Regulation of the glutamate dehydrogenase activity in rat islets of Langerhans and its consequence on insulin release

Inhibition of glutamate dehydrogenase and insulin secretion by KHG26377 does not involve ADP-ribosylation by SIRT4 or deacetylation by SIRT3

We investigated the mechanisms involved in KHG26377 regulation of glutamate dehydrogenase (GDH) activity, focusing on the roles of SIRT4 and SIRT3. Intraperitoneal injection of mice with KHG26377 reduced GDH activity with concomitant repression of glucose-induced insulin secretion. Consistent with their known functions, SIRT4 ribosylated GDH and reduced its activity, and SIRT3 deacetylated GDH, increasing its activity. However, KHG26377 did not affect SIRT4-mediated ADP-ribosylation/ inhibition or SIRT3-mediated deacetylation/activation of GDH. KHG26377 had no effect on SIRT4 protein levels, and did not alter total GDH, acetylated GDH, or SIRT3 protein levels in pancreatic mitochondrial lysates. These results suggest that the mechanism by which KHG26377 inhibits GDH activity and insulin secretion does not involve ADP-ribosylation of GDH by SIRT4 or deacetylation of GDH by SIRT3.

Delineation of glutamate pathways and secretory responses in pancreatic islets with β-cell-specific abrogation of the glutamate dehydrogenase

The amino acid profile and the secretory responses of glutamate dehydrogenase (GDH)-deficient β-cells are characterized. This study shows that GDH is essential for both insulin release and net glutamate synthesis evoked by glucose. Adding cellular glutamate restored the full development of glucose-stimulated insulin secretion, showing the requirement for permissive glutamate levels.

In pancreatic β-cells, glutamate dehydrogenase (GDH) modulates insulin secretion, although its function regarding specific secretagogues is unclear. This study investigated the role of GDH using a β-cell–specific GDH knockout mouse model, called βGlud1^−/−. The absence of GDH in islets isolated from βGlud1^–/– mice resulted in abrogation of insulin release evoked by glutamine combined with 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid or l-leucine. Reintroduction of GDH in βGlud1^–/– islets fully restored the secretory response. Regarding glucose stimulation, insulin secretion in islets isolated from βGlud1^–/– mice exhibited half of the response measured in control islets. The amplifying pathway, tested at stimulatory glucose concentrations in the presence of KCl and diazoxide, was markedly inhibited in βGlud1^–/– islets. On glucose stimulation, net synthesis of glutamate from α-ketoglutarate was impaired in GDH-deficient islets. Accordingly, glucose-induced elevation of glutamate levels observed in control islets was absent in βGlud1^–/– islets. Parallel biochemical pathways, namely alanine and aspartate aminotransferases, could not compensate for the lack of GDH. However, the secretory response to glucose was fully restored by the provision of cellular glutamate when βGlud1^–/– islets were exposed to dimethyl glutamate. This shows that permissive levels of glutamate are required for the full development of glucose-stimulated insulin secretion and that GDH plays an indispensable role in this process.

The complex regulation of human glud1 and glud2 glutamate dehydrogenases and its implications in nerve tissue biology

Mammalian glutamate dehydrogenase (GDH) is a housekeeping mitochondrial enzyme (hGDH1 in the human) that catalyses the reversible inter-conversion of glutamate to α-ketoglutarate and ammonia, thus interconnecting amino acid and carbohydrate metabolism. It displays an energy sensing mechanism, which permits enzyme activation under low cellular energy states. As GDH is at the crossroads of important metabolic pathways, a tight control of its activity is essential. Indeed, to fulfill its role in metabolism and cellular energetics, mammalian GDH has evolved into a highly regulated enzyme subject to allosteric modulation by diverse compounds. The recent emergence (<23 million years ago) in apes and humans of a hGDH2 isoenzyme with distinct regulatory properties, as well as, the detection of gain-of-function variants in hGDH1 and hGDH2 that affect the nervous system, have introduced additional complexities. The properties of the two highly homologous human GDHs were studied using purified recombinant hGDH1 and hGDH2 obtained by expression of the corresponding cDNAs in Sf21 cells. Results showed that, in contrast to hGDH1 that maintains substantial basal activity (35-40% of its maximal capacity), hGDH2 displays low basal activity (3-8% of maximal) that is remarkably responsive to activation by rising levels of ADP and/or l-leucine. This is primarily due to the Arg443Ser evolutionary change, which also made hGDH2 markedly sensitive to estrogens and neuroleptic drugs. In contrast to hGDH1, which is subject to potent GTP inhibition, hGDH2 has dissociated its function from this energy switch, being able to metabolize glutamate even when the Krebs cycle generates GTP levels sufficient to inactivate the housekeeping hGDH1. Our data also show that spermidine, a polyamine thought to reduce oxidative stress and to prolong survival, and EGCG, a green tea polyphenol, inhibit hGDH2 at lower concentrations than hGDH1. The implications of these findings in nerve tissue biology are discussed.

