Interfacial ionization and partitioning of membrane-bound local anesthetics

Prediction of negative food effect induced by bile micelle binding on oral absorption of hydrophilic cationic drugs

The purpose of the present study was to quantitatively predict the negative food effect induced by bile micelle binding on the oral absorption of hydrophilic cationic drugs. The intrinsic membrane permeability and bile micelle unbound fraction of 12 model drugs (7 tertiary amines, 3 quaternary ammoniums, and 2 neutral drugs) were calculated from the experimental Caco-2 permeability data (P_app) under fasted and fed conditions. From these input data, the fraction of a dose absorbed (Fa) was predicted using the gastrointestinal unified theoretical framework, a mechanism-based oral absorption model. The predicted Fa ratio (fed/fasted) was then compared with the in vivo fed/fasted area under the plasma concentration-time curve ratio (AUCr). The AUCr values of tertiary amines and neutral drugs were appropriately predicted (absolute average fold error (AAFE) = 1.19), whereas those of quaternary ammoniums were markedly underestimated (AAFE = 4.70). The P_app ratio (fed/fasted) predicted AUCr less quantitatively (AAFE = 1.30 for tertiary amines and neutral drugs). The results of the present study would lead to a better understanding of negative food effect on oral drug absorption.

Dynamic Protonation Dramatically Affects the Membrane Permeability of Drug-like Molecules

Permeability (P_m) across biological membranes is of fundamental importance and a key factor in drug absorption, distribution, and development. Although the majority of drugs will be charged at some point during oral delivery, our understanding of membrane permeation by charged species is limited. The canonical model assumes that only neutral molecules partition into and passively permeate across membranes, but there is mounting evidence that these processes are also facile for certain charged species. However, it is unknown whether such ionizable permeants dynamically neutralize at the membrane surface or permeate in their charged form. To probe protonation-coupled permeation in atomic detail, we herein apply continuous constant-pH molecular dynamics along with free energy sampling to study the permeation of a weak base propranolol (PPL), and evaluate the impact of including dynamic protonation on P_m. The simulations reveal that PPL dynamically neutralizes at the lipid-tail interface, which dramatically influences the permeation free energy landscape and explains why the conventional model overestimates the assigned intrinsic permeability. We demonstrate how fixed-charge-state simulations can account for this effect, and propose a revised model that better describes pH-coupled partitioning and permeation. Our results demonstrate how dynamic changes in protonation state may play a critical role in the permeation of ionizable molecules, including pharmaceuticals and drug-like molecules, thus requiring a revision of the standard picture.

Pulsatile release from pH triggered imidazoline switchable surfactant liposomes

The incorporation of an imidazoline (IDZ) based switchable surfactant into the lipid membrane of a liposome produces a system that can be triggered to release its solute upon pH change.

Lipid Librations at the Interface with the Na,K-ATPase

Transitions between conformational substates of membrane proteins can be driven by torsional librations in the protein that may be coupled to librational fluctuations of the lipid chains. Here, librational motion of spin-labeled lipid chains in membranous Na,K-ATPase is investigated by spin-echo electron paramagnetic resonance. Lipids at the protein interface are targeted by using negatively charged spin-labeled fatty acids that display selectivity of interaction with the Na,K-ATPase. Echo-detected electron paramagnetic resonance spectra from native membranes are corrected for the contribution from the bilayer regions of the membrane by using spectra from dispersions of the extracted membrane lipids. Lipid librations at the protein interface have a flat profile with chain position, whereas librational fluctuations of the bilayer lipids increase pronouncedly from C-9 onward, then flatten off toward the terminal methyl end of the chains. This difference is accounted for by increased torsional amplitude at the chain ends in bilayers, while the amplitude remains restricted throughout the chain at the protein interface with a limited lengthening in correlation time. The temperature dependence of chain librations at the protein interface strongly resembles that of the spin-labeled protein side chains, suggesting solvent-mediated transitions in the protein are driven by fluctuations in the lipid environment.

Water penetration profile at the protein-lipid interface in Na,K-ATPase membranes

The affinity of ionized fatty acids for the Na,K-ATPase is used to determine the transmembrane profile of water penetration at the protein-lipid interface. The standardized intensity of the electron spin echo envelope modulation (ESEEM) from (2)H-hyperfine interaction with D2O is determined for stearic acid, n-SASL, spin-labeled systematically at the C-n atoms throughout the chain. In both native Na,K-ATPase membranes from shark salt gland and bilayers of the extracted membrane lipids, the D2O-ESEEM intensities of fully charged n-SASL decrease progressively with position down the fatty acid chain toward the terminal methyl group. Whereas the D2O intensities decrease sharply at the n = 9 position in the lipid bilayers, a much broader transition region in the range n = 6 to 10 is found with Na,K-ATPase membranes. Correction for the bilayer population in the membranes yields the intrinsic D2O-intensity profile at the protein-lipid interface. For positions at either end of the chains, the D2O concentrations at the protein interface are greater than in the lipid bilayer, and the positional profile is much broader. This reveals the higher polarity, and consequently higher intramembrane water concentration, at the protein-lipid interface. In particular, there is a significant water concentration adjacent to the protein at the membrane midplane, unlike the situation in the bilayer regions of this cholesterol-rich membrane. Experiments with protonated fatty acid and phosphatidylcholine spin labels, both of which have a considerably lower affinity for the Na,K-ATPase, confirm these results.

