintenance of mitochondrial structure, membrane integrity, and in cell differentiation and development. GSH plays key roles in the response to several stress situations in fungi. For example, GSH is an important antioxidant molecule, which reacts non-enzymatically with a series of reactive oxygen species. In addition, the response to oxidative stress also involves GSH biosynthesis enzymes, NADPH-dependent GSH-regenerating reductase, glutathione S-transferase along with peroxide-eliminating glutathione peroxidase and glutaredoxins. Some components of the GSH-dependent antioxidative defence system confer resistance against heat shock and osmotic stress. Formation of protein-SSG mixed disulfides results in protection against desiccation-induced oxidative injuries in lichens. Intracellular GSH and GSH-derived phytochelatins hinder the progression of heavy metal-initiated cell injuries by chelating and sequestering the metal ions themselves and/or by eliminating reactive oxygen species. In fungi, GSH is mobilized to ensure cellular maintenance under sulfur or nitrogen starvation. Moreover, adaptation to carbon deprivation stress results in an increased tolerance to oxidative stress, which involves the induction of GSH-dependent elements of the antioxidant defence system. GSH-dependent detoxification processes concern the elimination of toxic endogenous metabolites, such as excess formaldehyde produced during the growth of the methylotrophic yeasts, by formaldehyde dehydrogenase and methylglyoxal, a by-product of glycolysis, by the glyoxalase pathway. Detoxification of xenobiotics, such as halogenated aromatic and alkylating agents, relies on glutathione S-transferases. In yeast, these enzymes may participate in the elimination of toxic intermediates that accumulate in stationary phase and/or act in a similar fashion as heat shock proteins. GSH S-conjugates may also form in a glutathione S-transferases-independent way, e.g. through chemical reaction between GSH and the antifugal agent Thiram. GSH-dependent detoxification of penicillin side-chain precursors was shown in Penicillium sp. GSH controls aging and autolysis in several fungal species, and possesses an anti-apoptotic feature.

Drosophila melanogaster glutamate-cysteine ligase activity is regulated by a modifier subunit with a mechanism of action similar to that of the mammalian form

Glutamate-cysteine ligase (GCL) plays an important role in regulating glutathione homeostasis. In mammals, it comprises a catalytic (GCLC) and modifier (GCLM) subunit. The existence of a modifier subunit in invertebrates has not been described to date. We now demonstrate that GCL from Drosophila melanogaster has a functional modifier subunit (DmGCLM). A putative DmGCLM was obtained as an expressed sequence tag with 27% identity to human GCLM at the amino acid level. D. melanogaster GCLC (DmGCLC) and the candidate DmGCLM were expressed separately in Escherichia coli, purified, mixed, and then subjected to gel filtration, where they eluted as an approximately 140-kDa complex. DmGCLC co-immunoprecipitated with DmGCLM from S2 cell extracts, suggesting that they also associate in vivo. Enzyme kinetic analyses showed that DmGCLC has a K(m) for glutamate of 2.88 mm, but when complexed with DmGCLM, the K(m) for glutamate is 0.45 mm. Inhibition of DmGCLC activity by glutathione was found to be competitive with respect to glutamate (K(i) = 0.03 mm), whereas inhibition of the GCL complex was mixed (K(i) = 0.67 mm), suggesting allosteric effects. In accordance with this, DmGCLC and DmGCLM have the ability to form reversible intermolecular disulfide bridges. A further mechanism for control of D. melanogaster GCL was found to be induction of DmGCLC by tert-butylhydroquinone in S2 cells. DmGCLM levels were, however, unaffected by tert-butylhydroquinone.

The gamma-glutamylcysteine synthetase of Onchocerca volvulus

The tripeptide glutathione (GSH) plays an important role in the maintenance of the intracellular thiol redox state and in detoxification processes. The intracellular GSH level depends on glutathione reductase as well as on GSH synthesis. The first and rate limiting step in the synthetic pathway is catalysed by gamma-glutamylcysteine synthetase (gamma-GCS). The gamma-GCS was partially purified from the filarial parasite Onchocerca volvoulus and preliminary steady state kinetics were performed. The Ki-value for L-buthionine-S,R-sulphoximine (BSO), a specific inhibitor of gamma-GCS, was determined to be 0.13 microM, which is 54-fold lower than the Ki-value for the mammalian enzyme. Filarial gamma-GCS was also inhibited by cystamine with a Ki-value of 3.9 microM compared with 22.2 microM determined for the rat enzyme. Further, the cDNA and the gene of the O. volvulus gamma-GCS were cloned and sequenced. The gene of 5762 bp is composed of 14 exons and 13 introns. Southern blot analysis indicates that the gamma-GCS gene is present as a single-copy gene. In accordance with Northern blot analysis, the entire cDNA sequence encompasses 2377 bp. At its 5' end a nematode-specific spliced leader 130 bp upstream of the first in frame methionine was identified. The cDNA encodes a polypeptide of 652 amino acids with 50 and 69% sequence identity to the human and the Caenorhabditis elegans counterparts, respectively. The filarial gamma-GCS is proposed as a potential drug target.

