A comparison of triton X-100 and the bile salt taurocholate as micellar ionophores or fusogens in phospholipid vesicular membranes. A 1H NMR method using the lanthanide probe ion Pr3+

Ionophore-catalyzed cation transport between phospholipid inverted micelles manifest in DNMR

Studies of hyperfine shifts of lipid 31P resonances due to hydrated phospholipid inverted micelles in benzene are presented. Systems with distinct resonances from micelles containing no paramagnetic ions, and from micelles containing a single praseodymium(III) or a single europium(III) ion (and three nitrate counterions) have been generated. The addition of an ionophoric antibiotic from Streptomyces lasaliensis, lasalocid-A (X537A). causes both resonances to broaden and. with further additions, coalesce and eventually resharpen as a single line. Dilution of only the ionophore reverses these spectral changes. This is interpreted as a manifestation of dynamic NMR (DNMR. exchange broadening): i.e., that the ionophore catalyzes the equilibrium exchange of metal ions from micelle to micelle to the point where it becomes fast on the NMR time scale. This exchange is inhibited by protons or other competitive metal ions. We have simulated the spectra with a total lineshape analysis program and have thus extracted the average preexchange lifetimes for various concentrations of the antibiotic. We find a reasonably good first-order dependence on lasalocid-A concentration in each of several different experiments. This is in contrast to the higher order concentration dependences often observed by others using different techniques employing bilayer membranes. We favor a diffusional carrier mechanism involving surface aggregates of lasalocid-A for our process. This leads to the implication that a higher order concentration dependence found for a bilayer system arises from a different mechanism. The ionophore valinomycin does not catalyze rapid exchange in our system.

Indole-3-acetic acid-mediated transport of Mn2+ and other ions across phosphatidylinositol vesicular membranes as determined by 31P-NMR

The indolic plant hormone, indole-3-acetic acid (IAA), mediated the transport of Mn2+ and other ions into small unilamellar vesicles prepared from soybean phosphatidylinositol (PI) and this process has been studied using 31P nuclear magnetic resonance spectroscopy (NMR). The rate of Mn2+ movement into PI vesicles is dependent on IAA concentration and temperature with an IAA stoichiometry of 4.1 and an activation energy of 16.8 kcal mol-1 derived for the rate-determining process. These values are altered by low concentrations of endogenous ions (which can be removed by treatment with EDTA) present in the PI. With non-EDTA-treated PI, values of 2.3 and 23.0 kcal mol-1 were obtained for the stoichiometry and activation energy, respectively. These values indicate that (a) IAA interacts with PI membranes; (b) IAA-induced changes in membrane permeability can be substantially modulated by ions and (c) IAA very significantly influences the rate of movement of some (but possibly not all) cations across PI membranes. Such effects are also modified by the oxidation state of the PI.

On the effect of lysophosphatidylcholine, platelet activating factor and other surfactants on calcium permeability in sarcoplasmic reticulum vesicles

The effect of low concentrations of lysophosphatidylcholine (LPC), platelet-activating factor (PAF) and other surfactants (Triton X-100, C12E8, sodium dodecyl sulfate, sodium cholate and sodium deoxycholate) on membrane permeability of native sarcoplasmic reticulum vesicles and sarcoplasmic reticulum lipid vesicles, has been studied. Triton X-100, C12E8, sodium dodecyl sulfate, sodium cholate and sodium deoxycholate were all able to permeabilize membranes at concentrations of surfactants below their critical micellar concentration (CMC) in both lipid and native vesicles, being the K0.5 of calcium release from native vesicles lower than that from lipid vesicles. The values of these K0.5 were well correlated with the corresponding CMC values for each type of membrane. However, both LPC and PAF behaved in a different way since, although they induced permeabilization of the native vesicles at values of K0.5 close to their CMC, their K0.5 values for permeabilizing vesicles, prepared by using lipids extracted from sarcoplasmic reticulum, were much higher than their corresponding CMC.

The effect of n-alcohols on vesicular permeability induced at the lipid phase transition temperature: a 1H-NMR study

The effect of a series of n-alcohols on the permeability of small, unilamellar dipalmitoyl phosphatidylcholine (DPPC), dimyristoyl phosphatidylcholine (DMPC) and distearoyl phosphatidylcholine (DSPC) vesicles at the gel-to-liquid crystal phase transition temperature was investigated. It was found that the permeability took the form of the transient lysis of a fraction of the population of vesicles. The effect on this lysis of the n-alcohols was seen to be very chain-length dependent, with a minimum at n = 8 (octan-1-ol) for DPPC vesicles. A similar minimum was observed in the presence of 0.1 mM Triton X-100, but the detergent could then interact with certain of the alcohols to produce permanent channels. The results are discussed in terms of the semi-empirical model of Brasseur et al. (1985) Biochim. Biophys. Acta 814, 227-236, for the interaction of the n-alcohols with a DPPC membrane. The effect of various n-alcohols on the outer and inner monolayers of DPPC vesicles was also studied and the results related to their fluidising effect, allowing channels to open at the phase transition temperature.