Multiple binding sites for fatty acids on the potassium channel KcsA

Interactions of fatty acids with the potassium channel KcsA were studied using Trp fluorescence quenching and electron paramagnetic resonance (EPR) techniques. The brominated analogue of oleic acid was shown to bind to annular sites on KcsA and to the nonannular sites at each protein-protein interface in the homotetrameric structure with binding constants relative to dioleoylphosphatidylcholine of 0.67 ± 0.04 and 0.87 ± 0.08, respectively. Mutation of the two Arg residues close to the nonannular binding sites had no effect on fatty acid binding. EPR studies with a spin-labeled analogue of stearic acid detected a high-affinity binding site for the fatty acid with strong immobilization. Fluorescence quenching studies with the spin-labeled analogue showed that the binding site detected in the EPR experiments could not be one of the annular or nonannular binding sites. Instead, it is proposed that the EPR studies detect binding to the central hydrophobic cavity of the channel, with a binding constant in the range of ~0.1-1 μM.

Electron spin resonance in membrane research: protein-lipid interactions from challenging beginnings to state of the art

Conventional electron paramagnetic resonance (EPR) spectra of lipids that are spin-labelled close to the terminal methyl end of the acyl chains are able to resolve the lipids directly contacting the protein from those in the fluid bilayer regions of the membrane. This allows determination of both the stoichiometry of lipid-protein interaction (i.e., number of lipid sites at the protein perimeter) and the selectivity of the protein for different lipid species (i.e., association constants relative to the background lipid). Spin-label EPR data are summarised for 20 or more different transmembrane peptides and proteins, and 7 distinct species of lipids. Lineshape simulations of the two-component conventional spin-label EPR spectra allow estimation of the rate at which protein-associated lipids exchange with those in the bulk fluid regions of the membrane. For lipids that do not display a selectivity for the protein, the intrinsic off-rates for exchange are in the region of 10 MHz: less than 10x slower than the rates of diffusive exchange in fluid lipid membranes. Lipids with an affinity for the protein, relative to the background lipid, have off-rates for leaving the protein that are correspondingly slower. Non-linear EPR, which depends on saturation of the spectrum at high radiation intensities, is optimally sensitive to dynamics on the timescale of spin-lattice relaxation, i.e., the microsecond regime. Both progressive saturation and saturation transfer EPR experiments provide definitive evidence that lipids at the protein interface are exchanging on this timescale. The sensitivity of non-linear EPR to low frequencies of spin exchange also allows the location of spin-labelled membrane protein residues relative to those of spin-labelled lipids, in double-labelling experiments.

Activation of membrane cholesterol by 63 amphipaths

A few membrane-intercalating amphipaths have been observed to stimulate the interaction of cholesterol with cholesterol oxidase, saponin and cyclodextrin, presumably by displacing cholesterol laterally from its phospholipid complexes. We now report that this effect, referred to as cholesterol activation, occurs with dozens of other amphipaths, including alkanols, saturated and cis- and trans-unsaturated fatty acids, fatty acid methyl esters, sphingosine derivatives, terpenes, alkyl ethers, ketones, aromatics and cyclic alkyl derivatives. The apparent potency of the agents tested ranged from 3 microM to 7 mM and generally paralleled their octanol/water partition coefficients, except that relative potency declined for compounds with >10 carbons. Some small amphipaths activated cholesterol at a membrane concentration of approximately 3 mol per 100 mol of bilayer lipids, about equimolar with the cholesterol they displaced. Lysophosphatidylserine countered the effects of all these agents, consistent with its ability to reduce the pool of active membrane cholesterol. Various amphipaths stabilized red cells against the hemolysis elicited by cholesterol depletion, presumably by substituting for the extracted sterol. The number and location of cis and trans fatty acid unsaturations and the absolute stereochemistry of enantiomer pairs had only small effects on amphipath potency. Nevertheless, potency varied approximately 7-fold within a group of diverse agents with similar partition coefficients. We infer that a wide variety of amphipaths can displace membrane cholesterol by competing stoichiometrically but with only limited specificity for weak association with phospholipids. Any number of other drugs and experimental agents might do the same.

