Glutamine synthetase of pea leaves: divalent cation effects, substrate specificity, and other properties

Genetic Diversity in Nitrogen Fertiliser Responses and N Gas Emission in Modern Wheat

Crops assimilate nitrogen (N) as ammonium via the glutamine synthetase/glutamate synthase (GS/GOGAT) pathway which is of central importance for N uptake and potentially represents a bottle neck for N fertiliser-use efficiency. The aim of this study was to assess whether genetic diversity for N-assimilation capacity exists in wheat and could be exploited for breeding. Wheat plants rapidly, within 6 h, responded to N application with an increase in GS activity. This was not accompanied by an increase in GS gene transcript abundance and a comparison of GS1 and GS2 protein models revealed a high degree of sequence conservation. N responsiveness amongst ten wheat varieties was assessed by measuring GS enzyme activity, leaf tissue ammonium, and by a leaf-disc assay as a proxy for apoplastic ammonia. Based on these data, a high-GS group showing an overall positive response to N could be distinguished from an inefficient, low-GS group. Subsequent gas emission measurements confirmed plant ammonia emission in response to N application and also revealed emission of N2O when N was provided as nitrate, which is in agreement with our current understanding that N2O is a by-product of nitrate reduction. Taken together, the data suggest that there is scope for improving N assimilation capacity in wheat and that further investigations into the regulation and role of GS-GOGAT in NH3 emission is justified. Likewise, emission of the climate gas N2O needs to be reduced, and future research should focus on assessing the nitrate reductase pathway in wheat and explore fertiliser management options.

RNA-Seq, physiological, and biochemical analysis of burley tobacco response to nitrogen deficiency

To explore the effects of nitrogen deficiency in burley tobacco, two varieties were cultivated and subjected to conditions of sufficient and deficient nitrogen. The natural characteristics of varieties TN90 and TN86 during tobacco cultivation were similar for nitrogen metabolism. Both carbon and nitrogen metabolism were significantly affected by reducing amounts of applied nitrogen. Under nitrogen-deficient conditions, average leaf biomass, root weight, photosynthetic rate (Pn), pigment levels, total nitrogen, and nitrate content of TN86 and TN90 were significantly decreased by 52.88%, 69.19%, 22.65%, 46.80%, 37.42%, and 79.15%, respectively (p < 0.01). Nicotine and soluble reducing sugar contents were significantly decreased by 96.67% and 95.12%, respectively, in TN86 roots (p < 0.01), which was consistent with the reductions in root surf area, average diameter, and root volume. Nitrogen deficiency induced 6318 differentially expressed genes in both TN90 and TN86, which were highly expressed. In total, 428 upregulated genes were analysed and found to be mainly enriched in the MAPK signalling pathway, sesquiterpenoid and triterpenoid biosynthesis, and arginine and proline metabolism. Meanwhile, 213 downregulated genes were analysed and found to be mainly enriched in photosynthesis, nitrogen metabolism, and amino acid biosynthesis. Reduced pigment content and Pn may result in low carbohydrate formation and decreased leaf biomass in burley tobacco under nitrogen-deficient conditions.

Nitrogen use plasticity in response to light intensity in neotropical tree species of distinct functional groups

Environmental conditions influence the use of different nitrogen (N) sources by plants. We hypothesized that an increase in light intensity favors the use of nitrate (NO₃ ^- ) relative to ammonium (NH₄ ⁺ ) by seedlings of neotropical tree species from different functional groups, that is, Cecropia pachystachya (a shade-intolerant species), Cariniana estrellensis (a shade-tolerant canopy species), and Guarea kunthiana (a shade-tolerant understory species). We analyzed the growth and N metabolism in seedlings simultaneously provided with NH₄ ⁺ and NO₃ ^- , under lower (LL) and higher (HL) light intensity. ¹⁵ N incorporation into amino acids was monitored after incubation with ¹⁵ N-labeled NH₄ ⁺ or NO₃ ^- . Under HL, all species showed decreased leaf area ratio, and increased growth, nitrate reductase activity and assimilated N content. Cecropia pachystachya increased the use of both N sources under HL, with substantial increases in ¹⁵ N-amino acids derived from ¹⁵ NO₃ ^- (12.5- and 4.0-fold in roots and leaves, respectively) and ¹⁵ NH₄ ⁺ (4.5- and 3.0-fold in roots and leaves, respectively). Guarea kunthiana showed the greatest plasticity in N use, as the assimilation of ¹⁵ NO₃ ^- in roots and leaves increased substantially under HL (11.2- and 17.0-fold, respectively). Cariniana estrellensis increased the assimilation of ¹⁵ NH₄ ⁺ in roots and ¹⁵ NO₃ ^- in leaves under HL. Therefore, the responses of N use strategies to light intensity varied with the species according to their ecological characteristics.

