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S Nir

Publications and source records attributed to S Nir.

At least 91 records · Page 5Linked to original sources
PubMed1985

Kinetics of Ca2+-induced fusion of cardiolipin-phosphatidylcholine vesicles: correlation between vesicle aggregation, bilayer destabilization, and fusion.

We have investigated the kinetics of Ca2+-induced aggregation and fusion of large unilamellar vesicles composed of an equimolar mixture of bovine heart cardiolipin and dioleoylphosphatidylcholine. Mixing of bilayer lipids was monitored with an assay based on resonance energy transfer (RET) and mixing of aqueous vesicle contents with the Tb/dipicolinate assay. The results obtained with either assay were analyzed in terms of a mass action kinetic model, providing separate rate constants for vesicle aggregation and for the fusion reaction proper. At different Ca2+ concentrations, either at 25 degrees C or at 37 degrees C, aggregation rate constants derived from the data obtained with the RET assay were the same as those derived from the Tb/dipicolinate data, indicating that mixing of bilayer lipids occurred only during vesicle aggregation events that resulted in mixing of aqueous contents as well. At 25 degrees C, identical fusion rate constants were obtained with either assay, indicating that at this temperature the probability of lipid mixing and that of aqueous contents mixing, occurring after vesicle aggregation, were the same. The fusion rate constants for the RET assay increased more steeply with increasing temperature than the fusion rate constants derived from the Tb/dipicolinate data. As a result, at 37 degrees C the tendency of the vesicles, after aggregation, to mix lipids was slightly higher than their tendency to mix aqueous contents. The aggregation rate constants increased steeply with Ca2+ concentrations increasing in a narrow range (9.5-11 mM), indicating that, in addition to a Ca2+-dependent charge neutralization on the vesicle surface, structural changes in the lipid bilayer are involved in the aggregation process.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium↗
PubMed1985

Effect of cholesterol on Ca2+-induced aggregation and fusion of sonicated phosphatidylserine/cholesterol vesicles.

Small unilamellar vesicles composed of phosphatidylserine (PS) and cholesterol at various ratios were employed in studying the effect of cholesterol on Ca2+-induced vesicle aggregation and fusion using the Tb/dipicolinic acid assay. The leakage of preencapsulated Tb3+ was also measured. The analysis of the data provided estimates for the rate of aggregation C11, and the rate of fusion per se, f11. An increase in cholesterol contents results in a decrease in C11 values. Similarly, aggregation of PS/cholesterol vesicles is slower than that of PS vesicles in the presence of 650 mM NaCl. With 100 or 200 mM NaCl, the overall fusion rate of PS/cholesterol vesicles is slower than that of PS vesicles; the rate being reduced by an increase in cholesterol contents. With 600 mM NaCl, the overall fusion rate of PS/cholesterol 9:1 vesicles is faster than that of PS vesicles, and results are well-simulated by assuming no delay in vesicle aggregation up to dimers. Emerging f11 values are larger in PS/cholesterol than in PS vesicles. An analysis of fusion kinetics of several lipid concentrations shows that f11 values of PS/cholesterol 3:1 vesicles are 5-times larger than those of PS vesicles, when fusion occurs in a medium containing 200 mM NaCl and 1.5 mM Ca2+. The increase in Na+ concentration from 100 to 200 mM, or 600 mM results in a 50- or 150-fold reduction in f11 values of PS vesicles. It is suggested that incorporation of cholesterol in PS vesicles results in enhancement of Ca2+-induced fusogenic capacity.

Calcium↗
PubMed1985

Temperature dependence of divalent cation induced fusion of phosphatidylserine liposomes: evaluation of the kinetic rate constants.

The effect of temperature and divalent cation binding (Ca2+, Sr2+, Ba2+) on the kinetic rate constants of aggregation and fusion of large phosphatidylserine liposomes is measured for the first time. Fusion is monitored by the Tb3+/dipicolinate assay. Fusion rate constants increase with temperature (15-35 degrees C) in a roughly linear fashion. These rate constants are not otherwise sensitive to whether the temperature is above or below the phase transition temperature of the Ba2+ or Sr2+ complex of phosphatidylserine, as measured by differential scanning calorimetry. Hence, the isothermal transition of the acyl chains from liquid-crystalline to gel phase induced by the cations is not the driving force of the initial fusion event. The aggregation rate constants increase with temperature, and it is the temperature dependence of the energetics of close approach of the liposomes which underlies this increase. On the other hand, the aggregation becomes more reversible at higher temperatures, which has also been observed with monovalent cation induced liposome aggregation where there is no fusion. Calculations on several cases show that the potential energy minimum holding the liposome dimer aggregates together is approximately 5-6 kT deep. This result implies that the aggregation step is highly reversible; i.e., if fusion were not occurring, no stable aggregates would form.

