Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: CAS:6893-26-1 (glutamate)
 73,096 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Pseudomonas aeruginosa has a pair of distinct ornithine carbamoyltransferases. The anabolic ornithine carbamoyltransferase encoded by the argF gene catalyzes the formation of citrulline from ornithine and carbamoylphosphate. The catabolic ornithine carbamoyltransferase encoded by the arcB gene promotes the reverse reaction in vivo; although this enzyme can be assayed in vitro for citrulline synthesis, its unidirectionality in vivo is determined by its high concentration at half maximum velocity for carbamoylphosphate ([S]0.5) and high cooperativity toward this substrate. We have isolated mutant forms of catabolic ornithine carbamoyltransferase catalyzing the anabolic reaction in vivo. The corresponding arcB mutant alleles on a multicopy plasmid specifically suppressed an argF mutation of P. aeruginosa. Two new mutant enzymes were obtained. When methionine 321 was replaced by isoleucine, the mutant enzyme showed loss of homotropic cooperativity at physiological carbamoylphosphate concentrations. Substitution of glutamate 105 by lysine resulted in a partial loss of the sigmoidal response to increasing carbamoylphosphate concentrations. However, both mutant enzymes were still sensitive to the allosteric activator AMP and to the inhibitor spermidine. These results indicate that at least two residues of catabolic ornithine carbamoyltransferase are critically involved in positive carbamoylphosphate cooperativity: glutamate 105 (previously known to be important) and methionine 321. Mutational changes in either amino acid will affect the geometry of helix H2, which contains several residues required for carbamoylphosphate binding.
 ...
PMID:Methionine-321 in the C-terminal alpha-helix of catabolic ornithine carbamoyltransferase from Pseudomonas aeruginosa is important for positive homotropic cooperativity. 785 49

The substitution of alanine for glutamate at position 105 (E105A) of the allosteric ornithine carbamoyltransferase (OTCase) of Pseudomonas aeruginosa abolishes the carbamoylphosphate (CP) cooperativity observed in the wild-type enzyme. A kinetic analysis of [E105A]OTCase was performed in order to determine the mechanism of the reaction. The results of initial velocity and inhibition studies are consistent with an ordered mechanism with CP as the first substrate to add to the enzyme. In addition, similar studies have been made using the wild-type enzyme in the presence of the activator, phosphate. The results are similar to those obtained with [E105A]OTCase indicating that the residue E105 is critical for the allosteric transition of the wild-type enzyme. The activities of the wild-type allosteric OTCase and of [E105A]OTCase have been studied in the pH range 5.8-8.2 in the absence and in the presence of positive and negative effectors. The sigmoid saturation of OTCases by CP has been analyzed according to the Hill equation. At low pH values, CP cooperativity is low in the wild-type enzyme but cooperativity and [S]CP0.5 values increase markedly with pH. For [E105A]OTCase, the saturation by CP is hyperbolic at all pH values; in this modified enzyme, the presence of spermidine, an allosteric inhibitor of the wild-type enzyme, results in an inhibition which induces CP cooperativity. Thus, the ionization of the residue E105 apparently results in a conformational change in the wild-type enzyme which modifies the catalytic site. Since the [E105A] enzyme retains the heterotropic effects of the wild-type enzyme, other structural features are required for the allosteric transition in the wild-type catabolic OTCase.
 ...
PMID:Steady-state kinetics and analysis of pH dependence on wild-type and a modified allosteric Pseudomonas aeruginosa ornithine carbamoyltransferase containing the replacement of glutamate 105 by alanine. 810 5

