Neuronal Glutamine Utilization: Glutamine/Glutamate Homeostasis in Synaptosomes

The synaptosomal metabolism of glutamine was studied under in vitro conditions that simulate depolarization in vivo. With [2-15N]glutamine as precursor, the [glutamine]i was diminished in the presence of veratridine or 50 mM KCl, but the total amounts of [15N]glutamate and [15N]aspartate formed were either equal to those of control incubations (veratridine) or higher (50 mM [KCl]). This suggests that depolarization decreases glutamine uptake and independently augments glutaminase activity. Omission of sodium from the medium was associated with low internal levels of glutamine which indicates that influx occurs as a charged Na(+)-amino acid complex. It is postulated that a reduction in membrane potential and a collapse of the Na+ gradient decrease the driving forces for glutamine accumulation and thus inhibit its uptake and enhance its release under depolarizing conditions. Inorganic phosphate stimulated glutaminase activity, particularly in the presence of calcium. At 2 mM or lower [phosphate] in the medium, calcium inhibited glutamine utilization and the production of glutamate, aspartate, and ammonia from glutamine. At a high (10 mM) medium [phosphate], calcium stimulated glutamine catabolism. It is suggested that a veratridine-induced increase in intrasynaptosomal inorganic phosphate is responsible for the enhancement of flux through glutaminase; calcium affects glutaminase indirectly by modulating the level of free intramitochondrial [phosphate]. Because phosphate also lowers the Km of glutaminase for glutamine, augmentation of the amino acid breakdown may occur even when depolarization lowers [glutamine]i. Reducing the intrasynaptosomal glutamate to 26 nmol/mg of protein had little effect on glutamine catabolism, but raising the pH to 7.9 markedly increased formation of glutamate and aspartate. It is concluded that phosphate and H+ are the major physiologic regulators of glutaminase activity.     

Glutamine uptake inZymomonas mobilis

Glutamine was transported inZymomonas mobilis by a mechanism following Michaelis-Menten kinetics with a Km value for glutamine 8 x 10^–5 M and a Vmax value of 15.4 nmol.mg^–1, min^–1 or 40 nmol.mg^–1.min^–1 for cells growing on complete medium or minimal medium respectively. The transport of glutamine was energy-dependent and more or less specific for glutamine when cell were grown on rich media. Evidence provided via spheroplasts suggests the possible involvement of a periplasmic component in this transport system.     

谷氨酰胺合成酶与Wnt信号转导通路

GS（glutamine synthetase）或GLuL（glutamate-ammonia ligase）,即谷氨酰胺合成酶,为人体内重要的功能酶,催化谷氨酸与氨生成谷氨酰胺。在体内氮的代谢中,尤其在维持氨离子和谷氨酰胺的稳定中发挥着重要的作用。GS表达和活性的异常常会导致人体很多疾病的发生。近年来研究发现GS表达和活性的异常与Wnt信号通路的异常密切相关。     

谷氨酰胺的药理作用

谷氨酰胺是肠粘膜上皮细胞,肾小管细胞,淋巴细胞,巨噬细胞及成纤维细胞等的重要能量物质。在促进免疫功能,维持肠道机能,改善酸碱平衡失调及提高机体对创伤、感染等应急的适应方面有极其重要的药理作用。动物实验及临床研究表明谷氨酰胺及其二肽和类似物没有任何毒性及不良反应,如何将谷氨酰胺应用于临床将是今后的主要研究方向。     

谷氨酰胺代谢与肿瘤

“谷氨酰胺代谢”是肿瘤细胞除Warburg效应外又一重要的能量代谢方式。迅速增殖的肿瘤细胞消耗谷氨酰胺（glutamine,Gin）来提供生长和增殖所需的能量和生物大分子原料,维持细胞内氧化还原稳态和参与细胞内信号通路的转导。肿瘤中原癌基因与抑癌基因的突变会影响Gin代谢。结合先进的诊断技术和研究手段将有利于了解肿瘤中Gln的生化代谢,揭示Gin代谢的关键环节,为依赖Gin的肿瘤提供治疗新策略。     

Derepression of the Glutamine Synthetase in Neuroblastoma Cells at Low Concentrations of Glutamine

Abstract: Regulation of the biosynthesis of glutamine synthetase was studied in neuroblastoma cells (Neuro-2A) by use of a recently developed, sensitive radioisotopic assay. The removal of glutamine from the culture medium of these cells for 24 h resulted in a 10-fold increase in glutamine synthetase specific activity (15-fold after 2 weeks) compared with the basal level found in cells grown in the presence of 2 m M glutamine. Following the growth of these cells for 2 weeks in the presence of various concentrations of glutamine, a negative linear correlation was observed between the specific activity of glutamine synthetase (from 1.7 to 0.14 unit/mg) and the concentration of glutamine in the growth medium (from 0.5 to 2 m M ). Cycloheximide or actinomycin D blocked the increase in glutamine synthetase activity observed in the absence of glutamine. These results suggest that the removal of glutamine led to the induction of glutamine synthetase by stimulating new enzyme synthesis. The enzyme was not degraded, but only diluted, by growth upon readdition of glutamine to the medium. The influence of glutamine depletion is also reported for C-6 glioma cells and glial cells in primary cultures.     

