Mutations in the glutathione synthetase gene cause 5-oxoprolinuria

Glutathione-mediated redox regulation in Cryptococcus neoformans impacts virulence

The fungal pathogen Cryptococcus neoformans is well adapted to its host environment. It has several defence mechanisms to evade oxidative and nitrosative agents released by phagocytic host cells during infection. Among them, melanin production is linked to both fungal virulence and defence against harmful free radicals that facilitate host innate immunity. How C. neoformans manipulates its redox environment to facilitate melanin formation and virulence is unclear. Here we show that the antioxidant glutathione is inextricably linked to redox-active processes that facilitate melanin and titan cell production, as well as survival in macrophages and virulence in a murine model of cryptococcosis. Comparative metabolomics revealed that disruption of glutathione biosynthesis leads to accumulation of reducing and acidic compounds in the extracellular environment of mutant cells. Overall, these findings highlight the importance of redox homeostasis and metabolic compensation in pathogen adaptation to the host environment and suggest new avenues for antifungal drug development.

Definition of the Neurotoxicity-Associated Metabolic Signature Triggered by Berberine and Other Respiratory Chain Inhibitors

To characterize the hits from a phenotypic neurotoxicity screen, we obtained transcriptomics data for valinomycin, diethylstilbestrol, colchicine, rotenone, 1-methyl-4-phenylpyridinium (MPP), carbaryl and berberine (Ber). For all compounds, the concentration triggering neurite degeneration correlated with the onset of gene expression changes. The mechanistically diverse toxicants caused similar patterns of gene regulation: the responses were dominated by cell de-differentiation and a triggering of canonical stress response pathways driven by ATF4 and NRF2. To obtain more detailed and specific information on the modes-of-action, the effects on energy metabolism (respiration and glycolysis) were measured. Ber, rotenone and MPP inhibited the mitochondrial respiratory chain and they shared complex I as the target. This group of toxicants was further evaluated by metabolomics under experimental conditions that did not deplete ATP. Ber (204 changed metabolites) showed similar effects as MPP and rotenone. The overall metabolic situation was characterized by oxidative stress, an over-abundance of NADH (>1000% increase) and a re-routing of metabolism in order to dispose of the nitrogen resulting from increased amino acid turnover. This unique overall pattern led to the accumulation of metabolites known as biomarkers of neurodegeneration (saccharopine, aminoadipate and branched-chain ketoacids). These findings suggest that neurotoxicity of mitochondrial inhibitors may result from an ensemble of metabolic changes rather than from a simple ATP depletion. The combi-omics approach used here provided richer and more specific MoA data than the more common transcriptomics analysis alone. As Ber, a human drug and food supplement, mimicked closely the mode-of-action of known neurotoxicants, its potential hazard requires further investigation.

Gss deficiency causes age-related fertility impairment via ROS-triggered ferroptosis in the testes of mice

Glutathione synthetase (GSS) catalyzes the final step in the synthesis of glutathione (GSH), a well-established antioxidant. Research on the specific roles of the Gss gene during spermatogenesis remains limited due to the intricate structure of testis. In this study, we identified pachytene spermatocytes as the primary site of GSS expression and generated a mouse model with postnatal deletion of Gss using Stra8-Cre (S8) to investigate the role of GSS in germ cells. The impact of Gss knockout on reducing male fertility is age-dependent and caused by ferroptosis in the testis. The 2-month-old S8/Gss-/- male mice exhibited normal fertility, due to a compensatory increase in GPX4, which prevented the accumulation of ROS. With aging, there was a decline in GPX4 and an increase in ALOX15 levels observed in 8-month-old S8/Gss-/- mice, resulting in the accumulation of ROS, lipid peroxidation, and ultimately testicular ferroptosis. We found that testicular ferroptosis did not affect spermatogonia, but caused meiosis disruption and acrosome heterotopia. Then the resulting aberrant sperm showed lower concentration and abnormal morphology, leading to reduced fertility. Furthermore, these injuries could be functionally rescued by inhibiting ferroptosis through intraperitoneal injection of GSH or Fer-1. In summary, Gss in germ cells play a crucial role in the resistance to oxidative stress injury in aged mice. Our findings deepen the understanding of ferroptosis during spermatogenesis and suggest that inhibiting ferroptosis may be a potential strategy for the treatment of male infertility.

Case report: A Chinese patient with glutathione synthetase deficiency and a novel glutathione synthase mutation

Glutathione synthetase deficiency (GSSD) is an autosomal-recessive metabolic disorder caused by glutathione synthetase (GSS) gene mutations. No more than 90 cases of GSSD have been reported worldwide; thus, the spectrum of GSS mutations and the genotype-phenotype association remain unclear. Here, we present a severely affected infant carrying a compound heterozygous GSS variation, c.491G > A, and a novel variant of c.1343_1348delTACTTC. We also summarize the clinical manifestations, treatment protocol, prognosis, and genetic characteristics of previously reported GSSD cases in China. In this case study, our patient presented with tachypnea, jaundice, intractable metabolic acidosis, and hemolytic anemia. Urinary-organic acid analysis revealed elevated 5-oxoproline levels. Further, this patient showed improved outcomes owing to early diagnosis and the timely administration of vitamins C and E. Therefore, our study indicates that in clinical cases of unexplained hemolytic anemia and metabolic acidosis, GSSD should be considered. Additionally, genetic testing and antioxidant application might help identify GSSD and improve the prognosis.

Skeletal muscle and erythrocyte redox status is associated with dietary cysteine intake and physical fitness in healthy young physically active men

PURPOSE

To investigate the association between redox status in erythrocytes and skeletal muscle with dietary nutrient intake and markers of physical fitness and habitual physical activity (PA).

METHODS

Forty-five young physically active men were assessed for body composition, dietary nutrient intake, muscle strength, cardiorespiratory capacity and habitual PA. Blood and muscle samples were collected to estimate selected redox biomarkers. Partial correlation analysis was used to evaluate the independent relationship of each factor with redox biomarkers.

RESULTS

Dietary cysteine intake was positively correlated (p < 0.001) with both erythrocyte (r = 0.697) and muscle GSH (0.654, p < 0.001), erythrocyte reduced/oxidized glutathione ratio (GSH/GSSG) (r = 0.530, p = 0.001) and glutathione reductase (GR) activity (r = 0.352, p = 0.030) and inversely correlated with erythrocyte protein carbonyls (PC) levels (r = - 0.325; p = 0.046). Knee extensors eccentric peak torque was positively correlated with GR activity (r = 0.355; p = 0.031) while, one-repetition maximum in back squat exercise was positively correlated with erythrocyte GSH/GSSG ratio (r = 0.401; p = 0.014) and inversely correlated with erythrocyte GSSG and PC (r = - 0.441, p = 0.006; r = - 0.413, p = 0.011 respectively). Glutathione peroxidase (GPx) activity was positively correlated with step count (r = 0.520; p < 0.001), light (r = 0.406; p = 0.008), moderate (r = 0.417; p = 0.006), moderate-to-vigorous (r = 0.475; p = 0.001), vigorous (r = 0.352; p = 0.022) and very vigorous (r = 0.326; p = 0.035) PA. Muscle GSSG inversely correlated with light PA (r = - 0.353; p = 0.022).

