Inhibition of γ-Glutamylcysteine Synthetase by l-Methionine-S-sulfoximine

Diphenylphosphinylhydroxylamine (DPPH) Affords Late-Stage S-imination to access free-NH Sulfilimines and Sulfoximines

Sulfilimines, as potential aza-isosteres of sulfoxides, are valued as building blocks, auxiliaries, ligands, bioconjugation handles, and as precursors to versatile S(VI) scaffolds including sulfoximines and sulfondiimines. Here, we report a thioether imination methodology that exploits O-(diphenylphosphinyl)hydroxyl amine (DPPH). Under mild, metal-free, and biomolecule-compatible conditions, DPPH enables late-stage S-imination on peptides, natural products, and a clinically trialled drug, and shows both excellent chemoselectivity and broad functional group tolerance. This methodological report is extended to an efficient and high-yielding one-pot reaction for accessing free-NH sulfoximines with diverse substrates including ones of potential clinical importance. In the presence of a rhodium catalyst, sulfoxides are S-iminated in higher yields to afford free-NH sulfoximines. S-imination was validated on an oxidatively delicate amatoxin to give sulfilimine and sulfoximine congeners. Interestingly, these new sulfilimine and sulfoximine-amatoxins show cytotoxicity. This method is further extended to create sulfilimine and sulfoximine-Fulvestrant and buthionine analogues.

Can the post-ruminal urea release impact liver metabolism, and nutritional status of beef cows at late gestation?

We aimed to evaluate the effects of post-ruminal supply of urea (PRU) on nutritional status, and liver metabolism of pregnant beef cows during late gestation. Twenty-four Brahman dams, pregnant from a single sire, and weighing 545 kg ± 23 kg were confined into individual pens at 174 ± 23 d of gestation, and randomly assigned into one of two dietary treatments up to 270 d of gestation: Control (CON, n = 12), consisting of a basal diet supplemented with conventional urea, where the cows were fed with diets containing 13.5 g conventional urea per kg dry matter; and PRU (PRU, n = 12), consisting of a basal diet supplemented with a urea coated to extensively prevent ruminal degradation while being intestinally digestible, where the cows were fed with diets containing 14,8 g urea protected from ruminal degradation per kg dry matter. Post-ruminal supply of urea reduced the urine levels of 3-methylhistidine (P = 0.02). There were no differences between treatments for dry matter intake (DMI; P = 0.76), total digestible nutrient (TDN) intake (P = 0.30), and in the body composition variables, such as, subcutaneous fat thickness (SFT; P = 0.72), and rib eye area (REA; P = 0.85). In addition, there were no differences between treatments for serum levels of glucose (P = 0.87), and serum levels of glucogenic (P = 0.28), ketogenic (P = 0.72), glucogenic, and ketogenic (P = 0.45) amino acids, neither for urea in urine (P = 0.51) as well as urea serum (P = 0.30). One the other hand, enriched pathways were differentiated related to carbohydrate digestion, and absorption, glycolysis, pyruvate metabolism, oxidative phosphorylation, pentose phosphate pathway, and biosynthesis of amino acids of the exclusively expressed proteins in PRU cows. Shifting urea supply from the rumen to post-ruminal compartments decreases muscle catabolism in cows during late gestation. Our findings indicate that post-ruminal urea supplementation for beef cows at late gestation may improve the energy metabolism to support maternal demands. In addition, the post-ruminal urea release seems to be able to trigger pathways to counterbalance the oxidative stress associated to the increase liver metabolic rate.

Direct formation and site-selective elaboration of methionine sulfoximine in polypeptides

Sulfoximines are emerging moieties for medicinal and biological chemistry, due in part to their efficacy in selective inhibition of amide-forming enzymes such as γ-glutamylcysteine synthetase. While small-molecule sulfoximines such as methionine sulfoximine (MSO) and its derivatives are well studied, structures with methionine sulfoximine residues within complex polypeptides have been generally inaccessible. This paper describes a straightforward means of late-stage one-step oxidation of methionine residues within polypeptides to afford NH-sulfoximines. We also present chemoselective subsequent elaboration, most notably by copper(ii)-mediated N-H cross-coupling at methionine sulfoximine residues with arylboronic acid reagents. This development serves as a strategy to incorporate diverse sulfoximine structures within natural polypeptides, and also identifies the methionine sulfoximine residue as a new site for bioorthogonal, chemoselective bioconjugation.

Synthesis and Transformations of NH-Sulfoximines

Recent years have seen a marked increase in the occurrence of sulfoximines in the chemical sciences, often presented as valuable motifs for medicinal chemistry. This has been prompted by both pioneering works taking sulfoximine containing compounds into clinical trials and the concurrent development of powerful synthetic methods. This review covers recent developments in the synthesis of sulfoximines concentrating on developments since 2015. This includes extensive developments in both S-N and S-C bond formations. Flow chemistry processes for sulfoximine synthesis are also covered. Finally, subsequent transformations of sulfoximines, particularly in N-functionalization are reviewed, including N-S, N-P, N-C bond forming processes and cyclization reactions.

CHOK1SV GS-KO SSI expression system: A combination of the Fer1L4 locus and glutamine synthetase selection

There are an ever-increasing number of biopharmaceutical candidates in clinical trials fueling an urgent need to streamline the cell line development process. A critical part of the process is the methodology used to generate and screen candidate cell lines compatible with GMP manufacturing processes. The relatively large amount of clone phenotypic variation observed from conventional "random integration" (RI)-based cell line construction is thought to be the result of a combination of the position variegation effect, genome plasticity and clonal variation. Site-specific integration (SSI) has been used by several groups to temper the influence of the position variegation effect and thus reduce variability in expression of biopharmaceutical candidates. Following on from our previous reports on the application of the Fer1L4 locus for SSI in CHOK1SV (10E9), we have combined this locus and a CHOK1SV glutamine synthetase knockout (GS-KO) host to create an improved expression system. The host, CHOK1SV GS-KO SSI (HD7876), was created by homology directed integration of a targetable landing pad flanked with incompatible Frt sequences in the Fer1L4 gene. The targeting vector contains a promoterless GS expression cassette and monoclonal antibody (mAb) expression cassettes, flanked by Frt sites compatible with equivalent sites flanking the landing pad in the host cell line. SSI clones expressing four antibody candidates, selected in a streamlined cell line development process, have mAb titers which rival RI (1.0-4.5 g/L) and robust expression stability (100% of clones stable through the 50 generation "manufacturing window" which supports commercial manufacturing at 12,000 L bioreactor scale).

