Nitrogen assimilation in Rhodopseudomonas acidophila

Rhodopseudomonas acidophila strain 7050 assimilated ammonia via a constitutive glutamine synthetase/glutamate synthase enzyme system.Glutamine synthetase had a K _m for NH ₄ ⁺ of 0.38 mM whilst the nicotinamide adenine dinucleotide linked glutamate synthase had a K _m for glutamine of 0.55 mM. R. acidophila utilized only a limited range of amino acids as sole nitrogen sources: l-alanine, glutamine and asparagine. The bacterium did not grow on glutamate as sole nitrogen source and lacked glutamate dehydrogenase. When R. acidophila was grown on l-alanine as the sole nitrogen source in the absence of N₂ low levels of a nicotinamide adenine dinucleotide linked l-alanine dehydrogenase were produced. It is concluded, therefore, that this reaction was not a significant route of ammonia assimilation in this bacterium except when glutamine synthetase was inhibited by methionine sulphoximine. In l-alanine grown cells the presence of an active alanine-glyoxylate aminotransferase and, on occasions, low levels of an alanine-oxaloacetate aminotransferase were detected. Alanine-2-oxo-glutarate aminotransferase could not be demonstrated in this bacterium.Abreviations ADH alanine dehydrogenase - GDH glutamate dehydrogenase - GS glutamine synthetase - GOGAT glutamate synthase - MSO methionine sulphoximine     

Fixation of [13N]N2 and transfer of fixed nitrogen in the Anthoceros-Nostoc symbiotic association

The initial product of fixation of [¹³N]N₂ by pure cultures of the reconstituted symbiotic association between Anthoceros punctatus L. and Nostoc sp. strain ac 7801 was ammonium; it accounted for 75% of the total radioactivity recovered in methanolic extracts after 0.5 min and 14% after 10 min of incubation. Glutamine and glutamate were the primary organic products synthesized from [¹³N]N₂ after incubation times of 0.5–10 min. The kinetics of labeling of these two amino acids were characteristic of a precursor (glutamine) and product (glutamate) relationship. Results of inhibition experiments with methionine sulfoximine (MSX) and diazo-oxonorleucine were also consistent with the assimilation of N₂-derived NH ₄ ⁺ by Anthoceros-Nostoc through the sequential activities of glutamine synthetase (EC 6.3.1.2) and glutamate synthase (EC 1.4.7.1), with little or no assimilation by glutamate dehydrogenase (EC 1.3.1.3). Isolated symbiotic Nostoc assimilated exogenous ¹³NH ₄ ⁺ into glutamine and glutamate and their formation was inhibited by MSX, indicating operation of the glutamine synthetase-glutamate synthase (GS-GOGAT) pathway: However, relative to free-living cultures, isolated symbiotic Nostoc assimilated 80% less exogenous ammonium into glutamine and glutamate, implying that symbiotic Nostoc could assimilate only a fraction of N₂-derived NH ₄ ⁺ . This implication was tested by using Anthoceros associations reconstituted with wild-type or MSX-resistant strains of Nostoc incubated with [¹³N]N₂ in the presence of MSX. The results of these experiments indicated that, in situ, symbiotic Nostoc assimilated about 10% of the N₂-derived NH ₄ ⁺ and that NH ₄ ⁺ was made available to Anthoceros tissue where it was apparently assimilated by the GS-GOGAT pathway. Since less than 1% of the fixed N₂ was lost to the suspension medium, it appears that transfer of NH ₄ ⁺ from symbiont to host tissue was very efficient in this extracellular symbiotic association.Abbreviations DON 6-diazo-5-oxo-l-norleucine - GDH glutamate dehydrogenase - GOGAT glutamate synthase - GS glutamine synthetase - MSX l-methionine-dl-sulfoximine     

Assimilation of exogenous and dinitrogen-derived13NH 4 + byAnabaena azollae separated fromAzolla caroliniana Willd

