Osmotic behavior of liposomes of phosphatidylcholine and phosphatidylsulfocholine as a function of lipid concentration

A mechanistic study on the destabilization of whole inactivated influenza virus vaccine in gastric environment

Oral immunization using whole inactivated influenza virus vaccine promises an efficient vaccination strategy. While oral vaccination was hampered by harsh gastric environment, a systematic understanding about vaccine destabilization mechanisms was not performed. Here, we investigated the separate and combined effects of temperature, retention time, pH, and osmotic stress on the stability of influenza vaccine by monitoring the time-dependent morphological change using stopped-flow light scattering. When exposed to osmotic stress, clustering of vaccine particles was enhanced in an acidic medium (pH 2.0) at ≥25°C. Fluorescence spectroscopic studies showed that hyper-osmotic stress at pH 2.0 and 37°C caused a considerable increase in conformational change of antigenic proteins compared to that in acidic iso-osmotic medium. A structural integrity of membrane was destroyed upon exposure to hyper-osmotic stress, leading to irreversible morphological change, as observed by undulation in stopped-flow light scattering intensity and transmission electron microscopy. Consistent with these analyses, hemagglutination activity decreased more significantly with an increasing magnitude of hyper-osmotic stress than in the presence of the hypo- and iso-osmotic stresses. This study shows that the magnitude and direction of the osmotic gradient has a substantial impact on the stability of orally administrated influenza vaccine.

Structure of polymerizable lipid bilayers. V. Synthesis, bilayer structure and properties of diacetylenic ether and ester lipids

Four diacetylenic phosphatidylcholines (PC's) have been synthesized and the structures of bilayers of these lipids have been determined at low resolution by low-angle X-ray diffraction. The PC's all have 18-carbon chains but differ with respect to the ether/ester linkage at the sn-1 and sn-2 positions and the relative position of the diacetylene moiety: diester-PC (1): 1,2-bis(octadeca-4',6'-diynoyl)-sn-glycero-3-phosphocholine diester-PC (2): 1-(octadeca-4',6'-diynoyl)-2-(octadeca-5',7'-diynoy l)-sn- glycero-3-phosphocholine diester-PC (3): 1,2-bis(octadeca-8',10'-diynoyl)-sn-glycerol-3-phosphocholin e diether-PC (4): 1-O-(octadeca-4',6'-diynyl)-2-O-(octadeca-5",7"-din yl)-sn- glycero-3-phosphocholine Only (1) exhibits the typical bilayer profile, whereas (2), (3) and (4) show evidence of interdigitation and/or significant disorder. Only (1) polymerized effectively upon illumination with 254 nm light, turning deep blue in seconds, indicating the formation of long, well-ordered polydiacetylenic structures. Liposomes of these derivatives were tested for permeability by osmotic swelling. Polymerized liposomes of (1) underwent osmotic swelling with urea, glycerol, and acetamide more rapidly than did liposomes of stearoyl-oleoyl-PC, but the initial rates of osmotic swelling of polymerized liposomes of (1) were 3-10-times lower than those of unpolymerized liposomes of (1). Blue polymerized multilayer samples of (1) exhibited an irreversible thermochromic transition to red at approx. 40 degrees C. Differential scanning calorimetry with liposome suspensions of (1) revealed an endotherm at 28.3 degrees C with a transition enthalpy of 40 J/g. PC (1) is a potentially useful diacetylenic lipid which exhibits facile, complete polymerization and a bilayer thickness comparable to that of biomembrane lipids.

