Isotope Exchange Studies Research Articles

A new synthesis of adenosine 5'-([.gamma.(R)-17O,18O]-.gamma.-thiotriphosphate) and its use to determine the stereochemical course of the activation of glutamate by glutamine synthetase

A new synthetic route to adenosine 5'-([gamma(R)-17O,18O]-gamma-thiotriphosphate) is described which combines chemical methods for introducing the heavy oxygen isotopes and enzymic methods for achieving the enantiospecificity. This material was used as a substrate for the activation of glutamate catalyzed by glutamine synthetase from Salmonella typhimurium. Analysis of the chirality of the [16O,17O,18O]thiophosphate produced showed that the reaction proceeds with inversion of configuration on phosphorus. This result, taken together with the positional isotope exchange studies of Midelfort and Rose [Midelfort, C. F., & Rose, I.A. (1976) J. Biol. Chem. 251, 5881-5887], demonstrates that the activation of glutamate to form gamma-glutamyl phosphate proceeds by a direct "in-line" transfer of the phosphoryl group.

Studies of isotopic exchange between gaseous hydrogen and palladium hydride powder

A gas flow apparatus has been constructed and used to study the isotopic exchange reaction occurring between the solid and gas phases in hydrogen (deuterium) gas flows directed through packed-powder beds of β-phase palladium deuteride (hydride). Spontaneous Raman light scattering is employed to obtain a real-time analysis of the isotopic composition of the gas (H 2, D 2, HD) exiting from the bed. A parametric rate-equation model is described which depicts the time-dependent behavior of the isotopic exchange process. The exchange mechanism is assumed to be rate-limited by processes occurring on the surface of the powder. The fundamental kinetic parameter of the model is the isotopic exchange probability, p, which is the probability that an isotopic exchange event occurs during a collision of a gas-phase atom with the surface. Isotope effects between the gas and solid phases are explicitly included in terms of the isotope separation factor, α. Results of the model are compared with the experimental measurements and, using a literature value of α = 2.4, good agreement is obtained for p ≈ 10 −7. In view of the importance of the isotope effects in the hydrogen/palladium system and the range of α values reported for the β-phase in the literature, the sensitivity of the model results to a variation in the value of α is examined.

Substrate activity of synthetic formyl phosphate in the reaction catalyzed by formyltetrahydrofolate synthetase.

Formyl phosphate, a putative enzyme-bound intermediate in the reaction catalyzed by formyltetrahydrofolate synthetase (EC 6.3.4.3), was synthesized from formyl fluoride and inorganic phosphate [Jaenicke, L. v., & Koch, J. (1963) Justus Liebigs Ann. Chem. 663, 50-58], and the product was characterized by 31P, 1H, and 13C nuclear magnetic resonance (NMR). Measurement of hydrolysis rates by 31P NMR indicates that formyl phosphate is particularly labile, with a half-life of 48 min in a buffered neutral solution at 20 degrees C. At pH 7, hydrolysis occurs with P-O bond cleavage, as demonstrated by 18O incorporation from H2(18)O into Pi, while at pH 1 and pH 13 hydrolysis occurs with C-O bond cleavage. The substrate activity of formyl phosphate was tested in the reaction catalyzed by formyltetrahydrofolate synthetase isolated from Clostridium cylindrosporum. Formyl phosphate supports the reaction in both the forward and reverse directions. Thus, N10-formyltetrahydrofolate is produced from tetrahydrofolate and formyl phosphate in a reaction mixture that contains enzyme, Mg(II), and ADP, and ATP is produced from formyl phosphate and ADP with enzyme, Mg(II), and tetrahydrofolate present. The requirements for ADP and for tetrahydrofolate as cofactors in these reactions are consistent with previous steady-state kinetic and isotope exchange studies, which demonstrated that all substrate subsites must be occupied prior to catalysis. The k cat values for both the forward and reverse directions, with formyl phosphate as the substrate, are much lower than those for the normal forward and reverse reactions. Kinetic analysis of the formyl phosphate supported reactions indicates that the low steady-state rates observed for the synthetic intermediate are most likely due to the sequential nature of the normal reaction.

Changes in intestinal glucose transport over the lifespan of the rat

Age-related changes in intestinal glucose absorption were studied using everted intestinal sacs and brush border membrane vesicles prepared from male F344 rats. Glucose uptake by everted intestinal sacs was greatest in young (2–3-month-old) as compared with adult (12–14-month-old) and old (24-month-old) rats. The greatest decrease in glucose uptake occurred between 2 and 12 months. The addition of phloridzin reduced glucose uptake to similar levels in all age groups, suggesting that the age-related change was in the carrier-mediated component of glucose transport. In order to localize the site of decreased carrier-mediated glucose transport, experiments were performed using brush border membrane vesicles. Vesicular glucose uptake in the presence of Na was significantly greater in vesicles prepared from 2-month-old rats (133 ± 18 pmol/mg/s), compared with those prepared from 12-month-old rats (82 ± 13 pmol/mg/s). Kinetic studies performed under non-equilibrium conditions demonstrated that the major effect of age was on the Na-dependent component of the brush border transport system. There was a reduction in the V max from 335 ± 37 pmol/mg/s in the young to 217 ± 22 pmol/mg/s in the adult, but there was no change in the K m. Isotope exchange studies performed under equilibrium conditions confirmed a decrease in the activity of the glucose transporter with age. No age-related changes in Na uptake by brush border membrane vesicles were observed. These findings suggest that a decrease in the number and/or activity of Na-linked glucose carriers may account for the decrease in intestinal glucose transport with age.