CONCLUSION

These results indicate that dietary cysteine intake may be a critical element for the regulation of glutathione metabolism and redox status in two different tissues pinpointing the independent significance of cysteine for optimal redox regulation. Musculoskeletal fitness and PA levels may be predictors of skeletal muscle, but not erythrocyte, antioxidant capacity.

TRIAL REGISTRATION

Registry: ClinicalTrials.gov, identifier: NCT03711838, date of registration: October 19, 2018.

Prognostic and therapeutic significance of a novel ferroptosis related signature in colorectal cancer patients

Increasing studies have highlighted the importance of ferroptosis in colorectal cancer (CRC). However, how to use ferroptosis-related genes (FRGs) to predict the prognosis and guide the treatment of CRC remains unknown. To build a prognostic prediction model using the GEO and TCGA databases and explored a therapeutic strategy for CRC patients based on FRGs. A total of 60 FRGs were identified and three of them including ACACA, GSS, and NFS1 were associated with the prognosis of CRC. Using Lasso regression analysis, an FRGs signature was constructed and validated as an independent prognostic predictor. Then we developed a nomogram based on the FRGs signature and clinical prognostic factors to predict the prognosis of CRC patients, which was better than the traditional TNM staging system. Single-sample gene set enrichment analysis (ssGSEA) was further performed for the functional analysis and suggested that JAK-STAT signaling, Ras signaling pathway, MAPK signaling pathway, and PI3K-Akt signaling pathway were significantly enriched in CRC patients with higher FRGs risk score. Interestingly, CRC cells with higher FRGs risk score were more sensitive to RSL3. Knocking down GSS and NFS1 increased the FRGs risk score and the sensitivity of CRC cells to RSL3. For the clinic use, we screened 75 FDA-approved cancer drugs and found that Fludarabine phosphate could decrease the expression of GSS and NFS1 most. Fludarabine phosphate, in combination with RSL3, showed a strong synergistic effect on CRC cells. Together, this study identified a potent prognostic model and provided an alternative individualized treatment for CRC patients.

Blood transcriptome analysis: Ferroptosis and potential inflammatory pathways in post-traumatic stress disorder

BACKGROUND

Transcriptome-wide analysis of peripheral blood in post-traumatic stress disorder (PTSD) indicates widespread changes in immune-related pathways and function. Ferroptosis, an iron-dependent regulated cell death, is closely related to oxidative stress. However, little is known as to whether ferroptosis plays a role in PTSD.

METHODS

We conducted a comprehensive analysis of combined data from six independent peripheral blood transcriptional studies in the Gene Expression Omnibus (GEO) database, covering PTSD and control individuals. Differentially expressed genes (DEGs) were extracted by comparing PTSD patients with control individuals, from which 29 ferroptosis-related genes (FRGs) were cross-matched and obtained. The weighted gene co-expression network analysis (WGCNA), the Extreme Gradient Boosting (XGBoost) model with Bayesian Optimization, and the least absolute shrinkage and selection operator (LASSO) Cox regression were utilized to construct a PTSD prediction model. Single-sample Gene Set Enrichment Analysis (ssGSEA) and CIBERSORT revealed the disturbed immunologic state in PTSD high-risk patients.

RESULTS

Three crucial FRGs (ACSL4, ACO1, and GSS) were identified and used to establish a predictive model of PTSD. The receiver operating characteristic (ROC) curve verifies its risk prediction ability. Remarkably, ssGSEA and CIBERSORT demonstrated changes in cellular immunity and antigen presentation depending on the FRGs model.

CONCLUSION

These findings collectively provide evidence that ferroptosis may change immune status in PTSD and be related to the occurrence of PTSD, which may help delineate mechanisms and discover treatment biomarkers for PTSD.

Polymorphisms of genes related to phase II metabolism and resistance to clopidogrel

Clopidogrel is an antiplatelet drug commonly used to prevent coagulation. This review aimed to investigate the effect of polymorphisms of G6PD, GCLC, GCLM, GSS, GST, GSR, HK and GLRX genes on clopidogrel during phase II metabolism through exploring previous studies. The results revealed that low glutathione plasma levels caused by several alleles related to these genes could affect the bioactivation process of the clopidogrel prodrug, making it unable to inhibit platelet aggregation perfectly and thus leading to severe consequences in patients with a high risk of blood coagulation. However, the study recommends platelet reactivity tests to predict clopidogrel efficacy rather than studying gene mutations, as most of these mutations are rare and other nongenetic factors could affect the drug's efficacy.

Dietary Cysteine Intake is Associated with Blood Glutathione Levels and Isometric Strength

Glutathione is the most abundant cellular antioxidant and regulates redox homeostasis. Healthy individuals with certain antioxidant inadequacies/deficiencies exhibit impairments in physiological functions. The aim was to investigate whether low levels of dietary cysteine intake are associated with a) lower erythrocyte glutathione, b) increased plasma F₂-isoprostanes, and c) impaired muscle function. Towards this aim, we recorded the dietary intake of the three amino acids that synthesize glutathione (i. e., glutamic acid, cysteine, and glycine) in forty-one healthy individuals, and subsequently measured erythrocyte glutathione levels. Maximal isometric strength and fatigue index were also assessed using an electronic handgrip dynamometer. Our findings indicate that dietary cysteine intake was positively correlated with glutathione levels (r=0.765, p<0.001). In addition, glutathione levels were negatively correlated with F₂-isoprostanes (r=- 0.311, p=0.048). An interesting finding was that glutathione levels and cysteine intake were positively correlated with maximal handgrip strength (r=0.416, p=0.007 and r=0.343, p=0.028, respectively). In conclusion, glutathione concentration is associated with cysteine intake, while adequate cysteine levels were important for optimal redox status and muscle function. This highlights the importance of proper nutritional intake and biochemical screening with the goal of personalized nutrition.

Subnormothermic Machine Perfusion of Steatotic Livers Results in Increased Energy Charge at the Cost of Anti-Oxidant Capacity Compared to Normothermic Perfusion

There continues to be significant debate regarding the most effective mode of ex situ machine perfusion of livers for transplantation. Subnormothermic (SNMP) and normothermic machine perfusion (NMP) are two methods with different benefits. We examined the metabolomic profiles of discarded steatotic human livers during three hours of subnormothermic or normothermic machine perfusion. Steatotic livers regenerate higher stores of ATP during SNMP than NMP. However, there is a significant depletion of available glutathione during SNMP, likely due to an inability to overcome the high energy threshold needed to synthesize glutathione. This highlights the increased oxidative stress apparent in steatotic livers. Rescue of discarded steatotic livers with machine perfusion may require the optimization of redox status through repletion or supplementation of reducing agents.