Rates of spontaneous exchange of synthetic radiolabeled sterols between lipid vesicles

14C-labeled sterols with structural variation in the polar function [3 alpha-OH, 3-O(CH2)2O-(CH2)2O(CH2)2OH, 3 alpha-NH2, 3 beta-NH2, and 3-OC(O)CHN = N] and at the 7 position (7-oxo, 7 alpha-OH, and 7 beta-OH) were synthesized and incorporated into unilamellar vesicles for studies of the rates of transfer to an excess of acceptor vesicles. Cholesterol, cholestanol, and epicholesterol underwent full exchange in a single kinetic pool, and 90% of the 3 alpha-triethoxycholesterol was exchangeable in one pool. Biphasic kinetics with full exchangeability were observed for cholesterylamines, which bear a positive charge at the 3 position; the slow phase reflects the high activation energy for inner-to-outer leaflet movement of the charged lipid. Biphasic kinetics were also found for cholesteryl diazoacetate, indicating that this photoaffinity probe and cholesterol have different mechanisms of transfer. Sterols that are more hydrophilic than cholesterol as estimated by reversed-phase high-performance chromatography (elution with acetonitrile-2-propanol, 4:1 v/v, with varying proportions of water) gave faster exchange rates than cholesterol, whereas sterols that are more hydrophobic gave slower exchange rates. However, the rates of [14C]sterol desorption from the lipid-water interface are not correlated with the relative sterol hydrophobicity as estimated by the logarithm of the capacity factors using acetonitrile-2-propanol-water as the mobile phase. These studies suggest that the interaction of sterols with phospholipids provides the principal physical-chemical basis for determining the rates of spontaneous exchange of sterols between bilayers.

Chemical synthesis and physiological activity of sulfonium analogues of platelet activating factor

Phosphatidylsulfocholine (PSC), the sulfonium analogue of phosphatidylcholine (PC), occurs naturally in some diatoms. The replacement of the [formula; see text] group by a [formula; see text] results in an increase in the polar head group size in PSC relative to that of PC, consistent with the observed increase in permeability of PSC bilayers towards urea. It was of interest to see whether replacement of the [formula; see text] group in platelet activating factor (PAF) by an [formula; see text] group leads to any change in platelet aggregation or other physiological activity. Synthesis of the sulfonium analogue of PAF was carried out by suitable modifications of known procedures. The PAF-sulfonium analogue was found to have almost the same platelet aggregating activity as PAF itself, in the concentration range 1-20 microM, but a much lower activity in the range 0.01-1 microM. The analogue had little or no effect on the platelet aggregation activity of PAF when added in the concentration range 0.01-1 microM and had about half the hypotensive activity of PAF towards hypertensive CDF male rats. The sulfonium analogue, however, was much more cytotoxic to HL-60 cells than PAF itself, in the concentration range 0-15 microM; replacement of the acetate group by a benzyl group increased the cytotoxicity to the level of that of the methoxy analogue of PAF. Thus, replacement of the [formula; see text] group by a [formula; see text] group in the polar head group region of PAF results in a relatively small change in its platelet aggregation activity and a decrease in its hypotensive activity, but greatly increases its antitumor activity.

Comparison of steady-state fluorescence polarization and urea permeability of phosphatidylcholine and phosphatidylsulfocholine liposomes as a function of sterol structure

The well-known reduction in the permeability properties of liposomes of dimyristoylphosphatidylcholine (DMPC) by sterols has also been demonstrated for its sulfonium analog (DMPSC) in which the N+(CH3)3 group of choline is replaced by S+(CH3)2. We have now compared the effects of 25 mol% 24-methylenecholesterol and cholesterol on the initial rates of urea permeation into dipalmitoyl-PC (DPPC) and dipalmitoyl-PSC (DPPSC) liposomes above the gel-to-liquid-crystalline phase transition temperature and found a greater reduction with 24-methylenecholesterol/DPPSC than with cholesterol/DPPSC liposomes but little difference between the two sterols in DPPC liposomes. Fluorescence polarization studies, using diphenylhexatriene as a probe, show that polarization (P) values are considerably higher in DMPSC liposomes containing 20 and 30 mol% 24-methylenecholesterol than in DMPC liposomes containing 20 and 30 mol% cholesterol. Higher P values were also obtained in DMPSC liposomes containing other 24-alkyl-substituted sterols (beta-sitosterol, ergosterol and campesterol) than in DMPC liposomes containing the same sterols. Reduced permeability rates in PSC liposomes containing 24-alkyl-substituted sterols are correlated with higher polarization values, reflecting an increased degree of order and/or motion in these liposomes compared with liposomes from the corresponding PC. These results suggest that alkyl substitution at C-24 of the sterol molecule results in tighter interactions with the sulfonium analog of PC than with PC.

