Mechanisms of gamma-glutamylcysteine ligase regulation

Phospholipid biosynthesis modulates nucleotide metabolism and reductive capacity

Phospholipid and nucleotide syntheses are fundamental metabolic processes in eukaryotic organisms, with their dysregulation implicated in various disease states. Despite their importance, the interplay between these pathways remains poorly understood. Using genetic and metabolic analyses in Saccharomyces cerevisiae, we elucidate how cytidine triphosphate usage in the Kennedy pathway for phospholipid synthesis influences nucleotide metabolism and redox balance. We find that deficiencies in the Kennedy pathway limit nucleotide salvage, prompting compensatory activation of de novo nucleotide synthesis and the pentose phosphate pathway. This metabolic shift enhances the production of antioxidants such as NADPH and glutathione. Moreover, we observe that the Kennedy pathway for phospholipid synthesis is inhibited during replicative aging, indicating its role in antioxidative defense as an adaptive mechanism in aged cells. Our findings highlight the critical role of phospholipid synthesis pathway choice in the integrative regulation of nucleotide metabolism, redox balance and membrane properties for cellular defense.

Glutamate rescues heat stress-induced apoptosis of Sertoli cells by enhancing the activity of antioxidant enzymes and activating the Trx1-Akt pathway in vitro

Heat stress reduces the number of Sertoli cells, which is closely related to an imbalanced redox status. Glutamate functions to maintain the equilibrium of redox homeostasis. However, the role of glutamate in heat treated Sertoli cells remains unclear. Herein, Sertoli cells from 3-week-old piglets were treated at 44 °C for 30 min (heat stress). Glutamate levels increased significantly following heat stress treatment, followed by a gradual decrease during recovery, while glutathione (GSH) showed a gradual increase. The addition of exogenous glutamate (700 μM) to Sertoli cells before heat stress significantly reduced the heat stress-induced apoptosis rate, mediated by enhanced levels of antioxidant substances (superoxide dismutase (SOD), total antioxidant capacity (TAC), and GSH) and reduced levels of oxidative substances (reactive oxygen species (ROS) and malondialdehyde (MDA)). Glutamate addition to Sertoli cells before heat stress upregulated the levels of glutamate-cysteine ligase, modifier subunit (Gclm), glutathione synthetase (Gss), thioredoxin (Trx1) and B-cell leukemia/lymphoma 2 (Bcl-2), and the ratio of phosphorylated Akt (protein kinase B)/total Akt. However, it decreased the levels of Bcl2-associated X protein (Bax) and cleaved-caspase 3. Addition of the inhibitor of glutaminase (Gls1), Bptes (Bis-2-(5-phenylacetamido-1,3,4-thiadiazol-2-yl)ethyl sulfide, 30 μM)to Sertoli cells before heat stress reversed these effects. These results inferred that glutamate rescued heat stress-induced apoptosis in Sertoli cells by enhancing activity of antioxidant enzymes and activating the Trx1-Akt pathway. Thus, glutamate supplementation might represent a novel strategy to alleviate the negative effect of heat stress.

Glutathione and glutathione-dependent enzymes: From biochemistry to gerontology and successful aging

The tripeptide glutathione (GSH), namely γ-L-glutamyl-L-cysteinyl-glycine, is an ubiquitous low-molecular weight thiol nucleophile and reductant of utmost importance, representing the central redox agent of most aerobic organisms. GSH has vital functions involving also antioxidant protection, detoxification, redox homeostasis, cell signaling, iron metabolism/homeostasis, DNA synthesis, gene expression, cysteine/protein metabolism, and cell proliferation/differentiation or death including apoptosis and ferroptosis. Various functions of GSH are exerted in concert with GSH-dependent enzymes. Indeed, although GSH has direct scavenging antioxidant effects, its antioxidant function is substantially accomplished by glutathione peroxidase-catalyzed reactions with reductive removal of H2O2, organic peroxides such as lipid hydroperoxides, and peroxynitrite; to this antioxidant activity also contribute peroxiredoxins, enzymes further involved in redox signaling and chaperone activity. Moreover, the detoxifying function of GSH is basically exerted in conjunction with glutathione transferases, which have also antioxidant properties. GSH is synthesized in the cytosol by the ATP-dependent enzymes glutamate cysteine ligase (GCL), which catalyzes ligation of cysteine and glutamate forming γ-glutamylcysteine (γ-GC), and glutathione synthase, which adds glycine to γ-GC resulting in GSH formation; GCL is rate-limiting for GSH synthesis, as is the precursor amino acid cysteine, which may be supplemented as N-acetylcysteine (NAC), a therapeutically available compound. After its cell export, GSH is degraded extracellularly by the membrane-anchored ectoenzyme γ-glutamyl transferase, a process occurring, as GSH synthesis and export, in the γ-glutamyl cycle. GSH degradation occurs also intracellularly by the cytoplasmic enzymatic ChaC family of γ-glutamyl cyclotransferase. Synthesis and degradation of GSH, together with its export, translocation to cell organelles, utilization for multiple essential functions, and regeneration from glutathione disulfide by glutathione reductase, are relevant to GSH homeostasis and metabolism. Notably, GSH levels decline during aging, an alteration generally related to impaired GSH biosynthesis and leading to cell dysfunction. However, there is evidence of enhanced GSH levels in elderly subjects with excellent physical and mental health status, suggesting that heightened GSH may be a marker and even a causative factor of increased healthspan and lifespan. Such aspects, and much more including GSH-boosting substances administrable to humans, are considered in this state-of-the-art review, which deals with GSH and GSH-dependent enzymes from biochemistry to gerontology, focusing attention also on lifespan/healthspan extension and successful aging; the significance of GSH levels in aging is considered also in relation to therapeutic possibilities and supplementation strategies, based on the use of various compounds including NAC-glycine, aimed at increasing GSH and related defenses to improve health status and counteract aging processes in humans.

