Biomedical subjects

M Deleers

Publications and source records attributed to M Deleers.

At least 19 recordsLinked to original sources

PubMed1995

Piracetam-induced changes to membrane physical properties. A combined approach by 31P nuclear magnetic resonance and conformational analysis.

Piracetam, Nootropil (2-oxo-1-pyrrolidine acetamide), is a drug promoting erythrocyte deformability. To establish the mode of action of this compound, we have investigated its influence on the organization of model phospholipid membranes. 31P NMR data show that the drug induces a structural modification in liposomes made of phosphatidylcholine and phosphatidylethanolamine. Our conformational analysis results have allowed the interpretation of the effect of piracetam on these model membranes: the specific interaction between the drug molecules and the phosphate headgroups induces a new organization of the lipids favouring formation of mobile drug-phospholipid complexes that exhibit an isotropic-type signal in the 31P NMR spectra.

Liposomes↗

PubMed1993

31P NMR study of the parameters influencing the formation of non-bilayer phases in model membrane.

The influence of various parameters on the formation of non-bilayer phases in mixed cardiolipin/phosphatidylcholine liposomes have been examined by 31P NMR. The Ca++ concentration, the Ca++/cardiolipin ratio and also the phospholipid concentration determine the proportions of the different phases detected on the spectra. In particular, an increase of the cardiolipin concentration favours the induction of isotropic and hexagonal phases. By considering this phospholipid concentration dependence, it is possible to reconcile previous apparently contradictory data on the Ca++ threshold for inducing fusion of this model membrane.

Calcium↗

PubMed1989

Physico-chemical properties of prostaglandins and related pharmacological compounds. A theoretical study on conformational related activity.

Thromboxane A2, prostaglandin H2, a series of chemically stable cyclic endoperoxide analogues (U 46619, U 44069, ONO 11113, 9, 11, diazo PGH2 and SQ 26655) and different isomers of SQ 26655 were analysed for their spatial configuration by conformational analysis in a simulated membrane-water interface environment with a "structure tree" procedure already described for prostaglandins, leukotrienes and lipoxins. The conformers derived from the structure tree and with a high probability of existence are presented. A new method allows one to visualize the surface charge density of the calculated molecules. The spatial configuration and the surface charge density of each molecule are compared to their known order of competition binding to the putative TXA2/PGH2 receptor of platelets. The conformational and charge density analysis merely shows that the different stereochemistry of these molecules lead to spatial conformation, that mimics (agonists), or that are far from (antagonists) the TXA2/PGH2 conformation.

Chemical Phenomena↗

PubMed1989

Ultrastructural, physico-chemical and conformational study of the interactions of gentamicin and bis(beta-diethylaminoethylether) hexestrol with negatively-charged phospholipid layers.

Aminoglycoside antibiotics such as gentamicin, which are fully hydrophilic, and cationic amphiphilic drugs such as bis(beta-diethylaminoethylether)hexestrol (DEH), are both known to inhibit lysosomal phospholipases and induce phospholipidosis. This enzymatic inhibition is probably related to the neutralization of the surface negative charges on which the lysosomal phospholipases A1 and A2 are dependent to express fully their activities (Mingeot-Leclerq et al., Biochem Pharmacol 37: 591-599, 1988). Using negatively charged liposomes, we show by 31P NMR spectroscopy that both gentamicin and DEH cause a significant restriction in the phosphate head mobility and, in sonicated vesicles, the appearance of larger bilayer structures. Both DEH and gentamicin increased the apparent size of sonicated negatively charged liposomes (but not of neutral liposomes) as measured by quasi-elastic light scattering spectroscopy. Examination of replicas from freeze-etched samples, however, revealed that gentamicin caused aggregation of liposomes, whereas DEH induced their fusion and the formation of intramembranous roundly shaped structures. Only DEH caused a significant decrease of the fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene, a fluorescent lipid-soluble probe. In addition, DEH, but not gentamicin, interfered with the bilayer to hexagonal phase transition occurring in dioleoyl- and dielaidoylphosphatidylethanolamine liposomes upon warming, and caused the appearance of an isotropic signal suggestive of the formation of inverted micelles. In computer-aided conformational analysis of the molecules at a simulated air-water interface, gentamicin was shown to display a largely-open crescent shape. When surrounded by phosphatidylinositol molecules, it remained as such at the interface which it locally mis-shaped, establishing close contact with the negatively charged phospho groups. In contrast, DEH could be oriented perpendicularly to the interface, with its two cationic groups associated with the phospho groups, and its phenyl- and diethylethandiyl moieties deeply inserted between and interacting with the aliphatic chains. Thus, although both agents cause lysosomal phospholipases inhibition, the differences in their interactions with negatively-charged bilayers is likely to result in a different organization of the phospholipids accumulated in vivo, which could lead to different toxicities.