Barium↗
PubMed1983

Binding of monovalent cations to phosphatidylserine and modulation of Ca2+- and Mg2+-induced vesicle fusion.

The effect of several monovalent cations on the Ca2+-induced aggregation and fusion of sonicated phosphatidylserine (PS) vesicles is studied by monitoring the mixing of internal compartments of the fusing vesicles using the Tb/dipicolinic acid assay. The dissociation of the fluorescent Tb-dipicolinate complex which accompanies Ca2+-induced vesicle fusion is determined directly and is due to leakage of contents and entry of medium into vesicles. PS vesicles do not fuse when the medium contains only monovalent cations (at pH 7.4), regardless of the cation concentration or whether there is aggregation of the vesicles. A mass-action kinetic analysis of the data provides estimates for the rate of aggregation, C11, and for the rate of fusion per se, f11. Values of f11 increase dramatically with reduction in monovalent cation concentration and are primarily determined by binding ratios of Ca2+ or Mg2+ per PS. With 300 mM of monovalent cations, the fusion per se is essentially rate-limiting to the overall fusion process and values of f11 are significantly larger with the monovalent cations which bind the least, i.e., according to the sequence tetramethylammonium greater than K+ greater than Na+ greater than Li+. With monovalent cations in concentrations of 100 mM or less, the aggregation is rate-limiting to the fusion and the overall initial fusion rates are determined by an interplay between aggregation and fusion rates. Under conditions of fast aggregation, the Ca2+-induced fusion of small PS vesicles can occur within milliseconds or less.

Animals↗
PubMed1981

Interactions of La2+ with phosphatidylserine vesicles: binding, phase transition, leakage, 31P-NMR and fusion.

The interaction of La2+ with phosphatidylserine vesicles is studied by differential scanning calorimetry, 140La binding, 31P-NMR chemical shifts and relaxation rates, carboxyfluorescein and [14C]sucrose release, X-ray diffraction and freeze-fracture electron microscopy. In the presence of La3+ concentrations above 1 mM and an incubation temperature of 38 degrees C, i.e., at the phase transition temperature of the complex La/phosphatidylserine, the binding ratio of La/lipid exceeds a 1/3 ratio, reaching saturation at a 1/2 ratio. Analysis, employing a modified Gouy-Chapman equation, indicates a significant increase in the intrinsic binding constant of La/phosphatidylserine when the La3+ concentrations exceeds the threshold concentration for leakage. The analysis illustrates that at the molecular level the binding of La3+ can be comparable to or even weaker than that of Ca2+, but that even when present at smaller concentrations La3+ competes with and partially displaces Ca2+ from membranes or other negatively charged surfaces. The results suggest that the sequence La3+ greater than Ca2+ greater than Mg2+ reflects both the binding strength of these cations to phosphatidylserine as well as their ability to induce leakage, enhancement of 31P spin-lattice relaxation rates, fusion and other structural changes. The leakage, fusion, and other structural changes are more pronounced at the phase transition temperature of the La/lipid complex.

Electron Spin Resonance Spectroscopy↗
PubMed1981

Calcium- and magnesium-induced fusion of mixed phosphatidylserine/phosphatidylcholine vesicles: effect of ion binding.

The aggregation, leakage, and fusion of pure PS (phosphatidylserine) and mixed PS/PC (phosphatidylcholine) sonicated vesicles were studied by light scattering, the release of encapsulated carboxyfluorescein, and a new fusion assay which monitors the mixing of the internal compartments of fusing vesicles. On a time scale of 1 min the extent of fusion was considerably greater than leakage. The Ca2+ and Mg2+ concentrations required to induce fusion increased when the PS content of the vesicles was decreased, and/or when the NaCl concentration was increased. Calculations employing a modified Gouy-Chapman equation and experimentally determined intrinsic binding constants of Na+ and Ca2+ to PS were shown to predict correctly the amount of Ca2+ bound in mixed PS/PC vesicles. For vesicles composed of either pure PS or of mixtures with PC in 100 mM NaCl (4:1 and 2:1 PS/PC); the induction of fusion (on a time scale of minutes) occurred when the amount of Ca or Mg bound/PS molecule exceeded 0.35-0.39. The induction of fusion for both pure PS and PS/PC mixed vesicles (with PS exceeding 50%) can be explained by assuming that destabilization of these vesicles requires a critical binding ratio of divalent cations to PS.

Binding Sites↗
PubMed1981

Aggregation of colloidal particles modeled as a dynamical process.

