Permeability properties of phospholipid membranes: Effect of cholesterol and temperature

Measurement of the glucose permeation rate across phospholipid bilayers using small unilamellar vesicles. Effect of membrane composition and temperature

Small unilamellar vesicles were used to measure the permeability of saturated phosphatidylcholine bilayers to glucose. The presented method circumvents most of the common restriction of classical permeability experiments. Increasing the fatty acid chain length of the lipids reduced the permeation rate significantly. Raising the temperature above that of the lipid phase transition drastically increased membrane permeability. Arrhenius plots demonstrated the activation energy to be independent of membrane composition and the phase-state of the lipids. The permeation process is discussed in terms of a constant energy to disrupt all hydrogen bonds between permeant and aqueous solvent prior to penetrating the membrane. The magnitude of the permeability coefficient is partly determined by a unfavourable change in entropy of activation on crossing the water/lipid interface. All results indicate that the penetration of the dehydrated permeant into the hydrophobic barrier is the rate-limiting step in the permeation of glucose.

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.

Preparation and characterization of large dioctadecyldimethylammonium chloride liposomes and comparison with small sonicated vesicles

Dioctadecyldimethylammonium chloride (DODAC) unilamellar liposomes with a mean external diameter of 0.5 micron and sharp gel-to-liquid-crystalline phase transition temperatures (Tc) were obtained by chloroform vaporization and compared with small sonicated DODAC vesicles. Sucrose, impermeant through large DODAC liposomes and sonicated vesicles, was used for internal volume determinations. The internal volumes for large DODAC liposomes and sonicated DODAC vesicles were 9.0 +/- 1.3 and 0.13 +/- 0.2 l/mol, respectively. Ideal osmometer behaviour, towards KCl (0-50 mM) and sucrose, was observed only for large DODAC liposomes. Sonicated DODAC vesicles were osmotically non-responsive towards sucrose and flocculated upon addition of KCl. At temperatures near the Tc, a steep increase in the initial shrinkage rate and a minimum for the total extent of shrinkage were observed for large DODAC liposomes. Large DODAC liposomes are proposed as an adequate synthetic membrane model.

Ultrastructural characterization of cholesterol distribution in toad bladder using filipin

The polyene antibiotic filipin was used to characterize the cholesterol distribution in the membranes of the toad bladder epithelium in freeze-fracture replicas. The apical membranes of granular and mitochondria-rich cells incorporate moderate amounts of filipin while the basolateral membranes of both cell types incorporate substantially greater amounts. Intracellular membranes, in general, take up very little filipin. The major exception to this is the granule membrane, which appears to be rich in cholesterol. An inverse correlation was found between the density of filipin-sterol complexes in the apical membrane and the incidence of granules in the cytoplasm. This suggests that fusion of granules with the apical membrane may be responsible for variation in the concentration of cholesterol in the apical membrane. Thirty minutes following vasopressin exposure, there is no consistent change in the cholesterol content of the apical membrane of granular cells as measured by the incidence of filipin-sterol complexes. The lack of change in the amount of membrane cholesterol indicates that the vasopressin-induced increase in transepithelial water permeability is not mediated by a change in cholesterol content of the apical membrane.

Effect of the lipid phase transition on the kinetics of H+/OH- diffusion across phosphatidic acid bilayers

The kinetics of H+/OH- diffusion across dimyristoyl phosphatidic acid bilayer membranes was measured by following the absorbance of the pH-sensitive indicator Cresol red (o-cresolsulfonphthalein) entrapped in single lamellar vesicles after rapidly changing the external pH in a stopped-flow apparatus. The H+/OH- permeability coefficient was found to be in the 10(-5) to 10(-3) cm . s-1 range. The lipid phase transition has a strong influence on the permeation kinetics as the permeability coefficients in the liquid-crystalline phase are drastically higher. The permeability shows no maximum at the phase transition temperature as is the case for other ions, but displays a similar temperature dependence as water permeation. This is also reflected in the high activation energy of approx. 20 kcal/mol and supports the hypothesis (Nichols, J.W. and Deamer, D.W. (1980) Proc. Natl. Acad. Sci. U.S.A. 77, 2038-2042) of H+/OH- permeation via hydrogen bonded water molecules. A second slower kinetic phase is also observed, where the permeation is obviously controlled by counterion diffusion. The temperature dependence of this slow process displays the for ion diffusion characteristic maximum in the permeability at the phase-transition temperature.

