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PubMedDate not supplied

Metabolic response of muscle to alanine, glutamine, and valine supplementation during severe illness.

BACKGROUND: Alanine and glutamine are released from muscle in response to critical illness. Subsequent depletion of glutamine from muscle is proposed as a principal factor in the limitation of muscle protein synthesis in severely ill patients. The objective of this study was to assess the peripheral metabolic response to enteral supplementation of alanine, glutamine, and valine in critically ill patients. METHODS: Isotopic tracers of alanine, glutamine, and phenylalanine were given IV to 6 critically ill patients and 6 healthy volunteers. Blood sampling from the femoral artery and vein along with muscle biopsies provided assessment of leg (ie, muscle) kinetics. Measurements were obtained during enteral nutrition alone and then with combined alanine (11.25 g), glutamine (7.5 g) and valine (11.25 g) supplementation for 3 hours. RESULTS: Compared with healthy volunteers, critically ill patients had significantly reduced concentrations of alanine and glutamine in arterial plasma (p < .05), which increased significantly with amino acid supplementation. Muscle glutamine concentrations were significantly less in the patients and were not significantly affected by supplementation. Alanine and glutamine transport into and out of muscle and the rates of alanine and glutamine incorporation into and production from muscle were not affected by supplementation. Phenylalanine kinetics, as a marker of muscle protein metabolism, were not significantly altered by alanine, glutamine, and valine intake. CONCLUSIONS: These results demonstrate that alanine, glutamine, and valine administration fails to significantly affect muscle glutamine availability or muscle protein metabolism. These findings suggest that accelerated muscle catabolism in critically ill patients is not in response to any deficiency in alanine or glutamine availability.

Adult↗
PubMed2005

Effects of tryptophan and valine administration on behavioral pharmacology of haloperidol.

Development of antipsychotics with slight/no extra-pyramidal symptoms (EPS) and/or other side effects is one of the exploring fields of drug research. Haloperidol is a high potency typical neuroleptic used in the treatment of schizophrenia but produces muscles related side effects commonly known as EPS. These effects are not produced following the administration of atypical neuroleptics such as clozapine. A severe side effect of clozapine treatment is however, agranulocytosis. This involves investigation on the mechanism by which a typical neuroleptic acting via serotonergic mechanism tends to produce less or no EPS. The present study was, therefore, designed to determine the effect of serotonin precursor tryptophan and a large neutral amino acid other than tryptophan (valine) on the modulation of haloperidol induced catalepsy and akinesia. Cataleptic effects of the drug and activity reducing effects were monitored on inclined surface and in an activity box or open field respectively. The results are discussed in the context of a role of tryptophan and valine induced changes of brain serotonin in modifying the extrapyramidal and monoaminergic effects of the typical neuroleptic haloperidol. In the present study administration of TRP and valine decreased activity in rats, haloperidol-induced catalepsy' was not modulated by prior administration of tryptophan or valine. Brain serotonin levels were elevated by haloperidol treatment and correlated very well with the behavioral response. These findings suggest a possible serotonergic involvement in neuroleptic induced tardive dyskinesia and an amelioration of the disorder through TRP supplementation.

Animals↗
PubMed1990

Conversion of ammonia or urea into essential amino acids, L-leucine, L-valine, and L-isoleucine, using artificial cells containing an immobilized multienzyme system and dextran-NAD+. 2. Yeast alcohol dehydrogenase for coenzyme recycling.