The synthesis of citrulline from glutamine was quantified in enterocytes from pre-weaning (14-21 days old) and post-weaning (29-58 days old) pigs. The cells were incubated at 37 degrees C for 30 min in Krebs-Henseleit bicarbonate buffer (pH 7.4) containing 0, 0.5, 2 and 5 mM glutamine. Oxygen consumption was linear during the 30 min incubation period. The rates of citrulline synthesis were low or negligible in enterocytes from 14-21-day-old pigs, but increased 10-20-fold in the cells from 29-58-day-old pigs. This marked elevation of citrulline synthesis coincided with an increase in the activity of pyrroline-5-carboxylate synthase with the animal's post-weaning growth. In contrast, decreases in the activities of phosphate-dependent glutaminase, ornithine aminotransferase, ornithine carbamoyltransferase and carbamoyl-phosphate synthase were observed as the age of the pigs increased. The concentrations of carbamoyl phosphate in enterocytes from pre-weaning pigs were higher than, or similar to, those in the cells from post-weaning pigs. It is possible that the low rate of citrulline synthesis from glutamine in enterocytes from pre-weaning pigs was due to a limited availability of ornithine, rather than that of carbamoyl phosphate. We suggest that this limited availability of ornithine in pre-weaning-pig enterocytes results from (i) the low rate of pyrroline-5-carboxylate synthesis from glutamate, due to the low activity of pyrroline-5-carboxylate synthase, and (ii) the competitive conversion of pyrroline-5-carboxylate into proline. Our present findings on the developmental aspect of citrulline synthesis in pig enterocytes may offer a biochemical mechanism for the previous observations that arginine is a nutritionally essential amino acid for suckling piglets, but not for adult pigs.
 ...
PMID:Synthesis of citrulline from glutamine in pig enterocytes. 816 28

The sparse-fur (spf) mutant mouse has an X-linked deficiency of ornithine transcarbamylase and develops congenital hyperammonemia similar to that seen in human patients. We studied the effect of sodium benzoate (2.5, 5 and 10 mmol/kg body wt) on ammonia, glutamine and glutamate, as well as various intermediates of energy metabolism in brain and liver of normal CD-1/Y and hyperammonemic spf/Y mice. The ammonia concentration of brain was decreased with 2.5 mmol sodium benzoate in spf/Y mice, whereas higher doses resulted in a significant increase in both liver and brain. Cerebral glutamine content decreased generally in a dose-dependent manner, both in normal and affected mice, following treatment with various doses of sodium benzoate. Cerebral glutamate concentrations were increased only in spf mice treated with sodium benzoate, whereas ATP and acetyl CoA were decreased (P < 0.001), in both normal and affected mice, indicating that glutamine synthesis may be affected by ATP availability. Free CoA levels were decreased (P < 0.05) only in liver in both groups of treated mice, whereas pyruvate concentrations were elevated (P < 0.05) in affected mice following sodium benzoate administration. The results demonstrate that a dose of 2.5 mmol sodium benzoate/kg body wt has a beneficial effect in reducing cerebral ammonia with a concomitant decrease in glutamine. However, the results suggest that many of the metabolite changes observed following higher doses of benzoate could be due to depletion of ATP, free CoA and acetyl CoA levels, possibly secondary to benzoyl CoA accumulation. The response of the spf/Y mouse to sodium benzoate was different from that of the control CD-1/Y mouse, which could be due to its urea cycle dysfunction and a chronic hyperammonemic state. Hence, the spf/Y mouse may be the ideal animal model for studying the pharmacology of sodium benzoate in hyperammonemic disorders at both the cerebral and hepatic levels.
 ...
PMID:Effect of sodium benzoate on cerebral and hepatic energy metabolites in spf mice with congenital hyperammonemia. 842 7

Alterations of excitatory amino acid neurotransmitters have previously been described in brain in congenital ornithine transcarbamylase (OTC) deficiency. In order to further elucidate the role of the glutamatergic neurotransmitter system in OTC deficiency, densities of binding sites for [3H]MK801, an NMDA receptor antagonist ligand were measured by quantitative receptor autoradiography in the brains of chronically hyperammonemic sparse-fur mice (spf), mutant mice with a congenital defect of OTC. [3H]MK801 binding site densities were significantly reduced by up to 57% (p < 0.01) in 16 out of 17 brain regions of OTC-deficient mice. Such changes could result from either neuronal cell loss in these animals or from "down-regulation" of these sites as a consequence of exposure to increased extracellular concentrations of glutamate or quinolinic acid, two known endogenous NMDA receptor ligands previously found to be increased in brain in chronic hyperammonemic syndromes. Reduced NMDA receptor densities in congenital OTC deficiency could represent an adaptive mechanism of protection against further excitotoxic brain injury.
 ...
PMID:Loss of [3H]MK801 binding sites in brain in congenital ornithine transcarbamylase deficiency. 883 Feb 85

