首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 31 毫秒
1.
[15N]-depleted (NH4)2SO4 applied to the soil in 1985 resulted in residual labeling of about 16% of the storage nitrogen (N) pool of mature walnut ( Juglans regia L. cv. Serr) trees in 1987. Application of [15N]-depleted (NH4)2SO4 fertilizer to a different set of mature walnut trees in 1987 allowed monitoring of the kinetics and utilization of N from current year uptake in 1987 and resulted in >20% labeling of fruit N following completion of leaf expansion. Redistribution of storage N to the new growth predominated during the spring flush of growth although N derived from the soil during current-year uptake contributed increasingly during leaf expansion. Labeled N from current year uptake accumulated preferentially in the leaves as compared with reproductive organs during leaf expansion but subsequent to leaf expansion, fruit were more highly labeled with N derived from current-year uptake than leaves. Pistillate flower abortion was coincident with an apparent competition for N among developing vegetative and reproductive organs and preceded the period of significant N contribution from current-year uptake.  相似文献   

2.
Abstract: The rate of glutamate synthesis from leucine by the branched-chain aminotransferase was measured in rat brain in vivo at steady state. The rats were fed exclusively by intravenous infusion of a nutrient solution containing [15N]leucine. The rate of glutamate synthesis from leucine, determined from the rate of increase of brain [15N]glutamate measured by 15N NMR and the 15N enrichments of brain and blood leucine analyzed by gas chromatography-mass spectrometry, was 0.7–1.8 µmol/g/h at a steady-state brain leucine concentration of 0.25 µmol/g. A comparison of the observed fractional 15N enrichments of brain leucine (0.42 ± 0.03) and glutamate (0.21 ± 0.015) showed that leucine provides ∼50% of glutamate nitrogen under our experimental condition. From the observed rate (0.7–1.8 µmol/g) and the known K m of the branched-chain aminotransferase for leucine (1.2 m M ), the rate of glutamate synthesis from leucine at physiological brain leucine concentration (0.11 µmol/g) was estimated to be 0.35–0.9 µmol/g/h, with leucine providing ∼25% of glutamate nitrogen. The results strongly suggest that plasma leucine from dietary source, transported into the brain, is an important external source of nitrogen for replenishment of brain glutamate in vivo. Implications of the results for treatment of maple-syrup urine disease patients with leucine-restricted diet are discussed.  相似文献   

3.
Effects of Ketone Bodies on Astrocyte Amino Acid Metabolism   总被引:5,自引:1,他引:4  
Abstract: The effects of acetoacetate and 3-hydroxybutyrate on glial amino acid metabolism were studied in primary cultures of astrocytes. The exchange of nitrogen among amino acids was measured with 15N as a metabolic probe and gas chromatography-mass spectrometry as a tool with which to quantify isotope abundance. Addition of either acetoacetate or 3-hydroxybutyrate (5 m M ) to the incubation medium did not alter the initial rate of appearance of [15N]glutamate in the glia, but it did inhibit transamination of glutamate to [15N]aspartate. Addition of acetoacetate also inhibited formation of [2-15N]glutamine, but 3-hydroxybutyrate had a stimulatory effect. The presence in the medium of sodium acetate (5 m M ) was also associated with diminished production of [15N]aspartate and [2-15N]glutamine with [15N]glutamate as precursor. Studies with [2-15N]glutamine as precursor indicated that treatment of the astrocytes with ketone bodies did not alter flux through the glutaminase pathway. Nor did the presence of the ketone bodies reduce significantly the flux of nitrogen from [15N]GABA to [2-15N]glutamine when the former species served as a metabolic tracer. The concentration of internal citrate increased in the presence of acetoacetate, 3-hydroxybutyrate, and acetate. Studies with purified sheep brain glutamine synthetase showed that citrate inhibited this enzyme. These findings are considered in terms of the known anticonvulsant effect of a ketogenic diet.  相似文献   