Glutamine potentiates TNF-alpha-induced tumor cytotoxicity

L-glutamine (Gln) sensitizes tumor cells to tumor necrosis factor (TNF)-alpha-induced cytotoxicity. The type and mechanism of cell death induced by TNF-alpha was studied in Ehrlich ascites tumor (EAT)-bearing mice fed a Gln-enriched diet (GED; where 30% of the total dietary nitrogen was from Gln). A high rate of Gln oxidation promotes a selective depletion of mitochondrial glutathione (mtGSH) content to approximately 58% of the level found in tumor mitochondria of mice fed a nutritionally complete elemental diet (standard diet, SD). The mechanism of mtGSH depletion involves a glutamate-induced inhibition of GSH transport from the cytosol into mitochondria. The increase in reactive oxygen intermediates (ROIs) production induced by TNF-alpha further depletes mtGSH to approximately 35% of control values, which associates with a decrease in the mitochondrial transmembrane potential (MMP), and elicits mitochondrial membrane permeabilization and release of cytochrome c. Mitochondrial membrane permeabilization was also found in intact tumor cells cultured with a Gln-enriched medium under conditions of buthionine sulfoximine (BSO)-induced selective GSH synthesis inhibition. Enforced expression of the bcl-2 gene in tumor cells could not avoid the glutamine- and TNF-alpha-induced cell death under conditions of mtGSH depletion. However, addition of GSH ester, which delivers free intracellular GSH and increases mtGSH levels, preserved cell viability. These findings show that glutamine oxidation and TNF-alpha, by causing a change in the glutathione redox status within tumor mitochondria, activates the molecular mechanism of apoptotic cell death.     

Glutamine transport in brain mitochondria

Gln is transported into rat brain synaptic and non-synaptic mitochondria by a protein catalyzed process. The uptake is significantly higher in synaptic than in non-synaptic mitochondria. The transport is inhibited by the amino acids Glu, Asn and Asp, and by the TCA cycle intermediates succinate, malate and 2-OG. The inhibition by 2-OG is counteracted by AOA and is therefore assumed to be due to transamination of 2-OG, whereby Glu is formed. This presumes that Glu also binds to an inhibitory site on the matrix face of the inner membrane. The transport is complex and cannot be explained by the simple uniport mechanism which has been proposed for renal (Schoolwerth and LaNoue, 1985), and liver mitochondria (Soboll et al., 1991). Thus, Gln transport is stimulated by respiration and by the proton electrochemical gradient. Since it is indicated that both the neutral Gln zwitterion and the Gln anion are transported, there are probably different uptake mechanisms, but not necessarily different carriers. Gln may be transported by an electroneutral mechanism as a proton compensated anion, as well as electrophoretically as a zwitterion with a proton, and probably also by diffusion as a zwitterion. The properties of the brain mitochondrial Gln uptake mechanisms are also not identical with those of a purified renal Gln transporter. It is possible that the Gln transport is controlled by more than one protein, which may be situated on distinct species in a heterogeneous mitochondrial population. Since Gln is assumed to participate in energy production as well as in the synthesis of nucleic acid components and proteins in brain mitochondria, the control of Gln uptake in these organelles may be important.     

Glutamine homeostasis and mitochondrial dynamics

Glutamine is a multifaceted amino acid that plays key roles in many metabolic pathways and also fulfils essential signaling functions. Although classified as non-essential, recent evidence suggests that glutamine is a conditionally essential amino acid in several physiological situations. Glutamine homeostasis must therefore be exquisitely regulated and mitochondria represent a major site of glutamine metabolism in numerous cell types. Glutaminolysis is mostly a mitochondrial process with repercussions in organelle structure and dynamics suggesting a tight and mutual control between mitochondrial form and cell bioenergetics. In this review we describe an updated account focused on the critical involvement of glutamine in oxidative stress, mitochondrial dysfunction and tumour cell proliferation, with special emphasis in the initial steps of mitochondrial glutamine pathways: transport into the organelle and hydrolytic deamidation through glutaminase enzymes. Some controversial issues about glutamine catabolism within mitochondria are also reviewed.     