Quantification of glutathione with high throughput live-cell imaging

Reduction oxidation (redox) reactions are central in life and altered redox state is associated with a spectrum of human diseases. Glutathione (GSH) is the most abundant antioxidant in eukaryotic cells and plays critical roles in maintaining redox homeostasis. Thus, measuring intracellular GSH level is an important method to assess the redox state of organism. The currently available GSH probes are based on irreversible chemical reactions with glutathione and can't monitor the real-time glutathione dynamics. Our group developed the first reversible reaction based fluorescent probe for glutathione, which can measure glutathione levels at high resolution using a confocal microscope and in the bulk scale with a flow cytometry. Most importantly it can quantitatively monitor the real-time GSH dynamics in living cells. Using the 2 ^nd generation of GSH probe, RealThiol (RT), this study measured the GSH level in living Hela cells after treatment with varying concentrations of DL-Buthionine sulfoximine (BSO) which inhibits GSH synthesis, using a high throughput imaging system, Cytation™ 5 cell imaging reader. The results revealed that GSH probe RT at the concentration of 2.0 µM accurately monitored the BSO treatment effect on GSH level in the Hela cells. The present results demonstrated that the GSH probe RT is sensitive and precise in GSH measurement in living cells at a high throughput imaging platform and has the potential to be applied to any cell lines.

The Protective Role of Glutathione on Zinc-Induced Neuron Death after Brain Injuries

Glutathione (GSH) is necessary for maintaining physiological antioxidant function, which is responsible for maintaining free radicals derived from reactive oxygen species at low levels and is associated with improved cognitive performance after brain injury. GSH is produced by the linkage of tripeptides that consist of glutamic acid, cysteine, and glycine. The adequate supplementation of GSH has neuroprotective effects in several brain injuries such as cerebral ischemia, hypoglycemia, and traumatic brain injury. Brain injuries produce an excess of reactive oxygen species through complex biochemical cascades, which exacerbates primary neuronal damage. GSH concentrations are known to be closely correlated with the activities of certain genes such as excitatory amino acid carrier 1 (EAAC1), glutamate transporter-associated protein 3-18 (Gtrap3-18), and zinc transporter 3 (ZnT3). Following brain-injury-induced oxidative stress, EAAC1 function is negatively impacted, which then reduces cysteine absorption and impairs neuronal GSH synthesis. In these circumstances, vesicular zinc is also released into the synaptic cleft and then translocated into postsynaptic neurons. The excessive influx of zinc inhibits glutathione reductase, which inhibits GSH's antioxidant functions in neurons, resulting in neuronal damage and ultimately in the impairment of cognitive function. Therefore, in this review, we explore the overall relationship between zinc and GSH in terms of oxidative stress and neuronal cell death. Furthermore, we seek to understand how the modulation of zinc can rescue brain-insult-induced neuronal death after ischemia, hypoglycemia, and traumatic brain injury.

Oxidants and Antioxidants in the Redox Biochemistry of Human Red Blood Cells

Red blood cells (RBCs) are exposed to both external and internal sources of oxidants that challenge their integrity and compromise their physiological function and supply of oxygen to tissues. Autoxidation of oxyhemoglobin is the main source of endogenous RBC oxidant production, yielding superoxide radical and then hydrogen peroxide. In addition, potent oxidants from other blood cells and the surrounding endothelium can reach the RBCs. Abundant and efficient enzymatic systems and low molecular weight antioxidants prevent most of the damage to the RBCs and also position the RBCs as a sink of vascular oxidants that allow the body to maintain a healthy circulatory system. Among the antioxidant enzymes, the thiol-dependent peroxidase peroxiredoxin 2, highly abundant in RBCs, is essential to keep the redox balance. A great part of the RBC antioxidant activity is supported by an active glucose metabolism that provides reducing power in the form of NADPH via the pentose phosphate pathway. There are several RBC defects and situations that generate oxidative stress conditions where the defense mechanisms are overwhelmed, and these include glucose-6-phosphate dehydrogenase deficiencies (favism), hemoglobinopathies like sickle cell disease and thalassemia, as well as packed RBCs for transfusion that suffer from storage lesions. These oxidative stress-associated pathologies of the RBCs underline the relevance of redox balance in these anucleated cells that lack a mechanism of DNA-inducible antioxidant response and rely on a complex and robust network of antioxidant systems.

Comprehensive Review of γ-Glutamyl Peptides (γ-GPs) and Their Effect on Inflammation Concerning Cardiovascular Health

γ-Glutamyl peptides (γ-GPs) are a group of peptides naturally found in various food sources. The unique γ-bond potentially enables them to resist gastrointestinal digestion and offers high stability in vivo with a longer half-life. In recent years, these peptides have caught researchers' attention due to their ability to impart kokumi taste and elicit various physiological functions via the allosteric activation of the calcium-sensing receptor (CaSR). This review discusses the various food sources of γ-glutamyl peptides, different synthesis modes, allosteric activation of CaSR for taste perception, and associated multiple biological functions they can exhibit, with a special emphasis on their role in modulating chronic inflammation concerning cardiovascular health.

Camellia oil Enhances Plasma Antioxidant Metabolism and Improves Plasma Lipid Metabolism in High-fat Diet-fed Rats

Living on a high-fat, high-calorie, and high-protein diet for a long period may compromise human immunity due to the long-term accumulation of free radicals and plasma lipids. The antioxidant and lipid-lowering compounds (ie polyphenols and vitamin E) in Camellia oil help to decrease the risk of numerous ailments, including cardiovascular disease (CVD), and obesity.

The aims of this study were to study the hypolipidemic and antioxidant effects of Camellia oil in high-fat-fed rats and to promote the high-value use of camellia resources. The high-fat-fed rats were administrated with 2.5, 7.5, and 15 mL/kg BW Camellia oil (Camellia oil group), and 10 mg/kg BW atorvastatin (atorvastatin group), respectively, and compared with a model group (only fed with high fat) and a control group (fed with basal diet). Several parameters were measured, including (1) body weight (BW), liver-to-BW ratio; (2) plasma total cholesterol (TC), triglyceride (TG), high-density lipoprotein cholesterol (HDL-C), and low-density lipoprotein cholesterol (LDL-C); and (3) alanine aminotransferase (ALT), alanine aminotransferase (AST), superoxide dismutase (SOD), glutathione peroxidase (GSH-Px) activity, model driven architecture (MDA) content, lipid metabolism-related genes, and antioxidant-related genes in liver tissue.

Compared with the model group, the high-fat-fed rats in the Camellia oil and atorvastatin group had significantly lower BW and liver-to-BW ratio ( P < .01), plasma TC, TG, and LDL-C levels and ALT and AST activities, but higher HDL-C levels. The relative expressions of ACAT1, DGAT2, FAS, and SREBP genes were significantly reduced in the Camellia oil and atorvastatin groups, while the relative expressions of LCAT, UCP2, MCD, and CPT-1 genes were significantly increased. The rats in the Camellia oil group showed significantly higher SOD and GSH-Px activities, significantly lower MDA content, and significantly higher relative expression of antioxidant genes (eg SOD1, GPx1, CAT, and Gclm). Thus, atorvastatin and Camellia oil exhibited significant hypolipidemic and antioxidant effects, which were better at a dose of 7.5 mL/kg (BW) of Camellia oil. Therefore, Camellia oil becomes a potential new natural resource for future research and development of antioxidant and hypolipidemic drugs, nutraceuticals, and additives.

