Locations and dynamical perturbations for lipids of cationic forms of procaine, tetracaine, and dibucaine in small unilamellar phosphatidylcholine vesicles as studied by nuclear Overhauser effects in 1H nuclear magnetic resonance spectroscopy

Combined in-silico and in-vitro experiments support acid-base equilibrium as a tool to estimate the localization depth of 4-nitrophenol within a phospholipid bilayer

The interaction and location of 4-nitrophenol (PNP) in biomembranes are relevant in the bioaccumulation and potentiation of the intensive toxic effects of this persistent organic pollutant. In this work, in-silico analyses predicted that, in a fluid phospholipid bilayer, the minimum energy of the protonated (PNPH) and deprotonated (PNP^-) species is located within the glycerol and choline region, respectively. This was experimentally confirmed by acid-base equilibrium experiments and theory, allowing the estimation of the mean location of PNP within a bilayer region with a dielectric constant D = 50.6 compatible with the phosphate/choline moiety of egg-yolk phosphatidylcholine unilamellar (EPC) vesicles. The comparison with the D = 43.2 value obtained in Triton X-100 micelles allow predicting a mean surface potential of ψ = 25.37 mV for the EPC-water interface. Changes in the chemical shifts and longitudinal relaxation times of EPC hydrogens by ¹H NMR confirm the deeper location of the PNPH within the glycerol region and at the choline region (PNP^-) at higher pH. Intermolecular PNP^-EPC dipolar interactions within the choline region was also demonstrated at pH 10.2 using ROESY experiments. Additional information was obtained trough ³¹P NMR, that detected an increase in the anisotropy at the membrane interface after insertion of PNP which probably act as a bridge between choline moieties rigidizing the crystalline structure at that spot. Concluding, here we provide experimental support to the "pH-piston hypothesis" proposed some decades ago in the pharmaceutical field, and that reinforce the importance of the environmental conditions (e.g. pH) to modulate the bioavailability of this highly toxic pollutant.

Effect of tetracaine on dynamic reorganization of lipid membranes

To understand the intrinsic influence of a drug on lipid membranes is of critical importance in pharmacological science. Herein, we report fluorescence microscopy analysis of the interaction between the local anesthetic tetracaine (TTC) and planar supported lipid bilayers (SLBs), as model membranes. Our results show that TTC increases lipid chain mobility, destabilizes the SLBs and remarkably induces membrane disruption and solubilization. Upon TTC binding, a local curvature change in the bilayer was observed, which led to the subsequent formation of up to 20-μm-long flexible lipid tubules as well as the formation of micron-size holes. Quantitative analysis revealed that membrane solubilization process can be divided into two distinct different stages as a function of TTC concentration. In the first stage (<800 μM), the bilayer disruption profiles fit well to a Langmuir isotherm, while in the second stage (800 μM-25 mM), TTC solubilizes the membrane in a detergent-like manner. Notably, the onset of membrane solubilization occurred below the critical micelle concentration (cmc) of TTC, indicating a local accumulation of the drug in the membrane. Additionally, cholesterol increases the insertion of TTC into the membrane and thus promotes the solubilization effect of TTC on lipid bilayers. These findings may help to elucidate the possible mechanisms of TTC interaction with lipid membranes, the dose dependent toxicity attributed to local anesthetics, as well as provide valuable information for drug development and modification.

Mechanism of local anesthetic-induced disruption of raft-like ordered membrane domains

BACKGROUND

Because ordered membrane domains, called lipid rafts, regulate activation of ion channels related to the nerve pulse, lipids rafts are thought to be a possible target for anesthetic molecules. To understand the mechanism of anesthetic action, we examined influence of representative local anesthetics (LAs); dibucaine, tetracaine, and lidocaine, on raft-like liquid-ordered (L_o)/non-raft-like liquid-disordered (L_d) phase separation.