Regulation of Ammonium Cellular Levels is An Important Adaptive Trait for the Euhalophytic Behavior of Salicornia europaea

Salinization of agricultural land is a devastating phenomenon which will affect future food security. Understanding how plants survive and thrive in response to salinity is therefore critical to potentiate tolerance traits in crop species. The halophyte Salicornia europaea has been used as model system for this purpose. High salinity causes NH4+ accumulation in plant tissues and consequent toxicity symptoms that may further exacerbate those caused by NaCl. In this experiment we exposed Salicornia plants to five concentrations of NaCl (0, 1, 10, 50 and 200 mM) in combination with two concentrations of NH4Cl (1 and 50 mM). We confirmed the euhalophytic behavior of Salicornia that grew better at 200 vs. 0 mM NaCl in terms of both fresh (+34%) and dry (+46%) weights. Addition of 50 mM NH4Cl to the growth medium caused a general growth reduction, which was likely caused by NH4+ accumulation and toxicity in roots and shoots. When plants were exposed to high NH4Cl, high salinity reduced roots NH4+ concentration (-50%) compared to 0 mM NaCl. This correlates with the activation of the NH4+ assimilation enzymes, glutamine synthetase and glutamate dehydrogenase, and the growth inhibition was partially recovered. We argue that NH4+ detoxification is an important trait under high salinity that may differentiate halophytes from glycophytes and we present a possible model for NH4+ detoxification in response to salinity.

Biochemical, Physiological and Transcriptomic Comparison between Burley and Flue-Cured Tobacco Seedlings in Relation to Carbohydrates and Nitrate Content

Burley tobacco is a genotype of chloroplast-deficient mutant with accumulates high levels of tobacco-specific nitrosamines (TSNAs) which would induce malignant tumors in animals. Nitrate is a principle precursor of tobacco-specific nitrosamines. Nitrate content in burley tobacco was significantly higher than that in flue-cured tobacco. The present study investigated differences between the two tobacco types to explore the mechanisms of nitrate accumulation in burley tobacco. transcripts (3079) related to the nitrogen and carbon metabolism were observed. Expression of genes involved in carbon fixation, glucose and starch biosynthesis, nitrate translocation and assimilation were significantly low in burley tobacco than flue-cured tobacco. Being relative to flue-cured tobacco, burley tobacco was significantly lower at total nitrogen and carbohydrate content, nitrate reductase and glutamine synthetase activities, chlorophyll content and photosynthetic rate (Pn), but higher nitrate content. Burley tobacco required six-fold more nitrogen fertilizers than flue-cured tobacco, but both tobaccos had a similar leaf biomass. Reduced chlorophyll content and photosynthetic rate (Pn) might result in low carbohydrate formation, and low capacity of nitrogen assimilation and translocation might lead to nitrate accumulation in burley tobacco.

Difference between Burley Tobacco and Flue-Cured Tobacco in Nitrate Accumulation and Chemical Regulation of Nitrate and TSNA Contents

Tobacco-specific nitrosamines (TSNAs) are harmful carcinogens, with nitrate as a precursor of their formation. Nitrate content is considerably higher in burley tobacco than in flue-cured tobacco, but little has been reported on the differences between types of nitrate accumulation during development. We explored nitrate accumulation prior to harvest and examined the effects of regulatory substances aimed at decreasing nitrate and TSNA accumulation. In growth experiments, nitrate accumulation in burley and flue-cured tobacco initially increased but then declined with the highest nitrate content observed during a fast-growth period. When treating tobacco crops with molybdenum (Mo) during fast growth, nitrate reductase activity in burley tobacco increased significantly, but the NO3-N content decreased. These treatments also yielded significant reductions in NO3-N and TSNA contents. Therefore, we suggest that treatment with Mo during the fast-growth period and a Mo-Gfo (Mo-glufosinate) combination at the maturity stage is an effective strategy for decreasing nitrate and TSNAs during cultivation.

Glutamine synthetase activity in leaves of Zea mays L. as influenced by magnesium status

The total capacity of the GS-mediated ligation of free ammonium and glutamate to form glutamine in the leaves of maize plants is not impaired upon severe magnesium starvation. Magnesium deficiency does not obligatorily lead to the decreased total protein concentrations in the leaves. Magnesium (Mg) is an integral component of the enzyme glutamine synthetase (GS), having both a structural and a catalytic role. Moreover, Mg is relevant for the post-translational regulation of the GS. Glutamine synthetase is one of the key enzymes in nitrogen assimilation, ligating-free ammonium (NH4 (+)) to glutamate to form glutamine and it is therefore crucial for plant growth and productivity. This study was conducted in order to test whether a severe Mg-deficiency impairs the total capacity of the GS-catalyzed synthesis of glutamine in maize leaves. Maize was grown hydroponically and the GS activity was analyzed dependent on different leaf developmental stages. Glutamine synthetase activity in vitro assays in combination with immune-dot blot analysis revealed that both the total activity and the abundance of glutamine synthetase was not impaired in the leaves of maize plants upon 54 days of severe Mg starvation. Additionally, it was shown that Mg deficiency does not obligatorily lead to decreased total protein concentrations in the leaves, as assayed by Bradford protein quantification. Moreover, Mg resupply to the roots or the leaves of Mg-deficient plants reversed the Mg-deficiency-induced accumulation of free amino acids in older leaves, which indicates impaired phloem loading. The results of our study reveal that the total GS-mediated primary or secondary assimilation of free NH4 (+) is not a limiting enzymatic reaction under Mg deficiency and thus cannot be accountable for the observed restriction of plant growth and productivity in Mg-deficient maize.