Osmotic water permeability in glycoprotein containing liposomes

The kinetics of osmotic water permeability in proteoliposomes containing alpha 1-acid glycoprotein was investigated by means of stopped-flow spectrophotometry. A biphasic time-course of scattered light with time was registered. The rate constants calculated from fits to an exponential function in the first phase were proportional to the final medium osmolarity. The apparent second order rate constants Kapp (Osm-1 sec-1) were determined at different glycoprotein concentrations in the original mixture for preparation of proteoliposomes. The value of Kapp at lipid:glycoprotein weight ratio = 1 was plotted in Arrhenius coordinates. The calculated activation energy for water permeation through the lipid bilayer suggests that eventual channel mechanism may be involved due to the presence of glycoprotein molecule in the liposomes.

Permeability behavior of liposomes prepared from fatty acids and fatty acid methyl esters

The permeability properties of liposomes prepared at pH 8.7 from a fatty acid and either methyl oleate or methyl elaidate, with or without cholesterol, were investigated. The fatty acids used were oleic acid, elaidic acid, and the selenium-containing fatty acids 9-selenaheptadecanoic acid and 13-selenaheneicosanoic acid. The liposomes trapped sucrose and carboxyfluorescein. Their volume change resulting from osmotic shock was directly proportional to the change in absorbance (light scattering). Liposomes prepared from oleic acid and either methyl oleate or methyl elaidate underwent osmotic swelling much more slowly than liposomes prepared from elaidic acid and either methyl oleate or methyl elaidate. Incorporation of cholesterol decreased the initial rate of erythritol permeation, especially in liposomes containing methyl oleate. The swelling rates of liposomes prepared with the selenium-containing fatty acids indicated that incorporation of methyl elaidate gave more tightly packed bilayers than did incorporation of methyl oleate. The effect of cholesterol on the initial rate of erythritol influx was greater in oleic acid and elaidic acid liposomes than in selenium-containing fatty acid liposomes, indicating that the large bulk of the selenium heteroatom suppresses the ability of cholesterol to interact with the hydrocarbon chain.

Osmotic water permeability through liposomes in the presence of alpha 1-acid glycoprotein

Alpha 1-acid glycoprotein (orosomucoid) from human blood serum was isolated in pure form and then reconstituted into large multilamellar liposomes, consisting of a binary mixture of hen-egg phosphatidylcholine and cholesterol. These liposomes were found to be osmotically sensitive. The osmotic water permeability of proteoliposomes was determined by light-scattering measurements of the osmotic volume changes after mixing with hyperosmotic solutions of potassium salts and aminoglycoside antibiotics. The initial rate of water outflow was measured as a function of glycoprotein concentration in the mixture for the preparation of proteoliposomes. This can serve as an indication for membrane permeability to the solutes used in these experiments. It was shown that aminoglycoside antibiotics passed much faster across the membrane than potassium salts, in the presence of glycoprotein in the liposomes. A recognition pattern in the osmotic behavior of these proteoliposomes was assumed.

Effects of purified ginseng saponins on multilamellar liposomes

To elucidate the mode of interaction between purified ginseng saponins and liposomes, the effects of ginseng saponins at concentrations below and above their critical micelle concentration on the osmotic behavior of liposomes were observed. Liposomes composed of egg phosphatidylcholine (PC), phosphatidic acid (PA) and/or cholesterol (Ch) behave as an ideal osmometer and this activity was described by the linear relationship between (1/A450)3/2 and Cin/Cout. Total saponin prepared from the lateral roots of ginseng cultivated in Korea interacted with multilamellar liposomes and prevented them from behaving as an ideal osmometer. 20-S-Protopanaxadiol saponin showed similar activity but 20-s-protopanaxatriol saponin had weak activity. Ginsenoside Rb1 whose genin is 20-S-protopanaxadiol, disturbed osmotic behavior of Ch-free liposomes but not Ch-containing liposomes as monomers and lyzed both liposomes as micelles. Rg1 whose genin is 20-S-protopanaxatriol, showed no activity on liposomes with or without Ch. It can be suggested that ginseng saponins may interact with and destroy liposomal membranes and that structure of sugar moiety is an important factor to their activities. Ch is not a target in this interaction but suppresses the activity of ginseng saponins.

