Permeability properties of phospholipid membranes: Effect of cholesterol and temperature

pH gradient loading of anthracyclines into cholesterol-free liposomes: enhancing drug loading rates through use of ethanol

Application of cholesterol-free liposomes as carriers for anticancer drugs is hampered, in part, because of standard pH gradient based loading methods that rely on incubation temperatures above the phase transition temperature (Tc) of the bulk phospholipid to promote drug loading. In the absence of cholesterol, liposome permeability is enhanced at these temperatures which, in turn, can result in the collapse of the pH gradient and/or unstable loading. Doxorubicin loading studies, for example, indicate that the drug could not be loaded efficiently into cholesterol-free DSPC liposomes. We demonstrated that this problem could be circumvented by the addition of ethanol as a permeability enhancer. Doxorubicin loading rates in cholesterol-free DSPC liposomes were 6.6-fold higher in the presence of ethanol. In addition, greater than 90% of the added doxorubicin was encapsulated within 2 h at 37 degrees C, an efficiency that was 2.3-fold greater than that observed in the absence of ethanol. Optimal ethanol concentrations ranged from 10% to 15% (v/v) and these concentrations did not significantly affect liposome size, retention of an aqueous trap marker (lactose) or, most importantly, the stability of the imposed pH gradient. Cryo-transmission electron micrographs of liposomes exposed to increasing concentrations of ethanol indicated that at 30% (v/v) perturbations to the lipid bilayer were present as evidenced by the appearance of open liposomes and bilayer sheets. Ethanol-induced increased drug loading was temperature-, lipid composition- and lipid concentration-dependent. Collectively, these results suggest that ethanol addition to preformed liposomes is an effective method to achieve efficient pH gradient-dependent loading of cholesterol-free liposomes at temperatures below the Tc of the bulk phospholipid.

The endoplasmic reticulum membrane is permeable to small molecules

The lumen of the endoplasmic reticulum (ER) differs from the cytosol in its content of ions and other small molecules, but it is unclear whether the ER membrane is as impermeable as other membranes in the cell. Here, we have tested the permeability of the ER membrane to small, nonphysiological molecules. We report that isolated ER vesicles allow different chemical modification reagents to pass from the outside into the lumen with little hindrance. In permeabilized cells, the ER membrane allows the passage of a small, charged modification reagent that is unable to cross the plasma membrane or the lysosomal and trans-Golgi membranes. A larger polar reagent of approximately 5 kDa is unable to pass through the ER membrane. Permeation of the small molecules is passive because it occurs at low temperature in the absence of energy. These data indicate that the ER membrane is significantly more leaky than other cellular membranes, a property that may be required for protein folding and other functions of the ER.

Thermosensitive chitosan-based hydrogel containing liposomes for the delivery of hydrophilic molecules

A novel injectable in situ gelling thermosensitive chitosan-beta-glycerophosphate (C-GP) formulation has been recently proposed for tissue repair and drug delivery. The system can sustain the release of macromolecules over a period of several hours to a few days. However, with low-molecular-weight hydrophilic compounds, the release is generally completed within 24 h. In this study, liposomes were added to the C-GP solution and their effect on the viscoelastic properties of the system and release kinetics of encapsulated carboxyfluorescein was investigated. The gelation rate and gel strength were slightly increased by the presence of the liposomes. The in vitro release profiles demonstrated controlled delivery over at least 2 weeks. The release rate strongly depended on the liposome size and composition (i.e. addition of cholesterol), and on the presence of phospholipase in the release medium. The kinetics was not substantially modified when using liposomes prepared with a negatively-charged lipid or a lipid having a high phase transition temperature. These results indicate that the liposome-C-GP system rapidly gels at body temperature, and can sustain the delivery of low-molecular-weight hydrophilic compounds. A mathematical model was proposed to characterize the release kinetics.

Do sterols reduce proton and sodium leaks through lipid bilayers?

Proton and/or sodium electrochemical gradients are critical to energy handling at the plasma membranes of all living cells. Sodium gradients are used for animal plasma membranes, all other living organisms use proton gradients. These chemical and electrical gradients are either created by a cation pumping ATPase or are created by photons or redox, used to make ATP. It has been established that both hydrogen and sodium ions leak through lipid bilayers at approximately the same rate at the concentration they occur in living organisms. Although the gradients are achieved by pumping the cations out of the cell, the plasma membrane potential enhances the leakage rate of these cations into the cell because of the orientation of the potential. This review proposes that cells use certain lipids to inhibit cation leakage through the membrane bilayers. It assumes that Na(+) leaks through the bilayer by a defect mechanism. For Na(+) leakage in animal plasma membranes, the evidence suggests that cholesterol is a key inhibitor of Na(+) leakage. Here I put forth a novel mechanism for proton leakage through lipid bilayers. The mechanism assumes water forms protonated and deprotonated clusters in the lipid bilayer. The model suggests how two features of lipid structures may inhibit H(+) leakage. One feature is the fused ring structure of sterols, hopanoids and tetrahymenol which extrude water and therefore clusters from the bilayer. The second feature is lipid structures that crowd the center of the bilayer with hydrocarbon. This can be accomplished either by separating the two monolayers with hydrocarbons such as isoprenes or isopranes in the bilayer's cleavage plane or by branching the lipid chains in the center of the bilayers with hydrocarbon. The natural distribution of lipids that contain these features are examined. Data in the literature shows that plasma membranes exposed to extreme concentrations of cations are particularly rich in the lipids containing the predicted qualities. Prokaryote plasma membranes that reside in extreme acids (acidophiles) contain both hopanoids and iso/anteiso- terminal lipid branching. Plasma membranes that reside in extreme base (alkaliphiles) contain both squalene and iso/anteiso- lipids. The mole fraction of squalene in alkaliphile bilayers increases, as they are cultured at higher pH. In eukaryotes, cation leak inhibition is here attributed to sterols and certain isoprenes, dolichol for lysosomes and peroxysomes, ubiquinone for these in addition to mitochondrion, and plastoquinone for the chloroplast. Phytosterols differ from cholesterol because they contain methyl and ethyl branches on the side chain. The proposal provides a structure-function rationale for distinguishing the structures of the phytosterols as inhibitors of proton leaks from that of cholesterol which is proposed to inhibit leaks of Na(+). The most extensively studied of sterols, cholesterol, occurs only in animal cells where there is a sodium gradient across the plasma membrane. In mammals, nearly 100 proteins participate in cholesterol's biosynthetic and degradation pathway, its regulatory mechanisms and cell-delivery system. Although a fat, cholesterol yields no energy on degradation. Experiments have shown that it reduces Na(+) and K(+) leakage through lipid bilayers to approximately one third of bilayers that lack the sterol. If sterols significantly inhibit cation leakage through the lipids of the plasma membrane, then the general role of all sterols is to save metabolic ATP energy, which is the penalty for cation leaks into the cytosol. The regulation of cholesterol's appearance in the plasma membrane and the evolution of sterols is discussed in light of this proposed role.