Carbamoyl phosphate synthetase I (CPSase I) catalyzes the first reaction of the urea cycle in mammalian ureotelic species. The positive allosteric cofactor N-acetyl-L-glutamate (AGA) is required for CPSase I activity and is important for regulation of the urea cycle. A similar enzyme, CPSase III, catalyzes this reaction in fish; CPSase III differs from CPSase I in that it utilizes glutamine as the nitrogen-donating substrate instead of ammonia. AGA also stimulates the CPSase III-catalyzed reaction, but is not absolutely required for activity if the glutamine concentration is high. There has been no report of the presence or function of AGA in fish. Here we report that AGA is present in those species and tissues of fish that have significant levels of CPSase III and urea cycle activity; the levels of AGA were higher in liver of adult gulf toadfish (Opsanus beta) and spiny dogfish shark (Squalus acanthias), both of which have high CPSase III activity, than in bass (Micropterus salmoides) or trout (Oncorhynchus mykiss), which have much lower or no CPSase III activity, respectively. In the toadfish the levels of AGA in liver and muscle tissue were considerably higher in the fed than in the fasting state, as is observed in mammalian species; in liver, but not in muscle, the level of AGA increased when the toadfish were confined (stressed), which has been shown to induce a ureotelic response. Toadfish muscle had CPSase III and ornithine carbamoyltransferase activities; the increase in AGA concentration in muscle when fed suggests that the presence of these first two enzymes of the urea cycle in muscle may be physiologically significant. The results indicate that the fish investigated have physiologically significant levels of AGA and that the levels correlate with parameters related to urea cycle activity.
 ...
PMID:N-acetyl-L-glutamate and the urea cycle in gulf toadfish (Opsanus beta) and other fish. 946 20

The allosteric catabolic ornithine carbamoyltransferase (OTCase) from Pseudomonas aeruginosa, a dodecamer build up of four trimers of identical subunits, shows strong carbamoylphosphate homotropic co-operativity. Its activity is allosterically inhibited by spermidine and activated by AMP. Modified forms of the enzyme exhibiting substantial alterations in both homotropic and heterotropic interactions were recently obtained. We report here the first detailed kinetic characterization of homotropic and heterotropic modulations in allosteric wild-type and in engineered OTCases. Homotropic co-operativity for the saturation either by citrulline or arsenate was also observed when arsenate was utilised as an alternate substrate of the reverse reaction. Amino acid substitution of glutamate 105 by a glycine produces an enzyme devoid of homotropic interactions between the catalytic sites for carbamoylphosphate. This mutant, which is blocked in an active conformation, is still sensitive to the allosteric effector AMP, which increases affinity with respect to the substrate, carbamoylphosphate. It is also observed that homotropic co-operative interactions do not reappear in the E105G enzyme upon strong inhibition by the allosteric inhibitor of the wild-type enzyme, spermidine.Replacement of residues 34 to 101 of the native enzyme by the homologous amino acids of anabolic Escherichia coli OTCase produces a trimeric enzyme which retains reduced homotropic co-operativity. Activation by AMP and inhibition by spermidine of this chimaeric OTCase do not affect carbamoylphosphate homotropic co-operativity. AMP acts by reducing the concentration of substrate at half maximum velocity while spermidine acts in the inverse way. These observations indicate that in the two mutant forms of OTCase, homotropic and heterotropic interactions can be uncoupled and therefore must involve different molecular mechanisms. Furthermore, the results of stimulation of enzyme activity by phosphate, arsenate, pyrophosphate and phosphonoacetyl-l-ornithine on wild-type and mutant OTCases suggest that the physiological substrate phosphate, besides acting at the catalytic site, may act at an allosteric site. On the other hand, pyrophosphate and phosphonoacetyl-l-ornithine activation results exclusively from interactions of this effector with the active site residues.
 ...
PMID:Allosteric regulation in Pseudomonas aeruginosa catabolic ornithine carbamoyltransferase revisited: association of concerted homotropic cooperative interactions and local heterotropic effects. 978 77

