Methionine Sulphoximine Research Articles

Regulation of ammonium and potassium uptake by glutamine and asparagine in Pisum arvense plants

Pisum arvense plants were subjected to 5 days of nitrogen deprivation. Then, in the conditions that increased or decreased the root glutamine and asparagine pools, the uptake rates of 0.5 mM NH4 + and 0.5 mM K+ were examined. The plants supplied with 1 mM glutamine or asparagine took up ammonium and potassium at rates lower than those for the control plants. The uptake rates of NH4 + and K+ were not affected by 1 mM glutamate. When the plants were pre-treated with 100 µM methionine sulphoximine, an inhibitor of glutamine synthesis, the efflux of NH4 + from roots to ambient solution was enhanced. On the other hand, exposure of plants to methionine sulphoximine led to an increase in potassium uptake rate. The addition of asparagine, glutamine or glutamate into the incubation medium caused a decline in the rate of NH4 + uptake by plasma membrane vesicles isolated from roots of Pisum arvense, whereas on addition of methionine sulphoximine increased ammonium uptake. The results indicate that both NH4 + and K+ uptake appear to be similarly affected by glutamine and asparagine status in root cells.

Multinuclear NMR Spectroscopy Studies on NH4Cl-Induced Metabolic Alterations and Detoxification Processes in Primary Astrocytes and Glioma Cells

Glutamine synthesis, the major pathway of ammonia detoxification, and the intracellular concentration of organic osmolytes in primary astrocytes and F98 glioma cells were investigated with multinuclear magnetic resonance spectroscopy. Acute exposure to ammonia (3 h incubation with NH₄Cl) raised the concentration of glutamine and other amino acids, such as glutamate and aspartate, and decreased myo-inositol, hypotaurine, and taurine concentrations. The loss of these osmolytes was partially reversed by co-treatment with the glutamine synthetase inhibitor, methionine sulphoximine. Glutamate, the precursor of glutamine, is provided by stimulated anaplerotic flux via pyruvate carboxylase and glutamate dehydrogenase activity. Thus, the glutamine increase and myo-inositol decrease observed by in vivo magnetic resonance spectroscopy on patients with hepatic encephalopathy may be due to the disturbed osmoregulation in astrocytes caused by accumulation of glutamine and the subsequent loss of organic osmolytes.

Methionine sulphoximine does not inhibit pea and wheat glutamate dehydrogenase

The addition of methionine sulphoximine (MSX) for 2 days at concentrations ranging from 0.6 to 2.0 mM, completely inhibited glutamine synthetase (EC 6.3.1.2) in the roots and leaves of 12 day old wheat (Triticum vulgare) and pea (Pisum sativum). However MSX had no effect on glutamate dehydrogenase (GDH; EC 1.4.1.2) activity or subunit composition in the roots of wheat or pea. In the leaves of pea, MSX stimulated a 4.5 fold increase in NADH-dependent glutamate dehydrogenase activity, that correlated with the synthesis of a second enzyme subunit and a dramatic alteration in the GDH isoenzyme pattern following native PAGE. Although MSX also caused a 2.4 fold increase in GDH activity in wheat leaves, this increase was not accompanied by a change in subunit composition or isoenzyme pattern. It is concluded that MSX does not inhibit GDH in the leaves or roots of wheat and pea seedlings, but it may cause the onset of premature senescence, which results in an increase in GDH activity in the leaves, as has been shown previously.

