Phosphatidylserine Monolayers Research Articles

Adjuvant lipopeptide interaction with model membranes

The cationic lipohexapeptide Pam 3Cys–Ser–(Lys) 4 is a synthetic model for the triacylated N-terminal part of bacterial lipoproteins, and it is used as an adjuvant and macrophage activator. The amphiphilic lipopeptide was injected below a phosphatidylserine monolayer at the air–water interface. It interacted with the interface, as seen by a decrease in the surface potential (Δ V), and it was inserted in the monolayer, until surface charge neutralization was reached, as seen by the parallel increases of Δ V and of the surface pressure. No insertion occurred above 29 mN/m. The interaction kinetics was sensitive to ionic strength and to the nature of acidic phospholipids and of their acyl chains, but the final equilibrium was independent of these factors. Addition of the lipopeptide to large unilamellar vesicles (LUVs) induced their aggregation, and an exchange of lipids between fluorophor-labelled and non-labelled LUVs. However, no fusion was observed, just as reported for polylysine. The lipopeptide strongly inhibited calcium-induced fusion of PS LUVs, in contrast to the published effect of polylysine. This was probably due to inhibition of calcium fixation on liposomes, since it was observed that the lipopeptide efficiently displaced 45 Ca 2+ from a PS monolayer. In addition, a phospholipid segregation was observed in SUVs for a few ten micromolar of the lipopeptide.

The relationship between the binding of ATP and calcium to annexin IV. Effect of nucleotide on the calcium-dependent interaction of annexin with phosphatidylserine.

With the use of ATP analogues, we have found that porcine liver annexin (Anx) IV can be covalently labelled with 8-azido[gamma-32P]-ATP in the presence of Ca2+ (Kd 4.2 microM) and that the labelling is prevented by asolectin/cholesterol liposomes or chelation of calcium ions. On the other hand, non-covalent binding of 2'-(or 3')-O-(2,4,6-trinitrophenyl)adenosine 5'-triphosphate (TNP-ATP) to AnxIV occurs optimally in the presence of liposomes and Ca2+ (Kd 7 microM). These observations were further confirmed by the results of intrinsic fluorescence quenching of AnxIV with various nucleotides, suggesting the existence of a relationship between Ca(2+)-, phospholipid- and ATP-binding sites within the annexin molecule. The interaction of AnxIV with nucleotides does not significantly affect its in vitro properties concerning the binding to phosphatidylserine (PS) monolayers.

Surface charge density measurements on mercury electrodes covered by phospholipid monolayers

A novel method to measure the surface charge density at a hanging mercury drop electrode (HMDE) coated with a selfassembled phospholipid monolayer mimicking a biological membrane is described. The charge density is obtained by analogical integration of the capacitive current which flows at constant applied potential as a consequence of a slight contraction of the mercury drop. The contraction must be carried out while keeping the neck of the lipidcoated mercury drop in contact with the lipid film spread on the surface of the electrolytic solution. The validity of this procedure is tested by comparison with differential capacity measurements carried out at a lipidcoated HMDE, fully immersed into the solution, under otherwise identical conditions. The potential of this method in the investigation of the properties of selfassembled lipid monolayers, either pure or incorporating lipophilic species, is briefly outlined. The method is applied to the determination of the charge density of tetraphenylphosphonium cations adsorbed in the polar head region of a phosphatidylserine monolayer supported by the HMDE.

Interaction of annexins IV and VI with phosphatidylserine in the presence of Ca2+: monolayer and proteolytic study.

Annexins, Ca2+- and phospholipid-binding proteins are known to bind to artificial and biological membranes in a calcium-dependent manner. However, the precise mechanism of the annexin-membrane interactions still remains to be studied in detail. In this paper we describe the results of studies on the interactions of the annexin/Ca complexes with phospholipids, obtained by the Wilhelmy balance method of assessing the surface pressure of a phospholipid monolayer. We show that the annexin IV/Ca as well as annexin VI/Ca complexes significantly reduce the surface pressure of a phosphatidylserine monolayer, when its initial value is close to collapse pressure. The effect is highly specific for monolayers composed of phosphatidylserine and strongly sensitive to pH and ionic strength. The most pronounced changes have been observed at pH 7.0-7.5, at a protein/Ca molar ratio of 1:2 for annexin IV and 1:4 for annexin VI. In the presence of sodium chloride at concentrations exceeding 400mM this effect was almost completely abolished. The obtained results point to the mainly electrostatic character of the annexin/phosphatidylserine interactions. In addition, using large multilamellar lipid vesicles and serine proteases, we demonstrate that annexins, when bound in a ternary complex with phospholipids and calcium ions, are partially protected against proteolysis. Our observation that annexin molecules, complexed with calcium ions, are protected against proteolytic attack in the presence of PS liposomes does not have to be necessarily explained in terms of partial penetration of protein within the membrane bilayer.