METHODS

Impact of LAs on the phase separation was observed by fluorescent microscopy. LA-induced perturbation of the L_o and L_d membranes was examined by DPH anisotropy measurements. Incorporation of LAs to the membranes was examined by fluorescent anisotropy of LAs. The biding location of the LAs was indicated by small angle x-ray diffraction (SAXD).

RESULTS

Fluorescent experiments showed that dibucaine eliminated the phase separation the most effectively, followed by tetracaine and lidocaine. The disruption of the phase separation can be explained by their disordering effects on the L_o membrane. SAXD and other experiments further suggested that dibucaine's most potent perturbation of the L_o membrane is attributable to its deeper immersion and bulky molecular structure. Tetracaine, albeit immersed in the L_o membrane as deeply as dibucaine, less perturbs the L_o membrane probably because of its smaller bulkiness. Lidocaine hardly reaches the hydrophobic region, resulting in the weakest L_o membrane perturbation.

CONCLUSION

Dibcaine perturbs the L_o membrane the most effectively, followed by tetracaine and lidocaine. This ranking correlates with their anesthetic potency.

GENERAL SIGNIFICANCE

This study suggests a possible mechanistic link between anesthetic action and perturbation of lipid rafts.

Monitoring drug–lipid membrane interactions via a molecular rotor probe

A label-free sensing method based on membrane viscosity changes to study the interactions between small drug molecules and lipid bilayers.

Sensing small molecule interactions with lipid membranes by local pH modulation

Herein, we utilized a label-free sensing platform based on pH modulation to detect the interactions between tetracaine, a positively charged small molecule used as a local anesthetic, and planar supported lipid bilayers (SLBs). The SLBs were patterned inside a flow cell, allowing for various concentrations of tetracaine to be introduced over the surface in a buffer solution. Studies with membranes containing POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) yielded an equilibrium dissociation constant value of Kd = 180 ± 47 μm for this small molecule-membrane interaction. Adding cholesterol to the SLBs decreased the affinity between tetracaine and the bilayers, while this interaction tightened when POPE (1-hexadecanoyl-2-(9-Z-octadecenoyl)-sn-glycero-3-phosphoethanolamine) was added. Studies were also conducted with three negatively charged membrane lipids, POPG (1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (sodium salt)), POPS (1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-l-serine (sodium salt)), and ganglioside GM1. All three measurements gave rise to a similar tightening of the apparent Kd value compared with pure POPC membranes. The lack of chemical specificity with the identity of the negatively charged lipid indicated that the tightening was largely electrostatic. Through a direct comparison with ITC measurements, it was found that the pH modulation sensor platform offers a facile, inexpensive, highly sensitive, and rapid method for the detection of interactions between putative drug candidates and lipid bilayers. As such, this technique may potentially be exploited as a screen for drug development and analysis.

Tetracaine-membrane interactions: effects of lipid composition and phase on drug partitioning, location, and ionization

Interactions of the local anesthetic tetracaine with unilamellar vesicles made of dimyristoyl or dipalmitoyl phosphatidylcholine (DMPC or DPPC), the latter without or with cholesterol, were examined by following changes in the drug's fluorescent properties. Tetracaine's location within the membrane (as indicated by the equivalent dielectric constant around the aromatic fluorophore), its membrane:buffer partition coefficients for protonated and base forms, and its apparent pK(a) when adsorbed to the membrane were determined by measuring, respectively, the saturating blue shifts of fluorescence emission at high lipid:tetracaine, the corresponding increases in fluorescence intensity at this lower wavelength with increasing lipid, and the dependence of fluorescence intensity of membrane-bound tetracaine (TTC) on solution pH. Results show that partition coefficients were greater for liquid-crystalline than solid-gel phase membranes, whether the phase was set by temperature or lipid composition, and were decreased by cholesterol; neutral TTC partitioned into membranes more strongly than the protonated species (TTCH(+)). Tetracaine's location in the membrane placed the drug's tertiary amine near the phosphate of the headgroup, its ester bond in the region of the lipids' ester bonds, and associated dipole field and the aromatic moiety near fatty acyl carbons 2-5; importantly, this location was unaffected by cholesterol and was the same for neutral and protonated tetracaine, showing that the dipole-dipole and hydrophobic interactions are the critical determinants of tetracaine's location. Tetracaine's effective pK(a) was reduced by 0.3-0.4 pH units from the solution pK(a) upon adsorption to these neutral bilayers, regardless of physical state or composition. We propose that the partitioning of tetracaine into solid-gel membranes is determined primarily by its steric accommodation between lipids, whereas in the liquid-crystalline membrane, in which the distance between lipid molecules is larger and steric hindrance is less important, hydrophobic and ionic interactions between tetracaine and lipid molecules predominate.