Effect of methionine sulphoximine on13 N‐ammonium fluxes in the roots of barley and squash seedlings

Abstract13 N‐Iabelled experiments were carried out with barley and squash seedlings to analyse the effect of methionine sulphoximine (MSO), as an inhibitor of the ammonium assimilation, on NH4+ fluxes in the roots at the level (150 μM NH4+) of the high affinity uptake system (HATS). MSO pretreatment in the root medium significantly accelerated NH4+ influx and efflux in NH4+ ‐fed plants of both species. Concomitantly, the level of NH4+ extracted from the root and shoot tissues was strongly elevated whereas the translocation of13 N‐Iabelled organic substances (presumably amino products) into the shoots was markedly reduced. The onset of the increased NH4+ influx did not occur immediately but was indicated about 1 h after the application of MSO to the nutrient solution. Also, MSO did not directly affect the uptake of13 NO3−. The reason why MSO treatment leads to an acceleration of the NH4+ influx against an increasing level in the cell tissue remains obscure. In squash, an intermediate of NO3− metabolism is thought to suppress NH4+ uptake since NH4+ influx was high in N‐free‐ and NH4+ ‐grown seedlings but was reduced about threefold in NO3− ‐fed squash plants; the suppression was not eliminated by MSO treatment.

Ammonia emissions to the atmosphere from leaves of wild plants and Hordeum vulgare treated with methionine sulphoximine

Using field plots, three species (Mercurialis perennis L., Rubus fruticosus L., and Trientalis europaea L.) were tested for their potential to emit gaseous ammonia to the atmosphere. Canopies were misted with 5 mM methionine sulphoximine (MSO) to inhibit glutamine synthetase (GS), the enzyme of ammonium assimilation. Leaf tissue NH4+ concentration of control plants was 0·03–0·1 μmol g−1 f. wt. Although NH4+ accumulated in the leaf tissue of MSO‐treated plants of all three species to similar concentrations (6‐10 μmol g−1 f. wt after 4 d), emissions were only detected from the leaves of M. perennis, with potential rates of 2·5 nmol m−2 leaf s−1. Experiments carried out in a controlled environment confirmed this rate of emission over 9 d, during which time leaf tissue ammonium increased to 66 μmol g−1 f. wt. Comparisons with Hordeum vulgare grown under the same conditions showed that tissue NH4+ concentration reached a plateau of about 40 μmol g −1f. wt after 2 d. Emissions of NH3 during the 5 d of treatment reached a maximum rate of 10 nmol m−2 s−1 by the third day.Apoplastic pH of the plants was determined, and it is suggested that this is an important factor explaining the differences in NH3 emission between species. The higher the apoplastic pH, the greater the likelihood of loss of NH3 from sub‐stomatal spaces to the atmosphere. T. europaea (non‐emitter) had an apoplastic pH of 5, R. fruticosus (non‐emitter) a pH of c. 5·6, whereas that of M. perennis (emitter) was c. pH 6·3. The apoplastic pH is thought to be dictated in part by the N nutrition of a species, nitrophilous species tending to have high pH. Without NO3− fertilization, H. vulgare had an apoplastic pH of 6·8 but this increased to 7·3, 3 d after feeding with NO3−.Short‐term fumigation (2 h) of shoots of H. vulgare with 60 μg (≈32 mg NH3 m−3) of labelled gaseous 15N‐NH3 (in the absence of MSO) showed that a substantial proportion (60%) of the applied label was found in the leaves, as well as in stems and roots (3%). There was also a change in amino acid pools, with an increase in shoot amino acids and a decrease in those in the root, while tissue NH4+ was very low in both shoots and roots. This provided indirect evidence that some of the applied label had been incorporated into an organic form. Following the fumigation treatment, emissions of NH3 were collected for 3 h, then c. 6·5 μg of N was recovered, of which c. 17% was 15N‐labelled. Some of this label could have resulted from desorption of NH3 from leaf surfaces, but it was more likely that the remaining 14N isotope was from sub‐stomatal emissions of NH3.It is argued that non‐nitrophilous plants tend to rely on mixed sources of N (NO3−, NH4+ or organic‐N) and are more likely to favour root rather than shoot assimilation. Under these circumstances, their apoplastic pH is relatively low (compared with that of nitrophiles, which tend to assimilate NO3− mainly in their shoots), and at atmospheric concentrations most wild species are likely to be net assimilators, rather than emitters, of atmospheric ammonia.