Adsorption of Tetraphenylphosphonium and Tetraphenylborate in Self-Assembled Phosphatidylcholine and Phosphatidylserine Monolayers Deposited on Mercury Electrodes

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTAdsorption of Tetraphenylphosphonium and Tetraphenylborate in Self-Assembled Phosphatidylcholine and Phosphatidylserine Monolayers Deposited on Mercury ElectrodesMaria Rosa Moncelli, Roberto Herrero, Lucia Becucci, and Rolando GuidelliCite this: J. Phys. Chem. 1995, 99, 24, 9940–9951Publication Date (Print):June 1, 1995Publication History Published online1 May 2002Published inissue 1 June 1995https://doi.org/10.1021/j100024a042RIGHTS & PERMISSIONSArticle Views66Altmetric-Citations34LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-Alerts

Interaction of melittin with lipid membranes

Interaction of melittin with lipid membranes was studied systematically with respect to its adsorption onto membranes, its effect on membrane leakage and fusion, and micellization at various melittin/lipid ratios. It was found that melittin has a strong affinity for adsorption onto lipid membranes. The analysis of the measured electrophoretic mobilities by use of a Gouy-Chapman double layer theory, shows that melittin is adsorbed onto the phosphatidylserine membrane several times more than the phosphatidylcholine membrane. However, it was observed that the phosphatidylcholine membrane is more susceptible to membrane leakage, vesicle fusion and micellization at a lower level of melittin adsorbed than the phosphatidylserine membrane. For small unilamellar phosphatidylcholine vesicles in 0.1 M NaCl, membrane leakage started at melittin to lipid ratio of 1:2000, a large increase in the rate of membrane leakage occurred at a ratio of about 1:500 or higher, membrane fusion occurred at a ratio of 1:200, and membrane micellization at a ratio of 1:10. On the other hand, for small unilamellar phosphatidylserine vesicles, the respective concentrations of melittin to result in membrane leakage, vesicle fusion, and membrane micellization were several times higher. Surface pressure measurements of lipid monolayers showed that the increase in surface pressure of the phosphatidylcholine monolayer due to the presence of melittin in the subphase solution was greater than that for the phosphatidylserine monolayer at any melittin concentration in the subphase solution. These experimental results indicate that melittin tends to be adsorbed on the surface of the negatively charged phosphatidylserine membrane due to the electrostatic binding so that the melittin molecule can stay out more on the surface of the membrane, while melittin appears to be adsorbed more into the hydrophobic membrane core for the electrically neutral phosphatidylcholine membrane.

Electrostatic and hydrophobic interactions are involved in factor Va binding to membranes containing acidic phospholipids.

The interaction of factor Va with phospholipid monolayers was studied using alternating current polarography. For these studies a hanging mercury drop electrode is positioned in contact with the monolayer at the air-monolayer interface. Factor Va introduced into the aqueous subphase beneath the monolayer caused alterations in the capacitance of the circuit. When factor Va was introduced beneath monolayers containing 13-25% mole fraction phosphatidylserine (PS), 87-75% mole fraction phosphatidylcholine (PC), a significant capacitance increase occurred, which is interpreted to be the result of the penetration of the factor Va molecules through the monolayer. No penetration or associated capacitance changes were observed with monolayers composed of pure PC or pure PS. Polarography experiments were also conducted with the electrode placed in the aqueous subphase to measure the binding of factor Va to pure PS monolayers. This approach has allowed detection of factor Va (at nM concentrations) adsorption to a pure PS monolayer. Thus the membrane adsorption process can be distinguished from factor Va penetration through the monolayer. These results suggest that there are two types of interaction of the factor Va molecule with PS-containing monolayers. The interaction with the PS component of the monolayer is essential for binding the factor Va (at nM concentrations) that occurs by surface adsorption. For mixed PC/PS monolayers, PS adsorption is followed by penetration of the protein through the membrane.