Altered pharmacological properties of liposome-associated human interferon-alpha

Human interferon-alpha was associated in different ways with positively (stearylamine) and negatively (phosphatidylserine) charged phosphatidylcholine multilamellar vesicles, depending on the presence or absence of a cholesterol component. Inclusion of cholesterol resulted in interferon that was significantly (P = 0.0001) more deeply internalized within the liposomes, such that detergent disruption was necessary before most of the interferon activity was expressed. Interferon was stably associated with stearylamine-containing liposomes, both with and without a cholesterol component. However, inclusion of cholesterol in the phosphatidylserine-containing liposomes was necessary for stable association of the interferon for more than 2 days at 4 degrees C or for more than 24 h at 37 degrees C. After intramuscular injection into mice, liposome-associated interferon in reverse-phase evaporation vesicles was retained at the local site of injection significantly longer than free interferon. Even 3 days after intramuscular injection, stearylamine-containing liposomes with or without cholesterol resulted in local interferon levels that were comparable to the peak levels obtained 2 to 4 h after free interferon was injected. In contrast, free interferon was not detectable in the local muscles 24 h after injection of 10(4.6) U. Liposomes containing phosphatidylserine and cholesterol resulted in intermediate levels of local interferon retention; without a cholesterol component, phosphatidylserine-containing liposomes resulted in no increased local interferon retention compared with the results when free interferon was injected.

Spin label reduction kinetics, a procedure to study the effect of drugs on membrane permeability: the effects of monosodium urate, dimethyl sulfoxide and amphotericin B

A method is presented to study the effect of drugs on membrane permeability. It is based on the reduction of a spin label trapped in the internal aqueous compartment(s) of membranes by ascorbate ions added to the bulk aqueous phase. The decay of the electron spin resonance signal of the spin label as a function of time gives an indication of the effect of added agents on the permeability of membranes. To demonstrate the technique, the effect on model membranes of egg phosphatidylcholine of the gout-implicated compound monosodium urate, the aprotic solvent dimethyl sulfoxide and the polyene antibiotic amphotericin B were examined. Monosodium urate did not affect the permeability, casting doubt on a proposed mechanism whereby the agent disrupts the membranes via hydrogen bonding. Dimethyl sulfoxide promoted a gradual increase in rate of solute passage across cholesterol-containing model membranes. Amphotericin B had a pronounced effect on the permeability of cholesterol-containing membranes, causing nearly total loss of paramagnetism immediately after addition. Some aspects of the mechanism of action of the drugs are discussed as well as the advantages and disadvantages of the method. The experiments also allow the evaluation of the effect of surface charge and cholesterol on the dimensions of model membranes.

Kinetics of hydrogen ion diffusion across phospholipid vesicle membranes

The membrane-impermeant, pH-sensitive fluorescence probe 8-hydroxy-1,3,6-pyrenetrisulfonate can be entrapped within the internal aqueous compartment of unilamellar phospholipid vesicles, where it serves as a reliable indicator of internal aqueous hydrogen ion concentration [Clement, N. R., & Gould, J. M. (1981) Biochemistry 20, 1534-1539]. When the external (medium) pH of a suspension of soybean phospholipid vesicles was rapidly changed from 8.2 to 6.65, the rate of subsequent H+ influx into the vesicles, measured as the change in pyranine fluorescence, was limited (in KCl media) by the rate of charge-compensating counterion redistributions. The half-time for the pyranine fluorescence change (corresponding to an internal pH change from 8.2 to 7.43), which was several minutes in the absence of valinomycin, could be decreased to approximately 300 ms, but not further, by the K+ ionophore valinomycin. Proton ionophores such as gramicidin or bis(hexafluoroacetonyl)acetone (1799), on the other hand, decreased the time required for transmembrane H+ equilibration to less than 1 ms. These findings indicate that the intrinsic permeability of unilamellar vesicle membranes to hydrogen ions is surprisingly high and much greater than the observed permeabilities of other small ions.

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.