Ligaza γ-glutamylocysteiny – od molekularnych mechanizmów regulacji aktywności enzymatycznej do implikacji terapeutycznych

Glutation (γ-glutamylocysteinyloglicyna, GSH) jest najbardziej rozpowszechnionym tiolowym antyoksydantem wytwarzanym w cytozolu wszystkich komórek ssaków, który pełni ważną rolę ochronną przed stresem oksydacyjnym. GSH jest syntetyzowany de novo przez sekwencyjne działanie dwóch enzymów: ligazy γ-glutamylocysteiny (GCL) i syntetazy glutationowej (GS). GCL katalizuje pierwszy etap biosyntezy GSH, którego produktem jest γ-glutamylocysteina (γ-GC). GCL jest heterodimerycznym enzymem zbudowanym z podjednostki katalitycznej (GCLc) i modulatorowej (GCLm), kodowanych przez dwa różne geny. Podjednostki GCL podlegają złożonej regulacji zarówno na poziomie przed-, jak i potranslacyjnym. Zmiany w ekspresji i aktywności GCL mogą zaburzać poziom GSH i homeostazy redoks. Przyczyną wielu przewlekłych schorzeń związanych ze stresem oksydacyjnym jest upośledzenie aktywności katalitycznej GCL oraz spadek stężenia GSH. Badania przedkliniczne sugerują, że podawanie egzogennej γ-GC podwyższa wewnątrzkomórkowe GSH przez dostarczenie brakującego substratu i może wykazywać potencjał jako terapia uzupełniająca w chorobach związanych z deplecją GSH.

Glutathione in the Nervous System as a Potential Therapeutic Target to Control the Development and Progression of Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a rare neurological disorder that affects the motor neurons responsible for regulating muscle movement. However, the molecular pathogenic mechanisms of ALS remain poorly understood. A deficiency in the antioxidant tripeptide glutathione (GSH) in the nervous system appears to be involved in several neurodegenerative diseases characterized by the loss of neuronal cells. Impaired antioxidant defense systems, and the accumulation of oxidative damage due to increased dysfunction in GSH homeostasis are known to be involved in the development and progression of ALS. Aberrant GSH metabolism and redox status following oxidative damage are also associated with various cellular organelles, including the mitochondria and nucleus, and are crucial factors in neuronal toxicity induced by ALS. In this review, we provide an overview of the implications of imbalanced GSH homeostasis and its molecular characteristics in various experimental models of ALS.

Effect of pristine fullerene on acquisition, consolidation and retrieval memory in wistar rats

The present study evaluated the effect of fullerene (C₆₀) under in vitro conditions, in hippocampus homogenates from rats and on the induction of behavioral disabilities. Exposure to in vitro C₆₀ led to an increase in the concentration of reactive oxygen species (ROS) and lipid peroxidation (LPO) of hippocampus treated with of fullerene and suspension. These results indicate that the oxidative stress caused by the exposure to C₆₀ was in part related to an absence of an antioxidant response. In this sense, one-trial inhibitory avoidance task were performed and results showed that fullerene at 0.2 and 0.45 μm impaired the acquisition and consolidation of short and long-term memory. Further, enzymatic analysis in rat hippocampus were not significantly different, however, there was an increase in the content of LPO and ROS produced by fullerene. Overall, the results indicates that fullerene possess neurotoxic properties that impairs behavior and promotes oxidative stress.

Lysine harvesting is an antioxidant strategy and triggers underground polyamine metabolism

Both single and multicellular organisms depend on anti-stress mechanisms that enable them to deal with sudden changes in the environment, including exposure to heat and oxidants. Central to the stress response are dynamic changes in metabolism, such as the transition from the glycolysis to the pentose phosphate pathway-a conserved first-line response to oxidative insults1,2. Here we report a second metabolic adaptation that protects microbial cells in stress situations. The role of the yeast polyamine transporter Tpo1p3-5 in maintaining oxidant resistance is unknown6. However, a proteomic time-course experiment suggests a link to lysine metabolism. We reveal a connection between polyamine and lysine metabolism during stress situations, in the form of a promiscuous enzymatic reaction in which the first enzyme of the polyamine pathway, Spe1p, decarboxylates lysine and forms an alternative polyamine, cadaverine. The reaction proceeds in the presence of extracellular lysine, which is taken up by cells to reach concentrations up to one hundred times higher than those required for growth. Such extensive harvest is not observed for the other amino acids, is dependent on the polyamine pathway and triggers a reprogramming of redox metabolism. As a result, NADPH-which would otherwise be required for lysine biosynthesis-is channelled into glutathione metabolism, leading to a large increase in glutathione concentrations, lower levels of reactive oxygen species and increased oxidant tolerance. Our results show that nutrient uptake occurs not only to enable cell growth, but when the nutrient availability is favourable it also enables cells to reconfigure their metabolism to preventatively mount stress protection.

Plant glutathione biosynthesis revisited: redox-mediated activation of glutamylcysteine ligase does not require homo-dimerization

Plant γ-glutamylcysteine ligase (GCL), catalyzing the first and tightly regulated step of glutathione (GSH) biosynthesis, is redox-activated via formation of an intramolecular disulfide bond. In vitro, redox-activation of recombinant GCL protein causes formation of homo-dimers. Here, we have investigated whether dimerization occurs in vivo and if so whether it contributes to redox-activation. FPLC analysis indicated that recombinant redox-activated WT (wild type) AtGCL dissociates into monomers at concentrations below 10^-6 M, i.e. below the endogenous AtGCL concentration in plastids, which was estimated to be in the micromolar range. Thus, dimerization of redox-activated GCL is expected to occur in vivo To determine the possible impact of dimerization on redox-activation, AtGCL mutants were generated in which salt bridges or hydrophobic interactions at the dimer interface were interrupted. WT AtGCL and mutant proteins were analyzed by non-reducing SDS-PAGE to address their redox state and probed by FPLC for dimerization status. Furthermore, their substrate kinetics (K _M, V _max) were compared. The results indicate that dimer formation is not required for redox-mediated enzyme activation. Also, crystal structure analysis confirmed that dimer formation does not affect binding of GSH as competitive inhibitor. Whether dimerization affects other enzyme properties, e.g. GCL stability in vivo, remains to be investigated.