Voltammetry of TlI, CdII, CuII, PbII and EuIII at phosphatidylserine-coated mercury electrodes

The characteristics of the electrochemical reduction of metal ions at a negatively charged lipid-coated electrode are described. The metal-ion voltammetry is related to the permeability of the electroactive ion in the lipid film and to interactions between the electrolyte and electroactive ions and the lipid surface. The cyclic voltammetry of Tl, Cd, Cu, Pb and Eu was investigated at a phosphatidylserine (PS)-coated mercury electrode in solutions of different pH, ionic composition and ionic concentration. Significant reduction peaks of TlI, CdII, CuII and PbII are observed within the potential region where the lipid maintains its stable orientation on the electrode thus these ions are significantly more permeable in PS monolayers than in dioleoyl phosphatidylcholine (DOPC) monolayers. The voltammetric peaks of CdII in solution pH 8.1 and the voltammetric peaks of CuII and EuIII in solution pH > 5.0 are characteristic of the irreversible reduction of adsorbed electroactive species whereas those for TlI and PbII represent the quasi-reversible and irreversible reduction respectively of the dissolved species. With a lowering in solution pH, from 10.4, 8.1, 7.0, 5.8 and 5.6 for TlI, CdI, PbII, CuII and EuIII respectively, the reduction current peaks of all the electroactive ions decrease in height. Specifically, an increased concentration of univalent ion electrolyte and the presence of univalent electrolyte ions with greater binding affinity for PS suppress the Tl voltammograms. In solution pH about one unit below the intrinsic second pKa value of PS (3.6) or in the presence of Mg2+ electrolyte or with small concentrations of La3+ ions in a univalent ion electrolyte, the voltammograms of all the electroactive ions begin to resemble those for the reduction of these metal ions at a DOPC coated electrode indicating that the lipid layer has become relatively impermeable to ions. It is evident that in solution pH ca. 8.1, CdII is adsorbed on PS. The association of CdII with PS decreases with a lowering in solution pH but EuIII and CuII are significantly adsorbed on PS at lower solution pH (5.9) thus CuII binds more strongly to PS than CdII. In solution pH < 5, the association of EuIII and CuII ions with the PS surface decreases with a lowering in pH. The effects of solution pH on the voltammograms relate directly to the protonation of PS and the lowering of the average monolayer potential. This lessens the electrostatic interaction of electroactive ions with the monolayer surface lowering their concentration in this region and their apparent permeability. It also alters the structure of the lipid. The influence of varying concentration and type of monovalent electrolyte cations on the TlI reduction currents is due to both a screening effect on the monolayer potential and a binding of ions to the lipid which lowers the TlI concentration adjacent to the lipid surface. Electrolyte Mg2+ and small concentrations of La3+ in a univalent ion electrolyte bind strongly to the lipid also lowering the electroactive ion concentration at the lipid surface as well as altering the lipid structure.

Voltammetry of retinal in phospholipid monolayers adsorbed on mercury

The redox and proton-transferring behaviour of retinal in biological membranes is of fundamental importance in physiological processes. The phospholipid-coated mercury electrode is an ideal membrane model with which to investigate these problems. All- trans, 13- cis and 9- cis retinal were incorporated into monolayers of dioleoyl phosphatidylcholine (DOPC), dimyristoyl phosphatidylcholine (DMPC), egg phosphatidylcholine (egg PC), phosphatidylserine (PS) and mixed monolayers of DOPC and egg phosphatidylethanolamine (PE) which were adsorbed onto a mercury electrode. The electrochemical technique used to measure charge transfer was cyclic voltammetry. Retinal is irreversibly reduced in phospholipid monolayers, in contrast to retinol and retinoic acid which are electro-inactive. Above solution pH 5, retinal forms an association with PS and PE in mixed retinal + PS and retinal + (4DOPC + PE) monolayers respectively. This association is electrochemically reduced at more positive potentials than the retinal in DOPC. The reduction of this association becomes more significant with increasing solution pH, and in solution pH > 9 the reduction is successive on each voltage sweep until all the retinal has formed the product. Experimental evidence indicates that retinal reduction in phospholipid layers is a two-electron process and the rate-controlling step is the second electron transfer which is preceded by protonation. The retinal + PS and retinal + PE associations are already partly protonated in solutions of pH < 8. In monolayers of DOPC, proton transfer becomes limiting at high scan rates and the limiting effect is alleviated by reducing monolayer coverage which renders the monolayer more permeable to proton-transferring species.

