Inducible Accumulation of mRNA for NADH-Dependent Glutamate Synthase in Rice Roots in Response to Ammonium Ions

After growth for 26 d in water, rice seedlings were transferredto a medium containing 1 mM NH⁴Cl. The mRNA for NADH-glutamatesynthase was markedly increased in the roots within 3 h. Methioninesulfoximine completely inhibited this accumulation, suggestingthat NH⁴⁺ is not a direct inducer of this process. (Received July 11, 1997; Accepted September 10, 1997)     

Glutamate Dehydrogenase Is Not Essential for Glutamate Formation by Corynebacterium glutamicum

Two Corynebacterium glutamicum strains, one being glutamate dehydrogenase (GDH) negative and the other possessing 11-fold-higher specific GDH activity than the parental wild type, were constructed and used to analyze the role of GDH in C. glutamicum. The results indicate (i) that GDH is dispensable for glutamate synthesis required for growth and (ii) that although a high level of GDH increases the intracellular glutamate pool, the level of GDH has no influence on glutamate secretion.     

Glutamate Dehydrogenase Activity in Lemna perpusilla 6746: The Effects of Sucrose,Glucose and Fructose Addition to Growth Media

Lemna perpusilla Torr, strain 6746 clones were maintained under conditions of continuous illumination with various concentrations of sucrose, glucose or fructose added to the growth, medium. After two weeks of growth, plants were harvested and either assayed for total glutamate dehydrogenase activity or fractionated into one chloroplast-rich and one mitochondria-rich preparation and then assayed for glutamate dehydrogenase activity. In all assays for glutamate dehydrogenase it was necessary to add bovine serum albumin to the extraction medium in order to obtain sufficient enzyme activity for accurate and reproducible results. The presence of sucrose in the growth medium reduced glutamate dehydrogenase activity in all studies. When samples containing intact organelles were assayed, sucrose inhibition of activity appeared to occur primarily in the chloroplast fraction. Glucose, on the other hand, increased glutamate dehydrogenase activity in the chloroplast-rich fractions. Upon freeze-thawing differences between the various treatments were less obvious. The results from these studies indicate possible differences in sugar uptake and/or utilization in Lemna perpusilla.     

The Role of Glutamine Synthetase and Glutamate Dehydrogenase in Cerebral Ammonia Homeostasis

In the brain, glutamine synthetase (GS), which is located predominantly in astrocytes, is largely responsible for the removal of both blood-derived and metabolically generated ammonia. Thus, studies with [¹³N]ammonia have shown that about 25?% of blood-derived ammonia is removed in a single pass through the rat brain and that this ammonia is incorporated primarily into glutamine (amide) in astrocytes. Major pathways for cerebral ammonia generation include the glutaminase reaction and the glutamate dehydrogenase (GDH) reaction. The equilibrium position of the GDH-catalyzed reaction in vitro favors reductive amination of α-ketoglutarate at pH 7.4. Nevertheless, only a small amount of label derived from [¹³N]ammonia in rat brain is incorporated into glutamate and the α-amine of glutamine in vivo. Most likely the cerebral GDH reaction is drawn normally in the direction of glutamate oxidation (ammonia production) by rapid removal of ammonia as glutamine. Linkage of glutamate/α-ketoglutarate-utilizing aminotransferases with the GDH reaction channels excess amino acid nitrogen toward ammonia for glutamine synthesis. At high ammonia levels and/or when GS is inhibited the GDH reaction coupled with glutamate/α-ketoglutarate-linked aminotransferases may, however, promote the flow of ammonia nitrogen toward synthesis of amino acids. Preliminary evidence suggests an important role for the purine nucleotide cycle (PNC) as an additional source of ammonia in neurons (Net reaction: l-Aspartate?+?GTP?+?H₂O?→?Fumarate?+?GDP?+?P_i?+?NH₃) and in the beat cycle of ependyma cilia. The link of the PNC to aminotransferases and GDH/GS and its role in cerebral nitrogen metabolism under both normal and pathological (e.g. hyperammonemic encephalopathy) conditions should be a productive area for future research.     