Skeletal muscle amino acid uptake is lower and alanine production is greater in late gestation intrauterine growth-restricted fetal sheep hindlimb

In a sheep model of intrauterine growth restriction (IUGR) produced from placental insufficiency, late gestation fetuses had smaller skeletal muscle mass, myofiber area, and slower muscle protein accretion rates compared with normally growing fetuses. We hypothesized that IUGR fetal muscle develops adaptations that divert amino acids (AAs) from protein accretion and activate pathways that conserve substrates for other organs. We placed hindlimb arterial and venous catheters into late gestation IUGR (n = 10) and control (CON, n = 8) fetal sheep and included an external iliac artery flow probe to measure hindlimb AA uptake rates. Arterial and venous plasma samples and biceps femoris muscle were analyzed by mass spectrometry-based metabolomics. IUGR fetuses had greater abundance of metabolites enriched within the alanine, aspartate, and glutamate metabolism pathway compared with CON. Net uptake rates of branched-chain AA (BCAA) were lower by 42%-73%, and muscle ammoniagenic AAs (alanine, glycine, and glutamine) were lower by 107%-158% in IUGR hindlimbs versus CON. AA uptake rates correlated with hindlimb weight; the smallest hindlimbs showed net release of ammoniagenic AAs. Gene expression levels indicated a decrease in BCAA catabolism in IUGR muscle. Plasma purines were lower and plasma uric acid was higher in IUGR versus CON, possibly a reflection of ATP conservation. We conclude that IUGR skeletal muscle has lower BCAA uptake and develops adaptations that divert AAs away from protein accretion into alternative pathways that sustain global energy production and nitrogen disposal in the form of ammoniagenic AAs for metabolism in other organs.

The role of astrocytes in seizure generation: insights from a novel in vitro seizure model based on mitochondrial dysfunction

Approximately one-quarter of patients with mitochondrial disease experience epilepsy. Their epilepsy is often severe and resistant towards conventional antiepileptic drugs. Despite the severity of this epilepsy, there are currently no animal models available to provide a mechanistic understanding of mitochondrial epilepsy. We conducted neuropathological studies on patients with mitochondrial epilepsy and found the involvement of the astrocytic compartment. As a proof of concept, we developed a novel brain slice model of mitochondrial epilepsy by the application of an astrocytic-specific aconitase inhibitor, fluorocitrate, concomitant with mitochondrial respiratory inhibitors, rotenone and potassium cyanide. The model was robust and exhibited both face and predictive validity. We then used the model to assess the role that astrocytes play in seizure generation and demonstrated the involvement of the GABA-glutamate-glutamine cycle. Notably, glutamine appears to be an important intermediary molecule between the neuronal and astrocytic compartment in the regulation of GABAergic inhibitory tone. Finally, we found that a deficiency in glutamine synthetase is an important pathogenic process for seizure generation in both the brain slice model and the human neuropathological study. Our study describes the first model for mitochondrial epilepsy and provides a mechanistic insight into how astrocytes drive seizure generation in mitochondrial epilepsy.

Superfluous glutamine synthetase activity in Chinese Hamster Ovary cells selected under glutamine limitation is growth limiting in glutamine-replete conditions and can be inhibited by serine

Passaging and expansion of animal cells in lean maintenance medium could result in periods of limitation of some nutrients. Over time, such stresses could possibly result in selection of cells with metabolic changes and contribute to heterogeneity. Here, we investigate whether selection of Chinese Hamster Ovary (CHO) cells under glutamine limitation results in changes in growth under glutamine-replete conditions. In glutamine-limiting medium, compared to control cells passaged in glutamine-rich medium, the selected cells showed higher glutamine synthetase (GS) activity and attained a higher peak viable cell density (PVCD). Surprisingly, in glutamine-replete conditions, selected cells still showed a higher GS activity but a lower PVCD. We show that in glutamine-replete medium, PVCD of selected cells was restored on (a) inhibition of GS activity with methionine sulfoximine, (b) supplementation of aspartate-without affecting GS activity, and (c) supplementation of serine, which is reported to inhibit GS in vitro. Consistent with the reported effect of serine, inhibition of GS activity was observed upon serine supplementation along with reduced growth of cells under glutamine-limiting conditions. The latter observation is important for the design of glutamine-free culture medium and feed used for GS-CHO and GS-NS0. In summary, we show that CHO cells selected under glutamine limitation have superfluous GS activity in glutamine-replete medium, which negatively affects their PVCD. This may be due to its effect on availability of aspartate which was the limiting nutrient for the growth of selected cells in glutamine-replete conditions.

Therapeutic effects of methionine sulfoximine in multiple diseases include and extend beyond inhibition of glutamine synthetase

INTRODUCTION

Methionine sulfoximine (MSO), a well-characterized inhibitor of glutamine synthetase, displays significant therapeutic benefits in animal models for several human diseases. This amino acid might therefore be a viable candidate for drug development to treat diseases for which there are few effective therapies. Areas covered: We describe the effects of MSO on brain swelling occurring in overt hepatic encephalopathy resulting from liver failure, the effects of MSO on excitotoxic damage involved in amyotrophic lateral sclerosis (ALS) or resulting from stroke, and the effects of MSO on a model for an inflammatory immune response involved in a range of diseases. We conclude that these results imply the existence of another therapeutic target for MSO in addition to glutamine synthetase. Expert opinion: We summarize the various diseases for which MSO treatment might be a candidate for drug development. We discuss why MSO has limited enthusiasm in the scientific and medical communities for use in humans, with a rebuttal to those negative opinions. And we conclude that MSO should be considered a candidate drug to treat brain swelling involved in overt hepatic encephalopathy and diseases involving an inflammatory immune response.

Glutathione in the human brain: Review of its roles and measurement by magnetic resonance spectroscopy

We review the transport, synthesis and catabolism of glutathione in the brain as well as its compartmentation and biochemistry in different brain cells. The major reactions involving glutathione are reviewed and the factors limiting its availability in brain cells are discussed. We also describe and critique current methods for measuring glutathione in the human brain using magnetic resonance spectroscopy, and review the literature on glutathione measurements in healthy brains and in neurological, psychiatric, neurodegenerative and neurodevelopmental conditions In summary: Healthy human brain glutathione concentration is ∼1-2 mM, but it varies by brain region, with evidence of gender differences and age effects; in neurological disease glutathione appears reduced in multiple sclerosis, motor neurone disease and epilepsy, while being increased in meningiomas; in psychiatric disease the picture is complex and confounded by methodological differences, regional effects, length of disease and drug-treatment. Both increases and decreases in glutathione have been reported in depression and schizophrenia. In Alzheimer's disease and mild cognitive impairment there is evidence for a decrease in glutathione compared to age-matched healthy controls. Improved methods to measure glutathione in vivo will provide better precision in glutathione determination and help resolve the complex biochemistry of this molecule in health and disease.

