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1.
Nitrogen assimilation in citrus trees   总被引:1,自引:0,他引:1  
Assimilation of 15N-ammonium and 15N-nitrate was examined in 3-year-old satsuma mandarin (Citrus unshiu Marcovitch) trees. Experiments were designed to establish the time course of incorporation of nitrogen just taken up into amino compounds. In fine roots, absorbed 15N-ammonium was actively incorporated into glutamine and then into glutamic acid and asparagine. When feeding 15N-nitrate, glutamic acid and asparagine were actively synthesized, but glutamine synthesis was comparatively low as compared with that in ammonium feeding. In current leaves and fruits, a clear difference in the labelling patterns of amino acids was found between the ammonium and nitrate feedings. The amino acid most markedly labelled was asparagine in the ammonium feeding and glutamine in the nitrate feeding. Considering the most heavily labelled component in leaves and fruits, the main form of the nitrogen components transported upward in the xylem was discussed.  相似文献   

2.
In Myrica gale L. plants the assimilation of ammonia released by symbiotic Frankia was observed by 15N2 labelling and subsequent analysis of the isotopic enrichment of nodule amino acids over time by single ion monitoring gas chromatography-mass spectrometry. In detached nodules of Myrica , glutamine was the first amino acid labelled at 30 s and subsequently the amino acids glutamate, aspartate, alanine and γ-amino butyric acid (GABA) became labelled. This pattern of labelling is consistent with the incorporation of ammonium via glutamine synthetase [GS; EC 6.3.1.2]. No evidence for the ammonium assimilation via glutamate dehydrogenase [GDH; EC 1.4.1.2] was observed as glutamate became labelled only after glutamine. Using attached nodules and pulse-chase labelling, we observed synthesis of glutamine, glutamate, aspartate, alanine, GABA and asparagine, and followed the transport of fixed nitrogen in the xylem largely as glutamine and asparagine. Estimation of the cost of nitrogen fixation and asparagine synthesis in Myrica nodules suggests a minimum of one sucrose required per asparagine produced. Rapid translocation of recently fixed nitrogen was observed in Myrica gale nodules as 80% of the nitrogen fixed during a 1-h period was translocated out of the nodules within 9 h. The large pool of asparagine that is present in nodules may buffer the transport of nitrogen and thus act to regulate nitrogen fixation via a feedback mechanism.  相似文献   

3.
—Glucose is a major precursor of glutamate and related amino acids in the retina of adult rats. 14C from labelled glucose appears to gain access to a large glutamate pool, and the resulting specific activity of glutamate labelled from glucose is always higher than that of glutamine or the other amino acids. Radioactive acetate appeared to label a small glutamate pool. The specific activity of glutamine labelled from acetate relative to that of glutamate was always greater than 1.0. Other precursors of the small glutamate pool were found to include glutamate, aspartate, GABA, serine, leucine and sodium bicarbonate. The level of radioactivity present in retinae incubated with [U-14C]glucose or [1-14C]sodium acetate was reduced in the presence of 10?5m -ouabain. Under these conditions, the relative specific activity of glutamine labelled from [1-14C]sodium acetate was lowered, but it was raised when [U-14C]glucose was used as substrate. Ouabain also considerably reduced the synthesis of GABA from [1-14C]sodium acetate. In all cases ouabain caused a fall in the tissue levels of the amino acids. Aminooxyacetic acid (10?4m ) almost completely abolished the labelling of GABA from both [U-14C]glucose and [1-14C]sodium acetate, while the RSA of glutamine labelled from the latter substrate was significantly increased. Aminooxyacetic acid raised the tissue concentration of glutamate, but caused a fall in the tissue concentrations of glutamine, aspartate and GABA. The results suggest that there are separate compartments for the metabolism of glutamate in retina and that these can be modified in different ways by different drugs.  相似文献   