Anabaena azollae was isolated fromAzolla caroliniana by the gentle roller method and differential centrifugation. Incubation of suchAnabaena preparations for 10 min with [¹³N]N₂ resulted in the formation of four radioactive compounds; ammonium, glutamine, glutamate and alanine. Ammonium accounted for 66% of the total radioactivity recovered and 58% of the ammonium was in an extracellular fraction. Since essentially no extracellular¹³N-labeled organic compounds were found, it appears that ammonium is the compound most probably made available toAzolla during dinitrogen-dependent growth of the association.The kinetics of incorporation of exogenous¹³NH ₄ ⁺ into glutamine and glutamate were characteristic of a precursor (glutamine)-product (glutamate) relationship and consistent with assimilation by the glutamine synthetase-glutamate synthase pathway. The results of experiments using the glutamine synthetase inhibitor, methionine sulfoximine, the glutamate synthase inhibitor, diazo-oxonorleucine, and increasing the ammonium concentration to greater than 1 mM, provided evidence for assimilation primarily by the glutamine synthetase-glutamate synthase pathway with little or no contribution from biosynthetic glutamate dehydrogenase.While showing that N₂ fixation and NH ₄ ⁺ assimilation were not tightly coupled metabolic processes in symbioticAnabaena, these results reflect a composite picture and do not indicate the extent to which ammonium assimilatory enzymes might be regulated in filaments associated with specific stages in theAzolla-Anabaena developmental profile.Non-standard abbreviations DON 6-Diazo-5-oxo-l-norleucine - GDH glutamate dehydrogenase - GOGAT glutamate synthase - GS glutamine synthetase - MSX l-methionine-Dl-sulfoximine     

N2 fixation and NH 4 + assimilation in the thermophilic anaerobes Clostridium thermosaccharolyticum and Clostridium thermoautotrophicum

Inorganic nitrogen metabolism in the obligate anaerobic thermophiles Chlostridium thermosaccharolyticum and Clostridium thermoautotrophicum differs in several respects. C. thermosaccharolyticum contains a nitrogenase as inferred from NH ₄ ⁺ repressible C₂H₂ reduction, a glutamine synthetase which is partially repressed by ammonium, very labile glutamate synthase activities with both NADH and NADPH, NADPH-dependent glutamate dehydrogenase, and NH ₄ ⁺ -dependent asparagine synthetase. C. thermoautotrophicum contains no nitrogenase, but glutamine synthetase, no glutamate synthase, no glutamate dehydrogenase, but a NADH-dependent alanine dehydrogenase and a NH ₄ ⁺ -dependent asparagine synthetase.Abbreviation GOGAT glutamine-oxoglutarate amidotransferase amidotransferase (glutamate synthase)     

Enzymes of ammonia assimilation and their regulation inAzospirillum lipoferum strain D-2

Azospirillum lipoferum strain D-2 possesses the following enzymes for the assimilation of N₂ and NH ₄ ⁺ : nitrogenase, glutamine synthetase, NADPH-dependent glutamate synthase, NADH-/NADPH-dependent glutamate dehydrogenase, and NADH-dependent alanine dehydrogenase. Nitrogenase and glutamine synthetase are repressed, whereas glutamate dehydrogenase and alanine dehydrogenase are induced by NH ₄ ⁺ . Glutamine synthetase activity is modulated by both repression and depression and also by adenylylation.     