Glutamine Synthetase: Role in Neurological Disorders

Glutamine synthetase (GS) is an ATP-dependent enzyme found in most species that synthesizes glutamine from glutamate and ammonia. In brain, GS is exclusively located in astrocytes where it serves to maintain the glutamate-glutamine cycle, as well as nitrogen metabolism. Changes in the activity of GS, as well as its gene expression, along with excitotoxicity, have been identified in a number of neurological conditions. The literature describing alterations in the activation and gene expression of GS, as well as its involvement in different neurological disorders, however, is incomplete. This review summarizes changes in GS gene expression/activity and its potential contribution to the pathogenesis of several neurological disorders, including hepatic encephalopathy, ischemia, epilepsy, Alzheimer's disease, amyotrophic lateral sclerosis, traumatic brain injury, Parkinson's disease, and astroglial neoplasms. This review also explores the possibility of targeting GS in the therapy of these conditions.

Inhibition of human glutamine synthetase by L-methionine-S,R-sulfoximine-relevance to the treatment of neurological diseases

At high concentrations, the glutamine synthetase inhibitor L-methionine-S,R-sulfoximine (MSO) is a convulsant, especially in dogs. Nevertheless, sub-convulsive doses of MSO are neuroprotective in rodent models of hyperammonemia, acute liver disease, and amyotrophic lateral sclerosis and suggest MSO may be clinically useful. Previous work has also shown that much lower doses of MSO are required to produce convulsions in dogs than in primates. Evidence from the mid-20th century suggests that humans are also less sensitive. In the present work, the inhibition of recombinant human glutamine synthetase by MSO is shown to be biphasic-an initial reversible competitive inhibition (K i 1.19 mM) is followed by rapid irreversible inactivation. This K i value for the human enzyme accounts, in part, for relative insensitivity of primates to MSO and suggests that this inhibitor could be used to safely inhibit glutamine synthetase activity in humans.

Quantitative analysis reveals dominance of gliogenesis over neurogenesis in an adult brainstem oscillator

Neural stem/progenitor cells in the neurogenic niches of the adult brain are widely assumed to give rise predominantly to neurons, rather than glia. Here, we performed a quantitative analysis of the resident neural progenitors and their progeny in the adult pacemaker nucleus (Pn) of the weakly electric fish Apteronotus leptorhynchus. Approximately 15% of all cells in this brainstem nucleus are radial glia-like neural stem/progenitor cells. They are distributed uniformly within the tissue and are characterized by the expression of Sox2 and Meis 1/2/3. Approximately 2-3% of them are mitotically active, as indicated by expression of proliferating cell nuclear antigen. Labeling of proliferating cells with a single pulse of BrdU, followed by chases of up to 100 days, revealed that new cells are generated uniformly throughout the nucleus and do not undergo substantial migration. New cells differentiate into S100+ astrocytes and Hu C/D+ small interneurons at a ratio of 4:1, reflecting the proportions of the total glia and neurons in this brain region. The continuous addition of new cells leads to a diffuse growth of the Pn, which doubles in volume and total cell number over the first 2 years following sexual maturation of the fish. However, the number of pacemaker and relay cells, which constitute the oscillatory neural network, remains constant throughout adult life. We hypothesize that the dominance of gliogenesis is an adaptation to the high-frequency firing of the oscillatory neurons in this nucleus.

Haloacetonitriles: metabolism and toxicity

The haloacetonitriles (HANs) exist in drinking water exclusively as byproducts of disinfection. HANs are found in drinking water more often, and in higher concentrations, when surface water is treated by chloramination. Human exposure occurs through consumption of finished drinking water; oral and dermal contact also occurs, and results from showering, swimming and other activities. HANs are reactive and are toxic to gastrointestinal tissues following oral administration. Such toxicity is characterized by GSH depletion, increased lipid peroxidation, and covalent binding of HAN-associated radioactivity to gut tissues. The presence of GSH in cells is an important protective mechanism against HAN toxicity; depletion of cellular GSH results in increased toxicity. Some studies have demonstrated an apparently synergistic effect between ROS and HAN administration, that may help explain effects observed in GI tissues. ROS are produced in gut tissues, and in vitro evidence indicates that ROS may contribute to the degradation and formation of reactive intermediates from HANs. The rationale for ROS involvement may involve HAN-induced depletion of GSH and the role of GSH in scavenging ROS. In addition to effects on GI tissues, studies show that HAN-derived radiolabel is found covalently bound to proteins and DNA in several organs and tissues. The addition of antioxidants to biologic systems protects against HAN-induced DNA damage. The protection offered by antioxidants supports the role of oxidative stress and the potential for a threshold in han-induced toxicity. However, additional data are needed to substantiate evidence for such a threshold. HANs are readily absorbed from the GI tract and are extensively metabolized. Elimination occurs primarily in urine, as unconjugated one-carbon metabolites. Evidence supports the involvement of mixed function oxidases, the cytochrome P450 enzyme family and GST, in HAN metabolism. Metabolism represents either a detoxification or bioactivation process, depending on the particular HAN and the enzyme involved. HANs can inhibit CYP2E1-mediated metabolism, an effect which may be dependent on a covalent interaction with the enzyme. In addition, HAN compounds inhibit GST-mediated conjugation, but this effect is reversible upon dialysis, indicating that the interaction does not represent covalent binding. No subchronic studies of HAN toxicity are available in the literature. However, studies show that HANs produce developmental toxicity in experimental animals. The nature of developmental toxicity is affected by the type of administration vehicle, which renders interpretation of results more difficult. Skin tumors have been found following dermal application of HANs, but oral studies for carcinogenicity are negative. Pulmonary adenomas were increased following oral administration of HANs, but the A/J strain of mice employed has a characteristically high background rate of such tumors. HANs interact with DNA to produce unscheduled DNA repair, SCE and reverse mutations in Salmonella. HANs did not induce micronuclei or cause alterations in sperm head morphology in mice, but did induce micronuclei in newts. Thus, there is concern for the potential carcinogenicity of HANs. It would be valuable to delineate any relationship between the apparent threshold for micronuclei formation in newts and the potential mechanism of toxicity involving HAN-induced oxidative stress. Dose-response studies in rodents may provide useful information on toxicity mechanisms and dose selection for longer term toxicity studies. Additional studies are warranted before drawing firm conclusions on the hazards of HAN exposure. Moreover, additional studies on HAN-DNA and HAN-protein interaction mechanisms, are needed. Such studies can better characterize the role of metabolism in toxicity of individual HANs, and delineate the role of oxidative stress, both of which enhance the capacity to predict risk. Most needed, now, are new subchronic (and chronic) toxicity studies; the results of such well-planned, controlled, conducted, interpreted and published investigations would be valuable in establishing margins of safety for HANs in human health risk assessment.