4.
To obtain evidence of the site of conversion of [U-14C]glucose into glutamate and related amino acids of the brain, a mixture of [U-14C]glucose and [3H]glutamate was injected subcutaneously into rats. [3H]Glutamate gave rise to several 3H-labelled amino acids in rat liver and blood; only 3H-labelled glutamate, glutamine or γ-aminobutyrate were found in the brain. The specific radioactivity of [3H]glutamine in the brain was higher than that of [3H]glutamate indicating the entry of [3H]glutamate mainly in the ‘small glutamate compartment’. The 14C-labelling pattern of amino acids in the brain and liver after injection of [U-14C]glucose was similar to that previously reported (Gaitonde et al., 1965). The specific radioactivity of [14C]glutamine in the blood and liver after injection of both precursors was greater than that of glutamate between 10 and 60 min after the injection of the precursors. The extent of labelling of alanine and aspartate was greater than that of other amino acids in the blood after injection of [U-14C]glucose. There was no labelling of brain protein with [3H]glutamate during the 10 min period, but significant label was found at 30 and 60 min. The highest relative incorporation of [14C]glutamate and [14C]aspartate in rat brain protein was observed at 5 min after the injection of [U-14C]glucose. The results have been discussed in the context of transport of glutamine synthesized in the brain and the site of metabolism of [U-14C]glucose in the brain.  相似文献   

5.
Cut, fruiting shoots of Lupinus albus L. supplied with 14C-and 15N-labelled L-asparagine, L-glutamine, L-aspartic acid,or L-glutamic acid through the transpiration stream readilytransferred the labelled carbon and nitrogen of each compoundto pods and seeds of fruits. A time course of labelling of phloemsap collected from petioles and fruit tips following feedingof labelled asparagine indicated that xylem to phloem exchangein leaflets was an immediate and effective route of transferof the amide to fruits and that this and the loading onto phloemof additional asparagine from unlabelled pools of the amidein stems furnished a major source of the nitrogen for fruitfilling. Xylem to phloem exchange of nitrogen was accomplishedin different ways for each amino acid. The amide nitrogen ofasparagine was transferred mainly in the unmetabolized compound,the nitrogen of aspartate and glutamate largely in a wide rangeof amino acids synthesized in the leaf, and the amide nitrogenof glutamine was transferred in a manner intermediate betweenthese extremes. Glutamine and asparagine were the principalphloem solutes labelled with nitrogen from any of the suppliedcompounds, but the photosynthetically produced amino acids,glutamate, aspartate, serine, alanine, and valine also became15N-labelled in phloem. The main pathway for glutamine synthesisin vegetative parts of the shoot appeared to be by amidationof glutamate, but asparagine was not considered to be derivedsimilarly from aspartate. Leaflets metabolized glutamine morereadily than asparagine, but in each case the amide nitrogenwas used for synthesis of a variety of amino acids and the carbonwas recovered largely in non-amino compounds.  相似文献   

6.
—(1) The effects of aminooxyacetic acid, ouabain and Ca2+ on the compartmentation of amino acid metabolism have been studied in slices of brain incubated with sodium-[1-14C]acetate, l-[U-14C]glutamate and l-[U-14C]aspartate as tracer metabolites. (2) Aminooxyacetic acid (10-3 m) inhibited the labelling of aspartate from [14C]acetate and [14C]glutamate, as well as the incorporation of label from [14C]aspartate into glutamate and glutamine. It also inhibited the labelling of GABA from all three radioactive precursors, as would be anticipated if there was inhibition of several transaminases as well as glutamate decarboxylase. The RSA of glutamine labelled from [1-14C]acetate was increased. This finding indicated that the glutamate pool which is utilized for glutamine formation is associated with glutamate dehydrogenase, and this enzyme appears to be related to the ‘synthetic tricarboxylic acid cycle’. AOAA exerted its major inhibitory effects on the citric acid‘energy cycle’with which transaminases are associated. (3) Ouabain (10-5 m) inhibited the labelling of glutamine to a much greater extent than the labelling of glutamate from [1-14C]acetate. It also caused leakage of amino acids from the tissue into the medium. Its effect on the glutamate–glutamine system was interpreted to be a selective inhibition of the 'synthetic’citric acid cycle. (4) The omission of Ca2+ from the incubation medium was associated with formation of glutamine with RSA less than 1·0 when labelled from [U-14C]glutamate, [U-14C]aspartate and lower than normal when labelled from [1-14C]acetate.  相似文献   