Protein-lipid interactions with Fusobacterium nucleatum major outer membrane protein FomA: spin-label EPR and polarized infrared spectroscopy

FomA, the major outer membrane protein of Fusobacterium nucleatum, was expressed and purified in Escherichia coli and reconstituted from detergent in bilayer membranes of phosphatidylcholines with chain lengths from C(12:0) to C(17:0). The conformation and orientation of membrane-incorporated FomA were determined from polarized, attenuated total reflection, infrared (IR) spectroscopy, and lipid-protein interactions with FomA were characterized by using electron paramagnetic resonance (EPR) spectroscopy of spin-labeled lipids. Approximately 190 residues of membranous FomA are estimated to be in a beta-sheet configuration from IR band fitting, which is consistent with a 14-strand transmembrane beta-barrel structure. IR dichroism of FomA indicates that the beta-strands are tilted by approximately 45 degrees relative to the sheet/barrel axis and that the order parameter of the latter displays a discontinuity corresponding to hydrophobic matching with fluid C(13:0) lipid chains. The stoichiometry ( N b = 23 lipids/monomer) of lipid-protein interaction from EPR demonstrates that FomA is not trimeric in membranes of diC(14:0) phosphatidylcholine and is consistent with a monomeric beta-barrel of 14-16 strands. The pronounced selectivity of interaction found with anionic spin-labeled lipids places basic residues of the protein in the vicinity of the polar-apolar membrane interfaces, consistent with current topology models. Comparison with similar data from the 8- to 22-stranded E. coli outer membrane proteins, OmpA, OmpG, and FhuA, supports the above conclusions.

Evaluation of the interaction of propranolol with 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) liposomes: the Langmuir model

The interaction of the amine containing beta-receptor blocking agent propranolol (Ppn) with dimyristoylphosphatidylcholine (DMPC) vesicles was studied. Using a centrifugation assay, the protonated as well as unprotonated amount of the drug sorbed was verified, whereas the binding of the protonated Ppn was deduced from the surface charge density of the vesicles as calculated from electrophoretic mobility measurements. Assuming a 1:1 binding, a Langmuir model with only two parameters was found to be sufficient to fit all experimental data. Sensitivity analysis revealed that the estimated values of these parameters were reliable and independent from each other. These parameters were truly intrinsic, as electrostatic interactions were accounted for in the model. It was found that the pKa of Ppn shifted from 9.24, when dissolved in water, downward by 1.34 units upon sorption, indicating that the intrinsic partition coefficient of the unprotonated Ppn was about 22 times higher than that of the protonated analog. In addition, a significant increase in the affinity of both Ppn analogs with increasing salt concentration was found. Theoretical analysis revealed that the Langmuir sorption model may be considered as a partitioning model with decreasing partition coefficient as the sorbed amount increases. Thus, the Langmuir model provides a better fit than a simple partition model at conditions that induce a substantial amount of propranolol sorbed, such as high pH and high propranolol concentrations.

Solubility and dissolution profile assessment in drug discovery

The purposes of the review are to: a) Provide a comprehensible introduction of the-state-of-the-art sciences of solubility and dissolution, b) introduce typical technologies to assess solubility and dissolution, and c) propose the best practice strategy. The theories of solubility and dissolution required in drug discovery were reviewed especially from the view point of oral absorption. The physiological conditions in the gastrointestinal fluid in humans and animals were then briefly summarized. Technologies to assess solubility and dissolution in drug discovery were then introduced. Recently, these technologies have been improved by the laboratory automation and computational technologies. Finally, the strategies to apply these technologies for a drug discovery project were discussed.

Lipid-protein interactions with the Na,K-ATPase

Studies of lipid interactions with membranous Na,K-ATPase by using electron spin resonance spectroscopy in conjunction with spin-labelled lipids are reviewed. The lipid stoichiometry, selectivity and exchange dynamics at the lipid-protein interface can be determined, in addition to information on the configuration and rotational dynamics of the protein-associated lipid chains. These parameters, particularly the stoichiometry and selectivity, are related directly to the intramembranous structure of the Na,K-ATPase, and can be used to check the integrity of extensively trypsinised preparations.

Selective Entrapment of the Cationic Form of Norfloxacin within Anionic Sodium Dodecyl Sulfate Micelles at Physiological pH and its Effect on the Drug Photodecomposition†

The binding of the photosensitizing fluoroquinolone (FQ) antibiotic norfloxacin (NX) to sodium dodecyl sulfate (SDS) micelles and the photoreactivity of the NX/SDS complex under physiological pH conditions are investigated by means of absorption and emission spectroscopy, steady-state and laser flash photolysis. It is shown that the photolabile zwitterionic form of NX, which is dominant at physiological pH, is not the most abundant species in the presence of SDS micelles. This medium exhibits a high preference for the cationic form of the drug, which is selectively and successfully entrapped within the micellar cage (K(ass) = 6 x 10(4) M(-1) +/- 3000), becoming the largely dominant species at neutral pH. The effect of this trapping is drastically reflected on both efficiency and nature of the drug photodecomposition. It is observed that the photostability of NX incorporated in the micellar pseudophase increases of more than one order of magnitude if compared to that of the "free" drug. Furthermore, the radical photodecomposition mechanism occurring in phosphate buffered solution is suppressed by the micellar medium and the low photodegradation observed seems to take place preferentially through an ionic pathway. Hopefully, the results presented herein may contribute to a better understanding of the bio-distribution of NX in biological systems and provide helpful and stimulating information in order to get the control of FQ photoreactivity under physiological pH conditions.

pH effects on drug interactions with lipid bilayers by liposome electrokinetic chromatography

Liposome electrokinetic chromatography (LEKC) provides convenient and rapid methods for studying drug interactions with lipid bilayers using liposomes as a pseudostationary phase. LEKC was used to determine the effects of pH on the partitioning of basic drugs into liposomes composed of zwitterionic phosphatidylcholine (PC), anionic phosphatidylglycerol (PG), and cholesterol, which mimic the composition of natural cell membranes. An increase in pH results in a smaller degree of ionization of the basic drugs and consequently leads to a lower degree of interaction with the negatively charged membranes. From the LEKC retention data, the fractions of drugs distributed in the bulk aqueous and the liposome phase were determined at various pH values. Finally, lipid mediated shifts in the ionization constants of drugs were examined.