4.
Uptake of [15N]-ammonia was more sensitive to UV-B exposure than the total 14CO2 fixation rate of Lithodesmium variabile Takano. Short-term UV-B radiation (15 min) had practically no effect on the kinetics of [15N]-ammonia, whereas there was an effect on [14C]-bicarbonate uptake rate. A significant reduction was found after 30 and 60 min UV-B stress. The time course of photosynthetic uptake of 15NH4Cl at several wavelengths was markedly depressed at shorter wavelengths (irradiation with WG 280). A short-term (11 min) exposure to ultraviolet radiation had no influence on the [14C]-labeled photosynthetic products. However, the [15N]-label of several amino acids and the ratio of [15N]-glutamine to [15N]-glutamic acid varied after irradiation with different ultraviolet wavebands. The results are discussed with reference to UV damage to the key enzymes of nitrogen metabolism.  相似文献   

5.
Abstract: Thiosphaera pantotropha has been reported to denitrify aerobically and nitrify heterotrophically. However, recent evidence has indicated that these properties (particularly aerobic denitrification) have been lost. The occurrence and levels of aerobic denitrification and heterotrophic nitrification by T. pantotropha in chemostat cultures have therefore been re-evaluated. Only low nitrate reduction rates were observed: the apparent nitrogen loss was of the same order of magnitude as the combined error in the calculated nitrogen consumption. However, 15N mass spectrometry revealed low aerobic denitrification rates (about 10% of the rates originally published by this group). Heterotrophic nitrification rates were about a third of previous observations. N2 and N2O were both produced from NH4+, NO3 and NO2. Periplasmic nitrate reductase was present in aerobically grown cells.  相似文献   

6.
SUMMARY. 1. Time-course measurements of NH4+ and NO3uptake were made on the natural phytoplankton populations in a eutrophic lake at a time when these nutrients were at their lowest annual concentration.
2. Both NH4+ and NO3 uptake was increased at least five-fold during the first 5 min of incubation following near saturating pulses of these nutrients.
3. Elevated uptake was also observed following low level (∼2μg N 1−1) pulses of NH4+ and NO3, but substrate depletion during the first hour of incubation may have been partially responsible for this apparent enhancement.
4. Incorporation of I5N into TCA-insoluble material (protein) following the saturating NH4+ pulse was increased less than total cellular 15N uptake, whereas no elevation of 15N incorporation into protein was observed following a saturating NO3pulse.
5. The percentage of I5N incorporated into protein, with respect to total cellular uptake, was ∼32% and ∼12% for NH4+ and NO3, respectively, following 5 h of incubation.  相似文献   

7.
The role of methionine as a precursor in mugineic acid (MA) biosynthesis was studied by feeding 15N-ammonium sulfate, 14C-amino acids, and [1-14C, 15N]-methionine to iron-deficient barley roots ( Hordeum vulgare L. cv. Minorimugi), grown hydroponically. The incorporation of isotopes into amino acids was also examined. Methionine appears to be the most efficient precursor of the mugineic acid family (MAs) of phytosiderophores; homoserine was also incorporated into the MAs, but other amino acids such as glutamate, alanine, and γ-amino butyric acid did not act as precursors of MAs. Carbon-14 and 15N of methionine were incorporated into MAs. This specific incorporation of 14C and 15N indicated that the nitrogen atoms of MAs were derived from two molecules of methionine. It is suggested that deoxymugineic acid (DMA) is probably the first phytosiderophore to be synthesized on the biosynthetic pathway of MAs.  相似文献   

