Properties and the locations of a set of fluorescent probes sensitive to the fluidity gradient of the lipid bilayer

Properties of phosphatidylcholine bilayers as revealed by mixed-acyl phospholipid fluorescent probes containing n-(9-anthroyloxy) fatty acids

A series of five mixed-acyl phosphatidylcholine (PC) fluorescent probes having the structure 1-caproyl-2-(n-(9-anthroyloxy)-acyl)-sn-glycero-3-phosphocholine, where the sn-2 anthroyloxy-labeled acyl chain is stearic acid (n = 2,6,9,12) or palmitic acid (n = 16), have been prepared. These probes have been used to study the thermal behavior and transbilayer organization of 1,2-distearoyl-PC (C(18)C(18)PC), 1-stearoyl-2-caproyl-PC (C(18)C(10)PC), and 1-caproyl-2-stearoyl-PC (C(10)C(18)PC) multilamellar dispersons. These probes reported the noninterdigitated gel to liquid-crystalline phase transition of C(18)C(18)PC at 55.1 degrees C and the mixed-interdigitated gel to liquid-crystalline phase transitions of C(18)C(10)PC and C(10)C(18)PC at 19.1 and 10.1 degrees C, respectively. The results suggest that, upon cooling, the C(18)C(10)PC liquid-crystalline phase transforms to the mixed-interdigitated gel phase by way of a partially interdigitated gel-phase intermediate. Isothermal plots of anisotropy versus the position of the anthroyl moiety on the sn-2 acyl chain of the PC probes were used to construct transbilayer anisotropy profiles of the gel phases of the three bilayer systems. These anisotropy profiles can serve as 'interdigitation signatures' that clearly distinguish the noninterdigitated from the mixed-interdigitated gel-phase bilayer organization. In the liquid-crystalline phase, the anisotropy profiles suggest that the dynamic motions of the disordered acyl chains of the mixed-acyl PCs are influenced by the interpenetration of the chains from the opposing leaflets of the bilayer.

Lipid chain order and dynamics at different bilayer depths in liposomes of several phosphatidylcholines studied by differential polarized phase fluorescence

The influence of acyl chain length, double bond number and position on the structural and dynamic properties of phosphatidylcholine bilayers in the liquid crystalline state was studied. The range and rate of the rotation of 1,6-diphenyl-1,3,5-hexatriene and a set of n-(9-anthroyloxy)stearates(n = 2, 7 and 12), which are located at different depths in the lipid bilayers, were measured using differential polarized phase fluorometry in multilamellar liposomes of several unsaturated synthetic phosphatidylcholines. For the anthroyloxy-stearate probes, two rotational modes, 'in' and 'out' of the plane of the anthroyl aromatic ring, were partially resolved by measuring at different excitation wavelengths. The data obtained here indicate that in lecithins containing the same acyl chain in sn-1 and sn-2, the introduction of double bonds towards the deep interior of the lipid bilayer decreases the order in the external region of the bilayer and the viscous resistance to the rotation of all the probes tested. On the contrary, double bonds located towards the external region of the bilayer increase order and viscous resistance to the rotation of probes located in the external region. Increasing acyl chain length has different effects depending on the probe and on the rotational mode considered. Measurements in mixed chain lecithins indicate that the influence of increasing unsaturation in sn-2 has different consequences depending on the acyl group that is present in sn-1. Similarly, increasing acyl chain length in sn-1 influences differently the bilayer properties depending on the fatty acid that is present in sn-2.

Fluorescence spectroscopy of 9-anthroyloxy fatty acids in solvents

A series of anthroyloxy fatty acid (AF) fluorescent probes, with the anthroyloxy group covalently linked at various positions along the alkyl chain, were studied in solvents exhibiting a wide range of polarity and hydrogen-bond donor (Hd) and acceptor (Ha) ability. These probes were sensitive to the solvent polarity as reflected by the Stokes' shift observed in steady state fluorescence. As determined by multi-linear regression analysis of the observed Stokes' shift and solvent parameters, such as orientation polarizability (delta f), Hd and Ha of the solvents, all the probes were sensitive to the Hd of solvents but were not affected by the Ha of solvents except the 2-AF. Due to the proximity of the polar headgroup to the fluorophore, it appears that some intramolecular hydrogen-bonding is present in 2-AF, an interaction that is sensitive to the pH of the solvent, but is less sensitive to the Hd and Ha of the solvents. Fluorescence lifetimes measured by the multi-frequency phase-modulation technique in mixtures of hexane and ethanol reflect a modified Stern-Volmer behavior suggesting the second solvent, ethanol, specifically interacts with the probe, in part through collisional quenching. Also, the lifetime data were sensitive to very low concentrations of the second solvent (0-0.1%, by vol.). The results from this study provide insight into the intrinsic differences between the different AF positions that must be taken into consideration while investigating the dynamics of lipid bilayer systems. Moreover, this study illustrates the utility and resolving power of lifetime based measurements needed for the interpretation of heterogeneous biophysical environments.

Lipid effects on aggregation of pulmonary surfactant protein SP-C studied by fluorescence energy transfer

The self-association of pulmonary surfactant protein SP-C in lipid vesicles was studied using fluorescence energy transfer. Bovine SP-C was labeled with two fluorescent probes, succinimidyl 6-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]hexanoate and eosin isothiocyanate, on the amino terminus of the protein, producing NBD-SP-C and EITC-SP-C, respectively. The N-terminus of SP-C was relatively immobile between 20 and 37 degrees C, as demonstrated by high fluorescence anisotropy of NBD-SP-C and EITC-SP-C. The mobility increased at the transition of the lipid to the fluid phase. Using fluorescence energy transfer, with NBD-SP-C as the donor and EITC-SP-C as the acceptor, a high degree of SP-C/SP-C association was found below 25 degrees C, decreasing to very little self-association above 42 degrees C in 7:1 1,2-dipalmitoylphosphatidylcholine-1,2-dipalmitoylphosphatidylglycerol (DPPC-DPPG) vesicles. The fraction of SP-C aggregated below 37 degrees C in 7:1 DPPC-DPPG was estimated from the observed energy transfer to be more than 70% of total SP-C. In various lipid mixtures, self-association of SP-C was dependent on the presence of at least some gel-phase lipids. In a lipid mixture resembling pulmonary surfactant, gradually increasing self-association was observed below 38 degrees C. The relation of the present data to the state of aggregation of SP-C in pulmonary surfactant is discussed.

Erythrocyte membrane structure in bipolar affective disorder: a non-replication

Fifteen bipolar patients were compared with sixteen controls in an attempt to replicate the findings of Pettegrew et al. in 1982 of decreased fluidity in the hydrocarbon core of the erythrocyte membrane. No significant differences were seen between groups. The present control group shows very similar membrane characteristics to the original control series; however, the bipolar patient group is not similar. Possible explanations are discussed.

