Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Azaserine”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 487 records · Page 27Linked to original sources
PubMed1993

Separate de Novo and Salvage Purine Pools Are Involved in the Biosynthesis of Theobromine but Not Caffeine in Leaves of Coffea arabica L.

In Coffea arabica leaves, the purine ring of theobromine (3,7-dimethylxanthine) and caffeine (1,3,7-trimethylxanthine) is provided by de novo purine biosynthesis: (a) [14C]glycine, [14C]bicarbonate, and [14C]formate were incorporated into inosine 5[prime]- monophosphate (IMP), sum of adenine nucleotides ([sigma]Ade), theobromine, and caffeine; and (b) incorporation of [14C]formate into IMP, [sigma]Ade, theobromine, and caffeine was inhibited by azaserine, a known inhibitor of de novo purine biosynthesis. Capacity of coffee leaves to salvage added purines was demonstrated by incorporation of [14C]hypoxanthine into [sigma]Ade and the incorporation of [14C]adenosine, [14C]adenine, [14C]inosine, and [14C]hypoxanthine into both theobromine and caffeine. Consistent with synthesis of theobromine from two separate purine nucleotide pools, one synthesized de novo and one via salvage, added xanthine 5[prime]-monophosphate (XMP), inosine, or hypoxanthine failed to reduce the incorporation of [14C]formate into theobromine but diluted the specific radioactivity of [14C]adenosine and [14C]adenine incorporated into theobromine. Evidence that theobromine is not the immediate precursor of caffeine is provided: (a) [14C]xanthine was incorporated into caffeine but not into theobromine; (b) exogenous xanthine diluted the specific radioactivity of caffeine synthesized from [14C]adenine and [14C]hypoxanthine but caused accumulation of radiolabel in theobromine; (c) allopurinol, a known inhibitor of the conversion of hypoxanthine to xanthine, reduced incorporation of [14C]adenine and [14C]hypoxanthine into caffeine but caused accumulation of radiolabel in theobromine; and (d) incorporation of [14C]formate into caffeine, but not into theobromine, was reduced by added XMP, inosine, or hypoxanthine.

Journal Article↗
PubMed1994

A Metabolic Control Analysis of the Glutamine Synthetase/Glutamate Synthase Cycle in Isolated Barley (Hordeum vulgare L.) Chloroplasts.

Ammonia assimilation in chloroplasts occurs via the glutamine synthetase/glutamate synthase (GS/GOGAT) cycle. To determine the extent to which these enzymes contribute to the control of ammonia assimilation, a metabolic control analysis was performed on isolated barley (Hordeum vulgare L.) leaf chloroplasts. Pathway flux was measured polarographically as ammonium-plus-2-oxoglutarate-plus-glutamine-dependent O2 evolution in illuminated chloroplasts. Enzyme activity was modulated by titration with specific, irreversible inhibitors of GS (phosphinothricin) and GOGAT (azaserine). Flux control coefficients (CJ0E0) were determined (a) by differentiation of best-fit hyperbolic curves of the data sets (flux versus enzyme activity), and (b) from estimates of the deviation indices (D/[prime]E0). Both analyses gave similar values for the coefficients. The control coefficient for GS was relatively high and the value did not change significantly with changes in 2-oxoglutarate concentration (C/0E0 = 0.58 at 5 mM 2-oxoglutarate and 0.40 at 20 mM 2-oxoglutarate). The control coefficient for GOGAT decreased with decreasing glutamine concentrations, from 0.76 at 20 mM glutamine to 0.19 at 10 mM glutamine. Thus, at high concentrations of glutamine, GOGAT exerts a major control over flux with a significant contribution also from GS. At lower concentrations of glutamine, however, GOGAT exerts far less control over pathway flux.

Journal Article↗
PubMed1968

Metabolism of cytidine and uridine in bean leaves.