The existence of monomeric metaphosphate in hydroxylic solvent: a positional isotope exchange study

The phosphoryl transfer reactions of adenosine 5′-[α,β-18O]diphosphate trianion in acetonitrile, acetonitrile–t-butyl alcohol, and neat t-butyl alcohol are all accompained by closely similar bridge-to-non-bridge scrambling in the reisolated starting material; the simplest interpretation involves a monomeric metaphosphate intermediate even in the presence of hydroxylic solvent.

Study of kinetics, equilibrium and isotope exchange in ion exchange systems

Donnees theoriques et cas de l'echange des ions Na + -Mg 2+ -UO 2 2+ sur un echangeur de cations fortement acide: Ostion KS08

Study of kinetics, equilibrium and isotope exchange in ion exchange systems

Resultats obtenus en presence des echangeurs d'ions Ostion KS (contenant de 1,5 a 12% de DVB) et Amberlite IR 120, et des solutions aqueuses de nitrates

Isotope exchange study of bis/diethyl dithio carbamate/cadmium/II/ complex

Isotope exchange behaviour of bis/diethyl dithio carbamate/cadmium/II/ complex has been studied at 25°C and 35°C varying the concentrations of both metal ion and the complex. The results show that the complex is kinetically labile and the concentration has a significant effect on the rate of the reaction. Increase in temperature also effects the rate of exchange.

Comment on the paper „Kinetics, equilibrium and isotope exchange in ion exchange systems” by Plicka et al.

Comments are given on the kinetic studies of isotope exchange in heterogeneous systems, reported in the paper by J. PLICKA, J. CABICAR, A. GOSMAN and K. ŜTAMBERG, published in J. Radioanal. Nucl. Chem., 88 (1985) 325–336. Both experimental procedure and interpretation of the results are discussed.

Initial rate and isotope exchange studies of rat skeletal muscle hexokinase.

The kinetic mechanism of rat skeletal muscle hexokinase (hexokinase II) was investigated in light of a proposal by Cornish-Bowden and his co-workers (Gregoriou, M., Trayer, I. P., and Cornish-Bowden, A. (1983) Eur. J. Biochem. 134, 283-288). These investigators reported that the kinetic mechanism is ordered, with glucose adding before ATP and ADP dissociating from hexokinase before glucose-6-P. In addition, these workers suggest that glucose-6-P and ATP add to allosteric sites on hexokinase. We investigated the mechanism of action of hexokinase II by studying initial rate kinetics in the nonphysiological direction and by isotope exchange at chemical equilibrium. The former experiments were carried out in the absence of inhibitors and then with AMP, which is a competitive inhibitor of ADP, and with glucose 1,6-bisphosphate, a competitive inhibitor of glucose-6-P. The findings from these experiments suggest that the kinetic mechanism is rapid equilibrium Random Bi Bi. Isotope exchange at equilibrium studies also supports the random nature of the muscle hexokinase reaction; however, they also suggest that the mechanism is partially ordered, i.e. there is a preferred pathway associated with the branched mechanism. Approximately two-thirds of the flux through the hexokinase reaction involves the glucose on first glucose-6-P off last branch of the Random Bi Bi mechanism. These results imply that the kinetic mechanism is steady state Random Bi Bi. There is some evidence to suggest that glucose-6-P binds to an allosteric site on muscle hexokinase, but none to suppose that ATP binds allosterically. Analysis of the mechanism of Gregoriou et al. suggests that it is at variance with the findings of this report as well as with data available from other laboratories.

Study of hydrogen isotope exchange on calcium, strontium and barium hydrides

The isotope exchange of hydrogen on rare-earth metal hydrydes has been studied. When passing from Ca to Sr and Ba, the activation energy decreases and, correspondingly, the rate of exchange increases. A similar dependence is observed for the isotope exchange of oxygen on the corresponding oxides and of nitrogen on nitrides.

On the mechanism of specific phosphate adsorption by hydroxylated mineral surfaces: A review

Abstract The mechanism of specific phosphate adsorption by hydroxy‐lated mineral surfaces comprises two aspects: the phosphate‐hydroxyl surface reaction and the configuration of the adsorbed phosphate ion. Evidence pointing to ligand exchange as the mechanism of the phosphate‐surface hydroxyl reaction include kinetics of adsorption and desorption; hydroxyl ion release; infrared spectroscopy, and stereochemical calculations. Data pertaining to the coordination of adsorbed phosphate on hydroxy‐lated mineral surfaces have not been conclusive overall. Isotopic exchange experiments and studies of desorption kinetics do not provide definitive information on surface coordination. Measurements of hydroxyl ion release and crystallographic calculations provide support for the existence of both monodentate and bidentate surface complexes of phosphate ions. Infrared spectroscopic investigations suggest a binuclear complex on dried, phosphated goethite. However, these studies cannot be extrapolated automatically to so...