Fluorescence anisotropy studies on the interaction of the short chain n-alkanols with stratum corneum lipid liposomes (SCLL) and distearoylphosphatidylcholine (DSPC)/distearoylphosphatidic acid (DSPA) liposomes

Previously, the action of the short chain n-alkanols (from C1 to C5) and isopropanol as possible enhancers on the transport of lipophilic and polar/ionic permeants across hairless mouse skin was investigated. In the present study, the steady-state fluorescence anisotropy was measured as a means of estimating the changes in fluidity caused by the n-alkanols at different depths in the stratum corneum lipid liposomes (SCLL). Some selected experiments with the distearoylphosphatidylcholine (DSPC)/distearoylphosphatidic acid (DSPA) liposomes were performed for relative comparisons. The effects of the n-alkanols on polarity sensitive parameters such as fluorescence lifetimes, fluorescence quantum yield ratios, and emission maxima were studied in the SCLL. The polarity of the bilayer decreased as the fluorescent probe was placed closer to the bilayer center and the n-alkanols did not alter this gradient. Assessment of the depth-dependent effects of the n-alkanols using SCLL showed that most of the significant changes in fluidity induced by the n-alkanols were observed at intermediate depths (C2-C9) and there was little or no increase in fluidity in the deep hydrophobic region close to the bilayer center. These results suggest that the short chain n-alkanols work as effective 'fluidizing' agents at the intermediate depths (C2-C9) in the bilayer.

Single photon radioluminescence. II. Signal detection and biological applications

A quantitative theory for excitation of fluorescent molecules by beta decay electrons is reported in the accompanying manuscript; experimental detection methods and biological applications are reported here. The single photon signals produced by an excited fluorophore (single photon radioluminescence, SPR) provide quantitative information about the distance between radioisotope and fluorophore. Instrumentation was constructed for SPR signal detection. Photons produced in a 0.5-ml sample volume were detected by a cooled photomultiplier and photon counting electronics. To minimize electronic noise and drift for detection of very small SPR signals, a mechanical light chopper was used for gated-signal detection, and a pulse height analyzer for noise rejection. SPR signals of approximately 1 cps were reproducibly measurable. The influence of inner filter effect, sample turbidity, and fluorophore environment (lipid, protein, and carbohydrate) on SPR signals were evaluated experimentally. SPR was then applied to measure lipid exchange kinetics, ligand binding, and membrane transport, and to determine an intermolecular distance in an intact membrane. (a. Lipid exchange kinetics.) Transfer of 12-anthroyloxystearic acid (12-AS) from sonicated lipid vesicles and micelles to vesicles containing 3H-cholesterol was measured from the time course of increasing SPR signal. At 22 degrees C, the half-times for 12-AS transfer from vesicles and micelles were 3.3 and 1.1 min, respectively. (b. Ligand binding.) Binding of 3H-oleic acid to albumin in solution, and 3H-2,2'-dihydro-4,4'-diisothiocyanodisulfonic stilbene (3H-H2DIDS) to band 3 on the erythrocyte membranes were detected by the radioluminescence of the intrinsic tryptophans. The SPR signal from 5 microCi 3H-oleic acid bound to 0.3 mM albumin decreased from 13 +/- 2 cps to 3 +/- 2 cps upon addition of nonradioactive oleic acid, giving 2.7 high affinity oleic acid binding sites per albumin. The SPR signal from 1 microCi 3H-H2DIDS bound selectively to erythrocyte band 3 in erythrocyte ghosts (1.5 mg protein/ml) was 2.2 +/- 0.8 cps. (c. Membrane transport). Dilution of J774 macrophages loaded with 3H-3-O-methylglucose and BCECF gave a decreasing SPR signal with a half-time of 81 s due to methylglucose efflux; the SPR measurement of the efflux rate was in agreement with a conventional tracer efflux rate determination by filtration. 20 microM cytochalasin B inhibited efflux by 97%. (d. Distance determination.) The SPR signal from erythrocyte membranes labeled with 27 microCi 3H-oleic acid and 10 microM of fluorescein-labeled wheat germ agglutinin was 5.7 +/- 0.5 cps, giving an average glycocalyx-to-bilayer distance of 5 nm. The results establish methods for experimental detection of SPR signals and demonstrate the applications of radioluminescence to the measurement of lipid exchange kinetics, ligand binding, membrane transport, and submicroscopic distances in intact membranes in real time.

Effect of ionic strength on the transfer of 1-pyrenemethyl-3 beta-hydroxy-22,23-bisnor-5-cholenate between bilayer vesicles containing phosphatidylserine

The influence of ionic strength or the concentration of K+ ([K+]) of the aqueous phase on the spontaneous transfer of cholesterol between negatively charged bilayer vesicles composed of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylserine (DPPS) (1:1, mole:mole) was studied using a pyrene-labelled cholesterol analogue, 1-pyrenemethyl-3 beta-hydroxy-22,23-bisnor-5-cholenate (PMC), as the probe. The decrease in PMC excimer fluorescence was best fitted to a bi-exponential function. Increasing [K+] from 0.1 M to 0.3 M had little effect on the shorter half-time (1.4 +/- 0.2 min) but increased the longer half-time from 16.3 +/- 1.9 min to 26.7 +/- 2.1 min. Fluorescence quenching and titration of an interface-located fluorophore, 1-anilinonaphthalene-8-sulfonic acid (ANS) revealed an increase in interfacial hydrophobicity upon increasing in ionic strength. The physical state of the acyl chains was not affected by ionic strength as indicated by a constant PMC excimer:monomer fluorescence intensity ratio. However, an increase in enthalpy change of the lipid phase transition from 15.7 kJ/mol ([K+] = 0.1 M) to 21.3 kJ/mol ([K+] = 0.3 M), together with a slight increase in the transition temperature, implies that interactions between adjacent molecules in the charged lipid bilayer vesicles became stronger at higher ionic strength. Our results suggest that the van der Waals attraction between PMC and phospholipid molecules could be affected by conformation changes in the charged head group region brought about by changes of ionic strength in the aqueous phase, with consequent effects on the desorption of cholesterol from the bilayer surface.

Nonmonotonic alterations in the fluorescence anisotropy of polar head group labeled fluorophores during the lamellar to hexagonal phase transition of phospholipids

The temperature dependence of the fluorescence anisotropy of polar head group labeled fluorophores (i.e., N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)dipalmitoyl-L- alpha-phosphatidylethanolamine or N-(lissamine rhodamine B sulfonyl)dipalmitoyl-L-alpha-phosphatidylethanol- amine) incorporated into multiple phosphatidylethanolamine molecular species was parabolic, possessing minima (dr/dT = 0) that precisely correlated with the respective lamellar (L alpha) to hexagonal (HII) phase transition temperature of each species. The parabolic alterations in the thermotropic behavior of these fluorophores were due to increased motional constraints in the polar head group region during heating (dr/dT greater than 0), because significant alterations in the fluorescence lifetimes of these probes during the phase transition did not occur. The sensitivity inherent in identification of peak minima was exploited to determine the lamellar to hexagonal phase transition temperatures of several homogeneous molecular species of plasmenylethanolamine (e.g., the transition temperature of 1-O-(Z)-hexadec-1'-enyl-2-octadec-9'- enoyl-sn-glycero-3-phosphoethanolamine was 28 degrees C). Experiments using ethanolamine glycerophospholipids containing either an ester or a vinyl ether linkage at the sn-1 position demonstrated that introduction of the vinyl ether constituent increased the propensity of these species to assume the hexagonal phase. Collectively, these results identify and substantiate a new technique for the characterization of the lamellar to hexagonal phase transition in phospholipids that requires only small amounts of phospholipids present in dilute membrane suspensions.