7.
1. The testis of the ram secretes considerable amounts of amino acids (200μmoles/day) into the fluid collected from the efferent ducts. The principal amino acid in this testicular fluid is glutamate, which is present in concentrations about eight times those in testicular lymph or in blood from the internal spermatic vein. 2. The concentration of glutamate in seminal plasma from the tail of the epididymis is about ten times that in testicular fluid, and, though glutamate is the major amino acid in ejaculated seminal plasma, its concentration is less than in epididymal plasma. 3. After the intravenous infusion of [U-14C]glucose, labelled glutamate was found in the testicular fluid. Radioactivity was also detected in alanine, glycine, serine plus glutamine and aspartate. Alanine had the highest specific activity, about 50% of the specific activity of blood glucose. 4. When [U-14C]glutamate was infused, the specific activity of glutamate in testicular fluid was only about 2% that in the blood plasma. 5. Testicular and ejaculated ram spermatozoa oxidized both [U-14C]glutamate and [U-14C]leucine to a small extent, but neither substrate altered the respiration from endogenous levels. 6. No radioactivity was detected in testicular spermatozoal protein after incubation with [U-14C]glutamate or [U-14C]leucine. Small amounts of radioactivity were detected in protein from ejaculated ram spermatozoa after incubation with [U-14C]glutamate. 7. The carbon of [U-14C]glucose was incorporated into amino acids by testicular spermatozoa; most of the radioactivity occurred in glutamate.  相似文献   

8.
A subconvulsant dose of sodium fluoroacetate inhibited the metabolic utilization of intracerebrally-administered N-acetyl-l -[U-14C]asparticacid and the labelling of glutamine from this precursor in mouse brain, but not the labelling of glutamate or aspartate. A convulsant dose also inhibited the utilization of l -[U-14C]aspartic acid. When intraperitoneal injection of a convulsant dose of sodium fluoroacetate was followed by intracerebral injection of N-acetyl-l -[U-14C]asparticacid, the levels of N-acetylaspartate, aspartate and glutamate in brain were lowered, while the glutamine content was increased. The specific radioactivity of glutamine relative to that of glutamate was much lower when these compounds were labelled from l -[U-14C]aspartic acid than when N-acetyl-l -[U-14C]aspartic acid was used as the precursor. Intracerebral injection of tracer amounts of l -[U-14C]aspartic acid reduced the content of N-acetylaspartate in brain and raised the glutamine content. Sodium fluoroacetate had no additional effect on the relative specific radioactivity of glutamine or the content of N-acetylaspartate, aspartate, glutamate or glutamine when l -[U-14C]aspartic acid was the precursor. We consider the results to be consistent with a selective inhibition both by sodium fluoroacetate and by exogenous aspartic acid of the tricarboxylic acid cycle in brain associated with the biosynthesis of glutamine. We suggest that the activity of this pathway may regulate the metabolism of N-acetylaspartate and aspartate.  相似文献   

9.
Abstract— The effects of supramaximal electrical stimulation on the metabolism of amino acids and proteins in incubated superior cervical ganglia of the rat were studied by the use of a gas-liquid chromatographic (GLC) assay procedure. Stimulation at 5 Hz for 2 h caused an apparent increase in tissue levels of free amino acids, with alanine, serine, glycine, valine, threonine, isoleucine and aspartate (+ asparagine) most noticeably affected. The amino acid composition (partial) of the TCA-insoluble proteins of resting and stimulated ganglia was approximately the same after 60 min of incubation, but there was less TCA-insoluble protein in the stimulated ganglia. The addition of amino acids (at plasma concentrations) to the standard media had no apparent affect on the amino acid composition of this protein fraction. Stimulation for 0 , 5 h initially increased the efflux of alanine, valine, proline and ornithine into the incubation media but prolonged stimulation (for 4–0 h) decreased the efflux of alanine, serine, glycine and isoleucine and increased the efflux of lysine into the incubation media. The leakage of amino acids from the ganglia appeared to be a sodium-dependent process. The incorporation of 14C from [U-14C]glucose into glutamate (+ glutamine) and aspartate (+ asparagine) was greater in stimulated than in resting ganglia. However, the conversion of glutamate carbons from [U-14C]l -glutamate into aspartate was not affected by stimulation. Incorporation of 14C from [U-14C]glucose into glycine and serine was apparently not affected by stimulation during the 60 min of incubation. However, serine was the only amino acid which exhibited a higher specific radioactivity in stimulated ganglia than in resting ganglia incubated for 4 h in standard media. Lithium ions had the apparent specific effect of increasing the labelling with 14C from [U-14C]glucose into ornithine, and increasing the efflux and overall metabolism of serine in the ganglia. Incorporation of 14C from [U-14C]glucose into proteins was lower in the stimulated than in the resting ganglia if compensation was made for the higher radioactivity available in the total free amino acid pool of the stimulated ganglia. The rate of 14C incorporation from [U-14C]glutamate into the TCA-insoluble proteins of resting ganglia was greater when no other amino acids at concentrations approximating plasma levels were added to the bathing media; this rate was lower in stimulated than in resting ganglia.  相似文献   