Antioxidants in Personalized Nutrition and Exercise

The present review highlights the idea that antioxidant supplementation can be optimized when tailored to the precise antioxidant status of each individual. A novel methodologic approach involving personalized nutrition, the mechanisms by which antioxidant status regulates human metabolism and performance, and similarities between antioxidants and other nutritional supplements are described. The usefulness of higher-level phenotypes for data-driven personalized treatments is also explained. We conclude that personally tailored antioxidant interventions based on specific antioxidant inadequacies or deficiencies could result in improved exercise performance accompanied by consistent alterations in redox profile.

Glutathione metabolism in cancer progression and treatment resistance

Bansal and Simon discuss strategies to block glutathione synthesis and utilization pathways to inhibit tumor propagation and treatment resistance.

Glutathione (GSH) is the most abundant antioxidant found in living organisms and has multiple functions, most of which maintain cellular redox homeostasis. GSH preserves sufficient levels of cysteine and detoxifies xenobiotics while also conferring therapeutic resistance to cancer cells. However, GSH metabolism plays both beneficial and pathogenic roles in a variety of malignancies. It is crucial to the removal and detoxification of carcinogens, and alterations in this pathway can have a profound effect on cell survival. Excess GSH promotes tumor progression, where elevated levels correlate with increased metastasis. In this review, we discuss recent studies that focus on deciphering the role of GSH in tumor initiation and progression as well as mechanisms underlying how GSH imparts treatment resistance to growing cancers. Targeting GSH synthesis/utilization therefore represents a potential means of rendering tumor cells more susceptible to different treatment options such as chemotherapy and radiotherapy.

A case of severe glutathione synthetase deficiency with novel GSS mutations

Glutathione synthetase deficiency (GSSD) is a rare inborn error of glutathione metabolism with autosomal recessive inheritance. The severe form of the disease is characterized by acute metabolic acidosis, usually present in the neonatal period with hemolytic anemia and progressive encephalopathy. A case of a male newborn infant who had severe metabolic acidosis with high anion gap, hemolytic anemia, and hyperbilirubinemia is reported. A high level of 5-oxoproline was detected in his urine and a diagnosis of generalized GSSD was made. DNA sequence analysis revealed the infant to be compound heterozygous with two mutations, c.738dupG in exon 8 of GSS gene resulting in p.S247fs and a repetitive sequence in exon 3 of GSS gene. Treatment after diagnosis of GSSD included supplementation with antioxidants and oral sodium hydrogen bicarbonate. However, he maintained a variable degree of metabolic acidosis and succumbed shortly after his parents requested discontinuation of therapy because of dismal prognosis and medical futility when he was 18 days old.

A Rare Cause of Neonatal Hemolytic Anemia: Glutathione Synthetase Deficiency

BACKGROUND

Isolated hemolysis or hemolytic anemia and 5-oxoprolinuria are 2 distinct medical conditions in the clinical spectrum associated with glutathione synthetase deficiency.

CLINICAL OBSERVATION

A 1-day-old female baby presented with anemia and respiratory distress. Her hemoglobin level was 9.5 g/dL and the total serum bilirubin level was 5.6 mg/dL. Metabolic acidosis was detected in her blood gas analysis. Metabolic acidosis recurred despite treatment and further investigation was required. Her 5-oxoproline level was 3815 mmol/mol creatinine in urine organic acid analysis, and a homozygous mutation [p.R125H (c.374G>A)] was found in the glutathione synthetase gene.

CONCLUSIONS

GSD has been observed in very few patients and is rarely considered in the differential diagnosis of hemolytic anemia in newborns.

Five Chinese patients with 5-oxoprolinuria due to glutathione synthetase and 5-oxoprolinase deficiencies

OBJECTIVE

5-Oxoprolinuria is a rare inherited metabolic disorder caused by a defective gamma-glutamyl cycle resulting from mutations in the genes encoding 5-oxoprolinase (OPLAH) and glutathione synthetase (GSS). No inherited 5-oxoprolinuria case has been reported in mainland China until now. In this study, clinical, biochemical, and genetic aspects of five Chinese 5-oxoprolinuria patients with OPLAH or GSS gene mutations were investigated.

METHODS

Three boys and two girls from five unrelated Chinese families with symptomatic 5-oxoprolinuria were identified within the past 3years in Peking University First Hospital. OPLAH and GSS genes were analyzed.

RESULTS

Patients were hospitalized between the age of 13days to 1year and 3months for hypersomnia, developmental retardation, feeding deficiency, vomiting, icterus and recurrent pneumonia. All patients had significantly elevated urine 5-oxoproline. Three novel mutations (c.1904G>A and c.2813_2815delGGG in Patient 1, c.2978G>T in Patient 2) on OPLAH, on GSS, one novel mutation (c.1252C>T in Patient 3) and a reported mutation (c.491G>A in Patients 3-5) were detected. Patient 4 has homozygous mutation c.491G>A, the others are heterozygous. After treatment by l-carnitine, vitamin E, B1, B2 and coenzyme Q10, three patients with GSS deficiency improved, but the two 5-oxoprolinase-deficient patients did not respond to treatment.

CONCLUSIONS

5-Oxoprolinase deficiency and GSS deficiency share some clinical and biochemical features. Genetic analysis is important for the deferential diagnosis. In this study, five Chinese patients had severe central nervous system damage. Antioxidant treatments were proved effective for the three patients with GSS deficiency but not for the two patients with 5-oxoprolinase deficiency.

Urine Metabolomics by (1)H-NMR Spectroscopy Indicates Associations between Serum 3,5-T2 Concentrations and Intermediary Metabolism in Euthyroid Humans

CONTEXT

3,5-Diiodo-L-thyronine (3,5-T2) is a thyroid hormone metabolite which exhibited versatile effects in rodent models, including the prevention of insulin resistance or hepatic steatosis typically forced by a high-fat diet. With respect to euthyroid humans, we recently observed a putative link between serum 3,5-T2 and glucose but not lipid metabolism.

OBJECTIVE

The aim of the present study was to widely screen the urine metabolome for associations with serum 3,5-T2 concentrations in healthy individuals.

STUDY DESIGN AND METHODS

Urine metabolites of 715 euthyroid participants of the population-based Study of Health in Pomerania (SHIP-TREND) were analyzed by (1)H-NMR spectroscopy. Multinomial logistic and multivariate linear regression models were used to detect associations between urine metabolites and serum 3,5-T2 concentrations.

RESULTS

Serum 3,5-T2 concentrations were positively associated with urinary levels of trigonelline, pyroglutamate, acetone and hippurate. In detail, the odds for intermediate or suppressed serum 3,5-T2 concentrations doubled owing to a 1-standard deviation (SD) decrease in urine trigonelline levels, or increased by 29-50% in relation to a 1-SD decrease in urine pyroglutamate, acetone and hippurate levels.