Ammonium assimilation in an Arthrobacter sp. “fluorescens”

Ammonium assimilation was studied in a nitrogenfixing Arthrobacter strain grown in both batch and ammonium-limited continuous cultures. Arthrobacter sp. fluorescens grown in nitrogen-free medium or at low ammonium levels assimilated NH ₄ ⁺ via the glutamine synthetase/glutamate synthase pathway. When ammonium was in excess it was assimilated via the alanine dehydrogenase pathway. Very low levels of glutamate dehydrogenase were found, irrespective of growth conditions.Abbreviations GS glutamine synthetase - GOGAT glutamine oxoglutarate aminotransferase - GDH glutamate dehydrogenase - ADH alanine dehydrogenase - GOT glutamate oxaloacetate transaminase - GPT glutamate pyruvate transaminase     

Isolation of auxotrophic mutants in support of ammonia assimilation via glutamine synthetase in Methanobacterium ivanovii

Various enzymes involved in the initial metabolic pathway for ammonia assimilation by Methanobacterium ivanovii were examined. M. ivanovii showed significant activity of glutamine synthetase (GS). Glutamate synthase (GOGAT) and alanine dehydrogenase (ADH) were present, wheras, glutamate dehydrogenase (GDH) was not detected. When M. ivanovii was grown with different levels of NH ₊ ⁴ (i.e. 2, 20 or 200 mM), GS, GOGAT and ADH activities varied in response to NH ₊ ⁴ concentration. ADH was not detected at 2 mM level, but its activity increased with increased levels of NH ₊ ⁴ in the medium. Both GS and GOGAT activities increased with decreasing concentrations of NH ₊ ⁴ and were maximum when ammonia was limiting, suggesting that at low NH ₊ ⁴ levels, GS and GOGAT are responsible for ammonia assimilation and at higher NH ₊ ⁴ levels, ADH might play a role. Metabolic mutants of M. ivanovii that were auxotrophic for glutamine were obtained and analyzed for GS activity. Results indicate two categories of mutants: i) GS-deficient auxotrophic mutants and ii) GS-impaired auxotrophic mutants.Abbreviations GS Glutamine synthetase - GOGAT glutamate synthase - GDH glutamate dehydrogenase - ADH alanine dehydrogenase     

15N2 Incorporation and metabolism in the lichen Peltigera aphthosa Willd

The Nostoc in the cephalodia of the lichen Peltigera aphthosa Willd. fixed ¹⁵N₂ and the bulk of the nitrogen fixed was continuously transferred from it to its eukaryotic partners (a fungus and a green alga, Coccomyxa sp.). Kinetic studies carried out over the first 30 min, after exposure of isolated cephalodia to ¹⁵N₂, showed that highest initial ¹⁵N₂-labelling was into NH ₄ ⁺ . After 12 min little further increase in the NH ₄ ⁺ label occurred while that in the amide group of glutamine and in glutamate continued to increase. The ¹⁵N-labelling of the amino group of glutamine and of aspartate increased more slowly, followed by an increase in the labelling of alanine. When total incorporation of ¹⁵N-label was calculated, the overall pattern was found to be rather similar except that, throughout the experiment, the total ¹⁵N incorporated into glutamate was about six times greater than that into the amide group of glutamine. Pulse chase experiments, in which ¹⁴N₂ was added to cephalodia previously exposed to ¹⁵N₂, showed that the NH ₄ ⁺ pool rapidly became depleted of ¹⁵N-label, followed by decreases in the labelling of glutamate, the amide group of glutamine and aspartate. The ¹⁵N-labelling of alanine, however, continued to increase for a period. When isolated cephalodia were treated with L-methionine-SR-sulphoximine, an inhibitor of glutamine synthetase (EC 6.3.1.2), and azaserine, an inhibitor of glutamate synthase (EC 2.6.1.53), there was no detectable labelling in glutamine although the ¹⁵N-labelling of glutamate increased unimpaired. On treating the cephalodia with amino-oxyacetate, an inhibitor of aminotransferase activity, the alanine pool decreased. Evidence was obtained that glutamine synthetase and glutamate synthase were located in the Nostoc, and that glutamate dehydrogenase (EC 1.4.1.4) and various amino-transferases were located in the cephalodial fungus. Possible implications of these findings are discussed.Abbreviations MSX L-methionine-SR-sulphoximine - AOA amino-oxyacetate - HEPES N-2-hydroxymethylpiperazine-N-2-ethane sulphonic acid - Tris tris-(hydroxymethyl) methylamine - GS glutamine synthetase - GOGAT glutamate synthase - GDH glutamate dehydrogenase - GPT glutamate-pyruvate aminotransferase - APT aspartate-pyruvate aminotransferase - ADH alanine dehydrogenase - GOT glutamate-oxaloacetate aminotransferase     