Liposome targeting to tumors using vitamin and growth factor receptors

Liposome-encapsulated anticancer drugs reveal their potential for increased therapeutic efficacy and decreased nonspecific toxicities due to their ability to enhance the delivery of chemotherapeutic agents to solid tumors. Advances in liposome technology have resulted in the development of ligand-targeted liposomes capable of selectively increasing the efficacy of carried agents against receptor-bearing tumor cells. Receptors for vitamins and growth factors have become attractive targets for ligand-directed liposomal therapies due to their high expression levels on various forms of cancer and their ability to internalize after binding to the liposomes conjugated to receptors' natural ligands (vitamins) or synthetic agonists (receptor-specific antibodies and synthetic peptides). This chapter summarizes various strategies and advances in targeting liposomes to vitamin and growth factor receptors in vitro and in vivo with special emphasis on two extensively studied liposome-targeting systems utilizing folate receptor and HER2/neu growth factor receptor.

Preparation and properties of arsonolipid containing liposomes

Arsonolipids are analogs of phosphonolipids which have a chemically versatile head group. In preliminary cell culture studies, liposomes composed solely of arsonolipids or of phosholipid-arsonolipid mixtures, demonstrate a specific toxicity against cancer cells (Gortzi et al., unpublished results). The possibility of using such formulations as an alternative of arsenic trioxide with or without combination of other cytostatic agents (encapsulated in their aqueous interior) prompted the investigation of their physicochemical characteristics. Herein we compared the characteristics of arsonolipid containing vesicles with different lipid compositions. Experimental results and morphological observations reveal that non-sonicated formulations have different structures and stability (when both membrane integrity and aggregation are taken into account) depending on the acyl chain length of the arsonolipid. When phospholipids and especially cholesterol are included in their membranes almost all arsonolipids studied produce more stable vesicles. An interesting aspect of these arsonolipid containing vesicles is also their negative surface charge, which may be modulated by mixing phospholipids with arsonolipids. Sonicated vesicles have smaller sizes and profoundly higher stability, especially when containing cholesterol and phosphatidylcholine mixed with arsonolipids. The only exception is that of the arsonolipid with the C(12) acyl chain which was observed to produce long tubes which break down to cubes by sonication. In conclusion, these initial studies demonstrate that sonicated vesicles composed of arsonolipid and phospholipid mixtures mixed with cholesterol posses the stability required to be used as an arsonolipid delivery system. In addition, although cryo-electron microscopy demonstrated that the sonicated vesicles are elliptical in shape, their encapsulation efficiency is not significantly lower than sonicated phospholipid liposomes. Thereby, these vesicles may be also used for the delivery of other drug molecules which can be sufficiently retained in their aqueous interior.

Interactions of peptides with liposomes: pore formation and fusion

Leakage from liposomes induced by several peptides is reviewed and a pore model is described. According to this model peptide molecules become incorporated into the vesicle bilayer and aggregate reversibly or irreversibly within the surface. When a peptide aggregate reaches a critical size, peptide translocation can occur and a pore is formed. With the peptide GALA the pores are stable and persist for at least 10 minutes. The model predicts that for a given lipid/peptide ratio, the extent of leakage should decrease as the vesicle diameter decreases, and for a given amount of peptide bound per vesicle less leakage would be observed at higher temperatures due to the increase in reversibility of surface aggregates of the peptide. Effect of membrane composition on pore formation is reviewed. When cholesterol was included in the liposomes the efficiency of inducation of leakage by the peptide GALA was reduced due to reduced binding and increased reversibility of surface aggregation of the peptide. Phospholipids which contain less ordered acyl-chains and have a slightly wedge-like shape, can better accommodate peptide surface aggregates, and consequently insertion and translocation of the peptide may be less favored. Demonstrations of antagonism between pore formation and fusion are presented. The choice of factors which promote vesicle aggregation, e.g., larger peptides, increased vesicle and peptide concentration results in enhanced vesicle fusion at the expense of formation of intravesicular pores. FTIR studies with HIV-1 fusion peptides indicate that in systems where extensive vesicle fusion occurred the beta conformation of the peptides was predominant, whereas the alpha conformation was exhibited in cases where leakage was the main outcome. Antagonism between leakage and fusion was exhibited by 1-palmitoyl-2-oleoylphosphatidylglycerol vesicles, where the order of addition of peptide (HIV(arg)) or Ca(2+)dictated whether pore formation or vesicle fusion would occur. The current study emphasizes that the addition of Ca(2+), which promotes vesicle aggregation can also reduce peptide translocation in isolated vesicles.

Stability of SUV liposomes in the presence of cholate salts and pancreatic lipases: effect of lipid composition

The effect of bile salts (sodium cholate and sodium taurocholate), and pancreatic lipases on the structural integrity of SUV liposomes of different lipid compositions was studied. Liposomal membrane integrity was judged by bile salt or pancreatin-induced release of vesicle encapsulated 5,6-carboxyfluorescein, and vesicle size distribution before and after incubations. Bile salt concentration was 10 mM, while a saturated solution of pancreatin (mixed with equal volume of liposomes) was utilized. Results agree with earlier studies, demonstrating the instability of liposomes composed of lipids with low transition temperatures (PC and DMPC) in presence of cholates. Addition of cholesterol (1:1 lipid:chol molar ratio) does not substantially increase the encapsulated molecule retention. Nevertheless, liposomes composed of lipids with high transition temperatures (DPPC, DSPC and SM), retain significantly higher amounts of encapsulated material, under all conditions studied. Furthermore, the vesicles formed by mixing cholesterol with these lipids will possibly be sufficiently stable in the gastrointestinal tract for long periods of time. Sizing results reveal that in most cases release of encapsulated molecules is mainly caused by their leakage through holes formed on the lipid bilayer. However, in stearylamine containing DPPC and DSPC vesicles, the cholate-induced drastic decrease in vesicle size suggests total liposome disruption as the possible mechanism of encapsulated material immediate release.