8.
In order to establish the relationship of nitrogen nutrition to root formation in vitro, efficiency of various forms of nitrogen supply was investigated in micropropagated shoots of Pinus pinaster. Nitrate (3.3 m M as N) containing medium supplemented with glutamine (2 m M as N) resulted in optimal rooting (control medium) whereas the rooting response was inhibited by the presence of glutamate (2 m M as N) in the medium. Study of 15NO3 behaviour in shoots cultured on the control medium showed a marked increase of 15N incorporation during the second half of a period of auxin treatment (total duration 16 days). The majority of the nitrogen incorporated from the medium was accumulated into protein in the rooting zone. Autoradiography of [3H]-thymidine in this reacting zone showed strong DNA synthesis at this time. Addition of glutamate both prevented the root formation process and resulted in a marked slowdown of nitrate uptake and decrease of protein synthesis.  相似文献   

9.
The osmotic role of nitrate during aftermath growth of Lolium perenne L. cv. Réveille was investigated. Plants were grown from seed in a controlled environment using a liquid medium with 1.0 m M NH4NO3 as nitrogen source.
Eight-week-old plants were cut 4.0 cm above the root system and then harvested over a 14-day period of regrowth on the same initial nutrient solution, except that nitrate was 15N labelled. Throughout the experimental period, nitrate storage and reduction in roots were low. In stubble and especially in leaves, nitrate accumulated during the first 6 days of regrowth whereas nitrate reduction mainly occurred after this period. Analyses of carbohydrate, chloride and potassium contents in stubble and leaves showed that the accumulation of nitrate osmotically compensated for the decrease in soluble sugars during the first 6 days of regrowth.
The cumulative osmotic potential of sugars, chloride and nitrate in differently treated plants was studied in stubble and leaves. Compared with uncut plants, the lower carbohydrate concentrations found in cut plants regrowing on 1.0 m M NH4NO3 were compensated for by an accumulation of nitrate. During aftermath growth on low nitrogen nutrition (0.2 m M NH4NO3), chloride replaced nitrate, supporting the proposed osmotic function of nitrate.
It is concluded that nitrate is involved in the osmotic adjustment of ryegrass during regrowth after cutting.  相似文献   

10.
To investigate nitrogen assimilation in Lolium perenne L. colonized by the arbuscular mycorrhizal (AM) fungus Glomus fasciculatum (Thax. sensu Gerd.), nitrate uptake, key enzyme activities, and 15N incorporation into free amino acids were measured. After a 4-h labelling period with [15N]nitrate, 15N content was higher in roots and shoots of AM-plants than in those of control plants. Glutamine synthetase (GS) and nitrate reductase (NR) activities were increased in shoots of AM-plants, but not in roots. More label was incorporated into amino acids in shoots of AM plants. Glutamine, glutamate, alanine and γ-aminobutyric acid were the major sinks for 15N in roots and shoots of control and AM plants. Interactions between mycorrhizal colonization, phosphate and nitrate nutrition and NR activity were investigated in plants which received different amounts of phosphate or nitrate. In shoots of control plants, NR activity was not stimulated by high levels of phosphate nutrition but was stimulated by high levels of nitrate. At 4 m M nitrate in the nutrient solution, NR activity was similar in control and AM plants. We concluded that mycorrhizal effects on nitrate assimilation are not mediated via improved phosphate nutrition, but could be due to improved nitrogen uptake and translocation.  相似文献   

11.
Roots and leaves of Zea mays L. cv. Ganga Safed-2 seedlings grown with nutrient solution containing either 10 m M KNO3 or NH4Cl or 5 m M NH4NO3 had considerably higher glutamate synthase (NADH, EC 1.4.1.14) activity than the corresponding organs from seedlings grown without any nitrogen. The supply of inorganic nitrogen for a short time, i.e. 3 h, to roots and leaves excised from seedlings grown without nitrogen also increased the enzyme activity in these organs. This increase was more pronounced with nitrate than with ammonium nitrogen. When excised roots and leaves from NH4NO3-grown seedlings were incubated in a minus nitrogen medium for 24 h, the enzyme activity declined considerably. This decline was inhibited to some extent by nitrogen, especially by nitrate. Inorganic nitrogen prevented similarly the decline in in vitro enzyme activity during 24 h storage at 25°C, more regularly for the root than for the leaf enzyme. The experiments demonstrate the role of inorganic nitrogen in the regulation of glutamate synthase activity.  相似文献   

