Influence of membrane viscosity on the lateral and transverse mobility of carboxylic ionophores (original) (raw)
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INFLUENCE OF MEMBRANE FLUIDITY UPON IONOPHOREMEDIATED CALCIUM TRANSPORT IN LIPOSOMES
Biomedical research, 1980
The influence of membrane fluidity upon ionophore-mediated ion transport was investigated in liposomes containing X537A. Liposomes of increasing viscosity (1.5-13.3 poise) were prepared from different lipid mixtures. Although, the different lipids exerted virtually no effect upon the ionophoretic capacity of X537A in a twophase bulk system, the rate of X537A-mediated Ca transport in the liposomes was dramatically affected by changes in fluidity. Both the absolute and relative increment in "5Ca transport rate attributable to a constant increase in X537A concentration tightly depended on the liposome viscosity, suggesting that an increase in fluidity augments both the transverse and lateral mobility of the ionophoretic molecules. It is postulated that the fluidity of biological membranes represents a major regulatory factor in the efflciency of ion transport as mediated by native ionophores.
The Effect of Cholesterol on the Lateral Diffusion of Phospholipids in Oriented Bilayers
Biophysical Journal, 2003
Pulsed field gradient NMR was utilized to directly determine the lipid lateral diffusion coefficient for the following macroscopically aligned bilayers: dimyristoylphosphatidylcholine (DMPC), sphingomyelin (SM), palmitoyloleoylphosphatidylcholine (POPC), and dioleoylphosphatidylcholine (DOPC) with addition of cholesterol (CHOL) up to ;40 mol %. The observed effect of cholesterol on the lipid lateral diffusion is interpreted in terms of the different diffusion coefficients obtained in the liquid ordered (l o ) and the liquid disordered (l d ) phases occurring in the phase diagrams.Generally, the lipid lateral diffusion coefficient decreases linearly with increasing CHOL concentration in the l d phase for the PC-systems, while it is almost independent of CHOL for the SM-system. In this region the temperature dependence of the diffusion was always of the Arrhenius type with apparent activation energies (E A ) in the range of 28-40 kJ/mol. The l o phase was characterized by smaller diffusion coefficients and weak or no dependence on the CHOL content. The E A for this phase was significantly larger (55-65 kJ/mol) than for the l d phase. The diffusion coefficients in the two-phase regions were compatible with a fast exchange between the l d and l o regions in the bilayer on the timescale of the NMR experiment (100ms). Thus, strong evidence has been obtained that fluid domains (with size of mm or less) with high molecular ordering are formed within a single lipid bilayer. These domains may play an important role for proteins involved in membrane functioning frequently discussed in the recent literature.The phase diagrams obtained from the analysis of the diffusion data are in qualitative agreement with earlier published ones for the SM/CHOL and DMPC/ CHOL systems. For the DOPC/CHOL and the POPC/CHOL systems no two-phase behavior were observed, and the obtained E A :s indicate that these systems are in the l d phase at all CHOL contents for temperatures above 258C.
Journal of Molecular Evolution, 1994
Liposomes formed from egg-yolk phosphatidylcholine:egg-yolk phosphatidate (molar ratio 2:1) containing pBR322 DNA and DNase I were induced to form, with divalent cations, bilayer/nonbilayer phase transitions of phosphatidate which allowed cation diffusion into liposomes; then cation diffusion was measured by the activation of the hydrolysis of DNase I on DNA. The formation of phosphatidate transitions on liposomes was demonstrated by freeze-fracture and 3lp NMR, and a direct correlation between the formation of phosphatidate transitions and the transbilayer diffusion of cations was found: only Ca 2+ and Mn 2÷, which induce phase transitions, were able to penetrate liposomes and triggered the DNase I activity; in addition, Ca 2÷ at higher concentrations (10 mM) caused fusion of liposomes, whereas Mn 2÷ did not, suggesting that transitions induced by Mn ~+ participated only in the diffusion of this ion; furthermore, Mg 2+ neither formed phase transitions nor triggered the enzymatic activity. The liposomes studied represent more dynamic structures that can form phosphatidate structures involved in both (1) the interchange of divalent cations with the surroundings, thereby modulating encapsulated enzymes, and (2) the fusion of lipid vesicles probably implicated in the enrichment of liposomal content in the early Precambian Earth.