Molecular analysis of two mutants from Lotus japonicus deficient in plastidic glutamine synthetase: functional properties of purified GLN2 enzymes

Two photorespiratory mutants from Lotus japonicus, namely Ljgln2-1 and Ljgln2-2, deficient in plastidic glutamine synthetase (GLN2), were analysed at the molecular level. Both mutants showed normal levels of Gln2 mRNA, indicating that they were affected post-transcriptionally. Complete sequencing of full-length Gln2 cDNAs revealed the presence of a single point mutation on each mutant, leading to G85R and L278H amino acid replacements, respectively. Different types of experimental approaches, including heterologous expression and complementation tests in Escherichia coli, showed that both GLN2 mutant proteins completely lacked of biosynthetic and transferase enzyme activities. Moreover, it was also shown that while GLN2-1 mutant protein was assembled into a less stable inactive octamer, GLN2-2 mutant protein was unable to acquire a proper quaternary structure and was rapidly degraded. Therefore, the mutations analysed are the first of their type affecting the stability and/or the quaternary structure of the GLN2 enzyme. The kinetic parameters of purified recombinant GLN2 were determined. The enzyme showed positive cooperativity towards ammonium and Mg(2+). Thiol compounds stimulated by twofold the biosynthetic activity but not the transferase activity of recombinant GLN2 and were able to alter the kinetics towards glutamate of the enzyme. Moreover, the biosynthetic activity of recombinant GLN2 was stimulated by more than tenfold by the presence of free Mg(2+).

Functional importance of Asp56 from the alpha-polypeptide of Phaseolus vulgaris glutamine synthetase. An essential residue for transferase but not for biosynthetic enzyme activity

Replacement of Asp56 by site-directed mutagenesis of the alpha-gene from Phaseolus vulgaris glutamine synthetase heterologously expressed in Escherichia coli produces a complete loss of transferase enzyme activity, thus revealing essentiality of the residue for this particular enzyme activity. This happens independent of Asp56 being replaced by Ala or Glu, suggesting that the essentiality of this residue cannot be attributed to its negative electrical charge. However, a high level of glutamine synthetase biosynthetic specific activity (referred to glutamine synthetase protein, as determined immunologically), is present in D56A and D56E mutants, suggesting that Asp56 is an example of a residue that has a different role in the catalytic mechanism of both enzyme activities of this protein. Km for ATP, glutamate and Mg2+, as well as energy of activation, can be altered as a consequence of the performed mutations. However, the Km and catalytic efficiency for ammonium remains unaffected. Therefore, the catalytic role of Asp56 in the alpha-polypeptide of higher plant glutamine synthetase is quite different from the role proposed for its highly conserved homologue in bacteria (Asp50 in E. coli), which has been associated with binding and deprotonation of ammonium. On the other hand, we also show other results indicating that Asp56 is important in the spatial conformation of the active site and/or the protein, Asp56 being a crucial residue in the salting-out aggregation properties of the enzyme.

Purification and Properties of Glutamine Synthetase in Leaves and Roots of Pinus banksiana Lamb

A method is described for the purification of glutamine synthetase (GS; EC. 6.3.1.2) from the leaves and roots of Pinus banksiana Lamb., a conifer which utilizes ammonium as its primary nitrogen source. The enzyme was purified to apparent homogeneity by a procedure involving salt fractionation as well as ion-exchange, size exclusion, and affinity chromatography. Since the final preparation produced two bands on SDS polyacryamide gels but only one band on a nondenaturating gel, it is concluded that the two subunits (44 and 40 kilodaltons, respectively) are part of a single enzymatic protein which shows GS activity. The pH optimum for leaf GS ranged between 6.2 and 6.5, one pH unit lower than the values reported for higher plants which utilize primarily nitrate nitrogen. Magnesium requirements for GS in P. banksiana were different for leaves and roots, showing V(max)/2 values of 2.5 and 8 millimolar, respectively at 5 millimolar ATP. Furthermore, K(m) values for ammonium were higher for the enzyme in leaves (33.1 micromolar) than in roots (19.2 micromolar). K(m) values for ATP and for glutamate, on the other hand, were similar for the two tissues. A polyclonal antibody was produced against the purified leaf GS. Western blots of leaf homogenates produced two bands, the lighter one being more abundant. The same pattern was found when immunodetection was performed using an anti GS IgG produced against purified GS from Phaseolus nodules thus indicating common antigenic determinants. At least 30% of total GS was recovered in a plastid-fraction of dark-grown calli produced from the basal part of P. banksiana hypocotyls.

