Interaction of the D-isomer of gamma-methylene glutamate with an active site thiol of gamma-glutamylcysteine synthetase

gamma-Glutamylcysteine synthetase has a thiol group in the vicinity of its glutamate-binding site. During efforts to find a covalently bound inhibitor, interaction of the enzyme with gamma-methylene glutamate was examined because this analog of glutamate, which has an alpha,beta-unsaturated moiety, would be expected to bind at the glutamate site and might react with an active site thiol. gamma-Methylene glutamate, which is not a significant substrate, inhibits the enzyme competitively toward glutamate. Preincubation of the enzyme with gamma-methylene DL-glutamate led to substantial inactivation which was dependent upon the presence of Mg2+ or Mn2+; glutamate protected against inactivation. Inactivation was observed with the D-isomer of gamma-methylene glutamate, but not with the corresponding L-isomer. The inactivated enzyme contains close to 1 mol of gamma-methylene glutamate/mol of enzyme. Studies in which enzyme inactivated by treatment with [14C]gamma-methylene glutamate was hydrolyzed indicate that gamma-methylene glutamate reacts with an active site thiol.     

Increased susceptibility of carbamylated glutamate dehydrogenase to proteolysis

Glutamate dehydrogenase is very susceptible to carbamylation which results in loss of activity. The effect of a number of proteolytic enzymes (pronase, trypsin and chymotrypsin) on native and carbamylated glutamate dehydrogenase was tested. In all cases, the carbamylated enzyme was at least twice as susceptible to proteolysis as the native enzyme. Antibodies were prepared against glutamate dehydrogenase and carbamylated glutamate dehydrogenase; the carbamylated enzyme was antigenically indistinguishable from the native enzyme. Preliminary experiments indicate that the carbamylated glutamate dehydrogenase is taken up by ascites tumor cells while glutamate dehydrogenase is not. It seems possible that the effects described can be extrapolated to degradation by lysosomes and to other covalently modified enzymes.     

Phosphorylation of NAD-dependent glutamate dehydrogenase from yeast.

The NAD-dependent glutamate dehydrogenase from Candida utilis was isolated from 32P-labeled cells following enzyme inactivation promoted by glutamate starvation and found to exist in a phosphorylated form. Analysis of purified, fully active NAD-dependent glutamate dehydrogenase (a form) and inactive NAD-dependent glutamate dehydrogenase (b form) for alkalilabile phosphate revealed that the a form contained 0.09 +/- 0.06 mol of phosphate/mol of enzyme subunit and b form 1.25 +/- 0.06 mol of phosphate/mol of enzyme subunit. Phosphorylation caused a 10-fold reduction in enzyme specific activity. Dephosphorylation (release of 32P) and enzyme reactivation occurred on incubation with cell-free yeast extracts, indicating the presence of a phosphoprotein phosphatase in such preparations.     

Occurrence of thermolabile and regulatory NAD-linked glutamate dehydrogenase in Pseudomonas fluorescens.

NAD-linked glutamate dehydrogeanse [EC 1.4.1.2] was detected together with NADP-linked glutamate dehydrogenase [EC 1.4.1.4] and aspartase [EC 4.3.1.1] in Pseudomonas fluorescens cells. The three enzymes were distinctly separated by DEAE-Sephadex column chromatography. The NAD-linked enzyme was extremely thermolabile and was rapidly inactivated even at temperatures as low as 35--40 degrees C. The combined addition of NAD+ and glutamate, however, effectively stabilized the enzyme. The glutamate saturation profile of the NAD-linked enzyme exhibited cooperativity with a Hill coefficient (n) of 1.4. ATP inhibited the enzyme in an allosteric manner, increasing the n value to 2.2. These results suggest a novel type of metabolic regulation shared by the three enzymes in the biosynthesis and catabolism of amino acids.     

Purification and properties of glutamate synthase from Thiobacillus thioparus.