The metabolism of cytidine-2-(14)C and uridine-2-(14)C was studied in discs cut from leaflets of bean plants (Phaseolus vulgaris L.). Cytidine was degraded to carbon dioxide and incorporated into RNA at about the same rates as was uridine. Both nucleosides were converted into the same soluble nucleotides, principally uridine diphosphate glucose, suggesting that cytidine was rapidly deaminated to uridine and then metabolized along the same pathways. However, cytidine was converted to cytidine diphosphate and cytidine triphosphate more effectively than was uridine. Cytidine also was converted into cytidylic acid of RNA much more extensively and into RNA uridylic acid less extensively than was uridine. Azaserine, an antagonist of reactions involving glutamine (including the conversion of uridine triphosphate to cytidine triphosphate), inhibited the conversion of cytidine into RNA uridylic acid with less effect on its incorporation into cytidylic acid. On the other hand, it inhibited the conversion of orotic acid into RNA cytidylic acid much more than into uridylic acid. The results suggest that cytidine is in part metabolized by direct conversion to uridine and in part by conversion to cytidine triphosphate through reactions not involving uridine nucleotides.

Journal Article↗
PubMed1976

In vivo conversion of 5-oxoproline to glutamate by higher plants.

l-(U-(14)C)-5-oxoproline (pyrollidone carboxylic acid or pyroglutamic acid) was infiltrated into detached leaves of a number of species and incubated for 1 to 6 hours. In every case, conversion to labeled glutamate and glutamine was observed. The amount converted varied from 1 to 64% of the total label fed depending on the species. The ratio of glutamate-(14)C to glutamine-(14)C ranged from 5 in Vicia faba to 1 in sugar beet. This ratio could be affected by preinfiltrating various compounds before allowing the uptake of the 5-oxoproline. When l-methionine-dl-sulfoximine was prefed to sugar beet leaves, the glutamate-glutamine ratio increased from 1 to 10. Prior treatment of V. faba leaves with azaserine resulted in essentially only labeled glutamine being recovered. Preinfiltration with NaF or ATP gave similar results in that the glutamate-glutamine ratio was greatly decreased. The results are consistent with glutamate being produced from the 5-oxoproline and then being converted to glutamine.

Journal Article↗
PubMed1977

Ammonium and amino acids as regulators of nitrate reductase in corn roots.

When amino acids or ammonia are added to plant systems, the effects on the development of nitrate-dependent nitrate reductase activity are variable. In addition, amino acids added singly or as casein hydrolysate may not support a normal growth. A physiologically correct mixture of amino acids, one similar in composition to amino acids released by the endosperm, has been shown to support normal growth and protein synthesis in corn (Zea mays) embryos. In this investigation, we have used the mixture of corn amino acids to determine whether amino acids have an effect on the appearance or disappearance of nitrate reductase activity. The results show that these amino acids partially inhibit the induction of nitrate reductase in corn roots. The effect is more pronounced in mature root than in root tip sections. When glutamine and asparagine are included along with the "corn amino acid mixture," the inhibition is more severe. Amino acids or amino acid analogues added singly to the induction medium have a similar effect: i.e. when the induction of nitrate reductase is inhibited in the root tips (lysine, canavanine, azaserine, azetidine-2-carboxylic acid, dl-4-azaleucine, asparagine, and glutamine), that inhibition is more severe in mature root sections. Arginine enhanced the recovery of nitrate reductase in root tips but inhibited it in mature root sections. The effect of the amino acids is apparently on some phase of the induction processes (i.e. the uptake or distribution of nitrate or a direct effect on the synthesis of the enzyme) and not on the turnover of the enzyme.

Journal Article↗
PubMed1977

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.

Journal Article↗
PubMed1978

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.

Journal Article↗
PubMed1979

A comparison of glutamate synthase obtained from maize endosperms and roots.

Glutamate synthase (EC 2.6.1.53) has been examined in developing endosperms and roots of maize. KCl is required for maximum activity in each tissue. The effect with KCl is seen with buffer strength of 25 to 100 millimolar in the assay. The optimum concentration for the enzyme from endosperm is 20 millimolar and for the enzyme from root tissue the saturating concentration is about 20 millimolar. In root material the enzyme is labile but activity can be restored if KCl is added to the assay. Divalent cations such as Mg(2+) or Mn(2+) also activate the enzyme to some extent.In each case NADH or NADPH can serve as reductant. The reaction is insensitive to alpha-aminooxyacetate, but is inhibited by glutamate, the glutamate analogs methionine sulfoximine and methionine sulfone, and by the glutamine analogs azaserine and albizziin.

