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D Combes

Publications and source records attributed to D Combes.

35 records · Page 2Linked to original sources
PubMed1994

Effect of polyols on alpha-chymotrypsin thermostability: a mechanistic analysis of the enzyme stabilization.

The influence of the synthetic substrate (N-acetyl-L-tyrosine ethyl ester) and the different polyols (ethylene glycol, glycerol, erythritol, xylitol and sorbitol) on the thermostability of alpha-chymotrypsin at 60 degrees C have been studied. The results obtained showed an important stabilizing effect in the presence of both additives. In order to describe the kinetics of enzyme stabilization, the experimental results were analyzed by a four-parameters deactivation model with excellent agreement. In all cases, alpha-chymotrypsin exhibited non-first-order deactivation kinetics, corresponding to a two-step unimolecular mechanism, where the main protective effect of polyols was observed in the first-step of the deactivation profile. Thus, the presence of polyols increased the level of activity stabilization (alpha 1), and decreased the first-order deactivation rate constant (k1). Additionally, the experimental results were analyzed as a function of both, the change in the standard free energy of denaturation (delta(delta Gzero)), and a protective effect, defined as the ratio of alpha-chymotrypsin half-lives (with and without polyols), showing in both cases a clear stabilizing effect of these polyhydroxylic cosolvents for the enzyme. The overall protective effect of polyols was also simultaneously related to their concentration and their water-activity depressing power.

Chymotrypsin↗
PubMed1994

Use of supercritical CO2 for bone delipidation.

Supercritical carbon dioxide was used for bone delipidation. It appeared that this technology is very efficient since supercritical CO2 is able to diffuse into microporous solids much better than liquids and that it has a good solvent capacity for lipids. This extraction is the ideal first step of any bone processing because microporosity of bone tissue becomes much more accessible, which may enhance osteoconduction once implanted. Moreover, it is safe since it involves no toxic chemical and is potentially usable with allografts as well as xenografts.

Animals↗
PubMed1993

Tetrodotoxin-sensitive dendritic spiking and control of axonal firing in a lobster mechanoreceptor neurone.

1. A primary mechanosensory neurone, the anterior gastric receptor (AGR) associated with gastric mill muscle in the lobster foregut was examined in vitro with extra- and intra-cellular recording techniques to understand processes of dendritic integration and dendro-axonal communication. 2. AGR has a 'T'-shaped geometry; its two long (> 3 mm) primary dendrites project distally to spatially separate, stretch sensitive terminals and converge centrally onto a common apical neurite that leads to a bipolar soma and single axon. 3. The receptor's bilateral dendrites are independently capable of generating action potentials. These appear to be Na+ dependent since they are blocked by tetrodotoxin, but not by Co2+ or a lack of Ca2+ in the bath saline. 4. Both dendrites are autogenically active, although impulses in the dendrite with the higher intrinsic excitability may cross over and activate the trigger zone on the contralateral side. Moreover, spikes arising on either dendrite do not actively invade the soma, but are conveyed as decremented potentials to a third trigger zone on the initial axon segment. 5. Focal applications of TTX (tetrodotoxin) demonstrated the existence and allowed precise definition of a central membrane compartment of AGR that appears to lack in functional Na+ channels. This inexcitable region includes the soma, the apical neurite and the central branch point of the two dendrites. A failure to observe collision block of bilateral dendritic potentials as they traverse the neurite supported this conclusion. 6. Horseradish peroxidase injections and staining revealed two morphological features of the apical neurite that differed markedly from other regions of the cell. In addition to a relatively large diameter, the neurite's plasma membrane is heavily convoluted and coiled to form a lamellar transverse profile. This latter feature may itself contribute to membrane inexcitability while the former is consistent with an elevated space constant for electrotonic conduction. 7. It is concluded that the inhomogeneous distribution of membrane excitability in AGR enhances the integrative capability of the receptor's dendrites, permitting mechanical input at diverse loci to be encoded and processed prior to transformation into axonal discharge.

Action Potentials↗
PubMed1991

alpha-Chymotrypsin in plastein synthesis: influence of substrate concentration on enzyme activity.

The role of alpha-chymotrypsin in the plastein reaction was studied using a peptic hydrolysate of albumin as substrate. Study of this reaction simultaneously by different methods showed that the plastein reaction is enzyme catalyzed and is highly dependent on environmental conditions. A gel permeation chromatography study of the plastein reaction showed simultaneous increases in the high- and low-molecular-weight oligopeptide fractions; a transpeptidation mechanism may be involved in the reaction. A study of the effect of substrate concentration on the plastein reaction catalyzed by alpha-chymotrypsin showed a profile with both hydrolytic and synthetic activities. This effect was also observed when the reaction course was followed by quantification of the free amino groups at different substrate concentrations, showing that a condensation mechanism is responsible for the synthetic activity when the substrate concentration is very high. These results have led us to conclude that the plastein reaction involves a transpeptidation and/or condensation mechanism, which is a function of the substrate concentration.