Decreased insulin secretion in islets from protein malnourished rats is associated with impaired glutamate dehydrogenase function: effect of leucine supplementation

We herein studied the role of glutamate dehydrogenase (GDH), in response to leucine (LEU) supplementation, upon insulin secretion of malnourished rats. Weaned male Wistar rats were fed normal-protein (17%) or low-protein diet (6%, LP) for 8 weeks. Half of the rats of each group were supplemented with LEU (1.5%) in the drinking water for the following 4 weeks. Gene and protein expressions, static insulin secretion, and cytoplasmic Ca(2+) oscillations were measured. Glutamate dehydrogenase messenger RNA was 58% lower in LP islets, and LEU supplementation augmented it in 28%. The LP islets secreted less insulin when exposed to 20 mmol/L LEU, 20 mmol/L LEU + 2 mmol/L glutamine (with or without 5 mmol/L aminooxyacetic acid, a branched chain aminotransferase inhibitor, or 20 μmol/L epigallocatechin gallate, a GDH inhibitor), 20 mmol/L α-ketoisocaproate, glutamine + 20 mmol/L β-2-aminobicyclo[2.2.1]heptane-2-carboxylic acid (a GDH activator), and 22.2 mmol/L glucose. Leucine supplementation augmented insulin secretion to levels found in normal-protein islets in all the above conditions, an effect that was blunted when islets were incubated with epigallocatechin gallate. The glutamine + β-2-aminobicyclo[2.2.1]heptane-2-carboxylic acid-induced increased [Ca(2+)](i) and oscillations were higher than those for LP islets. Leucine supplementation normalized these parameters in LP islets. Impaired GDH function was associated with lower insulin release in LP islets, and LEU supplementation normalized insulin secretion via restoration of GDH function. In addition, GDH may contribute to insulin secretion through ameliorations of Ca(2+) handling in LP islets.

Inhibitory effects of KHG26377 on glutamate dehydrogenase activity in cultured islets

GDH has been known to be related with hyperinsulinismhyperammonemia syndrome. We have screened new drugs with a view to developing effective drugs modulating GDH activity. In the present work, we investigated the effects of a new drug, KHG26377 on glutamate formation and GDH activity in cultured rat islets. When KHG26377 was added to the culture medium for 24 h prior to kinetic analysis, the V(max) of GDH was decreased by 59% whereas K(m) is not significantly changed. The concentration of glutamate decreased by 50% and perfusion of islets with KHG26377 reduced insulin release by up to 55%. Our results show that KHG26377 regulates insulin release by inhibiting GDH activity in primary cultured islets and support the previous studies for the connection between GDH activity and insulin release. Further studies are required to determine in vivo effects and pharmacokinetics of the drug.

Tissue specificity of mitochondrial glutamate pathways and the control of metabolic homeostasis

Glutamate is implicated in numerous metabolic and signalling functions that vary according to specific tissues. Glutamate metabolism is tightly controlled by activities of mitochondrial enzymes and transmembrane carriers, in particular glutamate dehydrogenase and mitochondrial glutamate carriers that have been identified in recent years. It is remarkable that, although glutamate-specific enzymes and transporters share similar properties in most tissues, their regulation varies greatly according to particular organs in order to achieve tissue specific functions. This is illustrated in this review when comparing glutamate handling in liver, brain, and pancreatic beta-cells. We describe the main cellular glutamate pathways and their specific functions in different tissues, ultimately contributing to the control of metabolic homeostasis at the organism level.

A mathematical model of the mitochondrial NADH shuttles and anaplerosis in the pancreatic beta-cell

The pancreatic beta-cells respond to an increased glycolytic flux by secreting insulin. The signal propagation goes via mitochondrial metabolism, which relays the signal to different routes. One route is an increased ATP production that, via ATP-sensitive K(+) (K(ATP)) channels, modulates the cell membrane potential to allow calcium influx, which triggers insulin secretion. There is also at least one other "amplifying" route whose nature is debated; possible candidates are cytosolic NADPH production or malonyl-CoA production. We have used mathematical modeling to analyze this relay system. The model comprises the mitochondrial NADH shuttles and the mitochondrial metabolism. We found robust signaling toward ATP, malonyl-CoA, and NADPH production. The signal toward NADPH production was particularly strong. Furthermore, the model reproduced the experimental findings that blocking the NADH shuttles attenuates the signaling to ATP production while retaining the rate of glucose oxidation (Eto K, Tsubamoto Y, Terauchi Y, Sugiyama T, Kishimoto T, Takahashi N, Yamauchi N, Kubota N, Murayama S, Aizawa T, Akanuma Y, Aizawa S, Kasai H, Yazaki Y, Kadowaki T. Science 283: 981-985, 1999) and provides an explanation for this apparent paradox. The model also predicts that the mitochondrial malate dehydrogenase reaction may proceed backward, toward malate production, if the activity of malic enzyme is sufficiently high. An increased fatty acid oxidation rate was found to attenuate the signaling strengths. This theoretical study has implications for our understanding of both the healthy and the diabetic beta-cell.