Semipermeable nylon-polyethylenimine artificial cells containing leucine dehydrogenase (EC 1.4.1.9), alcohol dehydrogenase (EC 1.1.1.1), urease (EC 3.5.1.5), and dextran-NAD+ were prepared. Artificial cells could convert ammonia or urea into L-leucine, L-valine, and L-isoleucine. For batch conversion in 20.0 mM of ammonium acetate substrate solutions, in 2 h 0.2 ml of artificial cells could produce 4.48 mumol of L-leucine, 9.98 mumol of L-valine, or 5.96 mumol of L-isoleucine. The corresponding conversion ratios were 22.4, 49.9, and 29.8%. In 20.0 mM of urea substrate solutions, 13.71 mumol of L-leucine, 16.12 mumol of L-valine, or 13.44 mumol of L-isoleucine was produced and the conversion ratios were 68.6, 80.6, and 67.2%. The substrate specificity of leucine dehydrogenase for the reductive amination was determined. Of the three branched-chain amino acids produced, the production rates of L-valine were the highest. The apparent Km values were as follows: 0.32 mM for alpha-ketoisocaproate, 1.63 mM for alpha-ketoisovalerate, and 0.73 mM for Dl-alpha-keto-beta-methyl-n-valerate. The leucine dehydrogenase multienzyme system had a good storage stability. It retained 72.0% of the original activity with artificial cells were stored at 4 degrees C for 6 weeks. The optimum conversion pH and temperature were 8.5-9.0 and 35-40 degrees C. The effects of urea and ammonium salts on conversion rate were also studied. The relative activities in ammonium salts solutions were 45.1-75.9% of those in urea solutions.

Alcohol Dehydrogenase↗
PubMed1990

Identification of valine 177 as a mutation altering specificity for transport of sugars by the Escherichia coli lactose carrier. Enhanced specificity for sucrose and maltose.

A mutant of the Escherichia coli lactose carrier has been selected (in an invertase-positive strain) based on its ability to grow on 6 mM sucrose in a manner dependent upon lactose carrier induction by isopropyl-1-thio-beta-D-galactopyranoside. The mutant was cloned, and DNA sequencing revealed a point mutation in lacY which changed alanine 177 to valine. The valine 177 mutation increased the transport rate for both [14C]sucrose and the maltose analog 4-nitrophenyl-alpha-maltoside. The potency for inhibition of beta-ONPG transport by several sugars containing the glucopyranosyl moiety (maltose, cellobiose, or palatinose) was increased significantly relative to the parental carrier. Similar experiments showed that the mutation did not affect the affinity for such commonly studied substrates as 4-nitrophenyl-alpha-D-galactopyranoside and beta-D-galactopyranosyl-1-thio-beta-D-galactopyranoside. These data indicate that gross structural alteration of the galactoside binding site cannot account for increased transport of sucrose and maltose by the valine 177 mutant. We conclude that effects of the valine 177 mutation are not limited strictly to changes in observed sugar affinity and that sugar-specific changes in turnover number may be an important determinant of the altered spectrum of sugar specificities exhibited by the Val-177 carrier. These phenomena may be related to the effect of this mutation on proton recognition (described in King, S.C., and Wilson, T.H. (1990) J. Biol. Chem. 265, 9645-9651).

Biological Transport↗
PubMed1996

Cloning, sequencing, overexpression in Escherichia coli, and inactivation of the valine dehydrogenase gene in the polyether antibiotic producer Streptomyces cinnamonensis.

The catabolism of branched chain amino acids, especially valine, appears to play an important role in furnishing building blocks for macrolide antibiotic biosynthesis. To determine for the first time the importance of valine dehydrogenase (vdh) in polyether antibiotic biosynthesis, the vdh gene from Streptomyces cinnamonensis has been cloned and sequenced. The enzyme (M(r)37,718 Da) has been produced in large amounts in an active form in the E. coli cytoplasm using a T7 RNA-polymerase expression system. Upon inactivation of the gene in S. cinnamonensis by a double-crossover mechanism, a hyg::vdh mutant was isolated that was devoid of vdh activity. Upon growth in chemically defined media, as well as a complex medium optimised for monensin production, the mutant and wild-type grew equally well and reached the same levels of monensin production. In both strains a valine transaminase activity could be detected that provides an alternative route for converting valine into 2-oxoisovaleric acid. The results show that vdh is not essential for normal growth of S. cinnamonensis, and its inactivation does not significantly affect normal levels of monensin production in this strain.

Amino Acid Oxidoreductases↗
PubMed2001

Psychrophilic valine dehydrogenase of the antarctic psychrophile, Cytophaga sp. KUC-1: purification, molecular characterization and expression.