Inhibition of the Nrf2/HO-1 Axis Suppresses the Mitochondria-Related Protection Promoted by Gastrodin in Human Neuroblastoma Cells Exposed to Paraquat

Mitochondria are double-membrane organelles involved in the transduction of energy from different metabolic substrates into adenosine triphosphate (ATP) in mammalian cells. The oxidative phosphorylation system is comprised by the activity of the respiratory chain and the complex V (ATP synthase/ATPase). This system is dependent on oxygen gas (O₂) in order to maintain a flux of electrons in the respiratory chain, since O₂ is the final acceptor of these electrons. Electron leakage from this complex system leads to the continuous generation of reactive species in the cells. The mammalian cells exhibit certain mechanisms to attenuate the consequences originated from the constant exposure to these reactive species. In this context, the transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) and one of the enzymes whose expression is modulated by Nrf2, heme oxygenase-1 (HO-1), take a central role in inducing cytoprotection in humans. Mitochondrial abnormalities are observed during intoxication and in certain diseases, including neurodegeneration. Mitochondrial protection promoted by natural compounds has attracted the attention of researchers due to the promising effects these agents induce experimentally. In this regard, we examined here whether and how gastrodin (GAS), a phenolic glucoside, would prevent the paraquat (PQ)-induced mitochondrial impairment in the SH-SY5Y cells. The cells were exposed to GAS (25 μM) for 4 h prior to the challenge with PQ at 100 μM for additional 24 h. The silencing of Nrf2 by siRNA or the inhibition of HO-1 by ZnPP IX suppressed the GAS-elicited cytoprotection. Therefore, GAS promoted mitochondrial protection by an Nrf2/HO-1-dependent manner.

Role for Cystathionine γ Lyase (CSE) in an Ethanol (E)-Induced Lesion in Fetal Brain GSH Homeostasis

Earlier, we reported that gestational ethanol (E) can dysregulate neuron glutathione (GSH) homeostasis partially via impairing the EAAC1-mediated inward transport of Cysteine (Cys) and this can affect fetal brain development. In this study, we investigated if there is a role for the transulfuration pathway (TSP), a critical bio-synthetic point to supply Cys in E-induced dysregulation of GSH homeostasis. These studies utilized an in utero E binge model where the pregnant Sprague⁻Dawley (SD) rat dams received five doses of E at 3.5 g/kg by gastric intubation beginning embryonic day (ED) 17 until ED19 separated by 12 h. The postnatal day 7 (PN7) alcohol model employed an oral dosing of 4 g/kg body weight split into 2 feedings at 2 h interval and an iso-caloric and iso-volumic equivalent maltose-dextrin milk solution served as controls. The in vitro model consisted of cerebral cortical neuron cultures from embryonic day (ED) 16⁻17 fetus from SD rats and differentiated neurons from ED18 rat cerebral cortical neuroblasts. E concentrations were 4 mg/mL. E induced an accumulation of cystathionine in primary cortical neurons (PCNs), 2nd trimester equivalent in utero binge, and 3rd trimester equivalent PN7 model suggesting that breakdown of cystathionine, a required process for Cys supply is impaired. This was associated with a significant reduction in cystathionine γ-lyase (CSE) protein expression in PCN (p < 0.05) and in fetal cerebral cortex in utero (53%, p < 0.05) without a change in the expression of cystathionine β-synthase (CBS). Concomitantly, E decreased Cse mRNA expression in PCNs (by 32% within 6 h of exposure, p < 0.05) and in fetal brain (33%, p < 0.05). In parallel, knock down of CSE in differentiated rat cortical neuroblasts exaggerated the E-induced ROS, GSH loss with a pronounced caspase-3 activation and cell death. These studies illustrate the importance of TSP in CSE-related maintenance of GSH and the downstream events via Cys synthesis in neurons and fetal brain.

Effects of GSH1 and GSH2 Gene Mutation on Glutathione Synthetases Activity of Saccharomyces cerevisiae

In this paper, three mutants from wild Saccharomyces cerevisiae HBU2.558, called U2.558, UN2.558, and UNA2.558, were screened by UV, sodium nitrite, Atmospheric and room temperature plasma, respectively. Glutathione production of the three mutants increased by 41.86, 72.09 and 56.76%, respectively. We detected the activity of glutathione synthetases and found that its activity was improved. Amino acid sequences of three mutant colonies were compared with HBU2.558. Four mutants: Leu51→Pro51 (L51P), Glu62→Val62 (E62V), Ala332→Glu332 (A332E) and Ser653→Gly653 (S653G) were found in the analysis of γ-glutamylcysteine ligase. L51 is located adjacently to the two active sites of GCL/E/Mg²⁺/ADP complex in the overall GCL structure. L51P mutant spread distortion on the β-sheet due to the fact that the φ was changed from -50.4° to -40.2°. A mutant Leu54→Pro54 (L54P) was found in the analysis of glutathione synthetase, and L54 was an amino acid located between an α-helix and a β-sheet. The results confirm that introduction of proline located at the middle of the β-sheet or at the N- or C-terminal between α-helix and β-sheet or, i.e., L51P and L54P, changed the φ, rigidity, hydrophobicity and conformational entropy, thus increased protein stability and improved the enzyme activity.

Mathematical modeling of the methionine cycle and transsulfuration pathway in individuals with autism spectrum disorder

Previous research has shown a connection between metabolic abnormalities in the methionine cycle and transsulfuration pathway and autism spectrum disorder. Using clinical data from a case-control study investigating measurements of transmethylation and transsulfuration metabolites, a steady-state model of these metabolites in liver cells was developed and participant-specific parameters were identified. Comparison of mean parameter values and parameter distributions between neurotypical study participants and those on the autism spectrum revealed significant differences for four model parameters. Sensitivity analysis identified the parameter describing the rate of glutamylcysteine synthesis, the rate-limiting step in glutathione production, to be particularly important in determining steady-state metabolite concentrations. These results may provide insight into key reactions to target for potential intervention strategies relating to autism spectrum disorder.