10.
Analysis of soil solution from forest sites dominated by Eucalyptus grandis and Eucalyptus maculata indicates that soluble forms of organic nitrogen (amino acids and protein) are present in concentrations similar to those of mineral nitrogen (nitrate and ammonium). Experiments were conducted to determine the extent to which mycorrhizal associations might broaden nitrogen source utilization in Eucalyptus seedlings to include organic nitrogen. In isolation, species of ectomycorrhizal fungi from northern Australia show varying abilities to utilize mineral and organic forms of nitrogen as sole sources. Pisolithus sp. displayed strongest growth on NH4+, glutamine and asparagine, but grew poorly on protein, while Amanita sp. grew well both on mineral sources and on a range of organic sources (e.g. arginine, asparagine, glutamine and protein). In sterile culture, non-mycorrhizal seedlings of Eucalyptus grandis and Eucalyptus maculata grew well on mineral sources of nitrogen, but showed no ability to grow on sources of organic nitrogen other than glutamine. In contrast, mycorrhizal seedlings grew well on a range of organic nitrogen sources. These observations indicate that mycorrhizal associations confer on species of Eucalyptus the ability to broaden their resource base substantially with respect to nitrogen. This ability to utilize organic nitrogen was not directly related to that of the fungal symbiont in isolation. Seedlings mycorrhizal with Pisolithus sp. were able to assimilate sources of nitrogen (in particular histidine and protein) on which the fungus in pure culture appeared to grow weakly. Experiments in which plants were fed 15N-labelled ammonium were undertaken in order to investigate the influence of mycorrhizal colonization on the pathway of nitrogen metabolism. In roots and shoots of all seedlings, 15N was incorporated into the amide group of glutamine, and label was also found in the amino groups of glutamine, glutamic acid, γ-aminobutyric acid and alanine. Mycorrhizal colonization appeared to have no effect on the assimilation pathway and metabolism of [15N]H4+; labelling data were consistent with the operation of the glutamate synthase cycle in plants infected with either Pisolithus sp. (which in isolation assimilates via the glutamate synthase cycle) or Elaphomyces sp. (which assimilates via glutamate dehydrogenase). It is likely that the control of carbon supply to the mycorrhizal fungus from the host may have a profound effect on both the assimilatory pathway and the range of nitrogen sources that can be utilized by the association.  相似文献   

11.
—During anoxia induced by the administration of potassium cyanide, [U-14C]glucose was injected intraperitoneally into adult mice and they were decapitated at 5, 15 and 30 min after the injection. After freeze-drying in vacuo, differences in the uptake of radioactive carbon from [U-14C]glucose into free amino acids (glutamate + glutamine, aspartate + asparagine, GABA, alanine and glycine) in mouse cerebral neocortex, cerebellar hemisphere, caudate nucleus, thalamus, hypothalamus and medulla oblongata were investigated (by macroautoradiography and GLC separation) and compared with those obtained under normal conditions. (1) During anoxia, autoradiographical densities in the thalamus and medulla oblongata were higher than that in the cerebral neocortex and caudate nucleus. (2) Among specific radioactivities (d.p.m./μmol) of free amino acids, alanine gave the highest value during anoxia, except in the cerebellar hemisphere and hypothalamus at 5 min and the medulla oblongata at 30 min. (3) During anoxia, the specific radioactivities of alanine and glycine in each brain region did not significantly decrease at 15 and 30 min compared with those under normal conditions. During anoxia, the specific radioactivity of glutamate + glutamine in the cerebellar hemisphere and hypothalamus did not significantly decrease compared with the normal conditions, while that of GABA, aspartate + asparagine and glutamate + glutamine in the cerebral neocortex, caudate nucleus, thalamus and medulla oblongata showed an increase. (4) The percentage decrease of glutamate + glutamine and aspartate + asparagine at 5 and 15 min was highly significant in the cerebral neocortex and caudate nucleus.  相似文献   