CONCLUSION

Our findings in humans confirmed the metabolic effects of circulating 3,5-T2 on glucose and lipid metabolism, oxidative stress and enhanced drug metabolism as postulated before based on interventional pharmacological studies in rodents. Of note, 3,5-T2 exhibited a unique urinary metabolic profile distinct from previously published results for the classical thyroid hormones.

L-Cysteine metabolism and its nutritional implications

L-Cysteine is a nutritionally semiessential amino acid and is present mainly in the form of L-cystine in the extracellular space. With the help of a transport system, extracellular L-cystine crosses the plasma membrane and is reduced to L-cysteine within cells by thioredoxin and reduced glutathione (GSH). Intracellular L-cysteine plays an important role in cellular homeostasis as a precursor for protein synthesis, and for production of GSH, hydrogen sulfide (H(2)S), and taurine. L-Cysteine-dependent synthesis of GSH has been investigated in many pathological conditions, while the pathway for L-cysteine metabolism to form H(2)S has received little attention with regard to prevention and treatment of disease in humans. The main objective of this review is to highlight the metabolic pathways of L-cysteine catabolism to GSH, H(2)S, and taurine, with special emphasis on therapeutic and nutritional use of L-cysteine to improve the health and well-being of animals and humans.

Metabolic phenotyping applied to pre-clinical and clinical studies of acetaminophen metabolism and hepatotoxicity

Acetaminophen (APAP, paracetamol, N-acetyl-p-aminophenol) is a widely used analgesic that is safe at therapeutic doses but is a major cause of acute liver failure (ALF) following overdose. APAP-induced hepatotoxicity is related to the formation of an electrophilic reactive metabolite, N-acetyl-p-benzoquinone imine (NAPQI), which is detoxified through conjugation with reduced glutathione (GSH). One method that has been applied to study APAP metabolism and hepatotoxicity is that of metabolic phenotyping, which involves the study of the small molecule complement of complex biological samples. This approach involves the use of high-resolution analytical platforms such as NMR spectroscopy and mass spectrometry to generate information-rich metabolic profiles that reflect both genetic and environmental influences and capture both endogenous and xenobiotic metabolites. Data modeling and mining and the subsequent identification of panels of candidate biomarkers are typically approached with multivariate statistical tools. We review the application of multi-platform metabolic profiling for the study of APAP metabolism in both in vivo models and humans. We also review the application of metabolic profiling for the study of endogenous metabolic pathway perturbations in response to APAP hepatotoxicity, with a particular focus on metabolites involved in the biosynthesis of GSH and those that reflect mitochondrial function such as long-chain acylcarnitines. Taken together, this body of work sheds much light on the mechanism of APAP-induced hepatotoxicity and provides candidate biomarkers that may prove of translational relevance for improved stratification of APAP-induced ALF.

Inborn defects in the antioxidant systems of human red blood cells

Red blood cells (RBCs) contain large amounts of iron and operate in highly oxygenated tissues. As a result, these cells encounter a continuous oxidative stress. Protective mechanisms against oxidation include prevention of formation of reactive oxygen species (ROS), scavenging of various forms of ROS, and repair of oxidized cellular contents. In general, a partial defect in any of these systems can harm RBCs and promote senescence, but is without chronic hemolytic complaints. In this review we summarize the often rare inborn defects that interfere with the various protective mechanisms present in RBCs. NADPH is the main source of reduction equivalents in RBCs, used by most of the protective systems. When NADPH becomes limiting, red cells are prone to being damaged. In many of the severe RBC enzyme deficiencies, a lack of protective enzyme activity is frustrating erythropoiesis or is not restricted to RBCs. Common hereditary RBC disorders, such as thalassemia, sickle-cell trait, and unstable hemoglobins, give rise to increased oxidative stress caused by free heme and iron generated from hemoglobin. The beneficial effect of thalassemia minor, sickle-cell trait, and glucose-6-phosphate dehydrogenase deficiency on survival of malaria infection may well be due to the shared feature of enhanced oxidative stress. This may inhibit parasite growth, enhance uptake of infected RBCs by spleen macrophages, and/or cause less cytoadherence of the infected cells to capillary endothelium.

Glutathione synthesis

BACKGROUND

Glutathione (GSH) is present in all mammalian tissues as the most abundant non-protein thiol that defends against oxidative stress. GSH is also a key determinant of redox signaling, vital in detoxification of xenobiotics, and regulates cell proliferation, apoptosis, immune function, and fibrogenesis. Biosynthesis of GSH occurs in the cytosol in a tightly regulated manner. Key determinants of GSH synthesis are the availability of the sulfur amino acid precursor, cysteine, and the activity of the rate-limiting enzyme, glutamate cysteine ligase (GCL), which is composed of a catalytic (GCLC) and a modifier (GCLM) subunit. The second enzyme of GSH synthesis is GSH synthetase (GS).

SCOPE OF REVIEW

This review summarizes key functions of GSH and focuses on factors that regulate the biosynthesis of GSH, including pathological conditions where GSH synthesis is dysregulated.

MAJOR CONCLUSIONS

GCL subunits and GS are regulated at multiple levels and often in a coordinated manner. Key transcription factors that regulate the expression of these genes include NF-E2 related factor 2 (Nrf2) via the antioxidant response element (ARE), AP-1, and nuclear factor kappa B (NFκB). There is increasing evidence that dysregulation of GSH synthesis contributes to the pathogenesis of many pathological conditions. These include diabetes mellitus, pulmonary and liver fibrosis, alcoholic liver disease, cholestatic liver injury, endotoxemia and drug-resistant tumor cells.

GENERAL SIGNIFICANCE

GSH is a key antioxidant that also modulates diverse cellular processes. A better understanding of how its synthesis is regulated and dysregulated in disease states may lead to improvement in the treatment of these disorders. This article is part of a Special Issue entitled Cellular functions of glutathione.

Systems biology tools for toxicology

An important goal of toxicology is to understand and predict the adverse effects of drugs and other xenobiotics. For pharmaceuticals, such effects often emerge unexpectedly in man even when absent from trials in vitro and in animals. Although drugs and xenobiotics act on molecules, it is their perturbation of intracellular networks that matters. The tremendous complexity of these networks makes it difficult to understand the effects of xenobiotics on their ability to function. Because systems biology integrates data concerning molecules and their interactions into an understanding of network behaviour, it should be able to assist toxicology in this respect. This review identifies how in silico systems biology tools, such as kinetic modelling, and metabolic control, robustness and flux analyse, may indeed help understanding network-mediated toxicity. It also shows how these approaches function by implementing them vis-à-vis the glutathione network, which is important for the detoxification of reactive drug metabolites. The tools enable the appreciation of the steady state concept for the detoxification network and make it possible to simulate and then understand effects of perturbations of the macromolecules in the pathway that are counterintuitive. We review how a glutathione model has been used to explain the impact of perturbation of the pathway at various molecular sites, as would be the effect of single-nucleotide polymorphisms. We focus on how the mutations impact the levels of glutathione and of two candidate biomarkers of hepatic glutathione status. We conclude this review by sketching how the various systems biology tools may help in the various phases of drug development in the pharmaceutical industry.