Fluorescence Polarization↗

PubMed1988

Conformational analysis of lipoxin A, lipoxin B and their trans-isomers.

Lipoxin A and lipoxin B (LXA and LXB) are formed from the oxygenation of arachidonic acid by interactions between the 5- and 15-lipoxygenases of human leukocytes. Each compound displays highly stereospecific biological actions. Here, we present a computational description of the following compounds: lipoxin A, (5S,6R,15S)-trihydroxy-7,9,13-trans-11-cis-eicosatetraenoic acid; 11-trans-lipoxin A, (5S,6R,15S)-trihydroxy-7,9,11,13-trans-eicosatetraenoic acid; lipoxin B, (5S,14R,15S)-trihydroxy-6,10,12-trans-8-cis-eicosatetraenoic acid; and 8-trans-lipoxin B, (5S,14R,15S)-trihydroxy-6,8,10,12-trans-eicosatetraenoic acid. The analyses considered van der Waals energy, electrostatic interactions, torsional potential, and alterations in electrostatic forces. Additional analyses were carried out with each of the four compounds forming complexes with one calcium ion. Each compound gave very different conformers. Both lipoxin A and lipoxin B can form globular conformations, while their all-trans isomers form rigid extended structures. When complexes with each of these compounds and one calcium ion were examined (i.e., (LXA)2Ca: (11-trans-LXA)2Ca), both LXA and LXB formed several flexible conformations including crumpled, wrapped or extended conformations. In this situation, LXA showed a higher probability than LXB to wrap around one Ca2+. In contrast, the two all-trans isomers always lead to extended conformations. Results from the present study illustrate that changes in the stereochemistry of LXA and LXB lead to unique conformations which may underlie the different biological actions of these compounds. Moreover, they indicate that the conformations of eicosanoids can change while in aqueous or hydrophobic environments (i.e., biomembranes).

Arachidonate 15-Lipoxygenase↗

PubMed1988

Computed conformational analysis of lipoxins and their ionic complexes.

The possible molecular conformations of four structurally and biologically different lipoxins derivatives were predicted by a systematic structure tree theoretical analysis. This method takes into account the London-Van der Waals energy of interaction, the electrostatic interaction, the rotation energy of the torsional angles and the energy of transfer through a possible lipid-water interface. Finally, the conformers derived from the structure tree and with a high probability of existence were submitted to the energy minimization procedure. The most probable conformers of lipoxin A: 5S,6R,15S-trihydroxy-7,9,13 trans-11 cis-eicosatetraenoic acid (LXA); 11 trans lipoxin A: 5S,6R,15S-trihydroxy-7,9,11,13 trans-eicosatetraenoic acid (11t-LXA); lipoxin B: 5S,14R 15S-trihydroxy-6,10,12 trans-8 cis-eicosatetraenoic acid (LXB) and 8 trans lipoxin B: 5S,14R,15S-trihydroxy- 6,8,10,12 trans-eicosatetraenoic acid (8t-LXB) in their isolated form or when forming complexes with one calcium ion are presented. The four isolated compounds lead to vastly different conformations. Lipoxin A can form the most globular conformer while lipoxin B seems to be slightly more extended. The all trans isomer of lipoxin B forms an extended conformer and 11 trans lipoxin A gives a fully extended molecule. Complexes of a pair of these compounds with one calcium ion were shown to lead to vastly different conformations. Both (LXA) 2Ca and (LXB)2Ca form crumpled or extended structure, the LXA molecules being more wrapped around Ca2+ than LXB molecules. The (11t-LXA)2Ca and (8t-LXB)2Ca complexes present a high probability of extended conformations. Our description merely shows that the peculiar stereochemistry of these molecules lead to equilibria between conformers or to very static conformers, the flexibility and rigidity of which being probably relevant in view of their different biological activities.