A simple in vitro model to study the release kinetics of liposome encapsulated material

A simple in vitro model was developed to study the release kinetics of liposome encapsulated material in the presence of biologic components. Liposomes were embedded in an agarose gel (bottom layer) formed in a glass vial and separated from the receptor compartment buffer by a second layer of agarose gel (top layer). To follow the release of liposomal contents, aqueous space markers differing in molecular weight (from 205 Dalton to 17500 Dalton) were encapsulated. The isotonic buffer in the receptor was completely changed at various time points and the amount of marker released from the agarose matrix containing the liposomes into the receptor medium determined. The release of non-encapsulated markers from the gel followed a time0.5 relationship with about 75% of a 17500 Dalton protein being released from the matrix in 48 h. In the same period, about 7% of the intact liposomes added to the agarose gel appeared in the receptor phase. The release of calcein from various liposome compositions including: (A) egg phosphatidylcholine (EPC)/egg phosphatidylglycerol (EPG) 9:1, (B) dioleoylphosphatidylethanolamine (DOPE)/cholesterylhemisuccinate (CHEMS) 2:1, and (C) dioleoylphosphatidylglycerol (DOPC)/dioleoylphosphatidylglycerol (DOPG) 2:1 was measured. Components of the biological milieu such as serum proteins and calcium influenced release of encapsulated material. This in vitro model is a convenient and reproducible system that permits the study of the release of high molecular weight molecules such as proteins from liposomal formulations in the presence of serum. It may find applications with respect to release of proteins from a variety of colloidal drug delivery systems.

Effect of bovine serum on the phase transition temperature of cholesterol-containing liposomes

The phase transition temperature of liposomes composed of dipalmitoylphosphatidylcholine (DPPC)/hydrogenated soy phosphatidylcholine (HSPC) at a 2:1 molar ratio was estimated in buffer, 30% and 50% bovine serum by monitoring the leakage of encapsulated self quenched doxorubicin (Dox) from the vesicles when exposed to a temperature increasing from 30-52 degrees C. The results showed that bovine serum caused a slight decrease in the phase transition temperature from 44 to 41 degrees C in 50% serum. Addition of 50% cholesterol to this liposomal composition resulted in the disappearance of transition temperature in buffer and 30% serum, whereas 50% bovine serum resulted in the reappearance of the transition temperature at 46 degrees C. The data suggest that bovine serum affects the transition temperature of liposomes in a concentration-dependent manner, and this effect is more pronounced in cholesterol-rich liposomes. The time course for the release of Dox from both kinds of liposomes (cholesterol rich and cholesterol free) during incubation in 50% bovine serum, at temperatures close to the transition temperature (42 degrees, 45 degrees C), was followed. The results showed an increased leakage of Dox from both kinds of liposomes, at both temperatures. However, liposomes with high cholesterol content released more drug at 42 degrees than at 45 degrees C. The size of these liposomes was monitored in 10% bovine serum at 25 degrees C for a period of 1 h using photon correlation spectroscopy. The data showed no variation in the size of both cholesterol-rich and cholesterol-free liposomes for this period, which indicates that bovine serum does not affect the size of either cholesterol-rich or cholesterol-free liposomes.

The direct cause of photodamage-induced lysosomal destabilization

Whether membrane lipid photoperoxidation is the immediate cause for lysosomal lysis is still unclear. In this study, we investigated the direct causal factor of photoinduced lysosomal destabilization in a K+-containing solution. Methylene blue (MB)-mediated photodamage caused lysosomal membrane lipid peroxidation and loss of membrane fluidity. Compared with unirradiated lysosomes, the photodamaged lysosomes significantly lost enzyme latency in an isotonic K+-containing solution during a 20-min period of incubation. It indicates an increase in lysosomal K+ permeability. The inward K+ permeation of photodamaged lysosomes was further proved by a K+-induced elevation of internal membrane potential. In addition, the photodamaged lysosomes displayed an increased osmotic sensitivity, showing that MB-mediated photodamage promotes lysosomal osmotic fragility. Although these photoinduced alterations occurred, the lysosomes were relatively stable in an isotonic sucrose medium. In contrast, the organelle destabilized in a photodamage-dependent fashion in an isotonic K+-containing solution. The results indicate that membrane lipid peroxidation does not definitely destabilize lysosomes. The direct cause for the lysosomal destabilization is photoinduced osmotic imbalance across its membrane via an increased K+ uptake, while the increase in osmotic sensitivity favors the destabilization of photodamaged lysosomes.

Effect of cholesterol on the tight insertion of cytochrome b5 into large unilamellar vesicles

When cytochrome b5 is added to large unilamellar vesicles (LUVs) of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), it binds predominantly in a 'loose,' or transferable form. Prolonged incubation of 30 degrees C leads to insertion in the physiological 'tight,' nontransferable form, with a halftime for the loose --> tight conversion of approx. 9 days. In this study, the effect of cholesterol on the rate of tight insertion was determined. Tight binding was assayed by depleting the LUVs of loose cytochrome b5 with an excess of SUV acceptors and then separating the liposome populations by gel-filtration or velocity sedimentation. Incorporation of cholesterol into the LUVs was found to markedly increase the rate of tight insertion, even though cholesterol decreases the equilibrium binding constant and saturation level of protein binding. The effect is not a continuously increasing function of cholesterol content, but attains a maximum at 20-25% mol%, where the rate enhancement is approx. 10-fold over baseline. At higher cholesterol levels, the rate decreases, returning to baseline at 40 mol% cholesterol. These observations are highly unusual in that cholesterol generally decreases the membrane binding affinity and the permeability of solutes, and does so as a monotonic function of cholesterol concentration (above the liquid-crystalline phase transition of the phospholipids). It is suggested that tight insertion is enhanced by lipid-protein packing mismatches and by bilayer fluidity; the former increases monotonically with increasing cholesterol whereas the latter decreases monotonically. At 20-25 mol% cholesterol the optimum balance of these physical properties is obtained for tight insertion.