12.
Properties of glutamate dehydrogenase from developing maize endosperm   总被引:2,自引:0,他引:2  
Glutamate dehydrogenase (EC 1.4.1.3) activity was assayed in homogenates of maize ( Zea mays L. inbred lines Oh43 and Oh43o2) endosperm during development. During the period 20–35 days after pollination anabolic (aminative) activities were higher than catabolic (deaminating) ones. In order to study the regulation of GDH activity, glutamine or glutamate were injected into the ear peduncle before sample harvesting. The amination and deamination reactions showed similar behaviour with different nitrogen sources: glutamine increased, whereas glutamate decreased, both aminative and deaminative reactions. Partially purified enzyme was active with NADH and NADPH in a ratio 9:1. In Tris-HCl buffer a broad optimum at pH 7.6–8.9 and pH 6.8–8.9 was observed with NADH and NADPH, respectively, NADH activity was activated by Ca2+. Saturation curves for (NH4)2SO4 and NADH showed normal Michaelis-Menten kinetics in the presence of 1 m M Ca2+, but substrate inhibition occurred without Ca2+. The enzyme was inactivated by EDTA. The effect of EDTA was reversed by Ca2+ and Mn2+, but not by Cu2+ and Mg2+.  相似文献   

13.
Response of nitrogen metabolism to boron toxicity in tomato plants   总被引:1,自引:0,他引:1  
Boron (B) toxicity has become important in areas close to the Mediterranean Sea where intensive agriculture has been developed. The objective of this research was to study the effects of B toxicity (0.5 m m and 2.0 m m B) on nitrogen (N) assimilation of two tomato cultivars that are often used in these areas. Leaf biomass, relative leaf growth rate (RGRL), concentration of B, nitrate (NO3), ammonium (NH4+), organic N, amino acids and soluble proteins, as well as nitrate reductase (NR), nitrite reductase (NiR), glutamine synthase (GS), glutamate synthetase (GOGAT) and glutamate dehydrogenase (GDH) activities were analysed in leaves. Boron toxicity significantly decreased leaf biomass, RGRL, organic N, soluble proteins, and NR and NiR activities. The lowest NO3 and NH4+ concentration in leaves was recorded when plants were supplied with 2.0 m m B in the root medium. Total B, amino acids, activities of GS, GOGAT and GDH increased under B toxicity. Data from the present study prove that B toxicity causes inhibition of NO3 reduction and increases NH4+ assimilation in tomato plants.  相似文献   

14.
Four-year-old seedlings of Scots pine ( Pinus sylvestris L.) were exposed to filtered air (FA), and to FA supplemented with NH3 (60 and 240 μg m−3) in controlled-environment chambers for 14 weeks. Exposure to the higher NH3 concentration resulted in an increased activity of glutamine synthetase (GS, EC 6.3.1.2), and an increase in the concentrations of soluble proteins, total nitrogen, free amino acids and leaf pigments in the needles. The GS activity (μmol g−1 fresh weight h−1) in the needle extract increased to levels 69% higher than in FA and the soluble protein concentration to levels 22% higher. Total nitrogen concentration in the needles was 42% higher than in FA, while the free amino acid concentration was 300% higher, which was caused by an increase in arginine, glutamate, aspartate and glutamine. Chlorophyll a , chlorophyll b and carotenoid concentrations were 29, 38 and 11% higher, respectively. Neither the glutamate dehydrogenase (GDH, EC 1.4.1.2) activity nor the concentrations of free NH4+ and glucose in the needles were affected by exposure to NH3. After NH3 fumigation at 240 μg m−3 the starch concentration decreased by 39% relative to the FA. The results indicate that the metabolism of Scots pine acclimates to concentrations of NH3 which are 3 to 10 times higher than the average concentration in areas with intensive stock farming. The possible mechanisms underlying acclimation to NH3 are discussed.  相似文献   