Conjugated linoleic acid isomers strongly improve the redox status of bovine mammary epithelial cells (BME-UV1)

Some studies have shown the protective effects of conjugated linoleic acid (CLA) isomers against oxidative stress and lipid peroxidation in animal models, but no information is available about CLA and changes in oxidative status of the bovine mammary gland. The objectives of the study were to assess in vitro the effect of CLA on the cellular antioxidant response of bovine mammary cells, to examine whether CLA isomers could play a role in cell protection against the oxidative stress, and to study the molecular mechanism involved. For the study, BME-UV1 cells, a bovine mammary epithelial cell line, were used as the experimental model. The BME-UV1 cells were treated with complete medium containing 50 µM cis-9,trans-11 CLA (c9,t11 CLA), trans-10,cis-12 CLA (t10,c12 CLA), and CLA mixture (1:1, cis-9,trans-11: trans-10,cis-12 CLA). To monitor cellular uptake of CLA isomers, cells and culture medium were collected at 0, 3, and 48 h from CLA addition for lipid extraction and fatty acid analyses. To assess the cellular antioxidant response, glutathione (GSH/GSSH), NADPH, and γ-glutamyl-cysteine ligase activity was measured after 48 h from addition of CLA. Cytoplasmic superoxide dismutase, glutathione peroxidase, glutathione S-transferase, and glutathione reductase activities and mRNA were also determined. Intracellular reactive oxygen species and thiobarbituric acid reactive substance production were assessed in cells supplemented with CLA isomers. Cell viability after 3h to H2O2 exposure was assessed to evaluate and to compare the potential protection of different CLA isomers against H2O2-induced oxidative stress. Mammary cells readily picked up all CLA isomers, their accumulation was time dependent, and main metabolites at 48 h are two 18:3 isomers. The CLA treatment induced an intracellular GSH increase, matched by high concentration of NADPH, and an increase of γ-glutamyl-cysteine ligase activity mainly in cells treated with the t10,c12 CLA isomer. The CLA isomer treatment of bovine mammary cells increased superoxide dismutase, glutathione peroxidase, and glutathione S-transferase activity and decreased glutathione reductase activity, but no changes in gene expression of these antioxidant enzymes were observed. Cells supplemented with CLA isomers showed a reduction in intracellular reactive oxygen species and thiobarbituric acid reactive substance levels. All CLA isomers were able to enhance cell resistance against H2O2-induced oxidative stress. These suggest an antioxidant role of CLA, in particular of t10,c12 CLA, by developing a significantly high redox status in cells.

Transcriptional responses of glutathione transferase genes in Ruditapes philippinarum exposed to microcystin-LR

Glutathione Transferases (GSTs) are phase II detoxification enzymes known to be involved in the molecular response against microcystins (MCs) induced toxicity. However, the individual role of the several GST isoforms in the MC detoxification process is still unknown. In this study, the time-dependent changes on gene expression of several GST isoforms (pi, mu, sigma 1, sigma 2) in parallel with enzymatic activity of total GST were investigated in gills and hepatopancreas of the bivalve Ruditapes philippinarum exposed to pure MC-LR (10 and 100 µg/L). No significant changes in GST enzyme activities were found on both organs. In contrast, MC-LR affected the transcriptional activities of these detoxification enzymes both in gills and hepatopancreas. GST transcriptional changes in gills promoted by MC-LR were characterized by an early (12 h) induction of mu and sigma 1 transcripts. On the other hand, the GST transcriptional changes in hepatopancreas were characterized by a later induction (48 h) of mu transcript, but also by an early inhibition (6 h) of the four transcripts. The different transcription patterns obtained for the tested GST isoforms in this study highlight the potential divergent physiological roles played by these isoenzymes during the detoxification of MC-LR.

Certain carotenoids enhance the intracellular glutathione level in a murine cultured macrophage cell line by inducing glutamate-cysteine-ligase

SCOPE

Glutathione (GSH) increases in RAW264 murine macrophage cells exposed to β-carotene or β-cryptoxanthin, however, the underlying mechanism has not been clarified. In the present study, we investigated the expression of glutamate-cysteine-ligase (GCL), the rate-limiting enzyme in GSH synthesis, in these cells.

METHODS AND RESULTS

Both the protein and mRNA expression of GCL increased in a β-carotene concentration-dependent manner. Buthionine sulfoximine, a GCL inhibitor, abolished the β-carotene-induced GSH increase without affecting the β-carotene-induced GCL protein expression. Both cycloheximide, a translation inhibitor, and actinomycin D, a transcription inhibitor, completely suppressed the β-carotene-induced GCL protein expression and the concomitant GSH increase. Actinomycin D inhibited the β-carotene-induced Gcl mRNA expression as well. Similarly to β-carotene, β-cryptoxanthin upregulated the GCL protein expression, but lutein did not. The c-Jun N-terminal kinase (JNK) inhibitor, SP600125, suppressed the β-carotene-induced GSH increase, whereas a p38 mitogen-activated protein kinase inhibitor or an extracellular signal-regulated kinase 1/2 inhibitor did not. The JNK inhibitor also suppressed the β-carotene-induced GCL protein expression, and consistently β-carotene induced JNK phosphorylation.

CONCLUSION

These findings revealed that certain carotenoids induce the Gcl mRNA expression in RAW264 cells and subsequently the GCL protein expression, which concomitantly enhances the intracellular GSH level, in a JNK pathway-related manner.

Low-level domoic acid protects mouse cerebellar granule neurons from acute neurotoxicity: role of glutathione

Domoic acid (DomA) is a potent marine neurotoxin. By activating α-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid/kainate receptors, DomA induces oxidative stress-mediated apoptotic cell death in neurons. The effect of prolonged (10 days) exposure to a low, nontoxic concentration (5nM) of DomA on acute (intermediate concentration) neurotoxicity of this toxin was investigated in cerebellar granule neurons (CGNs) from wild-type mice and mice lacking the glutamate cysteine ligase (GCL) modifier subunit (Gclm (/)). CGNs from Gclm (/) mice have very low glutathione (GSH) levels and are very sensitive to DomA toxicity. In CGNs from wild-type mice, prolonged exposure to 5nM DomA did not cause any overt toxicity but reduced oxidative stress-mediated apoptotic cell death induced by exposure to an intermediate concentration (100nM for 24h) of DomA. This protection was not observed in CGNs from Gclm (/) mice. Prolonged DomA exposure increased GSH levels in CGNs of wild-type but not Gclm (/) mice. Levels of GCLC (the catalytic subunit of GCL) protein and mRNA were increased in CGNs of both mouse strains, whereas levels of GCLM protein and mRNA, activity of GCL, and levels of GCL holoenzyme were only increased in CGNs of wild-type mice. Chronic DomA exposure also protected wild-type CGNs from acute toxicity of other oxidants. The results indicate that CGNs from Gclm (/) mice, which are already more sensitive to DomA toxicity, are unable to upregulate their GSH levels. As Gclm (/) mice may represent a model for a common human polymorphism in GCLM, such individuals may be at particular risk for DomA-induced neurotoxicity.