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.

All fourMycobacterium tuberculosis glnA genes encode glutamine synthetase activities but only GlnA1 is abundantly expressed and essential for bacterial homeostasis

Glutamine synthetases (GS) are ubiquitous enzymes that play a central role in every cell's nitrogen metabolism. We have investigated the expression and activity of all four genomic Mycobacterium tuberculosis GS - GlnA1, GlnA2, GlnA3 and GlnA4 - and four enzymes regulating GS activity and/or nitrogen and glutamate metabolism - adenylyl transferase (GlnE), gamma-glutamylcysteine synthase (GshA), UDP-N-acetylmuramoylalanine-D-glutamate ligase (MurD) and glutamate racemase (MurI). All eight genes are located in multigene operons except for glnA1, and all are transcribed in M. tuberculosis; however, some are not translated or translated at such low levels that the enzymes escape detection. Of the four GS, only GlnA1 can be detected. Each of the eight genes, as well as the glnA1-glnE-glnA2 cluster, was expressed separately in Mycobacterium smegmatis, and its gene product was characterized and assayed for enzymatic activity by analysing the reaction products. In M. smegmatis, all four recombinant-overexpressed GS are multimeric enzymes exhibiting GS activity. Whereas GlnA1, GlnA3 and GlnA4 catalyse the synthesis of L-glutamine, GlnA2 catalyses the synthesis of D-glutamine and D-isoglutamine. The generation of mutants in M. tuberculosis of the four glnA genes, murD and murI demonstrated that all of these genes except glnA1 are nonessential for in vitro growth. L-methionine-S,R-sulphoximine (MSO), previously demonstrated to inhibit M. tuberculosis growth in vitro and in vivo, strongly inhibited all four GS enzymes; hence, the design of MSO analogues with an improved therapeutic to toxic ratio remains a promising strategy for the development of novel anti-M. tuberculosis drugs.

Structure of Mycobacterium tuberculosis glutamine synthetase in complex with a transition-state mimic provides functional insights

Glutamine synthetase catalyzes the ligation of glutamate and ammonia to form glutamine, with the resulting hydrolysis of ATP. The enzyme is a central component of bacterial nitrogen metabolism and is a potential drug target. Here, we report a high-yield recombinant expression system for glutamine synthetase of Mycobacterium tuberculosis together with a simple purification. The procedure allowed the structure of a complex with a phosphorylated form of the inhibitor methionine sulfoximine, magnesium, and ADP to be solved by molecular replacement and refined at 2.1-A resolution. To our knowledge, this study provides the first reported structure for a taut form of the M. tuberculosis enzyme, similar to that observed for the Salmonella enzyme earlier. The phospho compound, generated in situ by an active enzyme, mimics the phosphorylated tetrahedral adduct at the transition state. Some differences in ligand interactions of the protein with both phosphorylated compound and nucleotide are observed compared with earlier structures; a third metal ion also is found. The importance of these differences in the catalytic mechanism is discussed; the results will help guide the search for specific inhibitors of potential therapeutic interest.

An astrocyte toxin influences the pattern of breathing and the ventilatory response to hypercapnia in neonatal rats

Recent in vitro data suggest that astrocytes may modulate respiration. To examine this question in vivo, we treated 5-day-old rat pups with methionine sulfoximine (MS), a compound that alters carbohydrate and glutamate metabolism in astrocytes, but not neurons. MS-treated pups displayed a reduced breathing frequency (f) in baseline conditions relative to saline-treated pups. Hypercapnia (5% CO(2)) increased f in both groups, but f still remained significantly lower in the MS-treated group. No differences between treatment groups in the responses to hypoxia (8% O(2)) were observed. Also, MS-treated rats showed an enhanced accumulation of glycogen in neurons of the facial nucleus, the nucleus ambiguus, and the hypoglossal nucleus, structures that regulate respiratory activity and airway patency. An altered transfer of nutrient molecules from astrocytes to neurons may underlie these effects of MS, although direct effects of MS upon neurons or upon peripheral structures that regulate respiration cannot be completely ruled out as an explanation.

The stability of pseudopeptides bearing sulfoximines as chiral backbone modifying element towards proteinase K

Incorporation of sulfoximines as backbone modifying element results in two new pseudopeptide bonds which display enhanced (bond A) and strongly reduced reactivity (bond B) towards hydrolysis by Proteinase K.

Tor1/2 regulation of retrograde gene expression in Saccharomyces cerevisiae derives indirectly as a consequence of alterations in ammonia metabolism

Retrograde genes of Saccharomyces cerevisiae encode the enzymes needed to synthesize alpha-ketoglutarate, required for ammonia assimilation, when mitochondria are damaged or non-functional because of glucose fermentation. Therefore, it is not surprising that a close association exists between control of the retrograde regulon and expression of nitrogen catabolic genes. Expression of these latter genes is nitrogen catabolite repression (NCR)-sensitive, i.e. expression is low with good nitrogen sources (e.g. glutamine) and high when only poor (e.g. proline) or limiting nitrogen sources are available. It has been reported recently that both NCR-sensitive and retrograde gene expression is negatively regulated by glutamine and induced by treating cells with the Tor1/2 inhibitor, rapamycin. These conclusions predict that NCR-sensitive and retrograde gene expression should respond in parallel to nitrogen sources, ranging from those that highly repress NCR-sensitive transcription to those that elicit minimal NCR. Because this prediction did not accommodate earlier observations that CIT2 (a retrograde gene) expression is higher in glutamine than proline containing medium, we investigated retrograde regulation further. We show that (i) retrograde gene expression correlates with intracellular ammonia and alpha-ketoglutarate generated by a nitrogen source rather than the severity of NCR it elicits, and (ii) in addition to its known regulation by NCR, NAD-glutamate dehydrogenase (GDH2) gene expression is down-regulated by ammonia under conditions where NCR is minimal. Therefore, intracellular ammonia plays a pivotal dual role, regulating the interface of nitrogen and carbon metabolism at the level of ammonia assimilation and production. Our results also indicate the effects of rapamycin treatment on CIT2 transcription, and hence Tor1/2 regulation of retrograde gene expression occur indirectly as a consequence of alterations in ammonia and glutamate metabolism.

