谷氨酸生产菌S9114中的谷氨酸脱氢酶的研究

谷氨酸脱氢酶 (GDH)是谷氨酸生物合成的关键酶 ,谷氨酸棒杆菌S91 1 4是目前我国味精工业应用最广泛的生产菌种 ,其谷氨酸脱氢酶的研究尚未见报道。分离纯化该菌中的谷氨酸脱氢酶 ,研究其辅酶组成 ,对揭示谷氨酸脱氢酶的分子结构和性质 ,提高谷氨酸产率很有必要。将培养至对数期中期的细胞离心收集并用含适量DTT、ED TA的Tris_HCl缓冲液 (pH 7 5 )洗涤 ,用Frenchpressurecellpress破碎 ,离心去除菌体碎片得无细胞抽提液。然后使用 KTA_10 0快速纯化系统经DEAE_纤维素柱、疏水柱 (HIC)、G_2 0 0凝胶过滤柱层析得到纯化大约 70倍的以NAD PH为辅酶的GDH和部分纯化的以NADH辅酶的GDH。这两个酶分别对NADPH、NADH高度专一 ,不能相互代替。经HPLC和SDS_PAGE测得前一种酶的分子量和亚基分子量分别为 188kD和 32kD ,表明该酶为具有相同亚基的六聚体。酶活性测定使用HITACHIU_30 0 0分光光度计利用NAD(P)H在 340nm氧化的初速度进行。蛋白质含量测定利用Bradford方法进行 ,并以牛血清白蛋白为标准蛋白。纯化结果表明S91 1 4中确实存在两种GDH ,其中以NADH为辅酶的GDH尚未见报道。和某些具有两种GDH的微生物一样 ,S91 1 4可能也是以NADPH为辅酶的GDH参与谷氨酸的合成代谢 ,以NADH为辅酶的GDH参与谷氨酸的分解代谢。     

Activation of Glutamate Dehydrogenase by Leucine and Its Nonmetabolizable Analogue in Rat Brain Synaptosomes

Leucine and beta-(+/-)-2-aminobicyclo[2.2.1]heptane-2-carboxylic acid (BCH) stimulated, in a dose-dependent manner, reductive amination of 2-oxoglutarate in rat brain synaptosomes treated with Triton X-100. The concentration dependence curves were sigmoid, with 10-15-fold stimulations at 15 mM leucine (or BCH); oxidative deamination of glutamate also was enhanced, albeit less. In intact synaptosomes, leucine and BCH elevated oxygen uptake and increased ammonia formation, consistent with stimulation of glutamate dehydrogenase (GDH). Enhancement of oxidative deamination was seen with endogenous as well as exogenous glutamate and with glutamate generated inside synaptosomes from added glutamine. With endogenous glutamate, the stimulation of oxidative deamination was accompanied by a decrease in aspartate formation, which suggests a concomitant reduction in flux through aspartate aminotransferase. Activation of reductive amination of 2-oxoglutarate by BCH or leucine could not be demonstrated even in synaptosomes depleted of internal glutamate. It is suggested that GDH in synaptosomes functions in the direction of glutamate oxidation, and that leucine may act as an endogenous activator of GDH in brain in vivo.     

Immunocytochemical Characterization of Glutamate Dehydrogenase in the Cerebellum of the Rat

The immunocytochemical distribution of glutamate dehydrogenase was studied in the cerebellum of the rat using antibodies made in rabbit and guinea pig against antigen purified from bovine liver. Antiserum was found to block partially enzymatic activity both of the purified enzyme and of extracts of the rat cerebellum. Using immunoblots of proteins of rat cerebellum, a major immunoreactive protein and several minor immunoreactive proteins were detected with antiserum. Only a single immunoreactive protein was detected using affinity-purified antibody preparations. This protein migrates with a molecular weight identical to that of the subunit of glutamate dehydrogenase. Further evidence that the antibodies were selective for glutamate dehydrogenase in rat cerebellum was obtained through peptide mapping. Purified glutamate dehydrogenase and the immunoreactive protein from rat cerebellum generated similar patterns of immunoreactive peptides. No significant cross-reaction was observed with glutamine synthetase. Immunocytochemistry was done on cryostat- and Vibratome-cut sections of the cerebellum of rats that had been perfused with cold 4% paraformaldehyde. Glial cells were found to be the most immunoreactive structures throughout the cerebellum. Most apparent was the intense labeling of Bergmann glial cell bodies and fibers. In the granule cell layer, heavy labeling of astrocytes was seen. Purkinje and granule cell bodies were only lightly immunoreactive, whereas stellate, basket, and Golgi cells were unlabeled. Labeling of presynaptic terminals was not apparent. These findings suggest that glutamate dehydrogenase, like glutamine synthetase, is enriched in glia relative to neurons.     