We found the occurrence of valine dehydrogenase in the cell extract of a psychrophilic bacterium, Cytophaga sp. KUC-1, isolated from Antarctic seawater and purified the enzyme to homogeneity. The molecular mass of the enzyme was determined to be approximately 154 kDa by gel filtration and that of the subunit was 43 kDa by SDS/PAGE: the enzyme was a homotetramer. The enzyme required NAD+ as a coenzyme, and catalyzed the oxidative deamination of L-valine, L-isoleucine, L-leucine and the reductive amination of alpha-ketoisovalerate, alpha-ketovalerate, alpha-ketoisocaproate, and alpha-ketocaproate. The reaction proceeds through an iso-ordered bi-bi mechanism. The enzyme was highly susceptible to heat treatment and the half-life at 45 degrees C was estimated to be 2.4 min. The kcat/Km (micro(-1).s(-1)) values for L-valine and NAD+ at 20 degrees C were 27.48 and 421.6, respectively. The enzyme showed pro-S stereospecificity for hydrogen transfer at the C4 position of the nicotinamide moiety of coenzyme. The gene encoding valine dehydrogenase was cloned into Escherichia coli (Novablue), and the primary structure of the enzyme was deduced on the basis of the nucleotide sequence of the gene encoding the enzyme. The enzyme contains 370 amino-acid residues, and is highly homologous with S. coelicolor ValDH (identity, 46.7%) and S. fradiae ValDH (43.1%). Cytophaga sp. KUC-1 ValDH contains much lower numbers of proline and arginine residues than those of other ValDHs. The changes probably lead to an increase in conformational flexibility of the Cytophaga enzyme molecule to enhance the catalytic activity at low temperatures.

Amino Acid Oxidoreductases↗
PubMed2000

Transport of amino acids (L-valine, L-lysine, L-glutamic acid) and sucrose into plasma membrane vesicles isolated from cotyledons of developing pea seeds.

Transport of the amino acids L-valine, L-lysine, and L-glutamic acid and of sucrose was studied in plasma membrane vesicles isolated from developing cotyledons of pea (Pisum sativum L. cv. Marzia). The vesicles were obtained by aqueous polymer two-phase partitioning of a microsomal fraction and the uptake was determined after the imposition of a H(+)-gradient (DeltapH, inside alkaline) and/or an electrical gradient (Deltapsi, inside negative) across the vesicle membrane. In the absence of gradients, a distinct, time-dependent uptake of L-valine was measured, which could be enhanced about 2-fold by the imposition of DeltapH. The imposition of Deltapsi stimulated the influx of valine by 20%, both in the absence and in the presence of DeltapH. Uptake of L-lysine was more strongly stimulated by Deltapsi than by DeltapH, and its DeltapH-dependent uptake was enhanced about 6-fold by the simultaneous imposition of Deltapsi. In the absence of gradients the uptake of L-glutamic acid was about 2-fold higher than that of L-valine, but it was not detectably affected by DeltapH or Deltapsi. Although the transport of sucrose was very low, a stimulating effect of DeltapH could be clearly demonstrated. The results lend further support to the contention that during seed development cotyledonary cells employ H(+)-symporters for the active uptake of sucrose and amino acids.

Amino Acids↗
PubMed1988

Effect of peptide histidine valine on cardiovascular and respiratory function in normal subjects.

Non-adrenergic inhibitory nerves may have an important role in regulating airway calibre. A recently discovered peptide, peptide histidine valine, is a potent relaxer of airway smooth muscle in vitro and has been proposed as a possible neurotransmitter in this tissue. The cardiovascular and respiratory effects of graded infusions of this peptide (2.5-10 pmol kg-1 min-1) have been examined in six normal subjects in a placebo controlled, randomised double blind study. The mean (SEM) peak plasma concentration of peptide histidine valine during the highest infusion rate was 2392 (170) pmol/l, representing a 29 fold increase above the basal concentration. This was accompanied by flushing, a significant increase in heart rate of 28 (3.7) beats/min and skin temperature of 1.8 degrees (0.16 degrees) C, but no effect on systolic or diastolic blood pressure. Despite these high plasma concentrations of the peptide and the substantial tachycardia and increase in skin blood flow, there was no change in partial expiratory flow at 40% of vital capacity (Vp40) or in the airway response to inhaled histamine (geometric PD40 9.37 and 9.73 mumol during saline and peptide histidine valine infusion respectively). Although these findings provide no support for a physiological role of peptide histidine valine in controlling airway function in healthy subjects, important effects of locally released peptides in the vasoactive intestinal peptide family cannot be excluded.