Journal Article↗
PubMed1982

Stimulation of ammonia and 2-oxoglutarate-dependent o(2) evolution in isolated chloroplasts by dicarboxylates and the role of the chloroplast in photorespiratory nitrogen recycling.

Intact chloroplasts isolated from spinach (Spinacia oleracea L.) leaves showed a light-dependent O(2) evolution (5.5 +/- 0.75 micromoles per milligram chlorophyll per hour) when supplied with ammonia and 2-oxoglutarate. This (ammonia, 2-oxoglutarate)-dependent O(2) evolution was stimulated 2- to 4-fold by the dicarboxylates, malate, succinate, fumarate, glutarate, and l-tartarate. Evolution of O(2) in the presence of malate was dependent on the presence of both 2-oxoglutarate and NH(4)Cl; malate with only either 2-oxoglutarate and NH(4)Cl alone did not support O(2) evolution. Furthermore, in the presence of malate, the amount of O(2) evolved was solely dependent on the amount of NH(4)Cl or 2-oxoglutarate added and malate did not affect the ratio of O(2) evolved to NH(4)Cl or 2-oxoglutarate consumed. Studies with inhibitors (2-(3,4-dichlorophenyl)-1,1-dimethyl urea, methionine sulfoximine, and azaserine) indicated that the above activity was directly linked to glutamine synthetase and glutamate synthase activity in the chloroplast and was not caused by the metabolism of malate. The V(max)/2 of (ammonia, 2-oxoglutarate)-dependent O(2) evolution was reached at 32 micromolar NH(4)Cl and 6 millimolar (approximately) 2-oxoglutarate in the absence of malate, and at 22 micromolar NH(4)Cl and 73 micromolar 2-oxoglutarate when malate (3 millimolar) was present.Intact chloroplasts isolated from pea (Pisum sativum) leaves also showed a stimulation of (ammonia, 2-oxoglutarate)-dependent O(2) evolution by malate. However glutamine was required for this activity even though glutamine with only either NH(4)Cl or 2-oxoglutarate did not respond to malate stimulation.The measured rates of (ammonia, 2-oxoglutarate)-dependent O(2) evolution in isolated spinach chloroplasts in the presence of malate were about 19.5 +/- 4.5 micromoles O(2) evolved per milligram chlorophyll per hour. This is adequate to sustain photorespiratory NH(3) recycling and the refixation of NH(3) arising from NO(3) under ambient conditions in the light. The role of the chloroplast in photorespiratory NH(3) recycling and the nature of the associated transport of 2-oxoglutarate into the chloroplast is discussed.

Journal Article↗
PubMed1982

Glutamate synthase from rice leaves.

Ferredoxin-dependent glutamate synthase (EC 1.4.7.1) from rice leaves (Oryza sativa L. cv Delta) was purified 206-fold with a final specific activity of 35.9 mumoles glutamate formed per min per milligram protein by a procedure including ammonium sulfate fractionation, DEAE-cellulose chromatography, Sephacryl S-300 gel filtration, and ferredoxin-Sepharose affinity chromatography. The purified enzyme yielded a single protein band on polyacrylamide gel electrophoresis. Molecular weight of the native enzyme was estimated to be 224,000 daltons by Sepharose 6B gel filtration. Electrophoresis of the dissociated enzyme in sodium dodecyl sulfate-polyacrylamide gel gave a single protein band which corresponds to the subunit molecular weight of 115,000 daltons. Thus, it is concluded that the glutamate synthase is composed of two polypeptidic chains exhibiting the same molecular weight. Spectrophotometric analysis indicated that the enzyme is free of iron-sulfide and flavin. The pH optimum was 7.3. The enzyme had a negative cooperativity (Hill number of 0.70) for glutamine, and its K(m) value increased from 270 to 570 mum at a glutamine concentration higher than 800 mum. K(m) values for alpha-ketoglutarate and ferredoxin were 330 and 5.5 mum, respectively. Asparagine and oxaloacetate could not be substituted for glutamine and alpha-ketoglutarate, respectively. Enzyme activity was not detected with pyridine nucleotides as electron donors. Azaserine and several divalent cations were potent inhibitors. The purified enzyme was stabilized by dithiothreitol.