Oxo-4-methylpentanoic acid directs the metabolism of GABA into the Krebs cycle in rat pancreatic islets

OMP (oxo-4-methylpentanoic acid) stimulates by itself a biphasic secretion of insulin whereas L-leucine requires the presence of L-glutamine. L-Glutamine is predominantly converted into GABA (gamma-aminobutyric acid) in rat islets and L-leucine seems to promote its metabolism in the 'GABA shunt' [Fernández-Pascual, Mukala-Nsengu-Tshibangu, Martín del Río and Tamarit-Rodríguez (2004) Biochem. J. 379, 721-729]. In the present study, we have investigated how 10 mM OMP affects L-glutamine metabolism to uncover possible differences with L-leucine that might help to elucidate whether they share a common mechanism of stimulation of insulin secretion. In contrast with L-leucine, OMP alone stimulated a biphasic insulin secretion in rat perifused islets and decreased the islet content of GABA without modifying its extracellular release irrespective of the concentration of L-glutamine in the medium. GABA was transaminated to L-leucine whose intracellular concentration did not change because it was efficiently transported out of the islet cells. The L-[U-14C]-Glutamine (at 0.5 and 10.0 mM) conversion to 14CO2 was enhanced by 10 mM OMP within 30% and 70% respectively. Gabaculine (250 microM), a GABA transaminase inhibitor, suppressed OMP-induced oxygen consumption but not L-leucine- or glucose-stimulated respiration. It also suppressed the OMP-induced decrease in islet GABA content and the OMP-induced increase in insulin release. These results support the view that OMP promotes islet metabolism in the 'GABA shunt' generating 2-oxo-glutarate, in the branched-chain alpha-amino acid transaminase reaction, which would in turn trigger GABA deamination by GABA transaminase. OMP, but not L-leucine, suppressed islet semialdehyde succinic acid reductase activity and this might shift the metabolic flux of the 'GABA shunt' from gamma-hydroxybutyrate to succinic acid production.

Protein-sensitive hypoglycemia without leucine sensitivity in hyperinsulinism caused by K(ATP) channel mutations

OBJECTIVE

Because children with congenital hyperinsulinism (HI) caused by recessive loss of function mutations in the adenosine triphosphate (ATP)-dependent potassium channel (K(ATP)-HI) are not leucine sensitive, we evaluated for protein-induced hypoglycemia with oral protein tolerance tests.

STUDY DESIGN

Blood glucose and insulin concentrations were measured every 15 minutes for 3 hours after an oral protein load in children with K(ATP)-HI (n = 11) and compared with those of children with glutamate dehydrogenase HI (n = 12) and control subjects (n = 12).

RESULTS

Similar to children with glutamate dehydrogenase HI, patients with K(ATP)-HI displayed protein-induced hypoglycemia (10/11) with blood glucose concentrations declining by 17 to 69 mg/dL. In contrast, oral protein had little effect on blood glucose concentrations in control subjects.

CONCLUSIONS

Protein-induced hypoglycemia is a feature of K(ATP)-HI, despite the absence of leucine sensitivity. The results indicate that amino acids can stimulate insulin secretion via a glutamate dehydrogenase- and K(ATP) channel-independent pathway.

In beta-cells, mitochondria integrate and generate metabolic signals controlling insulin secretion

Pancreatic beta-cells are unique neuroendocrine cells displaying the peculiar feature of responding to nutrients, principally glucose, as primary stimulus. This requires translation of a metabolic substrate into intracellular messengers recognized by the exocytotic machinery. Central to this signal transduction mechanism, mitochondria integrate and generate metabolic signals, thereby coupling glucose recognition to insulin secretion. In response to a glucose rise, nucleotides and metabolites are generated by mitochondria and participate, together with cytosolic calcium, to the stimulation of insulin exocytosis. This review describes the mitochondrion-dependent pathways of regulated insulin secretion. In particular, importance of cataplerotic and anaplerotic processes is discussed, with special attention to the mitochondrial enzyme glutamate dehydrogenase. Mitochondrial defects, such as mutations and reactive oxygen species production, are presented in the context of beta-cell failure in the course of type 2 diabetes.