Utilization of l-alanine by Rhodopseudomonas acidophila

Rhodopseudomonas acidophila strain 7050 can satisfy all its nitrogen and carbon requirements from l-alanine. Addition of 100 M methionine sulfoximine to alanine grown cultures had no effect on growth rate indicating that deamination of alanine via alanine dehydrogenase and re-assimilation of the released NH ₄ ⁺ by glutamine synthetase/glutamate synthase was an insignificant route of nitrogen transfer in this bacterium. Determination of aminotransferase activities in cell-free extracts failed to demonstrate the presence of direct routes from alanine to either aspartate or glutamate. The only active aminotransferase involving l-alanine was the alanine-glyoxylate enzyme (114–167 nmol·min^–1·mg^–1 protein) which produced glycine as end-product. The amino group of glycine was further transaminated to yield aspartate via a glycineoxaloacetate aminotransferase (117–136 nmol·min^–1 ·mg^–1 protein). No activity was observed when 2-oxoglutarate was substituted for oxaloacetate. The formation of glutamate from aspartate was catalysed by aspartate-2-oxoglutarate aminotransferase (85–107 nmol·min^–1·mg^–1 protein). Determinations of free intracellular amino acid pools in alanine and alanine+100 M methionine sulfoximine grown cells showed the predominance of glutamate, glycine and aspartate, providing further evidence that in alanine grown cultures R. acidophila satisfies its nitrogen requirements for balanced growth by transamination.Abbreviations ADH alanine dehydrogenase - GDH glutamate dehydrogenase - GS glutamine synthetase - GOGAT glutamate synthase - MSO methionine sulfoximine - GOT glutamate-oxaloacetate aminotransferase - GPT glutamate-pyruvate amino-transferase - AGAT alanine-glyoxylate aminotransferase - GOAT glycine-oxaloacetate aminotransferase - GOTAT glycine-2-oxoglutarate aminotransferase - AOAT alanine-oxaloacetate aminotransferase     

Photoproduction of ammonium ion from N2 inRhodospirillum rubrum

NH ₄ ⁺ excretion was undetectable in N₂-fixing cultures ofRhodospirillum rubrum (S-1) and nitrogenase activity in these cultures was repressed by the addition of 10 mM NH ₄ ⁺ to the medium. The glutamate analog,l-methionine-dl-sulfoximine (MSX), derepressed N₂ fixation even in the presence of 10 mM extracellular NH ₄ ⁺ . When 10 mg MSX/ml was added to cultures just prior to nitrogenase induction they developed nitrogenase activity (20% of the control activities) and excreted most of their fixed N₂ as NH ₄ ⁺ . Nitrogenase activities and NH ₄ ⁺ production from fixed N₂ were increased considerably when a combined nitrogen source, NH ₄ ⁺ (>40 moles NH ₄ ⁺ /mg cell protein in 6 days) orl-glutamate (>60 moles NH ₄ ⁺ /mg cell protein in 6 days) was added to the cultures together with MSX.Biochemical analysis revealed thatR. rubrum produced glutamine synthetase and glutamate synthase (NADP-dependent) but no detectable NADP-dependent glutamate dehydrogenase. The specific activity of glutamine synthetase was observed to be maximal when nitrogenase activity was also maximal. Nitrogenase and glutamine synthetase activities were repressed by NH ₄ ⁺ as well as by glutamate.The results demonstrate that utilization of solar energy to photoproduce large quantities of NH ₄ ⁺ from N₂ is possible with photosynthetic bacteria by interfering with their regulatory control of N₂ fixation.     