Reversal of multidrug resistance by surfactants

Cremophor EL, a pharmacologically inactive solubilising agent, has been shown to reverse multidrug resistance (MDR). Using flow cytometric evaluation of equilibrium intracellular levels of daunorubicin (DNR), we found that eight other surface active agents will also reverse MDR. All the active detergents contain polyethoxylated moieties but have no similarities in their hydrophobic components. The properties of three polyethoxylated surfactants that showed the lowest toxicities, Cremophor, Tween 80 and Solutol HS15, were examined in more detail. The concentrations of Tween 80 and Solutol required to reverse DNR exclusion were 10-fold lower than for Cremophor. However while concentrations greater than or equal to 1:10(2) of the former two surfactants resulted in breakdown of cells, even 1:10 of Cremophor did not lyse cells. Studies of the effects of Cremophor on the uptake and efflux of DNR in normal and MDR cell types showed that Cremophor increases intracellular DNR primarily by locking the rapid efflux from the cells. This blockage of drug efflux may be mediated by a substantial alteration in the fluidity of cell membranes induced by Cremophor, as shown by decreased fluorescence anisotropy of a membrane probe. Consistent with these data, coinjection of adriamycin plus Cremophor into mice carrying a multidrug resistant P388 transplantable tumour significantly increased the survival time of the mice compared with adriamycin treatment alone.

Fluorenyl fatty acids as fluorescent probes for depth-dependent analysis of artificial and natural membranes

The main objective of depth-dependent fluorescent probes is to provide information at a distinct position in the membrane hydrophobic core. We report here a series of fluorenyl fatty acids which can probe both artificial and natural membranes at different depths. Long-chain acids (C4, C6, and C8) are attached to fluorene chromophore on one side, and a hydrophobic tail (C4) is attached on the other side, so that on incorporation in membranes the carboxyl end of the molecule is oriented toward the membrane-water interface and the hydrophobic tail points toward the membrane interior. These acids can be readily partitioned into membranes. The disposition of these fluorenyl fatty acids in membranes was studied by fluorescence quenching using iodide as a water-soluble and 9,10-dibromostearic acid as a lipid-soluble quencher. The results obtained indicate that attachment of a hydrophobic tail is essential for effective alignment of depth-dependent fluorescent probes. The length of the hydrophobic tail was varied and an n-butyl chain was found to be most effective. In all cases, the compounds with a hydrophobic tail were found to be probing the membrane deeper than their counterparts with no hydrophobic tail. Further, the compounds with hydrophobic tails were more strongly immobilized in the membrane as indicated by fluorescence polarization studies. However, the effect of such a tail varied with membrane type. Thus in artificial membranes an n-butyl chain was found to be extremely important for effective monitoring by shallow probes like 4-(2'-fluorenyl)butyric acid, whereas in erythrocyte ghost membranes the same n-butyl tail was found to be more desirable for deeper probes like 8-(2'-fluorenyl)octanoic acid.(ABSTRACT TRUNCATED AT 250 WORDS)

Fluorescence lifetime imaging

We describe a new fluorescence imaging methodology in which the image contrast is derived from the fluorescence lifetime at each point in a two-dimensional image and not the local concentration and/or intensity of the fluorophore. In the present apparatus, lifetime images are created from a series of images obtained with a gain-modulated image intensifier. The frequency of gain modulation is at the light-modulation frequency (or a harmonic thereof), resulting in homodyne phase-sensitive images. These stationary phase-sensitive images are collected using a slow-scan CCD camera. A series of such images, obtained with various phase shifts of the gain-modulation signal, is used to determine the phase angle and/or modulation of the emission at each pixel, which is in essence the phase or modulation lifetime image. An advantage of this method is that pixel-to-pixel scanning is not required to obtain the images, as the information from all pixels is obtained at the same time. The method has been experimentally verified by creating lifetime images of standard fluorophores with known lifetimes, ranging from 1 to 10 ns. As an example of biochemical imaging we created life-time images of Yt-base when quenched by acrylamide, as a model for a fluorophore in distinct environments that affect its decay time. Additionally, we describe a faster imaging procedure that allows images in which a specific decay time is suppressed to be calculated, allowing rapid visualization of unique features and/or regions with distinct decay times. The concepts and methodologies of fluorescence lifetime imaging (FLIM) have numerous potential applications in the biosciences. Fluorescence lifetimes are known to be sensitive to numerous chemical and physical factors such as pH, oxygen, temperature, cations, polarity, and binding to macromolecules. Hence the FLIM method allows chemical or physical imaging of macroscopic and microscopic samples.

Partitioning of (+-)-5,6-dihydro-6-phenyl-2-n-alkyl-imidazo- [2,1-b]thiazoles into large unilamellar liposomes: a steady-state fluorescence quenching study

The interaction of the tetramisole derivative (+-)-5,6-dihydro-6-phenyl-imidazo[2,1-b]thiazole and a number of its 2-n-alkyl homologues (-ethyl through -n-pentyl and -n-heptyl) with large unilamellar phosphatidylcholine/phosphatidylethanolamine/dipalmitoylphosphatidic acid (2:1:0.06, w/w) vesicles was studied by means of steady-state fluorescence quenching using 8-(2-anthryl)octanoic acid as membrane probe. Linear Stern-Volmer plots were obtained for each derivative, indicating dynamic quenching. The slopes of the plots decreased with increasing liposomal concentration. For four short-chain homologues (-H, -ethyl, -n-propyl and -n-butyl), the respective membrane partition coefficients Kp and bimolecular quenching rate constants kq were determined from the plots of the reciprocal of the apparent quenching rate constant (kappq)-1 against the lipid volume fraction alpha L of the liposomes. The partition coefficients increased with increasing chain-length of the tetramisoles. A linear relationship was found between the free energy of partitioning and the number of methylene units of the homologues (-delta G degrees per methylene group = 1.6 +/- 0.1 kJ mol-1). For the n-pentyl and n-heptyl derivatives, the fluorescence quenching technique did not allow one to determine their membrane partition coefficients. Analysis of the fluorescence intensity measurements with Scatchard plots gave further evidence for the partitioning nature of the tetramisole derivatives' association with the liposomal membranes.