Biochimica Et Biophysica Acta - Biomembranes, 1977
Charge-pulse relaxation experiments of valinomycin-mediated Rb ÷ transport have been carried out in order to study the influence of membrane structure on carrier kinetics. From the experimental data the rate constants of association (ha) and dissociation (kD) of the ion-carrier complex as well as the rate constants of translocation of the complex (kMs) and of the free carrier (ks) could be obtained. The composition of the planar bilayer membrane was varied in a wide range. In a first series of experiments, membranes made from glycerolmonooleate dissolved in different n-alkanes (n-decane to n-hexadecane), as well as solvent-free membranes made from the same lipid by the Montal-Mueller technique were studied. The translocation rate constants ks and kMS were found to differ by less than a factor of two in the membranes of different solvent content. Much larger changes of the rate constants were observed if the structure of the fatty acid residue was varied. For instance, an increase in the number of double bonds in the C:0 fatty acid from one to four resulted in an increase of ks by a factor of seven and in an increase of kMS by a factor of twenty-four. The stability constant K = kR/k D of the ion-carrier complex as well as the translocation rate constants ks and kMs were found to depend strongly on the nature of the polar headgroup of the lipid. The incorporation of cholesterol into glycerolmonooleate membranes reduced kR, kMS and k s up to seven-fold.
Influence of Cholesterol and Water Content on Phospholipid Lateral Diffusion in Bilayers
Langmuir, 2003
The dependence of lipid lateral diffusion on temperature, cholesterol, and water contents has been studied in oriented bilayers in three phosphatidylcholine lipid systems and one sphingomyelin system. The lateral diffusion of lipids is found to be reduced both by the addition of cholesterol and by decreasing the water content. This reduction can be ascribed to the increase in the ordering of the lipid acyl chains upon cholesterol addition and/or reduced water content. The dependence of the lateral diffusion coefficient on the water content is similar in both the liquid ordered and the liquid disordered phase, while the apparent activation energy for the diffusion process is larger in the liquid ordered phase. We also report an anomalous increase in the lipid diffusion upon small additions of cholesterol to bilayer systems at low water content and temperatures. The increased diffusion is tentatively explained by a reduction of lipid chain entanglements.
Influence of sterols on ion transport through lipid bilayer membranes
Biochimica et Biophysica Acta (BBA) - Biomembranes, 1978
Charge-pulse relaxation experiments with the negatively charged lipophilic ions, dipicrylamine and tetraphenylborate, (as well as with the positively charged carrier system Rb*-valinomycin) have been carried out in order to study the influence of sterols on the ion transport through the lipid bilayer membrane. The mol fraction of the sterols (cholesterol, epicholesterol, ergosterol, stigmasterol, dihydrocholsterol, epicoprostanol and cholesterololeate) as referred to total lipid was varied in a wide range (mol fractions 0-0.8). The monoolein/sterol or dioleoylphosphatidylcholine/sterol mixtures were dissolved in n-hexadecane in order to minimize effects of the sterol on the membrane thickness. Cholesterol had a strong influence on the transport of the lipophilic ions. Its incorporation into monoolein membranes increased the rate constant k i of translocation up to 8-fold, but incorporation into phosphatidylcholine membranes had virtually no influence on k i. The other sterols with one hydroxy group and cholesterololeate had no influence on the rate constant or the partition coefficient ~. The results are discussed on the basis of a possible change of dipole potential of the membrane caused by'cholesterol and its derivatives. In the case of valinomycin-mediated Rb ÷ transport only cholesterol had a strong influence on transport properties. The rate constants of association (kR) as well as the rate constants of translocation of the complex (kMS) and of the free carrier (ks} were reduced by incorporation of cholesterol up to eightfold. The decrease of ks and kMS are possibly caused by a decrease of membrane fluidity, whereas the decrease of kR may be due to an increase of surface potential. The different action of cholesterol on the two transport systems is discussed under the assumption that the adsorption plane of the lipophilic ion is located more towards the aqueous side and that of the ion-carrier complexes more towards the hydrocarbon side of the dipole layer.