5-Oxoprolinuria in Heterozygous Patients for 5-Oxoprolinase (OPLAH) Missense Changes

The inherited 5-oxoprolinuria is primarily suggestive of genetic defects in two enzymes belonging to the gamma-glutamyl cycle in the glutathione (GSH) metabolism: the glutathione synthetase (GSS) and the 5-oxoprolinase (OPLAH). The GSS deficiency is the best characterized of the inborn errors of GSH metabolism, whereas the OPLAH deficiency is questioned whether it is a disorder or just a biochemical condition with no adverse clinical effects. Recently, the first human OPLAH mutation (p.H870Pfs) was reported in homozygosis in two siblings who suffered from 5-oxoprolinuria with a benign clinical course. We report two unrelated patients who manifested massive excretion of 5-oxoproline in urine. In both probands, the blood GSH levels were normal and no mutations were found in the GSS gene. The mutational screening of the OPLAH gene, which included the codified sequences, the intronic flanking sequences, the promoter sequence, and a genetic analysis in order to detect large deletions and/or duplications, showed that each patient only harbors one missense mutation in heterozygosis. The in silico analyses revealed that each one of these OPLAH mutations, p.S323R and p.V1089I, could alter the proper function of this homodimeric enzyme. In addition, clinical symptoms manifest in these two probands were not related to GSH cycle defects and, therefore, this study provides further evidence that oxoprolinuria may present as epiphenomenon in several pathological conditions and confound the final diagnosis.

Glutathione is essential for early embryogenesis--analysis of a glutathione synthetase knockout mouse

Glutathione (GSH) is present in all mammalian tissues and plays a crucial role in many cellular processes. The second and final step in the synthesis involves the formation of GSH from gamma-glutamylcysteine (γ-GC) and glycine and is catalyzed by glutathione synthetase (GS). GS deficiency is a rare autosomal recessive disorder, and is present in patients with a range of phenotypes, from mild hemolytic anemia and metabolic acidosis to severe neurologic disorders or even death in infancy. The substrate for GS, γ-GC, has been suggested as playing a protective role, by substituting for GSH as an antioxidant in GS deficient patients. To examine the role of GS and GSH metabolites in development, we generated mice deficient in GSH by targeted disruption of the GS gene (Gss). Homozygous mice died before embryonic day (E) 7.5, but heterozygous mice survived with no distinct phenotype. GS protein levels and enzyme activity, as well as GSH metabolites, were investigated in multiple tissues. Protein levels and enzyme activity of GS in heterozygous mice were diminished by 50%, while GSH levels remained intact. γ-GC could not be detected in any investigated tissue. These data demonstrate that GSH is essential for mammalian development, and GSH synthesis via GS is an indispensable pathway for survival.

Enzymatic defects underlying hereditary glutamate cysteine ligase deficiency are mitigated by association of the catalytic and regulatory subunits

Glutamate cysteine ligase (GCL) deficiency is a rare autosomal recessive trait that compromises production of glutathione, a critical redox buffer and enzymatic cofactor. Patients have markedly reduced levels of erythrocyte glutathione, leading to hemolytic anemia and, in some cases, impaired neurological function. Human glutamate cysteine ligase is a heterodimer comprised of a catalytic subunit (GCLC) and a regulatory subunit (GCLM), which catalyzes the initial rate-limiting step in glutathione production. Four clinical missense mutations have been identified within GCLC: Arg127Cys, Pro158Leu, His370Leu, and Pro414Leu. Here, we have evaluated the impacts of these mutations on enzymatic function in vivo and in vitro to gain further insight into the pathology. Embryonic fibroblasts from GCLC null mice were transiently transfected with wild-type or mutant GCLC, and cellular glutathione levels were determined. The four mutant transfectants each had significantly lower levels of glutathione relative to that of the wild type, with the Pro414Leu mutant being most compromised. The contributions of the regulatory subunit to GCL activity were investigated using a Saccharomyces cerevisiae model system. Mutant GCLC alone could not complement a glutathione deficient strain and required the concurrent addition of GCLM to restore growth. Kinetic characterizations of the recombinant GCLC mutants indicated that the Arg127Cys, His370Leu, and Pro414Leu mutants have compromised enzymatic activity that can largely be rescued by the addition of GCLM. Interestingly, the Pro158Leu mutant has kinetic constants comparable to those of wild-type GCLC, suggesting that heterodimer formation is needed for stability in vivo. Strategies that promote heterodimer formation and persistence would be effective therapeutics for the treatment of GCL deficiency.

Disruption of redox homeostasis and induction of apoptosis by suppression of glutathione synthetase expression in a mammalian cell line

The stable HepG2 transfectants anti-sensing expression of the glutathione synthetase (GS) gene exhibited delayed cell growth and increased reactive oxygen species (ROS) level. After the treatment with hydrogen peroxide, the intracellular ROS level was much higher in the stable transfectants than in the vector control cells. However, the GSH levels decreased more significantly in the stable transfectants than in the vector control cells, in the presence of hydrogen peroxide. Hydrogen peroxide-induced apoptosis of the stable transfectants was notably higher than that of the vector control cells. The GS anti-sense RNAs rendered the HepG2 cells more sensitive to growth arrest caused by glucose deprivation. They also sensitized the HepG2 cells to cadmium chloride (Cd) and nitric oxide (NO)-generating sodium nitroprusside (SNP). In brief, the results confirm that GS plays an important role in the defense of the human hepatoma cells against oxidative stress by reducing apoptosis and maintaining redox homeostasis.

Alternative RNA splicing in expression of the glutathione synthetase gene in human cells

Ubiquitous free radical production occurs continuously in cells and tissues. Glutathione is the most abundant mammalian antioxidant, and is synthesized by glutathione synthetase (GSS). Therefore, GSS plays an important role in defending the cell against reactive oxygen species. The expression of GSS has been studied in human cells; however, sequence information about alternative splicing variants of GSS mRNA has not been reported. In the present study, we identified a novel alternative splicing variant (ASV) of the GSS gene in 10 human normal tissues and five human cancer cell lines. The deleted transcript of GSS was characterized by an in-frame deletion of 333 bp, corresponding to the complete loss of exons 4 and 5. Thus this GSS ASV causes protein truncation. We quantified the mRNA of GSS ASV in human normal tissues using real-time PCR. The ASV was detected in colon, kidney, lung, liver, placenta, peripheral blood and uterus, but not in heart, skeletal muscle and spleen tissue. Our results provide a basis for more detailed studies on the regulation of GSS, and for further evaluation of this and other possible roles of GSS. Understanding the regulation of GSS expression is very important for the development of new strategies for controlling the development of GSH-based redox homeostasis.