Iodide penetration into lipid bilayers as a probe of membrane lipid organization

The quenching efficiency of iodide as a penetrating fluorescence quencher for a membrane-associated fluorophore was utilized to measure the molecular packing of lipid bilayers. The KI quenching efficiency of tryptophan-fluorescence from melittin incorporated in DMPC bilayer vesicles peaks at the phase transition temperature (24 degrees C) of DMPC, whereas acrylamide quenching efficiency does not depend on temperature. The ability of iodide to penetrate the hydrocarbon region of the bilayer was examined by measuring the fluorescence quenching of the pyrene-phosphatidylcholine incorporated into DMPC vesicles (pyrene was attached to the 10th carbon of the sn-2 chain). The quenching efficiency of pyrene by iodide again shows a maximum at the lipid phase transition. We conclude that iodide penetrates the membrane hydrocarbon region at phase transition through an increased number of bilayer defects. The magnitude of change in quenching efficiency of iodide during lipid phase transition provides a sensitive technique to probe the lipid organization in membranes.

Compression of lipid membranes as observed at varying membrane positions

We have measured the microscopic isothermal compressibility of dioleoyl- and dimyristyl-phosphatidylcholine multilayers and bilayers as a function of membrane depth by the pressure dependence of the polarization of a series of anthroyloxy fatty acids. In both systems, within experimental error, the compressibility did not change with membrane depth. The magnitudes of the compressibilities matched those of organic solids and those reported for dipalmitoylphosphatidylcholine multilayers from neutron diffraction measurements (Braganza, L. F., and D. L. Worcester. 1986. Biochemistry, 25:7484-7488). The bilayer compressibility decreased with temperature and this decrease was similar with membrane depth consistent with the isotropic thermal expansion of membranes previously observed (Scarlata, S. 1989. Biophys. J. 55:1215-1223). The vertical compressibility in the z direction is much lower than the horizontal (xy planes) for probes that lie parallel to the hydrocarbon chains which is consistent with an increase in bilayer thickness. The compressibility for probes that lie perpendicular to the hydrocarbon chains is more isotropic due to their limited spatial access to the z plane.

Fluorescent membrane probes incorporating dipyrrometheneboron difluoride fluorophores

The spectroscopic properties of a new series of fatty acid analogs in which a dipyrrometheneboron difluoride fluorophore forms a segment of the acyl methylene chain are presented and their characteristics as fluorescent membrane probes are examined. When incorporated as a low mole fraction component in model phospholipid membranes, the probes retain the principal characteristics of the parent fluorophore: green fluorescence emission with high quantum yield, extensive spectral overlap, and low environmental sensitivity. The fluorescence quantum yield is typically two to three times that of comparable membrane probes based on the nitrobenzoxadiazole fluorophore. The spectral overlap results in a calculated Förster energy transfer radius (Ro) of about 57 A. Consequently, increasing fluorescence depolarization and quenching are observed as the mole fraction of the probe species incorporated in the membrane is increased. Low environmental sensitivity is manifested by retention of high quantum yield emission in aqueous dispersions of fatty acids. Partition coefficient data derived from fluorescence anisotropy measurements and iodide quenching experiments indicate that in the presence of fluid phase phospholipid bilayers the aqueous fraction of fatty acid is very small. Fluorescence intensity and anisotropy responses to phospholipid phase transitions are examined and found to be indicative of nonrandom fluorophore distribution in the gel phase. It is concluded that the spectroscopic properties of the fatty acid probes and their phospholipid derivatives are particularly suited to applications in fluorescence imaging of cellular lipid distribution and membrane level studies of lateral lipid segregation.

Effect of increased chain packing on gramicidin-lipid interactions

To study the effect of lipid packing on the dynamics of membrane proteins, the changes in the rotational motion of gramicidin tryptophans with increased packing brought about by high hydrostatic pressure through fluorescence spectroscopy were determined. In fluid phase dimyristoylphosphatidylcholine, the rotational motion of the residues decreased slightly with increased packing, but in the gel phase a significant reversible increase was observed. The magnitude of this increase was temperature dependent and much greater at lower temperatures. Quenching studies show that the increase in rotational motion is not due to a change in the location of the peptide in the membrane under pressure. Aromatic ring stacking between residues 9 and 15 appears to be stabilized under pressure, and there is no evidence of pressure-induced changes in peptide aggregation. The increase in rotational motion could be caused by a destabilization of hydrogen bonds between the indole hydrogens and the lipid head group oxygens due to an increase in the thickness of the compressible lipid bilayer with pressure without a concomitant lengthening of the peptide. These results indicate that specific interactions between lipids and proteins may play a major role of regulating the dynamics of membrane proteins.

Membrane structural dynamics of plasma membranes of living human prostatic carcinoma cells differing in metastatic potential

Rigidity of the outer hemileaflet of the plasma membrane of two prostatic carcinoma cell lines with different metastatic potential, 1-LN and 1-LN-EMS-10, was assessed by steady-state anisotropy, using a battery of fluorescent probes. The "bulk" membrane rigidity sensed by diphenylhexatriene, trimethylammonio-DPH, 1-palmitoyl-2-[DPH-ethylcarbonyl]-phosphatidylcholine, and 10-pyrenedecanoic acid indicated slightly higher rigidity in the membrane of the highly metastatic line (1-LN). This was accompanied by 26% greater mole fraction of cholesterol and 9% lower phospholipid, resulting in 40% greater cholesterol/phospholipid ratio. Phosphatidylethanolamine was increased 12%, but corresponding decreases in phosphatidylserine and phosphatidylinositol resulted in no significant change in molar ratio of choline/noncholine phospholipids. Whereas unsaturation index was slightly higher in 1-LN, fatty acids of 1-LN plasma membranes contained 15% more 18:1, 43% more 20:4, 26% more 22:4, and 38% less 18:2. Anisotropy gradients were determined for the two cell lines using a series of n-(9-anthroyloxy) fatty acid probes with n = 2, 3, 6, 7, 9, 12, and 16. Gradients differed only in position of anisotropy maxima, which occurred with n = 6, in 1-LN, and n = 7, in 1-LN-EMS-10. Possible relationships between observed anisotropy gradients and differences in membrane cholesterol and fatty acid composition are discussed.

Comparisons of steady-state anisotropy of the plasma membrane of living cells with different probes

We have used an extended Perrin equation which was in agreement with literature data for steady-state anisotropy (rSS) for a wide variety of artificial and isolated biological membranes labeled with various probes (Van der Meer et al. (1986) Biochim. Biophys. Acta 854, 38-44 to obtain the static component (r infinity) for the intact plasma membranes of living cells. We show that lipid structural order parameters can be obtained for DPH and TMA-DPH in the plasma membranes of intact cells. We have examined the relationship between 'fractional limiting hindered anisotropy', r infinity/r0, which is related to the lipid structural order parameter, of DPH, TMA-DPH, DPHpPC, and a series of depth-dependent probes (n-(9-anthroyloxy) fatty acids, with n = 2-16), using data from 19 cell types. There was a linear relationship between r infinity/r0 values of DPH and TMA-DPH, but the relationship between either of these probes was non-linear with respect to DPHpPC or the series of fatty acid probes. The relationship between r infinity/r0 values of DPHpPC and the series of fatty acid probes was linear, suggesting that they not only undergo similar motions in the membrane, but also experience similar types of restriction to motion, a type which is different from that experienced by DPH and TMA-DPH. We show that for the plasma membranes of living cells, 'second degree' order parameters can be estimated for DPH and TMA-DPH, and propose that the parameter r infinity/r0, or the 'fractional limiting hindered anisotropy', analogous to a 'first degree' order parameter, can be estimated for DPHpPC and the depth-dependent fatty acid probes to evaluate the density of membrane packing.