Regulation of peanut glutamate dehydrogenase by methionine sulphoximine

Peanut glutamate dehydrogenase (GDH) was electrophoretically purified to homogeneity. Rotofor IEF fractionated the peanut GDH to 7 isoelectric (charge) isomers, which focused in the pH 5–8 range. Western blot analysis of the charge isomers using anti-GDH serum showed that methionine sulphoximine (MSX) treatment suppressed the b-subunit (69 KDa), but enhanced the a-subunit (45 KDa), and α-subunit (46 KDa) of the enzyme. The MSX-mediated suppression of the b-subunit increased the NH 4 + K m value of the acidic charge isomers from 7.7 mM in the control to 50 mM in the MSX-treated peanut, and also increased the 2-ketoglutarate K m value of the basic charge isomers from 0.4 mM in the control to 7.0 mM in the MSX treatment. Therefore, the control peanut could salvage NH 4 + with its GDH activity. But by increasing the NH 4 + K m value, the MSX rendered the enzyme ineffective in NH 4 + salvage. In the deamination direction, despite the enhancement of the a-, and α-subunits by MSX, the basic charge isomers of GDH had very high K m value for l-glu (50 mM), similar to that (25 mM) of the control. Thus, the GDHs of the control, and MSX treatment could not function in the deamination reaction in vivo. These results show that the treatment of peanut with MSX impaired the amination function of GDH.

Purification and properties of glutamine synthetase from the cellular slime mouldDictyostelium discoideum

Glutamine synthetase (GS) from the cellular slime mould Dictyostelium discoideum was purified to apparent electrophoretic homogeneity with a final yield of 21.7%. The native enzyme appeared to be a GS-II type enzyme. SDS-PAGE of the final preparation revealed a single band of 43.5 kDa. The enzyme has a native molecular mass of 376 kDa, determined using Superose 6, indicating that the enzyme is likely to be an octamer of identical subunits. Dictyostelium discoideum GS has an optimal temperature of 42.5 degrees C, although it is thermolabile in the absence of L-glutamate and (or) Mg(2+)-ATP. The enzyme exhibits a K(m) for L-glutamate, ATP, and NH4Cl of 2.18, 0.18, and 0.11 mM, respectively, in the L-glutamine synthetic reaction with an optimal pH of 7.9. GS from D. discoideum does not appear to be significantly inhibited by various end products of L-glutamine metabolism, although it is potently inhibited by methionine sulphoximine. These properties are those expected for an enzyme for which the primary function is the assimilation of ammonia.

Aspects of glutamine metabolism in human tumour cells.

The importance of the non-essential amino acid, glutamine, to the proliferation of human tumour cells was investigated. All of the cells studied had a high activity of phosphate-dependent glutaminase and were found to utilise glutamine from the culture medium during long term culture. The rate of cell proliferation, determined by [6-3H]-thymidine incorporation, was dependent on glutamine concentration with the exception of Hs578T and MOLT 4 cells. The glutamine concentration giving half maximal growth (ED50) ranged from 0.02-0.24 mM. The glutamine analogue, acivicin [L-(alpha S,5S)-alpha-amino-3-chloro-4,5-dihydro-5-isoxazoleacetic acid], markedly inhibited cell proliferation in the absence of glutamine. However, in the presence of glutamine acivicin only caused inhibition of proliferation in certain cell lines. Replacement of glutamine in the culture medium by glutamate resulted in an increase in the rate of cell proliferation when compared with rates in the absence of glutamine with no glutamate supplement. In addition, cells grown in the presence of the glutamine synthetase inhibitor, methionine sulphoximine, showed a marked decrease in proliferation. These data suggested the presence of glutamine synthetase in human tumour cells, which was confirmed by radiochemical assay of maximal glutamine synthetase activity. The breast tumour cells Hs578T, with high glutamine synthetase activity, were able to proliferate at rates similar to that in the presence of glutamine, when glutamine-deficient medium was supplemented with purine nucleosides. However, the breast tumour cells MCF7, with low glutamine synthetase activity, were unable to proliferate at comparable rates in the presence of either purine or pyrimidine nucleoside supplements.