Chemical Phenomena↗

PubMed1987

Aluminum-induced lipid phase separation and membrane fusion does not require the presence of negatively charged phospholipids.

The interaction of Aluminum with phosphatidyl serine lipid vesicles containing variable amounts of phosphatidyl ethanolamine, phosphatidyl choline and cholesterol has been studied by lipid phase separation monitored by fluorescence quenching. The interaction of Al3+ with neutral phospholipid membranes has also been investigated. Maximal lipid phase separation can be demonstrated in mixed phosphatidyl ethanolamine-cholesterol vesicles when using concentrations of aluminum between 87.5 and 125 microM. Millimolar concentrations of Ca2+, Mn2+, Cd2+ and Zn2+ were without any effect. Aluminum also induced fusion of phospholipid membranes monitored by resonance energy transfer between N-(7-nitro-2,1,3, benzoxadiazol-4 yl) phosphatidyl ethanolamine and N-(lissamine Rhodamine B-sulfonyl) phosphatidyl ethanolamine, either when containing low amounts of phosphatidyl serine (12.5%) or without any negatively charged phospholipid. Aluminum-induced fusion of liposomes was also monitored by the fluorescence of the terbium-dipicolinic acid complex (Tb-DPA3-) formed during fusion of vesicles containing either Tb-(citrate)6- complex or sodium salt of dipicolinic acid.

Aluminum↗

PubMed1986

Synergistic effects of micromolar concentrations of Zn2+ and Ca2+ on membrane fusion.

Resonance Energy Transfer between N-(7-nitro-2,1,3 benzoxadiazol -4 yl) phosphatidyl ethanolamine and N-Lissamine-Rhodamine B sulfonyl) phosphatidyl ethanolamine embedded in two different populations of small unilamellar vesicles made of phosphatidyl serine has been used to study the fusion process induced by Zn2+ and Ca2+. Lipid intermixing demonstrating fusion of liposome membranes can already be observed at 125 and 250 mumol/l of Zn2+. After short time pre-incubations with micromolar concentrations of Zn2+ as low as 150 mumol/l, Ca2+ induces an instantaneous increase of vesicle fusion. The lipid intermixing induced by micromolar concentrations of Ca2+ (250-500 mumol/l) could be increased up to 4 times when pre-incubated with 150 or 200 mumol/l of Zn2+. The effect of 1 mM of Ca2+ alone on lipid intermixing can be mimicked by 150 mumol/l of Zn2+ followed by 500 mumol/l of Ca2+. Our data demonstrate that Zn2+ and Ca2+ act synergistically to affect cation-induced membrane fusion. We suggest that Zn2+ specifically alters the physical state of phospholipid membranes making them more prone to calcium-triggered fusion.

Calcium↗

PubMed1986

Na+-H+ exchange in the process of glucose-induced insulin release from the pancreatic B-cell. Effects of amiloride on 86Rb, 45Ca fluxes and insulin release.

The effect of amiloride, an inhibitor of Na+-H+ exchange, on intracellular pH (pHi), 86Rb outflow, 45Ca outflow and insulin release from pancreatic rat islets was examined. In the 0.1-1 mM range, amiloride transiently reduced pHi of glucose-deprived islets and allowed glucose to induce a sustained decrease in pHi of the islet cells. Amiloride reproduced the effect of glucose to decrease 86Rb and 45Ca outflow. In the presence of glucose (5.6 mM or more), amiloride (100 microM) acted synergistically with the sugar to reduce K+ outflow, and to stimulate 40Ca inflow and insulin release from perifused islets. These results add strong support to the view that the generation of protons through the metabolism of glucose represents an important step in the process of glucose-induced release. The stimulation by glucose of Na+-H+ exchange apparently masks and even overcomes the glucose-induced decrease in pHi otherwise expected from the increase in catabolic fluxes.

