Biomedical subjects

J F Hervagault

Publications and source records attributed to J F Hervagault.

28 records · Page 2Linked to original sources

PubMed1986

Dissipative structures in a two-cell system: Numerical and experimental approaches.

It has been shown that the coupling between the photoreduction of the oxidized form of dichloroindophenol (an artificial electron acceptor) by thylakoids and the incident light intensity can lead to the appearance of multiple steady states when the system is operated under open conditions. In the present work, a numerical study and experimental evidence are presented on the occurrence of dissipative structures in an arrangement of two continuously stirred tank reactors with mutual mass exchange of dichloroindophenol through an inert membrane. The stable spatial structures are generated by the creation of transient internal and external asymmetries. A nontrivial hysteresis effect between symmetric and asymmetric stable steady states has been observed.

Journal Article↗

PubMed1986

Theoretical and experimental study on the competition of NANA-aldolase and cytidine-5'-monophosphosialate-synthase for their common substrate N-acetylneuraminic acid.

The competition of NANA-Aldolase and Cytidine-5'-monophosphosialate-Synthase for their common substrate NANA has been studied, with, (i) the two enzymes in solution, (ii) NANA-Aldolase in solution and Cytidine-5'-monophosphosialate-Synthase immobilized in an artificial membrane (diffusion coefficient: 1.2 X 10(-3) cm2 h-1). The relation of the reaction rates for both enzyme was found 1:1 in case (i) and 2:1 in case (ii), in favor of NANA-Aldolase. These results are in agreement with the results obtained by computer simulation, where the Michaelian assumption and the diffusion effects had been considered. It was also calculated that the regulation of this branch point for the metabolic pathway of NANA is dependent on the input of NANA produced by the previous steps of the pathway and not on the concentration of CTP (second substrate of Cytidine-5'-monophosphosialate-Synthase) or the parameters controlling the diffusion of NANA. Computer simulations were performed by numerical analysis.

Binding, Competitive↗

PubMed1985

Experimental evidence for multistability in a photobiochemical system.

The kinetic behavior of a typical Hill reaction catalyzed by thylakoids and using the oxidized form of 2,6-dichloroindophenol (DCPIPox) as the artificial electron acceptor, is considered. Here, the light absorption process and the reduction of DCPIPox are autocatalytically coupled, leading to the occurrence of multiple steady states with respect to either the acceptor concentration or the incident light intensity. Experimental evidence is presented for both cases and the emergence of autocatalysis is discussed. The effect of the spatial arrangement on the global behavior of the system described is emphasized.

2,6-Dichloroindophenol↗

PubMed1983

Experimental evidence and theoretical discussion for long-term oscillations of phosphofructokinase in a compartmentalized system.

The activity of rabbit muscle phosphofructokinase (EC 2.7.1.11) has been followed as a function of time under conditions where the enzyme is separated from the bulk solution by an inert membrane. An enzymatic coupling assay allows continuous measurement of the variations in NADH concentration, which is directly related to the enzyme catalytic activity. For given concentrations of substrates (ATP and Fru6P) in the outside reservoir and a given ratio between diffusion coefficients of both substrates, the activity of phosphofructokinase exhibits an oscillatory behavior during a period of about 5 h. The phenomenon is explained in terms of coupling between diffusion of metabolites and non-linear enzyme reaction.

Animals↗

PubMed1983

Multiple steady states and oscillatory behavior of a compartmentalized phosphofructokinase system.

This paper discusses interacting diffusion and reaction in an open enzyme system. The enzyme, rabbit muscle phosphofructokinase (PFK; ATP:D-fructose-6-phosphate 1-phosphotransferase, EC 2.7.1.11), is inhibited strongly by excess of the substrate ATP. Metabolites diffuse through an inert membrane separating the enzyme from the bulk reacting medium. We demonstrate that such a simple system is able to account for the existence of both oscillatory behavior (limit cycle) and multiple steady states (hysteresis) as well as for the sudden transitions between stable and periodic behaviors. Experimental evidence for time oscillations is given.

Adenosine Triphosphate↗

PubMed1981

Modulation of phosphofructokinase behavior by chemical modifications during the immobilization process.