Glycogen accumulated in the brain following insults is not degraded during a subsequent period of ischemia

The primary objective of this study was to attempt to induce excessive intraglial acidosis during ischemia by subjecting rats to an initial insult which leads to postinsult accumulation of glycogen, presumed to accumulate primarily in astrocytes. The initial insults were 15 min of transient forebrain ischemia, 30 min of hypoglycemic coma, and intraperitonial injection of methionine-sulphoximine (MSO). In the first two of these insults, glycogen content in neocortex increased to 6–7 mM · kg −1 after 6 h of recovery, and in MSO-treated animals even higher values were measured 24 h after administration (~12 mM · kg −1). In spite of this glycogen loading, the amount of lactate formed during a subsequent ischemic insult (of 5–30 min duration) did not exceed values usually obtained during complete ischemia in animals with normal glycogen contents (tissue lactate contents of ~15 mM · kg −1). This was because appreciable amounts of glycogen (3–7 mM · kg −1) remained undegraded even after 30 min of ischemia. The undigested part largely reflected the extra amount of glycogen accumulated after the initial insults. It is discussed whether this part is unavailable to degradation by phosphorylase.

Xylem exudation is related to nitrate assimilation pathway in detopped maize seedlings: use of nitrate reductase and glutamine synthetase inhibitors as tools

The xylem exudation of detopped 7-d-old seedlings of Zea mays L. doubled when KCl was present in the root medium compared to seedlings maintained on water. It was further enhanced when KCl was replaced by nitrogen compounds such as nitrate, ammonium and glutamine. The role of the nitrate assimilation pathway on the enhancement of xylem exudation rate was investigated using tungstate, an inhibitor of nitrate reductase (NR) activity, and phosphinothricin or methionine sulphoximine, inhibitors of glutamine synthetase (GS) activity. The sap levels of NO 3 - , NH 4 + , glutamine, and asparagine was used to ascertain the in vivo inhibition of both enzymes. The tungstate effects were also checked by measuring leaf in vitro NR activity and NR protein content. Xylem exudation rate of detopped seedlings fed with KNO 3 decreased when the nitrate assimilation pathway was blocked either at the NR or at GS sites. This decrease was prevented when urea (acting as NH 4 + supply) was given simultaneously with tungstate. KNO 3 does not act directly on exudation, but through the involvement of NH 4 + . The involvement of glutamine was also shown since GS inhibition resulted in a cancellation of the enhancing effect of KNO 3 on exudation. As change of exudation rate was not linked to change in sap osmolarity, it is assumed that the assimilation chain could modify root water conductance. The role of glutamine was discussed.

Ammonia emission from young barley plants: influence of N source, light/dark cycles and inhibition of glutamine synthetase

Barley (Hordeum vulgare L. cv. Golf) plants were grown at two different relative addition rates ; 0.1 and 0.2 d -1 of nitrate. Three to five days before measurements started the plants were transferred to a nutrient solution with 2 mM nitrate or ammonium. The ammonium-grown plants showed increased ammonium levels in both shoots and roots and also increased ammonium concentrations in xylem sap. Ammonia emission measured in cuvettes connected to an automatic NH 3 monitor was close to zero for nitrate-grown plants but increased to 0.59 and 0.88 nmol NH 3 m -2 s -1 for plants transferred to ammonium after growing at RA = 0.2 and 0.1 d -1 , respectively. In darkness, NH 3 emission decreased together with photosynthesis and transpiration, but increased rapidly when the light was turned on again. Addition of 0.5 mM methionine sulphoximine (MSO) to the plants caused an almost complete inhibition of both root and shoot glutamine synthetase (GS) activity after 24 h. Ammonia emission increased dramatically and photosynthesis and transpiration decreased in both nitrate- and ammonium-grown plants as a result of the GS inhibition. At the same time plant tissue and xylem sap ammonium concentrations increased, indicating the importance of GS in controlling plant ammonium levels and thereby NH 3 emission from the leaves.