The Regulation of the Synthesis of Glutamate Dehydrogenase in Excised Pea Roots by ExogenousL- aspartic andL- glutamic Acids

Exogenous L-aspartic and L-glutamic acids enhance glutamate dehydrogenase activity in isolated pea roots. The results obtained indicate that both ammo acids induce increased GDH synthesis.     

Histophotometric Evaluation of Glutamate Dehydrogenase Activity of the Rat Hippocampal Formation During Postnatal Development,with Special Reference to the Glutamate Transmitter Metabolism

Transmitter glutamate/aspartate synthesis is known to proceed along different metabolic pathways. In this light, the functional relevance of glutamate dehydrogenase in postnatally maturing glutamatergic/aspartatergic structures was studied by means of quantitative enzyme histochemistry. The basic requirements concerning the kinetics and calibration of the histochemical glutamate dehydrogenase reaction used were proved to be met in order to obtain valid quantitative data. The histochemically demonstrable activity of glutamate dehydrogenase (EC 1.4.1.3) in the hippocampal formation of the rat increased markedly during postnatal development. On day 30, the distribution pattern observed was similar to that in adult animals. While the enzyme activity rose within cell body layers from day 0 to day 30 by 240-285%, the increase in neuropil layers was found to be up to 830%. Maximum values were seen in the stratum lacunosum-moleculare of CA1 and CA3 and the stratum moleculare of the dentate fascia on day 30. Since the hippocampal neuropil is supposed to be copiously provided with glutamatergic (and aspartatergic?) structures which become functional in rats during the first weeks of postnatal life, the increase in enzyme activity is discussed to be primarily a consequence of maturing synaptic systems using glutamate and/or aspartate as transmitters.     

植物根内通气组织形成机理的研究进展

植物能否在湿地或淹涝环境中生长, 很大程度上取决于植物是否具有健全发达的通气组织。在结合形态学和分子生物学等方面研究的基础上, 概述了植物根内通气组织的形成过程, 主要涉及生理功能、诱导因子和相关酶等, 推测细胞程序性死亡是溶生性通气组织形成的机理, 乙烯在整体信号转导网络中起关键性中介作用。     

植物根内通气组织形成的研究进展

植物能否在湿地或淹涝环境中生长,很大程度上取决于植物是否具有健全发达的通气组织。在结合形态学和分子生物学等方面研究的基础上,概述了植物根内通气组织的形成过程,主要涉及生理功能、诱导因子和相关酶等,推测细胞程序性死亡是溶生性通气组织形成的机理,乙烯在整体信号转导网络中起关键性中介作用。     

The Regulation of Nitrate Reductase,Nitrite Reductase,and Glutamate Dehydrogenase in Excised Pea Roots by Some Exogenous Amino Acids

The effect of growth retarding amino acids (L-aspartic acid,L-leucine,L-methionine, andL-threonine) on nitrate reductase (NO₃R), nitrite reductase (NO₂R), and NADH₂ dependent glutamate dehydrogenase (GDH) in excised pea roots was followed,L-methionine andL-threonine slightly depressed NO₃R activity,L-aspartic acid enhanced NO₂R and GDH activities.L-methionine andL-threonine also slightly decreased nitrate uptake. The results obtained are discussed in connection with the growth effects of the amino acids investigated.     

