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Biomedical subjects

A Lejeune

Publications and source records attributed to A Lejeune.

At least 19 recordsLinked to original sources
PubMed2004

Thermodynamics from three-dimensional many-body fragmentation simulations on a cellular automaton model.

The thermal equilibrium of many-body systems subject to finite range interactions is investigated numerically, by means of a multipurpose 3D cellular automaton dynamic model developed by the authors. The numerical experiments, carried out at fixed number of bodies, volume and energy, demonstrate the formation of an equilibrium among 3D aggregates of bodies. The distribution of the aggregates against size obeys a power law of (negative) exponent tau approximately 2.2 (against 1.3 in 2D). Our experiments, indicating that the exponent is insensitive to the precise parameter values and the precise parametrization of the interactions, are consistent with the idea of the existence of a universality class corresponding to the thermal equilibrium. The numerical value for the exponent tau is in agreement with the theoretical thermal equilibrium analyses based on various other approaches, numerical and semianalytical, indicating that the cellular automaton approach provides an adequate methodology to investigate thermal equilibria. In this paper, as an illustration of this method, we refer to the problem of formation of clusters of nucleons in heavy ion collisions of nuclei leading on to fragmentation. The theoretical tau value, however, corresponding to the thermal equilibrium among the aggregation clusters, is 15 percent lower than the empirical value ( approximately 2.6 ) , as measured in laboratory nuclear fragmentation experiments induced by collision. There is then only a very approximate correspondence between the experimental and the thermal equilibrium value. On the basis of the results of this paper and of a previous paper of this series, we conjecture that the approximate agreement is due to a partial establishment of a thermodynamic equilibrium during the collision of the nuclei. The thermal equilibrium gives the main contribution to the observed tau value; the deviation from this possibly universal value is largely the consequence of the lack of full thermal equilibrium in actual laboratory experiments. This conjecture is extended to interpret the observed ubiquity of power laws of exponents exceeding 2.2, which refer to the distribution of various types of matter in 3D space.

Journal Article↗
PubMed2003

Dynamics of many-particle fragmentation in a cellular automaton model.

A three-dimensional cellular automaton model developed by the authors to deal with the dynamics of N-body interactions has been adapted to investigate the head-on collision of two identical bound clusters of particles, and the ensuing process of fragmentation. The range of impact energies is chosen low enough, to secure that a compound bound cluster can be formed. The model is devised to simulate the laboratory set-up of fragmentation experiments as monitored by 4 pi detectors. The particles interact via a Lennard-Jones potential. At low impact energies the numerical experiments following the dynamics of the individual particles indicate a phase of energy sharing among all the particles of the compound cluster. Fragments of all sizes are then found to evaporate from the latter cluster. The cluster sizes, measured in our setup by simulated 4 pi detectors, conform to a power law of exponent approximately 2.6. In an attempt to duplicate the laboratory caloric curves related, in particular, to nuclear fragmentation processes, we introduce several temperature parameters (kinetic temperature of nucleons, kinetic temperature of fragments, reaction equilibrium temperatures). Theoretical caloric curves are then constructed for those temperature parameters, we regard as physically most relevant. Our results show that different temperature definitions generate different curve patterns, indicating that the fragmentation system remains far from thermodynamic equilibrium. The pattern of the laboratory caloric curve for Au-Au collision experiments as derived from a recent analysis [NuPECC Report, 1997 (unpublished)] is reproduced qualitatively by our reaction temperatures.

Journal Article↗
PubMed2002

The pore of the leaf cavity of Azolla species: teat cell differentiation and cell wall projections.

The differentiation of the specialized secretory teat cells of the leaf cavity pore of Azolla species was investigated at the ultrastructural level with emphasis on their peculiar cell wall projections. The results indicated that the projections are formed as soon as the teat cells complete their differentiation and that their production is principally associated with changes in endoplasmic reticulum profiles. The number of projections increases with the teat cell age and is stimulated under salt and P deficiency stresses. Salt stress also promotes their emergence on Azolla species that under normal conditions do not produce projections. Cytochemical tests on different Azolla species showed that the projection composition is almost identical: proteins, acidic polysaccharides, and pectin are always detected. This study revealed that Azolla teat cell projections differ fundamentally from other types of hitherto described cell wall projections that are considered as remnant structures from cell separation. In contrast, in Azolla teat cells projections are actively produced and compounds are excreted by an exocytotic mechanism. The possible role of the projections in the symbiosis of Azolla spp. with Anabaena azollae is discussed.

