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Biomedical subjects

T Plesser

Publications and source records attributed to T Plesser.

15 recordsLinked to original sources
PubMed1996

Spiral waves, organizers of temporal and spatial order in excitable media.

The notion "excitable medium' appears to be a unifying concept for the description of a class of wave phenomena observed in many fields of research, from material science to medicine. The analysis of the spatio temporal organization in excitable media reveals that significantly different time series of local activity at various sites may have a common origin in cooperative dynamic excitation structures; rotating spirals. A spiral-shaped pattern emanates from a small organizing center, the core of the spiral. The motion of the spiral tip within the core determines the frequency and the wavelength of a spiral and the peculiar filter properties of an excitable medium allow for the formation of regular spirals even in the case of an highly irregular motion of the tip. Computer-assisted video techniques are recommended to measure these intimate neighbourhood of regular and irregular time-space structures in living matter.

Chemical Phenomena↗
PubMed1995

[Methodology for analysis of binary sequences in heart rate].

A qualitative and quantitative method for the description of heart rate variability is demonstrated. It is based on a symbolic coding of time series taken from heartbeat intervals and calculation of the value "entropy" as a measure for information. Entropy seems to be a propriate measure to gain insight into the temporal structure underlying the phenomenon of heart variability.

Electrocardiography↗
PubMed1987

Three-dimensional representation of chemical gradients.

Perspective display techniques are applied to chemical and biochemical data sets. These represent spatially distributed gradients of reactive compounds that participate in pattern-formation processes due to reaction-diffusion or reaction-convection coupling. The patterns form in thin solution layers and are observed as chemical waves in the Belousov-Zhabotinskii reaction, as convection-induced stationary structures during oscillating glycolysis in yeast cytoplasm, and as the diffusive spreading of enzyme-catalyzed metabolic turnover in a substrate layer. The digital data are measured with a two-dimensional spectrophotometer based on a computerized video equipment with high spatial, temporal and intensity resolution. By application of three-dimensional procedures detailed structural properties of chemical and biochemical model systems will be presented yielding localization of reaction and transport events.

Journal Article↗
PubMed1987

On the recognition of order and disorder.

We compare several algorithms for the recognition of ordered and disordered images. As image sources we use waves of the Belousov-Zhabotinskii reaction coupled to convective motion in a petri dish. This device allows reversibly the generation of periodic (ordered) and aperiodic (disordered) patterns. The best match between the parametric description and the observations is obtained by an "autodifference function". This function is computed by summing up intensity differences over all pairs of picture elements having a given distance on the picture plane. Then, the minimum of this function is determined upon variation of the distance. This algorithm is not only efficient for the recognition of order and disorder in "machine vision", but also plausible in biological visual perception.

Form Perception↗
PubMed1986

Two-dimensional spectrophotometry and pseudo-color representation of chemical reaction patterns.

The formation of spatio-temporal patterns in layers of chemical and biochemical media is observed and quantified by a two-dimensional spectrophotometer based on a computerized video equipment with high spatial, temporal, and intensity resolution. Images are shown, to some extent, in pseudo-colors. The use of pseudo-colors and three-dimensional perspective representations enhances the perception of the information contained in an image and the inherent order of the spatial structures.

Chemical Phenomena↗
PubMed1983

Analysis of progress curves. Interaction of pyruvate kinase from Escherichia coli with fructose 1,6-bisphosphate and calcium ions.