Fluorescence properties of methyl 8-(2-anthroyl) octanoate, a solvatochromic lipophilic probe

Fluorescence excitation and emission spectra, relative fluorescence quantum yield phi r and fluorescence lifetime tau of methyl 8-(2-anthroyl)-octanoate have been studied in a set of organic solvents covering a large scale of polarity and in the presence of water. In this probe, the 2-anthroyl chromophore exhibits quite remarkable and unique fluorescence properties. Thus, when going from n-hexane to methanol, the maximum emission wavelength lambda em max shifts from 404 nm to 492 nm while phi r and tau increase from 1 to 17.7 and from 0.91 ns to 13.5 ns, respectively. These increments are still more accentuated in the presence of water with estimated values of 526 nm for lambda em max, 27 for phi r and 20 ns for tau in this solvent. Because of the presence of a keto group which is a hydrogen bond acceptor and which can conjugate with the aromatic ring so as to provide the chromophore with a high dipole moment, the fluorescence properties of the probe strongly depend on the polarity of the surrounding medium. They can be accounted for in terms of general solvent effects (dipolar solute/solvent interactions) in the presence of aprotic solvents and in terms of specific solvent effects (hydrogen bonding) in protic solvents. Such properties of solvatochromism make the 2-anthroyl chromophore, after 8-(2-anthroyl)octanoic acid has been attached to phospholipids (E. Perochon and J.F. Tocanne (1991) Chem. Phys. Lipids 58, 7-17) a potential tool for studying microenvironmental polarity in biological membranes.

Compression of lipid membranes as observed at varying membrane positions

We have measured the microscopic isothermal compressibility of dioleoyl- and dimyristyl-phosphatidylcholine multilayers and bilayers as a function of membrane depth by the pressure dependence of the polarization of a series of anthroyloxy fatty acids. In both systems, within experimental error, the compressibility did not change with membrane depth. The magnitudes of the compressibilities matched those of organic solids and those reported for dipalmitoylphosphatidylcholine multilayers from neutron diffraction measurements (Braganza, L. F., and D. L. Worcester. 1986. Biochemistry, 25:7484-7488). The bilayer compressibility decreased with temperature and this decrease was similar with membrane depth consistent with the isotropic thermal expansion of membranes previously observed (Scarlata, S. 1989. Biophys. J. 55:1215-1223). The vertical compressibility in the z direction is much lower than the horizontal (xy planes) for probes that lie parallel to the hydrocarbon chains which is consistent with an increase in bilayer thickness. The compressibility for probes that lie perpendicular to the hydrocarbon chains is more isotropic due to their limited spatial access to the z plane.

Altered membrane anisotropy gradients of plasma membranes of living peripheral blood leukocytes in aging and Alzheimer's disease

Several reports have suggested that membrane rigidity, a term that refers to the relative motion of membrane constituents, is decreased in Alzheimer's Disease. Accordingly, a series of fluorescent membrane probes was used to evaluate the rigidity from the surface to the center of the outer hemi-leaflet of the plasma membrane of living neutrophils, monocytes and lymphocytes. Anisotropy, a parameter which increases with increasing membrane rigidity, was calculated from flow cytometric measurements of vertically and horizontally polarized components of the fluorescence emission of the probes. These preliminary experiments suggest that whereas membrane rigidity in certain regions of the plasma membrane of peripheral blood leukocytes is increased as expected in elderly controls, it is decreased in Alzheimer's disease.

pH-induced reorganization of synaptic membrane as revealed by fluorescence anisotropy and energy transfer

Two fluorescent probes, 2-(9-anthroyloxy)stearate and 12-(9-anthroyloxy)stearate, were used to investigate the effects of the neutralization of membrane charges on the organization of synaptic plasma membrane. Steady state fluorescence anisotropy measurements showed that a pH decrease provoked a rigidification of the synaptic membrane surface, whereas the bilayer core remained unaffected. The same effect was observed with negatively charged lipid vesicles. The relative distribution of proteins and the probes was estimated by fluorescence energy transfer from protein tryptophans to fluorescent probes: a pH decrease provoked an increase of the energy transfer, which was most pronounced with the surface probe, indicating an average closer packing between proteins and the probes. The modifications induced by a pH decrease were temperature dependent and were most marked at low temperatures. The results suggest that neutralization of the membrane charges provoked a redistribution of both membrane lipids and proteins. These findings are discussed in terms of a heterogeneous distribution of these membrane components.

Resolution of phospholipid conformational heterogeneity in model membranes by spin-label EPR and frequency-domain fluorescence spectroscopy

We have utilized both fluorescent and nitroxide derivatives of stearic acid as probes of membrane structural heterogeneity in phospholipid vesicles under physiological conditions, as well as conditions of varying ionic strengths and temperatures where spectral heterogeneity has been previously observed and attributed to multiple ionization states of the probes. To identify the source of this spectral heterogeneity, we have utilized complimentary measurements of the relaxation properties (lifetimes) and motion of both (a) spin labeled and anthroyloxy derivatives of stearic acid (i.e., SASL and AS) and (b) a diphenylhexatriene derivative of phosphatidylcholine (DPH-PC) in single component membranes containing dimyristoylphosphatidylcholine (DMPC). We use an 15N stearic-acid spin label for optimal sensitivity to membrane heterogeneity. The lifetime and dynamics of the fluorescent phospholipid analogue DPH-PC (with no ionizable groups over this pH range) were compared with those of AS, allowing us to discriminate between changes in membrane structure and the ionization of the label. The quantum yield and rotational dynamics of DPH-PC are independent of pH, indicating that changes in pH do not affect the conformation of the host phospholipids. However, both EPR spectra of SASL and the lifetime or dynamics of AS are affected profoundly by changes in solution pH. The apparent pKa's of these two probes in DMPC membranes were determined to be near pH 6.3, implying that at physiological pH and ionic strength these stearic-acid labels exist predominantly as a single ionized population in membranes. Therefore, the observed temperature- and ionic-strength-dependent alterations in the spectra of SASL as well as the lifetime or dynamics of AS in DMPC membranes at neutral pH are due to changes in membrane structure rather than the ionization of the probes. The possibility that ionic gradients across biological membranes induce alterations in phospholipid structures, thereby modulating lipid-protein interactions is discussed.