Glutamate inhibits protein phosphatases and promotes insulin exocytosis in pancreatic β-cells

In human type 2 diabetes mellitus, loss of glucose-sensitive insulin secretion from the pancreatic beta-cell is an early pathogenetic event, but the mechanisms involved in glucose sensing are poorly understood. A messenger role has been postulated for L-glutamate in linking glucose stimulation to sustained insulin exocytosis in the beta-cell, but the precise nature by which L-glutamate controls insulin secretion remains elusive. Effects of L-glutamate on the activities of ser/thr protein phosphatases (PPase) and Ca(2+)-regulated insulin exocytosis in INS-1E cells were investigated. Glucose increases L-glutamate contents and promotes insulin secretion from INS-1E cells. L-glutamate also dose-dependently inhibits PPase enzyme activities analogous to the specific PPase inhibitor, okadaic acid. L-glutamate and okadaic acid directly and non-additively promote insulin exocytosis from permeabilized INS-1E cells in a Ca(2+)-independent manner. Thus, an increase in phosphorylation state, through inhibition of protein dephosphorylation by glucose-derived L-glutamate, may be a novel regulatory mechanism linking glucose sensing to sustained insulin exocytosis.

Central nervous system hyperexcitability associated with glutamate dehydrogenase gain of function mutations

OBJECTIVE

To describe seizure phenotypes associated with the hyperinsulinism/hyperammonemia syndrome (HI/HA), which is caused by gain of function mutations in the enzyme glutamate dehydrogenase (GDH).

STUDY DESIGN

A retrospective review of records of 14 patients with HI/HA.

RESULTS

Nine patients had seizures as the first symptom of HI/HA, and six had seizures in the absence of hypoglycemia. No electroencephalogram (EEG) background abnormalities were identified. In four patients, EEG recordings during seizures in the setting of normal blood glucose contained generalized epileptiform discharges. EEGs of three of these patients showed 0.5- to 2-second generalized irregular spike-and-wave discharge at 3 to 6 Hz corresponding to eye blinks, eye rolling, or staring. The EEG of the fourth patient consisted of 20 seconds of generalized regular spike-and-wave discharge at 3 Hz in the clinical context of staring and unresponsiveness. In two patients, seizure control worsened with carbamezapine or oxcarbezapine treatment.

CONCLUSIONS

In patients with HI/HA, generalized seizures are common and can occur in the absence of hypoglycemia. The drugs carbamazepine and oxcarbazepine should be used with caution for treatment. Pathogenesis of epilepsy in these patients may be related to effects of GDH mutations in the brain, perhaps in combination with effects of recurrent hypoglycemia and chronic hyperammonemia.

Conversion into GABA (gamma-aminobutyric acid) may reduce the capacity of L-glutamine as an insulin secretagogue

We have carried out a detailed examination of L-glutamine metabolism in rat islets in order to elucidate the paradoxical failure of L-glutamine to stimulate insulin secretion. L-Glutamine was converted by isolated islets into GABA (gamma-aminobutyric acid), L-aspartate and L-glutamate. Saturation of the intracellular concentrations of all of these amino acids occurred at approx. 10 mmol/l L-glutamine, and their half-maximal values were attained at progressively increasing concentrations of L-glutamine (0.3 mmol/l for GABA; 0.5 and 1.0 mmol/l for Asp and Glu respectively). GABA accumulation accounted for most of the 14CO2 produced at various L-[U-14C]glutamine concentrations. Potentiation by L-glutamine of L-leucine-induced insulin secretion in perifused islets was suppressed by malonic acid dimethyl ester, was accompanied by a significant decrease in islet GABA accumulation, and was not modified in the presence of GABA receptor antagonists [50 micromol/l saclofen or 10 micromol/l (+)-bicuculline]. L-Leucine activated islet glutamate dehydrogenase activity, but had no effect on either glutamate decarboxylase or GABA transaminase activity, in islet homogenates. We conclude that (i) L-glutamine is metabolized preferentially to GABA and L-aspartate, which accumulate in islets, thus preventing its complete oxidation in the Krebs cycle, which accounts for its failure to stimulate insulin secretion; (ii) potentiation by L-glutamine of L-leucine-induced insulin secretion involves increased metabolism of L-glutamate and GABA via the Krebs cycle (glutamate dehydrogenase activation) and the GABA shunt (2-oxoglutarate availability for GABA transaminase) respectively, and (iii) islet release of GABA does not seem to play an important role in the modulation of the islet secretory response to the combination of L-leucine and L-glutamine.

Insulin secretion profiles are modified by overexpression of glutamate dehydrogenase in pancreatic islets

AIMS/HYPOTHESIS

Glutamate dehydrogenase (GDH) is a mitochondrial enzyme playing a key role in the control of insulin secretion. However, it is not known whether GDH expression levels in beta cells are rate-limiting for the secretory response to glucose. GDH also controls glutamine and glutamate oxidative metabolism, which is only weak in islets if GDH is not allosterically activated by L-leucine or (+/-)-2-aminobicyclo-[2,2,1]heptane-2-carboxylic acid (BCH).