15.
We report a novel use of the 15N dilution technique to detail the translocation of amino compounds in the legume Sesbania rostrata . The conventional 15N dilution technique follows the dilution of 15N within a labelled plant, as 14N2 is fixed by symbiotic bacteria. In our experiments, stem-nodulated Sesbania rostrata were enriched by feeding with 15N ammonium nitrate for 2 weeks, followed by a 1 week period where the only N available to the plants was via nitrogen fixation of atmospheric N2. We measured the composition, concentration and 15N enrichment of amino compounds in various plant tissues, both above and below the stem nodules, using GC-MS and isotopic abundance mass spectrometry techniques. Approximately 28% of the total N in the stem nodules was derived from internal plant sources. The ureides allantoic acid and allantoin were not abundant in xylem, leaf or nodule tissues. The amides asparagine and glutamine were the major export products from stem nodules although a wide range of other amino compounds are also synthesized. Amino acids within the nodules had a low level of enrichment, demonstrating that a small fraction (≈ 11%) was derived from outside the nodules, and significant cycling of N (28% of xylem N) through the root system was revealed by measurements of 15N distribution and amino acid concentrations.  相似文献   

16.
Over a period of several days, rhythmic changes in extracellular NH+4 concentration take place in cultures of the cyanobacterium Microcystis firma (Bré et Lenorm.) Schmidle, strain Gromov/St. Petersb. 398, under conditions of restricted CO2 supply and light/dark alternation. The changes are enhanced by nitrate supply. Among the various processes generating intracellular NH+4 (NH44 uptake, NO3 reduction, protein and amino acid degradation, photorespiration), NO3 reduction appears as the one most important. This can be concluded from experiments with and without nitrate and/or ammonium in the medium. In the presence of saturating CO2, continuous light, or continuous darkness, rhythmic NH+44 oscillations are not induced. Studies of the incorporation of NH+4 nitrogen by in vivo 15N-NMR show that if CO2 is supplied, 15N is accumulated in several components with the following time course: in the first hour in Gln (δ), in the second hour in the α-amino groups of most nonbranched amino acids, in the third hour in γ-aminobutyric acid (GABA), Orn (δ) and Lys (ε), and in the sixth hour in Ala. Carbon limitation, however, results in accumulation of label in the amide nitrogen of glutamine only.  相似文献   

17.
Helicobacter pylori utilises urea for amino acid synthesis   总被引:2,自引:0,他引:2  
Abstract Helicobacter pylori has one of the highest urease activities of all known bacteria. Its enzymatic production of ammonia protects the organism from acid damage by gastric juice. The possibility that the urease activity allows the bacterium to utilise urea as a nitrogen source for the synthesis of amino acids was investigated. H. pylori (NCTC 11638) was incubated with 50 mM urea, enriched to 5 atom% excess 15N, that is the excess enrichment of 15N above the normal background, in the presence of either NaCl pH 6.0, or 0.2M citrate pH 6.0. E. coli (NCTC 9001) was used as a urease-negative control. 15N enrichment was detected by isotope ratio mass spectrometry. H. pylori showed intracellular incorporation of 15N in the presence of citrate buffer pH 6.0 but there was no significant incorporation of 15N in unbuffered saline or by E. coli in either pH 6.0 citrate buffer or unbuffered saline. The intracellular fate of the urea-nitrogen was determined by means of gas chromatography/mass spectrometry following incubation with 15N enriched 5 mM urea in the presence of either 0.2 M citrate buffer pH 6.0 or 0.2 M acetate buffer pH 6.0. After 5 min incubation in either buffer the 15n label appeared in glutamate, glutamine, phenylalanine, aspartate and alanine. It appears, therefore, that at pH and urea concentrations typical of the gastric mucosal surface, H. pylori utilises exogenous urea as a nitrogen source for amino acid synthesis. The ammonia produced by H. pylori urease activity thus facilitates the organism's nitrogen metabolism at neutral pH as well as protecting it from acid damage at low pH.  相似文献   