Permeability and stability in buffer and in human serum of fluorinated phospholipid-based liposomes

The stability (with respect to encapsulated carboxyfluorescein release) of fluorinated liposomes and their membrane permeability have been investigated in buffer and in human serum as compared to conventional hydrogenated analogues. These fluorinated liposomes are made from highly fluorinated phosphatidylcholines and contain a fluorinated core within their membrane. In buffer and in their fluid state, the fluorinated liposomes retain much more efficiently their entrapped content and display lower membrane permeability coefficients than any of their hydrogenated counterparts. This indicates that the fluorinated core acts as a very efficient barrier to permeation. In terms of molecular structure/permeability relationships, the thicker the fluorinated lipophobic core, the more efficient the barrier to permeation. In their gel state, the fluorinated core has, however, almost no effect on permeation. Interestingly, some of the 'fluid' fluorinated liposomes were even less permeable than 'gel' or 'gel-like' ones, including egg phosphatidylcholines/cholesterol liposomes. Human serum destabilizes the 'fluid' fluorinated liposomes but to a lesser extent than the 'fluid' hydrogenated ones, indicating that the fluorinated lipophobic core inside the liposomal membrane protects the vesicles, possibly by reducing their interactions with serum components. 'Gel' or 'gel-like' fluorinated liposomes are significantly more stable in serum than in buffer. They are also more stable than conventional 'gel' or 'gel-like' liposomes.

The solute permeability of thylakoid membranes is reduced by low concentrations of trehalose as a co-solute

The different efficiencies of sucrose and trehalose in protecting isolated spinach (Spinacia oleracea L.) thylakoids against freeze-thaw damage is quantitatively related to their ability to reduce the solute loading of the vesicles during freezing. In the present paper we show that this effect is based on a reduction of the solute permeability of the membranes. Permeability was measured with 14C-labeled glucose at temperatures between 0 and 10 degrees C. Glucose permeability was reduced by both sucrose and trehalose, with trehalose effective at much lower concentrations than sucrose. An analysis of the temperature dependence of glucose permeability in the presence and absence of trehalose revealed that a 50% reduction in permeability resulted from a 10% increase in activation energy and a 30% decrease in activation entropy. Using the fluorescence probe 1,6-diphenyl-1,3,5-hexatriene (DPH), we found that the reduced permeability of the membranes in the presence of trehalose was unaccompanied by a reduction in lipid fluidity. This also excluded the possibility of a solute-induced liquid crystalline to gel phase transition. A reduced partitioning of the hydrophobicity-sensitive dye merocyanine 540 into thylakoids and into membranes containing 50% digalactosyldiacylglycerol in the presence of trehalose as compared to sucrose and glucose showed that the lipid headgroup region of these membranes became less accessible for solutes. No significant difference in merocyanine partitioning in the presence of trehalose as compared to sucrose or glucose was apparent when monogalactosyldiacylglycerol dispersions or phosphatidylcholine vesicles were investigated.

Microbial fatty acids and thermal adaptation

The existing literature on the role of fatty acids in microbial temperature adaptation is reviewed. Several modes of change of cellular fatty acids at varying environmental temperatures are shown to exist in yeasts and fungi, Gram-negative bacteria, and bacteria containing iso- and anteiso-branched fatty acids, as well as in a few Gram-positive bacteria. Consequently, the degree of fatty acid unsaturation and cyclization, fatty acid chain length, branching, and cellular fatty acid content increase, decrease, or remain unaltered on lowering the temperature. Moreover, microorganisms seem to be able to change from one mode or alter the cellular fatty acid profile temperature dependently to another on lowering the temperature, as well as even within the same growth temperature range, depending on growth conditions. Therefore, the effect of the temperature on cellular fatty acids appears to be more complicated than known earlier. However, similarities found in the modes of change of cellular fatty acids at varying environmental temperatures in several microorganisms within the above mentioned groups support the existence of a limited amount of common regulatory mechanisms. The models presented enable the prediction of temperature-induced changes occurring in the fatty acids of microorganisms, and enzymatic steps of the fatty acid biosynthesis that possibly are under temperature control.

Size distribution of liposomes by flow field-flow fractionation

The applicability of field-low fractionation (FFF) to the characterization of liposomes is discussed and theoretically described. Because of fundamental differences in their driving forces, sedimentation FFF and flow FFF measure different vesicle properties. Sedimentation FFF, although used previously to measure vesicle sizes and size distributions, is fundamentally a technique that measures the effective mass and mass distribution of particles. It is sensitive to small changes in the effective mass of either the biomembrane or its encapsulated load and thus is likely to be useful in characterizing such properties as drug loading, biomembrane volumes and areas, and distributions of these properties. Size characterization by sedimentation FFF can only be done by deducing size from effective mass. Flow FFF, by contrast, provides a direct measurement of vesicle size and size distribution. After demonstrating the high resolution and relative accuracy of size measurement of flow FFF by the separation of polystyrene latex standards, flow FFF was applied to two preparations of DSPC-DSPA liposomes that were sonicated under different temperature conditions. Fractograms and size distributions are reported as a function of sonication time. The rapid elimination of a large diameter tail to the distribution is shown to constitute a major mechanism for distribution narrowing. Finally, results are provided bearing on the reproducibility of size distribution measurements by flow FFF.

Sodium transport by an ionizable and a neutral mobile carrier: effects of membrane structure on the apparent activation energy

Temperature-jump relaxation experiments on Na+ transport by (221)C10-cryptand (ionizable mobile carrier) and nonactin (neutral mobile carrier) were carried out in order to study the effects of cholesterol and the degree of acyl chain unsaturation, and their temperature-dependence on ion transport through thin lipid membranes. The experiments were performed on large, negatively charged unilamellar vesicles (LUV) prepared from mixtures of phosphatidylcholine (egg phosphatidylcholine, dioleoylphosphatidylcholine and dilinoleolylphosphatidylcholine), phosphatidic acid and cholesterol (mole fractions 0-0.43), at various temperatures and carrier concentrations. The apparent rate constants of Na+ translocation by (221)C10 and nonactin increased with the carrier concentration, the degree of acyl chain unsaturation and the temperature. The incorporation of cholesterol into the membranes significantly reduced the carrier concentration-, acyl chain unsaturation- and temperature-dependence of this parameter. The apparent energy required to activate the transport decreased significantly with increasing (221)C10 concentrations and remained constant with increasing those of nonactin at any given cholesterol molar fraction and degree of acyl chain unsaturation. It increased significantly with increasing the cholesterol molar fraction at any given carrier concentration to an extent depending on the degree of acyl chain unsaturation. Our interpretation of the action of cholesterol on these transport systems is based on the assumption that the adsorption plane of Na(+)-(221)C10 and Na(+)-nonactin complexes is likely to be located towards the aqueous and the hydrocarbon side of the dipole layer, respectively. The results are discussed in terms of the structural, physico-chemical and electrical characteristics of carriers and complexes, and of the interactions occurring between an ionizable or a neutral mobile carrier and the membrane.