Cells of the unicellular cyanobacterium Synechocystis sp. strain PCC 6803 supplemented with micromolar concentrations of L-[(14)C]arginine took up, concentrated, and catabolized this amino acid. Metabolism of L-[(14)C]arginine generated a set of labeled amino acids that included argininosuccinate, citrulline, glutamate, glutamine, ornithine, and proline. Production of [(14)C]ornithine preceded that of [(14)C]citrulline, and the patterns of labeled amino acids were similar in cells incubated with L-[(14)C]ornithine, suggesting that the reaction of arginase, rendering ornithine and urea, is the main initial step in arginine catabolism. Ornithine followed two metabolic pathways: (i) conversion into citrulline, catalyzed by ornithine carbamoyltransferase, and then, with incorporation of aspartate, conversion into argininosuccinate, in a sort of urea cycle, and (ii) a sort of arginase pathway rendering glutamate (and glutamine) via Delta(1)pyrroline-5-carboxylate and proline. Consistently with the proposed metabolic scheme (i) an argF (ornithine carbamoyltransferase) insertional mutant was impaired in the production of [(14)C]citrulline from [(14)C]arginine; (ii) a proC (Delta(1)pyrroline-5-carboxylate reductase) insertional mutant was impaired in the production of [(14)C]proline, [(14)C]glutamate, and [(14)C]glutamine from [(14)C]arginine or [(14)C]ornithine; and (iii) a putA (proline oxidase) insertional mutant did not produce [(14)C]glutamate from L-[(14)C]arginine, L-[(14)C]ornithine, or L-[(14)C]proline. Mutation of two open reading frames (sll0228 and sll1077) putatively encoding proteins homologous to arginase indicated, however, that none of these proteins was responsible for the arginase activity detected in this cyanobacterium, and mutation of argD (N-acetylornithine aminotransferase) suggested that this transaminase is not important in the production of Delta(1)pyrroline-5-carboxylate from ornithine. The metabolic pathways proposed to explain [(14)C]arginine catabolism also provide a rationale for understanding how nitrogen is made available to the cell after mobilization of cyanophycin [multi-L-arginyl-poly(L-aspartic acid)], a reserve material unique to cyanobacteria.
 ...
PMID:Arginine catabolism in the cyanobacterium Synechocystis sp. Strain PCC 6803 involves the urea cycle and arginase pathway. 1064 27

In the arginine biosynthetic pathway of the vast majority of prokaryotes, the formation of ornithine is catalyzed by an enzyme transferring the acetyl group of N-alpha-acetylornithine to glutamate (ornithine acetyltransferase [OATase]) (argJ encoded). Only two exceptions had been reported-the Enterobacteriaceae and Myxococcus xanthus (members of the gamma and delta groups of the class Proteobacteria, respectively)-in which ornithine is produced from N-alpha-acetylornithine by a deacylase, acetylornithinase (AOase) (argE encoded). We have investigated the gene-enzyme relationship in the arginine regulons of two psychrophilic Moritella strains belonging to the Vibrionaceae, a family phylogenetically related to the Enterobacteriaceae. Most of the arg genes were found to be clustered in one continuous sequence divergently transcribed in two wings, argE and argCBFGH(A) ["H(A)" indicates that the argininosuccinase gene consists of a part homologous to known argH sequences and of a 3' extension able to complement an Escherichia coli mutant deficient in the argA gene, encoding N-alpha-acetylglutamate synthetase, the first enzyme committed to the pathway]. Phylogenetic evidence suggests that this new clustering pattern arose in an ancestor common to Vibrionaceae and Enterobacteriaceae, where OATase was lost and replaced by a deacylase. The AOase and ornithine carbamoyltransferase of these psychrophilic strains both display distinctly cold-adapted activity profiles, providing the first cold-active examples of such enzymes.
 ...
PMID:Evolution of arginine biosynthesis in the bacterial domain: novel gene-enzyme relationships from psychrophilic Moritella strains (Vibrionaceae) and evolutionary significance of N-alpha-acetyl ornithinase. 1069 66

We previously reported that alpha-motor neurons in organotypic cultures of rat spinal cord (OTC-SC) are resistant to excitotoxicity induced through NMDA receptors. Here we describe the effects of non-NMDA glutamate receptor agonists kainic acid (KA) and quisqualic acid (QUIS) on motor neurons in OTC-SC. Large ventral horn acetylcholinesterase-positive neurons (VHANs), most of which are motor neurons, were quite sensitive to QUIS and KA toxicity and displayed losses of 95% and 94%, respectively. Small VHANs were reduced by 41% and 61% only. Identical results were obtained in cultures stained for non-phosphorylated neurofilaments. These observations demonstrate that alpha-motor neurons are considerably more sensitive to KA and QUIS than to NMDA toxicity. The proposed excitotoxic mechanism of ALS, therefore, is most likely mediated through non-NMDA glutamate receptors.
 ...
PMID:Selective vulnerability of spinal cord motor neurons to non-NMDA toxicity. 1079 Aug 92


<< Previous 1 2 3 4 5 6 Next >>