Ammonia incorporation in Hydrogenomonas eutropha

Unusually stable NAD-specific glutamate dehydrogenase from the alkaliphile Amphibacillus xylanus

Nicotinamide adenine dinucleotide-specific glutamate dehydrogenase (NAD-GDH; EC 1.4.1.3) from Amphibacillus xylanus DSM 6626 was enriched 100-fold to homogeneity. The molecular mass was determined by native polyacrylamide electrophoresis and by gel filtration to be 260 kDa (+/- 25 kDa); the enzyme was composed of identical subunits of 45 (+/- 5) kDa, indicating that the native enzyme has a hexameric structure. NAD-GDH was highly specific for the coenzyme NAD(H) and catalyzed both the formation and the oxidation of glutamate. Apparent Km-values of 56 mM glutamate, 0.35 mM NAD (oxidative deamination) and 6.7 mM 2-oxoglutaric acid, 42 mM NH4Cl and 0.036 mM NADH (reductive amination) were measured. The enzyme was unusually resistant towards variation of pH, chaotropic agents, organic solvents, and was stable at elevated temperature, retaining 50% activity after 120 min incubation at 85 degrees C.

Purification and properties of glutamate dehydrogenase from Vigna unguiculata (L.) walp

Glutamate dehydrogenase (L-glutamate: NAD(+) oxidoreductase (deaminating), EC 1.4.1.2) from 2-day-old seedlings of Vigna unguiculata was purified more than 500-fold. The enzyme gave one diffuse protein band on polyacrylamide gel electrophoresis and its subunit molecular weight was 51,000±5% as determined by sodium-dodecyl-sulfate-gel electrophoresis. The enzyme was active with NADH, NAD(+) and NADPH in the ratio of 126:1:2 with pH optima of 8.0, 10.0 and 6.0 respectively. Inhibition of enzyme by EDTA was reversed by Ca(2+) and Mn(2+) while reduced glutathione reversed inhibition by p-hydroxymercuribenzoate. The enzyme was highly specific for L-glutamate and α-ketoglutarate showing very little or no activity with a range of other amino acids and keto acids. Values for Michaelis constants were: NAD(+), 0.285 mM; NADH, 0.059 mM; α-keto-glutarate, 1.79 mM; ammonium sulphate, 28.6 mM; L-glutamate, 16.7 mM.

Purification and properties of glutamate dehydrogenase from a thermophilic bacillus

A 250- to 300-fold purification of a nicotinamide adenine denucleotide phosphate (NADP)-dependent glutamate dehydrogenase (GDH, E.C. 1.4.1.4) with a yield of 60% from a thermophilic bacillus is described. More than one NADP-specific GDH was detected by polyacrylamide gel electrophoresis. The enzyme is of high molecular weight (approximately 2 X 10-6), similar to that of the beef and frog liver GDH. The pI of the thermophilic GDH is at pH 5.24. The enzyme is highly thermostable at the pH range of 5.8 to 9.0. The purified GDH, unlike the crude enzyme, was very labile at subzero temperatures. An unidentified factor(s) from the crude cell-free extract prevented the inactivation of the purified GDH at -70 C. Various reactants of the GDH system and D-glutamate also protected, to some extent, the enzyme from inactivation at -70 C. From the Michaelis constants for glutamate (1.1 X 10-2M), NADP (3 X 10-4M), ammonia (2.1 X 10-2M), alpha-ketoglutarate (1.3 X 10-3M), and reduced NADP (5.3 X 10-5M), it is suggested that the enzyme catalyzes in vivo the formation of glutamate from ammonia and alpha-ketoglutarate. The amination of alpha-ketoglutarate and deamination of glutamate by the thermophilic GDH are optimal at the pH values of 7.2 and 8.4, respectively.

Purification, properties, and regulation of glutamic dehydrogenase of Bacillus licheniformis

Cell-free extracts of Bacillus licheniformis and B. cereus were found to contain high specific activities of nicotinamide adenine dinucleotide phosphate (NADP)-dependent-l-glutamate dehydrogenase [EC 1.4.1.4; l-glutamate: NADP oxidoreductase (deaminating)]. Maximum specific activities were found in extracts of cells during the late exponential phase of growth when ammonium ion served as the sole source of nitrogen. Extremely low specific activities were detected throughout the growth cycle when l-glutamate or Casamino Acids served as the source of carbon and nitrogen. The enzyme was purified 55-fold from crude extracts of B. licheniformis, and apparent kinetic constants were determined. Sigmoidal saturation kinetics were not observed, and various adenylates had no effect on the enzyme. Repression of enzyme synthesis during growth on l-glutamate or Casamino Acids was partially overcome by additions of glucose or pyruvate, and this apparent derepression was totally abolished by inhibitors of ribonucleic acid and protein synthesis. Similarly, additions of l-glutamate or Casamino Acids to cells growing on glucose-ammonium ion resulted in strong repression of enzyme synthesis. It is suggested that the enzyme serves an anabolic role in metabolism. Nicotinamide adenine dinucleotide-dependent glutamate dehydrogenase activity was not detected in five species of Bacillus, irrespective of nutritional conditions or of the physiological age of cells.

pH-conditional, ammonia assimilation-deficient mutants of Hydrogenomonas eutropha: evidence for the nature of the mutation

Two amination-deficient mutants of Hydrogenomonas eutropha, characterized by pH-dependent linear growth on non-amino acid substrates, were investigated to determine the exact nature of the mutation. Glutamate dehydrogenase, the only aminating enzyme found in wild-type cells, was present at similar levels in mutant cells. Phenylalanine and aspartate, which allowed normal growth of the mutants, could transaminate 2-oxoglutarate to glutamate, whereas alanine, which does not support normal growth, could not transfer its amino nitrogen to form glutamate. In H. eutropha, l-alanine is apparently synthesized by beta-decarboxylation of aspartate. Studies with NH(4) (+) ions as the sole nitrogen source demonstrated that growth rates of the mutant strains were dependent on both extracellular pH and NH(4) (+) ion concentration. Comparison of these results revealed that the growth rate of mutant cultures was proportional to the concentration of extracellular NH(3). Wild-type cultures were not dependent on extracellular NH(3) since exponential growth rates did not vary with pH or NH(4) (+) ion concentration. The results suggest that the mutant strains lack an NH(4) (+) ion transport system and consequently are dependent on NH(3) diffusion which does not support optimal amination rates. The significance of the findings for the amino acid metabolism of H. eutropha is discussed.