METHODS

We constructed an adenovirus encoding for GDH to overexpress the enzyme in the beta-cell line INS-1E, as well as in isolated rat and mouse pancreatic islets. The secretory responses to glucose and glutamine were studied in static and perifusion experiments. Amino acid concentrations and metabolic parameters were measured in parallel.

RESULTS

GDH overexpression in rat islets did not change insulin release at basal or intermediate glucose (2.8 and 8.3 mmol/l respectively), but potentiated the secretory response at high glucose concentrations (16.7 mmol/l) compared to controls (+35%). Control islets exposed to 5 mmol/l glutamine at basal glucose did not increase insulin release, unless BCH was added with a resulting 2.5-fold response. In islets overexpressing GDH glutamine alone stimulated insulin secretion (2.7-fold), which was potentiated 2.2-fold by adding BCH. The secretory responses evoked by glutamine under these conditions correlated with enhanced cellular metabolism.

CONCLUSIONS/INTERPRETATION

GDH could be rate-limiting in glucose-induced insulin secretion, as GDH overexpression enhanced secretory responses. Moreover, GDH overexpression made islets responsive to glutamine, indicating that under physiological conditions this enzyme acts as a gatekeeper to prevent amino acids from being inappropriate efficient secretagogues.

Regulatory effects of 5'-deoxypyridoxal on glutamate dehydrogenase activity and insulin secretion in pancreatic islets

It has been known that glutamate, generated by glutamate dehydrogenase (GDH), acts as an intracellular messenger in insulin exocytosis in pancreatic beta cells. Here we demonstrate the correlation of GDH activity and insulin release in rat pancreatic islets perfused with 5'-deoxypyridoxal. Perfusion of islets with 5'-deoxypyridoxal, an effective inhibitor of GDH, reduced the islet GDH activity at concentration-dependent manner. Treatment of 5'-deoxypyridoxal up to 2 mM did not affect the cell viability. There was reduction in V(max) values on average about 60%, whereas no changes in K(m) values for substrates and coenzymes were observed. The concentration of GDH on the Western blot analysis and the level of GDH mRNA remained unchanged. The concentration of glutamate decreased by 52%, whereas the concentration of 2-oxoglutarate increased up to 2.3-fold in the presence of 5'-deoxypyridoxal. 5'-Deoxypyridoxal had no effects on inhibition by GTP and activation by ADP or L-leucine of islet GDH. In parallel with the inhibition of GDH activity, perfusion of islets with 5'-deoxypyridoxal reduced insulin release up to 2.5-fold. Although precise mechanism for correlation between GDH activity and insulin release remains to be studied further, our results suggest a possibility that the inhibitory effect of 5'-deoxypyridoxal on islet GDH activity may correlate with its effect on insulin release.

Glutaminolysis and insulin secretion: from bedside to bench and back

Identification of regulatory mutations of glutamate dehydrogenase (GDH) in a form of congenital hyperinsulinism (GDH-HI) is providing a model for basal insulin secretion (IS) and amino acid (AA)-stimulated insulin secretion (AASIS) in which glutaminolysis plays a key role. Leucine and ADP are activators and GTP is an inhibitor of GDH. GDH-HI mutations impair GDH sensitivity to GTP inhibition, leading to fasting hypoglycemia, leucine hypersensitivity, and protein-induced hypoglycemia, indicating the importance of GDH in basal secretion and AASIS. The proposed model for glutaminolysis in IS is based on GDH providing NADH and alpha-ketoglutarate (alpha-KG) to the Krebs cycle, hence increasing the beta-cell ATP-to-ADP ratio to effect insulin release. The process operates with 1) sufficient lowering of beta-cell phosphate potential (i.e., fasting) and when 2) AAs provide leucine for allosteric activation and glutamate from transaminations. To test this hypothesis, IS studies were performed in rat and GDH-HI mouse models. In the rat study, rat islets were isolated, cultured, and then perifused in Krebs-Ringer bicarbonate buffer with 2 mmol/l glutamine using 10 mmol/l 2-aminobicyclo[2,2,1]-heptane-2-carboxylic acid (BCH) or a BCH ramp after 50 or 120 min of glucose deprivation. In the GDH-HI mouse study, the H454Y GDH-HI mutation driven by the rat insulin promoter was created for H454Y beta-cell-specific expression. Cultured, isolated islets were perifused in leucine 0-10 mmol/l with 2 mmol/l glutamine 0-25 mmol/l, AA 0-10 mmol/l, or glucose 0-25 mmol/l. Rat islets displayed enhanced BCH-stimulated IS after 120 min of glucose deprivation, but not when energized by fuel. H454Y and control islets had similar glucose-stimulated IS, but H454Y mice had lower random blood glucose. Leucine-stimulated IS and AASIS occurred at lower thresholds and were greater in H454Y versus control islets. Glutamine stimulated IS in H454Y but not control islets. The clinical manifestations of GDH-HI and related animal studies suggest that GDH regulates basal IS and AASIS. Energy deprivation enhanced GDH-mediated IS, and H454Y mice were hypoglycemic, substantiating roles for GDH and its regulation by the phosphate potential in basal IS. Excessive IS from H454Y islets upon exposure to GDH substrates or stimuli indicate that regulation of GDH by the beta-cell phosphate potential plays a critical role in AASIS. These findings provide a foundation for defining pathways of basal secretion and AASIS, augmenting our understanding of beta-cell function.