The stratum corneum lipid thermotropic phase behavior

The stratum corneum, the outermost layer of mammalian skin, is considered the least permeable skin layer to the diffusion of water and other solutes. It is generally accepted that the intercellular lipid multilayer domain is the diffusional pathway for most lipophilic solutes. Fluidization of the lipid multilayers is believed to result in the loss of barrier properties of the stratum corneum. Current investigations address the lipid thermotropic phase behavior in terms of lipid alkyl chain packing, mobility and conformational order as measured by Fourier transform infrared (FTIR) spectroscopy. A solid-solid phase transition is observed with increased alkyl chain mobility followed by a gel to liquid-crystalline phase transition near 65 degrees C. These results further elucidate the role of lipid fluidity that may contribute to the transport properties of the stratum corneum.

Temperature-dependent effects of cholesterol on sodium transport through lipid membranes by an ionizable mobile carrier

Temperature-jump relaxation experiments on Na+ transport by (221)C10-cryptand were carried out in order to study the influence of cholesterol and its temperature-dependence on ion transport through thin lipid membranes. The experiments were performed on large, negatively charged unilamellar vesicles (LUV) prepared from mixtures of dioleoylphosphatidylcholine, phosphatidic acid and cholesterol (mole fractions 0-0.43), at various temperatures and carrier concentrations. The initial rates of Na+ transport and the apparent rate constants of its translocation by (221)C10 increased with the carrier concentration and the temperature. The incorporation of cholesterol into the membranes significantly reduced the carrier concentration- and temperature-dependence of these two parameters. The apparent energy required to activate the transport decreased significantly with increasing carrier concentrations at any given cholesterol molar fraction, and increased significantly with the cholesterol molar fraction at any given carrier concentration. Our interpretation of the action of cholesterol on this transport system is based on the assumption that the binding cavity of cryptands is likely to be located towards the aqueous side of the dipole layer. The results are discussed in terms of the structural, physico-chemical and electrical characteristics of carriers and complexes, and of the interactions occurring between an ionizable mobile carrier and the membrane.

Enzyme-catalyzed oxidation of cholesterol in mixed phospholipid monolayers reveals the stoichiometry at which free cholesterol clusters disappear

In this study, we have used cholesterol oxidase as a probe to study cholesterol/phospholipid interactions in mixed monolayers at the air/water interface. Mixed monolayers, containing a single phospholipid class and cholesterol at differing cholesterol/phospholipid molar ratios, were exposed to cholesterol oxidase at a lateral surface pressure of 20 mN/m (at 22 degrees C). At equimolar ratios of cholesterol to phospholipid, the average rate of cholesterol oxidation was fastest in unsaturated phosphatidylcholine mixed monolayers (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine and egg yolk phosphatidylcholine), intermediate in 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, and slowest in sphingomyelin monolayers (egg yolk or bovine brain sphingomyelin). The average oxidation rate in mixed monolayers was not exclusively a function of monolayer packing density, since egg yolk and bovine brain sphingomyelin mixed monolayers occupied similar mean molecular areas even though the measured average oxidation rate was different with these two phospholipids. This suggests that the phospholipid acyl chain composition influenced the oxidation rate. The importance of the phospholipid acyl chain length on influencing the average oxidation rate was further examined in defined phosphatidylcholine mixed monolayers. The average oxidation rate decreased linearly with increasing acyl chain lengths (from di-8:0 to di-18:0). When the average oxidation rate was examined as a function of the cholesterol to phospholipid (C/PL) molar ratio in the monolayer, the otherwise linear function displayed a clear break at a 1:1 stoichiometry with phosphatidylcholine mixed monolayers, and at a 2:1 C/PL stoichiometry with sphingomyelin mixed monolayers.(ABSTRACT TRUNCATED AT 250 WORDS)

Deuteron nuclear magnetic resonance study of the dynamic organization of phospholipid/cholesterol bilayer membranes: molecular properties and viscoelastic behavior

The influence of cholesterol on the dynamic organization of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) bilayers was studied by deuteron nuclear magnetic resonance (2H NMR) using unoriented and macroscopically aligned samples. Analysis of the various temperature- and orientation-dependent experiments were performed using a comprehensive NMR model based on the stochastic Liouville equation. Computer simulations of the relaxation data obtained from phospholipids deuterated at the 6-, 13- and 14-position of the sn-2 chain and cholesterol labeled at the 3 alpha-position of the rigid steroid ring system allowed the unambiguous assignment of the various motional modes and types of molecular order present in the system. Above the phospholipid gel-to-liquid-crystalline phase transition, TM, 40 mol % cholesterol was found to significantly increase the orientational and conformational order of the phospholipid with substantially increased trans populations even at the terminal sn-2 acyl chain segments. Lowering the temperature continuously increases both inter- and intramolecular ordering, yet indicates less ordered chains than found for the pure phospholipid in its paracrystalline gel phase. Trans-gauche isomerization rates on all phospholipid alkyl chain segments are slowed down by incorporated cholesterol to values characteristic of gel-state lipid. However, intermolecular dynamics remain fast on the NMR time scale up to 30 K below TM, with rotational correlation times tau R parallel for DMPC ranging from 10 to 100 ns and an activation energy of ER = 35 kJ/mol. Below 273 K a continuous noncooperative condensation of both phospholipid and cholesterol is observed in the mixed membranes, and at about 253 K only a motionally restricted component is left, exhibiting slow fluctuations with correlation times of tau R perpendicular greater than 1 microsecond. In the high-temperature region (T greater than TM), order director fluctuations are found to constitute the dominant transverse relaxation process. Analysis of these collective lipid motions provides the viscoelastic parameters of the membranes. The results (T = 318 K) show that cholesterol significantly reduces the density of the cooperative motions by increasing the average elastic constant of the membrane from K = 1 x 10(-11) N for the pure phospholipid bilayers to K = 3.5 x 10(-11) N for the mixed system.