Effect of carticaine on the sarcoplasmic reticulum Ca2+-adenosine triphosphatase. II. Cations dependence

Ca2+-ATPase is a major intrinsic protein in the sarcoplasmic reticulum (SR) from skeletal muscles. It actively transports Ca2+ from the cytoplasm to the SR lumen, reducing cytoplasmic [Ca2+] to promote muscle relaxation. Carticaine is a local anesthetic widely used in operative dentistry. We previously showed that carticaine inhibits SR Ca2+-ATPase activity and the coupled Ca(2+) uptake by isolated SR vesicles, and increases the rate of Ca2+ efflux from preloaded vesicles. We also found that these effects were antagonized by divalent cations, and concluded that they were mainly due to the direct interaction of carticaine with the Ca2+-ATPase protein. Here we present additional results on the modulation of the above effects of carticaine by Ca2+ and Mg2+. The activating effect of Ca2+ on the ATPase activity is competitively inhibited by carticaine, indicating a decreased Ca2+ binding to the high affinity Ca2+ transport sites. The activating effect of Mg2+ on the phosphorylation of Ca2+-ATPase by orthophosphate is also inhibited by carticaine. The anesthetic does not affect the reaction mechanism of the cations acting as cofactors of ATP in the catalytic site. On the basis of the present and our previous results, we propose a model that describes the effect of carticaine on the Ca2+-ATPase cycle.

Differential effects of uncharged aminoamide local anesthetics on phospholipid bilayers, as monitored by 1H-NMR measurements

We have collected evidences of a "transient site" for the local anesthetics (LA) lidocaine, etidocaine, bupivacaine and mepivacaine in sonicated egg phosphatidylcholine (EPC) vesicles. The effects of the uncharged anesthetic species at a fixed LA/EPC ratio inside the bilayer were measured by chemical shifts (C.S.) and longitudinal relaxation times (T(1)) of the lipid hydrogens. Two sort of changes were detected: (I) decrease, indicating specific orientation of the LA aromatic ring (measured as up-field C.S. changes by the short-range ring-current effect) and less rotational freedom (smaller T(1) values) for EPC hydrogens such as the two glycerol-CH(2) and the choline-CH(2) bound to the PO(4-) group, probably due to the nearby presence of the LA; (II) increase, indicating the aromatic ring is now perpendicular to the orientation observed before (causing down-field changes in C.S.) and larger T(1) values for all the choline and glycerol hydrogens, as a result of LA insertion behind these well-organized bilayer regions. The less hydrophobic, linear and nonlinear (lidocaine and mepivacaine, respectively) aminoamide analogs provide similar effects-described in I; their hydrophobic counterparts (etidocaine and bupivacaine) also produced comparable effects (depicted in II). The preferential positioning and orientation of each LA inside the bilayer is then determined by its hydrophobic and steric properties. We propose that this "transient site" in the lipid milieu exists also in biological membranes, where it can modulates the access of the uncharged LA species to its site(s) of action in the voltage-gated sodium channel.