Glutamate Dehydrogenase in Human Brain: Regional Distribution and Properties

Glutamate dehydrogenase (GDH) activity was studied in 17 regions of six human brains. Duration and conditions of the postmortem period did not affect enzyme activity. Specific activity ranged between 103 and 377 nmoles/min/mg protein at 25 degrees C and it was 10-fold higher than that found in leukocytes. Apart from exclusively white matter regions (corpus callosum and centrum ovale), there was a moderate regional distribution (2.5-fold variation), with highest values in the inferior olive and hypothalamus, and lowest in the cerebellum and lenticular nucleus. With alpha-ketoglutarate (alpha-KG), NADH, or NH4+ as variable substrate, the apparent Km values in human brain were Km alpha-KG = 1.9 X 10(-3) M, KmNADH = 0.21 X 10(-3) M, and KmNH4+ = 28 X 10(-3) M, and in leukocytes they were Km alpha-KG = 1.7 X 10(-3) M, KmNADH = 0.24 X 10(-3) M, and KmNH4+ = 28 X 10(-3) M. The effects of cofactors, inhibitor, and pH were similar in brain and leukocyte GDH.     

类产碱假单胞菌谷氨酸脱氢酶的提纯、鉴定及某些特性的初步研究

从类产碱假单胞菌纯化出电泳纯的谷氨酸脱氢酶,用聚丙烯酰胺梯度凝胶电泳和ＳＤＳ－聚丙烯酰胺凝胶电泳测得分子量为２９０ ｋＤ,亚基分子量为４７ ｋＤ,提示该酶为六聚体．该酶对ＮＡＤＰ（Ｈ）和底物均具有高度专一性,对谷氨酸、α－酮戊二酸及ＮＡＤＰ＋ 的Ｋｍ 值分别为：２８ ｍ ｍ ｏｌ／Ｌ、１．２ｍ ｍ ｏｌ／Ｌ及０．０６３ ｍ ｍ ｏｌ／Ｌ．用Ｈｉｌｌ作图法求得酶对ＮＨ＋４ 和ＮＡＤＰＨ 的［Ｓ］０．５分别为２４ ｍ ｍ ｏｌ／Ｌ和０．０３７ ｍ ｍ ｏｌ／Ｌ．最适反应温度为５０℃,催化氨化反应和脱氨反应的最适ｐＨ 分别为８．０和８．８,在热稳定性方面不及嗜热细菌的谷氨酸脱氢酶稳定．提纯的谷氨酸脱氢酶在低温（４℃）条件下,可在Ｔｒｉｓ－ＨＣｌ缓冲液中贮存半年以上,活力无明显下降,冷冻则可导致纯酶液迅速失活．氮源对菌体谷氨酸脱氢酶水平有显著影响．     

Purification and Characterization of Sorbitol Dehydrogenase from Bovine Brain

Sorbitol dehydrogenase (EC 1.1.1.14) was isolated from bovine brain and purified 3,000-fold to apparent homogeneity, as judged by polyacrylamide gel electrophoresis. The purified enzyme had a specific activity of 36 units/mg of protein; a molecular weight of 39,000 for each of the four identical subunits and 155,000 for the intact enzyme were determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and gel exclusion chromatography, respectively. The presence of one Zn2+ per subunit was confirmed by atom absorption spectroscopy; inactivation of the enzyme by metal-chelating agents points to the essential role that Zn2+ plays in the catalytically competent enzyme. The enzyme is also inactivated by thiol-blocking reagents; with respect to inactivation by sodium pyrophosphate, sorbitol dehydrogenase is different from closely related alcohol dehydrogenase.     