Binding of substance P to monolayers and vesicles made of phosphatidylcholine and/or phosphatidylserine

Analyses of interactions between substance P (SP) and phospholipids were performed by combined surface pressure and surface potential measurements in monolayers and by 13C-NMR experiments on liposomes. This study was carried out using synthetic SP molecules: [1- 13C-Gly 9]SP and [1- 13C-Gly 2]SP. Injection of SP into the aqueous subphase led to an expansion of phosphatidylcholine (PtdCho) or phosphatidylserine (PtdSer) monolayer surface area. An apparent association constant of SP for PtdSer was estimated to be around 10 6–10 7 M 1. The surface potential ΔV/ n varied linearly with the molecular area whereas the variation of surface pressure was biphasic, suggesting that at least two binding states contributed to the monolayer expansion. These two states Si (SP is inserted into the bilayer) and Ss (SP is stuck on the surface) were observed on vesicular membranes by 13C-NMR. The kinetic of interconversion between these two states can be estimated by NMR, the Ss state being the stablest one. No perpendicular insertion of SP into these vesicular preparations seemed to occur, as previously postulated. However, SP might form aggregates in contact with these model systems, leading to a loss of permeability of the lipid vesicles.

Divalent Cation-Induced Phase Transition of Phosphatidylserine Monolayer at the Polarized Oil–Water Interface and Its Influence on the Ion-Transfer Processes

Abstract The phase properties of dipalmitoylphosphatidylserine(DPPS) monolayer at a polarized nitrobenzene–water interface have been studied by measuring the double-layer capacitance of the interface, Cdl, under precise control of the potential drop across the interface. When a saturated DPPS monolayer was in contact with an aqueous 0.1 mol dm−3 LiCl solution, the Cdl took a value of 9.5 μF cm−2. The presence of Ca2+ or Mg2+ at greater than 2 mmol dm−3 in the aqueous phase induced a decrease in Cdl down to 1.5 μF cm−2, corresponding to the phase transition of the monolayer from a liquid-expanded to a condensed phase. The condensed DPPS monolayer was stable against a change in the applied potential across the interface between ΔOWφ=−0.14 and 0.10 V, where ΔOWφ is the potential of the aqueous phase with respect to that in the nitrobenzene phase. Cyclic voltammetric measurements of ion transfer showed that the DPPS monolayer in the liquid-expanded state decreased the rate of ClO4− transfer from the aqueous to the nitrobenzene phase, but did not affect the rate of tetramethylammonium ion, TMA+, transfer. This indicates the importance of the electrostatic interaction between the negatively charged DPPS monolayer and the transferring ions in determining the rate of ion transfer across the monolayer. In contrast, the condensed monolayer significantly diminished the rates of ion transfer for both TMA+ and ClO4− ions, suggesting that the presence of the condensed monolayer altered the rate-determining step and that the condensed monolayer exerted an additional hydrodynamic friction on the ion-transfer processes.

Phosphatidylserine monolayers as models for drug uptake into membranes and tissue

Phosphatidylserine monolayers as models for drug uptake into membranes and tissue

R 56865 and flunarizine displace Ca 2+ from phosphatidylserine monolayers in a stoichiometric manner