Purification and Characterization of Two Forms of Glutamine Synthetase from the Pedicel Region of Maize (Zea mays L.) Kernels

Maize (Zea mays L.) kernel pedicels, including vascular tissues, pedicel parenchyma, placento-chalazal tissue, and the surrounding pericarp, contained two forms of glutamine synthetase (EC 6.3.1.2), separable by anion exchange chromatography under mildly acidic conditions. The earlier-eluting activity (GS(p1)), but not the later-eluting activity (GS(p2)), was chromatographically distinct from the maize leaf and root glutamine synthetases. The level of GS(p1) activity changed in a developmentally dependent manner while GS(p2) activity was constitutive. GS(p1) and GS(p2) exhibited distinct ratios of transferase to hydroxylamine-dependent synthetase activities (5 and 23, respectively), which did not change with kernel age. Purified pedicel glutamine synthetases had native relative molecular masses of 340,000, while the subunit relative molecular masses differed slightly at 38,900 and 40,500 for GS(p1) and GS(p2), respectively. Both GS forms required free Mg(2+) with apparent K(m)s = 2.0 and 0.19 millimolar for GS(p1) and GS(p2), respectively. GS(p1) had an apparent K(m) for glutamate of 35 millimolar and exhibited substrate inhibition at glutamate concentrations greater than 90 millimolar. In contrast, GS(p2) exhibited simple Michaelis-Menten kinetics for glutamate with a K(m) value of 3.4 millimolar. Both isozymes exhibited positive cooperativity for ammonia, with S(0.5) values of 100 and 45 micromolar, respectively. GS(p1) appears to be a unique, kernel-specific form of plant glutamine synthetase. Possible functions for the pedicel GS isozymes in kernel nitrogen metabolism are discussed.

Photorespiratory ammonia does not inhibit photosynthesis in glutamate synthase mutants of Arabidopsis

Exposure of ferredoxin-dependent glutamate synthase (EC 1.4.7.1) mutants of Arabidopsis thaliana to photorespiratory conditions resulted in the accumulation of NH(4) (+) and the inhibition of photosynthesis. However, upon transfer from 2% O(2), 350 microliters per liter CO(2), to 21% O(2), 350 microliters per liter CO(2), net photosynthesis declined at a slower rate in methionine sulfoximine treated leaf discs relative to controls. The recovery of photosynthesis was also more rapid in MSO-treated leaf discs although ammonia levels were more than threefold higher. Photosynthesis in leaf discs treated with azaserine was inhibited more than controls when transferred to 21% O(2) and recovered less than controls when returned to 2% O(2) although NH(4) (+) levels were not significantly different. The results obtained are consistent with the view that the rapid inhibition of photosynthesis in the glutamate synthase mutants in photorespiratory conditions is not due to the accumulation of NH(4) (+) but rather to the depletion of amino donors for glyoxylate and the consequent effects of glyoxylate on the lack of return of carbon to the chloroplast.

Inactivation of Glutamine Synthetase by Tabtoxinine-beta-lactam : Effects of Substrates and pH

The inactivation of glutamine synthetase by tabtoxinine-beta-lactam, a phytotoxin produced by Pseudomonas syringae pv. tabaci, was shown to be irreversible. The chloroplast and cytosolic forms of the enzyme from pea leaves (Pisum sativum L.) were separated, purified, and found to be kinetically similar with K(m) values for glutamate of 6.7 and 4.3 millimolar and for ATP of 2.0 and 1.3 millimolar, respectively. Both forms were irreversibly inactivated by the toxin at equal rates. Using the chloroplast form, it was found that inactivation by tabtoxinine-beta-lactam required ATP. Glutamate and low levels of ammonia (<2 millimolar) slowed the rate of inactivation, whereas high levels of ammonia (5, 20, and 50 millimolar) accelerated it. The inactivation proceeded at a faster rate as the pH was increased from pH 6.5 to 7.5. The role which cellular compartmentalization could play in the inactivation is discussed.

Alternative Routes for the Synthesis of 5-Aminolevulinic Acid in Maize Leaves : II. Formation from Glutamate

Intact plastids from greening maize (Zea mays L.) leaves converted [(14)C]glutamate and [(14)C]2-ketoglutarate (KG) to [(14)C]5-aminolevulinic acid (ALA). Glutamate appeared to be the immediate precursor of ALA, while KG was first converted to glutamate, as shown by the effect of various inhibitors of amino acid metabolism. Plastids from greening leaves contained markedly higher activity as compared with etioplasts or chloroplasts. The synthesis of ALA by intact plastids was light dependent. The enzyme system resides in the stroma of plastids or may be lightly bound to membranes. The solubilized system showed maximal activity around pH 7.9 and required Mg(2+), ATP, and NADPH although dependence on the latter was not clear-cut. A relatively high level of activity could be extracted from etioplasts. Maximal activity was obtained from plastids of leaves which had been illuminated for 90 minutes, after which activity declined sharply. The enzyme system solubilized from plastids also catalyzed the conversion of putative glutamate 1-semialdehyde to ALA in a reaction which was not dependent on the addition of an amino donor.The system in maize greatly resembled the one which had been reported from barley. It is suggested that this system is the one responsible for the biosynthesis of ALA destined for chlorophyll formation.