Essential Active-Site Lysine of Brain Glutamate Dehydrogenase Isoproteins

Abstract: Two soluble forms of bovine brain glutamate dehydrogenase (GDH) isoproteins were inactivated by pyridoxal 5'-phosphate. Spectral evidence is presented to indicate that the inactivation proceeds through Schiff's base formation with amino groups of the enzyme. Sodium borohydride reduction of the pyridoxal 5'-phosphate-inactivated GDH isoproteins produced a stable pyridoxyl enzyme derivative that could not be reactivated by dialysis. The pyridoxyl enzyme was studied through fluorescence spectroscopy. No substrates or coenzymes separately gave complete protection against pyridoxal 5'-phosphate. A combination of 10 m M 2-oxoglutarate with 2 m M NADH, however, gave complete protection against the inactivation. Tryptic peptides of the isoproteins, modified with and without protection, resulted in a selective modification of one lysine. In both GDH isoproteins, the sequences of the peptide containing the phosphopyridoxyllysine were clearly identical to sequences of other GDH species.     

Solubilisation of a Glutamate Binding Protein from Rat Brain

Rat brain synaptic plasma membranes were solubilised in either 1% Triton X-100 or potassium cholate and subjected to batch affinity adsorption on L-glutamate/bovine serum albumin reticulated glass fibre. The fibre was extensively washed, and bound proteins eluted with 0.1 mM L-glutamate in 0.1% detergent, followed by repeated dialysis to remove the glutamate from the eluted proteins. Aliquots of the dialysed extracts were assayed for L-[3H]glutamate binding activity in the presence or absence of 0.1 mM unlabelled L-glutamate (to define displaceable binding). Incubations were conducted at room temperature and terminated by rapid filtration through nitrocellulose membranes. Binding to solubilised fractions could be detected only following affinity chromatography. Binding was saturable and of relatively low affinity: KD = 1.0 and 1.8 microM for Triton X-100 and cholate extracts, respectively. The density of binding sites was remarkably high: approximately 18 nmol/mg protein for Triton X-100-solubilised preparations, and usually double this when cholate was employed. Analysis of structural requirements for inhibition of binding revealed that only a very restricted number of compounds were effective, i.e., L-glutamate, L-aspartate, and sulphur-containing amino acids. Binding was not inhibited significantly by any of the selective excitatory amino acid receptor agonists--quisqualate, N-methyl-D-aspartate, or kainate. The implication from this study is that the glutamate binding protein is similar if not identical to one previously isolated and probably is not related to the pharmacologically defined postsynaptic receptor subtypes, unless solubilisation of synaptic membranes resulted in major alterations to binding site characteristics. Since solubilisation with Triton X-100 is known to preserve synaptic junctional complexes, it seems likely that the origin of the glutamate binding protein may be extrajunctional, although its functional role is unknown.     

Regional Development of Glutamate Dehydrogenase in the at Brain

The development of glutamate dehydrogenase enzyme activity in rat brain regions has been followed from the late foetal stage to the adult and through to the aged (greater than 2 years) adult. In the adult brain the enzyme activity was greatest in the medulla oblongata and pons greater than midbrain = hypothalamus greater than cerebellum = striatum = cortex. In the aged adult brain, glutamate dehydrogenase activity was significantly lower in the medulla oblongata and pons when compared to the 90-day-old adult value, but not in other regions. The enzyme-specific activity of nonsynaptic (free) mitochondria purified from the medulla oblongata and pons of 90-day-old animals was about twice that of mitochondria purified from the striatum and the cortex. The specific activity of the enzyme in synaptic mitochondria purified from the above three brain regions, however, remained almost constant.     

Nerve Tissue-Specific Human Glutamate Dehydrogenase that Is Thermolabile and Highly Regulated by ADP

Abstract: Glutamate dehydrogenase (GDH), an enzyme that is central to the metabolism of glutamate, is present at high levels in the mammalian brain. Studies on human leukocytes and rat brain suggested the presence of two GDH activities differing in thermal stability and allosteric regulation, but molecular biological investigations led to the cloning of two human GDH-specific genes encoding highly homologous polypeptides. The first gene, designated GLUD1, is expressed in all tissues (housekeeping GDH), whereas the second gene, designated GLUD2, is expressed specifically in neural and testicular tissues. In this study, we obtained both GDH isoenzymes in pure form by expressing a GLUD1 cDNA and a GLUD2 cDNA in Sf9 cells and studied their properties. The enzymes generated showed comparable catalytic properties when fully activated by 1 mM ADP. However, in the absence of ADP, the nerve tissue-specific GDH showed only 5% of its maximal activity, compared with ～40% showed by the housekeeping enzyme. Low physiological levels of ADP (0.05–0.25 mM) induced a concentration-dependent enhancement of enzyme activity that was proportionally greater for the nerve tissue GDH (by 550–1,300%) than of the housekeeping enzyme (by 120–150%). Magnesium chloride (1–2 mM) inhibited the nonactivated housekeeping GDH (by 45–64%); this inhibition was reversed almost completely by ADP. In contrast, Mg²⁺ did not affect the nonstimulated nerve tissue-specific GDH, although the cation prevented much of the allosteric activation of the enzyme at low ADP levels (0.05–0.25 mM). Heat-inactivation experiments revealed that the half-life of the housekeeping and nerve tissue-specific GDH was 3.5 and 0.5 h, respectively. Hence, the nerve tissue-specific GDH is relatively thermolabile and has evolved into a highly regulated enzyme. These allosteric properties may be of importance for regulating brain glutamate fluxes in vivo under changing energy demands.     