Purification and Properties of Insulin Receptor Ectodomain from Large-Scale Mammalian Cell Culture

Ectodomain of the exon 11+ form of the human insulin receptor (hIR) was expressed in the mammalian cell secretion vector pEE6.HCMV-GS, containing the glutamine synthetase gene. Following transfection of the hIR ectodomain gene into Chinese hamster ovary (CHO-K1) cells, clones were isolated by selecting for glutamine synthetase expression with methionine sulphoximine. The expression levels of ectodomain were subsequently increased by gene amplification. Production was scaled up using a 40-liter airlift fermenter in which the transfected CHO-K1 cells were cultured on microcarrier beads, initially in medium containing 10% fetal calf serum (FCS). By continuous perfusion of serum-free medium into the bioreactor, cell viability was maintained during reduction of FCS, which enabled soluble hIR ectodomain to be harvested for at least 22 days. Harvests were concentrated 20-fold by anion-exchange chromatography. Optimal recovery of ectodomain from early harvests containing large quantities of serum proteins was achieved by insulin-affinity chromatography, whereas in later harvests purification was achieved by multistep chromatography. Analysis of the purified hIR ectodomain showed that it had a molecular weight by sedimentation equilibrium analysis of 269,500. Amino-terminal amino acid sequence analysis showed that the ectodomain was correctly processed to α and β chains and that glycosylation characteristics were similar to those of native hIR. The integrity of the ectodomain was demonstrated by the recognition of conformation-dependent anti-hIR antibodies and by its binding of insulin (Kd∼2 × 10−9M). These results demonstrate the successful production and purification of hIR ectodomain by processes amenable to scale-up and in a form appropriate for structure/function studies of the ligand-binding domain of the receptor.

Metabolism of nitrate and ammonium in seedlings of Norway spruce (Picea abies) measured by in vivo 14N and 15N NMR spectroscopy

In vivo 15N and 14N nuclear magnetic resonance spectroscopy was used to investigate the assimilation of nitrate and ammonium in seedlings of Norway spruce (Picea abies [L.] Karst.). The main objective was to study accumulation of free NH+4 and examine to what extent the nitrogen source affects the composition of the free amino acid pools in roots, stems and needles. NH+4 concentrations in plants growing in the presence of 0.5–50 mM ammonium were quantified using 14N NMR. The NH+4 values in tissues ranged from 6 to 46 μmol (g fresh weight)−1. with highest concentrations in roots and needles. The tissue NH+4 peaked at 5.0 mM NH+4 in the medium. and failed to increase when NH+4 in the medium was increased to 50 mM, indicating metabolic control of the concentration of this cation in tissues. The 14N NMR spectra were used to estimate pH of the NH+4 storage pools. Based on the pH sensitivity of the quintet of 14NH+4 resonance, we suggest that the pH of the ammonium storage compartments in the roots and stems should be 3.7–3.8, and in needles 3.4–3.5, representing extremely low pH values of the tissue. 15N from nitrate or ammonium was first incorporated into the amide group of glutamine and then into α‐amino groups, confirming that the glutamine synthetase/ glutamate synthase cycle is the major route of nitrogen assimilation into amino acids and thus plays a role in lowering the levels of NH+4 in the cytoplasm. NH+4 can also be assimilated in roots in plants growing in darkness. The main 15N‐labelled amino acids were glutamine. arginine and alanine. Almost no 15N signals from needles were observed. Double labelling (δN + w, wN) of arginine is consistent with the operation of the ornithine cycle, and enrichment indicates that this cycle is a major sink of newly assimilated nitrogen. Nitrogen assimilation in roots in the presence of added methionine sulphoximine and glutamate indicated the catabolic action of glutamate dehydrogenase. The 15N NMR spectra of plants grown on 15N‐urea showed a marked increase in the labelling of ammonium and glutamine. indicating high urease activity. Amino acids were also quantified using high pressure liquid chromatography. Arginine was found to be an important transport form of nitrogen in the stem.