Phospholipids and bile acids as diffusional carriers of Na+ across nonpolar media

Phospholipids and bile acids, by virtue of their amphiphilic properties, can interact in nonpolar media forming "inverted" structures (micelles) which presumably have an hydrophilic core and might act as diffusional carriers (ionophores) of electrolytes across low dielectric constant media or lipid membranes. The Na+ ionophoretic capability of various purified phospholipids and the modulating effects of bile acids and phosphatidylcholine was examined by: (a) measurement of 22Na+ partition into the organic phase (chloroform) of a two-phase system and (b) direct measurement of the translocation of 22Na+ across a bulk chloroform phase separating two aqueous phases in a Pressman cell. All phospholipids tested, except for phosphatidylcholine, showed ionophoretic capability for Na+ at micromolar concentrations. Cardiolipin and phosphatidylserine were the most efficient Na+ carriers, comparable with monensin, an established Na+ ionophore. In contrast, cholic acid as well as other bile acids demonstrated only marginal or no Na+ ionophoretic capability. However, hydroxylated bile acids (particularly cholic acid), sodium dodecyl sulfate and Triton X-100, which can induce and stabilize inverted structures in lipid membranes, were able to increase 5- to 8-fold the phospholipid-mediated Na+ transport. Interaction of cardiolipin with Na+ in the chloroform phase followed a rectangular hyperbolic function with an apparent Kd within the physiological Na+ concentration range (16.9 +/- 5.1 mM). Addition of cholic acid to the cardiolipin-containing organic phase resulted in a 10-fold increase of maximal Na+ uptake and no change in apparent Kd. The effect of cholic acid on both cardiolipin-mediated Na+ partition and Na+ translocation across the chloroform phase showed a marked dependence on pH, being greater at pH 7.4.(ABSTRACT TRUNCATED AT 250 WORDS)

Coreconstitution of bacterial ATP synthase with monomeric bacteriorhodopsin into liposomes. A comparison between the efficiency of monomeric bacteriorhodopsin and purple membrane patches in coreconstitution experiments

The conditions for coreconstitution of a bacterial ATP synthase and bacteriorhodopsin into lecithin liposomes and for light driven ATP synthesis have been optimized. A rate of maximally 280 nmol ATP min-1 mg ATP synthase-1 was achieved with monomerized bacteriorhodopsin compared with a rate of up to 45 nmol ATP min-1 mg-1 found for proteoliposomes containing bacteriorhodopsin in the form of purple membrane patches. The different rates are explained by the finding that monomeric bacteriorhodopsin is more homogeneously distributed among the liposomes than the purple membrane patches. The final activities depended on both the purification method for the two proteins and the coreconstitution procedure. Furthermore, the ratio (lipid to bacteriorhodopsin to ATP synthase) could be optimized. Light-driven ATP synthesis depends also on the type of detergent used. The best result was obtained by deoxycholate. Also the relationship between proton translocation (by bacteriorhodopsin) and ATP synthesis activity was measured. A constant H+/ATP ratio was found at higher light intensities. This ratio increased strongly at lower light intensities.

Nasal absorption of insulin: enhancement by hydrophobic bile salts

We demonstrate that therapeutically useful amounts of insulin are absorbed by the nasal mucosa of human beings when administered as a nasal spray with the common bile salts. By employing a series of bile salts with subtle differences in the number, position, and orientation of their nuclear hydroxyl functions and alterations in side chain conjugation, we show that adjuvant potency for nasal insulin absorption correlates positively with increasing hydrophobicity of the bile salts' steroid nucleus. As inferred from studies employing various concentrations of unconjugated deoxycholate and a constant dose of insulin, insulin absorption begins at the aqueous critical micellar concentration of the bile salt and becomes maximal when micelle formation is well established. These and other data are consistent with the complementary hypotheses that bile salts act as absorption adjuvants by producing high juxtamembrane concentrations of insulin monomers via solubilization in mixed bile salt micelles and forming reverse micelles within nasal membranes, through which insulin monomers can diffuse through polar channels from the nares into the blood stream.