Regulation of glutathione synthesis

Glutathione (GSH) is a ubiquitous intracellular peptide with diverse functions that include detoxification, antioxidant defense, maintenance of thiol status, and modulation of cell proliferation. GSH is synthesized in the cytosol of all mammalian cells in a tightly regulated manner. The major determinants of GSH synthesis are the availability of cysteine, the sulfur amino acid precursor, and the activity of the rate-limiting enzyme, glutamate cysteine ligase (GCL). GCL is composed for a catalytic (GCLC) and modifier (GCLM) subunit and they are regulated at multiple levels and at times differentially. The second enzyme of GSH synthesis, GSH synthase (GS) is also regulated in a coordinated manner as GCL subunits and its up-regulation can further enhance the capacity of the cell to synthesize GSH. Oxidative stress is well known to induce the expression of GSH synthetic enzymes. Key transcription factors identified thus far include Nrf2/Nrf1 via the antioxidant response element (ARE), activator protein-1 (AP-1) and nuclear factor kappa B (NFkappaB). Dysregulation of GSH synthesis is increasingly being recognized as contributing to the pathogenesis of many pathological conditions. These include diabetes mellitus, pulmonary fibrosis, cholestatic liver injury, endotoxemia and drug-resistant tumor cells. Manipulation of the GSH synthetic capacity is an important target in the treatment of many of these disorders.

Inborn errors in the metabolism of glutathione

Glutathione is a tripeptide composed of glutamate, cysteine and glycine. Glutathione is present in millimolar concentrations in most mammalian cells and it is involved in several fundamental biological functions, including free radical scavenging, detoxification of xenobiotics and carcinogens, redox reactions, biosynthesis of DNA, proteins and leukotrienes, as well as neurotransmission/neuromodulation. Glutathione is metabolised via the gamma-glutamyl cycle, which is catalyzed by six enzymes. In man, hereditary deficiencies have been found in five of the six enzymes. Glutathione synthetase deficiency is the most frequently recognized disorder and, in its severe form, it is associated with hemolytic anemia, metabolic acidosis, 5-oxoprolinuria, central nervous system (CNS) damage and recurrent bacterial infections. Gamma-glutamylcysteine synthetase deficiency is also associated with hemolytic anemia, and some patients with this disorder show defects of neuromuscular function and generalized aminoaciduria. Gamma-glutamyl transpeptidase deficiency has been found in patients with CNS involvement and glutathionuria. 5-Oxoprolinase deficiency is associated with 5-oxoprolinuria but without a clear association with other symptoms. Dipeptidase deficiency has been described in one patient. All disorders are very rare and inherited in an autosomal recessive manner. Most of the mutations are leaky so that many patients have residual enzyme activity. Diagnosis is made by measuring the concentration of different metabolites in the gamma-glutamyl cycle, enzyme activity and in glutathione synthetase and gamma-glutamylcysteine synthetase deficiency, also by mutation analysis. Prenatal diagnosis has been preformed in glutathione synthetase deficiency. The prognosis is difficult to predict, as few patients are known, but seems to vary significantly between different patients. The aims of the treatment of glutathione synthesis defects are to avoid hemolytic crises and to increase the defense against reactive oxygen species. No treatment has been recommended for gamma-glutamyl transpeptidase, 5-oxoprolinase and dipeptidase deficiency.

Cloning and characterization of the human glutathione synthetase 5'-flanking region

GSH synthesis occurs through a two-step enzymatic reaction driven by GCL (glutamate-cysteine ligase; made up of catalytic and modifying subunits) and GSS (glutathione synthetase). In humans, oxidative stress regulates GCL expression in an antioxidant response element-dependent manner via Nrf2 [NFE (nuclear factor erythroid)-related factor 2]. In the rat, GSS and GCL are regulated co-ordinately by oxidative stress, and induction of GSS further increases GSH synthetic capacity. Transcriptional regulation of the human GSS has not been examined. To address this, we have cloned and characterized a 2.2 kb 5'-flanking region of the human GSS. The transcriptional start site is located 80 nt upstream of the translation start site. The human GSS promoter efficiently drove luciferase expression in Chang cells. Overexpression of either Nrf1 or Nrf2 induced the GSS promoter activity by 130 and 168% respectively. Two regions homologous to the NFE2 motif are demonstrated to be important for basal expression of human GSS, as mutation of these sites reduced the promoter activity by 66%. Nrf1, Nrf2 and c-Jun binding to these NFE2 sites under basal conditions was demonstrated using chromatin immunoprecipitation assays. In summary, two NFE2 sites in the human GSS promoter play important roles in the basal expression of GSS and, similar to the GCL subunits, the human GSS gene expression is also regulated by Nrf2.

Acetaminophen-induced anion gap metabolic acidosis and 5-oxoprolinuria (pyroglutamic aciduria) acquired in hospital

A rare cause of high anion gap acidosis is 5-oxoproline (pyroglutamic acid), an organic acid intermediate of the gamma-glutamyl cycle. Acetaminophen and several other drugs have been implicated in the development of transient 5-oxoprolinemia in adults. We report the case of a patient with lymphoma who was admitted for salvage chemotherapy. The patient subsequently developed fever and neutropenia and was administered 20.8 g of acetaminophen during 10 days. During this time, anion gap increased from 14 to 30 mEq/L (14 to 30 mmol/L) and altered mental status developed. After usual causes of high anion gap acidosis were ruled out, a screen for urine organic acids showed 5-oxoproline levels elevated at 58-fold greater than normal values. Predisposing factors in this case included renal dysfunction and sepsis. Clinicians need to be aware of this unusual cause of anion gap acidosis because it may be more common than expected, early discontinuation of the offending agent is therapeutic, and administration of N -acetylcysteine could be beneficial.

Genotype, enzyme activity, glutathione level, and clinical phenotype in patients with glutathione synthetase deficiency

Glutathione synthetase (GS) deficiency is a rare autosomal recessive disorder. The clinical phenotype varies widely, and nearly 30 different mutations in the GSS gene have been identified. In the present study, genotype, enzyme activity, metabolite levels and clinical phenotype were evaluated in 41 patients from 33 families. From some of the patients, data on glutathione (GSH) levels and gamma-glutamylcysteine levels in cultured fibroblasts were also available. Twenty-seven different mutations were found: 14 missense, 9 splice, 2 deletions, 1 insertion and 1 nonsense mutation. Twenty-three patients were homozygous and 18 were compound heterozygous. The moderate and severe clinical phenotypes could not be distinguished based on enzyme activity, GSH or gamma-glutamylcysteine levels in cultured fibroblasts. However, in fibroblasts, the residual GS activity was correlated with the GSH level. All mutations causing frameshifts, premature stop codons or aberrant splicing were associated with moderate or severe clinical phenotypes including haemolytic anaemia, 5-oxoprolinuria, and (in several forms) neurodevelopmental signs. The data indicate that additional genetic or environmental factors modify at least the moderate and severe phenotypes and that the clinical classification given to the patients may be influenced by variation in follow-up. The type of mutation involved can, to some extent, predict a mild versus a more severe phenotype.