Adult↗
PubMed1971

Capacity of human subjects to utilize keto analogues of valine and phenylalanine.

Three adult human subjects were maintained for 7 days (period I) on a protein-free formula diet containing the minimum daily requirements of the eight essential amino acids plus 40 g glycine. During the last 5 days of this period, the average daily nitrogen balances for the three subjects were +0.52, +0.71, and +0.30 g, respectively. During the next 7 days (period II), valine was withdrawn from the diet, and the glycine ration increased by an equimolar amount. During the last 5 days of period II, average daily nitrogen balances declined to -1.82, -1.61, and -1.87 g, respectively. In the final period of 7 days (period III), the keto analogue of valine, alpha-ketoisovaleric acid, was added to the diet in a quantity equimolar to the minimum daily requirement of valine. During the last 5 days of this period, average daily nitrogen balances improved to -0.02, -0.18, and -0.83 g, respectively. Analogous experiments in three subjects involved the withdrawal from the diet of phenylalanine (period II) and replacement by its keto analogue, phenylpyruvic acid (period III). The average daily nitrogen balances were as follows: period I: +1.04, +0.96, and +0.53 g; period II: -1.45, -1.83, and -1.94 g; period III: +0.07, +0.11, and -0.52 g. The data demonstrate that man can convert alpha-ketoisovaleric acid and phenylpyruvic acid to the corresponding essential amino acids, valine and phenylalanine. The efficiency of these conversions is considerably less than 100%.

Adult↗
PubMed2003

Effect of co administration of haloperidol and large neutral amino acids (tryptophan and valine) on rats striatal dopamine, serotonin and their metabolism.

Haloperidol is a high potency typical neuroleptic used in the treatment of schizophrenia. Administration of haloperidol produces muscles related side effects commonly known as extrapyramidal effects (EPS). These effects are not produced following the administration of atypical neuroleptics such as clozapine. A severe side effect of clozapine treatment is however, agranulocytosis. Development of antipsychotics with little/no EPS and/or other side effects is one of the exploring fields of drug research. This involves investigation on the mechanism by which a typical neuroleptic acting via serotonergic mechanism tends to produce less or no EPS. The present study is, therefore, designed to determine the effect of serotonin precursor tryptophan and a large neutral amino acid (valine) other than tryptophan on the modulation of neurochemical changes in the striatum. Neurochemical estimation were done by HPLC-EC. Present study showed that administration of tryptophan increased tryptophan, 5HT, 5HIAA and DA concentration in the striatum. DOPAC and HVA were not effected. Administration of valine increased DOPAC concentration in the striatum and did not alter tryptophan, 5HT, 5HIAA, DA and HVA concentration. Administration of the haloperidol increased HVA, 5HT and 5HIAA concentration. No effect was produced on tryptophan, DOPAC and DA levels. Valine administration followed by haloperidol injection did not alter striatal tryptophan, 5HT, DA, DOPAC and HVA concentration but decreased 5HIAA concentration. Administration of tryptophan followed by haloperidol injection increased tryptophan and 5HT concentrations and decreased DA levels. No effect was produced on 5HIAA, DOPAC and HVA concentrations. Administration of TRP increased plasma and brain concentration as well as DA levels in the striatum. Administration of valine did not decrease striatal TRP concentration while Haloperidol increased striatal 5-HT and 5-HIAA concentrations and no change in DA levels after haloperidol administration. whereas prior injection of TRP that increased 5HT concentration did not alter haloperidol-induced DA turnover in the brain.