Journal Article↗
PubMed1983

Effect of inhibitors on ammonia-, 2-oxoglutarate-, and oxaloacetate-dependent o(2) evolution in illuminated chloroplasts.

The evolution of O(2) in spinach chloroplasts in the presence of oxaloacetate (OAA) was inhibited by a wide range of dicarboxylates. In contrast, (ammonia, 2-oxoglutarate)-dependent O(2) evolution was stimulated by malate, succinate, fumarate, glutarate, maleiate, and l-tartrate although OAA has little effect. This increase in O(2) evolution was accompanied by a similar increase in (14)C incorporation from [5-(14)C]oxoglutarate into amino acids which was sensitive to azaserine inhibition. Glutamate and aspartate inhibited (ammonia, 2-oxoglutarate)-dependent O(2) evolution, but this inhibition was relieved by the addition of succinate, malate, or fumarate. OAA-dependent O(2) evolution also was inhibited by glutamate and aspartate, but succinate, malate, or fumarate had little effect on this inhibition. Phthalonate and n-butyl malonate inhibited (ammonia, 2-oxoglutarate)-dependent O(2) evolution competitively with respect to 2-oxoglutarate and uncompetitively with respect to malate. Both these inhibitors inhibited OAA-dependent O(2) evolution competitively. This evidence suggests that different mechanisms might be involved in the transport of OAA, 2-oxoglutarate, and malate into the chloroplasts.

Journal Article↗
PubMed1983

De Novo Purine Biosynthesis in Intact Cells of Cucurbita pepo.

The capacity of intact cells of roots excised from summer squash plants (Cucurbita pepo L. cv Early Prolific Straightneck) to synthesize purine nucleotides de novo was investigated. Evidence that purine nucleotides are synthesized de novo included: (a) demonstration of the incorporation of [1-(14)C]glycine, [2-(14)C]glycine, NaH(14)CO(3), and H(14)COONa into total adenine nucleotides; (b) observation that the addition of azaserine or aminopterin, known inhibitors of de novo purine synthesis in other organisms, blocked the incorporation of these precursors into adenine nucleotides; and (c) demonstration that the purine ring synthesized from these precursors was labeled in a manner consistent with the pathway for de novo purine biosynthesis found in microorganisms and animal tissues. Under optimal conditions, the activity of this pathway in roots excised from 2-day-old squash plants was 244 +/- 13 nanomoles (mean +/- standard error, n = 17) NaH(14)CO(3) incorporated into summation operatorAde (the sum of the adenine nucleotides, nucleoside and free base) per gram tissue during the 3-hour incubation period.The possible occurrence of alternative enzymic reactions for the first steps of de novo purine biosynthesis was also investigated. No conclusive evidence was obtained to support the operation of alternative enzymic reactions in the intact cell of C. pepo.

Journal Article↗
PubMed1984

Asparagine biosynthesis in soybean nodules.

Asparagine biosynthesis in soybean (Glycine max [L.] Merr.) nodules has been difficult to demonstrate due to the poor conversion of suspected immediate precursors to asparagine and the instability of the key enzyme asparagine synthetase. The present study was designed to explore the effects of two ammonium assimilation inhibitors on the metabolism of (14)CO(2) to [(14)C]asparagine and to demonstrate the existence in nodules of the enzyme asparagine synthetase. When detached nodules were incubated in (14)CO(2), radioactivity in asparagine (as a percentage of amino acid cpm) increased 10-fold over 4 hours. Vacuum infiltration of 10 mm methionine sulfoximine or 10 mm azaserine prior to (14)CO(2) incubations decreased both the rate of dark fixation and the radioactivity in the amino acid fraction. These inhibitors also decreased the recovery of label in aspartate and asparagine. These results, plus the sequence of labeling of metabolites from (14)CO(2), are consistent with a glutamine-dependent synthesis of asparagine from aspartate with oxalacetate as a precursor to aspartate.An enzyme catalyzing the ATP- and glutamine-dependent amidation of aspartic acid to form asparagine was isolated from soybean nodules. High levels of sulfhydryl protectants were required and the inclusion of glycerol and substrates in the extraction buffer helped to stabilize the enzyme. Enzyme activity in taproot nodules increased between 38 and 41 days after planting and peaked soon after flower initiation (45 days). The activity then declined to basal levels by 70 days. On a total enzyme activity basis, there was 170-fold more asparagine synthetase activity in the infected zone of the nodule than in the cortex, and 205-fold more activity in the cytosol than the bacteroid fraction. The enzyme has a broad pH maximum around pH 8.25, and the apparent K(m) values for the substrates aspartate, MgATP, and glutamine are 1.24 mm, 0.076 mm, and 0.16 mm, respectively. Ammonium ion can replace glutamine as the nitrogen donor, but the K(m) value of the enzyme for ammonium ion is 40-fold higher than that for glutamine.