Effects of carbon nanomaterials fullerene C₆₀ and fullerol C₆₀(OH)₁₈₋₂₂ on gills of fish Cyprinus carpio (Cyprinidae) exposed to ultraviolet radiation

In consequence of their growing use and demand, the inevitable environmental presence of nanomaterials (NMs) has raised concerns about their potential deleterious effects to aquatic environments. The carbon NM fullerene (C₆₀), which forms colloidal aggregates in water, and its water-soluble derivative fullerol (C₆₀(OH)₁₈₋₂₂), which possesses antioxidant properties, are known to be photo-excited by ultraviolet (UV) or visible light. To investigate their potential hazards to aquatic organisms upon exposure to UV sunlight, this study analyzed (a) the in vitro behavior of fullerene and fullerol against peroxyl radicals (ROO) under UV-A radiation and (b) the effects of these photo-excited NMs on oxidative stress parameters in functional gills extracted from the fish Cyprinus carpio (Cyprinidae). The variables measured were the total antioxidant capacity, lipid peroxidation (TBARS), the activities of the antioxidant enzymes glutathione reductase (GR) and glutamate cysteine ligase (GCL), and the levels of the non-enzymatic antioxidant glutathione (GSH). The obtained results revealed the following: (1) both NMs behaved in vitro as antioxidants against ROO in the dark and as pro-oxidants in presence of UV-A, the latter effect being reversed by the addition of sodium azide, which is a singlet oxygen (¹O₂) quencher; (2) fullerene induced toxicity with or without UV-A incidence, with a significant (p<0.05) increase in lipid peroxidation (with greater damage under illumination), a decrease in GCL activity, and the depletion of GSH stocks (under illumination), all of which were attributed to ¹O₂ generation; and (3) fullerol also decreased GCL activity and GSH formation (p<0.05) but without lipid damage. The overall results show that fullerene can be toxic with or without light incidence, whereas UV radiation seems to play a key role in the environmental toxicity of carbon NMs through ¹O₂ formation.

Identification of age-specific Nrf2 binding to a novel antioxidant response element locus in the Gclc promoter: a compensatory means for the loss of glutathione synthetic capacity in the aging rat liver?

NFE2-related factor 2 (Nrf2) transcriptionally governs the cellular response to harmful electrophiles, xenobiotics, and reactive oxygen species. Its nuclear levels decline with age (Suh et al., 2004a), which in part explains the age-related loss of phase II detoxification. However, little work has yet characterized how age affects Nrf2 DNA binding or the role that alterations to the Nrf2 transcriptional apparatus plays in modulating Nrf2-mediated gene expression. In this study, we used immunoprecipitation assays to show that Nrf2 bound to the active antioxidant response element (ARE) of the catalytic subunit of glutamate cysteine ligase (GCLC) is significantly lower in hepatic chromatin from aged vs. young rats. Moreover, the activity at this ARE locus is diminished during aging because of the presence of Bach1 and the absence of CREB-binding protein (CBP), a transcriptional repressor and co-activator, respectively. Further analysis reveals that Nrf2 occupies an alternate ARE site located -2.2 kb downstream from the normally active ARE binding site in livers of old rats, indicating an age-specific adaptation to maintain gene expression. Our results, thus, show that the conversion of Nrf2 binding from an active ARE to an alternative ARE element is not adequate to maintain basal expression of hepatic Gclc in old rats, which provides a potential mechanism for the age-related loss of glutathione synthetic and other phase II enzymes.

Integrated biological responses of zebrafish (Danio rerio) to analyze water quality in regions under anthropogenic influence

This study analyzed water quality in regions around Patos lagoon (Southern Brazil) that are under anthropogenic pressure. Water samples were collected from five different sites, including one used as a source for human consumption (COR) and others known to be influenced by human activities (IP). Danio rerio (Teleostei, Cyprinidae) organisms were exposed for 24h to these water samples, plus a control group. It was observed that: (1) reactive oxygen species levels were lower in COR and IP than in the control group; (2) glutamate-cysteine ligase (catalytic subunit) expression was higher in COR than in other sites; (3) exposure to all water samples affected long-term memory (LTM) when compared to control group. Thus, some water samples possess the ability to modulate the antioxidant system and to induce a decline in cognitive functions, as measured by LTM. The obtained results indicate that a combination of variables of different organization level (molecular, biochemical and behavioral) can be employed to analyze water quality in impacted regions.

Influence of a toxic Microcystis aeruginosa strain on glutathione synthesis and glutathione-S-transferase activity in common carp Cyprinus carpio (Teleostei: Cyprinidae)

We evaluated the effects of aqueous extracts of the cyanobacterium-producing microcystin (MC), Microcystis aeruginosa (strain RST9501), on detoxification capacity and glutathione (GSH) synthesis in liver, brain, gill, and muscle-as well as apoptotic protease (calpain) activity in liver and brain-in the common carp Cyprinus carpio (Teleostei: Cyprinidae). Experimental groups were defined as follows: (1) control (CTR); (2) carp treated with an aqueous extract from the toxic cyanobacteria M. aeruginosa in a final MC concentration of 25 μg/kg (MC 25); and (3) carp treated with an aqueous extract from the toxic cyanobacteria M. aeruginosa in a final MC concentration of 50 μg/kg (MC 50). Carp were gavaged with a cyanobacterial aqueous solution or MilliQ water (CTR group). The experiment was conducted for period of 48 h comprising two gavages with a 24-h interval between them. Some of the parameters analyzed in liver, brain, gill, and muscle included activity of the enzymes glutathione-S-transferase (GST), glutamate cysteine ligase (GCL), glutathione reductase (GR), and GSH concentration. We also evaluated GST pi concentration by Western blot as well as calpain activity in liver and brain samples. In carp liver from the MC 50 group, we observed a decrease in GST and GCL activity, which was accompanied by a decreased GSH concentration. In addition, liver calpain activity was highly induced in carp at both MC doses. Thus, MC ingestion affected the liver antioxidant status through decreasing the GSH concentration and the activity of the enzyme involved in its synthesis (GCL). It also decreased the MC detoxification capacity of the liver because total GST activity decreased, a result that cannot be ascribed to GST pi levels. Because GSH acts as an uncompetitive inhibitor of calpain, its decrease should explain the higher activity of this apoptotic enzyme. The main goal of this study was to show that a decrease in GSH concentration is related to decreased activity of GCL, the limiting enzyme involved in GSH synthesis. Because MCs are phosphatase inhibitors and GCL is allosterically inhibited by phosphorylation, the cellular hyperphosphorylation state induced by MC exposure could act as a modulator factor for antioxidant defenses.