Characterization of dicarboxylate stimulation of ammonia, glutamine, and 2-oxoglutarate-dependent o(2) evolution in isolated pea chloroplasts

Intact isolated chloroplasts from pea (Pisum sativum) leaves carried out light-dependent (NH(3), 2-oxoglutarate) and (glutamine, 2-oxoglutarate)-dependent O(2) evolution at rates of 3.3 +/- 0.7 (n = 7) and 6.0 +/- 0.4 (n = 5) micromoles per milligram chlorophyll per hour, respectively. Malate stimulated the rate of (NH(3), 2-oxoglutarate)-dependent O(2) evolution 2.1 +/- 0.5 (n = 7)-fold in the absence of glutamine, and 3.3 +/- 0.4 (n = 11)-fold in the presence of glutamine. Malate also stimulated (glutamine, 2-oxoglutarate)-dependent O(2) evolution in the presence of high concentrations of glutamine. The affinity (K(1/2)) of (NH(3), glutamine, 2-oxoglutarate)-dependent O(2) evolution for 2-oxoglutarate was estimated at 200 to 250 micromolar in the absence of malate and 50 to 80 micromolar when malate (0.5 millimolar) was present. In contrast to malate and various other dicarboxylates, aspartate, glutarate, and glutamate did not stimulate (NH(3), glutamine, 2-oxoglutarate)-dependent O(2) evolution in isolated pea chloroplasts. Using both in vitro assays and reconstituted chloroplast systems, malate was shown to have no effect on the activities of either glutamine synthetase or glutamate synthase.The concentration of malate required for maximal stimulation of O(2) evolution was dependent on the concentration of 2-oxoglutarate present. However, the small extent of the competition between malate and 2-oxoglutarate for uptake was not consistent with that predicted by the current ;single carrier' model proposed for the uptake of dicarboxylates into chloroplasts.

[N]NMR determination of asparagine and glutamine nitrogen utilization for synthesis of storage protein in developing cotyledons of soybean in culture

Solid-state [(15)N]NMR was used to measure the use of the amide and amino nitrogens of glutamine and asparagine for synthesis of storage protein in cotyledons of soybean (Glycine max L. cv. Elf) in culture. No major discrimination in the incorporation of the amide or amino nitrogens of glutamine into protein is apparent, but the same nitrogens of asparagine are used with a degree of specificity. During the first seven days in culture with asparagine as the sole nitrogen source, the amino nitrogen donates approximately twice as much nitrogen to protein as does the amide nitrogen. The use of the amide nitrogen increases with longer periods of culture. The reduced use of the amide nitrogen was confirmed by its early appearance as ammonium in the culture medium. The amide nitrogen of asparagine was found at all times to be an essential precursor for protein because of its appearance in protein in residues whose nitrogens were not supplied by the amino nitrogen. In addition, methionine sulfoximine inhibited growth completely on asparagine, indicating that some ammonium assimilation is essential for storage protein synthesis. These results indicate that in a developing cotyledon, a transaminase reaction is of major importance in the utilization of asparagine for synthesis of storage protein and that, at least in the early stages of cotyledon development, reduced activities of ammonium-assimilating enzymes in the cotyledon tissue or in other tissues of the seed or pod may be a limiting factor in the use of asparagine-amide nitrogen.

Characterization of glutamine synthetase from avian liver mitochondria

1. Glutamine synthetase has been purified to homogeneity from chicken liver mitochondria. 2. The native enzyme is an octamer composed of identical subunits with monomeric mol. wt of 42,000 dalton. 3. Apparent Kms for NH4+, ATP and glutamate were 0.5, 0.9 and 6 mM, respectively. D-Glutamate and L-alpha-hydroxyglutarate were utilized as substrates with activities approx. 40% those obtained with glutamate. Of several nucleotides tested, none were effective replacements for ATP. 4. Heavy metal ions were inhibitory as were Mn2+, Ca2+ and lanthanide ions. 5. Despite its different subcellular localization and physiological function, avian glutamine synthetase is markedly similar to the weakly-bound microsomal rat liver enzyme with respect to a number of physical and chemical properties.

Effects of carbon dioxide and oxygen on the regulation of photosynthetic carbon metabolism by ammonia in spinach mesophyll cells

Photosynthetic carbon metabolism of isolated spinach mesophyll cells was characterized under conditions favoring photorespiratory (PR; 0.04% CO(2) and 20% O(2)) and nonphotorespiratory (NPR; 0.2% CO(2) and 2% O(2)) metabolism, as well as intermediate conditions. Comparisons were made between the metabolic effects of extracellularly supplied NH(4) (+) and intracellular NH(4) (+), produced primarily via PR metabolism. The metabolic effects of (14)CO(2) fixation under PR conditions were similar to perturbations of photosynthetic metabolism brought about by externally supplied NH(4) (+); both increased labeling and intracellular concentrations of glutamine at the expense of glutamate and increased anaplerotic synthesis through alpha-ketoglutarate. The metabolic effects of added NH(4) (+) during NPR fixation were greater than those during PR fixation, presumably due to lower initial NH(4) (+) levels during NPR fixation. During PR fixation, addition of ammonia caused decreased pools and labeling of glutamate and serine and increased glycolate, glyoxylate, and glycine labeling. The glycolate pathway was thus affected by increased rates of carbon flow and decreased glutamate availability for glyoxylate transamination, resulting in increased usage of serine for transamination. Sucrose labeling decreased with NH(4) (+) addition only during PR fixation, suggesting that higher photosynthetic rates under NPR conditions can accommodate the increased drain of carbon toward amino acid synthesis while maintaining sucrose synthesis.