Journal Article↗
PubMed1984

Effect of ammonium on nitrate utilization by roots of dwarf bean.

The effect of exogenous NH(4) (+) on NO(3) (-) uptake and in vivo NO(3) (-) reductase activity (NRA) in roots of Phaseolus vulgaris L. cv Witte Krombek was studied before, during, and after the apparent induction of root NRA and NO(3) (-) uptake. Pretreatment with NH(4)Cl (0.15-50 millimolar) affected neither the time pattern nor the steady state rate of NO(3) (-) uptake.When NH(4) (+) was given at the start of NO(3) (-) nutrition, the time pattern of NO(3) (-) uptake was the same as in plants receiving no NH(4) (+). After 6 hours, however, the NO(3) (-) uptake rate (NUR) and root NRA were inhibited by NH(4) (+) to a maximum of 45% and 60%, respectively.The response of the NUR of NO(3) (-)-induced plants depended on the NH(4)Cl concentration. Below 1 millimolar NH(4) (+), the NUR declined immediately and some restoration occurred in the second hour. In the third hour, the NUR became constant. In contrast, NH(4) (+) at 2 millimolar and above caused a rapid and transient stimulation of NO(3) (-) uptake, followed again by a decrease in the first, a recovery in the second, and a steady state in the third hour. Maximal inhibition of steady state NUR was 50%. With NO(3) (-)-induced plants, root NRA responded less and more slowly to NH(4) (+) than did NUR.Methionine sulfoximine and azaserine, inhibitors of glutamine synthetase and glutamate synthase, respectively, relieved the NH(4) (+) inhibition of the NUR of NO(3) (-)-induced plants. We conclude that repression of the NUR by NH(4) (+) depends on NH(4) (+) assimilation. The repression by NH(4) (+) was least at the lowest and highest NH(4) (+) levels tested (0.04 and 25 millimolar).

Journal Article↗
PubMed1985

Metabolic Conversion of Amino Acids Loaded in the Vacuole of Chara australis Internodal Cells.

Vacuoles of internodal cells of Chara australis (or Chara corallina) were loaded with a 10 millimolar amount of various amino acids by a perfusion method and incubated under continuous light. After 20 to 24 hours, the cell sap was collected, and free amino acids in it and the rest of the cell (cytoplasm) were analyzed. The only amino acid metabolized completely was alanine. About 40 to 80% of the aspartic acid, glutamine, serine, and glycine were metabolized, whereas less than 30% of the threonine, asparagine, isoasparagine, isoleucine, phenylalanine, gamma-aminobutyric acid, lysine, and arginine were metabolized. The figure for glutamic acid fluctuated between 10 and 100%. The main metabolites of alanine were glutamine, glycine and ammonia, which accumulated in the vacuole. Alanine utilization was not affected by l-methionine-d,l-sulfoximine or azaserine, but was strongly inhibited by aminooxyacetate. The cell extract contained enough alanine aminotransferase activity to account for the rate of alanine metabolism.

Journal Article↗
PubMed1986

Pathways of Nitrogen Metabolism in Nodules of Alfalfa (Medicago sativa L.).