Analysis of glutathione: implication in redox and detoxification

BACKGROUND

Glutathione is a ubiquitous thiol-containing tripeptide, which plays a central role in cell biology. It is implicated in the cellular defence against xenobiotics and naturally occurring deleterious compounds, such as free radicals and hydroperoxides. Glutathione status is a highly sensitive indicator of cell functionality and viability. Its levels in human tissues normally range from 0.1 to 10 mM, being most concentrated in liver (up to 10 mM) and in the spleen, kidney, lens, erythrocytes and leukocytes. In humans, GSH depletion is linked to a number of disease states including cancer, neurodegenerative and cardiovascular diseases. The present review proposes an analysis of the current knowledge about the methodologies for measuring glutathione in human biological samples and their feasibility as routine methods in clinical chemistry. Furthermore, it elucidates the fundamental role of glutathione in pathophysiological conditions and its implication in redox and detoxification process.

TESTS AVAILABLE

Several methods have been optimised in order to identify and quantify glutathione forms in human biological samples. They include spectrophotometric, fluorometric and bioluminometric assays, often applied to HPLC analysis. Recently, a liquid chromatography-mass spectrometry technique for glutathione determination has been developed that, however, suffers from the lack of total automation and the high cost of the equipment.

CONCLUSION

Glutathione is a critical factor in protecting organisms against toxicity and disease. This review may turn useful for analysing the glutathione homeostasis, whose impairment represents an indicator of tissue oxidative status in human subjects.

Inhibition of glutamine synthetase in a549 cells during hyperoxia

High oxygen concentrations are used in the treatment of acute respiratory distress syndrome and hyaline membrane disease. Hyperoxia, however, can damage alveolar epithelial cells through the release of free oxygen radicals. Supplemental glutamine (Gln) has recently been shown to increase survival of A549 cells, a distal epithelial cell line, during hyperoxia (). We found that supplemental Gln (Gln+) is essential for cell growth in A549 cells. In room air, cells without supplemental Gln (Gln-) survived with BCL-2 levels similar to those of Gln+ cells, but cell growth was minimal. We also evaluated the role of glutamine synthetase (GS) in A549 cells during hyperoxia. L-methionine sulfoximine (MSO), an irreversible inhibitor of GS, was added to Gln+ and Gln- cells. In hyperoxia, Gln- cells had greater survival then Gln- cells treated with MSO. Supplemental Gln could rescue cells in hyperoxia from the effect of MSO, suggesting that GS, through the endogenous synthesis of Gln, could attenuate hyperoxic cell injury. In hyperoxia, cells treated with 10-mM concentrations of Gln had increased survival compared with cells receiving 2-mM concentrations. The higher concentration of Gln, however, did not decrease the percentage of cells undergoing necrosis.

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

Gamma-glutamylcysteine synthetase (gamma-GCS, glutamate-cysteine ligase), which catalyzes the first and rate-limiting step in glutathione biosynthesis, is present in many prokaryotes and in virtually all eukaryotes. Although all eukaryotic gamma-GCS isoforms examined to date are rapidly inhibited by buthionine sulfoximine (BSO), most reports indicate that bacterial gamma-GCS is resistant to BSO. We have confirmed the latter finding with Escherichia coli gamma-GCS under standard assay conditions, showing both decreased initial binding affinity for BSO and a reduced rate of BSO-mediated inactivation compared with mammalian isoforms. We also find that substitution of Mn2+ for Mg2+ in assay mixtures increases both the initial binding affinity of BSO and the rate at which BSO causes mechanism-based inactivation. Similarly, the specificity of E. coli gamma-GCS for its amino acid substrates is broadened in the presence of Mn2+, and the rate of reaction for some very poor substrates is improved. These results suggest that divalent metal ions have a role in amino acid binding to E. coli gamma-GCS. Electron paramagnetic resonance (EPR) studies carried out with Mn2+ show that E. coli gamma-GCS binds two divalent metal ions; Kd values for Mn2+ are 1.1 microm and 82 microm, respectively. Binding of l-glutamate or l-BSO to the two Mn2+/gamma-GCS species produces additional upfield and downfield X-band EPR hyperfine lines at 45 G intervals, a result indicating that the two Mn2+ are spin-coupled and thus apparently separated by 5 A or less in the active site. Additional EPR studies in which Cu2+ replaced Mg2+ or Mn2+ suggest that Cu2+ is bound by one N and three O ligands in the gamma-GCS active site. The results are discussed in the context of the catalytic mechanism of gamma-GCS and its relationship to the more fully characterized glutamine synthetase reaction.

The brain response to 2-deoxy glucose is blocked by a glial drug

Two brain regions - the basomedial hypothalamus and area postrema (AP) - react to changes in circulating glucose levels by altering feeding behavior and the secretion of pituitary and non-pituitary hormones. The precise identity of cells responding to glucose in these regions is uncertain. The recent detection of high-capacity glucose transporter proteins in astrocytes in these areas has suggested that astrocytes may play a role in glucose sensing by the brain. To test this hypothesis, rats were injected with either saline or methionine sulfoximine (MS), a compound that produces alterations in carbohydrate and glutamate metabolism in astrocytes. Eighteen hours later, rats were injected with either saline or 2-deoxy glucose (2-DG) and brain sections were stained to demonstrate 2-DG-activated neurons immunoreactive for Fos protein. MS-treated rats showed a 70% reduction in numbers of Fos+ neurons in the AP region (p<0.05). Also, specialized, Gomori+ astrocytes were particularly abundant in both glucose sensitive regions and showed a distribution identical to that reported for high-capacity glucose transporter proteins. These data suggest that specialized astrocytes influence the glucose-sensing function of the brain.