An essential role of phosphatidylglycerol in the formation of the osmotically stable liposomes of Escherichia coli phospholipids

A temperature sensitive auxotroph of Escherichia coli K-12 requiring unsaturated fatty acids can grow normally at 28 degrees C, but requires an osmotic stabilizer such as a high amount of salt or sugar in the medium for the growth at 42 degrees C. Namely, the apparent osmotic stability of the cells at 28 degrees C and 42 degrees C is quite different. The osmotic properties of liposomes of the phospholipids extracted from these cells were investigated. The osmotically induced volume change of the multilamellar liposomes was examined by the turbidimetric method. The liposomes prepared from cells grown at 28 degrees C can swell and shrink under a wide range of hypo-and hypertonic conditions. However, those from cells grown at 42 degrees C could not swell under hypotonic conditions. These results exhibit a good correlation between the apparent osmotic stability of E. coli cells and the osmotic properties of the liposomes prepared from the extracted total phospholipids. To clarify the role of each phospholipid component, the osmotic properties of the liposomes reconstituted from the purified phospholipid species were further investigated. The results clearly showed that phosphatidylglycerol is the key factor that stabilizes the membranes of E. coli phospholipids against osmotic pressure.

Isotopic light scattering of lipid vesicles. Water permeation and effect of alpha-tocopherol

The influence of the index of refraction of the solvent on light scattering properties of lecithin bilayer vesicles is described. Large vesicles (diameter 300 nm) are considered where one lamella separates the intravesicular compartment from the external medium. Stationary and transient cases are distinguished with special emphasis on the isotopic substitution of the solvent, i.e. H2O vs. D2O. Theoretical calculations based on the Mie theory of light-scattering are in accord with results from experiments. The two stationary cases considered serve to calibrate the numerical calculations and illustrate the capability of the method. Transient experiments allow the determination of permeation rates; in particular the D2O/H2O permeability coefficients can be obtained. Single component vesicular lecithin bilayers and ones containing tocopherol are compared. In the crystalline state the incorporation of tocopherol increases the fluidity of the lipid bilayer in parallel with the water permeation rate.

Interaction of sea cucumber saponins with multilamellar liposomes

The effect of three sea cucumber saponins, echinoside A, bivittoside D and holothurin A, on multilamellar liposomes was investigated. An ideal osmotic behavior of liposomes was described as a linear relationship between the reciprocal 3/2 s power of absorbance at 450 nm and the osmotic gradient across the membrane. Sea cucumber saponins at concentrations below critical micelle concentration (CMC) disturbed this linear relationship in liposomes composed of egg phosphatidylcholine, phosphatidic acid and cholesterol. Cholesterol-free liposomes were not susceptible to these saponins. Results of optical measurements were consistent with those of transmission electron microscopy, which showed saponin-induced changes in liposomal structure. The lytic activity of sea cucumber saponins on liposomes depended on their chemical structure. These results suggest that sea cucumber saponins as monomers can interact with liposomes and that cholesterol serve as a principal binding site for the sea cucumber saponins.

Phosphatidylsulfocholine bilayers. An infrared spectroscopic characterization of the polymorphic phase behavior

The thermal response of aqueous dispersions of phosphatidylsulfocholines (dimyristoyl-, dipalmitoyl- and distearoyl-) was studied by Fourier transform infrared spectroscopy. Comparison with that of the corresponding phosphatidylcholines showed several close resemblances, including the observation in the gel phase of a "pretransition" and of a "subtransition". The similarity in the thermotropic phase behavior of these two lipid classes is consistent with the total replacement of phosphatidylcholine by phosphatidylsulfocholine in certain marine diatoms.