The role of surface charge and hydrophilic groups on liposome clearance in vivo

The effect of negative surface charge and hydrophilic groups on liposome clearance from blood was investigated in mice using liposome-entrapped 67gallium-deferoxamine as a label. The presence of negatively-charged lipids may retard or accelerate liposome clearance. Physicochemical features contributing to optimal retardation of liposome clearance include a hydrophilic carbohydrate moiety and a sterically hindered negatively-charged group. The relevance of the negative charge steric effect is suggested by the finding that phosphatidylinositol phosphate (PIP) and trisialoganglioside (GT1) are less effective than phosphatidylinositol (PI) and monosialoganglioside (GM1), respectively, in retarding liposome clearance. The need for negative charge in addition to the carbohydrate group for optimal effect on retardation of clearance is indicated by the observation that asialoganglioside (AGM1) is less effective than GM1 in this respect. The negative charge effect is observed with liposome bilayers having both low and high temperature phase-transitions. Increasing the molar fraction of negatively-charged lipid (hydrogenated PI derived from soya) from 23 to 41% resulted in a dramatic acceleration of liposome clearance. The clearance-accelerating effect of the high negative charge was specifically directed to the liver with selective reduction of spleen uptake. Increasing liposome size also had an accelerating effect on clearance but in this case it was accompanied by a non-specific concomitant increase of both liver and spleen uptake.

A Cl(-)-translocating adenosinetriphosphatase in Acetabularia acetabulum. 2. Reconstitution of the enzyme into liposomes and effect of net charges of liposomes on chloride permeability and reconstitution

The Mono Q-III fraction, a Mg2(+)-ATPase, isolated from Acetabularia acetabulum was reconstituted into liposomes of various net charges prepared by the reversed-phase method and tested for a Cl(-)-translocating activity. The liposomes from a mixture of egg lecithin, dicetyl phosphate, and cholesterol (63:18:9 mole ratio, negative liposomes) and from a mixture of egg lecithin and cholesterol (63:9 mole ratio, neutral liposomes) were less leaky than positive liposomes from asolectin, and from a mixture of egg lecithin, stearylamine, and cholesterol (63:18:9 mole ratio). A significant increase in 36Cl- efflux from the negative and neutral liposomes was observed by addition of ATP in the presence of valinomycin after incorporation of the enzyme by short-term dialysis. The ATP-driven 36Cl- efflux was inhibited by addition of azide, an inhibitor of the ATPase. The preincubation of the enzyme with phenylglyoxal, an arginine-modifying reagent, inactivated ATP-mediated 36Cl- efflux, but the ATPase activity of the preparation was not affected. When chloride was replaced by 35SO4(2)-, no ATP-dependent 35SO4(2)- efflux was detectable from the proteoliposomes. Proton-translocating activity of the enzyme was also tested, and no fluorescent quenching of 9-ACMA was observed.

Enhanced fluorescence in indirect immunophenotyping by the use of fluorescent liposomes

Small unilamellar liposomes were optimised for cell phenotyping by indirect immunofluorescence. This involved selection and covalent attachment to the liposome of a polyspecific ligand for cell-bound antibody. For this purpose sheep anti-mouse antibody was preferred to protein A because of its ability to attach to cell-bound IgG1 as well as IgG2 at physiological pH. The maximally fluorescent concentration of encapsulated carboxyfluorescein was determined to be 20 mM and liposomes thus comprised gave up to a nine-fold increase in mean cell fluorescence when compared with sheep anti-mouse antibody conjugated to fluorescein isothiocyanate. There was no parallel increase in background fluorescence. Liposomes retained their targeting and fluorescence properties after 3 months storage. They could be sterilised and were as versatile in use as FITC-antibody conjugates.

A fluorescence and radiolabel study of sterol exchange between membranes

The fluorescent sterols delta 5,7,9(11),22-ergostatetraen-3 beta-ol (dehydroergosterol) and delta 5,7,9,(11)-cholestatrien-3 beta-ol (cholestatrienol) as well as [1,2-3H]cholesterol were utilized as cholesterol analogues to examine spontaneous exchange of sterol between 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) small unilamellar vesicles (SUV). Exchange of fluorescent sterols was monitored at 24 degrees C by release from self-quenching of polarization from the time of mixing without separation of donor and acceptor vesicles. The polarization curve for 35 mol% sterol in POPC best fitted a two-exponential function, with a fast-exchange rate constant k1 = 0.0217 min-1, 1t1/2 = 32 min, size pool 1 = 12%, and a slow rate constant k2 = 2.91.10(-3) min-1, 2t1/2 = 238 min, size pool 2 = 88%. In addition to the above two exchangeable pools of sterol, the data were consistent with the presence of a slowly or nonexchangeable pool, 42% of total sterol, that was highly dependent on sterol content. These results were confirmed by simultaneous monitoring of [1,2-3H]cholesterol radioactivity and dehydroergosterol fluorescence intensity after separation of donor and acceptor vesicles by ion-exchange column chromatography. Thus, dehydroergosterol or cholestatrienol exchange as measured by fluorescence parameters (polarization and/or intensity) provides two new methods to follow cholesterol spontaneous exchange. These methods allow resolution and quantitation of a shorter exchange t1/2 near 30 min previously not reported. Thus, the cholesterol desorption rate from membranes may be faster than previously believed. In addition, the presence of a slowly non-exchangeable pool was confirmed.

Effect of erythropoietin on membrane lipid peroxidation, superoxide dismutase, catalase, and glutathione peroxidase of rat RBC

Starved animals having low levels of erythropoietin in blood showed increased MDA, fluorescent pigments, and met-Hb values whereas the hemoglobin concentration decreased significantly on starvation. In vivo and in vitro studies with Ep reversed the effects of starvation and brought these values close to normal. The activities of the enzymes (SOD, catalase, GSH-PX, GR G6PD, and 6PGD) which protect the RBC membrane directly or indirectly from peroxidative threat, decreased on starvation and restored to normal levels after Ep treatment.

Compensatory mechanisms in erythrocyte lipids in patients with atherosclerosis

The quantitative composition of phospholipids and fatty acids of erythrocytes was investigated in patients with atherosclerosis. It was stated that the erythrocyte lipids of atherosclerotic patients contained smaller quantities of phosphatidylcholine and phosphatidylinositol, a significantly larger quantity of sphingomyelin, and higher sphingomyelin/phosphatidylcholine and cholesterol/phospholipid ratios. The existence of compensatory changes was stated, which was evident in the reduction of palmitic and stearic acids and the increase of linoleic and eicosatrienoic acids in erythrocyte phospholipids. These changes in fatty acid composition probably cause minimal changes in the membrane fluidity induced by an increased cholesterol/phospholipid and sphingomyelin/phosphatidylcholine ratios. This paper was the first evidence of occurrence of those changes in erythrocytes during spontaneous atherosclerosis in human.