The monovalent cationic drugs, R 56865 and flunarizine added to the subphase, displaced Ca 2+ from phosphatidylserine (PS) monolayers at pH 5 with EC 50 values of 1.2 × 10 −6 and 1.3 × 10 −6 mol/l. The EC 50 of flunarizine for the increase in surface pressure due to incorporation of the drug into the PS film was 1.8 × 10 −6 mol/l. The maximal binding of both drugs to PS monolayers was estimated from experiments in which the drug was added to the spreading solution for the monolayer. The maximal molar binding ratio of R 56865 was identical to twice the maximal Ca 2+ binding (2 × 0.13 Ca/PS) to the monolayer. The uptake of [ 14C]R 56865 into PS monolayers could be measured directly and was well described by a Langmuir isotherm in which the EC 50 for 45Ca displacement was taken as the concentration for half-maximal binding, and the molar ratio for twice the Ca 2+ binding was taken is maximal molar binding ratio. These results show that two molecules of the univalently charged R 56865 displace one molecule of Ca 2+. Maximal binding of the cationic drug flunarizine to PS monolayers showed a similar stoichiometry. 45Ca displacement data may thus be useful for predicting monolayer and perhaps also membrane concentrations of the charged species of cationic drugs.

Phase Changes of Phosphatidylserine in Monolayers

AbstractUsing a Langmuir film balance, experiments have been performed in order to study the interaction between Phosphatidylserine and mono and divalent ions. Compression isotherms of phosphatidylserine monolayers spread over subphases free of ions gave phase transitions highly dependent of the initial phospholipid quantity spread. The presence of 100 mM Na+ in the subphases prevented these transitions and expanded the monolayer. These effects could be reverted by 1 mM Ca+2 or 1 mM Mn+2. The penetration of all these ions in monolayers of Phosphatidylserine follows the same pattern, being Na+ the ion that higher pressure increases produces.

The importance of the amino terminus of the mitochondrial precursor protein apocytochrome c for translocation across model membranes

The importance of the various regions of the apocytochrome c molecule and the effect of covalent coupling of the heme group to the protein for the translocation across a model-membrane was studied by using chemically and enzymatically prepared fragments of horse heart apo- and holocytochrome c and model-membranes composed of phosphatidylserine and phosphatidylcholine. Binding experiments showed that fragments of apocytochrome c with the highest net positive charge have the highest affinity for negatively charged large unilamellar phosphatidylserine vesicles. Monolayer experiments demonstrated that the amino-terminal fragments were only able to penetrate into a phosphatidylserine monolayer whereas the carboxyl-terminal fragments, in addition, penetrated into a phosphatidylcholine monolayer although with a lower efficiency. The covalent coupling of the heme group to both a small amino-terminal fragment residue numbers 1-38 and to the entire precursor protein resulted in a marked decrease in the ability to penetrate into a phosphatidylserine monolayer. Translocation experiments with trypsin enclosed in vesicles, showed that only the amino-terminal fragments of the precursor protein and not carboxyl-terminal peptides or the heme-containing fragments of the mature protein were able to cross the bilayer and become digestible by trypsin at the opposite side of the bilayer. Circular dichroism measurements with the various peptides both in an aqueous and lipidic environment were performed to investigate the conformation of apocytochrome c after interaction with model-membranes. Implications of these data for the import of apocytochrome c into mitochondria will be discussed.

Penetration of morphine and naloxone in phosphatidylserine monolayers

The interaction between morphine or naloxone and phosphatidylserine (PS), has been studied using monomolecular layers as membrane model. The penetration kinetics of both molecules measured at different monolayer pressures and for increasing opiate concentrations has been determined. The results show that penetration of morphine presents a maximum at 15 mN · m −1 of initial PS pressure whereas that of naloxone has a maximum at 10 mN · m −1.

Reversibility of prothrombin adsorption on lipid monolayers

Desorption of tritium—labeled human prothrombin adsorbed on lipid monolayers was induced by overall dilution of the aqueous solution by exchange with cold prothrombin and by adding EDTA to remove Ca 2+. The adsorption—desorption isotherms of prothrombin on phosphatidylserine (PS) and on mixed PS—phosphatidylcholine (PC) monolayers at the air/water interface show a very strong hysteresis. Dilution by cold prothrombin decreases the surface radioactivity to less than the dilution factor in the solution. Addition of EDTA displaces prothrombin molecules adsorbed on a pure PS monolayer to the level of adsorption in the absence of Ca 2+; however, the displacement by EDTA from mixed PC—PS monolayers is less effective and the remaining surface concentration is higher than that obtained by adsorption in the absence of Ca 2+. Significant interaction with PC monolayers in the presence of Ca 2+ was observed. Penetration of prothrombin into the monolayer and the involvement of its interaction with the air/monolayer interface in analogy with other interfaces (e.g., mercury/monolayer) are considered. The significance of these results to prothrombin conformation and function on lipid membranes is discussed.