Journal Article↗
PubMed1989

delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine synthetase from Aspergillus nidulans. The first enzyme in penicillin biosynthesis is a multifunctional peptide synthetase.

A multienzyme catalyzing the formation of delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine, the first free intermediate in penicillin biosynthesis, was detected in an assay measuring the formation of tripeptide from L-[U-14C]valine in the presence of L-alpha-aminoadipic acid, L-cysteine, ATP, Mg2+ ions, and dithioerythritol. Enzyme was extracted from dry mycelium using a buffer with a high glycerol concentration and thiol protective agent to stabilize enzyme activity. In five steps the enzyme was purified 118-fold. It catalyzed ATP-pyrophosphate exchange in dependence of all three constituent amino acids, and the enzyme could be amino-acylated with L-[14C]valine. The molecular weight of the protein both native (in gel filtration chromatography) and denatured (polyacrylamide gel electrophoresis) was about 220 kDa. These data suggest that delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine synthetase consists of a single polypeptide chain and a multienzyme thiotemplate mechanism for the reaction sequence is postulated.

Aspergillus nidulans↗
PubMed1976

Hydrolytic action of aminoacyl-tRNA synthetases from baker's yeast: "chemical proofreading" preventing acylation of tRNA(I1e) with misactivated valine.

Phe-tRNAPhe-C-C-A, Val-tRNAVal-C-C-A, and Ile-tRNAIle-C-C-A, which accept their amino acid on the 2'-OH of the 3'-terminal adenosine, are hydrolyzed readily by their aminoacyl-tRNA synthetase. If the 3'terminal adenosine in these tRNAs is replaced by either 3'-deoxyadenosine or formycin, little if any hydrolysis can be observed. Correspondingly Ser-tRNASer-C-C-A which accepts serine on the 3'-OH of the 3'-terminal adenosine is hydrolyzed by seryl-tRNA synthetase, whereas Ser-tRNASer-C-C-2'dA and Ser-tRNASer-C-C-F are not. Tyr-tRNATyr-C-C-A and all modified Tyr-tRNATyr-C-C-N, which can accept tyrosine on either the 2'OH or the 3'-OH of the 3'terminal adenosine, are not hydrolyzed by tyrosyl-tRNA synthetase. The data can be rationalized assuming that hydrolysis takes place only if the amino acid is bound to the nonaccepting OH and hence is not positioned at the amino acid binding site upon formation of the complex between aminoacyl-tRNA and aminoacyl-tRNA synthetase. In the formycin-carrying tRNA, the amino acid bound to the nonaccepting OH seems to be inaccessible to the enzymatic groups responsible for hydrolysis. Val-tRNAIle-C-C-3'dA and Ile-tRNAIle-C-C-3'DA cannot be hydrolyzed by isoleucyl-tRNA synthetase. Val-tRNAIle-C-C-A is hydrolyzed by the enzyme five times more rapidly than Ile-tRNAIle-C-C-A. Whereas Ile-tRNAIle-C-C-F is absolutely stable, Val-tRNAIle-C-C-F si hydrolyzed immediately. As shown by the earlier finding that valine misactivated by isoleucyl-tRNA synthetase cannot be permanently transferred to tRNAIle-C-C-A but to tRNAIle-C-C-3'dA, the 3'-OH is essential for preventing transfer of misactivated valine. It thus appears that valine is hydrolyzed off Val-tRNAIle-C-C-N if it is bound to the accepting 2'-OH in the binding site for isoleucine. A hypothesis is offered attempting to explain the experimental observations in mechanistic terms. We consider the hydrolytic action of the aminoacyl-tRNA synthetases as a general mechanism of "chemical proofreading" in the protein biosynthesis.

Amino Acyl-tRNA Synthetases↗
PubMed1964

FORMATION OF VALINE BY STREPTOMYCIN-DEPENDENT ESCHERICHIA COLI.