Physiological and pathological aspects of GSH metabolism

The antioxidant glutathione is found in low levels in diseases in which increasing evidence implicate oxidative stress in the development of the disease, for example retinopathy of prematurity, necrotizing enterocolitis, bronchopulmonary dysplasia, patent ductus arteriosus and asthma. Glutathione is metabolized in the gamma-glutamyl cycle, which involves six different enzymes. The synthesis of glutathione is a two-step process in which the first step is catalysed by gamma-glutamylcysteine synthetase and the second step by glutathione synthetase. Glutathione synthetase deficiency is an autosomal recessive disease and the most common inborn error of the gamma-glutamyl cycle. Approximately 25% of patients with hereditary glutathione synthetase deficiency die during childhood. Patients present with a clinical picture ranging from compensated haemolytic anaemia to a complex disorder with additional symptoms like 5-oxoprolinuria, metabolic acidosis and central nervous system impairment. Even though the correlation between phenotype and genotype in these patients is complex, an indication of the phenotype can be based on the type of mutation involved. Also, there is a correlation between the glutathione synthetase activity and the level of glutathione in cultured fibroblasts. Inborn errors have also been described in three additional steps of the y-glutamyl cycle, namely gamma-glutamyl-transpeptidase, 5-oxoprolinase and gamma-glutamylcysteine synthetase.

CONCLUSION

The range of disorders in patients with inborn errors in the metabolism of glutathione illustrates the intricate metabolism of glutathione and its involvement in numerous essential processes in the cell. By studying these patients, further insight into the functions and metabolism of glutathione can be achieved.

Human hereditary glutathione synthetase deficiency: kinetic properties of mutant enzymes

Patients with hereditary glutathione synthetase deficiency suffer from haemolytic anaemia, 5-oxoprolinuria, metabolic acidosis, recurrent bacterial infections and various degrees of central nervous system dysfunction. To investigate the molecular basis of the mutations associated with this disease, seven naturally occurring missense mutations [L188P (Leu188-->Pro), D219A, D219G, Y270C, Y270H, R283C and P314L] were expressed using a His-tagged, Escherichia coli-based expression system. Effects of the mutations on kinetic properties, including negative co-operative binding of gamma-glutamyl substrate, were evaluated. The mutation P314L did not have any major effect on these parameters and was classified as a neutral mutation. The remaining mutations decreased V(max) to 2-27% of wild-type activity. Negative co-operativity for gamma-gluABA (L-gamma-glutamyl-L-alpha-aminobutyric acid) was abolished in five mutant recombinant enzymes, whereas for one mutant enzyme, this co-operativity changed from negative to positive. The structural consequences of the mutations were interpreted on the basis of the known structure of the wild-type enzyme.

Function of conserved residues of human glutathione synthetase: implications for the ATP-grasp enzymes

Glutathione synthetase is an enzyme that belongs to the glutathione synthetase ATP-binding domain-like superfamily. It catalyzes the second step in the biosynthesis of glutathione from gamma-glutamylcysteine and glycine in an ATP-dependent manner. Glutathione synthetase has been purified and sequenced from a variety of biological sources; still, its exact mechanism is not fully understood. A variety of structural alignment methods were applied and four highly conserved residues of human glutathione synthetase (Glu-144, Asn-146, Lys-305, and Lys-364) were identified in the binding site. The function of these was studied by experimental and computational site-directed mutagenesis. The three-dimensional coordinates for several human glutathione synthetase mutant enzymes were obtained using molecular mechanics and molecular dynamics simulation techniques, starting from the reported crystal structure of human glutathione synthetase. Consistent with circular dichroism spectroscopy, our results showed no major changes to overall enzyme structure upon residue mutation. However, semiempirical calculations revealed that ligand binding is affected by these mutations. The key interactions between conserved residues and ligands were detected and found to be essential for enzymatic activity. Particularly, the negatively charged Glu-144 residue plays a major role in catalysis.

On the role of the two extracytoplasmic substrate-binding domains in the ABC transporter OpuA

Members of two transporter families of the ATP-binding cassette (ABC) superfamily use two or even four extracytoplasmic substrate-binding domains (SBDs) for transport. We report on the role of the two SBDs in the translocation cycle of the ABC transporter OpuA from Lactococcus lactis. Heterooligomeric OpuA complexes with only one SBD or one functional and one non-functional SBD (inactivated by covalent linkage of a substrate mimic) have been constructed, and the substrate binding and transport kinetics of the purified transporters, reconstituted in liposomes, have been determined. The data indicate that the two SBDs of OpuA interact in a cooperative manner in the translocation process by stimulating either the docking of the SBDs onto the translocator or the delivery of glycine betaine to the translocator. It appears that one of these initial steps, but not the later steps in translocation or resetting of the system to the initial state, is rate determining for transport. These new insights on the functional role of the extracytoplasmic SBDs are discussed in the light of the current knowledge of substrate-binding-protein-dependent ABC transporters.

Decreased synthetic capacity underlies the age-associated decline in glutathione content in Fisher 344 rats

Although it is well documented that the concentration of glutathione (GSH), the most abundant intracellular free thiol and an important antioxidant, declines with age in many tissues of different animal species, the underlying mechanism is not well understood. In a previous study, we showed that the expression of the glutamate cysteine ligase genes was down-regulated with age, accompanied by a decline in GSH content in the liver, kidney, and lung of Fisher 344 rats. The aim of this study was to examine the age-associated changes in the activities of three other enzymes, which also play important roles in GSH biosynthesis, to further explore the mechanism underlying the age-associated decline in GSH content in Fisher 344 rats. The results showed for the first time that the activity and gene expression of glutathione synthase, which catalyzes the second reaction in de novo GSH synthesis, were also decreased with increased age in the lung and kidney, but not in the liver or heart. No age-associated change in the activity of either gamma-glutamyltranspeptidase or glutathione reductase was observed in any of the organs examined. The results further indicate that decreased GSH synthetic capacity is responsible for the age-associated decline in GSH content in Fisher 344 rats.