Immunochemical Characterization of Pyruvate Dehydrogenase Complex in Rat Brain

Pyruvate dehydrogenase complex (PDHC) in rat brain was studied immunochemically, using antibodies against the bovine kidney PDHC, by immunoblotting, immunoprecipitation, inhibition of enzyme activity, and enzyme-linked immunoabsorbent assay (ELISA). The immunoblots showed that the antibodies bound strongly to the alpha peptide of the pyruvate dehydrogenase (E1) component, and to the dihydrolipoyl transacetylase (E2) and the dihydrolipoyl dehydrogenase (E3) components of PDHC. A similar immunoblotting pattern was observed in all eight brain regions examined. On immunoblotting of the subcellular fractions, these PDHC peptides were observed in mitochondria and synaptosomes but not in the postmitochondrial supernatants. This agrees with other evidence that brain PDHC is localized in the mitochondria. These results, together with those from sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the immunoprecipitin, also showed that the alpha E1, beta E1, and E3 peptides of rat brain PDHC are very similar in sizes to those of the bovine kidney PDHC, being 42, 36, and 58 kD, respectively. The size of the E2 peptide, 66 kD, is different from that of bovine kidney E2, 73 kD. The relative abundance of PDHC protein in nonsynaptic mitochondria was compared by enzyme activity titration and ELISA. Both methods demonstrated that the amount of PDHC antigen in the mitochondria from cerebral cortex is greater than that in the olfactory bulb mitochondria. This is consistent with the results of the activity measurement. The ELISA also showed that the PDHCs in both mitochondrial populations are antigenically similar.(ABSTRACT TRUNCATED AT 250 WORDS)     

Biochemical Characterization and Localization of a Non-N-Methyl-D-Aspartate Glutamate Receptor in Rat Brain

The structure and distribution of non-N-methyl-D-aspartate glutamate receptors in the rat brain were studied using subunit-specific antibodies that recognize the receptor subunit GluR1. The GluR1 protein, a 106-kDa glycoprotein, appears predominantly in synaptic plasma membranes, where it is highly enriched in the postsynaptic densities. When synaptic plasma membranes are solubilized with the detergent 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate, high-affinity alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) binding and GluR1 immunoreactivity comigrate at a native Mr of 610,000. GluR1 is enriched in the hippocampus and cerebellar cortex but is present throughout the CNS. It is found on neuronal cell bodies and processes within most regions of the brain; within the cerebellum, however, it is localized to the Bergmann glia. These data suggest that the GluR1 protein is a subunit of multimeric AMPA-preferring glutamate receptors present on neurons and on specialized glia.     

Interactions Between the Glutamate and Glycine Recognition Sites of the N-Methyl-d-Aspartate Receptor from Rat Brain, as Revealed from Radioligand Binding Studies