Ammonium ion-dependent isomerization of glutamate dehydrogenase in relation to glutamate synthesis in maize

The glutamate dehydrogenase [GDH, EC 1.4.1.2] from maize seeds that were germinated in the presence of different concentrations of NH 4Cl had charge isomers in the pH 5.8–7.5 range. In a three-phase process, different NH 4Cl concentrations sequentially altered the binomial distribution of the GDH isoenzyme population. The first phase of GDH isomerization was by low (< 2 mM); the second was by medium (2–58 mM); and the third was by high ( >58 mM) NH 4Cl concentrations. Ammonium ion isomerized the GDH by regulating the hexameric subunit assembly reaction. The isomerization was abolished by methionine sulphoximine. The GDH isoenzyme population distribution that was induced by 25 mM NH 4Cl closely approached the theoretical binomial distribution, and it also gave the highest GDH activity. An analysis of the free amino acids of maize seeds, and comparison of the activities of GDH and glutamate synthase (GOGAT), in each of the three phases of GDH isomerization, showed that GDH was 25% more efficient than GOGAT in the synthesis of l-Glu.

Ammonium Metabolism in the Green Hydra Symbiosis.

Inhibitors of enzymes of ammonium assimilation were used to test if assimilation of ammonium in the green hydra-Chlorella symbiosis was due to host or symbionts. Both methionine sulphoximine (MSX, an inhibitor of glutamine synthetase, found in both host and symbionts) and azaserine (AZS, an inhibitor of 2-oxoglutarate amido transferase, not found in the host) inhibited ammonium uptake by the intact symbiosis. MSX was taken up and caused predictable changes in pools of glutamate and glutamine in both freshly isolated symbionts and cultured ex-symbiotic Chlorella. However, after treatment of the intact symbiosis with MSX, no MSX was found in the symbiotic Chlorella, and glutamine and glutamate pools of both host and symbionts were unaffected. Although both MSX and AZS inhibited ammonium uptake by Chlorella, MSX caused seven times as much ammonium release from the intact symbiosis as did AZS. AZS treatment of the intact symbiosis caused an increase in glutamine pools in both host and symbionts, and AZS also competitively inhibited glutamine uptake by Chlorella. Further, ammonium treatment of intact hydra did not affect the nitrogen status of the algal symbionts, although it did cause a small increase in the number of algae in each digestive cell of the host. It is suggested that primary ammonium assimilation in the green hydra symbiosis occurs by means of animal glutamine synthetase, and that the resulting glutamine may be taken up and further processed by the symbiotic algae. Freshly isolated symbionts were able to process glutamine into glutamate even when incubated at low pH, which causes them to release a substantial proportion of fixed carbon as maltose.

N and C mineralization in soil amended with the N immobilization inhibitor, methionine sulphoximine

N and C mineralization in soil amended with the N immobilization inhibitor, methionine sulphoximine

Ammonia Volatilization in Relation to Nitrogen Nutrition of Young Braßica napus Plants Growing with Controlled Nitrogen Supply

Summary Brasica napus plants were grown for a period of up to 6 weeks in solution culture with exponentially increasing amounts of nitrate at three different relative addition rates. In some of the experiments nitrate was replaced by ammonium during the last 3 days before measurements of shoot gas exchange and root nitrogen absorption. The contents of nitrate, ammonium and total nitrogen in shoots and roots increased with increasing relative addition rates. Uptake rates of nitrate and ammonium at an external concentration of 1 mM decreased with increasing plant N status. Ammonia emision from the shoots was measured on-line by use of an automatic NH 3 monitor. The emision ranged from 0.1 to 1.7 nmol m -2 leaf surface s -1 and was higher in ammonium than in nitrate supplied plants. Inhibition of glutamine synthetase by addition of 0.5 mM methionine sulphoximine markedly increased the amount of ammonia emitted from the leaves, indicating the importance of this enzyme in controlling the ammonia concentration in plant tisues.

