Studies on phosphatidylcholine model membranes. I. Size-heterogeneity effect on permeability measurements

Liposome permeability probed by laser light scattering

We have developed a laboratory project in which students prepare liposomes, expose them to hyperosmotic and hypoosmotic solutions, and follow the resulting shrinking and swelling (respectively) with laser light scattering. Each light intensity transient can be fit to an exponential decline or rise, with the decay constant (k) and the amplitude (ΔV_max ) being indirectly related to the kinetics and thermodynamics (respectively) of transmembrane osmotic flux. Students vary the experimental system by changing the types and concentrations of osmolytes such as alcohols, amides, and salts. Students then compare how these changes alter the rate and extent of osmotic flux. This upper division biochemistry laboratory project is a challenging and rewarding one that exposes students to a biomolecule (lipid) and a spectroscopic technique (laser spectroscopy) that are not commonly used in the undergraduate laboratory setting. © 2019 International Union of Biochemistry and Molecular Biology, 47(3):239-246, 2019.

Characterization of 5-fluorouracil loaded liposomes prepared by reverse-phase evaporation or freezing-thawing extrusion methods: study of drug release

Entrapment of the anti-tumoral drug 5-fluorouracil (5-FU) in unilamellar liposomes prepared by freeze-thawing extrusion technique (FATVET) and the reverse-phase evaporation method (REV) from natural (bovine brain) sphingomyelin (SM) and synthetic distearoylphosphatidylcholine (DSPC) phospholipids was studied. Reverse-phase evaporation vesicles obtained from DSPC sized through polycarbonate membranes of 0.2 micron pore size were found to entrap roughly double amounts of drug than did extruded liposomes (0.1 micron pore size); however, s-REV in these preparations were more heterogenous in vesicle size than FATVET. The rate of in vitro drug release from the liposomes was found to be dependent of the bilayer composition and the method used to prepare the vesicles. The permeability coefficient P obtained was approx. 10(-11) m/s. The results suggest that 5-FU release is kinetically controlled by an interfacial process seemingly dependent on the surface activity of the drug. Also, the physical state of the bilayer determines the retention capacity of the vesicles. Thus, liposomes consisting of distearoylphosphatidylcholine whose acyl chains were in a gel state at the working temperature (37 degrees C) retained 70% of encapsulated 5-FU after 1 h, whereas liposomes composed of natural bovine brain sphingomyelin retained only 15% over the same period.

Stopped-flow study of anesthetic effect on water-transport kinetics through phospholipid membranes. Interfacial versus lipid core ligands

We have compared ligand effects between polar and apolar anesthetic molecules upon water transport across phospholipid membranes by kinetic analysis of the osmotic swelling rate, using a stopped-flow technique. Chloroform and 1-hexanol were used as interfacial ligands, and carbon tetrachloride and n-hexane were used as their counterparts, representing lipid core action. Because anesthetics transform the solid-gel membrane into a liquid-crystalline state, and because phospholipid membranes display an anomaly in permeability at the phase transition, dimyristoylphosphatidylcholine vesicles were studied at temperatures above the main phase transition to avoid this anomaly. All these molecules increased the osmotic swelling rate. However, a significant difference was observed in the activation energy, delta Ep, between polar and apolar molecules; delta Ep was almost unaltered by the addition of polar molecules (chloroform and 1-hexanol), whereas it was decreased by apolar molecules (carbon tetrachloride and n-hexane). The obtained results were analyzed in terms of the dissolution-diffusion mechanism for water permeation across the lipid membrane. It is suggested that polar molecules affect water permeability by altering the partition of water between the membrane interior and water phase, and apolar molecules affect it by altering both the partition and the diffusion of water within the membrane interior.

Single channel gating events in tracer flux experiments. III. acetylcholine receptor-controlled Li+ efflux from sealed Torpedo marmorata membrane fragments

Filter assay measurements of Li+ efflux from acetylcholine receptor-containing vesicular Torpedo marmorata membrane fragments (microsacs) are presented. Techniques are introduced for: (a) inducing a complete emptying of the Li+ content of all microsacs containing one or more functionally intact receptors, and (b) for determining the distribution of internal volumes of the microsacs using filtration with membrane filters of different pore sizes. The flux amplitudes resulting for acetylcholine receptor-controlled Li+ efflux, when receptors are inhibited by alpha-bungarotoxin or inactivated by a neuroactivator-induced desensitization process, were measured. Amplitude analysis was used to determine characteristic parameters of the microsacs that may vary with the technique of preparation (e.g., the distribution in size and receptor content), as well as the mean single channel flux amplitude contribution (e-kt)infinity, which represents the mean reduction of the Li+ content of a microsac due to efflux from a single receptor-controlled channel closing due to inhibition or inactivation of the receptor. The ratio keff/ki was found to lie in the range 0.1 less than keff/ki less than 0.5, where keff and ki are, respectively, the rate constant for Li+-Na+ exchange flux and for the slow inactivation reaction mode of the acetylcholine receptor induced by carbamoylcholine at high concentrations.