12.
-Aminobutyric acid (GABA) accumulation occurs in cultured ricecells when ammonium is added to the medium [Kishinami and Ojima(1980) Plant Cell Physiol. 21: 581–589]. Whether thisphenomenon occurs in rice plant tissues was examined with respectto exogenously supplied auxins: 2,4-dichlorophenoxyacetic acid(2,4-D), indole-3-acetic acid (IAA) and naphthalene-acetic acid(NAA). In intact rice plants grown in medium containing ammonium withoutauxin, glutamine first increased, then asparagine graduallyincreased. In both shoots and roots, the asparagine contentbecame the highest among four amino acids after 4 days of cultureperiod. GABA did not increase at all, its level remaining lowin both shoots and roots throughout the culture period. GABA accumulation was observed in excised rice root tips whenthey were incubated in the medium containing ammonium in thepresence of 2,4-D, IAA or NAA. In the absence of auxin, however,excised rice root tips accumulated asparagine and glutamine,but not GABA. Rice root segments obtained from a region in whichroot cells had already developed to maturity did not accumulateGABA but asparagine and glutamine in the presence of both ammoniumand 2,4-D. These results suggest that GABA accumulation occurs in rapidlygrowing and dividing tissue, such as the apical meristem ofrice root in the presence of auxin during ammonium assimilation. (Received June 15, 1987; Accepted March 14, 1988)  相似文献   

13.
By macroautoradiography and by GLC separation, differences in the uptake of radioactive carbon from [U-14C]glucose into free amino acids (glutamate + glutamine, aspartate + asparagine, GABA, alanine and glycine) in mouse cerebral neocortex, hippocampus, thalamus and hypothalamus were investigated. (1) The autoradiographical densities in the thalamus, cerebral neocortex and hippocampus measured with a microdensitometer were higher than that in the hypothalamus at 5 min after subcutaneous injection. At 180 min, densities in the cerebral neocortex, hippocampus and hypothalamus were higher than that in thalamus. (2) The free amino acid levels determined by GLC varied with each brain region. (3) The specific radioactivity (d.p.m./μmol) of alanine in each brain region was higher than that of the other amino acids at 5 min after the injection. The specific radioactivity of GABA in the brain regions was clearly higher than that of (glutamate + glutamine), (aspartate + asparagine) and glycine at 5 and 15 min. (4) The autoradiographical data were in good agreement with the chemical data at 5 min but were different at 180 min. (5) Variations in specific radioactivity of each free amino acid among brain regions at 5 min were influenced greatly by existing free amino acid concentrations in each region.  相似文献   

14.
—The incorporation of radioactivity from labelled glucose, acetate, phenylalanine and proline into glutamate, aspartate and glutamine was measured in mice treated with methionine sulphoximine and in the control animals. The labelled precursors were injected and their incorporation determined before the onset of convulsions. The incorporation of radioactivity from labelled glucose into the dicarboxylic amino acids was reduced, in particular the incorporation into glutamine. The incorporation of radioactivity from labelled acetate and phenylalanine into glutamate and aspartate was increased by methionine sulphoximine, while the incorporation into glutamine was not changed very much. The labelling of glutamine, relative to glutamate, was reduced with all precursors, indicating that glutamine synthetase was inhibited in vivo by methionine sulphoximine. It is very likely that methionine sulphoximine affects many aspects of energy metabolism in brain; in particular the metabolism of glucose seems to be inhibited, while the rate of conversion of substrates other than glucose seems to be increased.  相似文献   

15.
Summary. Accumulation of amino acids was studied in rice roots of 3-day-old seedlings subjected for 48 h to anaerobic conditions. Alanine and Gaba were the main amino acids accumulated under anoxia. Their synthesis was strongly inhibited by MSX and AZA, inhibitors of glutamine synthetase and glutamate synthase. These activities increased after 8 h of anaerobic treatment and, by immunoprecipitation of 35S-labeled proteins, it was shown that glutamine synthetase and ferredoxin-dependent glutamate synthase were synthesized during the treatment. These findings indicate that the glutamine synthetase/glutamate synthase cycle play an important role in anaerobic amino acid accumulation. Received April 5, 1999  相似文献   