Glutamate dehydrogenase in developing endosperm, chloroplasts, and roots of castor bean

All the glutamate dehydrogenase activity in developing castor bean endosperm is shown to be located in the mitochondria. The enzyme can not be detected in the plastids, and this is probably not due to the inactivation of an unstable enzyme, since a stable enzyme can be isolated from castor bean leaf chloroplasts. The endosperm mitochondrial glutamate dehydrogenase consists of a series of differently charged forms which stain on polyacrylamide gel electrophoresis with both NAD(+) and NADP(+). The chloroplast and root enzymes differ from the endosperm enzyme on polyacrylamide gel electrophoresis. The amination reaction of all the enzymes is affected by high salt concentrations. For the endosperm enzyme, the ratio of activity with NADH to that with NADPH is 6.3 at 250 millimolar NH(4)Cl and 1.5 at 12.5 millimolar NH(4)Cl. K(m) values for NH(4) (+) and NAD(P)H are reduced at low salt concentrations. The low K(m) values for the nucleotides may favor a role for glutamate dehydrogenase in ammonia assimilation in some situations.

Regulation of Glutamine Synthetase by Light and during Nitrogen Deficiency in Synchronous Chlorella sorokiniana

A method is described to achieve density labeling of proteins in unicellular algae by using (13)CO(2). This is a satisfactory procedure especially for work on nitrogen metabolism. The increase in activity of glutamine synthetase (EC 6.3.1.2.) and glutamate synthase (EC 1.4.7.1.) in Chlorella sorokiniana mediated by a dark/light shift and by nitrogen starvation were investigated. Using the method of density labeling and isopycnic centrifugation, we demonstrated that the increase in enzyme activity after a dark/light shift is based on activation rather than de novo synthesis. The increase in enzyme activity after transfer to nitrogen-deficient medium is based both on activation and de novo synthesis.

Glutamine Synthetase in Rice: A COMPARATIVE STUDY OF THE ENZYMES FROM ROOTS AND LEAVES

Chromatographic, kinetic, and regulatory properties of glutamine synthetase in rice were investigated. By DEAE-Sephacel column chromatography, two forms (glutamine synthetase 1 and glutamine synthetase 2) were identified in leaves and one form (glutamine synthetase R) was identified in roots. Purification on hydroxyapatite and gel electrophoresis showed that glutamine synthetase R was distinct from the leaf enzymes. The three isoforms were purified to similar specific activities and their properties were studied. Heat lability, pH optimum about 8, K(m) for l-glutamate of 20 millimolar, and inhibition by glucosamine 6-phosphate were the main characteristics of glutamine synthetase 2. Heat stability, pH optimum about 7.5, K(m) for l-glutamate of 2 millimolar, and no effect of glucosamine 6-phosphate differentiated glutamine synthetase 1 from glutamine synthetase 2. Glutamine synthetase R was also a labile protein but its kinetic and regulatory properties were quite similar to those of glutamine synthetase 1. These results clearly demonstrate the existence of three isoforms of glutamine synthetase in rice, two of which are located in the leaves and the third in the roots.

On the Gaseous Exchange of Ammonia between Leaves and the Environment: Determination of the Ammonia Compensation Point

Whole shoots of Phaseolus vulgaris L. and other species were exposed to a range of partial pressures of gaseous ammonia in air and the resulting fluxes were measured. Net uptake is linear with partial pressure in the range 5 to 50 nanobars and is zero at a finite partial pressure, termed the ammonia compensation point. Below the compensation point, ammonia (or possibly other volatile amines) is evolved by the leaves. The compensation points in several species are near the low partial pressures found in unpolluted air and approximate to the K(m) of glutamine synthetase in vitro. In P. vulgaris L., the compensation point increases with temperature.

Role of mitochondrial glutamate dehydrogenase in the reassimilation of ammonia produced by glycine serine transformation

The ability of isolated pea-shoot mitochondria conditioned to incorporate ammonia into glutamate to reassimilate endogenously produced ammonia from glycine transformation was investigated. In the presence of 1 mM to 20 mM glycine less than 15% of the ammonia liberated was found to be incorporated into glutamate. Thus, a prominent role of mitochondrial glutamate dehydrogenase in the reassimilation of intramitochondrially produced ammonia can be excluded.

Influence of the axis on the enzymes of protein and amide metabolism in the cotyledons of mung bean seedlings

The growth of the mung bean (Vigna radiata) seedling is accompanied by the biosynthesis and accumulation of the endopeptidase vicilin peptidohydrolase and the catabolism of the reserve proteins in the cotyledons. If the axis is removed from the dry seeds and the cotyledons incubated on moist sand the accumulation of vicilin peptidohydrolase is reduced by 77% and the catabolism of reserve proteins slowed to 25% of the rate in intact seedlings. The cotyledons and the cotyledon exudate are rich in asparagine and this amino acid accounts for more than half of the reduced nitrogen exported from the cotyledons. Glutamine synthetase and asparagine synthetase, two key enzymes in the pathway of asparagine synthesis, are under temporal control in the cotyledons. Their activities increase 3.5- and 10-fold, respectively, then decline again. These increases in enzyme activity occur to the same extent in excised cotyledons and are prevented when the cotyledons are incubated in 5 micromolar cycloheximide. The results indicate that the axis may control certain key metabolic events in the cotyledons, such as the synthesis of vicilin peptidohydrolase, while many other anabolic activities may not depend on a growing axis.