Abstract: The N-methyl-d -aspartate (NMDA) receptor possesses two distinct amino acid recognition sites, one for glutamate and one for glycine, which appear to be allosterically linked. Using rat cortex/hippocampus P₂ membranes we have investigated the effect of glutamate recognition site ligands on [³H]glycine (agonist) and (±)4-trans-2-car-boxy-5,7-dichloro-4-[³H]phenylaminocarbonylamino-1,2,3,4-tetrahydroquinoline ([³H]l -689,560; antagonist) binding to the glycine site and the effect of glycine recognition site ligands on l -[³H]glutamate (agonist), dl -3-(2-carboxypiperazin-4-yl)-[³H]propyl-1 -phosphonate ([³H]-CPP; “C-7” antagonist), and cis-4-phosphonomethyl-2-[³H]piperidine carboxylate ([³H]CGS-19755; “C-5” antagonist) binding to the glutamate site. “C-7” glutamate site antagonists partially inhibited [³H]l -689,560 binding but had no effect on [³H]glycine binding, whereas “C-5” antagonists partially inhibited the binding of both radioligands. Glycine, d -serine, and d -cycloserine partially inhibited [³H]CGS-19755 binding but had little effect on l -[³H]-glutamate or [³H]CPP binding, whereas the partial agonists (+)-3-amino-1-hydroxypyrrolid-2-one [(+)-HA-966], 3R-(+)cis-4-methyl-HA-966 (l -687,414), and 1-amino-1-carboxycyclobutane all enhanced [³H]CPP binding but had no effect on [³H]CGS-19755 binding, and (+)-HA-966 and l -687,414 inhibited l -[³H]glutamate binding. The association and dissociation rates of [³H]l -689,560 binding were decreased by CPP and d -2-amino-5-phosphonopentanoic acid (“C-5”). Saturation analysis of [³H]l -689,560 binding carried out at equilibrium showed that CPP had little effect on the affinity or number of [³H]l -689,560 binding sites. These results indicate that complex interactions occur between the glutamate and glycine recognition sites on the NMDA receptor. In addition, mechanisms other than allosterism may underlie some effects, and the possibility of a steric interaction between CPP and [³H]l -689,560 is discussed.     

Enzymatic Regeneration of NAD in Enantioselective Oxidation of Secondary Alcohols: Glutamate Dehydrogenase Versus NADH Dehydrogenase

To improve yield and productivity of ketose in NAD-dependent polyol oxidations, two enzymatic methods for regeneration of the oxidized coenzyme form have been compared and partly optimized for the batch conversion of xylitol into D-xylulose and D-sorbitol into D-fructose. Polyol oxidation was catalyzed by xylitol dehydrogenase from the yeast Galactocandida mastotermitis. Reduction of OM₂ (apparently to H₂O) by partially purified NADH dehydrogenase complex from Corynebacterium callunae could drive alcohol oxidations better than reductive amination of EaL-ketoglutarate by glutamate dehydrogenase. A fed-batch procedure was developed that overcame inhibition of glutamate dehydrogenase by α-ketoglutarate (K_is 25 mM), thus increasing the productivity of ketose almost 2-fold. For D-fructose production from D-sorbitol (0.1-0.3M) yields of < 90% and productivities up to 1.30g/(L.h) have been obtained. High conversion of up to 50g/L xylitol into D-xylulose for which xylitol dehydrogenase exhibits an about 80-fold higher specificity constant than for D-fructose required complexation of the ketose product with borate. In comparison with reductive amination by glutamate dehydrogenase, advantages of using NADH-dehydrogenase catalyzed regeneration of NAD for ketose production are (i) avoidance of byproduct formation, (ii) cheaper substrate (02 versus α-ketoglutarate), and (iii) easier process control (batch versus fed-batch).     

黄色短杆菌GDK-9谷氨酸脱氢酶基因的克隆、序列分析及表达

利用PCR技术从黄色短杆菌GDK-9的基因组DNA中扩增出谷氨酸脱氢酶基因(gdh)片段(EC.1.4.1.4), 连到pUCm-T载体上测序。核酸序列分析结果表明, 该片段全长1927 bp, 包含一个ORF, 推测此ORF区编码一条448个氨基酸的多肽, 分子量约为48 kD。与已报道的gdh序列相似性为99.55%, 其中1190位碱基(C→A)突变导致了编码氨基酸的变化(Thr→Asn), 其它的碱基变化不影响编码的氨基酸。将gdh基因克隆入穿梭质粒pXMJ19中, 并转化E. coli XL-Blue和Brevibacterium flavum GDK-9, 经IPTG诱导后, SDS-PAGE电泳结果显示, 在预计位置出现明显的诱导蛋白条带, 分子量约为48.7 kD。谷氨酸发酵实验表明, 尽管谷氨酸脱氢酶GDH能明显提高胞内的谷氨酸含量, 但其不影响谷氨酸的分泌。     

Purification and Characterization of Aromatic Amine Dehydrogenase from Alcaligenes xylosoxidans