Glutamate synthase was purified about 250-fold from Thiobacillus thioparus and was characterized. The molecular weight was estimated as 280,000 g/mol. The enzyme showed absorption maxima at 280, 380, and 450 nm and was inhibited by Atebrin, suggesting that T. thioparus glutamate synthase is a flavoprotein. The enzyme activity was also inhibited by iron chelators and thiolbinding agents. The enzyme was specific for reduced nicotinamide adenine dinucleotide phosphate (NADPH) and alpha-ketoglutarate, but L-glutamine was partially replaced by ammonia as the amino donor. The Km values of glutamate synthase for NADPH, alpha-ketoglutarate, and glutamine were 3.0 muM, 50 muM, and 1.1 mM, respectively. The enzyme had a pH optimum between 7.3 and 7.8. Glutamate synthase from T. thioparus was relatively insensitive to feedback inhibition by single amino acids but was sensitive to the combined effects of several amino acids. Enzymes involved in glutamate synthesis in T. thioparus were studied. Glutamine synthetase and glutamate synthase, as well as two glutamate dehydrogenases (NADH and NADPH dependent), were present in this organism. This levels of glutamate synthase and glutamate dehydrogenase were similar in T. thioparus grown on 0.7 or 7.0 mM ammonium sulfate. The sum of the activities of both glutamate dehydrogenases was only 1/25 of that of glutamate synthase under the assay conditions. It was concluded that the glutamine pathway is important for ammonia assimilation in this autotrophic bacterium.     

Glutamate dehydrogenase--an activator of NAD-kinase in the rabbit liver

An electrophoretically homogeneous preparation of a NAD-kinase activator from rabbit liver was obtained and its physico-chemical properties were investigated. The molecular mass of the monomer and oligomer, pI, number of SH-groups per enzyme subunit and some other factors were determined. The similarity of activator properties to those of glutamate dehydrogenase and the revealed glutamate dehydrogenase activity of the NAD-kinase activator permitted to identify the latter as glutamate dehydrogenase. It was demonstrated that the enzyme activates NAD-kinase 2-4 times already at the glutamate dehydrogenase: NAD-kinase ratio of 2:1. The effect of glutamate dehydrogenase on the enzyme consists in an increase of Vmax; the KmNAD value for the NAD-kinase reaction remains thereby unchanged. The physiological role of the interaction between the two enzymes is discussed.     

Nitrogen regulation of glutamine synthetase in Neurospora crassa.

A higher activity of glutamine synthetase (EC 6.3.1.2) was found in Neurospora crassa when NH4+ was limiting as nitrogen source than when glutamate was limiting. When glutamate, glutamine or NH4+ were in excess, a lower activity was found. Immunological titration and sucrose gradient sedimentation of the enzyme established that under all these conditions enzyme activity corresponded to enzyme concentration and that the octamer was the predominant oligomeric form. When N. crassa was shifted from nitrogen-limiting substrates to excess product as nitrogen source, the concentration of glutamine synthetase was adjusted with kinetics that closely followed dilution by growth. When grown on limiting amounts of glutamate, a lower oligomer was present in addition to the octameric form of the enzyme. When the culture was shifted to excess NH4+, glutamine accululated at a high rate; nevertheless, there was only a slow decrease in enzyme activity and no modification of the oligomeric pattern.     

Purification and properties of glutamate dehydrogenase from the mantle muscle of the squid, Loligo pealeii. Role of the enzyme in energy production from amino acids.

1. The activity of glutamate dehydrogenase was measured in the tissues of the squid, Loligo pealeii. The enzyme occurs in high activity in digestive pouch, systemic heart, and all muscle tissues. 2. Glutamate dehydrogenase from mantle muscle is located intra-mitochondrially, has a molecular weight of 310,000, and is electrophoretically similar to the enzyme from all other squid tissues. 3. The enzyme from mantle muscle was purified 40-fold by elution from DEAE-cellulose and used for kinetic studies. The enzyme is NAD+-specific, activated by ADP, AMP, and leucine, and inhibited by GTP, GDP, ATP, and reaction products (in particular NADH). 4. Squid glutamate dehydrogenase shows an almost absolute dependence on ADP. The purified enzyme is activated over 100-fold by saturating concentrations of ADP (Ka = 0,75 7M); The pH optima are also altered significantly by ADP. 5. The enzyme appears to be kinetically adapted to favour glutamate oxidation in comparison to glutamate dehydrogenase from other resources. The evidence indicates that the primary role of glutamate dehydrogenase in squid mantle muscle is in regulating the catabolism of amino acids for energy production.     

