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Biomedical subjects

E A Boeker

Publications and source records attributed to E A Boeker.

At least 19 recordsLinked to original sources
PubMed1990

The kinetics and inhibition of p-nitrophenylacetate-hydrolysing esterases in a solitary bee, Megachile rotundata (Fab.).

1. The kinetics and inhibition of p-nitrophenylacetate hydrolysis by cytosolic esterases of female alfalfa leafcutting bees, Megachile rotundata (Fab.) was examined. 2. For p-nitrophenylacetate, the Km = 1.24 x 10(-4) M and Vmax = 2.29 x 10(-9) mol/s per mg protein. 3. Regarding four organophosphate insecticides, the mechanism of inhibition in all cases was mixed (competitive and uncompetitive) and, based on inhibition constants, the order of toxicity was naled greater than paraoxon greater than trichlorfon greater than oxydemeton methyl. 4. Comparisons are made to the honey bee, Apis mellifera.

Animals↗
PubMed1987

Analysis of enzyme kinetics by using integrated rate equations. Arginine decarboxylase.

We have used an integrated rate equation to analyse the reaction catalysed by the inducible arginine decarboxylase from Escherichia coli B. The stoichiometry Arginine----agmatine + CO2 is the simplest of the multiple-substrate/multiple-product cases. Twenty-one time courses were carried out at various initial concentrations of arginine and agmatine, and were then fitted to the integrated equation by using appropriate analytical procedures. Values were obtained for six of the seven possible kinetic constants, corresponding to kcat, KArg, the terms for competitive inhibition by agmatine, by CO2 and by agmatine and CO2 together, and the term for uncompetitive inhibition by agmatine. The uncompetitive constant for CO2 was indeterminate. Our results indicate that it is both practical and experimentally economical to obtain kinetic constants from full time courses.

Agmatine↗
PubMed1987

Analytical methods for fitting integrated rate equations. A discontinuous assay.

The integrated rate equation for reactions with stoichiometry A----P + Q is: e0t = -Cf . ln(1-delta P/A0) + C1 delta P + 1/2C2(delta P)2 where the coefficients C are linear or quadratic functions of the kinetic constants and the initial substrate and product concentrations. I have used the 21 progress curves described in the accompanying paper [Cox & Boeker (1987) Biochem. J. 245, 59-65] to develop computer-based analytical and statistical techniques for extracting kinetic constants by fitting this equation. The coefficients C were calculated by an unweighted non-linear regression: first approximations were obtained from a multiple regression of t on delta P and were refined by the Gauss-Newton method. The procedure converged in six iterations or less. The bias in the coefficients C was estimated by four methods and did not appear to be significant. The residuals in the progress curves appear to be normally distributed and do not correlate with the amount of product produced. Variances for Cf, C1 and C2 were estimated by four resampling procedures, which gave essentially identical results, and by matrix inversion, which came close to the others. The reliability of C2 can also be estimated by using an analysis-of-variance method that does not require resampling. The final kinetic constants were calculated by standard multiple regression, weighting each coefficient according to its variance. The weighted residuals from this procedure were normally distributed.

Analysis of Variance↗
PubMed1982

Initial rates. A new plot.

Excellent estimations of initial rates can be obtained from plots of delta P/t versus product formed (where P is the instantaneous concentration of the product). delta P/t is the chord from P0,t0 to P,t on an ordinary P-versus-t plot. When the chord is plotted as a function of product, the intercept at P0 of the resulting curve is necessarily dP/dt0. This curve approximates to a straight line extremely closely in all cases tested thus far. If delta P/t versus product is calculated from the integrated rate equation for a first-order reaction, and if a straight line is fitted through points representing the first 50% of the reaction, the discrepancy between the true initial rate and dP/dt0 estimated from the plot is 0.68%. For the most common form of the integrated rate equation for catalysed reactions the discrepancy varies between 0 and 0.90%. Because of the complexities of the integrated rate equations, catalysed second-order reactions have not been evaluated directly; uncatalysed reactions have been done instead. For a reaction with one reactant and two products, the discrepancy varies from 0.68 to 2.02%. For two reactants and one product, it varies from 0 to 0.68%; for two and two, 0 to 2.02%. The larger discrepancies occur only when unfavourable equilibrium constants are being overcome by the initial conditions.

Catalysis↗
PubMed1980

Metabolism of ethanol.

The first step in ethanol metabolism is carried out by two enzyme systems: Alcohol dehydrogenase and cytochrome P-450. The cytochrome P-450 system also detoxifies a wide variety of foreign compounds. On the basis of recent evidence, metabolic reasons are suggested for four well-known consequences of alcoholism: (a) The development of fatty liver; (b) the development of metabolic tolerance; (c) the occurrence of interactions between drugs and alcohol; and (d) the fact that many of the kilocalories attributable to ethanol do not seem to "count "count "count" when ethanol forms a major part of the diet.

Alcohol Oxidoreductases↗
PubMed1978

Arginine decarboxylase from Escherichia coli B: mechanism of dissociation from the decamer to the dimer.

The mechanism by which arginine decarboxylase dissociates from a decamer to a dimer has been examined by allowing a sulfhydryl group, available in the dimer but not the decamer, to react with 5,5'-dithiobis(2-nitrobenzoic acid). Initial rates of dissociation were obtained by following the resulting increase in absorbance at 412 nm in a stopped-flow spectrophotometer. The rate of dissociation increases linearly with the protein concentration and reaches a maximum as a function of the concentrations of 5,5'-dithiobis(2-nitrobenzoic acid), Na+, and 1/[H+]. Experiments in which the rate of dissociation was measured while one reagent was varied at fixed levels of a second indicate that dissociation requires three events: binding of one Na+ ion, dissociation of one proton, and the irreversible dissociation of subunits, in that order. The results also show that the decamer dissociates in stages rather than all at once. The activation energy for the overall process is 16 kcal/mol.

Arginine↗
PubMed1978

Mechanisms and rate equations for dissociating systems.

The results presented in the previous paper (Boeker, E.A. (1978), Biochemistry 17 (preceding paper in this issue) indicate that the dissociation of the decamer of arginine decarboxylase of Escherichia coli B is enhanced by Na+ and retarded by H+. In this system, substances which increase the rate of dissociation can be treated kinetically either as substrates or activators, and substances which retard dissociation can be treated as products or inhibitors. In addition, the events needed for dissociation can occur in an ordered or a random sequence, and the dissociation itself, from a decamer to five dimers, can be a sequential or a concerted process. In order to provide a framework for the experimental results, mechanisms for the dissociation of arginine decarboxylase that take all of these factors into account are described. In addition, it is shown that the usual methods of steady-state kinetics can be applied to these systems when true initial rates are measured; rate equations are presented for each mechanism. The results can be used for any dissociating of three or more subunits and will describe the dissociation of a dimer under certain conditions.

Arginine↗