The protein–lipid interface: perspectives from magnetic resonance and crystal structures

Lipid-protein interactions in membranes are dynamic, and consequently are well studied by magnetic resonance spectroscopy. More recently, lipids associated with integral membrane proteins have been resolved in crystals by X-ray diffraction, mostly at cryogenic temperatures. The conformation and chain ordering of lipids in crystals of integral proteins are reviewed here and are compared and contrasted with results from magnetic resonance and with the crystal structures of phospholipid bilayers. Various aspects of spin-label magnetic resonance studies on lipid interactions with single integral proteins are also reviewed: specificity for phosphatidylcholine, competition with local anaesthetics, oligomer formation of single transmembrane helices, and protein-linked lipid chains. Finally, the interactions between integral proteins and peripheral or lipid-linked proteins, as reflected by the lipid-protein interactions in double reconstitutions, are considered.

On the mechanism of drug-induced acceleration of phospholipid translocation in the human erythrocyte membrane

Small amphiphilic compounds (M(r)<200 Da) such as anaesthetics and hexane derivatives with different polar groups produced a concentration-dependent acceleration of the slow passive transbilayer movement of NBD-labelled phosphatidylcholine in the human erythrocyte membrane. Above a threshold concentration characteristic for each compound, the flip rate gradually increased at increasing concentrations in the medium. For compound concentrations required to produce a defined flip acceleration, corresponding membrane concentrations were estimated using reported octanol/water partition coefficients. The effective threshold membrane concentrations (50-150 mmol l(-1)) varied in the order: hexylamine>isoflurane=hexanoic acid>hexanol=chloroform>hexanethiol=1,1,2,2-tetrachloroethane>chlorohexane. Apolar hexane, which mainly distributes in the apolar membrane core, was much less effective and supersaturating concentrations were required to enhance flip. Localization of the drug at the lipid-water interface seems to be required for flip acceleration. Such a localization may increase the lateral pressure in this region and the bilayer curvature stress with concomitant decrease of order and rigidity at the interface. This unspecific bilayer perturbation is proposed to enhance the probability of formation of hydrophobic defects in the bilayer, facilitating penetration of the polar head group of the phospholipid into the apolar membrane core.

Drug permeation in biomembranes

In the past decades, it has become increasingly apparent that in addition to therapeutic effect, drugs need to exhibit favourable absorption, distribution, metabolism and excretion (ADME) characteristics to produce a desirable response in vivo. As the recent progress in drug discovery technology enables rapid synthesis of vast numbers of potential drug candidates, robust methods are required for the effective screening of compounds synthesized within such programs, so that compounds with poor pharmacokinetic properties can be rejected at an early stage of drug development. Furthermore, a viable in silico method would save resources by enabling virtual screening of drug candidates already prior to synthesis. This review gives a general overview of the approaches aimed at predicting biological permeation, one of the cornerstones behind the ADME behaviour of drugs. The most important experimental and computational models are reviewed. Physicochemical factors underlying the permeation process are discussed.

Liposome/water lipophilicity: methods, information content, and pharmaceutical applications

This review discusses liposome/water lipophilicity in terms of the structure of liposomes, experimental methods, and information content. In a first part, the structural properties of the hydrophobic core and polar surface of liposomes are examined in the light of potential interactions with solute molecules. Particular emphasis is placed on the physicochemical properties of polar headgroups of lipids in liposomes. A second part is dedicated to three useful methods to study liposome/water partitioning, namely potentiometry, equilibrium dialysis, and (1)H-NMR relaxation rates. In each case, the principle and limitations of the method are discussed. The next part presents the structural information encoded in liposome/water lipophilicity, in other words the solutes' structural and physicochemical properties that determine their behavior and hence their partitioning in such systems. This presentation is based on a comparison between isotropic (i.e., solvent/water) and anisotropic (e.g., liposome/water) systems. An important factor to be considered is whether the anisotropic lipid phase is ionized or not. Three examples taken from the authors' laboratories are discussed to illustrate the factors or combinations thereof that govern liposome/water lipophilicity, namely (a) hydrophobic interactions alone, (b) hydrophobic and polar interactions, and (c) conformational effects plus hydrophobic and ionic interactions. The next part presents two studies taken from the field of QSAR to exemplify the use of liposome/water lipophilicity in structure-disposition and structure-activity relationships. In the conclusion, we summarize the interests and limitations of this technology and point to promising developments.