An inhibitor of exported Mycobacterium tuberculosis glutamine synthetase selectively blocks the growth of pathogenic mycobacteria in axenic culture and in human monocytes: extracellular proteins as potential novel drug targets

Mycobacterium tuberculosis and other pathogenic mycobacteria export abundant quantities of proteins into their extracellular milieu when growing either axenically or within phagosomes of host cells. One major extracellular protein, the enzyme glutamine synthetase, is of particular interest because of its link to pathogenicity. Pathogenic mycobacteria, but not nonpathogenic mycobacteria, export large amounts of this protein. Interestingly, export of the enzyme is associated with the presence of a poly-L-glutamate/glutamine structure in the mycobacterial cell wall. In this study, we investigated the influence of glutamine synthetase inhibitors on the growth of pathogenic and nonpathogenic mycobacteria and on the poly-L-glutamate/glutamine cell wall structure. The inhibitor L-methionine-S-sulfoximine rapidly inactivated purified M. tuberculosis glutamine synthetase, which was 100-fold more sensitive to this inhibitor than a representative mammalian glutamine synthetase. Added to cultures of pathogenic mycobacteria, L-methionine- S-sulfoximine rapidly inhibited extracellular glutamine synthetase in a concentration-dependent manner but had only a minimal effect on cellular glutamine synthetase, a finding consistent with failure of the drug to cross the mycobacterial cell wall. Remarkably, the inhibitor selectively blocked the growth of pathogenic mycobacteria, all of which release glutamine synthetase extracellularly, but had no effect on nonpathogenic mycobacteria or nonmycobacterial microorganisms, none of which release glutamine synthetase extracellularly. The inhibitor was also bacteriostatic for M. tuberculosis in human mononuclear phagocytes (THP-1 cells), the pathogen's primary host cells. Paralleling and perhaps underlying its bacteriostatic effect, the inhibitor markedly reduced the amount of poly-L-glutamate/glutamine cell wall structure in M. tuberculosis. Although it is possible that glutamine synthetase inhibitors interact with additional extracellular proteins or structures, our findings support the concept that extracellular proteins of M. tuberculosis and other pathogenic mycobacteria are worthy targets for new antibiotics. Such proteins constitute readily accessible targets of these relatively impermeable organisms, which are rapidly developing resistance to conventional antibiotics.

The enzymes of glutathione synthesis: gamma-glutamylcysteine synthetase

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

Inhibition of the high-affinity uptake of D-[3H]aspartate in rate by L-alpha-aminoadipate and arachidonic acid

The mechanism of inhibition of the high-affinity sodium-dependent transport of D-[3H]aspartate by the gliotoxin, L-alpha-aminoadipate, and also by the endogenous fatty acid, arachidonic acid (cis-5,8,11,14 eicosatetraenoic acid), into rat brain synaptosomes has been investigated. L-alpha-Aminoadipate competitively inhibited the transport of D-[3H]aspartate with a K1 value of 192 microM. Superfusion of coronal slices of rat brain for 40 min with 1 mM L-alpha-aminoadipate reduced the glutathione concentration of the tissue by 20%. Neither glutamate nor kainate depleted the glutathione level of the slices. Pre-incubation of synaptosomes with arachidonic acid (10 microM) for 10-60 min produced a marked potentiation of the inhibition of D-[3H]aspartate transport, compared to experiments in which the acid was added concurrently with the D-[3H]aspartate ('co-incubation' experiments). Inhibition of D-[3H]aspartate transport by arachidonic acid was not blocked by addition of nordihydroguaretic acid to the pre-incubation medium. Staurosporine (50 nM) reduced the inhibition of transport occurring during pre-incubation with 10 microM arachidonic acid, and there was no longer any significant difference from the level of inhibition obtained in co-incubation experiments. Phorbol, 12-myristate, 13-acetate (1 microM) reduced the transport of D-[3H]aspartate to 73% of control after 20 min pre-incubation of the synaptosomes. This study highlights the fact that inhibition of glutamate transport may affect brain function in a number of different ways. Competitive inhibition by a structural analogue of glutamate, such as L-alpha-aminoadipate, leads to a reduction in the glutathione level, which may be an important factor in L-alpha-aminoadipate-mediated toxicity. On the other hand, the more long-term effects of non-competitive inhibition of glutamate transport by arachidonic acid, in a mechanism involving protein kinase C, may represent a physiological means for regulation of transporter activity in the brain.

Alteration in the glial cell metabolism of glutamate by kainate and N-methyl-D-aspartate

Incubation of coronal slices of rat brain with neurotoxic concentrations of kainate (300 microM) and N-methyl-D-aspartate (NMDA; 500 microM) for 40 min reduced the activity of the glial enzyme, glutamine synthetase, by 33% and 21%, respectively. The immunoreactivity of the neuronal enzyme, gamma gamma-enolase (neuron-specific enolase), was also decreased, but to a lesser extent than glutamine synthetase. Pre-incubation of the slices with L-methionine-S-sulphoximine (500 microM), an irreversible inhibitor of both glutamine synthetase and gamma-glutamylcysteine synthetase, before addition of either kainate or NMDA produced a supra-additive reduction in the activity of the enzyme in both cases. Neither kainate nor NMDA directly inhibited the activity of glutamine synthetase, but kainate did inhibit gamma-glutamylcysteine synthetase, a rate-limiting enzyme of the gamma-glutamyl cycle, which is responsible for maintaining glutathione levels within cells. Pre-incubation of the slices with L-NG-nitroarginine, a competitive inhibitor of nitric oxide synthase, effectively prevented the NMDA-induced reduction in glutamine synthetase and neuron specific enolase, but did not diminish the kainate-induced decrease in the activity of either enzyme. These results provide evidence that NMDA, as well as kainate, indirectly affects the activity of glutamine synthetase in brain slices, yet does so by a different mechanism from kainate. The results are discussed in terms of the possible mode of action of each toxin in inhibiting the glial cell metabolism of glutamate.

Inhibition of the glutamate transporter and glial enzymes in rat striatum by the gliotoxin, alpha aminoadipate

1. The effect of the gliotoxic analogue of glutamate, alpha aminoadipate, on the high affinity transport of D-[3H]-aspartate into a crude striatal P2 preparation, and on the activity of two enzymes of which glutamate is the substrate has been examined. 2. The L-isomer of alpha aminoadipate competitively inhibited the transport protein, with a Ki value of 192 microM, whereas the D-isomer of alpha aminoadipate was ineffective. The potent convulsant, L-methionine-S-sulphoximine, was also without effect on the activity of the glutamate transport protein. 3. L-alpha Aminoadipate was a competitive inhibitor of both glutamine synthetase, and gamma-glutamylcysteine synthetase, with Ki values of 209 microM and 7 mM respectively. Once again, the D-isomer of alpha aminoadipate was a far weaker inhibitor of either enzyme. 4. The results are discussed in terms of the mechanism of action of alpha aminoadipate in causing toxicity of glial cells.