Substitution of Ser for Arg-443 in the regulatory domain of human housekeeping (GLUD1) glutamate dehydrogenase virtually abolishes basal activity and markedly alters the activation of the enzyme by ADP and L-leucine

Human glutamate dehydrogenase (GDH) exists in GLUD1 (housekeeping) and in GLUD2-specified (brain-specific) isoforms, which differ markedly in their basal activity and allosteric regulation. To determine the structural basis of these functional differences, we mutagenized the GLUD1 GDH at four residues that differ from those of the GLUD2 isoenzyme. Functional analyses revealed that substitution of Ser for Arg-443 (but not substitution of Thr for Ser-331, Leu for Met-370, or Leu for Met-415) virtually abolished basal activity and totally abrogated the activation of the enzyme by l-leucine (1-10 mm) in the absence of other effectors. However, when ADP (0.025-0.1 mm) was present in the reaction mixture, l-leucine (0.3-6.0 mm) activated the mutant enzyme up to >2,000%. The R443S mutant was much less sensitive to ADP (SC(50) = 383.9 +/- 14.6 microm) than the GLUD1 GDH (SC(50) = 31.7 +/- 4.2 microm; p < 0.001); however, at 1 mm ADP the V(max) for the mutant (136.67 micromol min(-1) mg(-1)) was comparable with that of the GLUD1 GDH (152.95 micromol min(-1) mg(-1)). Varying the composition and the pH of the reaction buffer differentially affected the mutant and the wild-type GDH. Arg-443 lies in the "antenna" structure, in a helix that undergoes major conformational changes during catalysis and is involved in intersubunit communication. Its replacement by Ser is sufficient to impair both the catalytic and the allosteric function of human GDH.

Cassette mutagenesis and photoaffinity labeling of adenine binding domain of ADP regulatory site within human glutamate dehydrogenase

The adenine binding domain of the ADP site within human glutamate dehydrogenase (GDH) was identified by cassette mutagenesis at the Tyr187 position. The wild type GDH was activated 3-fold by ADP at a concentration of 1 mM at pH 8.0, whereas no significant activation by ADP was observed with the Tyr187 mutant GDH regardless of the size, hydrophobicity, and ionization of the side chains. Studies of the steady-state velocity of the mutant enzymes revealed essentially unchanged apparent K(m) values for 2-oxoglutarate and NADH, but an approximately 4-fold decrease in the respective apparent V(max) values. The binding of ADP to the wild type or mutant GDH was further examined by photoaffinity labeling with [alpha-(32)P]8-azidoadenosine 5'-diphosphate (8N(3)ADP). 8N(3)ADP, without photolysis, mimicked the stimulatory properties of ADP on GDH activity. Saturation of photoinsertion with 8N(3)ADP occurred with apparent K(d) values near 25 microM for the wild type GDH, and the photoinsertion of [alpha-(32)P]8N(3)ADP was decreased best by ADP in comparison to other nucleotides. Unlike the wild type GDH, essentially no photoinsertion was detected for the Tyr187 mutant GDH in the presence or absence of 1 mM ADP. For the wild type GDH, photolabel-containing peptide generated by tryptic digestion was identified in the region containing the sequence EMSWIADTYASTIG, and the photolabeling of this peptide was prevented >95% by the presence of 1 mM ADP during photolysis, whereas no such a peptide was detected for the Tyr187 mutant GDH in the presence or absence of ADP. These results with cassette mutagenesis and photoaffinity labeling demonstrate selectivity of the photoprobe for the ADP binding site and suggest that the photolabeled peptide is within the ADP binding domain of the human GDH and that Tyr187 is responsible for the efficient base binding of ADP to human GDH.