Nitrate Reductase and Glucose-6-phosphate Dehydrogenase Activities as Influenced by Leaf Age and Addition of Protein to Extraction Media

Lea fage influences the level of activity and the decay rate of the enzymes nitrate reductase (NR: E.C. 1.6.6.1) and glucose-6-phosphate dehydrogenase (G6PD; E.C. 1.1.1.49) extracted with and without protein in the extraction media. Such influence was determined in three plant species: corn (Zea mays L.), oats (Arena sativa L.), and tobacco (Nicotiana tabacum L.). Leaves of different ages were obtained from plants of various ages, or by removal of leaf blades from different positions on a single shoot. NR activity (per g fresh weight) declined as leaves of these plant species became older, especially when extraction was effected with conventional media (i.e. without added protein). The instability of NR in these extracts increased as leaves became older. Decay of NR in vitro was exponential with time. Addition of protein [3 %(w/v) casein or bovine serum albumin] to extraction media markedly increased the level of NR activity and its stability, especially in older leaves of all three plant species. Addition of protein did not affect the level of G6PD extracted from corn leaf blades, but slightly enhanced its activity in extracts from the oldest leaf blade of oats. G6PD activity also declined as leaf blades of corn and oats became older.     

Regional Development of Glutamate Dehydrogenase in the at Brain

The development of glutamate dehydrogenase enzyme activity in rat brain regions has been followed from the late foetal stage to the adult and through to the aged (greater than 2 years) adult. In the adult brain the enzyme activity was greatest in the medulla oblongata and pons greater than midbrain = hypothalamus greater than cerebellum = striatum = cortex. In the aged adult brain, glutamate dehydrogenase activity was significantly lower in the medulla oblongata and pons when compared to the 90-day-old adult value, but not in other regions. The enzyme-specific activity of nonsynaptic (free) mitochondria purified from the medulla oblongata and pons of 90-day-old animals was about twice that of mitochondria purified from the striatum and the cortex. The specific activity of the enzyme in synaptic mitochondria purified from the above three brain regions, however, remained almost constant.     

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 CO₂ 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, ¹⁵NH₄Cl was injected into the central pith of stem sections. The kinetics of ¹⁵N 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 ¹⁵N derived from ¹⁵NH₄Cl was almost completely inhibited. Injections of amido-¹⁵N glutamine demonstrated a potential for ¹⁵N 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.     

Cost and Advantage of Soil Channel Formation by Contractile Roots in Successful Plant Movement

In most cases, contractile roots not only produce a pullingforce on an underground plant (bulb, corm), but also push awaythe substratum and create a soil channel in which plant movementis made easier. Two specially-constructed experimental set-upsallowed the pulling and pushing activity of contractile rootsin underground plant movement to be measured under identicalstandard conditions. Our results show that root channel formationis always advantageous for the accomplishment of movement. Theextent of channel formation in relation to the size of the bulbor corm being moved was calculated as a percentage channel effect.A value of 100% indicates a channel effect sufficient to allowplant movement with no resistance. For smaller distances moved(20-30 mm) a small channel effect (10-40%) was found to be veryefficient. For more extensive movement (> 50 mm) a 100% channeleffect seems to be the optimum energetically. These systemsof movement commonly exist in nature.Copyright 1995, 1999 AcademicPress Contractile root, root contraction, soil channel, channel effect, plant movement     

Formation of Glycerol Dehydrogenase by Microorganisms

The distribution of glycerol dehydrogenase activity was studied with cell-free extracts of bacteria, yeasts, molds and actinomycetes. High activity was found in 4 strains of bacteria and in 3 strains of molds. The enzymes of bacteria were dependent on NAD⁺ and those of molds were dependent on NADP⁺. An isolated gram-positive bacterium, which showed the high activity, was identified as Cellulomonas sp. NT3060. The total and specific activities were associated with growth of this strain and reached the maximum at the early stationary phase. Significant high level activity was detected in cell-free extracts from glycerol and glucose media.     

Alcohol Dehydrogenase Activity in Relation to Flooding Tolerance in Roots

The ability of a number of plants to grow under conditions ofexperimental flooding has been examined. There was an increasein ethanol production under anaerobic conditions in those specieswhose growth was reduced by flooding. The period of floodinginduced a marked increase in alcohol dehydrogenase activityof the roots of these plants. Plants not adversely affectedby flooding showed no increase in ethanol production and noinduction of alcohol dehydrogenase activity. It is suggestedthat species in which such activation occurs are excluded fromwet areas because of the accumulation of toxic quantities ofethanol.     