Cholesterol enhances mouse hepatitis virus-mediated cell fusion

Mouse hepatitis virus (MHV) infection of the L-2 subline of mouse fibroblasts results in acute infection characterized by extensive cell fusion. In contrast, infection of the LM-K subline leads to virus persistence with reduced cell fusion. We undertook studies designed to elucidate the role of host cell membrane lipid composition and the cytoskeleton in modulating the fusion process and the resultant effect(s) on virus persistence. MHV-induced cell fusion proceeded normally in cells treated with cytoskeleton-disrupting drugs, cytochalasin B and colchicine. Modification of cell membrane fatty acid composition by supplementation of LM-K cells with arachidonic (C-20:4) or palmitic (C-16:0) acids had little effect on the extent of MHV-induced cell fusion or on virus replication. However, supplementation of both cell types with cholesterol (resulting in increased membrane cholesterol/fatty acid ratio) resulted in marked enhancement of virus-mediated cell fusion. The increase in cell membrane cholesterol did not enhance internalization of MHV suggesting that cholesterol primarily modulates a later event. This suggestion was confirmed by demonstrating cholesterol-enhancement of fusion in a contact fusion assay. Cholesterol-supplemented L-2 cells were less productive for virus replication than unsupplemented cells, in agreement with our previous observations that MHV replication is compromised by extensive cytopathic effect. Although cholesterol-supplemented LM-K cells showed increased susceptibility to MHV-mediated cell fusion, the extent of such susceptibility did not approach that observed in L-2 cells. Also, the property of LM-K cells to support MHV persistence was not abolished by cholesterol supplementation. Thus membrane fusion resistance and MHV persistence are modulated but not alleviated by cell membrane cholesterol content.

An ionotropic phase transition in phosphatidylcholine: cation and anion cooperativity

Evidence is presented for cooperative interaction between cations and anions specifically bound to dimyristoylphosphatidylcholine (DMPC). The cooperativity is with regard to an ion-induced (ionotropic) phase transition for the lipid and is signalled by a change in the luminescence from bound Tb3+. The intrinsic binding of Tb3+ to DMPC was determined from equilibrium dialysis experiments, using conventional methods to correct for electrostatic contributions. Preliminary results demonstrate great potential for infrared spectroscopy as a means to relate these Tb3+ luminescence studies to experiments involving less tractable cations. This work provides insight into the role of bound ions in modifying lateral phase behavior in phospholipid membranes.

Efficiency, Na+/K+ selectivity and temperature dependence of ion transport through lipid membranes by (221)C10-cryptand, an ionizable mobile carrier

The kinetics of Na+ and K+ transport across the membrane of large unilamellar vesicles (LUV) were determined at two pH's when transport was induced by (221)C10-cryptand (diaza-1,10-decyl-5-pentaoxa-4,7,13,16,21-bicyclo [8.8.5.] tricosane) at various temperatures, and by nonactin at 25 degrees C and (222)C10-cryptand at 20 and 25 degrees C. The rate of Na+ and K+ transport by (221)C10 saturated with the cation and carrier concentrations. Transport was noncooperative and exhibited selectivity for Na+ with respect to K+. The apparent affinity of (221)C10 for Na+ was higher and less pH-dependent than that for K+, and seven times higher than the affinity for Na+ of nonactin. Its enthalpy was higher than that of (222)C10 for K+ ions (20.5 vs. 1.7 kcal . mole-1). The efficiency of (221)C10 transport of Na+ was pH- and carrier concentration-dependent, and was similar to that of nonactin; its activation energy was similar to that for (222)C10 transport of K+ (35.5 and 29.7 kcal . mole-1, respectively). The reaction orders in cation n(S) and in carrier m(M), respectively, increased and decreased as the temperature rose, and were both independent of carrier or cation concentrations; in most cases, they varied slightly with the pH. n(S) varied with the cation at pH 8.7 and with the carrier for Na+ transport only, while m(M) always depended on the type of cation and carrier. Results are discussed in terms of the structural, physico-chemical and electrical characteristics of carriers and complexes.

Ca2+-induced fusion of large unilamellar phosphatidylserine/cholesterol vesicles

The effect of cholesterol on the Ca2+-induced aggregation and fusion of large unilamellar phosphatidylserine (PS) vesicles has been investigated. Mixing of aqueous vesicle contents was followed continuously with the Tb/dipicolinate assay, while the dissociation of pre-encapsulated Tb/dipicolinate complex was taken as a measure of the release of vesicle contents. Vesicles consisting of pure PS or PS/cholesterol mixtures at molar ratios of 4:1, 2:1 and 1:1 were employed at three different lipid concentrations, each at four different Ca2+ concentrations. The results could be well simulated in terms of a mass-action kinetic model, providing separately the rate constants of vesicle aggregation, c11, and of the fusion reaction itself, f11. In the analyses the possibility of deaggregation of aggregated vesicles was considered explicitly. Values of both c11 and f11 increase steeply with the Ca2+ concentration increasing from 2 to 5 mM. With increasing cholesterol content of the vesicles the value of c11 decreases, while the rate of the actual fusion reaction, f11, increases. Remarkably, the effect of cholesterol on both aggregation and fusion is quite moderate. The presence of cholesterol in the vesicle bilayer does not affect the leakage of vesicle contents during fusion.

Filipin-cholesterol binding in CNS axons prior to myelination: evidence for microheterogeneity in premyelinated axolemma

The distribution of cholesterol in axonal membrane of developing rat optic nerves prior to myelination was studied by freeze-fracture cytochemistry. Binding of the cholesterol-specific probe, filipin, to the axolemma of premyelinated axons was heterogeneous; this suggests the presence of microdomains of axolemma with different membrane composition and/or cytoskeletal/extracellular matrix association. Although the reasons for this binding pattern have not yet been determined, heterogeneity occurs prior to association of glia with the axon, and may reflect regional differences in lipid/sterol composition of the axonal membrane bilayer, or distribution of membrane-associated cytoskeleton. The distribution of intramembranous particles was not obviously associated with the pattern of filipin binding in early developing axons, however, as might have been expected from the attending differences in fluidity of the membrane microdomains. Microheterogeneity in axonal membranes of developing axons could have an influence on several membrane properties, and may be associated with processes important for growth and differentiation of axons.