16.
Nitrogen re-mobilization and changes in free amino acids werestudied as a function of time in leaves, stubble, and rootsduring ryegrass (Lolium perenne L.) re-growth. Experiments with15N labelling clearly showed that during the first days nearlyall the nitrogen in new leaves came from organic nitrogen re-mobilizedfrom roots and stubble. On the days of defoliation, stubblehad the highest content of free amino acids with 23 mg per gdry weight against 15 mg and 14 mg in leaves and roots, respectively.The major amino acids in leaves were asparagine (23% of totalcontent in free amino acids), aminobutyrate, serine, glutamine,and glutamate (between 7% and 15%) whereas in roots and stubblethe contribution of amides was high, especially asparagine (about50%). Re-growth after cutting was associated with a rapid increaseof the free amino acid content in leaves, with a progressivedecrease in roots while stubble content remained virtually unchanged.In leaves, asparagine increased from the first day of re-growth,while the aspartate level remained unchanged and glutamine increasedstrongly on the first day but decreased steadily during thenext few days of re-growth. Asparagine in stubble and rootschanged in opposite directions: in stubble it tended to increasewhereas in roots it clearly decreased. In contrast, stubbleand roots showed a similar decrease in glutamine. In these twoplant parts, as in leaves, aspartate remained at a low level.Results concerning free amino acids are discussed with referenceto nitrogen re-mobilization from source organs (stubble androots) to the sink organ (regrowing leaves). Key words: Lolium perenne L, re-growth, nitrogen, free amino acids, glutamine, asparagine  相似文献   

17.
Abstract— Hemisections of toad brains, when incubated in a physiological medium containing no glutamine. released considerable amounts of this amino acid into the medium. When glutamine was included in the medium at a concentration of 0.2 mm the net efflux from the tissue was reduced but not totally prevented. Although there was no net uptake of glutamine, the tissue did accumulate [U-14C]glu-tamine and some of this labelled glutamine was rapidly metabolized to glutamate, GABA and aspartate. The precursor-product relationship for the metabolism of glutamine to glutamate differed from the classic single compartment model in that the specific radioactivity of glutamate rose very quickly to approx one-tenth that of glutamine, but increased slowly thereafter. These data suggest that the [14C]glutamine was taken up into two metabolically distinct compartments and/or that some of the [14C]glutamine was converted to [14C]glutamate during the uptake process. The uptake of [14C]glutamine was diminished when the tissue was incubated in a non-oxygenated medium or when Na+ was omitted (substituted with sucrose) and K+ was concomitantly elevated. However, on a relative basis, the incorporation of radioactivity into glutamate and GABA was increased by these incubation conditions. The metabolism of glutamine to aspartate was greatly depressed when the tissue was not oxygenated. The glutamate formed from [U-14C]glutamine taken up by the tissue was converted to GABA at a faster rate than was glutamate derived from [U-14C]glucose. [U-14C]gly-cerol or exogenous [U-14C]glutamate. This suggests that glutamine was metabolized to GABA selectively; i.e. on a relative basis, glutamine served as a better source of carbon for the synthesis of GABA than did glucose, glycerol or exogenous glutamate. When the brain hemisections were incubated in the normal physiological medium with or without glutamine. there was very little efflux of glutamate, GABA or aspartate from the tissue. However when NaCl was omitted from the medium (substituted with sucrose) and K+ was elevated to 29 miu. a marked efflux of these three amino acids into the medium did occur, and over a period of 160min, the content of each amino acid in the tissue was depleted considerably. When glutamine (0.2 mm ) was included in the Na+ deficient-high K.+ medium, the average amount of glutamate, GABA and aspartate in the tissue plus the medium was greater than when glutamine was not included in the medium. Such data indicate that CNS tissues can utilize glutamine for a net synthesis of glutamate, GABA and aspartate. The results of this study provide further evidence in support of the concept that the functional (transmitter) pools of glutamate and GABA are maintained and regulated in part via biosynthesis from glutamine. One specific mechanism instrumental in regulating the content of glutamate in nerve terminals may be a process of glutamine uptake coupled to deamidation.  相似文献   