Interaction of Epirubicin HCl with Surfactants: Effect of NaCl and Glucose

The interaction of an antitumoural drug, Epirubicin HCl, with anionic (sodiumdodecylsulfate; SDS), cationic (cetyltrimethylammonium bromide; CTAB), and nonionic (t-octylphenoxypolyethoxyethanol; TX-100, polyoxyethylenesorbitanmonolaurate; Tween 20) surfactants has been studied by absorption spectra as a function of surfactant concentration ranging from the premicellar to postmicellar region. At the concentrations below the critical micelle concentration (CMC), the equilibrium complex formation constant between Epirubicin cations and SDS anions has been determined by Job's method. Above the CMC, binding constant (K(b)) of Epirubicin to various types of micelles has been calculated by means of the Benesi-Hildebrand Equation. The nonionic surfactant micelles showed stronger interaction than the ionic SDS micelles, and the binding tendency of Epirubicin followed the order: Tween 20 > TX-100 > SDS. Binding of Epirubicin also has been studied in the presence of NaCl and glucose because it is administered to patients intravenously in 0.9% NaCl or 5% glucose solution. The additives have been observed to affect the CMC of the surfactants and the Epirubicin-micelle binding constant appreciably. The presence of NaCl and glucose lowered the CMC of all the surfactants studied. The binding constant of Epirubicin decreased in the presence of NaCl but increased in the presence of glucose. The equilibrium complex formation constant between Epirubicin and SDS decreased in the presence of NaCl compared with purely aqueous media.

Dissociation constants of phenothiazine drugs incorporated in phosphatidylcholine bilayer of small unilamellar vesicles as determined by carbon-13 nuclear magnetic resonance spectrometric titration

The dissociation constants (pKms) of the phenothiazine drugs promazine, chlorpromazine, and triflupromazine, incorporated in the phosphatidylcholine (PC) bilayer of small unilamellar vesicles (SUV), were investigated by a 13C nuclear magnetic resonance (NMR) titration method employing their N-13CH3 (ionizable group) labelled derivatives. Use of the labelled drugs enabled direct observations of the ionization equilibrium of the N-dimethyl group. A second derivative spectrophotometric study proved that 95-98% of the phenothiazine species in the sample solutions (200 microM phenothiazine in the presence of 27 mM PC SUV) were incorporated into the PC bilayer, which simplified the calculation of pKm values by allowing that the phenothiazines in the aqueous phase could be neglected. The pKm values were calculated from the chemical shift dependence of the N-dimethyl 13C NMR signal on the pH value of sample solutions. The pKm values obtained were smaller than those measured in aqueous solutions by about one unit. The existence of cholesterol (30 mol%) in the PC bilayer showed little effect on the pKm values, suggesting that cholesterol in the bilayer does not largely affect the interfacial region where the N-dimethyl group of the incorporated phenothiazines is located. The results offered clear evidence for the pKm decrease and provided their precise values.

Binding of peptides with basic and aromatic residues to bilayer membranes: phenylalanine in the myristoylated alanine-rich C kinase substrate effector domain penetrates into the hydrophobic core of the bilayer

Electrostatic interactions with positively charged regions of membrane-associated proteins such as myristoylated alanine-rich C kinase substrate (MARCKS) may have a role in regulating the level of free phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) in plasma membranes. Both the MARCKS protein and a peptide corresponding to the effector domain (an unstructured region that contains 13 basic residues and 5 phenylalanines), MARCKS-(151-175), laterally sequester the polyvalent lipid PI(4,5)P2 in the plane of a bilayer membrane with high affinity. We used high resolution magic angle spinning NMR to establish the location of MARCKS-(151-175) in membrane bilayers, which is necessary to understand the sequestration mechanism. Measurements of cross-relaxation rates in two-dimensional nuclear Overhauser enhancement spectroscopy NMR experiments show that the five Phe rings of MARCKS-(151-175) penetrate into the acyl chain region of phosphatidylcholine bilayers containing phosphatidylglycerol or PI(4,5)P2. Specifically, we observed strong cross-peaks between the aromatic protons of the Phe rings and the acyl chain protons of the lipids, even for very short (50 ms) mixing times. The position of the Phe rings implies that the adjacent positively charged amino acids in the peptide are close to the level of the negatively charged lipid phosphates. The deep location of the MARCKS peptide in the polar head group region should enhance its electrostatic sequestration of PI(4,5)P2 by an "image charge" mechanism. Moreover, this location has interesting implications for membrane curvature and local surface pressure effects and may be relevant to a wide variety of other proteins with basic-aromatic clusters, such as phospholipase D, GAP43, SCAMP2, and the N-methyl-d-aspartate receptor.

Quantifying molecular partition into model systems of biomembranes: an emphasis on optical spectroscopic methods

Optical spectroscopies have been intensively used to determine partition coefficients by a plethora of methodologies. The present review is intended to give detailed and useful information for the determination of partition coefficients and addresses several relevant aspects, namely: (i) definition and calculation of the partition coefficient between aqueous and lipidic phases; (ii) partition coefficients vs. "binding" formalisms; (iii) advantages of spectroscopic methodologies over separation techniques; (iv) formalisms for various experimental approaches based on UV-Vis absorption or fluorescence parameters (fluorescence intensity, lifetime, anisotropy and quenching); (v) experimental hints, artifacts and model limitations; and (vi) a brief survey of nonoptical techniques.