Effect of reducing brain glutamine synthesis on metabolic symptoms of hepatic encephalopathy

Liver failure, or shunting of intestinal blood around the liver, results in hyperammonemia and cerebral dysfunction. Recently it was shown that ammonia caused some of the metabolic signs of hepatic encephalopathy only after it was metabolized by glutamine synthetase in the brain. In the present study, small doses of methionine sulfoximine, an inhibitor of cerebral glutamine synthetase, were given to rats either at the time of portacaval shunting or 3-4 weeks later. The effects on several characteristic cerebral metabolic abnormalities produced by portacaval shunting were measured 1-3 days after injection of the inhibitor. All untreated portacaval-shunted rats had elevated plasma and brain ammonia concentrations, increased brain glutamine and tryptophan content, decreased brain glucose consumption, and increased permeability of the blood-brain barrier to tryptophan. All treated rats had high ammonia concentrations, but the brain glutamine content was normal, indicating inhibition of glutamine synthesis. One day after shunting and methionine sulfoximine administration, glucose consumption, tryptophan transport, and tryptophan brain content remained near control values. In the 3-4-week-shunted rats, which were studied 1-3 days after methionine sulfoximine administration, the effect was less pronounced. Brain glucose consumption and tryptophan content were partially normalized, but tryptophan transport was unaffected. The results agree with our earlier conclusion that glutamine synthesis is an essential step in the development of cerebral metabolic abnormalities in hyperammonemic states.

Enhanced endogenous ornithine concentrations protect against tonic seizures and coma in acute ammonia intoxication

Pretreatment of mice with 5-fluoromethylornithine (5FMOrn), a selective inactivator of ornithine aminotransferase, diminishes the accumulation of ammonia in the brain after administration of ammonium acetate, and antagonizes ammonia-induced fatal tonic extensor convulsions. In about 50% of the treated animals the loss of the righting reflex and coma is prevented. Presumably these effects are based on the enhancement of urea formation by the increased liver ornithine concentrations. However, since brain ornithine concentrations are greatly enhanced by 5FMOrn, it is not excluded that ornithine has direct effects on cellular events involved in ammonia-induced seizure generation, even though 5FMOrn had no anticonvulsant properties in a series of established animal seizure models, including N-methyl-D,L-aspartate-induced convulsions. NMDA receptor antagonists are capable of preventing death, but do not protect against the generation of coma and tonic extensor convulsions in ammonium acetate intoxicated mice. Since no evidence was found for ammonia-induced glutamate release from rat hippocampus, there is no convincing evidence for the idea that the tonic convulsions are mediated by NMDA receptors. L-Methionine-D, L-sulfoximine (MSO)-induced seizures can be partially antagonized by pretreatment with 5FMOrn. However, the effect is considerably smaller than against ammonia-induced convulsions, although at the time of seizure onset brain ammonia levels of MSO-intoxicated mice were lower than in the animals receiving ammonium acetate. This suggests that MSO-convulsions are not entirely due to the elevation of brain ammonia concentrations, even though MSO administration mimics effects of ammonia on cortical inhibitory neuronal interactions.(ABSTRACT TRUNCATED AT 250 WORDS)

Variations in the effects of L-methionine-DL-sulfoximine on the activity of cerebral gamma-glutamyl transpeptidase in rats as a function of age

Changes in the activity of gamma-glutamyl transpeptidase (GGTP) and the effects of a subacute dose (150 mg/kg b.wt.) of methionine sulfoximine (MSO) on its activity were studied in cerebral cortex, cerebellum and brainstem of rats of 4 different age groups. GGTP activity increased with increasing age between days 10 and 180 after birth and decreased between 180 and 360 days of age in these 3 regions of the brain. Older animals showed convulsions and succumbed to the toxic effects of MSO. In the younger animals, wobbly gait and splayed leggedness were noticed in the earlier time periods and these animals recovered from the toxic effects of the drug. Following the administration of MSO, GGTP activity was suppressed in all the regions (excepting the cerebral cortex and cerebellum of 90-day-old animals). In 10- and 90-day-old animals). In 10- and 90-day-old animals, there was a complete recovery of enzyme activity which exceeded the control value as the time progressed. No such recovery was observed in 180- and 360-days-old animals. Parallel changes were observed in the GGTP activity and the behavioural pattern of the animals. These results were discussed in relation to the localization of this enzyme in astrocytes and capillaries.

Analytical and preparative separation of the diastereomers of L-buthionine (SR)-sulfoximine, a potent inhibitor of glutathione biosynthesis

Buthionine sulfoximine inhibits gamma-glutamylcysteine synthetase, the enzyme catalyzing the first reaction of glutathione (GSH) biosynthesis. GSH synthesis is blocked in animals or cultured cells exposed to buthionine sulfoximine, and GSH is substantially depleted in cells or tissues with moderate to high rates of GSH utilization. Studies reported to date have used DL-buthionine (SR)-sulfoximine or L-buthionine (SR)-sulfoximine, mixtures of four and two isomers, respectively. The present report describes a chiral solvent HPLC procedure for the analytical separation of the diastereomers of L-buthionine (SR)-sulfoximine and the separation of those isomers from the unresolved diastereomers of D-buthionine (SR)-sulfoximine. L-buthionine (R)-sulfoximine was isolated preparatively by repeated crystallization of L-buthionine (SR)-sulfoximine from water; L-buthionine (S)-sulfoximine was obtained by crystallization as the trifluoroacetate salt in ethanol/hexane mixtures. The absolute configuration, bond lengths and angles of L-buthionine (R)-sulfoximine were determined by X-ray diffraction. In vitro studies demonstrate that L-buthionine (R)-sulfoximine is a relatively weak inhibitor of rat kidney gamma-glutamylcysteine synthetase; binding is competitive with L-glutamate. L-buthionine (S)-sulfoximine is a tight-binding, mechanism-based inhibitor of the enzyme. Since L-buthionine sulfoximine is initially bound as a transition-state analogue, identification of the inhibitory diastereomer elucidates the steric relationships among ATP, glutamate, and cysteine within the active site. When administered to mice, L-buthionine (S)-sulfoximine (0.2 mmol/kg) was as effective as L-buthionine (SR)-sulfoximine (0.4 mmol/kg) in causing GSH depletion in liver, kidney, and pancreas. L-Buthionine (R)-sulfoximine (0.2 mmol/kg) did not cause significant GSH depletion in liver or pancreas. The L-(R)-diastereomer caused a modest GSH depletion in kidney that is tentatively attributed to interference with gamma-glutamylcyst(e)ine transport.