Identification of the GTP binding site of human glutamate dehydrogenase by cassette mutagenesis and photoaffinity labeling

It has been reported that the hyperinsulinism-hyperammonemia syndrome is caused by mutations in glutamate dehydrogenase (GDH) gene that affects enzyme sensitivity to GTP-induced inhibition. To identify the GTP binding site(s) within human GDH, mutant GDHs at Tyr-266 or Lys-450 position were constructed by cassette mutagenesis. More than 90% of the initial activities were remained at the concentration of GTP up to 300 microm for the Lys-450 mutant GDHs regardless of their size, hydrophobicity, and ionization of the side chains, whereas the wild type GDH and the Tyr-266 mutant GDHs were completely inhibited by 30 microm GTP. The binding of GTP to the wild type GDH or the mutant GDHs was further examined by photoaffinity labeling with 8-[gamma-(32)P]azidoguanosine 5'-triphosphate (8-N(3)-GTP). Saturation of photoinsertion with 8-N(3)-GTP occurred apparent K(d) values near 20 microm for the wild type GDH or the Tyr-266 mutant GDH, and the photoinsertion of 8-N(3)-[gamma-(32)P]GTP was significantly decreased in the presence of 300 microm GTP. Unlike the wild type GDH or the Tyr-266 mutant GDH, less than 10% of photoinsertion was detected in the Lys-450 mutant GDH, and the photoinsertion was not affected by the presence of 300 microm GTP. The results with cassette mutagenesis and photoaffinity labeling demonstrate selectivity of the photoprobe for the GTP binding site and suggest that Lys-450, but not Tyr-266, is required for efficient binding of GTP to GDH. Interestingly, studies of the steady-state velocity showed that both the wild type GDH and the Tyr-266 mutant GDHs were inhibited by ATP at concentrations between 10 and 100 microm, whereas less than 10% of the initial activities of the Lys-450 mutant GDHs were diminished by ATP. These results indicate that Lys-450, but not Tyr-266, may be also responsible for the ATP inhibition; therefore, ATP bound to the GTP site.

Molecular characterisation of glutamate dehydrogenase gene defects in Japanese patients with congenital hyperinsulinism/hyperammonaemia

Congenital hyperinsulinism and hyperammonaemia (CHH) is caused by dysregulation of glutamate dehydrogenase (GDH). We characterised the GDH gene in two Japanese patients with CHH. Patient 1 showed late-onset and mild hypoglycaemic episodes and mild hyperammonaemia, compared with patient 2. In GDH activity of lymphoblasts, patient 1 showed twofold higher basal GDH activity than control subjects and mild insensitivity for GTP inhibition. Patient 2 showed severe insensitivity for GTP inhibition, and similar allosteric stimulation by ADP in the controls. Genetic studies identified heterozygous and de novo L413V and G446D mutations in patients 1 and 2, respectively. COS cell expression study confirmed that both mutations were disease-causing gene. The insensitivity for GTP inhibition in L413V and G446D was emphasised in COS cell expression system as a result of the dosage effect of mutant GDH gene. L413V showed less impairment of GDH than G446D based on biochemical and genetic results, which was consistent with the clinical phenotype. Based on the structure of bovine GDH, G446D was located in GTP binding site of pivot helix and its surroundings, while L413V was located in alpha-helix of antenna-like structure. These different locations of mutations gave different effects on GDH enzyme. The antenna-like structure plays an important role in GDH activity.

Acute insulin responses to leucine in children with the hyperinsulinism/hyperammonemia syndrome

Mutations of glutamate dehydrogenase cause the hyperinsulinism/hyperammonemia syndrome by desensitizing glutamate dehydrogenase to allosteric inhibition by GTP. Normal allosteric activation of glutamate dehydrogenase by leucine is thus uninhibited, leading us to propose that children with hyperinsulinism/hyperammonemia syndrome will have exaggerated acute insulin responses to leucine in the postabsorptive state. As hyperglycemia increases beta-cell GTP, we also postulated that high glucose concentrations would extinguish abnormal responsiveness to leucine in hyperinsulinism/hyperammonemia syndrome patients. After an overnight fast, seven hyperinsulinism/hyperammonemia syndrome patients (aged 9 months to 29 yr) had acute insulin responses to leucine performed using an iv bolus of L-leucine (15 mg/kg) administered over 1 min and plasma insulin measurements obtained at -10, -5, 0, 1, 3, and 5 min. The acute insulin response to leucine was defined as the mean increase in insulin from baseline at 1 and 3 min after an iv leucine bolus. The hyperinsulinism/hyperammonemia syndrome group had excessively increased insulin responses to leucine (mean +/- SEM, 73 +/- 21 microIU/ml) compared with the control children and adults (n = 17) who had no response to leucine (1.9 +/- 2.7 microU/ml; P < 0.05). Four hyperinsulinism/hyperammonemia syndrome patients then had acute insulin responses to leucine repeated at hyperglycemia (blood glucose, 150-180 mg/dl). High blood glucose suppressed their abnormal baseline acute insulin responses to leucine of 180, 98, 47, and 28 microU/ml to 73, 0, 6, and 19 microU/ml, respectively. This suppression suggests that protein-induced hypoglycemia in hyperinsulinism/hyperammonemia syndrome patients may be prevented by carbohydrate loading before protein consumption.