谷氨酰胺发酵条件的研究

以黄色短杆菌ＳＧｒ和ＤＯＮｒ抗性菌株作为生产菌株,在１６升罐对产谷氨酰胺的发酵条件进行了研究。试验结果,产谷氨酰胺５．２２％,周期４４ｈ,转化率３３．３％。该变异菌株具有工业化开发的价值。     

Glutamine and the immune system

Summary Glutamine is utilised at a high rate by cells of the immune system in culture and is required to support optimal lymphocyte proliferation and production of cytokines by lymphocytes and macrophages. Macrophage-mediated phagocytosis is influenced by glutamine availability. Hydrolysable glutamine dipeptides can substitute for glutamine to support in vitro lymphocyte and macrophage functions. In man plasma and skeletal muscle glutamine levels are lowered by sepsis, injury, burns, surgery and endurance exercise and in the overtrained athlete. The lowered plasma glutamine concentrations are most likely the result of demand for glutaminne (by the liver, kidney, gut and immune system) exceeding the supply (from the diet and from muscle). It has been suggested that the lowered plasma glutamine concentration contributes, at least in part, to the immunosuppression which accompanies such situations. Animal studies have shown that inclusion of glutamine in the diet increases survival to a bacterial challenge. Glutamine or its precursors has been provided, usually by the parenteral route, to patients following surgery, radiation treatment or bone marrow transplantation or suffering from injury. In most cases the intention was not to stimulate the immune system but rather to maintain nitrogen balance, muscle mass and/or gut integrity. Nevertheless, the maintenance of plasma glutamine concentrations in such a group of patients very much at risk of immunosuppression has the added benefit of maintaining immune function. Indeed, the provision of glutamine to patients following bone marrow transplantation resulted in a lower level of infection and a shorter stay in hospital than for patients receiving glutamine-free parenteral nutrition.     

谷氨酰胺提取分离研究

谷氨酰胺是一种十分有用的氨基酸,它既可作为治疗药物,又可作为其它合成药物的前体。目前国内以谷氨酸为原料,采用化学合成法生产谷氨酰胺,供作试剂用,产量十分有限。日本用发酵法生产谷氨酰胺,年产1000吨,还有增加趋势。谷氨酰胺的销售价格比较高,经济效益可观。近年来国内有好几个研究小组都在进行谷氨酰胺发酵研究。我们是国内第一个谷氨酰胺研究组,已在5m~3发酵罐上取得中试成功。谷氨酰胺发酵液中混有谷氨酸,而这些少量的谷氨酸采用等电点沉淀法并不能将它们去除,影响谷氨酰胺的纯度。我们采用离子交换树脂法,成功地将谷氨酸从谷氨酰胺和谷氨酸的混合物中分离掉,得到单一的谷氨酰胺。在将发酵液上柱交换之前,必须对发酵液进行预处理,我们试验了4种不同方法,结果表明第4种方法更适合工厂使用。从我们使用过的几种树脂的分离效果及树脂的价格看,考虑到工     

Glutamine utilization by Rhizobium etli

We undertook the study of the use of glutamine (Gln) as the source of carbon and energy by Rhizobium etli. Tn5-induced mutagenesis allowed us to identify several genes required for Gln utilization, including those coding for two broad-range amino acid transporters and a glutamate dehydrogenase. The isolated mutants were characterized by the analysis of their capacity i) to grow on different media, ii) to transport Gln (uptake assays), and iii) to utilize Gln as the C energy source (CO2 production from Gln). We show that Gln is degraded through the citric acid cycle and that its utilization as the sole C source is related to a change in the bacterial cell shape (from bacillary to coccoid form) and a high susceptibility to a thiol oxidative insult. Both these data and the analysis of ntr-dependent promoters suggested that Gln-grown bacteria are under a condition of C starvation and N sufficiency, and as expected, the addition of glucose counteracted the morphological change and increased both the bacterial growth rate and their resistance to oxidative stress. Finally, a nodulation analysis indicates that the genes involved in Gln transport and degradation are dispensable for the bacterial ability to induce and invade developing nodules, whereas those involved in gluconeogenesis and nucleotide biosynthesis are strictly required.     

Glutamine Synthetase in Muscle Is Required for Glutamine Production during Fasting and Extrahepatic Ammonia Detoxification

The main endogenous source of glutamine is de novo synthesis in striated muscle via the enzyme glutamine synthetase (GS). The mice in which GS is selectively but completely eliminated from striated muscle with the Cre-loxP strategy (GS-KO/M mice) are, nevertheless, healthy and fertile. Compared with controls, the circulating concentration and net production of glutamine across the hindquarter were not different in fed GS-KO/M mice. Only a ∼3-fold higher escape of ammonia revealed the absence of GS in muscle. However, after 20 h of fasting, GS-KO/M mice were not able to mount the ∼4-fold increase in glutamine production across the hindquarter that was observed in control mice. Instead, muscle ammonia production was ∼5-fold higher than in control mice. The fasting-induced metabolic changes were transient and had returned to fed levels at 36 h of fasting. Glucose consumption and lactate and ketone-body production were similar in GS-KO/M and control mice. Challenging GS-KO/M and control mice with intravenous ammonia in stepwise increments revealed that normal muscle can detoxify ∼2.5 μmol ammonia/g muscle·h in a muscle GS-dependent manner, with simultaneous accumulation of urea, whereas GS-KO/M mice responded with accumulation of glutamine and other amino acids but not urea. These findings demonstrate that GS in muscle is dispensable in fed mice but plays a key role in mounting the adaptive response to fasting by transiently facilitating the production of glutamine. Furthermore, muscle GS contributes to ammonia detoxification and urea synthesis. These functions are apparently not vital as long as other organs function normally.