The modulation of ion channels by the inhalation general anaesthetics. A1H-NMR investigation using unilamellar phospholipid membranes

The modulation of a variety of mechanisms of channel-mediated transport across unilamellar phospholipid membranes by a range of halogenated inhalation general anaesthetics (chloroform, enflurane, halothane and methoxyflurane) was investigated using 1H-NMR spectroscopy. Transport of the probe ion Pr3+ across egg yolk phosphatidylcholine (PC) and dipalmitoyl phosphatidylcholine (DPPC) vesicular membranes in the presence of the channel forming polypeptides alamethicin 30 and melittin, and the polyene antibiotic nystatin, as well as the degree of vesicular lysis at the gel to liquid-crystal phase transition of DPPC vesicles was monitored. The observation that the inhalation general anaesthetics inhibit such membrane permeability independently of the channel system or type of lipid used, suggests that hydrogen-bonded water structure and/or hydrogen-bonding centres at dipolar lipid-polypeptide interfaces, can be likely sites of action of the general anaesthetics.

Effect of micellar lipids on rabbit intestinal brush-border membrane phospholipid bilayer integrity studied by 31P NMR

The effect of biliary salts and fatty acids on the bilayer structure of rabbit intestinal brush-border membranes was studied using the nonperturbing probe 31P NMR. The broad, asymmetric lineshape of the 31P NMR spectrum of isolated brush-border vesicles demonstrates that their component phospholipids are organized in extended bilayers. These membranes are not significantly perturbed by incubation with physiological concentrations of biliary salts (3, 9, 18 mM), demonstrating that the vesicles are highly stable, corresponding to their biological function. However, the emergence of a narrow peak superimposed on the broad lineshape indicates that a small proportion of the membrane phospholipids has reached isotropic motion, which may correspond to external or internal micellar structures. Incubation with mixed micelles of fatty acids and taurochlorate show that long-chain fatty acids enhance the membrane-perturbing effect of taurocholate while short-chain, water-soluble fatty acids do not, suggesting a difference in the absorption mechanisms.

Application of 1H-n.m.r. to the design of liposomes for oral use. Synergistic activity of bile salts and pancreatic phospholipase A2 in the induced permeability of small unilamellar phospholipid vesicles

H-n.m.r. spectroscopy of small unilamellar phospholipid vesicles in the presence of the lanthanide probe ion Dy3+ has been used to study the permeability of these liposomes induced by the bile salts (glycocholate and glycodeoxycholate) and pancreatic phospholipase A2. A marked synergism is demonstrated in the combined effects of these digestive agents in producing permeability of the vesicles to Dy3+. Changes in the 1H-n.m.r. spectrum of the vesicular phospholipid head-groups before permeability is induced, indicate that the products of the enzymic hydrolysis (lyso lipids and fatty acids) and transmembrane lipid exchange are involved in the permeability mechanism. The results are discussed in terms of the advantages of the use of n.m.r. techniques in the future design of liposomes for oral use.

A 1H-NMR investigation of the effects of ethanol and general anaesthetics on ion channels and membrane fusion using unilamellar phospholipid membranes

Using 1H-NMR of small unilamellar vesicles in the presence of the lanthanide probe ion Pr3+, the effects of ethanol, diethyl ether and chloroform on various mechanisms of channel-mediated transport were studied. The mechanisms include channel formation by the polypeptide Alamethicin 30 and vesicular lysis at the gel to liquid-crystal phase transition of the lipid. Channel stabilisation and membrane fusion induced by sub-critical micelle concentrations of Triton X-100 were also investigated. The observation that ethanol and diethyl ether increase membrane permeability and fusion while chloroform inhibits them suggests a common locus of action on the properties and structure of channel-associated water. This conclusion is discussed in terms of current theories of general anaesthesia.

Perturbation of the structure and function of a membranous Ca2+-ATPase by non-solubilizing concentrations of a non-ionic detergent

The present study characterizes the effect of octa(ethyleneglycol)-monododecylether (C12E8) on Ca2+-ATPase membranes, prepared from sarcoplasmic reticulum (SR). At low concentrations C12E8 is incorporated into the membrane (less than or equal to 0.2 g/g protein), without any solubilization or appreciable morphological changes of freeze-fracture replica. Binding studies of C12E8 to ATPase membranes and SR lipid liposomes suggest that the major part of the detergent interacts with lipid. Solubilization of ATPase membranes occurs at a free concentration of C12E8 close to the critical micellar concentration (c.m.c); at low temperatures (2 degrees C) phospholipid is extracted somewhat more easily than ATPase. Electron-spin resonance (ESR) spectra of appropriate spin labels, incorporated into ATPase membranes, show that C12E8 strongly increases the fluidity of the lipid phase and the rotational diffusion of ATPase in the membrane. The effect of C12E8 on the ESR spectra is indistinguishable from that produced by a rise in temperature. Incorporation of C12E8 alters the functional properties of Ca2+-ATPase in a characteristic way: V is decreased and the modulatory effect of high ATP concentrations is reduced, in contrast to what occurs by a rise of temperature. The intrinsic fluorescence of the protein is increased, especially in the absence of Ca2+, suggesting that C12E8 modified in particular the E* form (Ca2+-depleted conformation) of the enzyme. Furthermore, stopped-flow data indicate that C12E8 strongly activates the E* to E transition, which may account for the effect of the detergent on ATP modulation during steady-state ATP hydrolysis. It is concluded that C12E8 perturbs ATPase turnover by direct interaction with the enzyme, rather than by an indirect effect exerted via a change in the lipid phase or protein aggregation.