Lipid-protein interactions in Escherichia coli membranes over-expressing the sugar-H(+) symporter, GalP EPR of spin-labelled lipids

The D-galactose-H(+) symport protein (GalP) of Escherichia coli is a homologue of the human glucose transport protein, GLUT1. After amplified expression of the GalP transporter in E. coli, lipid-protein interactions were studied in gradient-purified inner membranes by using spin-label electron paramagnetic resonance (EPR) spectroscopy. Phosphatidylethanolamine, -glycerol, -choline and -serine, in addition to phosphatidic and stearic acids, were spin-labelled at the 14 C-atom of the sn-2 chain. EPR spectra of these spin labels at probe amounts in GalP membranes consist of two components. One component corresponds to a lipid population whose motion is restricted by direct interaction with the transmembrane sections of the integral protein. The other component corresponds to a lipid population with greater chain mobility, and is similar to the single-component EPR spectrum of the spin-labelled lipids in membranes of E. coli lipid extract. Quantitation of the protein-interacting spin-label component allows determination of the stoichiometry and selectivity of lipid-protein interactions. On average, approximately 20 mol of lipid are motionally restricted per 52 kDa of protein in GalP membranes. At the pH of the transport assay, there is relatively little selectivity between the different phospholipids tested. Only stearic acid displays a stronger preferential interaction with this protein.

Correlation between fluidising effects on phospholipid membranes and mitochondrial respiration of propofol and p-nitrosophenol homologues

Nitrosopropofol (2-6-diisopropyl-4-nitrosophenol) has dramatic consequences for respiration, ATP synthesis and the transmembrane potential of isolated rat liver mitochondria at concentrations at which propofol (2-6-diisopropylphenol) does not cause any apparent effects. These results correlate well with the observation that nitrosopropofol is also a stronger perturbing agent of phospholipid membranes. In this paper we verify the possible biological activity of different phenols and nitrosophenols on mitochondrial respiration. We then discuss their interactions with phospholipid liposomes, studied with differential scanning calorimetry, spin labelling techniques and UV-Vis spectrophotometry, in order to obtain information on drug distribution and the modifications they impose on lipid bilayer. The results of the experiments performed on mitochondria and model membranes prove an interesting correlation between the effects of the molecules on both systems.

Partitioning of uncharged local anesthetic benzocaine into model biomembranes

The partitioning of uncharged local anesthetic benzocaine (BzC) into molecular aggregates formed by cationic surfactant decylammonium chloride (DeAC) and phospholipid dipalmitoylphosphatidylcholine (DPPC) was studied from the surface tension and light transmittance measurements. The quantities concerning the partitioning of BzC, the compositions of BzC in the surface-adsorbed film and micelle and three kinds of differential partition coefficients corresponding to phase transitions of the DPPC bilayer membrane were evaluated from thermodynamic analysis of the experimental data. The surface-adsorbed film and micelle were more abundant in BzC than the aqueous solution and significantly large differential partition coefficients for the DPPC membranes were observed. The results clearly showed that the BzC molecules greatly partitioned into hydrophobic environments produced by surfactant-monolayer and phospholipid-bilayer membranes. The partitioning behavior of BzC was also compared with that of charged local anesthetic procaine hydrochloride (PC.HCl). It was shown that the PC.HCl molecule did not or hardly partition into such hydrophobic environments. The contrasting results of the partitioning between BzC and PC.HCl are attributable to the drastic decrease of hydrophilicity of BzC due to the lacking of ionic polar head group in comparison with PC.HCl.

Continuum solvent model studies of the interactions of an anticonvulsant drug with a lipid bilayer

Valproic acid (VPA) is a short, branched fatty acid with broad-spectrum anticonvulsant activity. It has been suggested that VPA acts directly on the plasma membrane. We calculated the free energy of interaction of VPA with a model lipid bilayer using simulated annealing and the continuum solvent model. Our calculations indicate that VPA is likely to partition into the bilayer both in its neutral and charged forms, as expected from such an amphipathic molecule. The calculations also show that VPA may migrate (flip-flop) across the membrane; according to our (theoretical) study, the most likely flip-flop path at neutral pH involves protonation of VPA pending its insertion into the lipid bilayer and deprotonation upon departure from the other side of the bilayer. Recently, the flip-flop of long fatty acids across lipid bilayers was studied using fluorescence and NMR spectroscopies. However, the measured value of the flip-flop rate appears to depend on the method used in these studies. Our calculated value of the flip-flop rate constant, 20/s, agrees with some of these studies. The limitations of the model and the implications of the study for VPA and other fatty acids are discussed.