Enzymes of ammonium metabolism in ectendomycorrhizal and ectomycorrhizal symbionts of pine

Ammonium assimilation enzymes from several strains of ectendo‐ and ectomycorrhizal fungi were assayed after three weeks culture on a buffered synthetic medium containing ammonium as sole nitrogen source. Activity of NADP‐dependent glutamate dehydrogenase (GDH, EC 1.4.1.4) of ectomycorrhizal strains was very low despite excellent mycelial growth. Only ectendomycorrhizal fungus MrgX isolated from roots of Pinus sylvestris showed high GDH activity. Similar results were obtained when the enzyme extracts were subjected to starch gel electrophoresis. Growth of the fungi, except ectendomycorrhizal MrgX, was arrested when inhibitors of glutamine synthetase (GS, EC 6.3.1.2) or glutamate synthase (GOGAT. EC 1.4.7.1) (methionine sulphoximine or albizine, respectively) were included in the culture medium. Glutamine synthetase activity was found in all fungi tested. The results suggest that the GS pathway for ammonium assimilation is potentially operative in ectomycorrhizal fungi and imply only a minor role for GDH in ammonium assimilation by the studied ectomycorrhizal symbionts of pine. Some physiological and ecological implications of these results are discussed.

Effects of methionine sulphoximine treatment on renal amino acid and ammonia metabolism in rats.

Renal glutamine metabolism in relation to ammoniagenesis has been extensively studied during chronic metabolic acidosis, when arterial glutamine levels are reduced. However, little is known about the effects of reduced glutamine delivery on renal glutamine and ammonia metabolism at physiological systemic pH values. Therefore, a model of decreased arterial glutamine concentrations at normal pH values was developed using methionine sulphoximine (MSO). Renal glutamine and ammonia metabolism was measured by determining fluxes and intracellular concentrations after an overnight fast in ether anaesthetized normal rats, MSO-treated rats and their pair-fed controls. Moreover, fluxes and intracellular concentrations of several other amino acids were determined concomitantly. After 2 and 4 days of MSO treatment, arterial glutamine concentrations were reduced to 55%, while arterial ammonia concentrations increased by 70%. Kidney glutamine uptake reduced, but systemic pH was unchanged. Fractional extraction of glutamine remained unchanged, suggesting that also in vivo net uptake of glutamine by the kidney at subnormal levels is related to arterial glutamine concentrations. As a result, at day 2 but not at day 4, the kidney reduced the net release of ammonia into the renal vein and thus reduced net renal ammonia addition to body ammonia pools. Therefore at day 2, the kidney seems to play an important role in adaptation to both hyperammonaemia and hypoglutaminaemia.

Inward and outward transport of ammonium in roots of maize and sorghum: contrasting effects of methionine sulphoximine

To examine the adaptive responses of ammonium efflux of nitrate-grown plants during first exposure to ammonium nutrition, isotopic flows of ammonium in and out of root tissue of maize (Zea mays L.) and sorghum (Sorghum bicolor L.) were delineated. Appreciable efflux of 14 N-ammonium occurred during the first 2 min of exposure, suggesting that ammonium efflux is characteristic of plants grown with nitrate as the sole source of nitrogen. Treatment with methionine sulphoximine (MSX) enhanced the initial 14 N-ammonium efflux 5-fold in maize but less than 2-fold in sorghum. Increasing ambient acidity from pH 6 to pH 4 had relatively little effect on 14 N-ammonium efflux in either species