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T Keleti

Publications and source records attributed to T Keleti.

At least 37 records · Page 2Linked to original sources
PubMed1982

Kinetics of coupled reactions catalyzed by aspartate aminotransferase and glutamate dehydrogenase.

Liver mitochondrial aspartate aminotransferase and glutamate dehydrogenase catalyze following sequence of reactions: see formula in text. In the presence of a slight excess of dehydrogenase, the time course of NADPH oxidation resulting from the overall reaction goes through a lag phase and reaches a linear phase. The slopes of the linear part of this curve is a linear function of transaminase concentration. At high concentration (approximately or equal to 10 microM) of both enzymes the lag phase, as observed after rapid mixing of the two enzymes in a Durrum stopped-flow spectrophotometer, is shorter, than that predicted from the kinetic parameters determined for the separate reactions catalyzed by each enzyme.

Animals↗
PubMed1979

pH and temperature dependence of the double inhibition of D-glyceraldehyde-3-phosphate dehydrogenase by ATP and quinaldate.

ATP and quinaldate, two inhibitors of D-glyceraldehyde-3-phosphate dehydrogenase, act antagonistically if [Pi] greater than 1 mM, at pH 7.5 and 25 degrees C. However, the type of interaction of the two inhibitors changes with pH and temperature. Antagonism is strongest at pH values near neutrality and decreases at higher pH. Above pH 9.5 the two inhibitors act nearly additively. At pH 8.5 and [Pi] = 5 mM there is antagonism above 20 degrees C between the two inhibitors whereas there is synergy below 20 degrees C. A lag period in the time course of the enzyme reaction was detected when both inhibitors were present. The lag period is a function of pH. Below pH 8.5 the pH-dependence of the lag period resembles a titration curve with a pKapp greater than 8.

Adenosine Triphosphate↗
PubMed1979

Inhibition of D-glyceraldehyde-3-phosphate dehydrogenase by ATP and quinaldate.

The inhibition of D-glyceraldehyde-3-phosphate dehydrogenase by ATP is of purely mixed type with respect to NAD (Ki=4.9 mM), purely uncompetitive with respect to D-glyceraldehyde-3-phosphate (Ki=9.4 mM) and partially uncompetitive with respect to inorganic phosphate (Ki=6.0 mM). Quinaldate is a purely mixed type inhibitor with respect to both NAD (Ki==10.0 mM) and D-glyceraldehyde-3-phosphate (Ki=15.3 mM), whereas purely non-competitive with respect to inorganic phosphate (Ki=11.0 mM). In the presence of quinaldate a lag period is observed in the time course of enzyme reaction. The duration of this lag period depends on both quinaldate and substrate concentrations.

Adenosine Triphosphate↗
PubMed1979

Double inhibition of D-glyceraldehyde-3-phosphate dehydrogenase and lactate dehydrogenase.

The simultaneous action of ATP (partially uncompetitive inhibitor with respect to Pi) and quinaldate (purely non-competitive inhibitor with respect to Pi) on D-glyceraldehyde-3-phosphate dehydrogenase was analyzed kinetically. The interaction constant [as defined by Keleti and Fajszi (1971) Math. Biosci. 12 197] of the two inhibitors for the D-glyceraldehyde-3-phosphate dehydrogenase-Pi complex is greater than 1, which means that the two inhibitors act antagonistically. The kinetic analysis of the double inhibition shows that there is no ATP-enzyme-quinaldate ternary complex, but a quaternary complex with Pi is formed. The interaction of the two inhibitors on the enzyme-Pi complex depends on substrate (Pi) concentration. The antagonistic effect of the two inhibitors becomes additive at low Pi concentrations (about 1 mM). The simultaneous action of oxalate (purely uncompetitive inhibitor with respect to NAD) and quinaldate (partially mixed type inhibitor with respect to NAD) on lactate dehydrogenase was also analyzed. Oxalate and quinaldate act antagonistically on lactate dehydrogenase. However, at low NAD concentrations (about 0.06 mM) or at high quinaldate and low oxalate concentrations (around 7 and 1.7 mM, respectively) the antagonism turns into the simple summation of the effects of the two inhibitors.

Adenosine Triphosphate↗
PubMed1978

Physico-chemical evidence for the interaction between aldolase and glyceraldehyde-3-phosphate dehydrogenase.