Initial organic products of assimilation of [N]ammonium and [N]nitrate by tobacco cells cultured on different sources of nitrogen

Glutamine is the first major organic product of assimilation of (13)NH(4) (+) by tobacco (Nicotiana tabacum L. cv. Xanthi) cells cultured on nitrate, urea, or ammonium succinate as the sole source of nitrogen, and of (13)NO(3) (-) by tobacco cells cultured on nitrate. The percentage of organic (13)N in glutamate, and subsequently, alanine, increases with increasing periods of assimilation. (13)NO(3) (-), used for the first time in a study of assimilation of nitrogen, was purified by new preparative techniques. During pulse-chase experiments, there is a decrease in the percentage of (13)N in glutamine, and a concomitant increase in the percentage of (13)N in glutamate and alanine. Methionine sulfoximine inhibits the incorporation of (13)N from (13)NH(4) (+) into glutamine more extensively than it inhibits the incorporation of (13)N into glutamate, with cells grown on any of the three sources of nitrogen. Azaserine inhibits glutamate synthesis extensively when (13)NH(4) (+) is fed to cells cultured on nitrate. These results indicate that the major route for assimilation of (13)NH(4) (+) is the glutamine synthetase-glutamate synthase pathway, and that glutamate dehydrogenase also plays a role, but a minor one. Methionine sulfoximine inhibits the incorporation of (13)N from (13)NO(3) (-) into glutamate more strongly than it inhibits the incorporation of (13)N into glutamine, suggesting that the assimilation of (13)NH(4) (+) derived from (13)NO(3) (-) may be mediated solely by the glutamine synthetase-glutamate synthase pathway.

Enzymology of Glutamine Metabolism Related to Senescence and Seed Development in the Pea (Pisum sativum L.)

The metabolism of glutamine in the leaf and subtended fruit of the aging pea (Pisum sativum L. cv. Burpeeana) has been studied in relation to changes in the protein, chlorophyll, and free amino acid content of each organ during ontogenesis. Glutamine synthetase [EC 6.3.1.2] activity was measured during development and senescence in each organ. Glutamate synthetase [EC 2.6.1.53] activity was followed in the pod and cotyledon during development and maturation. Maximal glutamine synthetase activity and free amino acid accumulation occurred together in the young leaf. Glutamine synthetase (in vitro) in leaf extracts greatly exceeded the requirement (in vivo) for reduced N in the organ. Glutamine synthetase activity, although declining in the senescing leaf, was sufficient (in vitro) to produce glutamine from all of the N released during protein hydrolysis (in vivo). Maximal glutamine synthetase activity in the pod was recorded 6 days after the peak accumulation of the free amino acids in this organ.In the young pod, free amino acids accumulated as glutamate synthetase activity increased. Maximal pod glutamate synthetase activity occurred simultaneously with maximal leaf glutamine synthetase activity, but 6 days prior to the corresponding maximum of glutamine synthetase in the pod. Cotyledonary glutamate synthetase activity increased during the assimilatory phase of embryo growth which coincided with the loss of protein and free amino acids from the leaf and pod; maximal activity was recorded simultaneously with maximal pod glutamine synthetase.We suggest that the activity of glutamine synthetase in the supply organs (leaf, pod) furnishes the translocated amide necessary for the N nutrition of the cotyledon. The subsequent activity of glutamate synthetase could provide a mechanism for the transfer of imported amide N to alpha amino N subsequently used in protein synthesis. In vitro measurements of enzyme activity indicate there was sufficient catalytic potential in vivo to accomplish these proposed roles.

Metabolism of Ammonium Ion and Glutamate in Relation to Nitrogen Supply and Utilization during Grain Development in Barley

Changes in the activity of a number of enzymes concerned with amino acid synthesis and metabolism were recorded for the endosperm, testa pericarp, and embryo of developing barley (Hordeum distichum L.) grains. Both glutamate-pyruvate transaminase and glutamate-oxaloacetate transaminase activities were present in all tissues and at all ages examined. Glutamate dehydrogenase activity was largely confined to endosperm while glutamine synthetase activity was mainly in the testa pericarp.Ammonium ion concentration was maximal in endosperm by 20 days after anthesis. Glutamate concentration varied in endosperm and was in the range of 3.5 to 8.5 mm between 20 and 45 days after anthesis. Significant levels of ammonium ion and glutamate were also present in the testa pericarp over the major part of the developmental period.

Effects of ammonia on carbon metabolism in photosynthesizing isolated mesophyll cells from Papaver somniferum L

Addition of ammonia to a suspension of photosynthesizing isolated mesophyll cells from P. somniferum quantitatively alters the pattern of carbon metabolism by increasing rates of certain key ratelimiting steps leading to amino-acid synthesis and by decreasing rates of rate-limiting steps in alternative biosynthetic pathways. Of particular importance is the stimulation of reactions mediated by pyruvate kinase and phosphoenolpyruvate carboxylase. The increased rates of these two reactions, which result in an increased flow of carbon into the tricarboxylic-acid cycle, correlate with a rapid rise in glutamine (via glutamine synthetase) which draws carbon off the tricarboxylic-acid cycle as α-ketoglutarate. Increased flux of carbon in this direction appears to come mainly at the expense of sucrose synthesis. The net effect of addition of ammonia to mesophyll cells is thus a redistribution of newly fixed carbon away from carbohydrates and into amino acids.