Different antigenic reactivities of bovine brain glutamate dehydrogenase isoproteins

The structural differences between two types of glutamate dehydrogenase (GDH) isoproteins (GDH I and GDH II), homogeneously isolated from bovine brain, were investigated using a biosensor technology and monoclonal antibodies. A total of seven monoclonal antibodies raised against GDH II were produced, and the antibodies recognized a single protein band that comigrates with purified GDH II on sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblot. Of seven anti-GDH II monoclonal antibodies tested in the immunoblot analysis, all seven antibodies interacted with GDH II, whereas only four antibodies recognized the protein band of the other GDH isoprotein, GDH I. When inhibition tests of the GDH isoproteins were performed with the seven anti-GDH II monoclonal antibodies, three antibodies inhibited GDH II activity, whereas only one antibody inhibited GDH I activity. The binding affinity of anti-GDH II monoclonal antibodies for GDH II (K(D) = 1.0 nM) determined using a biosensor technology (Pharmacia BIAcore) was fivefold higher than for GDH I (K(D) = 5.3 nM). These results, together with epitope mapping analysis, suggest that there may be structural differences between the two GDH isoproteins, in addition to their different biochemical properties. Using the anti-GDH II antibodies as probes, we also investigated the cross-reactivities of brain GDHs from some mammalian and an avian species, showing that the mammalian brain GDH enzymes are related immunologically to each other.

Hyperinsulinism and hyperammonemia in infants with regulatory mutations of the glutamate dehydrogenase gene

BACKGROUND

A new form of congenital hyperinsulinism characterized by hypoglycemia and hyperammonemia was described recently. We hypothesized that this syndrome of hyperinsulinism and hyperammonemia was caused by excessive activity of glutamate dehydrogenase, which oxidizes glutamate to alpha-ketoglutarate and which is a potential regulator of insulin secretion in pancreatic beta cells and of ureagenesis in the liver.

METHODS

We measured glutamate dehydrogenase activity in lymphoblasts from eight unrelated children with the hyperinsulinism-hyperammonemia syndrome: six with sporadic cases and two with familial cases. We identified mutations in the glutamate dehydrogenase gene by sequencing glutamate dehydrogenase complementary DNA prepared from lymphoblast messenger RNA. Site-directed mutagenesis was used to express the mutations in COS-7 cells.

RESULTS

The sensitivity of glutamate dehydrogenase to inhibition by guanosine 5'-triphosphate was a quarter of the normal level in the patients with sporadic hyperinsulinism-hyperammonemia syndrome and half the normal level in patients with familial cases and their affected relatives, findings consistent with overactivity of the enzyme. These differences in enzyme insensitivity correlated with differences in the severity of hypoglycemia in the two groups. All eight children were heterozygous for the wild-type allele and had a mutation in the proposed allosteric domain of the enzyme. Four different mutations were identified in the six patients with sporadic cases; the two patients with familial cases shared a fifth mutation. In two clones of COS-7 cells transfected with the mutant sequence from one patient, the sensitivity of the enzyme to guanosine 5'-triphosphate was reduced, findings similar to those in the child's lymphoblasts.

CONCLUSIONS

The hyperinsulinism-hyperammonemia syndrome is caused by mutations in the glutamate dehydrogenase gene that impair the control of enzyme activity.

Regulation of glutamate dehydrogenase by branched-chain amino acids in skeletal muscle from rats and chicks

Little information is available regarding the regulation of glutamate dehydrogenase in skeletal muscle. We investigated the regulation of glutamate dehydrogenase by branched-chain amino acids (BCAA) in skeletal muscles from rats and chicks and determined the effects of metabolic acidosis on the activity and regulation of this enzyme by BCAA in rat skeletal muscle. Skeletal muscle mitochondria were prepared from normal rats and chicks and acidotic rats. Mitochondrial glutamate dehydrogenase activity was measured in the presence or absence of BCAA. Metabolic acidosis was induced by feeding rats 1.5% NH4Cl as drinking water. Glutamate dehydrogenase activity was stimulated by leucine (P < 0.001) and isoleucine (P < 0.05) in rat muscles and by leucine (P < 0.05) in chick muscles in a concentration-dependent manner. Both leucine and isoleucine had their maximum effects at a concentration of 1 mM (45% by leucine and 27% by isoleucine in rat muscle; 36% by leucine in chick muscle). The maximum stimulatory effects of leucine and isoleucine in rat muscles were additive. Neither valine nor 2-oxoisocaproate had an effect on glutamate dehydrogenase activity in rat or chick muscles. In acidotic rats, the basal activity of skeletal muscle glutamate dehydrogenase was 1.8-fold (P < 0.01) greater than in control rats; leucine, isoleucine, and valine significantly increased glutamate dehydrogenase activity (maximally 86, 55 and 33%, respectively; P < 0.05). We conclude that glutamate dehydrogenase activity in skeletal muscle from rats and chicks is regulated by BCAA, and that a species difference exists between rats and chicks. Metabolic acidosis increases the activity of glutamate dehydrogenase and its sensitivity to BCAA.