The Journal of Membrane Biology, 1982
port have, in the main, utilized artificial membranes, both planar and vesicular. Systems of biological interest, viz., cells and organelles, resemble vesicles in size and geometry. Methods are, therefore, required to extend the results obtained with planar membranes to liposome systems. In this report we present an analysis of a fluorescence technique, using the divalent cation probe chlortetracycline, in small, unilamellar vesicles, for the study of divalent cation fluxes. An ion carrier (X537A) and a pore former (alamethicin) have been studied. The rate of rise of fluorescence signal and the transmembrane ion gradient have been related to transmembrane current and potential, respectively. A second power dependence of ion conduction-including the electrically silent portion thereof-on X537A concentration, has been observed. An exponential dependence of "current" on "transmembrane potential" in the case of alamethicin is also confirmed. Possible errors in the technique are discussed.
Ion Transport Across a Bilayer Lipid Membrane in the Presence of a Hydrophobic Ion or an Ionophore
Journal of nuclear and radiochemical sciences, 2005
The ion transport from one aqueous phase (W1) to another (W2) across a bilayer lipid membrane (BLM) in a cell system in the presence of a hydrophobic ion or an ionophore was investigated by voltammetry. The ion transport current was observed by addition of a small amount of hydrophobic ion such as tetraphenylborate, dipicrylaminate, etc. into W1 or/and W2 containing a hydrophilic salt serving as a supporting electrolyte. The hydrophobic ion was distributed into the BLM with the counter ion to hold the electroneutrality within the BLM. It was pointed out that the counter ion could transfer between W1 and W2 across the BLM since concentrations of the counter ion in WI, BLM, and W2 were so high as to cause the ion transfer current while concentrations of the hydrophobic ion were very low. The facilitated transports of alkali ions across a BLM containing valinomycin (Val) used as an ionophore were also investigated by considering the hydrophobicity of both the objective cation and the counter anion and the formation of the alkali metal ion-Val complex.
Influence of membrane structure on ion transport through lipid bilayer membranes
The Journal of Membrane Biology, 1978
Charge-pulse relaxation studies with the positively charged PV-K + complex (cyclo-(D-Val-L-Pro-L-Val-D-Pro)3) and the negatively charged lipophilic ion dipicrylamine (DPA-) have been performed in order to study the influence of structural properties on ion transport through lipid bilayer membranes. First, the thickness of monoolein membranes was varied over a wide range using different n-alkanes and solvent-free membranes. The thickness (d) of the hydrocarbon core of these membranes varied between 4.9 and 2.5 nm. For both transport systems the partition coefficient/~ was found to be rather insensitive to variations in d. The same was valid for the translocation rate constant kMs of PV-K § whereas a strong increase of the translocation rate constant k i of DPA-with decreasing d was observed. In a further set of experimental conditions the structure of the lipids, such as number and position of the double bonds in the hydrocarbon chain and its chain length as well as the nature of the polar head group, was varied. The translocation constant k~s of PV-K § transport was found to be much more sensitive to these variations than k~ of DPA- .
Lateral diffusion of saturated phosphatidylcholines in cholesterol-containing bilayers
Biophysics, 2007
Abstract⎯A pulsed field gradient NMR was used to study lateral diffusion in the cholesterol-containing oriented bilayers of saturated (dipalmitoyl-and dimyristoyl-) phosphatidylcholines, upon their limiting hydration. Similar dependences of lateral diffusion coefficients on temperature and cholesterol concentration were observed, which agree with phase diagram showing the presence of the regions of disordered and ordered liquid-crystalline phases and a two-phase region. Under the same conditions, the lateral diffusion coefficient of dipalmitoylphosphatidylcholine is lower, which agrees qualitatively with its larger molecular weight. The comparison of data for dipalmitoylphosphatidylcholine with previous results for dipalmitoylsphingomyelin−cholesterol bilayers under the same conditions, in spite of similarity of phase diagrams, shows large (two−three times) differences in the lateral diffusion coefficient and a different profile of its dependence on cholesterol concentration. The comparison of data for dimyristoylphosphatidylcholine with previous results shows that the values of lateral diffusion coefficient and the shape of its dependence on cholesterol concentration coincide at high concentrations (>15 mol%) but differ at lower concentrations The revealed disagreement may be caused by the fact that the measurements were carried out at different water content in the system. At limiting hydration (more than 35% of water), the lateral diffusion coefficient for lipids decreases when cholesterol concentration rises, while at water content about 25% (as a result of equilibrium hydration from vapors) the lateral diffusion coefficient of phosphatidylcholine may be independent of cholesterol concentration. This is the consequence of the denser packing of molecules in the bilayer at reduced water content, an effect that competes with the ordering effect of cholesterol.