Boronphenylalanine insertion in cationic liposomes for Boron Neutron Capture Therapy

Cationic liposomes are widely used as carriers of biomolecules specifically targeted to the cell nucleus. p-Boronphenylalanine (BPA) is a powerful anti-tumor agent for Boron Neutron Capture Therapy (BNCT). In this paper, (1)H and (13)C NMR was used to study the insertion of BPA in mixed liposomes, made up by the positively charged 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) and the zwitterionic 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE). The boronated drug was distributed between the water phase and the liposomes. The location site of BPA into the lipid bilayer was investigated and the boron-substituted aromatic ring was found inserted in the hydrophobic region, whereas the amino acidic group was oriented towards the aqueous environment. Further information was given by proton spin-lattice relaxation rates.

Effect of the glycosphingolipid, GM1 on localization of dibucaine in phospholipid vesicles: a fluorescence study

Interaction of the local anesthetic dibucaine with small unilamellar vesicles of dimyristoylphosphatidylcholine (DMPC) and dioleoylphosphatidylcholine (DOPC) containing different mole percents of monosialoganglioside (GM1) has been studied by fluorescence spectroscopy. Fluorescence measurements on dibucaine in the presence of phospholipid vesicles containing various amounts of GM1 yielded a pattern of variation of wavelength at emission maximum and steady-state anisotropy which indicated that the microenvironment of dibucaine is more hydrophobic and rigid in membranes that contain GM1 than in membranes without it. Experiments on quenching of fluorescence from membrane-associated dibucaine by potassium iodide showed reduced quenching efficiency with the increase in GM1 content of the vesicles, demonstrating lesser accessibility of the iodide quenchers to dibucaine in the presence of GM1, when compared to that in its absence. Total emission intensity decay profiles of dibucaine yielded two lifetime components of approximately 1 and approximately 2.8-3.1 ns with mean relative contributions of approximately 25 and approximately 75%, respectively. The mean lifetime in vesicles was 20-30% lower than in the aqueous medium and showed a definite increase in presence of GM1 from that in the absence of it. All the spectral properties point that dibucaine encountered regions of membrane containing significant amount of GM1 and penetrated deeper in hydrophobic core of the bilayer.

Spectroscopic evidence for a preferential location of lidocaine inside phospholipid bilayers

We examined the effect of uncharged lidocaine on the structure and dynamics of egg phosphatidylcholine (EPC) membranes at pH 10.5 in order to assess the location of this local anesthetic in the bilayer. Changes in the organization of small unilamellar vesicles were monitored either by electron paramagnetic resonance (EPR)-in the spectra of doxyl derivatives of stearic acid methyl esters labeled at different positions in the acyl chain (5-, 7-, 12- and 16-MeSL)-or by fluorescence, with pyrene fatty-acid (4-, 6-, 10- and 16-Py) probes. The largest effects were observed with labels located at the upper positions of the fatty-acid acyl-chain. Dynamic information was obtained by 1H-NMR. Lidocaine protons presented shorter longitudinal relaxation times (T(1)) values due to their binding, and consequent immobilization to the membrane. In the presence of lidocaine the mobility of all glycerol protons of EPC decreased, while the choline protons revealed a higher degree of mobility, indicating a reduced participation in lipid-lipid interactions. Two-dimensional Nuclear Overhauser Effect experiments detected contacts between aromatic lidocaine protons and the phospholipid-choline methyl group. Fourier-transform infrared spectroscopy spectra revealed that lidocaine changes the access of water to the glycerol region of the bilayer. A "transient site" model for lidocaine preferential location in EPC bilayers is proposed. The model is based on the consideration that insertion of the bulky aromatic ring of the anesthetic into the glycerol backbone region causes a decrease in the mobility of that EPC region (T(1) data) and an increased mobility of the acyl chains (EPR and fluorescence data).