Aromatic amine dehydrogenase was purified and characterized from Alcaligenes xylosoxidans IFO13495 grown on β-phenylethylamine. The molecular mass of the enzyme was 95.5 kDa. The enzyme consisted of heterotetrameric subunits (α₂β₂) with two different molecular masses of 42.3 kDa and 15.2 kDa. The N-terminal amino acid sequences of the α-subunit (42.3-kDa subunit) and the β-subunit (15.2-kDa subunit) were DLPIEELXGGTRLPP and APAAGNKXPQMDDTA respectively. The enzyme had a quinone cofactor in the β-subunit and showed a typical absorption spectrum of tryptophan tryptophylquinone-containing quinoprotein showing maxima at 435 nm in the oxidized form and 330 nm in the reduced form. The pH optima of the enzyme activity for histamine, tyramine, and β-phenylethylamine were the same at 8.0. The enzyme retained full activity after incubation at 70 °C for 40 min. It readily oxidized various aromatic amines as well as some aliphatic amines. The Michaelis constants for phenazine methosulfate, β-phenylethylamine, tyramine, and histamine were 48.1, 1.8, 6.9, and 171 μM respectively. The enzyme activity was strongly inhibited by carbonyl reagents. The enzyme could be stored without appreciable loss of enzyme activity at 4 °C for one month at least in phosphate buffer (pH 7.0).     

Aspartate Aminotransferase and Glutamate Dehydrogenase Activities in the Squid Giant Nerve

Abstract: The present study sought to investigate the presence and distribution of some enzymatic activities involved in the metabolism of glutamate in the giant nerve fiber of the tropical squid Sepioteuthis sepioidea . Specific activities of aspartate aminotransferase and glutamate dehydrogenase were evaluated in homogenates of the isolated giant fiber, extruded axoplasm, and axoplasm-free giant nerve fiber sheaths. The activities of both enzymes were present in the tissue. The specific activity of aspartate aminotransferase was similar in axoplasm and sheaths. However, the specific activity of glutamate dehydrogenase was an order of magnitude higher in the sheaths. This finding is discussed in the framework of the hypothesis that proposes that a differential distribution of the enzymes of the glutamatergic system between the axonal and neuroglial compartments forms part of a system of communication between these cells whose neuronal signal may be glutamate.     

基于分子伴侣策略提高NADPH依赖型醇脱氢酶的异源可溶性表达 *

目的:生物催化的氧化还原反应广泛应用于手性化合物的制备,其中很多反应涉及辅酶NADPH的原位再生。以异丙醇为辅助底物,利用醇脱氢酶再生NADPH,具有比酶活高、副产物丙酮易于分离等优势,受到越来越多的关注。选择极具应用潜力的来源于Clostridium beijerinckii的醇脱氢酶CbADH作为研究对象,针对其在大肠杆菌中的可溶性表达差、酶活低的瓶颈问题开展研究。方法: 首先通过引入诱导型质粒pGro7表达分子伴侣GroES-GroEL,将pET-28a(+)质粒表达CbADH的可溶性提高了3.57倍,酶活达到出发菌株的4.83倍。其次,考察了另外三种不同的分子伴侣表达策略:pET-28a(+)单质粒共表达、基因组强化表达GroES-GroEL和组成型改造pGro7/GroES-GroEL和pET-28a(+)/CbADH双质粒共表达。结果: 组成型改造pGro7和pET-28a(+)双质粒共表达策略的效果最优,其CbADH的可溶性表达提高了8.07倍,酶活达到了21.79U/mg DCW,是出发菌株的9.43倍。结论: 为CbADH的工业应用奠定了良好的基础,也为外源蛋白的可溶性表达提供了参考。     

Inhibition of Glutamate Dehydrogenase in Brain Mitochondria and Synaptosomes by Mg2+ and Polyamines: A Possible Cause for Its Low In Vivo Activity

Abstract: Magnesium and the polyamines putrescine, spermidine, and spermine inhibited the activity of glutamate dehydrogenase in permeabilized rat brain mitochondria in a concentration-dependent manner. The inhibitory effect was observed on both the reductive amination of 2-oxoglutarate and oxidative deamination of glutamate, as well as in the presence and absence of ADP and leucine, the allosteric activators of the enzyme. Kinetic studies at various concentrations of substrates showed that inhibition by magnesium and spermine was very pronounced at 2-oxoglutarate concentrations less than 0.5 m M and NADH levels less than 0.08 m M . The presence of the former compounds also accentuated the inhibitory effect of high concentrations of 2-oxoglutarate (>2.0 m M ) and NADH (>0.32 m M ). Addition of magnesium and spermine to suspensions of synaptosomes decreased the amount of ammonia produced from glutamate. It is suggested that polyamines and magnesium, normal constituents of mammalian brain, are responsible, at least in part, for the low glutamate dehydrogenase activity in vivo.