Cell Differentiation↗
PubMed2001

Quantitative analysis of the stabilization by substrate of Staphylococcus aureus PC1 beta-lactamase.

BACKGROUND: The stabilization of enzymes in the presence of substrates has been recognized for a long time. Quantitative information regarding this phenomenon is, however, rather scarce since the enzyme destroys the potential stabilizing agent during the course of the experiments. In this work, enzyme unfolding was followed by monitoring the progressive decrease of the rate of substrate utilization by the Staphylococcus aureus PC1 beta-lactamase, at temperatures above the melting point of the enzyme. RESULTS: Enzyme inactivation was directly followed by spectrophotometric measurements. In the presence of substrate concentrations above the K(m) values, significant stabilization was observed with all tested compounds. A combination of unfolding kinetic measurements and enzymatic studies, both under steady-state and non-steady-state regimes, allowed most of the parameters characteristic of the two concurrent phenomena (i.e. substrate hydrolysis and enzyme denaturation) to be evaluated. In addition, molecular modelling studies show a good correlation between the extent of stabilization, and the magnitude of the energies of interaction with the enzyme. CONCLUSIONS: Our analysis indicates that the enzyme is substantially stabilized towards heat-induced denaturation, independently of the relative proportions of non-covalent Henri-Michaelis complex (ES) and acyl-enzyme adduct (ES*). Thus, for those substrates with which the two catalytic intermediates are expected to be significantly populated, both species (ES and ES*) appear to be similarly stabilized. This analysis contributes a new quantitative approach to the problem.

Acylation↗
PubMed2001

[Long-term treatment of Alzheimer's disease: followup of a cohort of 255 patients treated with lacrine for four years].

We describe the follow-up of a cohort of 255 Alzheimer's disease (AD) patients (81 males, 174 females) treated by tacrine during 4 years. We performed the survey of hepatic, cholinergic and general tolérance. Drug efficacy was measured by MMS examination on weeks 0, 18, 30, 52, 104, 156 and 208. A total of 190 patients (74.5 percent) were dropped out of this study, 75 (29 percent) for adverse events. We found 85 hepatic (33 percent), 79 cholinergic (31 percent), 31 (12 percent) neuropsychiatric and 72 general (28 percent) side effects. In term of drug efficacy we observed a global decline of 2.5 MMS points during the first year and 2 MMS points between W52 and W156. Tacrine's symptomatic efficacy, defined as the number of patients improved or stabilized at W30, was present in 50 patients (46 percent) among the 109 patients reaching W30. The intensity of symptomatic efficacy was expressed by a 2.7 MMS points increase in 37 patients improved on W30. The long term effects of Tacrine, measured by the MMS score at one year, showed a positive impact as the MMS was 2.5 points above the expected score in non treated AD patients. This study raises the practical problem of optimal cholinesterase inhibitors use in AD and the theoretical question of long term action of cholinesterase inhibitors on cerebral lesions of AD.

Aged↗
PubMed1999

The in vivo association of BiP with newly synthesized proteins is dependent on the rate and stability of folding and not simply on the presence of sequences that can bind to BiP.

Immunoglobulin heavy chain-binding protein (BiP) is a member of the hsp70 family of chaperones and one of the most abundant proteins in the ER lumen. It is known to interact transiently with many nascent proteins as they enter the ER and more stably with protein subunits produced in stoichiometric excess or with mutant proteins. However, there also exists a large number of secretory pathway proteins that do not apparently interact with BiP. To begin to understand what controls the likelihood that a nascent protein entering the ER will associate with BiP, we have examined the in vivo folding of a murine lambdaI immunoglobulin (Ig) light chain (LC). This LC is composed of two Ig domains that can fold independent of the other and that each possess multiple potential BiP-binding sequences. To detect BiP binding to the LC during folding, we used BiP ATPase mutants, which bind irreversibly to proteins, as "kinetic traps." Although both the wild-type and mutant BiP clearly associated with the unoxidized variable region domain, we were unable to detect binding of either BiP protein to the constant region domain. A combination of in vivo and in vitro folding studies revealed that the constant domain folds rapidly and stably even in the absence of an intradomain disulfide bond. Thus, the simple presence of a BiP-binding site on a nascent chain does not ensure that BiP will bind and play a role in its folding. Instead, it appears that the rate and stability of protein folding determines whether or not a particular site is recognized, with BiP preferentially binding to proteins that fold slowly or somewhat unstably.