Transbilayer movement of 24Na in sonicated phosphatidylcholine vesicles exposed to frequency-modulated microwave radiation

Sonicated egg phosphatidylcholine vesicles loaded with 24Na+ were exposed at 20mW to a frequency-modulated (3 Hz) microwave field in the range of 2350 to 2550 MHz, or at 80 mW to a 2450-MHz CW (continuous wave) field, in a waveguide. The vesicle suspension absorbed microwaves at about 1 mW/ml and 25 mW/ml (CW experiment). The average temperature change of the irradiated suspension was less than 0.1 degree C from ambient. Leakage of 24Na+ from the vesicles for up to 19 hours was measured. No difference was noted in the movement of 24Na+ from the vesicles in the irradiated and control dispersions.

Ion and sugar permeabilities of lecithin bilayers: comparison of curved and planar bilayers

Na+ and sugar permeabilities of egg lecithin bilayers were measured using curved bilayers and planar bilayers as represented by single-bilayer vesicles and black lipid films, respectively. The Na+ permeability coefficient measured with single-bilayer vesicles at 25 degrees C is (2.1 +/- 0.6) x 10(-13) cm sec-1. Because of technical difficulties it has been impossible to measure ionic permeabilities of values lower than about 10(-10) cm sec-1 in planar (black) lipid bilayers using tracer methods. The D-glucose and D-fructose permeabilities were measured with both curved and planar bilayers. The permeability coefficients measured with vesicles at 25 degrees C are (0.3 +/- 0.2) x 10(-10) cm sec-1 for glucose and (4 +/- 1) x 10(-10) cm sec-1 for D-fructose; these are in reasonable agreement with the corresponding values obtained for planar (black) lipid bilayers which are (1.1 +/- 0.3) x 10(-10) cm sec-1 for D-fructose, respectively.

Effects of anesthetic alcohols on membrane transport processes in human erythrocytes

1. Anesthetic alcohols (pentanol, hexanol and heptanol) were found to increase the fluidity of red cell membrane lipids as monitored by the fluorescence depolarization of diphenylhexatriene. The relative potency of the alcohols was found to be parallel to their relative membrane/water partition coefficients. 2. Hexanol had biphasic effect on erythritol uptake by simple diffusion by red cells. At concentrations less than 9 mM, there was an approximately linear increase in erythritol permeability with increasing alcohol concentration. 3. The facilitated transport of uridine was markedly inhibited by hexanol. Hexanol at 6 mM produced a 65% inhibition of uridine (4 mM) uptake. Hexanol decreased both the apparent Km and V values for the equilibrium exchange of uridine. 4. The facilitated transport of galactose was only slightly inhibited by hexanol. 5. Hexanol was without effect on the passive and active fluxes of Na+ and K+ in red cells with altered cation contents. Cells that were slightly depleted of K+ and cells that were highly K+ -depleted were both insensitive to hexanol.

On the mechanism of regulation of hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase and of acyl coenzyme A:cholesterol acyltransferase by dietary fat