Bragg, P. D. (University of British Columbia, Vancouver, British Columbia, Canada), and W. J. Polglase. Formation of valine by streptomycin-dependent Escherichia coli. J. Bacteriol. 88:1006-1009. 1964.-The primary extracellular products of the aerobic catabolism of glucose in streptomycin-dependent Escherichia coli were found to be carbon dioxide and acetate, the ratio of these two products being dependent on the rate of aeration but independent of antibiotic. Secondary extracellular products of dependent E. coli were valine and lactic acid; the former was produced in the presence of antibiotic under aerobic conditions, and the latter metabolite was formed under conditions of either antibiotic depletion or oxygen deprivation. The fact that the same products were formed when either streptomycin or oxygen was limiting supports the hypothesis that the antibiotic is required to complete hydrogen transport in dependent E. coli. The formation of valine appears to represent an anomalous pathway of aerobic glucose catabolism, whereby a neutral, relatively highly reduced end product is excreted.

Alanine↗
PubMed1965

BIOCHEMICAL AND GENETIC GROUPING OF ISOLEUCINE-VALINE MUTANTS OF PSEUDOMONAS AERUGINOSA.

Pearce, L. E. (University of Otago, Dunedin, New Zealand), and J. S. Loutit. Biochemical and genetic grouping of isoleucine-valine mutants of Pseudomonas aeruginosa. J. Bacteriol. 89:58-63. 1965.-Isoleucine- and valine-requiring mutants of Pseudomonas aeruginosa, induced with ultraviolet and ethyl-methane-sulfonate, were isolated. The feeding properties and growth requirements of 56 of these mutants were examined, and evidence was found for at least four phenotypic groups, A, B, C, and D, controlling the biosynthesis of isoleucine and valine. Preliminary genetic mapping by use of bacteriophage-mediated transduction showed linkage between the mutant loci within groups A, B, and C. There was also evidence of linkage between the genes controlling groups A and B and some slight evidence of linkage between the other groups. There was insufficient evidence to suggest the arrangement of the genes.

Amino Acids↗
PubMed2004

Adducts of N-terminal valines in hemoglobin with isoprene diepoxide, a metabolite of isoprene.

Isoprene (2-methylbuta-1,3-diene) is a multi-site carcinogen in rodents. To evaluate the role of the diepoxide metabolite (1,2:3,4-diepoxy-2-methylbutane) in carcinogenesis, measurements of in vivo doses of the diepoxide are needed. The in vivo dose may be inferred from levels of reaction products with hemoglobin (Hb adducts). This report presents in vitro studies of the adduct formation by the diepoxide of isoprene with valinamide and oligopeptides as model compounds of N-terminal valines in hemoglobin (Hb). In the reaction with valinamide it was shown that isoprene diepoxide forms as the main product a ring-closed adduct, which is a pyrrolidine derivative [N,N-(2,3-dihydroxy-2-methyl-1,4-butadiyl)valinamide, MPyr-Val]. The analysis was performed by gas chromatography/mass spectrometry (GC/MS) (EI and PICI) after acetylation. The ring-closed adduct was also identified by liquid chromatography/electrospray ionization mass spectrometry (LC/ESI-MS) as the main product in the reaction between isoprene diepoxide and standard hepta- or (2H8)octapeptides, corresponding to the N-terminal peptides of the alpha-chains in mouse and rat Hb. These peptides, alkylated with isoprene diepoxide, to be used as internal standards and calibration standards for quantification of MPyr-adduct levels in vitro and in vivo, were analyzed with respect to the degree of MPyr-alkylation by two independent methods, amino acid analysis and HPLC-UV; similar results were obtained using these methods. A method for measurement of Hb adducts as modified peptides, used earlier to measure a similar adduct to N-terminal valines in Hb from the diepoxide of 1,3-butadiene, has in the present work been tested for application to isoprene diepoxide. The method is based on tryptic degradation of globin and LC/ESI-MS analysis of N-terminal Pyr-heptapeptides of the Hb alpha-chain enriched by HPLC. MPyr-adduct levels in isoprene diepoxide alkylated hemolysate from mouse erythrocytes incubated with different concentrations of isoprene diepoxide (2 and 10 mM) for 1 h were quantified. The adduct level was about 50 nmol/g alpha-chain Hb per mM x h. From the adduct levels the rate constant of isoprene diepoxide for reaction with N-terminal valine was calculated to be about 1.6 times faster than for diepoxybutane.