Lipid fluidity and composition of the erythrocyte membrane from healthy dogs and Labrador retrievers with hereditary muscular dystrophy

Erythrocyte membranes and their liposomes were prepared from clinically normal dogs and Labrador retrievers with hereditary muscular dystrophy. The "static" and "dynamic" components of fluidity of each membrane were then assessed by steady-state fluorescence polarization techniques using limiting hindered fluorescence anisotropy and order parameter values of 1,6-diphenyl-1,3,5-hexatriene (DPH) and fluorescence anisotropy values of DL-2-(9-anthroyl)-stearic acid and DL-12-(9-anthroyl)-stearic acid, respectively. Membrane lipids were extracted and analyzed by thin-layer chromatography and gas chromatography. The results of these studies demonstrated that the lipid fluidity of erythrocyte membranes, and their liposomes, prepared from dystrophic dogs were found to possess significantly lower "static and dynamic components of fluidity" than control counterparts. Analysis of the composition of membranes from dystrophic dogs revealed a higher ratio of saturated fatty acyl chain/unsaturated chains (w/w) and lower double-bond index. Alterations in the fatty acid composition such as decrease in levels of linoleic (18:2) and arachidonic (20:4) acids and increase in palmitic (16:0) and stearic (18:0) acids were also observed in the membranes of dystrophic animals. These associated fatty acyl alterations could explain, at least in part, the differences in membrane fluidity between dystrophic and control dogs.

Divalent cation-sensitive pores formed by natural and synthetic melittin and by Triton X-100

Leakage of ions and low-molecular-weight metabolites from Lettre cells is induced by synthetic melittin, as effectively as by melittin isolated from bee venom; in each case leakage is inhibited by Ca2+, Zn2+ or H+. Inhibition of leakage by divalent cations is reversible in that Lettre cells incubated with melittin (or with Triton X-100) in the presence of inhibitory amounts of Zn2+, when freed of Zn2+ by EGTA or by centrifugation, begin to leak (in Zn2(+)-sensitive manner). Electrorotation of Lettre cells is altered by melittin, compatible with membrane permeabilization; melittin plus Zn2+ does not alter electrorotation until Zn2+ (and unbound melittin) are removed. Melittin or Triton X-100 added to calcein-loaded liposomes induces leakage of calcein; divalent cations inhibit. Energy transfer between liposome-associated melittin and 2-, 7- or 12-(9-anthroyloxy)stearate (AS) is maximal with 12-AS; addition of Zn2+ has little effect. Circular dichroism spectra of melittin plus liposomes are unaffected by Zn2+. These results show that the formation of divalent cation-sensitive pores is not dependent on the presence of endogenous membrane proteins and that the action of divalent cations is not by displacement of melittin (or Triton) from the lipid bilayer.

Fluorescence quenching of a series of membrane probes measured in living cells by flow cytometry

The transverse location normal to the bilayer surface of a series of n-(9-anthroyloxy) fatty acid probes, where n = 2, 3, 6, 7, 9, 12, and 16, was determined by fluorescence quenching measurements with a flow cytometer. We show that the anthroyloxy moieties of the probes locate at a graded series of depths in the outer leaflet of the plasma membrane of living HeLa cells, in a manner similar to that previously observed for model membrane systems, and mitochondria. For different n, the efficiency of quenching with an aqueous phase quencher, Cu+2, was 2 greater than or equal to 3 greater than 6 greater than or equal to 7 greater than 9 greater than 12 greater than 16. Therefore, flow cytometry permits use of these probes for measurements of dynamic parameters related to membrane fluidity at different depths in the plasma membranes of living cells.

Fatty acid binding sites of rodent adipocyte and heart fatty acid binding proteins: characterization using fluorescent fatty acids

Murine adipocyte and rat heart fatty acid binding proteins (FABP) are closely related members of a family of cytosolic proteins which bind long-chain free fatty acids (ffa). The physical and chemical characteristics of the fatty acid binding sites of these proteins were studied using a series of fluorescent analogues of stearic acid (18:0) with an anthracene moiety covalently attached at seven different positions along the length of the hydrocarbon chain (AOffa). Previously, we used these probes to investigate the binding site of rat liver FABP (L-FABP) [Storch et al. (1989) J. Biol. Chem. 264, 8708-8713]. Here we extend those studies to adipocyte and heart FABP, two members of the FABP family which share a high degree of sequence homology with each other (62% identity) but which are less homologous with L-FABP (approximately 30%). The results show that the fluorescence emission spectra of AOffa bound to adipocyte FABP (A-FABP) are blue-shifted relative to heart FABP (H-FABP), indicating that AOffa bound to A-FABP are held in a more constrained configuration. For both proteins, constraint on the bound ffa probe is highest at the midportion of the acyl chain. Ffa are bound in a hydrophobic environment in both proteins. Excited-state lifetimes and fluorescence quantum yields suggest that the binding site of H-FABP is more hydrophobic than that of A-FABP. Nevertheless, acrylamide quenching experiments indicate that ffa bound to H-FABP are more accessible to the aqueous environment than are A-FABP-bound ffa.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma membrane fluidity gradients of human peripheral blood leukocytes

The shape of the fluidity gradient of the outer hemileaflet of the plasma membrane of normal, living, human white blood cells was determined using a series of n-(9-anthroyloxy) fatty acid probes where n = 2, 3, 6, 7, 9, 11, 12, and 16, to establish a baseline for future studies on the consequences of various pathological states. Fluorescence uptake and steady-state anisotropy values were obtained with a flow cytometer capable of continuous recording over time of vertical and horizontal emission intensities, with the output of these intensities as calculated anisotropy values. The fluorescence uptake of all of the membrane probes was rapid up to about 15 min. The magnitudes of the uptake of fluorescence was, for the n-(9-anthroyloxy) series, in the order 2 greater than 3 greater than 6 greater than 7 greater than 9 greater than 11 = 12 = 16 for neutrophils, lymphocytes, and monocytes. Anisotropy values were constant from 5 to 30 min after addition of the various probes. The orders of the anisotropy magnitudes, indicative of the shapes of the fluidity gradient, were, for neutrophils, 6 greater than 7 greater than 9 greater than 2 = 3 = 11 = 12 greater than 16, for lymphocytes, 7 greater than 6 greater than 9 greater than 11 greater than 2 = 3 greater than 11 = 12 greater than 16, and for monocytes, 9 greater than 7 greater than 6 greater than 11 greater than 2 = 3 greater than 12 greater than 16. The kinetics of anisotropy from 1 to 5 min after addition of the probes differed for each of the three cell types. Probes with an n-value less than or equal to the maxima (n = 6, neutrophils; n = 7, lymphocytes; n = 9, monocytes) rapidly (1.2 min) reached equilibrium, whereas those probes with n-values greater than the maxima took progressively longer times to equilibrate as n increased. This behavior is consistent with the existence of an energy barrier just below the approximate region sensed by the probes, which would correspond to just below 6AS for neutrophils, 7AS for lymphocytes, and 9AS for monocytes.