Preparation of unilamellar liposomes of intermediate size (0.1-0.2 mumol) by a combination of reverse phase evaporation and extrusion through polycarbonate membranes

Liposomes can be prepared by a combination of reverse phase evaporation and sequential extrusion through polycarbonate membranes. The vesicles have diameters in the range 0.05-0.5 micron and are mostly unilamellar as indicated by electon microscopy, capture volume, and availability of reactive groups to periodate oxidation. Sequential extrusion leads to a decrease in the encapsulation efficiency by 2-4-fold, depending upon the lipid composition. The inclusion of cholesterol at a 1 : 1 molar ratio of cholesterol-to-phospholipid increases both the mean size and the size heterogeneity of the liposomes as measured by negative-stain electron microscopy. The mean size of vesicles with an equal molar ratio of cholesterol-to-phospholipid after extrusion through a 0.1 micron membrane is 0.140 micron. Vesicles composed of phosphatidylglycerol/phosphatidylcholine (1 : 4) have a mean size of 0.08 micron after extrusion through a 0.1 micron membrane. The intermediate-size (0.1-0.2 micron) vesicles formed by this process have an aqueous space-to-lipid ratio of 3 : 5 and capture between 12 and 25% of the aqueous phase. The procedure is relatively simple, rapid, and yields almost quantitative recovery of vesicles that encapsulate a large percentage of the total aqueous space.

Binding of uranyl to phosphatidylcholine liposomes. Liposome aggregation effect on surface area

The binding of uranyl ion, UO2+2, to egg phosphatidylcholine liposomes was studied as a potential method for the determination of liposome surface areas. Unbound uranyl was determined spectrophotometrically as the Arsenazo III complex with centrifuge supernatant. There is an apparent positive cooperativity in uranyl binding at phosphatidylcholine concentrations above approx. 0.1 mM. The binding capacity per mol increases upon liposome dilution. The data are consistent with liposomes existing in a highly aggregated state. The binding constant in the limit of low concentration of bound uranyl was 9+/-3)-10(6) M-1 in 0.1 M NaCl, pH 4.1. At saturation about four uranyl ions are bound per 100 phosphatidylcholine molecules. Relative surface areas of different dispersions may be calculated from intercepts of extrapolated binding isotherms, and absolute surface areas may be calculated if a value for the uranyl-phosphatidylcholine stoichiometry is assumed.

Preparation and properties of vesicles enclosed by fatty acid membranes

Stable preparations of microscopic particles were obtained from long-chain fatty acids by mechanical agitation of evaporated films in presence of buffer solutions. Oleic and linoleic acids were used. Studies of osmotic swelling and shrinking of the particles indicated that they are enclosed by semipermeable membranes. The particles, which were named ufasomes, are also capable of entrapping glucose in spaces inaccessible to enzymes. It was concluded that the ufasomes closely resemble phospholipid liposomes in their structure and properties.

Interaction of apoliprotein C-III with phosphatidylcholine vesicles. Dependence of aproprotein-phospholipid complex formation on vesicle structure

We have studied the interaction of an apolipoprotein from human very low density lipoproteins (apoC-III) with egg yolk phosphatidylcholine in the form of single- and multi-bilayer vesicles. The reactivity of single-bilayer vesicles with apoC-III appears to be greater than that of the multi-bilayer vesicles according to several thermodynamic and spectrosconic criteria. In the complexes formed by the association of apoC-III with single-bilayer vesicles, the alpha-helical content of the peptide backbone and the apolarity of the environment around the tryptophan residues are greater than that observed in the complexes formed with the multibilayer vesicles. A higher yield and more homogeneous density distribution of lipid-apoprotein complexes results from the interaction of apoC-III with the single-bilayer vesicles relative to those obtained with the multi-bilayer vesicles. The enthalpy of association of apoC-III with phospholipid was greater for the single-shelled vesicles (25 kcal/mol apoC-III) than for the multi-shelled ones (18 kcal/mol apoC-III). The difference in reactivity of these two types of liposomes is not due to a difference in their fluidities since their fatty acid compositions are identical, but may be due to a difference in their areas of sterically accessible phospholipid, their permeabilities to the apoprotein, their radii of curvation, or a combination of these factors.

Studies on phosphatidylcholine model membranes. I. Iodine permeability measurement by specific ion electrode

An iodide specific ion electrode was used to measure iodide released from egg phosphatidylcholine liposomes after passage through an ion exchange column. The permeation process was shown to be a sum of two separate first order processes. The method permits linear initial rates of iodide release to be determined for the first 5 min of permeation. Iodide permeability in the absence of Tris buffer was found to be a decreasing function of pH. In the presence of Tris, iodide permeability went through a minimum ofrom ph 7.3-8.5. the permeability was found to decrease when cholesterol was incorporated into the liposomes.