Alkylation↗
PubMed1995

Allosteric regulation of biosynthetic threonine deaminase from Escherichia coli: effects of isoleucine and valine on active-site ligand binding and catalysis.

The sigmoidal steady-state kinetics of biosynthetic threonine deaminase from Escherichia coli is allosterically controlled by isoleucine and valine, the end-products of branched-chain amino acid biosynthesis. A basis for the regulation of threonine deaminase by heterotropic effectors has been studied by capitalizing on the intrinsic fluorescence of the essential pyridoxal phosphate cofactor in this enzyme in kinetic and equilibrium binding studies with the substrate analog D-threonine. D-Threonine binds cooperatively to four sites on the free enzyme, with an average dissociation constant of 19.8 mM. However, in the presence of saturating valine, or isoleucine, the D-threonine binding isotherms are noncooperative and characterized by dissociation constants of 3.9 and 24.8 mM, respectively. The rate of association of D-threonine with threonine deaminase in the presence of the regulatory ligands was biphasic. Analysis of the data in terms of a two-step scheme whereby the internal aldimine Schiff base in the initial encounter complex undergoes transimination with D-threonine to form an external aldimine yielded estimates for overall binding constants that were in good agreement with those determined from equilibrium binding isotherms. These analyses indicate that the positive allosteric effector valine acts solely to alter the binding of D-threonine to the active sites of threonine deaminase by shifting the equilibrium between a low-affinity and high-affinity state, consistent with predictions from a simple two-state model. However, isoleucine has a compound effect on the enzyme. The negative allosteric ligand promotes decreases in the rate and equilibrium constants for encounter complex formation, consistent with its preferential binding to the low-affinity state of the enzyme. In addition, however, isoleucine promotes a decrease in the transimination rate and equilibrium constants. Since transimination is generally considered to be protein-catalyzed in pyridoxal phosphate requiring enzymes, it is proposed that isoleucine affects both binding and catalysis in threonine deaminase, which provides a possible explanation for the inadequacy of a simple two-state model to describe the allosteric regulation of this enzyme.

Allosteric Regulation↗
PubMed1999

Valine 108, a chain-folding initiation site-belonging residue, crucial for the ribonuclease A stability.

Thermal denaturation of bovine pancreatic ribonuclease A and a set of its single variants, carrying replacements of hydrophobic residues in the postulated 106-118 chain folding initiation site, has been studied by differential scanning calorimetry. Ribonuclease A variants undergo a two-state thermal transition denaturation except for those with replacement of valine 108. Most mutations cause a significant destabilization of the protein compared to the wild-type, thus demonstrating the importance of hydrophobic residues at the 106-118 region in maintaining the stability of the molecule. Among them, those of valine 108 promote the greatest (14-27 degrees C) destabilization of the molecule. Therefore, valine 108 plays a crucial role for ribonuclease A stability.

Animals↗
PubMed1975

Transcriptional control of the isoleucine-valine messenger RNA's in E. coli K-12.

Hybridization of messenger ribonucleic acid (mRNA) isolated from Escherichia Coli K-12 to deoxyribonucleic acid (DNA) from lambdaCI857st68h80dilv was used to detect isoleucine-valine (ilv) specific mRNA. A number of strains partially constitutive for the isoleucine-valine enzymes had levels of ilv mRNA 2 to 3-fold higher than the parent strain. Starvation for any of the branched-chain amino acids resulted in a 20 to 23-fold increase in ilv mRNA as compared to repressed levels. These differences were not due to altered growth rates or to changes in the stability of ilv mRNA. These data indicate that regulation of the isoleucine-valine enzymes by multivalent repression occurs mainly at the level of transcription. Kinetics of elongation of ilv mRNA after repression are consistent with the assumption that the mechanism of multivalent repression involves the prevention of further initiations by RNA polymerase.

DNA↗