Modification of the adenylate cyclase activity of bovine thyroid plasma membranes by manipulating the ganglioside composition with a nonspecific lipid transfer protein

Gangliosides (GM1, GT1b, GD3) were incorporated in bovine thyroid plasma membranes using the nonspecific lipid transfer protein from beef liver. The transfer of GT1b or GD3 in the presence of 16 units of transfer protein was twice as high as that of GM1. However, taking into account the spontaneous exchange (approximately 8% for GT1b or GD3 and 1% for GM1) the transfer protein seemed to be more effective for GM1. Incorporation of these gangliosides in bovine thyroid plasma membranes caused a concentration dependent inhibition of the TSH-stimulated adenylate cyclase activity. The forskolin-stimulated adenylate cyclase activity was not significantly affected by ganglioside modification of the plasma membranes, indicating that the gangliosides do not act at the level of the catalyst of adenylate cyclase. Binding experiments on the other hand revealed that TSH binding to bovine thyroid plasma membranes was inhibited with the same order of efficacy (GT1b greater than GD3 greater than GM1) and to the same extent as their inhibitory effect on TSH stimulation. Therefore, this indicates that the ganglioside induced drop in TSH binding might be an important factor in the decrease in TSH-stimulated adenylate cyclase activity. Incorporation of GT1b or GD3 (approximately 11 nmol) in bovine thyroid plasma membranes, however, also induced a substantial decrease in cholera toxin-stimulated adenylate cyclase activity (approximately 30%) and to a lesser degree a decrease in NaF-stimulated activity (approximately 17%), whereas GM1 incorporation did not significantly affect these stimulated activities. These latter inhibitory effects were paralleled by changes in fluorescence steady-state anisotropy: GT1b modification of the plasma membranes provoked a slight increase in TMA-DPH anisotropy, whereas the anisotropy of DPH was substantially enhanced after incorporation of GD3 or GT1b. These results suggest that gangliosides might also interfere with the coupling between the alpha-subunit of the stimulatory GTP-binding regulatory protein and the catalyst of the adenylate cyclase system by affecting the membrane fluidity.

Glucose transport and microvillus membrane physical properties along the crypt-villus axis of the rabbit

Both transport function and microvillus membrane physical properties evolve as the enterocyte matures and migrates up the crypt-villus axis. We isolated enriched fractions of villus tip, mid-villus, and crypt enterocytes from which microvillus membrane vesicles were prepared. Using this material we characterized the alterations that occur in microvillus membrane fluidity as the rabbit enterocyte matures and correlated these with kinetic studies of glucose transport. With increasing maturity the microvillus membrane becomes more rigid due to both an increase in the cholesterol/phospholipid ratio and alterations in individual phospholipid subclasses. Maximal rates of glucose transport were greatest in microvillus membrane vesicles prepared from mature cells. However, the glucose concentration producing half-maximal rates of transport (Km) was significantly lower in crypt microvillus membrane vesicles, suggesting that a distinct glucose transporter existed in crypt enterocytes. This distinction disappeared when differences between membrane lipid environments were removed. By fluidizing villus-tip microvillus membrane vesicles, in vitro, to levels seen in the crypt microvillus membrane, we observed a reduction in the Km of this transport system. These data suggest that the kinetic characteristics of the sodium-dependent glucose transporter are dependent upon its local membrane environment.

Transverse location of anthracyclines in lipid bilayers. Paramagnetic quenching studies

Quenching of anthracycline fluorescence by a series of spin-labeled fatty acids was used to probe the transverse location of the drug in phosphatidylcholine bilayers in the form of small unilamellar vesicles. Stern-Volmer plots of the quenching data indicate that the fluorophore moiety of the anthracycline is intercalated into the hydrocarbon region of the bilayer, with deeper penetration observed in fluid-phase than in solid-phase vesicles. 31P-NMR parameters (T1 and nuclear Overhauser enhancement (NOE] are unaffected by the presence of drug, consistent with a binding site removed from the interfacial region. Comparison of intensity (F0/F) plots with lifetime (tau 0/tau) data shows that the predominant mechanism of anthracycline quenching by membrane-bound nitroxides is static. Since the membrane-bound drug is also accessible to quenching by I-, the binding site in the membrane must create a channel which is accessible to solvent. Two other fluorescent probes, 12-(9-anthroyloxy)stearate (12-AS) and diphenylhexatriene (DPH), were employed to confirm the results obtained with the anthracyclines, giving quenching data representative of their location in the bilayer.

The effect of transmembrane potential on the dynamic behavior of cell membranes

The relationship between transmembrane potential and lipid dynamics in the cytoplasmic membrane of mouse thymus cells has been investigated. Changes of transmembrane potential was followed by measuring the fluorescence emission of the anionic dye, bis-(1,3-dibutylbarbiturate)trimethine oxonol (diBa-C4-(3)). Assessment of lipid fluidity was carried out applying three fluorescent lipid probes, 1-[4-(trimethylammonium)phenyl]-6-phenyl-1,3,5-hexatriene (TMA-DPH), 12-(9-anthroyloxy)stearic acid (12-AS) and 1,6-diphenyl-1,3,5-hexatriene (DPH) used to monitor different structural regions of the bilayer. The fluorescence anisotropy of these probes was measured as a function of temperature at two values of transmembrane potential. In the case of DPH it proved to depend on the membrane potential in the higher temperature range (above 28 degrees C), while no such a dependence could be observed for DPH below this temperature range and for TMA-DPH and 12-AS in between 20 and 37 degrees C. These data suggest that changes in transmembrane potential are accompanied with some local alteration in membrane lipid dynamics and/or structure.

Vascular atherosclerotic disease: evaluation of the whole-blood filterability and red cell membrane microrheological pattern after intravenous administration of a single dose of pentoxifylline

A study was carried out in 10 patients with vascular atherosclerotic disease to investigate the effects of a single dose of 300 mg pentoxifylline, given by intravenous infusion over 30 minutes, on various macrorheological and microrheological parameters. Measurements were made at baseline, at the end of the intravenous infusion and again 30 minutes later, of whole-blood filterability, erythrocyte membrane fluidity (using pyrene as probe) and transverse fluidity gradient (using fluorescent fatty acids as probes). The results showed that there was a significant variation in whole-blood filterability and the red cell membrane fluidity gradient after pentoxifylline. No difference, however, was noted for the membrane fluidity. These data suggest, therefore, that pentoxifylline plays a role in affecting the red cell membrane microrheological pattern.

Lipid permeability of the intestinal microvillus membrane may be modulated by membrane fluidity in the rat

Fatty acids and cholesterol permeate across the intestinal microvillus membrane at rates dictated by the hydrophobicity of the permeating lipid and the permeability properties of the microvillus membrane. A theory has evolved suggesting that the chemical composition and physical properties of the microvillus membrane are important in determining microvillus membrane lipid permeability in vivo. This communication reports a test of this hypothesis. To compare in vivo membrane lipid permeability within the same intestinal region, but under conditions were membrane physical properties were radically altered, rats were fed an inhibitor of cholesterol synthesis. This resulted in the replacement of 87-90% of membrane cholesterol with its' precursor, 7-dehydrocholesterol. Marked changes in membrane physical properties were observed, including a reduction in the static and dynamic component of membrane fluidity within the jejunal microvillus membrane. These changes were limited primarily to the outer regions of the bilayer. Associated with these alterations was a pronounced reduction in membrane lipid permeability. Therefore, microvillus membrane lipid permeability, in vivo, appears to be correlated with physical properties of the bilayer, especially those of the superficial regions.