PH-dependent influence of membrane-incorporated flunarizine on Ca-binding to phosphatidylserine monolayer membranes

The pH-dependent 45Ca binding to phosphatidylserine monolayers was investigated. Ca binding increased with increasing pH. Between pH 10 and pH 11 a steep increase of Ca binding could be observed. This increase was interpreted to be due to complex Ca binding opposed to ionic binding at low pH. Flunarizine added to the spreading solution of the monolayer dose dependently displaced up to 100% Ca at pH 5 independently of phospholipid packing. At pH 11 less than 20% of Ca could be displaced by flunarizine. Intermediate results were found at pH 7. Flunarizine displaced less Ca from dense than from loosely packed monolayers at pH 7. The results suggest two binding states of flunarizine: ionic binding at low pH and apolar binding at high pH. The latter is much less effective in displacing Ca from phosphatidylserine monolayers. The Ca displacing properties of charged flunarizine may prevent a deleterious phospholipid reorientation within the membrane induced by the intracellular Ca rise during ischemia.

Interaction of erythrocyte protein 4.1 with phospholipids. A monolayer and liposome study

We have studied the interaction of purified human erythrocyte protein 4.1 with phospholipid membranes by monitoring both the increase in surface pressure of monolayers at the air/water interface and the change in permeability in liposomes to fluorescent molecules, in the presence of protein 4.1. Protein 4.1 penetrated into monolayers of brain phosphatidylserine (PS) and egg phosphatidylcholine (PC), even above surface pressures of 30 mN/m. Protein 4.1 increased the permeability of negatively charged PS, but not PC, liposomes, measured as the increase in fluorescence when encapsulated 1-aminonaphthalene-3,6,8-trisulfonic acid (ANTS) and p- xylenebispyridinium bromide (DPX) or carboxyfluorescein were released into the medium. The interaction of protein 4.1 with PS large unilamellar vesicles (LUV) was increased as the pH and the ionic strength were lowered, and decreased as the Ca 2+ or Mg 2+ concentrations and ionic strength were raised. In order to study the relevance of these measurements to the erythrocyte, we prepared LUV of synthetic lipid mixtures characteristic of both the inner and the outer membrane leaflets. Protein 4.1 increased the permeability of inner, but not outer, leaflet LUV at both pH 6.0 and 7.4. These observations suggest that negatively charged phospholipid domains around the protein 4.1 high-affinity protein-binding site(s) may contribute to the anchoring of protein 4.1 to the cytoplasmic surface of the red cell membrane.

Amantadine-induced lipidosis. A cytological and physicochemical study

The purpose of this study was to test whether or not the antiviral drug amantadine induces the structural features of lipidosis in intact animals (rats) and cultured cells, and to investigate the interactions between amantadine and phospholipids. Chlorphentermine was used as reference compound. When subchronically fed to rats at a daily dosage of approximately 180 mg/kg, amantadine induced ultrastructural symptoms of generalized lipidosis, the degree of which was, however, by far less marked than that previously reported for chlorphentermine. This was paralleled by the findings on cell cultures (rat peritoneal macrophages), where the lipidosis-inducing potency of amantadine was approximately 10-fold lower than that of chlorphentermine. As to drug-phospholipid interactions, amantadine had less marked effects than chlorphentermine upon the phase transition characteristics of phosphatidylcholine and phosphatidic acid; furthermore, amantadine was approximately 10-fold less potent than chlorphentermine in displacing Ca from phosphatidylserine monolayers. The present study has revealed a parallel between the comparatively low lipidosis-inducing efficacy inherent to amantadine and the comparatively low tendency to interact with phospholipids. It is suggested that the cage-like structure of the amantadine molecule hinders an effective intercalation of the drug into phospholipid aggregates, and that this is an essential factor responsible for the low inherent efficacy of amantadine with respect to lipidosis induction.