Increase in size of sonicated phospholipid vesicles in the presence of detergents

Triton X-100, sodium dodecylsulphate, sodium cholate, and beta-octylglucoside increase the size of sonicated, but not of unsonicated, phospholipid vesicles above the Tc gel to liquid-crystalline transition temperature. Lysophosphatidylcholine or glyceryl monooleate do not increase liposome size under these circumstances. The observed phenomenon of vesicle growth is virtually unaffected by phospholipid composition, surface potential, calcium ions, EDTA, or albumin. The presence of cholesterol makes the vesicles more susceptible to detergent-induced growth. The presence of detergents, even at concentrations much lower than required to observe any increase in vesicle size, favors the liberation of vesicle contents. These results may have implications concerning membrane fusion as well as the reconstitution of membrane proteins in the presence of detergents.

Lanthanide-ion transport across phospholipid vesicular membranes: a comparison of alamethicin 30 and A23187 using 1H-NMR spectroscopy

The kinetics of Pr3+ transport by the ionophores alamethicin 30 and A23187 across unilamellar phospho-lipid vesicular membranes has been compared by following the time-dependent changes in the 1H-NMR spectrum of the vesicles. The measured rates of transport allow stoichiometries of the transporting species to be deduced which are consistent with channel- and carrier-mediated mechanisms respectively. The method provides a useful complement to planar bilayer conductivity studies of these systems.

Nuclear magnetic resonance studies of cation transport across vesicle bilayer membranes

We analyze an increasingly popular NMR method analogous to the black lipid membrane (BLM) isotopic tracer experiment for the study of mediated cation transport but involving the preparation of vesicles with an environment asymmetric in that paramagnetic metal ions are present only outside the vesicles. This asymmetry is manifest in the NMR spectrum as two distinct resonances for magnetic nuclei in outside and inside lipid headgroups. As mediated transport begins and for the paramagnetic metal ions enter the vesicles, the inner headgroup resonance line shifts and changes shape with a time course containing much information on the actual ion transport mechanism. Processes by which the ions enter the vesicles one or a few at a time (such as via a diffusive carrier) are easily distinguishable from those by which the ions enter in large bursts (such as by pore activation). The limiting case where intervesicular mediator exchange is slow relative to cation transport (the situation for integral membrane proteins) is treated analytically. Computer simulated curves indicate conditions necessary for certain changes in the line shape which are analogous to the "current jumps" observed in BLM conductance studies. The theory derived allows estimates of the average number of ions entering the first few bursts, how often the bursts occur, and how they depend on the concentration of the mediating species in the vesicular membrane. Preliminary experimental spectra illustrating some of the various possible line shape behaviors are presented.

Monoclonal antibody covalently coupled to liposomes: specific targeting to cells

We have evaluated optimal conditions for coupling monoclonal antibody to small unilamellar liposomes. Coupling of an IgG2 alpha monoclonal anti-beta 2-microglobulin antibody, which reacts with human cells, was examined in detail. Liposomes were composed of dipalmitoyl lecithin and cholesterol, and variable quantities of phosphatidylethanolamine substituted with the heterobifunctional cross-linking reagent N-hydroxysuccinimidyl 3-(2-pyridyldithio) propionate (SPDP). They were reacted with antibody derivatized with the same reagent at a 5- to 20-fold molar excess, and activated by mild reduction. This degree of SPDP modification had no effect on the capacity of the antibody to bind to its target antigen. More than 40% of antibody could be reproducibly bound to liposomes, resulting in the coupling of from 1 to 10 antibody molecules per liposome (mean diameter:580 A). The coupling reaction did not lead to loss of carboxyfluorescein encapsulated within liposomes. At least 80% of liposomes carried nondenatured antibody, as confirmed by precipitation of liposomes and encapsulated carboxyfluorescein by Staphylococcus aureus, strain Cowan I. The liposome-coupled antibody retained its immunological specificity: only cells expressing human beta 2-microglobulin bound liposomes in vitro, and the binding was inhibited by the free antibody in solution. Results with antibodies of different antigens specificity confirm that the technique can be generally applied.