18.
Abstract: Metabolism of [U-13C5]glutamine was studied in primary cultures of cerebral cortical astrocytes in the presence or absence of extracellular glutamate. Perchloric acid extracts of the cells as well as redissolved lyophilized media were subjected to nuclear magnetic resonance and mass spectrometry to identify 13C-labeled metabolites. Label from glutamine was found in glutamate and to a lesser extent in lactate and alanine. In the presence of unlabeled glutamate, label was also observed in aspartate. It could be clearly demonstrated that some [U-13C5]glutamine is metabolized through the tricarboxylic acid cycle, although to a much smaller extent than previously shown for [U-13C5]glutamate. Lactate formation from tricarboxylic acid cycle intermediates has previously been demonstrated. It has, however, not been demonstrated that pyruvate, formed from glutamate or glutamine, may reenter the tricarboxylic acid cycle after conversion to acetyl-CoA. The present work demonstrates that this pathway is active, because [4,5-13C2]glutamate was observed in astrocytes incubated with [U-13C5]glutamine in the additional presence of unlabeled glutamate. Furthermore, using mass spectrometry, mono-labeled alanine, glutamate, and glutamine were detected. This isotopomer could be derived via the action of pyruvate carboxylase using 13CO2 produced within the mitochondria or from labeled intermediates that had stayed in the tricarboxylic acid cycle for more than one turn.  相似文献   

19.
Abstract. The 15N isotope was used to compare the uptake and the assimilation of NH4+ and NO3 nitrogen in ryegrass ( Lolium perenne L.) during regrowth after cutting. Uptake of nitrate-N, expressed per plant, was at all times greater than ammonium-N uptake and assimilation decreased in roots and stubble while its assimilation was maintained at a high level in leaves. It has been suggested that ammonium assimilation is directly related to the availability of carbohydrates in the sink organ (leaves) resulting from their remobilization from the source organs (roots and stubble). Nitrate reduction decreased in all organs, while the uptake of NO3 was still high. After this first period of regrowth, nitrogen assimilation both from nitrate and ammonium increased in all the plants. Nitrate reduction capacity (expressed in μg NO3-N reduced per g D.W. per d) is 7.5 and 22.5 times greater in leaves than in stubble and roots, respectively. Therefore, nitrogen assimilation in stubble and particularly in roots was mainly dependent on ammonium nitrogen.  相似文献   

20.
The effect of nitrogen on excretion and metabolism of glycolate in Anabaena cylindrica (CCAP 1403/2a) was studied. Glycidate, an inhibitor of glutamate:glyoxylate aminotransferase (EC 2.6.1.4), reduced the L-methionine-DL-sulfoximine-induced NH4+ release by ca 40%, while net CO2 fixation and C2H2 reduction were not lowered. This indicates that at least a part of the glyoxylate synthesized in A. cylindrica is metabolized via glycine to serine. Addition of NH4Cl or glutamate to the medium reduced the excretion of glycolate. At pH 9, under air, NH4Cl reduced the excretion by 10–30% and under high pO2 (0.03 kPa CO2 in O2) by about 80–90%. At pH 7.5, under high pO2, NH4Cl and glulamate reduced the excretion by about 40 and 80%, respectively. Also, the presence of NH4Cl stimulated the animation of glyoxylate under such conditions as shown by an increased glycine pool and a decreased glutamate pool. We suggest that nitrogen regulates the capacity of A. cylindrica to retain and recycle glycolate intracellularly and that glutamate serves as an amino donor in the conversion of glyoxylate to glycine.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号