Posttranslational modification and regulation of glutamate-cysteine ligase by the α,β-unsaturated aldehyde 4-hydroxy-2-nonenal

4-Hydroxy-2-nonenal (4-HNE) is a lipid peroxidation product formed during oxidative stress that can alter protein function via adduction of nucleophilic amino acid residues. 4-HNE detoxification occurs mainly via glutathione (GSH) conjugation and transporter-mediated efflux. This results in a net loss of cellular GSH, and restoration of GSH homeostasis requires de novo GSH biosynthesis. The rate-limiting step in GSH biosynthesis is catalyzed by glutamate-cysteine ligase (GCL), a heterodimeric holoenzyme composed of a catalytic (GCLC) and a modulatory (GCLM) subunit. The relative levels of the GCL subunits are a major determinant of cellular GSH biosynthetic capacity and 4-HNE induces the expression of both GCL subunits. In this study, we demonstrate that 4-HNE can alter GCL holoenzyme formation and activity via direct posttranslational modification of the GCL subunits in vitro. 4-HNE directly modified Cys553 of GCLC and Cys35 of GCLM in vitro, which significantly increased monomeric GCLC enzymatic activity, but reduced GCL holoenzyme activity and formation of the GCL holoenzyme complex. In silico molecular modeling studies also indicate these residues are likely to be functionally relevant. Within a cellular context, this novel posttranslational regulation of GCL activity could significantly affect cellular GSH homeostasis and GSH-dependent detoxification during periods of oxidative stress.

Rapid activation of glutamate cysteine ligase following oxidative stress

Glutamate cysteine ligase (GCL) catalyzes the rate-limiting step in the formation of the cellular antioxidant glutathione (GSH). The GCL holoenzyme consists of two separately coded proteins, a catalytic subunit (GCLC) and a modifier subunit (GCLM). Both GCLC and GLCM are controlled transcriptionally by a variety of cellular stimuli, including oxidative stress. This study addresses post-translational control of GCL activity, which increased rapidly in human lymphocytes following oxidative stress. Activation of GCL occurred within minutes of treatment and without any change in GCL protein levels and coincided with an increase in the proportion of GCLC in the holoenzyme form. Likewise, GCLM shifted from the monomeric form to holoenzyme and higher molecular weight species. Normal rat tissues also showed a distribution of monomeric and higher molecular weight forms. Neither GCL activation, nor the formation of holoenzyme, required a covalent intermolecular disulfide bridge between GCLC and GCLM. However, in immunoprecipitation studies, a neutralizing epitope associated with enzymatic activity was protected following cellular oxidative stress. Thus, the N-terminal portion of GCLC may undergo a change that stabilizes the GCL holoenzyme. Our results suggest that a dynamic equilibrium exists between low and high activity forms of GCL and is altered by transient oxidative stress. This provides a mechanism for the rapid post-translational activation of GCL and maintenance of cellular GSH homeostasis.

Oxidative stress generation by microcystins in aquatic animals: why and how

Microcystins (MICs) are potent toxins produced worldwide by cyanobacteria during bloom events. Phosphatases inhibition is a well recognized effect of this kind of toxins as well as oxidative stress. However, it is not fully understood why and how MICs exposure can lead to an excessive formation of reactive oxygen species (ROS) that culminate in oxidative damage. Some evidences suggest a close connection between cellular hyperphosphorylation state and oxidative stress generation induced by MICs exposure. It is shown, based on literature data, that MICs incorporation per se can be the first event that triggers glutathione depletion and the consequent increase in ROS concentration. Also, literature data suggest that hyperphosphorylated cellular environment induced by MICs exposure can modulate antioxidant enzymes, contributing to the generation of oxidative damage. This review summarizes information on MICs toxicity in aquatic animals, focusing on mechanistic aspects, and rise questions that in our opinion needs to be further investigated.

Responses of glutamate cysteine ligase and glutathione to oxidants in deer mice (Peromyscus maniculatus)

Sensitivities of a wildlife species, deer mice, to oxidants were evaluated. A single dose (1589 mg/kg body weight by intraperitoneal injection) of carbon tetrachloride, a typical hepatotoxicant, caused changes in GCL activity and GSH content in multiple organs of deer mice. Hepatic GCL activity and GSH content were depleted substantially (P<0.01), renal GCL activity increased (P<0.05). Blood, brain and heart GCL activities increased (P<0.05), whereas GSH contents decreased significantly. Deer mice were exposed to Pb, or Pb together with Cu and Zn via drinking water for 4 weeks. GCL activities were not significantly affected by treatments. GSH contents were increased significantly by Pb alone, Pb with medium and high concentrations of Cu and Zn. Effects of multi-metal-contaminated soil were investigated via lactational, juvenile and lifelong exposure to feed supplemented with soils. Metal-contaminated soils did not lead to significant effects in pups via lactation, 50-day exposure altered GSH content marginally, while 100-day exposure resulted in marked GCL activity depletion. After 100-day exposure, GCL activities of the medium soil-, high soil- and Pb-treated deer mice were only 53%, 40% and 46% of the control, respectively (P<0.0001).

Muscarinic receptors prevent oxidative stress-mediated apoptosis induced by domoic acid in mouse cerebellar granule cells

In mouse cerebellar granule neurons (CGNs) low concentrations of domoic acid (DomA) induce apoptotic cell death, which is mediated by oxidative stress; apoptosis is more pronounced in CGNs from Gclm (-/-) mice, which lack the modifier subunit of glutamate cysteine ligase (GCL) and have very low GSH levels. By activating M(3) muscarinic receptors, the cholinergic agonist carbachol inhibits DomA-induced apoptosis, and the anti-apoptotic action of carbachol is more pronounced in CGNs from Gclm (+/+) mice. Carbachol does not prevent DomA-induced increase in reactive oxygen species, suggesting that its anti-apoptotic effect is downstream of reactive oxygen species production. Carbachol inhibits DomA-induced activation of Jun N-terminal (JNK) and p38 kinases, increased translocation to mitochondria of the pro-apoptotic protein Bax, and activation of caspase-3. Carbachol activates extracellular signal-regulated kinases 1/2 (ERK1/2) MAPK and phospahtidylinositol-3 kinase (PI3K) in CGNs from both genotypes. However, while the protective effect of carbachol is mediated by ERK1/2 MAPK in CGNs from both mouse genotypes, inhibitors of PI3K are only effective at antagonizing the action of carbachol in CGNs from Gclm (+/+) mice. In CGNs from both Gclm (+/+) and (-/-) mice, carbachol induces a MAPK-dependent increase in the level of the anti-apoptotic protein Bcl-2. In contrast, carbachol causes a PI3K-dependent increase in GCL activity and of GSH levels only in CGNs from Gclm (+/+) mice. Such increase in GCL is not because of a transcriptionally-mediated increase in glutamate cysteine ligase catalytic subunit or glutamate cysteine ligase modifier subunit, but rather to an increase in the formation of the GCL holoenzyme. The results indicate that multiple pathways may contribute to the protective action of carbachol toward DomA-induced apoptosis. Compromised GCLM expression, which is also found in a common genetic polymorphism in humans, leads to lower GSH levels, which can exacerbate the neurotoxicity of DomA, and decreases the anti-apoptotic effectiveness of muscarinic agonists.