Study of kinetics, equilibrium and isotope exchange in ion exchange systems

Study of kinetics, equilibrium and isotope exchange in ion exchange systems

Pyruvate carboxylase: mechanisms of the partial reactions.

Data from isotopic exchange studies and from experiments with 32P- and 14C-labeled enzyme-bound intermediates support the following description of the first partial reaction: (Formula: see text). From studies of the transfer of the carboxyl-group from ENZ-biotin-CO2- to pyruvate or its analogues we propose that binding of the acceptor substrate induces the translation of carboxybiotin from the first to the second partial reaction site. The studies on the translocation of carboxybiotin can be summarized in the following reaction scheme: (Formula: see text). Where k+3 less than k+1, k-1, k+2 and k-2. Thus, the rate-limited step is governed by k+3 which represents the movement of carboxybiotin from the first subsite to the second.

Study of solid-state hydrogen isotope exchange in a Pd/organic compound system

A kinetic study of hydrogen isotope exchange was carried out in a H2/Pd membrane/organic compound system for a number of compounds in the 0.2–20 kPa H2 pressure range. The results suggest a low specificity of the reaction kinetics for the compounds used. Possible reaction mechanisms are discussed and analyzed.

An infrared study of isotopic exchange during methanation over supported rhodium catalysts: an inverse spillover effect

Infrared spectroscopy has been employed to study the reduction of carbon dioxide on supported catalyst films. The investigation included isotopic labeling using D/sub 2/ as the reduction gas. Isotopic exchange was observed for both CO/sub 2//D/sub 2/ and CH/sub 4//D/sub 2/ mixtures. The mechanism of this isotopic exchange involves migration of hydrogen from the support to the Rh sites, an ''inverse spillover effect''. A key intermediate in the dissociation of CO/sub 2/ on the supported Rh films was a carbonyl hydride species. 21 refs., 2 figs.

Isotope exchange studies of the rate of dissociation of CO2 on liquid iron oxides and CaO-saturated calcium ferrites

Measurements of the rates of dissociation of CO2 on liquid iron oxides and CaO-saturated liquid calcium ferrites have been made by the14CO2-CO isotope exchange technique. For temperatures up to about 1550 °C, the apparent first order rate constants for both melts are essentially inversely proportional to the equilibrium CO2/CO ratio over the range studied (≈0.4 to 12). Evidence is presented that the rate limiting step in the interfacial oxidation of these melts by CO2 is the dissociation of CO2- Rates of oxidation, in mol cm−2 s−1, in CO2-CO atmospheres are deduced to be given by the equations:v =( p CO 2 a o − 1 − p CO) exp(−15,900/T − 2.03) andv = (pCO2 α o -1 −pCO) exp(−3800/T − 6.93) for the iron oxides and CaO-saturated calcium ferrites, respectively, wherea 0 is the oxygen activity of the melt expressed as the equilibrium CO2/CO ratio and the pressures are in atmospheres. The strong dependence on the CO2/CO ratio is shown to be consistent with the need to transfer two charges to the adsorbing or dissociating CO2 molecule. Correlation with the existing surface tension data is inconclusive.

Studies of heterogeneous isotope exchange of Cd (II) between the solution and the surface layer formed on aluminium oxide and activated carbon

The dependence of the isotope exchange kinetic curves on I∞ I o is studied, where I ∞ denotes the equilibrium activity and I o is the initial activity of the solution. Experimental kinetic curves of isotope exchange of Cd (II) ions between the bulk solution and the surface layer formed on aluminium oxide and activated carbon (charcoal) are compared with theoretical calculations.

A positional isotope exchange study of the argininosuccinate lyase reaction.

15N nuclear magnetic resonance spectroscopy was used to follow the positional isotope exchange reaction of bovine liver argininosuccinate lyase. The enzyme was shown to catalyze the N-3-N-2 positional nitrogen exchange in [3-15N]argininosuccinate in the presence of excess arginase. The ratio of the positional isotope exchange rate and the rate for net substrate turnover is less than 0.15 at low levels of fumarate but increases to a limiting value of 1.8 at high fumarate. These data have been interpreted to mean that the dissociation of fumarate and arginine from the ternary enzyme complex is random although fumarate is released at least an order of magnitude faster than is arginine from this complex. The rate constant for the release of fumarate from enzyme-arginine-fumarate is at least 6 times faster than the turnover number of the reverse reaction of argininosuccinate lyase. The lower limit for the release of arginine from this same complex is 0.5.

Studies of isotopic exchange in solid state: healing of damaged molecules

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTStudies of isotopic exchange in solid state: healing of damaged moleculesAmar NathCite this: Acc. Chem. Res. 1984, 17, 3, 90–96Publication Date (Print):March 1, 1984Publication History Published online1 May 2002Published inissue 1 March 1984https://doi.org/10.1021/ar00099a002RIGHTS & PERMISSIONSArticle Views70Altmetric-Citations6LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (952 KB) Get e-Alerts Get e-Alerts