Glutathione deficiency produced by inhibition of its synthesis, and its reversal; applications in research and therapy

Glutathione, which is synthesized within cells, is a component of a pathway that uses NADPH to provide cells with their reducing milieu. This is essential for (a) maintenance of the thiols of proteins (and other compounds) and of antioxidants (e.g. ascorbate, alpha-tocopherol), (b) reduction of ribonucleotides to form the deoxyribonucleotide precursors of DNA, and (c) protection against oxidative damage, free radical damage, and other types of toxicity. Glutathione interacts with a wide variety of drugs. Despite its many and varied cellular functions, it is possible to achieve therapeutically useful modulations of glutathione metabolism. This article emphasizes an approach in which the synthesis of glutathione is selectively inhibited in vivo leading to glutathione deficiency. This is achieved through use of transition-state inactivators of gamma-glutamylcysteine synthetase, the enzyme that catalyzes the first and rate-limiting step of glutathione synthesis. The effects of marked glutathione deficiency, thus produced in the absence of applied stress, include cellular damage associated with severe mitochondrial degeneration in a number of tissues. Such glutathione deficiency is not prevented or reversed by giving glutathione. The cellular utilization of GSH involves its extracellular degradation, uptake of products, and intracellular synthesis of GSH. This is a normal pathway by which cysteine moieties are taken up by cells. Glutathione deficiency induced by inhibition of its synthesis may be prevented or reversed by administration of glutathione esters which, in contrast to glutathione, are readily transported into cells and hydrolyzed to form glutathione intracellularly. Research derived from this model has led to several potentially useful therapeutic approaches, one of which is currently in clinical trial. Thus, certain tumors, including those that exhibit resistance to several drugs and to radiation, are sensitized to these modalities by selective inhibition of glutathione synthesis. An alternative interpretation is suggested which is based on the concept that some resistant tumors have high capacity for glutathione synthesis and that such increased capacity may be as significant or more significant in promoting the resistance of some tumors than the cellular levels of glutathione. Therapeutic approaches are proposed in which normal cells may be selectively protected against toxic antitumor agents and radiation by cysteine- and glutathione-delivery compounds. Current studies suggest that research on other modulations of glutathione metabolism and transport would be of interest.

Studies on the mechanism of haloacetonitrile-induced gastrointestinal toxicity: interaction of dibromoacetonitrile with glutathione and glutathione-S-transferase in rats

The haloacetonitrile, dibromoacetonitrile (DBAN), is a direct-acting genotoxic agent that has been detected in drinking water. In a time course study, male Sprague-Dawley rats were treated with DBAN (75 mg/kg PO), and killed at 0.5, 1, 2, and 4 hr after treatment. In a dose response study, animals were treated orally with various doses of DBAN (25, 50, 75, and 100 mg/kg) and killed at one-half hour after treatment. Control animals received 1 ml/kg PO of the vehicle dimethyl sulfoxide (DMSO). In both experiments blood and organs were collected and stored at -80 degrees C until the time of analysis. At 0.5 hr after treatment, a single oral dose of DBAN caused a significant decrease of glutathione (GSH) concentrations in liver (54% of control) and stomach (6% of control). Hepatic GSH depletion was maximal at 0.5 hr and rebound to the control levels by 4 hr. In contrast, gastric GSH concentrations remained low at all time points. DBAN caused an insignificant change in both kidney and blood GSH levels. DBAN significantly inhibited glutathione-S-transferase (GST) activity in liver and stomach. Hepatic GST inhibition was maximal (34% of control) at 2 hr and minimal (80% of control) at 4 hr. Meanwhile, in the stomach GST activity was inhibited at 1 hr (60% of control) and remained low at all times after treatment. Both GSH depletion and GST inhibition were dose-dependent. This study indicates that GSH and GST play an important role in the metabolism and detoxification of DBAN in rats. The prolonged depletion of GSH and inhibition of GST in the gastrointestinal (GI) tissues suggest that the GI tract is a major target for DBAN toxicity.

λ-Glutamylcysteine synthetase in higher plants: catalytic properties and subcellular localization

λ-Glutamylcysteine synthetase activity (EC 6.3.2.2) was analysed in Sephacryl S-200 eluents of extracts from cell suspension cultures ofNicotiana tabacum L. cv. Samsun by determination of λ-glutamylcysteine as its monobromobimane derivative. The enzyme has a relative molecular mass (Mr) of 60000 and exhibits maximal activity at pH 8 (50% at pH 7.0 and pH9.0) and an absolute requirement for Mg(2+). With 0.2mM Cd(2+) or Zn(2+), enzyme activity was reduced by 35% and 19%, respectively. Treatment with 5 mM dithioerythritol led to a heavy loss of activity and to dissociation into subunits (Mr 34000). Buthionine sulfoximine andL-methionine-sulfoximine, known as potent inhibitors of λ-glutamylcysteine synthetase from mammalian cells, were found to be effective inhibitors of the plant enzyme too. The apparent Km values forL-glutamate,L-cysteine, and α-aminobutyrate were, respectively, 10.4mM, 0.19 mM, and 6.36 mM. The enzyme was completely inhibited by glutathione (Ki=0.42 mM). The data indicate that the rate of glutathione synthesis in vivo may be influenced substantially by the concentration of cysteine and glutamate and may be further regulated by feedback inhibition of λ-glutamylcysteine synthetase by glutathione itself. λ-Glutamylcysteine synthetase is, like glutathione synthetase, localized in chloroplasts as well as in the cytoplasm. Chloroplasts fromPisum sativum L. isolated on a Percoll gradient contained about 72% of the λ-glutamylcysteine synthetase activity in leaf cells and 48% of the total glutathione synthetase activity. In chloroplasts ofSpinacia oleracea L. about 61% of the total λ-glutamylcysteine synthetase activity of the cells were found and 58% of the total glutathione synthetase activity. These results indicate that glutathione synthesis can take place in at least two compartments of the plant cell.