Surface plasmon resonance characterization of drug/liposome interactions

Using Biacore's surface plasmon resonance-based biosensor technology, we developed experimental protocols and probed test conditions required to study drugs interacting with liposome surfaces. Liposome capture on hydrophobic alkane surfaces (Pioneer L1 chip) was reproducible and stable under variable conditions of pH, temperature, lipid content, cholesterol content, and buffer dimethylsulfoxide concentration. Importantly, drug binding responses were directly proportional to the amount of lipid captured, while the kinetics of drug binding and the magnitude of the responses correlated with a drug's chemical composition. In general, anionic drugs tended to rapidly dissociate from the surface, while cationic drugs displayed heterogeneous binding, suggesting partitioning within the lipid bilayer itself. The results illustrate how surface plasmon resonance can be used to establish passive transport properties of drugs.

Effect of cholesterol on interaction of dibucaine with phospholipid vesicles: a fluorescence study

Interaction of the local anesthetic dibucaine with small unilamellar vesicles of dimyristoylphosphatidylcholine (DMPC) and dioleoyl phosphatidylcholine (DOPC) containing different mol percents of cholesterol has been studied by fluorescence spectroscopy. Fluorescence measurements on dibucaine in presence of phospholipid vesicles containing various amounts of cholesterol yielded a pattern of variation of wavelength at emission maximum and steady-state anisotropy which indicated that the microenvironment of dibucaine is more polar and flexible in membranes that contain cholesterol than in membranes without cholesterol. Experiments on quenching of fluorescence from membrane-associated dibucaine by potassium iodide showed a marked increase in quenching efficiency as the cholesterol content of the vesicles was increased, demonstrating increased accessibility of the iodide quenchers to dibucaine in the presence of cholesterol, when compared to that in its absence. Total emission intensity decay profiles of dibucaine yielded two lifetime components of approximately 1 ns and approximately 2.8--3.1 ns with mean relative contributions of approximately 25 and approximately 75%, respectively. The mean lifetime in vesicles was 20--30% smaller than in the aqueous medium and showed a moderate variation with cholesterol content. Fluorescence measurements at two different temperatures in DMPC SUVs, one at 33 degrees C, above the phase transition temperature and another at 25 degrees C, around the main phase transition, indicated two different mode of dibucaine localization. At 25 degrees C dibucaine partitioned differentially in presence and absence of cholesterol. However, at 33 degrees C the apparent partition coefficients remained unaltered indicating differences in the microenvironment of dibucaine in presence and absence of cholesterol in the phospholipid membranes.

Interaction of local anesthetics with phospholipids in Langmuir monolayers

We have used epifluorescence microscopy to study the interactions of two local anesthetics of the "caine" family (tetracaine and dibucaine), with Langmuir monolayers of the phospholipid dipalmitoylphosphatidylcholine (DPPC). These results show that incorporation of either dibucaine or tetracaine causes significant changes in the domain shapes of the liquid condensed phase in monolayers. In particular, at low pH, where the charged cationic form of the local anesthetics predominates, local anesthetic: DPPC monolayers formed significantly less compact liquid condensed domains with highly ramified shapes, compared to DPPC-only controls. For high pH values at which both local anesthetics are electrically neutral, the liquid condensed domains in mixed monolayers resembled that of DPPC-only controls, indicating that these effects have their origins in electrostatic interactions between the local anesthetics and the phospholipid headgroups. Epifluorescence images obtained using the intrinsic fluorescence of dibucaine indicated that dibucaine partitions into both the liquid condensed and liquid expanded phases.

Elucidation of biphasic alterations on acetylcholinesterase (AChE) activity and membrane fluidity in the structure-functional effects of tetracaine on AChE-associated membrane vesicles