Spin-label electron paramagnetic resonance studies on the interaction of avidin with dimyristoyl-phosphatidylglycerol membranes

The interaction of avidin--a basic protein from hen egg-white--with dimyristoyl-phosphatidylglycerol membranes was investigated by spin-label electron paramagnetic resonance spectroscopy. Phosphatidylcholines, bearing the nitroxide spin label at different positions along the sn-2 acyl chain of the lipid were used to investigate the effect of protein binding on the lipid chain-melting phase transition and acyl chain dynamics. Binding of the protein at saturating levels results in abolition of the chain-melting phase transition of the lipid and accompanying perturbation of the lipid acyl chain mobility. In the fluid phase region, the outer hyperfine splitting increases for all phosphatidylcholine spin-label positional isomers, indicating that the chain mobility is decreased by binding avidin. However, there was no evidence for direct interaction of the protein with the lipid acyl chains, clearly indicating that the protein does not penetrate the hydrophobic interior of the membrane. Selectivity experiments with different spin-labelled lipid probes indicate that avidin exhibits a preference for negatively charged lipid species, although all spin-labelled lipid species indirectly sense the protein binding. The interaction with negatively charged lipids is relevant to the use of avidin in applications such as the ultrastructural localization of biotinylated lipids in histochemical studies.

Denervation alters protein-lipid interactions in membrane fractions from electrocytes of Electrophorus electricus (L.)

Protein-lipid interactions are studied in normal and denervated electrocytes from Electrophorus electricus (L.). Structural modifications of the lipid micro-environment encircling integral membrane proteins in membrane fractions presenting Na(+),K(+)-ATPase activity are investigated using ESR spectroscopy of stearic acid spin labeled at the 14th carbon (14-SASL). The microsomal fraction derived from the innervated electric organ exhibits, on a discontinuous sucrose gradient, a bimodal distribution of the Na(+),K(+)-ATPase activity, bands a and b. Band b is almost absent in microsomes from the denervated organ, and band a', with the same density as band a has lower Na(+),K(+)-ATPase activity. Band a' presents a larger ratio of protein-interacting lipids than band a. Analysis of the lipid stoichiometry at the protein interface indicates that denervation causes at least a twofold average decrease on protein oligomerization. Physical inactivity and denervation have similar effects on protein-lipid interactions. Denervation also influences the selectivity of proteins for fatty acids. Experiments in decreasing pH conditions performed to verify the influence of stearic acid negative charge on protein interaction revealed that denervation produces loss of charge selectivity. The observed modifications on molecular interactions induced by denervation may have importance to explain modulation of enzyme activity.

Changes in the flow properties of phospholipid dispersions induced by procaine hydrochloride. Effect of pH and temperature

Since local anaesthetics are known to interact with membrane lipids, we have examined the changes taken place by procaine hydrochloride in lipid matrices as a function of pH. Rheological methods might give useful information on the association of this anaesthetic with soybean lecithin. The procaine interacted with negatively charged phospholipid polar head groups at pH 4. This system exhibits a loosening in the tight arrangement of phospholipid molecules caused by the addition of procaine as a function of this anaesthetic's concentration. The flow enthalpy values as a function of procaine-lipid ratio shows biphasic behaviour and suggests a phase transition when the anaesthetic concentration goes from 10 to 14 mM and temperatures dip below 10 degrees C.

Acceleration of phospholipid flip-flop in the erythrocyte membrane by detergents differing in polar head group and alkyl chain length

The detergents, alkyltrimethylammonium bromide, N-alkyl-N, N-dimethyl-3-ammonio-1-propanesulfonate (zwittergent), alkane sulfonate, alkylsulfate, alkyl-beta-D-glucopyranoside, alkyl-beta-D-maltoside, dodecanoyl-N-methylglucamide, polyethylene glycol monoalkyl ether and Triton X-100, all produce a concentration-dependent acceleration of the slow passive transbilayer movement of NBD-labeled phosphatidylcholine in the human erythrocyte membrane. Above a threshold concentration, which was well below the CMC and characteristic for each detergent, the flip rate increases exponentially upon an increase of the detergent concentration in the medium. The detergent-induced flip correlates with reported membrane-expanding effects of the detergents at antihemolytic concentrations. From the dependence of the detergent concentration required for a defined flip acceleration on the estimated membrane volume, membrane/water partition coefficients for the detergents could be determined and effective detergent concentrations in the membrane calculated. The effective membrane concentrations are similar for most types of detergents but are 10-fold lower for octaethylene glycol monoalkyl ether and Triton X-100. The effectiveness of a given type of detergent is rather independent of its alkyl chain length. Since detergents do not reduce the high temperature dependence of the flip process the detergent-induced flip is proposed to be due to an enhanced probability of formation of transient hydrophobic structural defects in the membrane barrier which may result from perturbation of the interfacial region of the bilayer by inserted detergent molecules.

Surface active drugs: self-association and interaction with membranes and surfactants. Physicochemical and biological aspects

Many pharmacologically active compounds are of amphiphilic (or hydrophobic) nature. As a result, they tend to self-associate and to interact with biological membranes. This review focuses on the self-aggregation properties of drugs, as well as on their interaction with membranes. It is seen that drug-membrane interactions are analogous to the interactions between membranes and classical detergents. Phenomena such as shape changes, vesiculation, membrane disruption, and solubilization have been observed. At the molecular level, these events seem to be modulated by lipid flip-flop and formation of non-bilayer phases. The modulation of physicochemical properties of drugs by self-association and membrane binding is discussed. Pathological consequences of drug-membrane interaction are described. The mechanisms of drug solubilization by surfactants are reviewed from the physicochemical point of view and in relation to drug carrying and absorption by the organism.