Patterns of [13N]ammonium uptake and assimilation by Frankia HFPArl3

Nitrogen-starved cells of Frankia strain HFPArl3 incorporated [¹³N]-labeled ammonium into glutamine serine (glutamate, alanine, aspartate), after five-minute radioisotope exposures. High initial endogenous pools of glutamate were reduced, while total glutamine increased, during short term NH _inf4 ^sup+ incubation. Preincubation of cells in methionine sulfoximine (MSX) resulted in [¹³N]glutamine reduced by more than 80%, while [¹³N]glutamate and [¹³N]alanine levels increased. The results suggest that glutamine synthetase is the primary enzyme of ammonium assimilation, and that glutamate dehydrogenase and alanine dehydrogenase may also function in ammonium assimilation at low levels. Efflux of [¹³N]serine and lesser amounts of [¹³N]glutamine was detected from the Frankia cells. The identity of both Ser and Gln in the extracellular compartment was confirmed with gas chromatography/mass spectrometry. Serine efflux may be of significance in nitrogen transfer in Frankia.Abbreviations Pthr phosphothreonine - Aad -amino-adipate - MSX methionine sulfoximine     

Inorganic nitrogen metabolism in two cellulose-degrading clostridia

Inorganic nitrogen metabolism in two cellulose degrading clostridia, the mesophile Clostridium cellobioparum and the thermophile Clostridium thermocellum was investigated. Both strains show acetylene reduction (i.e. possibly nitrogenase activity), contain glutamine synthetase, glutamate dehydrogenase and glutamate-dependent transaminases. C. cellobioparum additionally contains a NADH-dependent glutamate synthase and a NH ₄ ⁺ -repressible glycine dehydrogenase (NADPH). Remarkably, acetylene reduction in C. thermocellum is not repressed by ammonium, casting doubt whether this activity is due to nitrogenase. The results are compared with the data from other saccharolytic clostridia.Abbreviation GOGAT glutamine-oxoglutarate amidotransferase (glutamate synthase)     

Nitrogen metabolism of Frankia strain,Cc01, Mg+, At4 and Hr18

The composition and levels of amino acids in four Frankia strains isolated from different actinorhizal plants, were determined. Minor differences in the amino acid profiles were noted with GLN (GLU) being the major amino acid in all four strains. Enzyme actives of ammonia metabolism, GS (glutamine synthetase), GOGAT (glutamate synthetase), and GDH (glutamate dehydrogenase), were also measured. In strains At4 and Hr18, GS and GOGAT activity levels were elevated in N₂-grown cells but significant amounts of GDH activity were present in ammonia-grown cells. No GDH was detected in strain Cc01 and Mg⁺. The characters of heat-stable and heat-labile GSs were described. In N₂-fixing cells, the ATP and amino acid content was much lower, but ammonia content was higher than in NH _inf4 ^sup+ -grown cells.     

Interactions between cyanobacterium and fungus during 15N2-incorporation and metabolism in the lichen Peltigera canina

On following N₂-incorporation and subsequent metabolism in the lichen Peltigera canina using ¹⁵N as tracer, it was found, over a 30 min period, that greatest initial labelling was into NH ₄ ⁺ followed by glutamate and the amide-N of glutamine. Labelling of the amino-N of glutamine, aspartate and alanine increased slowly. Pulse-chase experiments using ¹⁵N confirmed this pattern. On inhibiting the GS-GOGAT pathway using l-methionine-dl-sulphoximine and azaserine, ¹⁵N enrichment of glutamate, alanine and aspartate continued although labelling of glutamine was undetectable. From this and enzymic data, NH ₄ ⁺ assimilation in the P. canina thallus appears to proceed via GS-GOGAT in the cyanobacterium and via GDH in the fungus; aminotransferases were present in both partners. The cyanobacterium assimilated 44% of the ¹⁵N₂ fixed; the remainder was liberated almost exclusively as NH ₄ ⁺ and then assimilated by fungal GDH.Abbreviations ADH alanine dehydrogenase - APT aspartate-pyruvate aminotransferase - AOA aminooxyacetate - GDH glutamate dehydrogenase - GOT glutamate-oxaloacetate aminotransferase - GOGAT glutamate synthase - GPT glutamate-pyruvate aminotransferase - GS glutamine synthetase - HEPES 4-(2-hydroxyethyl)-1-piperazine ethanesulphonic acid - MSX l-methionine-dl-sulphoximine     