The activity and the kinetic properties of hydroxymethylglutaryl-CoA reductase and of acyl-CoA:cholesterol acyltransferase in the liver microsomal fraction have been compared between rats fed on either unsaturated or on saturated fat. When rats wre fed for 12h on a compounded diet containing either safflower seed oil or tristearin the composition of the fatty acyl chains of the microsomal phospholipids was shown to be relatively more unsaturated in the rats that received the unsaturated fat. The activity of hydroxymethylglutaryl-CoA reductase in the microsomal fraction was considerably reduced in rats fed on compounded diet containing unsaturated fat whereas this dietary condition resulted in a considerable increase in the activity of acyl-CoA:cholesterol acyltransferase. Similar effects were observed after feeding rats for 12 h on a commercial diet supplemented with either safflower seed oil or with tristearin. The addition of 2% cholesterol to the fat-supplemented diets resulted in both cases in a decrease in hydroxymethylglutaryl-CoA reductase and an increse in acyl-CoA:cholesterol acyltransferase activity as compared with the corresponding values from the rats fed on the fat-supplemented diets with no cholesterol. The Arrhenius plots of hydroxymethylgutaryl-CoA reductase in the microsomal fraction from rats fed on fat-supplemented commercial diet for 12 h showed breaks in the activation energy at 29.6 degrees C for the preparations from rats fed on tristearin and 28 degrees C for those from rats fed on safflower seed oil. The activation energy of the enzyme was lower above and higher below the break for the preparations from rat fed on the unsaturated fat-supplemented diet. Similar differences were obtained from the comparison of the Arrhenius plots in the preparations from rats fed on saturated fat and those in the preparations from rats fed on unsaturated fat when the diet was compounded and given to the animals for 36 h. The addition of 2% cholesterol to the commercial diet supplemented with either saturated or unsaturated fat resulted in Arrhenius plots with a constant activation energy between 37 and 22 degrees C for the enzyme in microsomal preparations from both groups of rats. The apparent Km value for hydroxymethylglutaryl-CoA was lower for the reductase in microsomal preparations from rats fed on the unsaturated fat as compared with that for the enzyme in microsomal preparations from rats fed on saturated fat. There was also a decrease in the apparent Km value for oleic acid for the acyltransferase from rats fed on unsaturated fat as compared with that for the enzyme in the microsomal preparation from the rats fed on saturated fat. The results of the present study are consistent with higher concentration of free cholesterol in endoplasmic reticular membrane in the environment of the reductase and that of acyltransferase following the administration of dietary unsaturated fat as compared with that following the administation of saturated fat.

Serum-induced leakage of liposome contents

Efflux of contents from small unilamellar vesicles of various compositions, conaining a highly quenched fluorescent compound (calcein, 175 mM) was determined as a function of temperature in the presence and absence of human serum. Efflux of calcein from the liposomes was monitored as an increase in fluorescence as calcein became dequenched upon release from the liposomes. The presence of serum significantly increased liposome leakage in all cases. Incorporation of increasing molar ratios of cholesterol into liposomes reduced leakage of calcein from liposomes incubated with buffer and with serum. Leakage was significantly faster from liposomes with an osmotic gradient across the membrane (higher inside) than from equiosmolar liposomes. The leakage of [14C]sucrose from egg lecithin liposomes at 37 degrees C was also dramatically increased in the presence of serum.

Characteristics of coumarin metabolism by liver microsomes from cobalt-pretreated mice

1. Cobalt pretreatment of male mice leads to increased liver microsomal mixed-function oxidation of various coumarin derivatives; 7-ethoxycoumarin, 7-hydroxycoumarin and coumarin are metabolized three to four times more rapidly than in control microsomes. 2. In Arrhenius plots of 7-ethoxycoumarin de-ethylation, the temperature at which phase transition occurs is not changed by cobalt pretreatment. However, at high temperature, above the transition point, the slope indicates that cobalt lowers the apparent activation energy of the reaction to 65% of control value. 3. Metabolism of 7-hydroxycoumarin by hepatic microsomes from cobalt-treated mice gave 3,7- , 6,7- and 7,8-dihydroxycoumarin, identified by g.l.c. and mass spectrometry.

Measurement of net proton-hydroxyl permeability of large unilamellar liposomes with the fluorescent pH probe, 9-aminoacridine

The fluorescent probe 9-aminoacridine was used to measure the rate of decay of experimentally established pH gradients across liposome membranes. From the rate of decay, separate permeability coefficients for protons (PH) and hydroxyls (POH) were calculated and summed to yield the net proton-hydroxyl permeability (Pnet). The net permeability of protons and hydroxyls was found to be approximately 10(-4) cm/s, six orders of magnitude greater than that measured for sodium and pyrophosphate ions under similar conditions. This suggests that protons and/or hydroxyls cross lipid bilayers by a different mechanism than do other monovalent cations and anions. In addition, the measurements provide a standard for net proton-hydroxyl permeability in pure phospholipid bilayers for comparison with biological membranes.