Tetracaine-induced biphasic structure-functional alterations were investigated in acetylcholinesterase (AChE)-associated membrane vesicles from the electric organ of Torpedo californica. Enzyme assays showed that tetracaine exhibits a biphasic effect on the activity of membrane-bound AChE: increasing it at low concentrations (< 12 mM) and decreasing it at high concentrations (> 12 mM). Fluorescence-polarization experiments demonstrated that tetracaine affects the fluidity of lipid hydrocarbon chains of these membranes in a biphasic manner: increasing it at < 20 mM and decreasing it at > 20 mM. This small molecule also alters the fluidity of the negatively charged lipid head group: increasing it at < 13 mM and remaining essentially at the same level at > 13 mM. The positively charged lipid head group is unaffected. Contrasting effects on AChE activity with changes in membrane fluidity showed that [tetracaine] for AChE activity is comparable to that for the fluidity of the negatively charged lipid head group (12 mM versus 13 mM), but lower than that for a biphasic effect on the fluidity of lipid hydrocarbon chains (12 mM versus 20 mM). Differential scanning microcalorimetry showed that, due to membrane protein-lipid interaction, the lipid-phase transition temperature (tml) is higher for AChE-associated membrane vesicles than for isolated lipids from these membranes. An overall disordering of the membranes by tetracaine, as inferred from the lowering of tml, was also demonstrated. These findings suggested that binding of tetracaine to the lipid polar head group and membrane protein-lipid interaction may contribute to a higher [tetracaine] in inducing a comparable biphasic effect on membrane fluidity. At high [tetracaine], charge interactions between the tetracaine cation and the negatively charged lipid head group may result in a new lipid phase in the membranes, which could reverse the increase in membrane fluidity, resulting in the observed biphasic effect. Although both tetracaine and alcohol are amphiphilic species, they exhibit distinctive structure-functional effects on the membranes, as shown by comparing the results obtained on tetracaine with those previously reported for alcohol. The present observations may have significant physiological implications and may be of importance in understanding the biochemical effects of tetracaine in correlation with its physiological impact.

A fluorescence spectroscopy study of the interaction of monocationic quinine with phospholipid vesicles. Effect of the ionic strength and lipid composition

The interaction of monocationic quinine with zwitterionic dimyristoyl phosphatidylcholine (DMPC) and mixed negatively-charged dimyristoylphosphatidyl glycerol (DMPG)/DMPC small unilamellar vesicles in the liquid-crystalline phase was investigated by steady-state fluorescence spectroscopy at pH 7 and 37 degrees C. The maximum fluorescence emission peak at 383 nm, upon excitation at 335 nm, shifts to lower wavelength and decreases its intensity as the ratio between the total lipid and quinine concentrations increases. This indicates that in the membrane-bound state quinine is in an environment of low polarity, more deeply buried when anionic DMPG is present in the vesicle. For monoprotonated quinine/DMPC system the corresponding association isotherms show that the extension of binding is slightly enhanced as the ionic strength decreases, whereas for mixed DMPG/DMPC vesicles at low ionic strength, the association of the drug is favoured as the percentage of anionic DMPG increases. The binding curves have been quantitatively analyzed by the binding and the partition models including in this latter an activity coefficient, gamma, to account for non ideal quinine interactions. It is demonstrated for both neutral and anionic membranes that the activity coefficient approaches the unity and that the deviation from ideality is mainly due to electrostatic forces.

Use of a novel method for determination of partition coefficients to compare the effect of local anesthetics on membrane structure

A new, simple procedure for the determination of partition coefficients (P) was developed based on spectral effects caused upon addition of solutes to spin labeled model lipid membranes, and on the knowledge of their water solubility. Values of P were determined for nine local anesthetics (LA), amino-esters and amino-amides. The results were in good agreement with those found by phase separation and by a more complex, previously reported, methodology (Lissi et al. (1990) Biochim. Biophys. Acta 1021, 46-50) applied to either EPR or fluorescence spectra of probes incorporated in the bilayers. Both the present and the previously reported procedures make use of effects on membrane structure evaluated by spectroscopic techniques and offer the advantage of not requiring phase separation. The spectral effects, indicative of a decrease in bilayer organization increased with LA concentration, reaching a maximum at the drug water solubility, indicating that partitioning in the membrane is limited by saturation of the aqueous phase. A thermodynamic analysis of the partition data according to Hill (Hill, M.W. (1974) Biochim. Biophys. Acta 356, 117-124) showed that the LAs did not display ideal behavior. Knowledge of the partition coefficients allowed a comparison between effects at the same drug concentration in the membrane. Within a given family (esters, acyclic amides, cyclic amides) no clear proportionality was observed between effect and LA hydrophobicity, as reflected in the partition coefficient. Rather, the membrane perturbing ability is a result of steric effects originating in the mismatch between anesthetic and phospholipid shapes.