Adenosine Triphosphatases↗
PubMed1999

Application of cellular automata to N-body systems.

A two-dimensional cellular automaton model is introduced to deal with the dynamics of a finite system of particles whose interactions are simulated by two-body step potentials. The method is illustrated for a potential approximating the standard Lennard-Jones potential, representative for the problem of heavy ion collisions in nuclear physics. From the cellular automaton dynamics thermodynamic equilibrium state variables are introduced in the usual way. The numerical experiments indicate the occurrence of a phase transition. Macroscopically the transition is marked by a singularity in the equation of state; microscopically it manifests itself by the formation of clusters of particles of all sizes, obeying a mass distribution in the form of a power law of exponent 1.35.

Journal Article↗
PubMed1998

A collapsed intermediate with nonnative packing of hydrophobic residues in the folding of TEM-1 beta-lactamase.

The kinetics of refolding of TEM-1 beta-lactamase from solution in guanidine hydrochloride have been investigated on the manual and stopped-flow mixing time scales. The kinetics of change of far-UV circular dichroism and of intrinsic and ANS fluorescence have been compared with changes in the quenching of fluorescence by acrylamide as a probe of the accessibility of solvent to tryptophan. The binding of ANS points to hydrophobic collapse in the very early stages of folding which take place in the burst phase. This is accompanied by regain of 60-65% of native ellipticity, indicating formation of a significant proportion of secondary structure. Also in the burst phase, the tryptophan residues, which are largely exposed to solvent in the native protein, become less accessible to acrylamide, and the intrinsic fluorescence increases markedly. An early intermediate is thus formed in which tryptophan is more buried than in the native protein. Further intermediates are formed over the next 20 s. Quenching by acrylamide increases during this period, as the transient nonnative state is disrupted and the tryptophan residue(s) become(s) reexposed to solvent. The two slowest phases are determined by the isomerization of incorrect prolyl isomers, but double jump tryptophan fluorescence and acrylamide quenching experiments show little, if any, effect of proline isomerization on the earlier phases. Hydrophobic collapse thus occurs to a folding intermediate in which there is a nonnative element of structure which has to rearrange in the later steps of folding, resulting in a nonhierarchical folding pathway. The C-terminal W290 is suggested as being involved in the nonnative intermediate. beta-Lactamase provides further evidence for the occurrence of nonnative intermediates in protein folding.

Acrylamide↗
PubMed1998

Beta-lactamases as models for protein-folding studies.

This review traces some of the key features of the folding of beta-lactamases and their relevance to the way proteins fold in general. Studies on the enzymes have highlighted the nature and role of equilibrium and transient condensed states. The kinetics of folding are multiphasic, and when monitored by acrylamide quenching of the tryptophan fluorescence, an early phase provides evidence for the transient accumulation of a nonnative intermediate involving burial of tryptophan in a nonpolar environment. Intermediate phases can be understood in terms of progressive folding of different parts of the molecule. The later, slow phases are associated with proline isomerization in the TEM-1 enzyme and, in its P167T mutant form, with isomerization from trans to cis of the E166 T167 peptide bond. Coupled with kinetic and X-ray crystallographic studies of the beta-lactamase from Staphylococcus aureus and its D179Q mutant, it appears that the final stage of folding is that of collapse and packing of the omega-loop on to the main body of the protein.

Bacterial Proteins↗
PubMed1997

Unexpected influence of a C-terminal-fused His-tag on the processing of an enzyme and on the kinetic and folding parameters.

The addition of a poly-His C-terminal extension, designed to facilitate the purification of the protein, to the beta-lactamase of a thermophilic Bacillus licheniformis strain modified the site of action of the signal peptidase. This resulted in the secretion of a protein with a different N-terminus, showing that this type of protein engineering might not always be as 'neutral' as generally assumed.

Bacillus↗
PubMed1997

Sciatic nerve regeneration through venous or nervous grafts in the rat.