The influence of fructose 1,6-bisphosphate and Ca2+ on the kinetics of pyruvate kinase from Escherichia coli K12 was studied (at pH 7.0 and 25 degrees C) by using the pH-stat method for the measurement of the reaction progress as well as initial-rate analysis. The data were analysed on the basis of a concerted model with three conformational states [Markus, Plesser, Boiteux, Hess & Malcovati (1980) Biochem. J. 189, 421-433] by using a novel procedure for a computer-directed treatment of progress curves [Markus & Plesser (1976) Biochem. Soc. Trans. 4, 361-364]. By addition of fructose 1,6-bisphosphate the sigmoid kinetics with respect to phosphoenolpyruvate and Mg2+ is abolished and the activity of the enzyme is described by classical saturation kinetics. This is explained by exclusive binding of fructose 1,6-bisphosphate at an allosteric site of the conformational state that forms the active complex. We observe that Ca2+ is an activator of the enzyme at low Mg2+ and Ca2+ concentrations; otherwise it is an inhibitor. These effects can be understood by assuming that Ca2+ has the same binding properties as Mg2+, although it does not allow a catalytic turnover.

Calcium↗
PubMed1983

Free energy dissipation of the pyruvate kinase reaction has a minimum at cell metabolite concentrations.

The ratio of substrates and products (mass action ratio) for the reaction catalyzed by the enzyme pyruvate kinase is measured under the constraint of constant reaction rate for pyruvate kinase (EC 2.7.1.40) from brewers yeast and Escherichia coli. For both organisms, a maximum of the ratio is found at concentrations comparable to those obtained from cell metabolite measurements. This observation suggests an optimum principle for free energy transduction in the glycolytic reaction pathway, as a maximum of the mass action ratio corresponds to a minimum dissipation of free energy.

Chemical Phenomena↗
PubMed1981

Analysis of progress curves in enzyme kinetics: bias and convergent set in the differential and in the integral method.

Two problems encountered in the analysis of progress curves are examined: 1. Systematic deviations due to errors in the initial solute concentrations make the least-squares method unsuitable. The improvements accomplished by the introduction of a proper weighting matrix are investigated. 2. Non-linear parameter optimization implies a dependence of the optimized parameters on their initial estimates, due to the existence of multiple minima. It is shown that the sensitivity of the optimized parameters on the initial estimates is reduced by fitting the slopes of the progress curves. A subsequent fit of the original progress curve data is recommended for refinement of the parameters.

Enzymes↗
PubMed1980

Analysis of progress curves. Rate law of pyruvate kinase type I from Escherichia coli.

Progress curves of the reaction catalysed by pyruvate kinase from Escherichia coli K12, designed to cover the four-dimensional concentration space of phosphoenolpyruvate, ADP, Mg2+ and ATP in the regulatory region, were recorded with the pH-stat method (pH 7.0 and 25 degrees C). Additional initial-rate measurement were performed to assess specific points. Two methods for the evaluation of progress curves were used: fitting the rate law to the rates obtained from the tangents of the progress curves and fitting the integrated rate law directly to the curves. Two models, both extensions of the concerted model given by Monod, Wyman & Changeux [(1965) J. Mol. Biol. 12, 88--118] with four protomers, could be fitted to the data within the experimental error. Model discrimination in favour of one of these models was possible by proper experimental design. In the selected model one conformational state of the enzyme forms the active complex. The active site of a second conformational state forms abortive complexes with Mg2+, causing strong inhibition at high Mg2+ concentrations. In the absence of ligands, most of the enzyme is in a third state that binds ATP at an allosteric site.

Adenosine Diphosphate↗
PubMed1975

Determination of the kinetic constants of glucose-6-phosphate 1-epimerase by non-linear optimization.

1. The overall kinetic constants of the reversible anomerisation of d-glucopyranose 6-phosphate from alpha to beta non-enzymatically as well as catalysed by glucose-6-phosphate 1-epimerase are determined by application of a novel computerized non-linear optimization technique. 2. The non-enzymic rate constants for the anomerisation of d-glucopyranose 6-phosphate from alpha to beta and reverse are 0.0658 and 0.0389s-minus 1, respectively. The Michaelis constants of the enzymic reaction are (see journal for formulas) with the turnover numbers of 1950s-minus 1 and 446s-minus 1 for the conversion of d-glucopyranose 6-phosphate from alpha to beta and reverse, respectively.

Animals↗