STUDIES ON THE REGULATION OF GABA SYNTHESIS: SUBSTRATE-PROMOTED DISSOCIATION OF PYRIDOXAL-5''-PHOSPHATE FROM GAD

The association between glutamate decarboxylase (GAD) and its cofactor, pyridoxal-5′-phos-phate (pyridoxal-P), was studied using 20,0000 supernatant of rat brain. In this preparation GAD required added pyridoxal-P to maintain a linear reaction rate beyond 5 min of incubation. Following exhaustive dialysis the enzyme was more than 83% saturated with cofactor indicating that the cofactor was tightly bound to the enzyme. When incubations were performed in the presence of glutamate and without added pyridoxal-P there was a progressive inactivation of the enzyme which was dependent on the glutamate concentration. This lost activity was almost completely recovered by addition of pyridoxal-P to the dialyzed glutamate-inactivated enzyme. The results suggest that glutamate inactivates GAD by promoting the dissociation of pyridoxal-P from the enzyme thereby producing inactive apoen-zyme which can be reactivated by combining with available pyridoxal-P. This interpretation is supported by the finding that progress curves for the reaction were accurately described over a 30 min incubation period and 10-fold glutamate concentration range by an integrated rate equation which takes the glutamate-promoted dissociation of cofactor into account. The progressive inactivation could not be attributed to denaturation of the enzyme, impurities in the substrate, effects of pH, depletion of substrate, protein concentration, sulfhydryl reagents or product inhibition. The results presented here also show that certain precautions must be adopted to accurately measure GAD activity in the absence of added pyridoxal-P as has been widely done in studies of drug action. Specifically, measurements must be made at short times of incubation and low concentrations of glutamate to minimize the glutamate-promoted inactivation of the enzyme.     

Effect of Lesions of the Olfactory Bulb on the Levels of Amino Acids and Related Enzymes in the Olfactory Cortex of the Guinea Pig

Abstract: Olfactory bulb removal and consequential degeneration of the lateral olfactory tract led to a decreasein the levels of glutaminase and malate dehydrogenase inthe ipsilateral olfactory cortex. These changes in enzyme activity may account for the well established decrease inthe levels of aspartate and glutamate in the olfactory cortex following ipsilateral bulbectomy. The level of glutamine synthetase, a glial marker enzyme, was slightly-increased while the activities of glutamate decarboxylase, glutamate dehydrogenase, and glutamate oxaloacetic transaminase were unchanged.     

Purification and characteristics of a gamma-glutamyl kinase involved in Escherichia coli proline biosynthesis.

gamma-Glutamyl kinase, the first enzyme of the proline biosynthetic pathway, was purified to homogeneity from an Escherichia coli strain resistant to the proline analog 3,4-dehydroproline. The enzyme had a native molecular weight of 236,000 and was apparently comprised of six identical 40,000-dalton subunits. Enzymatic activity of the protein was detectable only in assays containing highly purified gamma-glutamyl phosphate reductase, the second enzyme of the proline pathway. Plots of gamma-glutamyl kinase activity as a function of glutamate concentration were sigmoidal, with a half-saturation value for glutamate of 33 mM, whereas plots of enzyme activity as a function of ATP concentration displayed typical Michaelis-Menten kinetics with a Km for ATP of 4 X 10(-4) M. Enzyme activity was insensitive to the glutamate analog L-methionine-DL-sulfoximine, but ADP was a potent competitive inhibitor. Characteristics of the enzyme were compared with those of a gamma-glutamyl kinase partially purified from a 3,4-dehydroproline-sensitive E. coli. These results indicated that the only major difference was that the enzyme from the 3,4-dehydroproline-resistant strain was 100-fold less sensitive to feedback inhibition by proline.     

Properties of the recombinant ferredoxin-dependent glutamate synthase of Synechocystis PCC6803. Comparison with the Azospirillum brasilense NADPH-dependent enzyme and its isolated alpha subunit