Polarization of fluorescence measurements of aldolase and D-glyceraldehyde-3-phosphate dehydrogenase labeled with fluorescein isothiocyanate have been used to detect the possible formation of a soluble complex between the proteins. The results suggest an interaction between aldolase and D-glyceraldehyde-3-phosphate dehydrogenase with an apparent dissociation constant 3 X 10(-7) M and an apparent stoichiometry of two aldolase tetramers bound per tetramer of D-glyceraldehyde-3-phosphate dehydrogenase.

Fluoresceins↗
PubMed1978

Kinetic evidence for interaction between aldolase and D-glyceraldehyde-3-phosphate dehydrogenase.

The possibility of interaction between purified rabbit muscle aldolase and D-glyceraldehyde-3-phosphate dehydrogenase was studied by rapid kinetic methods, by analyzing the kinetics of the consecutive reaction catalyzed by the coupled enzyme system. The Km of the intermediary product, glyceraldehyde 3-phosphate, produced by aldolase was determined in the coupled reaction for glyceraldehyde-3-phosphate dehydrogenase. Its value corresponds to that of the aldehyde (active) form of glyceraldehyde 3-phosphate, although in the given conditions the aldehyde leads to diol interconversion is faster than the enzymic reaction catalyzed by glyceraldehyde-3-phosphate dehydrogenase. We suggest that above a certain concentration of the enzymes the glyceraldehyde 3-phosphate produced by aldolase gets direct access to glyceraldehyde-3-phosphate dehydrogenase without participating in the aldehyde leads to diol interconversion which otherwise would occur if the substrate were to mix with the bulk medium.

Animals↗
PubMed1977

Mechanism of action of D-glyceraldehyde-3-phosphate dehydrogenase.

A computer approximation with polynomial quotients was used to evaluate from experimental data the dependence of the initial velocity of D-glyceraldehyde-3-phosphate dehydrogenase reaction on the concentration of substrates. The initial velocity values were determined at optimum conditions, over a wide range of substrate concentrations and by interpolating the time curve of enzyme reaction as t leads to 0. A further computer approximation with polynomial quotients, without any implied hypothese, gave the best fit to the experimental results. The analysis of this final equation shows that two types of catalytic sites may exist. Due to the complexity of the system, the results are compatible either with the ordered binding or with rapid equilibrium random binding of substrates to each separate, but interacting type of sites. Previous experimental data showing the formation of abortive and dead-end complexes can be interpreted as kinetic effects, inherent in the mechanism. Results at variance with earlier data can be explained by the different experimental conditions.

Computers↗
PubMed1975

Sigmoidal substrate saturation curves in Michaelis-Menten mechanism as an artefact.

The conditions under which sigmoidal substrate saturation curves are to be expected in the simple Michaelis-Menten mechanism and the Michaelis-Menten mechanism combined with the concomitant partial inactivation of the enzyme have been determinee solved numerically for different sets of rate constants by computer simulation on the basis of the second order Runge-Kutta method. In order to simulate the real experimental conditions, the substrate saturation curves have also been derived from velocity values in the quasi-steady state, by using the non-linear least squares fitting method. In the framework of the Michaelis-Menten mechanism there is a sigmoidal relationship between initial velocity and substrate concentration only in the case of a Van Slyke mechanism, i.e. if k2 greater than k-1 and therefore K=k2/k1 is a "kinetic constant" if the velocity is determined in the quasi-steady state. If the enzyme is inactivated during the course of velocity measurement in the quasi-steady state, a sigmoidal or a degenerated hyperbolic saturation curve is obtained. A sigmoidal saturation curve can be obtained in the case of the Van Slyke mechanism, independent of the rate constant of the inactivation of the enzyme, or in the case of Michaelis-Menten or Briggs-Haldane mechanism, if k3 is sufficiently high. The inflexion point of such substrate saturation curves is determined by the rate constants, i.e. S0 less than or approximately k3/k1 and k-1/k1 and/or k2/k1. The higher the value of k3 the more pronounced is the sigmoidicity. The Michaelis constant can precisely be determined only if the velocities are measured in the very steady state at all substrate concentrations used. If the measurements are made in the quasi-steady state, the KM is always underestimated.

Computers↗