Veno-occlusive disease: cytokines, genetics, and haemostasis

Hepatic veno-occlusive disease (VOD) is a major cause of morbidity and mortality following high dose cytotoxic therapy for stem cell transplantation (SCT). Pre-existing liver damage, SCT-related therapy, and genetic polymorphisms all appear to increase the risk of developing VOD. Studies of biological markers during SCT suggest that cytokines, haemostasis, and hepatic drug metabolism via the glutathione pathway are all involved in the pathogenesis of VOD. Until recently, treatment options were limited and experimental therapies directed at the pathogenesis of the disease were mostly unsuccessful. However, Defibrotide, a relatively new agent that has modulatory effects on vascular endothelium, cytokine release, and haemostasis, has been used with some success in the management and prophylaxis of VOD. In the future, a better understanding of genetic polymorphisms and biological markers which may be important in the pathogenesis of VOD, may enable us to predict which patients are most likely to be affected.

The importance of glutathione in human disease

Reduced glutathione (GSH) is the most prevalent non-protein thiol in animal cells. Its de novo and salvage synthesis serves to maintain a reduced cellular environment and the tripeptide is a co-factor for many cytoplasmic enzymes and may also act as an important post-translational modification in a number of cellular proteins. The cysteine thiol acts as a nucleophile in reactions with both exogenous and endogenous electrophilic species. As a consequence, reactive oxygen species (ROS) are frequently targeted by GSH in both spontaneous and catalytic reactions. Since ROS have defined roles in cell signaling events as well as in human disease pathologies, an imbalance in expression of GSH and associated enzymes has been implicated in a variety of circumstances. Cause and effect links between GSH metabolism and diseases such as cancer, neurodegenerative diseases, cystic fibrosis (CF), HIV, and aging have been shown. Polymorphic expression of enzymes involved in GSH homeostasis influences susceptibility and progression of these conditions. This review provides an overview of the biological importance of GSH at the level of the cell and organism.

Large conformational changes in the catalytic cycle of glutathione synthase

Glutathione synthase catalyzes the final ATP-dependent step in glutathione biosynthesis, the formation of glutathione from gamma-glutamylcysteine and glycine. We have determined structures of yeast glutathione synthase in two forms: unbound (2.3 A resolution) and bound to its substrate gamma-glutamylcysteine, the ATP analog AMP-PNP, and two magnesium ions (1.8 A resolution). These structures reveal that upon substrate binding, large domain motions convert the enzyme from an open unliganded form to a closed conformation in which protein domains completely surround the substrate in the active site.

Role of AP-1 in the coordinate induction of rat glutamate-cysteine ligase and glutathione synthetase by tert-butylhydroquinone

GSH synthesis occurs via two enzymatic steps catalyzed by glutamate-cysteine ligase (GCL, made up of two subunits) and GSH synthetase (GS). Recently, we described coordinate induction of GCL subunits and GS. To study GS transcriptional regulation, we have cloned and characterized a 2.2-kb 5'-flanking region of the rat GS (GenBank accession number AF333982). One transcriptional start site is located at 51 nucleotides upstream of the translational start site. The rat GS promoter drove efficiently luciferase expression in H4IIE cells. Sequential deletion analysis revealed DNA regions that are involved in positive and negative regulation. One repressor identified was NF1. tert-Butylhydroquinone (TBH) exerted a dose- and time-dependent increase in the mRNA level and promoter activity of both GCL subunits and GS. TBH increased protein binding to several regions of the GS promoter, c-jun expression, and activator protein 1 (AP-1) binding activity to several of the putative AP-1-binding sites of the GS promoter. Blocking AP-1 binding with dominant-negative c-jun led to decreased basal expression and significantly blocked the TBH-induced increase in promoter activity and mRNA level of all three genes. In conclusion, AP-1 is required for basal expression of GCL and GS; while NF1 serves as a repressor of GS, increased AP-1 transactivation is the predominant mechanism for coordinate induction of GCL and GS expression by TBH.

Glutathione synthetase from Plasmodium falciparum

GSH is the major low-molecular-mass thiol in most organisms. The tripeptide maintains a reduced intracellular environment and protects cellular components from damaging oxidation. GSH is synthesized by the action of two ATP-dependent enzymic steps, in which gamma-glutamylcysteine synthetase (gamma-GCS) catalyses the ligation of glutamate and cysteine and subsequently glutathione synthetase (GS) adds glycine to the dipeptide. Recently it was shown that the synthesis of gamma-glutamylcysteine is crucial for the survival of the erythrocytic stages of the malaria parasite Plasmodium falciparum by using the specific gamma-GCS inhibitor buthionine sulphoximine. In order to investigate further the synthetic pathway of the tripeptide in the parasite, GS was cloned and expressed recombinantly. The deduced amino acid sequence of P. falciparum GS shares only a moderate degree of identity with other known GSs, but the residues responsible for substrate and co-factor binding are almost all conserved, with the exception of the ones involved in gamma-glutamylcysteine binding. The protein is active as a dimer, with a subunit molecular mass of 77 kDa, and the addition of reducing reagents such as dithiothreitol is essential in maintaining enzymic activity, indicating that thiol groups are important for stability and enzymic activity. The K(app)(m) values for gamma-glutamyl-alpha-aminobutyrate, ATP and glycine were determined to be 107.1 microM, 59.1 microM and 5.04 mM, respectively, and the V(max) of 5.24 +/- 0.7 micromol.min(-1).mg(-1) was in the same range as that of the mammalian enzymes. However, the negative co-operativity observed for gamma-glutamylcysteine binding to the rat enzyme was not found for the parasite protein. This may be due to the alteration of several amino acids in the gamma-glutamylcysteine-binding site.

Long-term clinical outcome in patients with glutathione synthetase deficiency

OBJECTIVE

The objective was to determine the long-term clinical outcome and the effects of treatment of patients with glutathione synthetase (GS) deficiency (n = 28).

METHODS

The diagnosis was based on demonstration of a marked decrease in GS activity in erythrocytes or cultured fibroblasts in all patients and was supported by finding a decrease in erythrocyte or fibroblast glutathione, presence of 5-oxoprolinuria, or both. The treatment varied but usually included correction of acidosis and supplementation with vitamins C and/or E.

RESULTS

Sixteen patients were severely affected with neurologic symptoms such as seizures and psychomotor retardation; 7 had died at the time of the study. None of the severely affected patients had been treated with both vitamins C and E from the neonatal period. No significant difference was found in GS activity between patients with or without neurologic symptoms or in erythrocyte or fibroblast glutathione levels. Five patients had recurrent bacterial infections.

CONCLUSION

On the basis of clinical symptoms, patients with GS deficiency can be classified into 3 phenotypes: mild, moderate, and severe. Our results indicate that early supplementation with vitamins C and E may improve the long-term clinical outcome.

Oxidative stress, human genetic variation, and disease

Oxidative stress has been implicated in numerous pathophysiological conditions and also aging. The tools for studying oxidative stress are now expanding as a result of the human genome effort and, in particular, expanding knowledge on human genetic variation. A few genetic variants, mostly in the form of single nucleotide polymorphisms of relevance to oxidative stress are already studied by a molecular epidemiologic approach. A review of the current knowledge on variant human genes that are directly implicated in human protection against oxidative stress is presented.