18.
The metabolism of GABA and other amino acids from various radioactive precursors has been studied in the rat substantia nigra using a sensitive double isotope dansyl derivative assay. Labelled acetate gave greater labelling of glutamate than of glutamine in substantia nigra slices whereas the reverse was the case for cerebral cortex slices. Unilateral transection of the striato-nigral pathway caused a parallel decrease in the GABA and GAD content of the substantia nigra. It also reduced the total synthesis of GABA from all labelled precursors used, namely acetate, glutamate and glucose. After incubation with [1-14C]acetate the specific activity of glutamate and aspartate, but not that of GABA, increased on the lesioned side compared with the normal side. The specific activity of glutamate, but not that of GABA or aspartate, decreased after incubation with [U-14C]glucose on the lesioned side compared with the normal side. The results could be explained by the previously proposed hypothesis concerning differential labelling of metabolic pools by the two precursors. [U-14C]Glutamate lead to increased labelling of GABA on the lesioned side relative to the normal side. Incubation of slices from substantia nigra with β-mercaptopropionic acid caused a decrease of labelling of GABA from glucose and acetate, probably as the result of GAD inhibition. The labelling pattern of the other amino acids, apart from that of glutamate which showed a decrease when synthesised from acetate, did not change appreciably.  相似文献   

19.
To investigate nitrogen assimilation and translocation in Zea mays L. colonized by the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus fasciculatum (Thax. sensu Gerd.), we measured key enzyme activities, 15N incorporation into free amino acids, and 15N translocation from roots to shoots. Glutamine synthetase and nitrate reductase activities were increased in both roots and shoots compared with control plants, and glutamate dehydrogenase activity increased in roots only. In the presence of [15N]ammonium, glutamine amide was the most heavily labeled product. More label was incorporated into amino acids in VAM plants. The kinetics of 15N labeling and effects of methionine sulfoximine on distribution of 15N-labeled products were entirely consistent with the operation of the glutamate synthase cycle. No evidence was found for ammonium assimilation via glutamate dehydrogenase. 15N translocation from roots to shoots through the xylem was higher in VAM plants compared with control plants. These results establish that, in maize, VAM fungi increase ammonium assimilation, glutamine production, and xylem nitrogen translocation. Unlike some ectomycorrhizal fungi, VAM fungi do not appear to alter the pathway of ammonium assimilation in roots of their hosts.  相似文献   

20.
Weissman , Gerard S. (Rutgers U., Camden, N. J.) Influence of ammonium and nitrate on the protein- and amino acids in shoots of wheat seedlings. Amer. Jour. Bot. 46(5): 339–346. 1959.—Total and protein nitrogen per shoot of wheat seedlings grown with endosperm attached increased at a steady rate during a 96-hr. growth period, and protein nitrogen, as a percentage of total nitrogen, remained constant at about 53%. Total and protein nitrogen concentration was greatest for 24-hr. shoots and declined as the shoots became older. Total and protein nitrogen were determined in 96-hr. shoots of seedlings grown with endosperm attached but also supplied with ammonium, nitrate, or both in the culture solution. Total nitrogen was greatest in shoots supplied with ammonium, but only 38% was in the form of protein. Maximum protein synthesis occurred in shoots grown in both ammonium and nitrate and protein nitrogen as a percentage of total nitrogen approximated that achieved in shoots lacking nitrogen in the culture solution. The protein amino acid composition of 48-, 72-, and 96-hr. shoots was very similar but differed from 24-hr. shoots which contained higher percentages of arginine and lysine and lower percentages of alanine and threonine. This may be correlated with the higher proportion of meristematic cells in 24-hr. shoots. The protein amino acids in shoots grown with ammonium resembled that of shoots lacking nitrogen in the culture solution, but nitrate shoot protein contained a higher percentage of arginine and a lower percentage of lysine. Nitrate may stimulate the formation of enzymes, possibly of a nitrate-reducing system, with high arginine- low lysine content. Free asparagine and glutamine were both at a maximum in ammonium shoots and at a minimum in nitrate shoots, but asparagine predominated in shoots supplied with ammonium while glutamine was greatest in nitrate shoots. Aspartic acid, asparagine, and glutamine appeared to have ammonia-storage functions, but glutamic acid appeared to be primarily concerned with protein synthesis. Amino acid accumulation was greatest in shoots supplied with both ammonium and nitrate. Protein synthesis in these appeared to be limited by inadequate concentrations of glutamic acid and proline. A hypothesis is proposed in explanation of the high glutamic acid concentration in shoots provided with ammonium and nitrate.  相似文献   

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