Aspects of nitrogen fixation in Chlorobium

Four strains of the green sulfur bacterium Chlorobium were studied in respect to nitrogen nutrition and nitrogen fixation. All strains grew on ammonia, N₂, or glutamine as sole nitrogen sources; certain strains also grew on other amino acids. Acetylene-reducing activity was detectable in all strains grown on N₂ or on amino acids (except for glutamine). In N₂ grown Chlorobium thiosulfatophilum strain 8327 1 mM ammonia served to switch-off nitrogenase activity, but the effect of ammonia was much less dramatic in glutamate or limiting ammonia grown cells. The glutamine synthetase inhibitor methionine sulfoximine inhibited ammonia switch-off in all but one strain. Cell extracts of glutamate grown strain 8327 reduced acetylene and required Mg²⁺ and dithionite, but not Mn²⁺, for activity. Partially purified preparations of Rhodospirillum rubrum nitrogenase reductase (iron protein) activating enzyme slightly stimulated acetylene reduction in extracts of strain 8327, but no evidence for an indigenous Chlorobium activating enzyme was obtained. The results suggest that certain Chlorobium strains are fairly versatile in their nitrogen nutrition and that at least in vivo, nitrogenase activity in green bacteria is controlled by ammonia in a fashion similar to that described in nonsulfur purple bacteria and in Chromatium.Non-common abbreviations MSX Methionine sulfoximine - MOPS 3-(N-morpholino) propane sulfonic acid This paper is dedicated to Professor Norbert Pfennig on the occasion of his 60th birthday     

A novel class of ammonium assimilation mutants of the photosynthetic bacterium Rhodobacter capsulatus

Nitrogen assimilation in Rhodobacter capsulatus has been shown to proceed via the coupled action of glutamine synthetase (GS) and glutamate synthase (GOGAT) with no measurable glutamate dehydrogenase (GDH) present. We have recently isolated a novel class of mutants of R. capsulatus strain B100 that lacks a detectable GOGAT activity but is able to grow at wild type rates under nitrogen-fixing conditions. While NH ₄ ⁺ -supported growth in the mutants was normal under anaerobic/photosynthetic conditions, the growth rate was decreased under aerobic conditions. Ammonium and methylammonium uptake experiments indicated that there was a clear difference in the ammonium assimilatory capabilities in these mutants under aerobic versus anaerobic growth. Regulation of expression of a nifH : : lacZ fusion in these mutants was not impaired. The possible existence of alternative ammonium assimilatory pathways is discussed.     

Utilization of nitrogen compounds and ammonia assimilation by chromatiaceae

Chromatium vinosum strain D, Thiocapsa roseopersicina strain 6311 and Ectothiorhodospira mobilis strain 8112 were grown anaerobically in the light with various single nitrogen sources. When substituted for NH₄Cl only glutamine and casamino acids supported good growth of all strains tested. Peptone and urea were utilized by C. vinosum and T. roseopersicina, glutamate, asparagine and nitrate only by C. vinosum. The strains were able to grow with molecular nitrogen; complete inhibition of this growth was observed in the presence of alanine with E. mobilis, and of alanine or asparagine with T. roseopersicina.Glutamate dehydrogenase, requiring either NADH or NADPH, NADH-linked glutamate synthase, and glutamine synthetase were demonstrate in the above organisms grown on NH₄Cl.     