The enzymes of glutathione synthesis: gamma-glutamylcysteine synthetase

The metabolite glutathione fulfills many important and chemically complex roles in protecting cellular components from the deleterious effects of toxic species. GSH combines with hydroxyl radical, peroxynitrite, and hydroperoxides, as well as reactive electrophiles, including activated phosphoramide mustard. This thiol-containing reductant also maintains so-called thiol-enzymes in their catalytically active form, and maintains vitamins C and E in their biologically active forms. The key step in glutathione synthesis, namely the ATP-dependent synthesis of gamma-glutamylcysteine, is the topic of this review. Details are presented on (a) the enzyme's purification and protein chemistry, (b) the successful cDNA cloning, and characterization of the genes responsible for the biosynthesis of this enzyme. After considering aspects of the role of overexpression of this synthetase in terms of cancer chemotherapy, attention is focused on post-translational regulation. The remainder of the review deals with the catalytic mechanism (including substrate specificity, reactions catalyzed, steady-state kinetics, and chemical mechanism) as well as the inhibition of the enzyme (via feedback inhibition, reaction with S-alkyl homocysteine sulfoximine inhibitors, the clinical use of buthionine sulfoximine with cancer patients, and inactivation by cystamine, chloroketones, and various nitric oxide donors).

The putative gamma-glutamylcysteine synthetase from Plasmodium falciparum contains large insertions and a variable tandem repeat

The tripeptide glutathione plays a pivotal role in the maintenance of the thiol redox state of the cell and for the detoxification of reactive oxygen species. Glutathione is synthesized in two consecutive reactions by y-glutamylcysteine synthetase (gamma-GCS) and glutathione synthetase, respectively. The former enzyme represents the rate limiting step of the synthetic pathway. We have cloned the cDNA and gene of a putative gamma-GCS from Plasmodium falciparum. The contiguous cDNA sequences obtained from various cDNA libraries of P. falciparum K1 and 3D7 encompass 4206 bp or 4038 bp and encode polypeptides of 1119 and 1063 amino acids, respectively. The deduced amino acid sequences show four regions of homology (identity: 31.3-43.9%) to human and Trypanosoma brucei gamma-GCS. These regions are interrupted by three large insertions between 94 and 239 amino acids. Within the first insert a variable repetitive motif was identified, which is responsible for the differing sizes of the sequences. We have analysed this phenomenon in five additional P. falciparum strains and found a high degree of variability in the number of the repeated octamer (Y/C)S(N/D)LQQ(Q/R). Therefore the predicted molecular mass of the proteins from different P. falciparum strains ranges from 124.4 to 133.2 kDa, which is almost twice that of the catalytic subunit of the human host enzyme. Isolation of three genomic clones revealed that the gene does not contain introns. P. falciparum gamma-GCS transcription peaks in trophozoites (24-30 h) suggesting that the antioxidant glutathione is predominantly produced at a time where hemoglobin degradation and the simultaneous formation of reactive oxygen species is maximal.

Trypanosoma brucei gamma-glutamylcysteine synthetase. Characterization of the kinetic mechanism and the role of Cys-319 in cystamine inactivation

The parasitic protozoan Trypanosoma brucei utilizes a conjugate of glutathione and spermidine, termed trypanothione, in place of glutathione to maintain cellular redox balance. The first committed step in the biosynthesis of glutathione and thereby trypanothione, is catalyzed by gamma-glutamylcysteine synthetase (gamma-GCS). We have determined the kinetic mechanism for T. brucei gamma-GCS. The kinetics are best described by a rapid equilibrium random ter-reactant mechanism, in which the model derived Kd values for the binding of L-Glu, L-alpha-aminobutyrate, and ATP to free enzyme are 2.6, 5.1, and 1.4 mM, respectively. However, significant dependences exist between the binding of some of the substrate pairs. The binding of either ATP or L-Glu to the enzyme increases the binding affinity of the other by 18-fold, whereas the binding of L-Glu or L-alpha-aminobutyrate decreases the binding affinity of the other by 6-fold. Similarly to the mammalian enzyme, cystamine is a time-dependent, irreversible inhibitor of T. brucei gamma-GCS. It has been suggested by several studies that cystamine labels an active site Cys residue essential for catalysis. Among the enzymes reported to be inactivated by cystamine, only one Cys residue is invariant (Cys-319 in T. brucei gamma-GCS). Mutation of Cys-319 to Ala in T. brucei gamma-GCS renders the enzyme insensitive to cystamine inactivation without significantly affecting the enzyme's catalytic efficiency, kinetic mechanism, or substrate affinities. These studies suggest that cystamine inactivates the enzyme by blocking substrate access to the active site and not by labeling an essential active site residue.