Intertissue Differences for the Role of Glutamate Dehydrogenase in Metabolism

The enzyme glutamate dehydrogenase (GDH) plays an important role in integrating mitochondrial metabolism of amino acids and ammonia. Glutamate may function as a respiratory substrate in the oxidative deamination direction of GDH, which also yields α-ketoglutarate. In the reductive amination direction GDH produces glutamate, which can then be used for other cellular needs such as amino acid synthesis via transamination. The production or removal of ammonia by GDH is also an important consequence of flux through this enzyme. However, the abundance and role of GDH in cellular metabolism varies by tissue. Here we discuss the different roles the house-keeping form of GDH has in major organs of the body and how GDH may be important to regulating aspects of intermediary metabolism. The near-equilibrium poise of GDH in liver and controversy over cofactor specificity and regulation is discussed, as well as, the role of GDH in regulation of renal ammoniagenesis, and the possible importance of GDH activity in the release of nitrogen carriers by the small intestine.     

Immunocytochemical Characterization of Glutamate Dehydrogenase in the Cerebellum of the Rat

The immunocytochemical distribution of glutamate dehydrogenase was studied in the cerebellum of the rat using antibodies made in rabbit and guinea pig against antigen purified from bovine liver. Antiserum was found to block partially enzymatic activity both of the purified enzyme and of extracts of the rat cerebellum. Using immunoblots of proteins of rat cerebellum, a major immunoreactive protein and several minor immunoreactive proteins were detected with antiserum. Only a single immunoreactive protein was detected using affinity-purified antibody preparations. This protein migrates with a molecular weight identical to that of the subunit of glutamate dehydrogenase. Further evidence that the antibodies were selective for glutamate dehydrogenase in rat cerebellum was obtained through peptide mapping. Purified glutamate dehydrogenase and the immunoreactive protein from rat cerebellum generated similar patterns of immunoreactive peptides. No significant cross-reaction was observed with glutamine synthetase. Immunocytochemistry was done on cryostat- and Vibratome-cut sections of the cerebellum of rats that had been perfused with cold 4% paraformaldehyde. Glial cells were found to be the most immunoreactive structures throughout the cerebellum. Most apparent was the intense labeling of Bergmann glial cell bodies and fibers. In the granule cell layer, heavy labeling of astrocytes was seen. Purkinje and granule cell bodies were only lightly immunoreactive, whereas stellate, basket, and Golgi cells were unlabeled. Labeling of presynaptic terminals was not apparent. These findings suggest that glutamate dehydrogenase, like glutamine synthetase, is enriched in glia relative to neurons.     

Glutamate Dehydrogenase Activity in Roots: Distribution in a Seedling and Storage Root, and the Effects of Red and Far-red Illuminations

Pisum seedling and Pastinaca storage roots contained high glutanrate dehydrogenase (GDH) activity in areas of reported rapid growth and high phytoctrome content. A similar distribution was observed for malate dehydrogenase. Freeze-thawings of mitochondrial preparations from Pisum roots always resulted in increases of GDH specific activity; however, the observed increases were much larger with basal than apical sections. Both intact and freeze-thawed mitochondrial preparations from seedling roots exhibited increases in GDH activity with time after isolation. In intact mitochondrial preparations from roots of etiolated seedlings, an increase in malate dehydrogenase activity was observed similar to that of GDH activity; however, no increased malate dehydrogenase activity was noted in preparations from light-grown seedlings. Illuminating Pisum seedlings with far-red light slowly increased GDH activity in roots over a period of two weeks. Since these observed increases were not due to direct exposure of roots to light, other factors were likely involved.