Amiloride↗

PubMed1986

Micromolar concentrations of Zn2+ potentiates Ca2+-induced phase separation of phosphatidyl serine containing liposomes.

Fluorescence quenching of 1-acyl-2-[6[(7 nitro-2,1,3-benzoxadiazol-4yl) amino]caproyl] phosphatidyl choline in small unilamellar vesicles consisting of phosphatidyl serine has been used to monitor the lipid phase separation induced by Zn2+ and Ca2+. Phase separation of vesicle membranes was observed with Zn2+ at concentrations as low as 125 microM. Low concentrations of Zn2+ required long incubation times to reach maximal quenching (120 minutes at 375 microM). When low concentrations of Ca2+ were added to the preparation during the developing phase of Zn2+-induced quenching, an explosive increase in fluorescence quenching was instantenously observed. Phase separation induced by sub-millimolar concentrations of Ca2+ could be increased at least 4 times when vesicles were pre-incubated with 250 microM of Zn2+.

4-Chloro-7-nitrobenzofurazan↗

PubMed1986

Neurotoxic cations induce membrane rigidification and membrane fusion at micromolar concentrations.

The effect of the neurotoxic cations aluminum, cadmium and manganese on membranes was examined in sonicated unilamellar vesicles containing phosphatidylserine and compared to the effect of Ca2+. Fusion of membranes was monitored by assessing the resonance energy transfer between N-(7-nitrobenz-2-oxa-1,3-diazol-4-y)phosphatidylethanolamine and N-(lissamine-rhodamine B-sulfonyl)phosphatidylethanolamine. Self-quenching of high concentrations of carboxyfluorescein in liposomes was used to demonstrate the release of molecules entrapped in liposomes to compare the kinetics of leakage and intermixing of lipid. Rigidification of membranes was evaluated by monitoring the fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene embedded in membranes containing phosphatidylserine and dipalmitoylphosphatidylcholine. Cation-induced lipid intermixing of vesicles membranes and release of dye from the vesicles occurred in the same concentration range. With aluminum, these effects were observed with concentrations less than 25 microM. Significant rigidification of vesicle membranes was apparent with less than 25 microM of Al3+. Similar effects could only be observed with concentrations of Cd2+ and Mn2+ at least one order of magnitude higher (200 and 400 microM, respectively).

Aluminum↗

PubMed1986

Protonophoretic activity of hypoglycemic sulfonylureas in black lipid membranes.

The ability of glibenclamide and of gliclazide to act as protonophores has been studied in black lipid membranes made of glycerol monoleate as a function of pH and drug concentration. Protons may effectively be transported across lipid membranes with a maximum at a pH close to the pKa of the sulfamide (pKa of glibenclamide = 5.4). It is suggested therefore that hypoglycemic sulfonylureas may act in part as Ca2+ inward and H+ outward ionophores through plasma membranes of pancreatic B cells.

Biological Transport↗

PubMed1986

Divergent calciphoretic properties of negatively charged phospholipids in a two-phase system and in liposomes.

The ionophoretic properties of several negatively charged phospholipids for transporting calcium ions in an organic phase and across membranes of lipid vesicles have been studied. The amounts of Ca2+ translocated in the organic phase depend on the lipid type but also on the nature of the hydrocarbon chains. Under our experimental conditions, Ca2+ outflow from liposomes is observable with phosphatidic acid derived from egg yolk (PA) and with bovine brain phosphatidyl serine (PS) while dipalmitoyl phosphatidic acid (DPPA), cardiolipin (CL), phosphatidyl inositol (PI) and phosphatidyl inositol diphosphate (PIP2) remained unactive as calcium ionophores through lipid membranes but active as calcium translocators in an organic phase.

Animals↗

PubMed1985

Structural considerations for calcium ionophoresis by prostaglandins.