Albumins↗
PubMed1990

Effect of diffusional resistances on the action pattern of immobilized alpha-amylase.

Alpha-amylase from Aspergillus oryzae has been immobilized onto corn grits and porous silica (specific areas 180 and 440 m2 g-1). Kinetic parameters of immobilized enzyme have been determined. Immobilization of alpha-amylase results in the formation of less polymerized products resulting in an apparent decrease in the number of transglycosylation reactions, for both maltotetraose and starch as substrates, when compared with free enzyme. Diffusional limitations for substrate and products have been quantified in the case of the three supports used. External diffusional resistances were important in all cases for the reaction products, whilst they became negligible for the substrate in the case of silica supports. Moreover, internal transfer limitations were identified with silica 180 m2 g-1 support. It was demonstrated that diffusional resistances were in direct relation to the apparent modification of the enzyme action pattern after immobilization.

Aspergillus oryzae↗
PubMed1989

The relationship between the glucose oxidase subunit structure and its thermostability.

The thermostability of glucose oxidase (beta-D-glucose: oxygen 1-oxidoreductase, EC 1.1.3.4) at 60 degrees C has been studied as a function of its concentration in various media (pure water and pure deuterium oxide). In deuterium oxide, glucose oxidase is more stable than in water, and two kinds of stabilizing effect have been observed: the medium-organization effect and the enzyme-concentration effect. This effect has been related to the glucose oxidase subunit structure. This enzyme contains four forms of subunit: monomer, dimer, trimer, and tetramer, which are all composed of the identical monomer. The monomers of glucose oxidase subunits are linked by the non-covalent bond. Only dimer and trimer possess the enzymatic activity. During glucose oxidase denaturing, monomers assemble into dimer, trimer, or tetramer. This redistribution behavior depends on the enzyme concentration and the nature of the medium.

Aspergillus niger↗
PubMed1980

Platelets in myeloproliferative disorders. I. A comparative evaluation with certain platelet function tests.

Certain platelet functions were evaluated in 24 patients with secondary polycythaemia (SP) and in a large number of patients suffering from myeloproliferative disorders (MD'S): 89 patients with chronic myeloid leukaemia (CML) at different stages of development, 58 with polycythaemia vera (PV), 23 with essential thrombocythaemia (ET), and 25 with agnogenic myeloid metaplasia (AMM). Bleeding time, epinephrine-induced platelet aggregation and adhesiveness agreed with those generally reported in the literature; they are independent of thrombocytosis, the haemoglobin level and the leucocyte count. Macrothrombocytosis, evaluated by an electronic method, was only found in CML, mainly during acute blast crisis. An increased percentage of light platelets was a constant feature in all groups except in the SP and in 20% of the PV. The most severe abnormalities were observed in AMM and CML in the acute stage; in the chronic phase of CML there is no correlation between the severity of platelet abnormalities and the survival of the patients.

Blood Platelets↗
PubMedDate not supplied

Effect of different proteases on bitterness of hemoglobin hydrolysates.

Hemoglobin was hydrolyzed by several enzymes (Proctase, Alcalase, Neutrase, papain). Hydrolysates were analyzed (degree of hydrolysis, gel permeation on Superose 12 column, tasting) and fractionated by ultrafiltration and 2-butanol extraction. The bitter peptides were isolated and identified. The results were compared with those already obtained with peptic hemoglobin hydrolysates. All the findings were confirmed. Ultrafiltration concentrated bitter compounds in the fraction corresponding to 500-5000 Da, and these compounds were selectively extracted by 2-butanol. All the bitter peptides belonged to the same fragment of the beta-chain of bovine hemoglobin. Finally, the use of a Superose 12 chromatographic column for easy detection of bitter hydrolysates without sensory analysis could be generalized for hemoglobin hydrolysates.

Amino Acid Sequence↗
PubMedDate not supplied

Four acetylcholinesterase genes in the nematode Caenorhabditis elegans.

Whereas a single gene encodes acetylcholinesterase (AChE) in vertebrates and most insect species, four distinct genes have been cloned and characterized in the nematode Caenorhabditis elegans. We found that ace-1 (mapped to chromosome X) is prominently expressed in muscle cells whereas ace-2 (located on chromosome I) is mainly expressed in neurons. Ace-x and ace-y genes are located in close proximity on chromosome II where they are separated by only a few hundred base pairs. The role of these two genes is still unknown.

Acetylcholinesterase↗