Aggregation kinetics of sonicated phosphatidylserine (PtdSer) vesicles in NaCl indicate that the process is fully reversible and dynamical, involving the rapid formation and dispersal of aggregates. Accordingly, the general mass action kinetic equations are analyzed with respect to the equilibrium state and the formation of higher order aggregates. For a general class of systems, the values for the mass average aggregate size at equilibrium are obtained from simple closed-form expressions. It is shown that an analysis of the aggregation equilibrium will yield estimates for the potential energy well that holds the aggregates together. A fit to the experimental data for kinetics of Na+-induced aggregation of the vesicles has been achieved by employing mass action kinetic equations that include the dissociation reactions. The threshold of NaCl concentration required for aggregation involves the clear distinction between the rate and extent of aggregation.

Colloids↗
PubMed1980

Mass action kinetics of phosphatidylserine vesicle fusion as monitored by coalescence of internal vesicle volumes.

The kinetics of Ca2+-induced fusion of sonicated phosphatidylserine vesicles is analyzed by means of the mass action model. The results of calculations are shown to simulate the experimental results for the mixing of aqueous vesicle volumes, release of vesicle contents and for the observed increase in light scattering [Wilschut, J., Düzgünes, N., Fraley, R., & Papahadjopoulos, D. (1980) Biochemistry (first of three papers in this issue)]. The calculations give the distribution of vesicle sizes during the initial stages of the fusion process and an estimate for the occurrence of multiple fusion events. It is estimated that during the first few seconds from the beginning of the fusion process in the above systems only a small fraction of the material trapped will leak during each fusion event. The fraction of material which leaks per fusion event is further reduced with increased Ca2+ concentrations. The values of the rates of fusion which describe the above experiments suggest that the rate limiting step of the overall fusion reaction is the aggregation and close approach of vesicles to each other rather than the fusion event per se.

Calcium↗
PubMed1980

Reversibility of sodium-induced aggregation of sonicated phosphatidylserine vesicles.

The kinetics of sodium-induced aggregation of sonicated phosphatidylserine vesicles has been studied as a function of sodium concentration and temperature. The concentration threshold for aggregation induced by monovalent sodium has been found to be 550 mM sodium by stopped-flow rapid-mixing techniques. This aggregation is completely reversible to changes in sodium ion concentration and to changes in temperature. The aggregation rate decreases with increasing temperature, indicating that the backward reaction rate increases more rapidly with temperature than does the forward rate.

Cell Aggregation↗
PubMed1979

Interactions of La3+ with phosphatidylserine vesicles. Binding, phase transition, leakage and fusion.

The interaction of La3+ with phosphatidylserine vesicles is elucidated by binding studies, differential scanning calorimetry, X-ray diffraction, freeze fracture electron microscopy, and release of vesicle contents. La3+ effectively competes with Ca2+ for phosphatidylserine binding sites. The saturation level is close to a La/lipid ratio of 1:3. A concentration of 0.1 mM of La3+ is sufficient to induce fusion between sonicated vesicles.

Calorimetry, Differential Scanning↗
PubMed1979

Specificity of Na+ binding to phosphatidylserine vesicles from a 23Na NMR relaxation rate study.

23Na NMR relaxation rate measurements show that Na+ binds specifically to phosphatidylserine vesicles and is displaced partially from the binding site by K+ and Ca2+ but to a considerably less extent by tetraethylammonium ion. The data indicate that tetraethylammonium ion affects the binding of Na+ only slightly, by affecting the surface potential through its presence in the double layer, without competing for a phosphatidylserine binding site. Values for the intrinsic binding constant for the Na+-phosphatidylserine complex that would be consistent with the competition experiments (and the dependence of the relaxation rate on concentration of free Na+) fall in the range 0.4--1.2 M-1 with a better fit towards the higher values. We conclude that in the absence of competing cations in solution an appreciable fraction of the phosphatidylserine sites could be associated with bound Na+ at 0.1 M Na+ concentration.

Calcium↗
PubMed1978

Specificity of Ca2+ and Mg2+ binding to phosphatidylserine vesicles and resultant phase changes of bilayer membrane structure.

Differences in the interaction of Ca2+ and Mg2+ with phosphatidylserine vesicles were revealed by binding studies, differential scanning calorimetry and X-ray diffraction. The two cations produced structurally different complexes with phosphatidylserine as evidenced by phase transition characteristics, lamellar spacings, and hydrocarbon packing. Ca2+ effectively completes with Mg2+ for phosphatidylserine binding sites. Analysis of the binding data showed that Ca2+ had a ten-fold greater intrinsic binding constant for phosphatidylserine.

Binding Sites↗