This study analyses the interest of isologous venous grafts filled with saline or with Schwann cells versus nerve grafts as guides for regeneration of the sciatic nerve in 35 Wistar rats. Electrophysiological parameters (conduction velocities and distal latencies of motor responses) and the functional index of De Medinacelli were measured several times from 1 month to 1 year after surgery. An histological analysis was performed on 2 control rats and on 3 rats killed 6 or 12 months after surgery: the total number of fibers was counted on a montage photoprint of the whole nerve, and the diameters of axons and the thickness of the myelin sheath were measured on digitized images. With a portion of nerve as guide, the regeneration is faster than with a vein. However, regeneration after 6 months is at least as good with a venous graft filled with Schwann cells, as assessed by electrophysiological, functional, and histological analysis. The addition of Schwann cells in grafted veins allows the nerve to regenerate through longer gaps than previously described (25 vs 15 mm). In order to assess the quality of nerve regeneration, functional, electrophysiological, and histological analysis are complementary.

Animals↗
PubMed1992

Target cell deformability determines the type of phagocytic mechanism used by Entamoeba histolytica-like, Laredo strain.

Morphological study of red blood cell phagocytosis by Entamoeba histolytica-like (Laredo strain) has shown that this amoeba is able to ingest by two distinct mechanisms. One is classical phagocytosis and the other is by suction or microphagocytosis. Rigidification of red blood cells by treatment with glutaraldehyde shows that there is a correlation between the deformability of the ingested cell and the type of phagocytosis observed. Indeed, as the red cells become more rigid, less microphagocytosis is observed. To demonstrate that this shift in phagocytic mechanisms is not induced by the modification of a surface receptor by the glutaraldehyde treatment, the amoebas were fed with erythrocyte ghosts. Since these have lost most of their hemoglobin content, they are less rigid than the intact erythrocytes. The ghosts, even after glutaraldehyde treatment, are always ingested by microphagocytosis. These results have therefore led us to conclude that the type of erythrocyte phagocytosis used by E histolytica-like (Laredo strain) is determined by the deformability of the targetted red blood cells.

Animals↗
PubMed1992

Cyclization characteristics of cyclodextrin glucanotransferase are conferred by the NH2-terminal region of the enzyme.

Cyclodextrin glucanotransferase (CGTase; EC 2.4.1.19) is produced mainly by Bacillus strains. CGTase from Bacillus macerans IFO3490 produces alpha-cyclodextrin as the major hydrolysis product from starch, whereas thermostable CGTase from Bacillus stearothermophilus NO2 produces alpha- and beta-cyclodextrins. To analyze the cyclization characteristics of CGTase, we cloned different types of CGTase genes and constructed chimeric genes. CGTase genes from these two strains were cloned in Bacillus subtilis NA-1 by using pTB523 as a vector plasmid, and their nucleotide sequences were determined. Three CGTase genes (cgt-1, cgt-5, and cgt-232) were isolated from B. stearothermophilus NO2. Nucleotide sequence analysis revealed that the three CGTase genes have different nucleotide sequences encoding the same amino acid sequence. Base substitutions were found at the third letter of five codons among the three genes. Each open reading frame was composed of 2,133 bases, encoding 711 amino acids containing 31 amino acids as a signal sequence. The molecular weight of the mature enzyme was estimated to be 75,374. The CGTase gene (cgtM) of B. macerans IFO3490 was composed of 2,142 bases, encoding 714 amino acids containing 27 residues as a signal sequence. The molecular weight of the mature enzyme was estimated to be 74,008. The sequence determined in this work was quite different from that reported previously by other workers. From data on the three-dimensional structure of a CGTase, seven kinds of chimeric CGTase genes were constructed by using cgt-1 from B. stearothermophilus NO2 and cgtM from B. macerans IFO3490. We examined the characteristics of these chimeric enzymes on cyclodextrin production and thermostability. It was found that the cyclization reaction was conferred by the NH2-terminal region of CGTase and that the thermostability of some chimeric enzymes was lower than that of the parental CGTases.

Amino Acid Sequence↗
PubMed1991

Non-toxic antiseptic irrigation with chlorhexidine in experimental revascularization in the rat.

The effect of different wound irrigation fluids upon femoral arteries and veins was investigated in the rat, using microsurgical techniques. Toxicity was evaluated by microscopical observation after selective staining of histological slides. Povidone-iodine, 10%, proved to be a very irritant solution, provoking an attack on the vascular endothelium and secondary thrombosis. Chlorhexidine at 0.05%, 0.02% and 0.001% was found by contrast to have a very low toxicity which was comparable to physiological saline. Experimental investigation of antiseptic solutions should not only include the determination of the antibacterial effect, but also the potential for cell toxicity, using an irrigation technique.

Animals↗