Parasite sulphur amino acid metabolism

This paper reviews current knowledge regarding the metabolism of the sulphur-containing amino acids methionine and cysteine in parasitic protozoa and helminths. Particular emphasis is placed on the unusual aspects of parasite biochemistry which may present targets for rational design of antiparasite drugs. In general, the basic pathways of sulphur amino acid metabolism in most parasites resemble those of their mammalian hosts, since the enzymes involved in (a) the methionine cycle and S-adenosylmethionine metabolism, (b) the trans-sulphuration sequence, (c) the transminative catabolism of methionine, (d) the oxidative catabolism of cysteine and (e) glutathione synthesis have been demonstrated variously in several helminth and protozoan species. Despite these common pathways, there also exist numerous differences between parasite and mammalian metabolism. Some of these differences are relatively subtle. For example, the biochemical properties (and primary amino acid structures) of certain parasite methionine cycle enzymes and S-adenosylmethionine decarboxylases differ from those of the corresponding mammalian enzymes, and nematodes and trichomonads possess a novel, non-mammalian form of the trans-sulphuration enzyme cystathionine beta-synthase. The most profound differences between parasite and mammalian biochemistry relate to a number of unusual enzymes and thiol metabolites found in parasitic protozoa. In certain protozoa the pathway for methionine recycling from 5'-methylthioadenosine differs markedly from the mammalian route, and involves 2 exclusively microbial enzymes. Trypanosomatid protozoa contain the non-mammalian antioxidant thiol compounds ovothiol A and trypanothione, together with unique trypanothione-linked enzymes. Specific anaerobic protozoa possess another exclusively microbial enzyme, methionine gamma-lyase, which catabolises methionine (and homocysteine); the physiological significance of these non-mammalian activities is not fully understood. These unusual features offer opportunities for chemotherapeutic exploitation, and in some cases represent metabolic similarities with bacteria. Additionally, some anaerobic protozoa contain unidentified thiols and this implies the presence of further unusual enzymes/pathways in these organisms. So far, no truly unique targets for chemotherapy have been found in helminth sulphur amino acid metabolism, and to some degree this reflects the relative lack of detailed study in the area.

Characterization of Trypanosoma brucei gamma-glutamylcysteine synthetase, an essential enzyme in the biosynthesis of trypanothione (diglutathionylspermidine)

The parasitic protozoan Trypanosoma brucei maintains redox balance by synthesizing a conjugate of glutathione and spermidine termed trypanothione. The first committed step in the biosynthesis of glutathione, and thereby trypanothione, is catalyzed by the enzyme gamma-glutamylcysteine synthetase (gammaGCS). We have cloned and sequenced the 2037-base pair gene coding for the catalytic subunit of T. brucei gammaGCS. T. brucei gammaGCS appears to be encoded by a single copy gene. A transcript of about 2.3 kilobases was observed in procyclic trypomastigotes. The deduced amino acid sequence of 679 amino acids shares 45, 41, and 36% sequence identity with mammalian, Caenorhabditis elegans, and yeast gammaGCS, respectively. The T. brucei gammaGCS gene was expressed in E. coli; the purified 77.4-kDa enzyme catalyzes the ligation of L-Glu to L-Cys with a kcat of 10 s-1, confirming that the gene encodes the functional catalytic subunit of gammaGCS. The apparent Km values measured for the three natural substrates L-Glu, L-Cys, and ATP are 0.24, 0.69, and 0.07 mM, respectively. Unlike the mammalian enzyme, L-alpha-aminobutyrate (apparent Km = 10 mM) is a poor substitute for L-Cys in the T. brucei gammaGCS-catalyzed reaction. T. brucei gammaGCS is feedback-inhibited by glutathione (apparent KI = 1.1 mM), and it is inactivated by cystamine and buthionine sulfoximine. The kinetic properties of recombinant T. brucei gammaGCS suggest that the substrate binding pocket and the mechanism of enzyme regulation differ from the mammalian enzyme, providing evidence that T. brucei gammaGCS could be a selective chemotherapeutic target for the treatment of trypanosomiasis.