3-[p-(6-phenyl)-1,3,5-hexatrienyl]phenylpropionic acid (PA-DPH): characterization as a fluorescent membrane probe and binding to fatty acid binding proteins

The negatively charged fluorophore 3-[p-(6-phenyl)-1,3,5-hexatrienyl]phenylpropionic acid (PA-DPH) was characterized by comparison with its parent compound DPH, and with cationic trimethylammonium-DPH (TMA-DPH). The molar absorption coefficient of PA-DPH (60,000 cm-1.mol-1) as well as its quantum yield (0.7) and fluorescence lifetime (5 ns) in fluid phase membranes are intermediate between DPH and TMA-DPH. Steady-state fluorescence polarization studies show that PA-DPH detects the phase transition of both neutral and anionic bilayers. In fluid phase membranes the absolute values of PA-DPH polarization are considerably higher than DPH and somewhat lower than TMA-DPH. The results suggest that like TMA-DPH, PA-DPH is anchored to the surface of the membrane by its charge, but that it is probing a region somewhat deeper along the bilayer normal. PA-DPH binds to rat hepatic fatty acid binding protein (hFABP) and bovine serum albumin at PA-DPH/protein molar ratios of 1.5:1 and at least 6:1, respectively. Native oleic acid competes with PA-DPH for binding to both proteins, suggesting that the two ligands compete for similar binding sites. The affinity of PA-DPH for hFABP is similar to that of oleic acid. Thus, PA-DPH should be useful both as an anionic fluorescent membrane probe and a long-chain free fatty acid analogue.

Oxygen diffusion-concentration in erythrocyte plasma membranes studied by the fluorescence quenching of anionic and cationic pyrene derivatives

Fluorescence quenching by oxygen of cationic [pyrene-(CH2)nN(CH3)3+; n = 1, 4, and 11] and anionic [pyrene-(CH2)nCO2-, n = 3, 8, 11, and 15] probes was investigated in erythrocyte plasma membranes (leaky) in order to assess the ability of oxygen molecules to interact with solutes located at different positions in the membrane. The pseudounimolecular quenching rate constants measured increase, both for cationic and anionic probes, when n increases. These results are interpreted in terms of an increased oxygen solubility toward the center of the membrane interior, and imply that lateral diffusion contributes more than transverse diffusion to total oxygen mobility. For all of the probes considered, quenching rates increase when n-alkanols are added. The effect observed increases when n decreases and when the size of the n-alkanol alkyl chain increases. Arrhenius-type plots for the quenching rate constants show noticeable downward curvatures. Average (0-40 degrees C) activation energies are approximately 6 kcal/mol.

Evaluation of the thermal coefficient of the resistance to fluorophore rotation in model membranes

The thermal coefficient of the frictional resistance to fluorophore rotation (b), a parameter related to the change in the local viscosity with temperature, was determined for anthroyloxy-fatty acid probes in micelles and dimyristoyl lecithin (DMPC) and dioleoyl lecithin (DOPC) unilamellar and multilamellar vesicles. The value of b and the percent change in anisotropy with temperature (%dA/dT) remained constant with membrane depth and only depended on composition. These parameters were also the same when either in-plane, or in-plane and out-of-plane fluorophore motions were observed. This result indicates that the membranes expand isotropically. The magnitude of b was found to be primarily dependent on the packing of the hydrocarbon chains with higher b values relating to more closely-packed chains. b was responsive to the gel to liquid crystal phase transition of DMPC and the bilayer to hexagonal phase transition of egg-phosphatidylethanolamine. When the enthalpy values for the fluorophore transfer from one phase to another are calculated, the values are larger than those measured by calorimetry and reflect a discrepancy between the microscopic enthalpy experienced by the fluorophore due to a change in environment versus the macroscopic enthalpy of the system as a whole.

Localization of adriamycin in model and natural membranes. Influence of lipid molecular packing

The interaction of adriamycin with lipids was studied in model (monolayers, small unilamellar vesicles, large multilamellar vesicles) and natural (chinese hamster ovary cell) membranes by measurement of fluorescence energy transfer and fluorescence quenching. 2-APam, 7-ASte, 12-ASte and anthracene-phosphatidylcholine were used as fluorescent probes in which the anthracene group is well located at graded depths in the membrane. Egg-yolk phosphatidylcholine and a 1/1 mixture of it with bovine brain phosphatidylserine were used in model membrane systems. Large fluorescence energy transfer was observed between these molecules as donors and the drug as acceptor. With liposomes, at pH 7.4 and over an adriamycin concentration range of 0-100 microM, the efficiency of energy transfer was 12-ASte greater than 7-ASte greater than 2-APam, with 100% energy transfer for 12-ASte above a drug concentration of 30 microM. At pH 5, where the fatty acids are buried deeper (0.45 nm) in the lipid bilayer due to protonation of the carboxyl group, the order of energy transfer 7-ASTe greater than 12-ASte = 2-APam was observed. Measurements of fluorescence quenching using the non-permeant Cu2+ ion as quencher and spectrophotometric assays indicated that around 40% of the adriamycin molecules were deeply embedded in the lipid bilayer. Adriamycin molecules thus appear to penetrate the lipid bilayer, with the aminoglycosyl group interacting with the lipid phosphate groups and the dihydroanthraquinone residue in contact with the lipid fatty acid chains. In contrast, fluorescence energy transfer and quenching studies on CHO cells showed that adriamycin penetrated the plasma membrane of these cells to a much more limited extent than in the model membrane systems. This can be related to the squeezing out of the drug from a film of phosphatidylcholine which was observed in monolayers by means of surface pressure, potential and fluorescence experiments. These observations indicated that the penetration of adriamycin into lipid bilayers strongly depends on the molecular packing of the lipid.

Quenching of fluorescence of pyrene-substituted lecithin by tetracyanoquinodimethane in liposomes

In this work we have applied a kinetic scheme derived from fluorescence kinetics of pyrene-labeled phosphatidylcholine in phosphatidylcholine membrane to explain the fluorescence quenching of 1-palmitoyl-2-(10-[pyrenl-yl]-sn-glycerol-3-phosphatidylchol ine (PPDPC) liposomes by tetracyanoquinodimethane (TCNQ). The scheme was also found to be applicable to neat PPDPC and the effect of the quencher could be attributed to certain steps of the proposed mechanism. The TCNQ molecules influence the fluorescence of pyrene moieties in PPDPC liposome in two ways. Firstly, an interaction between the quencher molecule and the pyrene monomer in the excited state quenches monomer fluorescence and effectively prevents the diffusional formation of the excimer. Secondly, an interaction between the quencher molecule and the excited dimer quenches the excimer fluorescence. The TCNQ molecule does not prevent the formation of the excimer in pyrene moieties aggregated in such a way that they require only a small rotational motion to attain excimer configuration. The diffusional quenching rate constant is calculated to be 1.0 x 10(8) M-1 s-1 for the pyrene monomer quenching and 1.3 x 10(7) M-1 s-1 for the pyrene excimer quenching. The diffusion constant of TCNQ is 1.5 x 10(-7) cm2 s-1 for the interaction radii of 0.8-0.9 nm. The TCNQ molecules are practically totally partitioned in the membrane phase.