Divalent cation binding to phospholipid veiscles. Dependence on temperature and lipid fluidity

Mn2+ binding to vesicles prepared from several different species of anionic phospholipids was determined as a function of temperature by electron paramagnetic resonance (EPR). The Mn2+ affinities of phosphatidylserine, cardiolipin and egg yolk phosphatidylglycerol all increased monitonically with temperature. Vesicles prepared from hydrogenated and natural (bovine) phosphatidylserine were monitored with respect to hydrocarbon chain fluidity as well as Mn2+ binding. Contrary to expectations based on surface potential considerations, the affinity of phosphatidylserine for divalent cations was apparently not lowered in going from the gel state to the lipid crystalline state of the bilayer. The results are instead consistent with an enhancement in cation affinity with increased lipid fluidity. Dipalmitoyl phosphatidylglycerol vesicle fluidity and Mn2+ binding were also studied with EPR. A large reduction in the measured Mn2+ affinity accompained melting of the phospholipid, but observed hysteresis in the temperature dependence of the binding render uncertain any simple explanation based on changes in surface potential. Supplementary light scattering data indicated that vesicle aggregation was involved in the hysteresis phenomena.

Threshold effects on the lectin-mediated aggregation of synthetic glycolipid-containing liposomes

Cholesterol analogs containing sugar residues linked by spacer groups to the cholesterol O can be incorporated into egg yolk lecithin small unilamellar liposomes. The synthetic glycolipid analogs distribute evenly on both sides of the bilayer. These liposomes are aggregated by the appropriate lectin. For example, when the sugar residue is a beta-galactoside the liposomes are aggregated by ricin and when it is an alpha-mannoside they are aggregated by Con A. The lectin-mediated aggregation of these liposomes is reversed by the addition of the appropriate sugar. The rates but not the extents of aggregation of these liposomes are highly sensitive to the amount of glycolipid incorporated. Below approximately 5% glycolipid incorporation the rate of the lectin-mediated aggregation of these liposomes is exceedingly slow, whereas above this level rapid aggregation proceeds. At all concentrations studied the synthetic glycolipids are incorporated in a unimodal fashion so that the observed threshold effects cannot be based on possible differences in the manner in which the glycolipids are incorporated at different concentrations. This conclusion is based on 1) studies with galactose oxidase that show that the percentage of galactose oxidation in a liposome prepared from a galactosyl-containing glycolipid is independent of glycolipid concentration, and 2) studies on the aggregation of liposomes containing mixed glycolipids in which the glycolipids are shown to behave independently. The importance of a critical density of membrane-bound receptors in order for aggregation to occur is discussed.

Effect of membrane cholesterol on potassium transport in Mycoplasma mycoides var. Capri (PG3)

Relationships between membrane lipid composition and physiological properties, particularly intracellular potassium levels, have been studied at 37 degrees C in Mycoplasma mycoides var. Capri (PG3). Native organisms grown on medium supplemented with either oleic acid plus palmitic acid or elaidic acid have identical growth characteristics, acidification properties and intracellular K content. On the other hand, when the cholesterol normally present in the membrane (20--25% of total lipids) is reduced to less than 2%, we observe: (1) the intracellular K content decreases (20 microgram K/mg cell protein instead of 40) and is independent of the phase of growth; (2) K passive permeability is drastically increased but K distribution remains in equilibrium with the transmembrane potential (delta psi); (3) organisms stop growing at pH 6.5 (instead of 5.2) and acidification is reduced by 40%, suggesting a large increase in H+ permeability, and (4) intracellular Na contents rise from 3 to 9 microgram Na/mg cell protein. Replenishing cholesterol in membranes of depleted cells results in a recovery of the high intracellular K level (35--40 microgram K/mg cell protein) and acidification properties. It is suggested that cholesterol affects the cation content via the increase in proton permeability which in turn controls the value of the delta psi responsible for the value of intracellular K equilibrium. Changes in K passive permeability, although related to the amount of cholesterol present in the plasma membrane, are probably not involved in the control of the intracellular K level.

Cholesterol esters and membrane permeability. A nuclear magnetic resonance (MNR) study

Cholesteryl esters have been incorporated into phospholipid vesicles up to 5 mole percent. Excess ester separates out into a separate phase which resembles the mesomorphic droplets of atherosclerosis. The incorporation of 5 mole percent cholesteryl palmitate is shown by 31P NMR studies to increase the permeability of the model membranes to ions 10-fold. The same incorporation of cholesteryl linoleate does not affect the membrane permeability. Implications of these findings, and the significance of the cholesteryl ester/free cholesterol ratio upon atherosclerosis is discussed.