Exposure of intact alfalfa nodules to (15)N(2) showed that in bacteroids the greatest flow of (15)N was to NH(3). Label was also detected in glutamic acid, aspartic acid, and asparagine (Glu, Asp and Asn), but at far lower levels. In the host plant cytosols, more (15)N was incorporated into Asn than into other compounds. Detached nodules were also used to study the metabolic pathway of N assimilation after exposure to (15)N(2) or vacuum infiltration with ((15)NH(4))(2)SO(4) in the presence or absence of different inhibitors of nitrogen assimilation: methionine sulfoximine (MSO), azaserine (AZA), or amino-oxyacetate (AOA). Treatment with MSO, an inhibitor of glutamine synthetase (GS), inhibited the flow of the label to glutamine (Gln)-amide, resulting in subsequently decreased label in Asnamide. Aza, which inhibits the formation of Glu from Gln by glutamate synthase (GOGAT), enhanced the labeling of the amide groups of both Gln and Asn, while that of Asn-amino decreased. When AOA was used to block the transamination reaction very little label was found in Asp and Asn-amino. The results are consistent with the role of GS/GOGAT in the cytosol for the assimilation of NH(3) produced by N(2) fixation in the bacteroids of alfalfa nodules. Asn, a major nitrogen transport compound in alfalfa, is mainly synthesized by a Gln-dependent amidation of Asp, according to feeding experiments using the (15)N-labeled amide group of glutamine. Data from (15)NH(4) (+) feeding support some direct amidation of Asp to form Asn.

Journal Article↗
PubMed1986

Nitrate and Ammonium Induced Photosynthetic Suppression in N-Limited Selenastrum minutum.

Nitrate-limited chemostat cultures of Selenastrum minutum Naeg. Collins (Chlorophyta) were used to determine the effects of nitrogen addition on photosynthesis, dark respiration, and dark carbon fixation. Addition of NO(3) (-) or NH(4) (+) induced a transient suppression of photosynthetic carbon fixation (70 and 40% respectively). Intracellular ribulose bisphosphate levels decreased during suppression and recovered in parallel with photosynthesis. Photosynthetic oxygen evolution was decreased by N-pulsing under saturating light (650 microeinsteins per square meter per second). Under subsaturating light intensities (<165 microeinsteins per square meter per second) NH(4) (+) addition resulted in O(2) consumption in the light which was alleviated by the presence of the tricarboxylic acid cycle inhibitor fluoroacetate. Addition of NO(3) (-) or NH(4) (+) resulted in a large stimulation of dark respiration (67 and 129%, respectively) and dark carbon fixation (360 and 2080%, respectively). The duration of N-induced perturbations was dependent on the concentration of added N. Inhibition of glutamine 2-oxoglutarate aminotransferase by azaserine alleviated all these effects. It is proposed that suppression of photosynthetic carbon fixation in response to N pulsing was the result of a competition for metabolites between the Calvin cycle and nitrogen assimilation. Carbon skeletons required for nitrogen assimilation would be derived from tricarboxylic acid cycle intermediates. To maintain tricarboxylic acid cycle activity triose phosphates would be exported from the chloroplast. This would decrease the rate of ribulose bisphosphate regeneration and consequently decrease net photosynthetic carbon accumulation. Stoichiometric calculations indicate that the Calvin cycle is one source of triose phosphates for N assimilation; however, during transient N resupply the major demand for triose phosphates must be met by starch or sucrose breakdown. The effects of N-pulsing on O(2) evolution, dark respiration, and dark C-fixation are shown to be consistent with this model.

Journal Article↗
PubMed1986

Ammonia Fixation via Glutamine Synthetase and Glutamate Synthase in the CAM Plant Cissus quadrangularis L.

Succulent stems of Cissus quadrangularis L. (Vitaceae) contain glutamine synthetase, glutamate synthase, and glutamate dehydrogenase. The CO(2) and water gas exchanges of detached internodes were typical for Crassulacean acid metabolism plants. During three physiological phases, e.g. in the dark, in the early illumination period after stomata closure, and during the late light phase with the stomata wide open, (15)NH(4)Cl was injected into the central pith of stem sections. The kinetics of (15)N labeling in glutamate and glutamine suggested that glutamine synthetase was involved in the initial ammonia fixation. In the presence of methionine sulfoximine, an inhibitor of glutamine synthetase, the incorporation of (15)N derived from (15)NH(4)Cl was almost completely inhibited. Injections of amido-(15)N glutamine demonstrated a potential for (15)N transfer from the amido group of glutamine into glutamate which was suppressed by the glutamate synthase inhibitor, azaserine. The evidence indicates that glutamine synthetase and glutamate synthase could assimilate ammonia and cycle nitrogen during all phases of Crassulacean acid metabolism.

Journal Article↗