Phosphofructokinase was immobilized within a protein membrane or on soluble protein polymers using glutaraldehyde as cross-linking reagent. The native enzyme was also modified chemically, using the cross-linking reagent alone. A comparative kinetic investigation of these preparations was carried out. The catalytic activity of the chemically modified enzyme and its affinity towards fructose 6-phosphate decreased significantly; the modified enzyme lost its cooperative properties and the allosteric regulation by AMP was affected. When the chemical treatment was performed in the presence of effectors (AMP or ATP) the allosteric transition induced by AMP was restored, suggesting that the cross-linking reagent modified the AMP regulatory sites, albeit no higher-substrate-affinity enzyme conformation was frozen. Molecular data showed that glutaraldehyde produced intramolecular then intermolecular bonds as its concentration increased. When the enzyme was immobilized into protein membranes or on soluble polymers, the enzyme behavior was quite similar: decrease of affinity towards fructose 6-phosphate but no changes in cooperative properties and modifications of allosteric transition induced by AMP. When AMP was present during the immobilisation process, the enzyme immobilized in this way was no longer sensitive to effectors, either AMP or ATP. It showed Michaelian behavior and higher substrate affinity quite similar to that of the native enzyme. The data suggested that a higher-substrate-affinity enzymatic form was most probably stabilized by immobilization.

Adenosine Monophosphate↗

PubMed1980

[Oscillations of phosphofructokinase in a compartmentalized system].

The analysis of an allosteric model for phosphofructokinase, exhibiting an inhibition by excess substrate (ATP) shows the possibility of sustained self-oscillations. This model is described by a system of, two ordinary differential equations taking into account the coupling between metabolites diffusion (Fick's first law) and enzyme reaction. The different behavior types of the system have been studied as a function of two main parameters, rho (ration between respective permeabilities for ATP and F6P) and sigma (valuation of the diffusional constraints importance). The research for instability conditions has been carried out by normal mode analysis.

Animals↗

PubMed1977

Experimental evidence for a kinetic and electrochemical memory in enzyme membranes.

The existence of hysteresis phenomena in artificial enzyme membranes due to the coupling of simple kinetic enzyme properties with diffusion transport processes is reported. The intramembrane pH of a urease coating on the surface of a glass pH electrode exhibits a hysteresis loop when the pH of the bulk solution varies cyclically. The steady-state potential of a urease membrane, as a function of the substrate concentration in the bulk solution, also exhibits a memory effect. The influence of the membrane's history on its overall behavior is visualized by electron microscopy. We interpret the results in terms of a coupling between the enzyme reactions and diffusion processes, without taking into account molecular effects.

Diffusion↗

PubMed1975

The kinetic behavior of an artificial bienzyme membrane.

Artificial membranes bearing immobilized enzymes can be used to study some effects of membrane structure on enzyme kinetic behavior. The bienzyme system described is a mixture of beta-glucosidase and glucose oxidase. Gluconolactone, the product of thesecond enzyme, is an inhibitor of the first one. The resulting feedback effect has been compared using a mixed two-enzyme membrane, two separated one-enzyme membranes, and astirred bienzyme solution. The feedback effect is quicker and more efficient in the two-enzyme membrane than in solution; it is slower and less efficient in the case of the separated one-enzyme membranes. Effects of enzyme proximity in the structure are discussed. Conclusions are drawn concerning the efficiency of feedback mechanisms when enzymes are embedded within a single structure.

Anaerobiosis↗

PubMed1975

Kinetic studies dealing with an immobilized bienzyme system.

The binding of enzymes into artificial membranes makes possible a study of the interaction between membrane structure and enzyme kinetics within a simple context. Artificial protein membranes bearing a bienzyme system (xanthine oxidase, uricase) are produced by using a co-crosslinking method. The inhibition of uricase was shown to be dependent not only on the concentration of inhibitor in the bulk solution, but also on the kinetic properties of the membrane-bound enzymes. In the presence of xanthine oxidase inside the structure the uricase inhibition by xanthine is less important than in solution. Under defined conditions the activity was found to be higher in the presence of inhibitor than in its absence. Due to diffusion limitations this specific bienzyme system is more efficient when immobilized inside a membrane than when in solution.

Binding, Competitive↗