Polarographic study of ammonia assimilation by isolated chloroplasts

Illuminated pea (Pisum sativum) chloroplasts catalyze (ammonia plus alpha-ketoglutarate [alpha-KG])-dependent O(2) evolution at rates which are commensurate with other estimates of the flux of assimilated nitrogen (mean of eight determinations, 8.3 mumole per mg chlorophyll per hour, sd 2.4). The reaction was usually initiated with 1 mm ammonia after preincubating chloroplasts in the presence of alpha-KG, ADP, pyrophosphate, and MgCl(2).Progressive increases in ammonia concentration gave V(max)/2 at 0.2 mm (approximately) and V(max) at about 1 mm. Higher concentrations were inhibitory; at 7 mm the rate was again about V(max)/2. The highest ratio of O(2) evolved per mol of ammonia supplied was 0.36.The (ammonia plus alpha-KG)-dependent reaction was inhibited by methionine sulfoximine, azaserine, and aspartate in the presence of amino-oxyacetate but not by amino-oxyacetate alone and not by l-glutamate. The rate of O(2) evolution in the presence of 1 mm ammonia and 2.5 mm alpha-KG was increased only slightly by addition of 5 mm glutamine. Similarly, the rate of O(2) evolution in the presence of 5 mm glutamine and 2.5 mm alpha-KG was increased only slightly by addition of 1 mm ammonia.The results are attributed to the incorporation of ammonia via glutamine synthetase and reductive transamination of the glutamine formed by photosynthetically coupled glutamate synthase using alpha-KG as the amino acceptor. Several lines of evidence rule out the possibility that photosynthetically coupled glutamate dehydrogenase is involved.

Amino Acid metabolism of pea leaves: diurnal changes and amino Acid synthesis from N-nitrate

In the young leaves of pea (Pisum sativum L.) plants, there was a diurnal variation in the levels of amino acids. In the light, total amino nitrogen increased for the first few hours, then stabilized; in the dark, there was a transient decrease followed by a gradual recovery. Asparagine, homoserine, alanine, and glutamine accounted for much of these changes. The incorporation of (15)N into various components of the young leaves was followed after supply of (15)N-nitrate. (15)N appeared most rapidly in ammonia, due to reduction in the leaf, and this process took place predominantly in the light. A large proportion of the primary assimilation took place through the amide group of glutamine, which became labeled and turned over rapidly; labeling of glutamic acid and alanine was also rapid. Asparagine (amide group) soon became labeled and showed considerable turnover. Slower incorporation and turnover were found for aspartic acid, gamma-aminobutyric acid, and homoserine. Synthesis and turnover of all of the amino acids continued at a low rate in the dark. gamma-Aminobutyric acid was the only compound found to label more rapidly in the dark than in the light.

An NADP-dependent Glutamate Dehydrogenase in Chloroplasts from the Marine Green Alga Caulerpa simpliciuscula

NADP-dependent glutamate dehydrogenase was partially purified from extracts of the marine siphonous green alga Caulerpa simpliciuscula. The enzyme had an apparent Km NH(4) (+) of 0.4 to 0.7 mm and was highly specific for NADPH, alpha-ketoglutarate, and ammonium ions.The bulk of the NADP-glutamate dehydrogenase was isolated with the chloroplast fraction in cell-free preparations of this alga and was released from these "chloroplast fractions" as a soluble enzyme on gentle lysis of chloroplast membranes.

Distribution of Nitrate-assimilating Enzymes between Mesophyll Protoplasts and Bundle Sheath Cells in Leaves of Three Groups of C(4) Plants

Intercellular distribution of enzymes involved in amino nitrogen synthesis was studied in leaves of species representing three C(4) groups, i.e. Sorghum bicolor, Zea mays, Digitaria sanguinalis (NADP malic enzyme type); Panicum miliaceum (NAD malic enzyme type); and Panicum maximum (phosphoenolpyruvate carboxykinase type). Nitrate reductase, nitrite reductase, glutamine synthetase, and glutamate synthase were predominantly localized in mesophyll cells of all the species, except in P. maximum where nitrite reductase had similar activity on a chlorophyll basis, in both mesophyll and bundle sheath cells. NADH-glutamate dehydrogenase was concentrated in the bundle sheath cells, while NADPH-glutamate dehydrogenase was localized in both mesophyll and bundle sheath cells. The activities of nitrate-assimilating enzymes, except for nitrate reductase, were high enough to account for the proposed in vivo rates of nitrate assimilation.Based on the differential centrifugation of cell homogenates of P. miliaceum, mesophyll chloroplasts appear to be the major site of nitrate assimilation since nitrite reductase, glutamine synthetase, glutamate synthase, and NADPH-glutamate dehydrogenase were primarily localized in the chloroplast fraction. Both the glutamine synthetase-glutamate synthase and glutamate dehydrogenase pathways were considered as alternative routes of amino nitrogen synthesis.