Ammonia assimilation and glutamate incorporation in coenzyme F420 derivatives ofMethanosarcina barkeri

Methanosarcina barkeri was able to grow on L-alanine and L-glutamate as sole nitrogen sources. Cell yields were 0.5 g/l and 0.7 g/l (wet wt), respectively. The mechanism of ammonia assimilation inMethanosarcina barkeri strain MS was studied by analysis of enzyme activities. Activity levels of nitrogen-assimilating enzymes in extracts of cells grown on different nitrogen sources (ammonia, 0.05–100 mM; L-alanine, 10 mM; L-glutamate, 10 mM) were compared. Activities of glutamate dehydrogenase, glutamate synthase, glutamine synthetase, glutamate oxaloacetate transaminase and glutamate pyruvate transaminase could be measured in cells grown on these three nitrogen sources. Alanine dehydrogenase was not detected under the growth conditions used. None of the measured enzyme activities varied significantly in response to the NH₄ ⁺ concentration. The length of the poly--glutamyl side chain of F₄₂₀ derivatives turned out to be independent of the concentration of ammonia in the culture medium.Abbreviations ADH alanine dehydrogenase - FO 7,8-didemethyl-8-hydroxy-5-deazariboflavin - GDH glutamate dehydrogenase - GOGAT glutamate synthase - GOT glutamate oxaloacetate transaminase - GPT glutamate pyruvate transaminase - GS glutamine synthetase - H₄MPT tetrahydromethanopterin     

Ammonia assimilation and metabolism byBeggiatoa alba

Beggiatoa alba B18LD was investigated for its pathways of ammonia assimilation. The increase in growth yields ofB. alba with excess acetate was linear from 0.1 to 2.0 mM ammonia.B. alba had strong glutamine synthetase (GS) and glutamate synthase (GOGAT) activities, irrespective of the ammonia concentration in the medium. Glutamate dehydrogenase activity was not found, and alanine dehydrogenase (aminating) was observed only whenB. alba was grown at high (2.0 mM) ammonia. Methionine sulfoximine, an inhibitor of GS, inhibited growth ofB. alba irrespective of the ammonia concentration in the medium. Thus it appears the primary pathway for ammonia assimilation inB. alba is via the GS-GOGAT pathway at both low and high ammonia concentrations. Preliminary experiments were unable to discern if theB. alba GS is modified by covalent modification.Non-standard abbreviations GS Glutamine synthetase - GOGAT glutamate-oxoglutarate aminotransferase - GDH glutamate dehydrogenase - ADH alanine dehydrogenase - MSX methionine sulfoximine - GOT glutamate-oxaloacetate aminotransferase - GPT glutamate-pyruvate aminotransferase     

The localisation of enzymes of nitrogen assimilation in maize leaves and their activities during greening

The activities of nitrate reductase (EC1.6.6.1), nitrite reductase (EC 1.6.6.4), glutamine synthetase (EC6.3.1.2), glutamate synthase (EC1.4.7.1) and NAD(P)H-dependent glutamate dehydrogenase (EC 1.4.1.3) were investigated in mesophyll and bundle sheath cells of maize leaves (Zea mays L.). Whereas nitrate and nitrite reductase appear to be restricted to the mesophyll and GDH to the bundle sheath, glutamine synthetase and glutamate synthase are active in both tissues.During the greening process, the activities of nitrate and nitrite reductase increased markedly, but glutamine synthetase, glutamate synthase and glutamate dehydrogenase changed little.Abbreviations BDH British Drug Houses - EDTA Ethylene diamine tetra-acetic acid - GDH Glutamate dehydrogenase - NADH Nicotinamide-adenine dinucleotide reduced form - NADPH Nicotnamide-adenine dinucleotide phosphate reduced form - PMSF Phenylmethyl sulphonyl fluoride