Low affinity binding of taurine to phospholiposomes and cardiac sarcolemma

A sarcolemma-enriched membrane fraction was prepared from the hearts of Sprague-Dawley rats and its ability to bind taurine (0.5-150 mM) was measured. In the absence of cations, the sarcolemma bound a maximum of 661 nmol taurine/mg protein, with a dissociation constant of 19.2 mM and a Hill coefficient of 1.9, indicating positive cooperativity. Scatchard analysis of taurine binding to sarcolemma gave a bell-shaped curve. Neither beta-alanine nor guanidinoethane sulfonate, inhibitors of taurine transport, affected the degree of taurine binding to sarcolemma. However, hypotaurine was an effective antagonist. Equimolar concentrations of Ca2+, Na+ or K+ also reduced taurine binding. Heterogeneous phospholipid vesicles of phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol, and phosphatidylserine (18:19:2:1) also bound taurine with positive cooperativity, yielding a bell-shaped Scatchard curve. The affinity of taurine for these mixed phospholipid vesicles was enhanced by the inclusion of cholesterol (50%). Taurine associated in a maximum ratio of 1:1 with homogeneous vesicles of phosphatidylcholine or phosphatidylserine. Vesicles of phosphatidylethanolamine bound taurine in a maximum ratio of 2:1, whereas those of phosphatidylinositol bound insignificant amounts of taurine. These studies demonstrate a low affinity binding to sarcolemma of taurine at concentrations normally present in rat heart. Similar levels of binding were observed in phospholipid vesicles, suggesting that the interaction of taurine with biological membranes involves phospholipids.

Cholesterol and the cell membrane

Recent studies concerning cholesterol, its behavior and its roles in cell growth provide important new clues to the role of this fascinating molecule in normal and pathological states.

Modification of HT 29 cell response to the vasoactive intestinal peptide (VIP) by membrane fluidization

We used liposomes made with phospholipids of fatty acid chain length ranging from C12:0 to C16:0 to modify the cAMP dependent protein kinase (PK) activity of HT 29 cells induced by VIP or forskolin. Both VIP and forskolin effects were inhibited in dilauroylphosphatidylcholine (DLPC) treated cells. PK activity was slightly lowered when cells were treated by dimyristoylphosphatidylcholine (DMPC) liposomes. However neither VIP nor forskolin-induced PK activities were affected with dipalmitoylphosphatidylcholine (DPPC) liposomes. Furthermore, the binding of [125I]VIP to DLPC treated cells was drastically lowered whereas no change was observed when cells were incubated with DMPC or DPPC liposomes. On the other hand, the interaction of HT 29 cells with DLPC vesicles provoked a decrease in membrane cholesterol content with subsequent increase in membrane fluidity. These findings provide evidence that, in HT 29 cells, the mechanisms of VIP-receptor interaction and of adenylate cyclase activation is lipid dependent and is regulated by membrane fluidity.

Role of interfacial structured water in membrane: osmotic properties of L-alpha-egg lecithin liposomes

The role of large amounts of membrane-bound water in regulating various functions of the membrane is not clear at present. We have investigated the effect of perturbing the interfacial water structure on the osmotic shrinkage properties, such as water permeability and extent of shrinkage of egg lecithin liposomes. Water structure was perturbed by a series of reagents which have been earlier reported to affect phase transition of dipalmitoyl phosphatidylcholine liposomes by perturbing interfacial water structure. Anomalous variations of osmotic shrinkage properties with concentration of structure maker and breaker reagents have been interpreted to arise from concentration-dependent structural transitions of the ordered water at the membrane-aqueous interface. Various modes of interaction of these reagents on interfacial structured water have been suggested. Influence of molecular size and functional groups on the molecule in actions of some structure makers and breakers were also observed.

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.

Lateral phase separation of phospholipids as a basis for increased permeability of membranes towards fluorescein and other chemical species

Using mouse spleen cells, before and after treatment with glutaraldehyde or mild hyperthermia, we observe a strong correlation between permeability to fluorescein and susceptibility to staining with N epsilon-dansyl-L-lysine (irrespective of the cells' ability to exclude trypan blue). We observe the same correlation using liposomes prepared from phosphatidylcholine and varying amounts of cholesterol. We have recently introduced N epsilon-dansyl-L-lysine as a fluorescent membrane stain, or "probe," whose uptake, we propose, is selective for phospholipid domains in membranes (G.M.K. Humphries & J.P. Lovejoy Biophys. J. 42:307-310, 1983; G.M.K. Humphries & J.P. Lovejoy J. Membrane Biol. 77:115-122, 1984). The results presented here are consistent with the hypothesis that the presence or absence of phospholipid domains in membranes also modifies their permeability toward fluorescein, and suggests that permeability towards other chemical species may be similarly affected. On the basis of work using liposomes, we believe this to be the case for carboxyfluorescein and for glucose.

25-Hydroxycholesterol-induced elevation in 45Ca uptake: correlation with depressed DNA synthesis

The mechanism whereby 25-hydroxycholesterol, an inhibitor of the synthesis of cholesterol, depresses DNA synthesis in cycling P815 mastocytoma cells was investigated. The uptake of 45Ca into P815 cells treated with 1 microgram/ml 25-hydroxycholesterol began to rise above control levels by 6 hours after initiation of treatment and was increased tenfold by 15 hours. Kinetic data of calcium uptake indicated the presence of at least two components of calcium uptake, fast and slow. The fast phase of calcium exchange at the cell surface was changed little by treatment with 25-hydroxycholesterol. The slow phase of calcium exchange with the intracellular compartment was markedly affected by treatment with the inhibitor, there being a large increase in the flux and half-time of uptake, and a fall in the rate constant. This resulted in a large elevation of the intracellular compartment size. Incorporation of [3H]thymidine into DNA began to decline between 9 and 12 hours posttreatment in these cultures. Uptake of calcium and depression of DNA synthesis were shown to be directly related to the dose of 25-hydroxycholesterol used. The changes in 45Ca uptake and DNA synthesis due to 25-hydroxycholesterol treatment were abolished by addition of exogenous cholesterol to the incubation medium. The results are consistent with the hypothesis that 25-hydroxycholesterol, by inhibiting cholesterol production, depresses DNA synthesis via an elevation in the uptake of calcium into the cell to a level incompatible with continued DNA replication.