The properties of the recombinant ferredoxin-dependent glutamate synthase of Synechocystis PCC6803 were determined by means of kinetic and spectroscopic approaches in comparison to those exhibited by the bacterial NADPH-dependent enzyme form. The ferredoxin-dependent enzyme was found to be similar to the bacterial glutamate synthase alpha subunit with respect to cofactor content (one FMN cofactor and one [3Fe-4S] cluster per enzyme subunit), overall absorbance properties, and reactivity of the FMN N(5) position with sulfite, as expected from the similar primary structure of ferredoxin-dependent glutamate synthase and of the bacterial NADPH-dependent glutamate synthase alpha subunit. The ferredoxin- and NADPH-dependent enzymes were found to differ with respect to the apparent midpoint potential values of the FMN cofactor and of the [3Fe-4S] cluster, which are less negative in the ferredoxin-dependent enzyme form. This feature is, at least in part, responsible for the efficient oxidation of L-glutamate catalyzed by this enzyme form, but not by the bacterial NADPH-dependent counterpart. At variance with earlier reports on ferredoxin-dependent glutamate synthase, in the Synechocystis enzyme the [3Fe-4S] cluster is not equipotential with the flavin cofactor. The present studies also demonstrated that binding of reduced ferredoxin to ferredoxin-dependent glutamate synthase is essential in order to activate reaction steps such as glutamine binding, hydrolysis, or ammonia transfer from the glutamine amidotransferase site to the glutamate synthase site of the enzyme. Thus, ferredoxin-dependent glutamate synthase seems to control and coordinate catalytic activities taking place at its subsites by regulating the reactions of the glutamine amidotransferase site. Association with reduced ferredoxin appears to be necessary, but not sufficient, to trigger the required activating conformational changes.     

Regulation of glutamate dehydrogenase in Bacillus subtilis.

The activity of the nicotinamide adenine dinucleotide-dependent glutamate dehydrogenase in Bacillus subtilis was influenced by the carbon source, but not the nitrogen source, in the growth medium. The highest specific activity for this enzyme was found when B. subtilis was grown in a minimal or rich medium that contained glutamate as the carbon source. It is proposed that glutamate dehydrogenase serves a catabolic function in the metabolism of glutamate, is induced by glutamate, and is subject to catabolite repression.     

Purification and some properties of mitochondrial glutamate:glyoxylate aminotransferase and mechanism of its involvement in glycolate pathway in Euglena gracilis z

Euglena contains glutamate:glyoxylate aminotransferase (GGT) both in mitochondria and in cytosol. Both isoforms were separated from each other by DEAE-cellulose chromatography. The mitochondrial enzyme had an apparent Km of 1.9 mM for glutamate and the cytosolic enzyme 52.6 mM. Mitochondrial GGT was further purified by ammonium sulfate fractionation, isoelectric focusing, and gel chromatography. It had a molecular weight of 141,000 and an isoelectric point of pH 4.88; the optimum pH was 8.5. Its apparent Km values for glutamate and for glyoxylate were 2.0 and 0.25 mM, respectively. In addition to glutamate, mitochondrial GGT used 5-hydroxytryptophan, tryptophan, and cysteine as amino donors in the transamination to glyoxylate. Alanine did not support the activity. The relative activity of the enzyme for amino acceptors on the transamination from glutamate was 4-hydroxyphenylpyruvate greater than phenylpyruvate greater than glyoxylate greater than hydroxypyruvate. Pyruvate and 2-oxoglutarate were not used in the reaction. Evidence that GGT functions mainly in the irreversible transamination between glutamate and glyoxylate is presented. The functional significance of GGT in the glycolate pathway of Euglena is also discussed.     

A rapid and efficient new method of purification of glutamate dehydrogenase by affinity chromatography on GTP-sepharose

The very high affinity for GTP of glutamate dehydrogenase was used to purify this enzyme by affinity chromatography. After periodic acid oxidation, GTP was covalently bound to an activated Sepharose. When crude mitochondrial extracts were applied on a column of this GTP-Sepharose, glutamate dehydrogenase was retained with very few other proteins. Glutamate dehydrogenase from rat liver was eluted with a KCl gradient with only one contaminating protein. From a pig heart mitochondrial extract the enzyme was purified 300-fold in one step. A chromatography on hydroxyapatite was sufficient to achieve the purification. This very simple technique avoids the long and troublesome crystallization steps generally involved in glutamate dehydrogenase purification.     

Role of NAD-linked glutamate dehydrogenase in nitrogen metabolism in Saccharomyces cerevisiae.

We cloned GDH2, the gene that encodes the NAD-linked glutamate dehydrogenase in the yeast Saccharomyces cerevisiae, by purifying the enzyme, making polyclonal antibodies to it, and using the antibodies to screen a lambda gt11 yeast genomic library. A yeast strain with a deletion-disruption allele of GDH2 which replaced the wild-type gene grew very poorly with glutamate as a nitrogen source, but growth improved significantly when the strain was also provided with adenine or other nitrogenous compounds whose biosynthesis requires glutamine. Our results indicate that the NAD-linked glutamate dehydrogenase catalyzes the major, but not sole, pathway for generation of ammonia from glutamate. We also isolated yeast mutants that lacked glutamate synthase activity and present evidence which shows that normally NAD-linked glutamate dehydrogenase is not involved in glutamate biosynthesis, but that if the enzyme is overexpressed, it may function reversibly in intact cells.     