Interaction of linear mono- and diamines with dimyristoylphosphatidylcholine and dimyristoylphosphatidylglycerol multilamellar liposomes

The effect of linear monoamines on dimyristoylphosphatidylglycerol and dimyristoylphosphatidylcholine multilamellar liposomes was studied as a function of their length and compared with the behavior of linear carboxylic acids. The role of the hydrophobic interactions was demonstrated and the free energy of the binding for each interacting carbon atom was determined. The thermotropic behavior of the liposomes was characterized by differential scanning calorimetry and it was shown that these molecules affect the temperature and the cooperativity of the gel to fluid state transition of the membrane differently. In particular, it appeared that membrane perturbation was maximum when the chain length of the amphipathic molecules ranged between 7 and 9 carbon atoms, with more pronounced effects in the case of monoamines. Molecules shorter than 3-4 carbon atoms did not produce any observable change in the transition temperature. The study was extended to linear alpha,omega-diamines to investigate the amphipathic character of long diamines and to investigate the role of bridging bonds established with neighboring phospholipids.

Biophysical studies of lipopeptide-membrane interactions

There are a number of naturally occurring motifs for lipidation of peptides and proteins. In cases in which this involves adding a single hydrocarbon chain to the peptide, it is either a fatty acid or an isoprenyl group. Lipopeptides will partition between membrane and aqueous phases. When only one hydrocarbon chain is attached to the peptide, the affinity of the lipopeptide for the membrane is only marginally increased over that of the free peptide. The resulting partitioning is largely determined by the extent of the interaction of the peptide moiety with the membrane. In contrast, lipidation involving two hydrocarbon chains, either as two single chains attached at distinct locations of the peptide or a double-chain lipid anchor, firmly attaches the lipopeptide to the membrane. This can allow the placement of specific binding sites on a membrane surface. Such a strategy can be used, for example, to place specific antibodies on the surface of drug-carrying liposomes for the purpose of targeting drug delivery. In addition, lipopeptides will alter the physical properties of membranes. One of these effects is to increase the bilayer to hexagonal phase transition temperature. Substances having this property may also alter functional properties of membranes. While it is unlikely that these changes in the biophysical properties of the membranes. While it is unlikely that these changes in the biophysical properties of the membrane are responsible for specific functions of lipopeptides, such changes may be used to modulate certain properties of a membrane, such as the rate of viral fusion.

Interaction of Chlorpromazine and Trifluoperazine with Anionic Sodium Dodecyl Sulfate (SDS) Micelles: Electronic Absorption and Fluorescence Studies

The characteristics of binding of two phenothiazine antipsychothic drugs, chlorpromazine (CPZ) and trifluoperazine (TFP), to anionic sodium dodecyl sulfate (SDS) monomers and/or micelles were investigated using electronic absorption and fluorescence spectroscopies. Binding constants K(b) and pK(a) values for the drugs in SDS micelles were estimated using the red shifts of the maximum absorption and changes in absorption upon alkalization or in the presence of surfactant. The pK(a) shift of CPZ due to its interaction with SDS micelles is about 0.7 unit to higher values, as compared to the reported value of pK(a) obtained in buffer around 9.3. For TFP the pK(a) shift is 0.4 unit to higher values compared to that in buffer, reported as 4.0. The electronic absorption spectroscopic data suggest a biphasic interaction as a function of detergent concentration which is quite dependent of the protonation states of the drugs. In the case of TFP a very strong binding takes place when the drug is fully protonated (pH 2.0) and a distinct binding takes place at stoichiometric (low) surfactant concentrations (interaction via surfactant monomers) and at higher concentrations (in the presence of micelles). Static fluorescence probe analysis using pyrene was used to study the nature of the phenothiazine-surfactant premicellar and self-aggregates. The I(3)/I(1) and I(475)/I(1) ratios associated to pyrene fluorescence vibronic bands and excimer intensities ratios, respectively, were monitored for several ratios [SDS]/[drug] and significant changes, dependent of the drug presence and its protonation state, have been observed revealing a hydrophobic microenvironment provided by TFP-SDS aggregates in comparison with CPZ both at pH 7.0 and 4.0. Static anisotropy was also used to monitor the changes of the self-aggregates and micellar packing in the presence of the phenothiazine drugs. In aqueous solutions the anisotropy of the fluorescent probe dipyridamole (DIP) is quite low, being around 0.005 at pH 7.0 and 0.025 at pH 4.0, and the addition of detergent leads to an increase in the values of anisotropy to 0.030 at pH 7.0 and 0.070 at pH 4.0. In the presence of the phenothiazine drugs, and in the premicellar detergent concentration range, the anisotropy of DIP increases to 0.134 and 0.111 (dependent on drug concentration) for CPZ and TFP, respectively, at pH 4.0. These results suggest that the presence of both phenotiazine drugs makes the premicellar aggregates more rigid by decreasing the probe mobility, and are consistent with a more polar localization of the CPZ in the micelles as compared with TFP. At pH 7.0 the anisotropy changes are smaller, suggesting a slight decrease in CMC induced by the phenothiazines. Copyright 2000 Academic Press.