The prostaglandins PGB2, PGE2 and PGF2 alpha were found to translocate calcium in a modified Pressman cell. At pH 7.40, PGB2 was more potent than PGE2 and than PGF2 alpha. When incorporated at a 1% molar ratio in liposomes made of cholesterol and different diacyl phosphatidyl choline, prostaglandins are able to mediate a slow calcium exchange diffusion. A significant prostaglandin-mediated calcium release that depends on the lipid matrix rigidity is observable at 37 degrees but not at 22 degrees. Conformational analysis of the complex formed by two molecules of prostaglandins and one calcium atom, either at a simulated membrane-water interface or in a simulated bulk lipid phase reveals rigid complexes with great distances between hydrophilic and hydrophobic gravity centres that predict low ionophoretic properties.

Calcium↗

PubMed1985

Micromolar concentrations of Al3+ induce phase separation, aggregation and dye release in phosphatidylserine-containing lipid vesicles.

The interaction of Al3+, Cd2+ and Mn2+ with phosphatidylserine-containing lipid vesicles was studied. Phase separation of vesicles was investigated by monitoring fluorescence quenching of the phospholipid analogue 1-palmitoyl-2-(6-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)] aminocaproyl)phosphatidylcholine (C6-NBD-PC). Aggregation was determined by turbidimetry and leakage of vesicles content during fusion was monitored by the fluorescence of released 6-carboxyfluorescein. Al3+ demonstrated quenching at less than 30 mumol/l with a maximum effect at 100 mumol/l. Al3+-induced aggregation and dye release from the lipid vesicles were observed in the same concentration range. The effect of Cd2+ and Mn2+ on quenching was much less pronounced and could only be demonstrated in the 0.1-1 mmol/l range. Increasing amounts of phosphatidylcholine or phosphatidylethanolamine in the vesicles decreased both Al3+-induced quenching and aggregation, whereas cholesterol only slightly increased aggregation without affecting quenching.

Aluminum↗

PubMed1985

Adriamycin effects on insulin secretion, Ca2+ movements and glucose oxidation in pancreatic islet cells.

The effects of Adriamycin on islet B cells has been investigated for insulin secretion, Ca movements and glucose oxidation in order to determine other toxic effect of the new antineoplastic drug than cardiotoxic effects. Our results suggest that therapeutic doses of Adriamycin could be highly toxic for endocrine secretory function. These effects are discussed in terms of interactions of the drug with the respiratory chain.

Animals↗

PubMed1985

Nutrient-induced changes in the pH of pancreatic islet cells.

Fluorescein rapidly accumulates in rat pancreatic islets exposed to fluorescein diacetate. The influence of environmental agents upon cellular pH was examined in fluorescein-labelled islets by recording their fluorescence intensity at 520 nm after excitation at 490 and 435 nm, respectively. Glucose caused a rapid, sustained and dose-related increase in cellular pH. Another nutrient secretagogue, 2-ketoisocaproic acid, also increased cellular pH. The stimulation of islet cells by non-nutrient secretagogues, e.g. by glibenclamide or in response to an increase in extracellular K+ concentration, decreased cellular pH, indicating that the nutrient-induced increase in cellular pH is not merely a consequence of stimulated Ca2+ inflow and/or insulin release. In either the presence of amiloride or absence of bicarbonate, glucose decreased cellular pH. These results strongly suggest that the acidification of islet cells which can be expected from the increased metabolism of glucose in glucose-stimulated islets is normally masked and overcome by stimulation of such processes as Na+/H+ and HCO3-/Cl- exchange.

Animals↗

PubMed1985

Glucose increases cytosolic Ca2+ activity in pancreatic islet cells.

Isolated cells prepared from rat pancreatic islets were labelled with the tetraacetoxymethyl ester of the fluorescent Ca2+ indicator quin-2. An increase in the extracellular concentration of glucose provoked a rapid and sustained increase in the fluorescence of the labelled cells. This indicates that glucose increases cytosolic Ca2+ activity in pancreatic islet cells.

Aminoquinolines↗