Optimization and application of glutamate cysteine ligase measurement in wildlife species

Glutamate cysteine ligase (GCL), synthesizing gamma-glutamylcysteine from glutamate and cysteine, is the rate-limiting enzyme in glutathione (GSH) biosynthesis. GCL activity measurement was optimized in tissues from deer mice, Sprague Dawley rats, and mallard ducks. Varying glutamic acid concentrations from 5 to 80 mM did not affect GCL activities markedly, whereas cysteine concentrations from 2.5 to 40 mM influenced GCL activities substantially. Optimal cysteine concentrations for deer mouse, Sprague Dawley rat, and mallard duck (respectively) were 30, 30, and 20 mM in liver, 10, 10, and 20 mM in kidney, 20, 20, and 30 mM in brain, and 30 mM in heart for all three species. Responses of mallard duck GCL activity to acid metalliferous water were evaluated. After subacute exposure, low doses increased GCL activity and GSH content in liver by 48.3% and 54.4%, respectively. High doses reduced GCL activities significantly in liver and kidney to 31.2% and 43.0% of the control, respectively.

Pro-oxidant shift in glutathione redox state during aging

The GSH:GSSG ratio, which is the primary determinant of the cellular redox state, becomes progressively more pro-oxidizing during the aging process due to an elevation in the GSSG content and a decline in the ability for de novo GSH biosynthesis. The K(m) of glutamate-cysteine ligase (GCL), the rate-limiting enzyme in de novo GSH biosynthesis, significantly increases during aging, which would adversely affect the ability for rapid GSH biosynthesis, especially under stressful conditions. Experimental studies suggest that age-related accumulation of homocysteine, an intermediate in the trans-sulfuration pathway, may be responsible for causing the loss of affinity between GCL and its substrates. Over-expression of GCL has been shown to prolong the life span of Drosophila by up to 50%, suggesting that perturbations in glutathione metabolism play a causal role in the aging process.

Structure, function, and post-translational regulation of the catalytic and modifier subunits of glutamate cysteine ligase

Glutathione (GSH) is a tripeptide composed of glutamate, cysteine, and glycine. The first and rate-limiting step in GSH synthesis is catalyzed by glutamate cysteine ligase (GCL, previously known as gamma-glutamylcysteine synthetase). GCL is a heterodimeric protein composed of catalytic (GCLC) and modifier (GCLM) subunits that are expressed from different genes. GCLC catalyzes a unique gamma-carboxyl linkage from glutamate to cysteine and requires ATP and Mg(++) as cofactors in this reaction. GCLM increases the V(max) and K(cat) of GCLC, decreases the K(m) for glutamate and ATP, and increases the K(i) for GSH-mediated feedback inhibition of GCL. While post-translational modifications of GCLC (e.g. phosphorylation, myristoylation, caspase-mediated cleavage) have modest effects on GCL activity, oxidative stress dramatically affects GCL holoenzyme formation and activity. Pyridine nucleotides can also modulate GCL activity in some species. Variability in GCL expression is associated with several disease phenotypes and transgenic mouse and rat models promise to be highly useful for investigating the relationships between GCL activity, GSH synthesis, and disease in humans.

Regulation of GCL activity and cellular glutathione through inhibition of ERK phosphorylation

Extracellular signal-regulated protein kinase (ERK), one of the mitogen-activated protein kinase, has been known to be involved in diverse cellular functions. In this work, we found that basically inhibition of this kinase in cultured cells markedly increased the gamma-glutamate-cysteine ligase (GCL; EC 6.3.2.2) activity, but without any considerable induction of the GCL genes. The increased GCL activity consequently elevated the cellular GSH level and eventually enhanced the cellular antioxidant capacity. Genetic inhibition of B-Raf, the upstream of ERK, also resulted in increased GCL activity and GSH level. Recent evidence also suggests that chronic pro-oxidant exposure results in the loss of ERK phosphorylation in vivo. Therefore, the findings in the present study suggest that inhibition of B-Raf/MEK/ERK pathway might be a promising physiological approach to up-regulate GCL activity and GSH. This study would also help us to understand the comprehensive role of the Raf/MEK/ERK pathway in overall physio/pathological conditions.

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.

Interaction between the catalytic and modifier subunits of glutamate-cysteine ligase

Glutamate-cysteine ligase (GCL) is the rate-limiting enzyme in the glutathione (GSH) biosynthesis pathway. This enzyme is a heterodimer, comprising a catalytic subunit (GCLC) and a regulatory subunit (GCLM). Although GCLC alone can catalyze the formation of l-gamma-glutamyl-l-cysteine, its binding with GCLM enhances the enzyme activity by lowering the K(m) for glutamate and ATP, and increasing the K(i) for GSH inhibition. To characterize the enzyme structure-function relationship, we investigated the heterodimer formation between GCLC and GCLM, in vivo using the yeast two-hybrid system, and in vitro using affinity chromatography. A strong and specific interaction between GCLC and GCLM was observed in both systems. Deletion analysis indicated that most regions, except a portion of the C-terminal region of GCLC and a portion of the N-terminal region of GCLM, are required for the interaction to occur. Point mutations of selected amino acids were also tested for the binding activity. The GCLC Cys248Ala/Cys249Ala and Pro158Leu mutations enzyme showed the same strength of binding to GCLM as did wild-type GCLC, yet the catalytic activity was dramatically decreased. The results suggest that the heterodimer formation may not be dependent on primary amino-acid sequence but, instead, involves a complex formation of the tertiary structure of both proteins.