THE PURIFICATION AND PROPERTIES OF RAT BRAIN GLUTAMATE DEHYDROGENASE

—A method is described for the preparation of glutamate dehydrogenase in a highly purified form from rat brain. Only one protein band was detected when the enzyme was subjected to electrophoresis on SDS polyacrylamide gels. The rat brain enzyme was essentially identical to the rat liver enzyme with respect to electrophoresis on SDS polyacrylamide gels, immunochemical properties and most kinetic parameters. However, the brain enzyme was much less reactive with glutamate, was more sensitive to inhibition by haloperidol, and was considerably more stable than the liver enzyme.     

Occurrence of ferredoxin-dependent glutamate synthase in plant cell fraction of soybean root nodules (Glycine max)

Ferredoxin-dependent glutamate synthase (EC 1.4.7.1) and NADH-dependent glutamate synthase (EC 1.4.1.14) have been identified in the plant cells of soybean nodules. Ferredoxin-dependent glutamate synthase is 2-fold more active than NADH-dependent enzyme in vitro. Ferredoxin-dependent glutamate synthase cross-reacts with IgG against ferredoxin-dependent glutamate synthase of rice green leaves, whereas NADH-dependent glutamate synthase does not recognize the IgG, indicating that there are two distinct enzyme proteins. Ferredoxin-dependent glutamate synthase is composed of polypeptide chain(s) of 165 kDa and has a high affinity to spinach leaf ferredoxin as an electron carrier.     

NAD and NADP-dependent glutamate dehydrogenase in Hydrogenomonas H 16

Summary Hydrogenomonas H 16 synthetized two chromatographically distinct forms of glutamate dehydrogenase which differed in their thermolability. One glutamate dehydrogenase utilized NAD, the other NADP as a coenzyme.Low specific activity of NAD-dependent glutamate dehydrogenase was found in cells grown with glutamate as sole nitrogen source or in cells grown with a high concentration of ammonium ions. In the presence of a low concentration of ammonium ions or in a nitrogen free medium, the specific activity of the NAD-dependent enzyme increased. Corresponding to the formation of the NAD-dependent glutamate dehydrogenase the enzyme glutamine synthetase was synthesized. The ratio of NAD-dependent glutamate dehydrogenase to glutamine synthetase activity differed only slightly in cells grown with different nitrogen and carbon sources.The NADP-dependent glutamate dehydrogenase was found in high specific activity in cells grown with an excess of ammonium ions. Under nitrogen starvation the formation of the NADP-dependent glutamate dehydrogenase ceased and the enzyme activity decreased.     

Rapid transfer of oxygens from inorganic phosphate to glutamine catalyzed by Escherichia coli glutamine synthetase.

Measurements are reported on certain isotopic fluxes during the net conversion of glutamine, ADP and Pi to glutamate, NH3, and ATP by Escherichia coli glutamine synthetase (adenylylated form, Mn2+ activated) in presence of a hexokinase/glucose trap to remove the ATP formed during the reaction. The results show that the transfer of oxygens from Pi to glutamine is the most rapid of the measured isotopic interchanges, over five oxygens from Pi being transferred to glutamine for each glutamate formed by net reaction. Under similar conditions, the oxygen transfer from Pi to glutamate, was stimulated somewhat by an increase in the glutamate concentration but inhibited by an increase in the ammonia concentration. The enzyme from brain or peas did not show the rapid transfer of 18O from Pi to glutamine shown by the E. coli enzyme. Deductions are also made from the data about the availability of the oxygens of gamma-carboxyl of bound glutamate for reaction. The most logical explanation of the results with the E. coli enzyme is that the gamma-carboxyl group of bound glutamate has sufficient rotational freedom so that under conditions of rapid substrate interconversion either carboxylate oxygen can participate in the reaction. The results with the pea enzyme are consistent with hindered rotation of the gamma-care additional findings make likely a relative order of certain catalytic steps for the E. coli enzyme as follows: ATP release less than NH3 release less than glutamate release less than substrate interconversion less than glutamine release and Pi release and glutamate release less than ADP release.