Local anesthetics destabilize lipid membranes by breaking hydration shell: infrared and calorimetry studies

Differential scanning calorimetry (DSC) showed that local anesthetics decreased the pretransition (L beta'-->P beta') temperature of dipalmitoylphosphatidylcholine (DPPC) vesicle membranes four- to five-fold more than the main transition (P beta'-->L alpha) temperature. Because pretransition is mainly a change in the hydrophilic head property (tilted-rippled), the stronger effect on the pretransition suggests that the primary action site of local anesthetics is the lipid-water interface. The interfacial effect was analyzed by Fourier-transform infrared spectroscopy (FTIR) in water-in-oil (CCl4) reversed micelles. FTIR showed that the local anesthetics released hydrogen-bonded water molecules from the phosphate (P = O bands) and glycerol (sn-2 C = O) moieties. The N-H stretching band of the local anesthetics was deconvoluted into two bands: hydrogen bonded to the phosphate moiety of the lipid and free (unbound to lipid). The formation constants between lipid P = O and anesthetic N-H were estimated in CCl4 from the spectral changes: 110 M-1 for lidocaine and 250 M-1 for dibucaine. This small difference in the formation constants cannot explain the ten-fold stronger effect on the phase-transition temperature of dibucaine over lidocaine. By comparing the local anesthetic adsorption to the air/water interface in the presence and absence of lipid monolayers, we have previously shown (Lin et al. (1980) Biochim. Biophys. Acta 598, 51-65) that lipid-anesthetics interaction involves three forces: lipophilic effect, hydrophobic effect, and anesthetic-anesthetic interaction. The anesthetic potency depends mainly on the hydrophobic effect (the difference in the standard molar free energies of local anesthetics in water and at the interface) and anesthetic-anesthetic interaction energy. The anesthetic-anesthetic interaction means cooperativity of local anesthetics for the interfacial density: local anesthetics condense at the membrane surface when there are enough anesthetic molecules present at the interface to attract more anesthetics. The present data suggest that anesthetic action is directed to the interface between water and macromolecule, whether it is lipid membranes or proteins.

Depth profiling of dibucaine in sarcoplasmic reticulum vesicles by fluorescence quenching

The location of molecules of the local anesthetic dibucaine in sarcoplasmic reticulum vesicles (SRV) was determined using the quenching of its intrinsic fluorescence by iodide and by nitroxide-labeled stearic acids (SASL) with the nitroxide group at different positions of the fatty acyl chain. The molar ratios of dibucaine to Ca(2+)-ATPase in the samples were less than 1. The acid-base titration of membrane bound dibucaine revealed a pK of 9.1, showing a negligible shift upon binding. The quenching data were obtained at pH 6.8 and are therefore related to protonated dibucaine. Quenching by iodide showed SRV-bound dibucaine to be more protected from collisions with iodide anion than dibucaine in buffer or even in neutral micelles. This shows the influence of negatively charged lipids in keeping iodide away from the ionic diffuse layer of the membrane surface where the dibucaine tertiary amine might be located. Analysis of the SASL quenching data indicates that dibucaine molecules are at a shallow position in the membrane bilayer. Their average depth was found to be at most that of the fourth carbon atom of the fatty acyl chain. The results do not exclude a preferential site for dibucaine in Ca(2+)-ATPase, but if there is such site it must be located at the protein/lipid interface.

Depth-dependent change in membrane fluidity by phenolic compounds in bovine platelets and its relationship with their effects on aggregation and adenylate cyclase activity

The effects of phenolic compounds on membrane fluidity of bovine blood platelets were investigated by studies on the fluorescence anisotropies of diphenylhexatriene (DPH) and its ionic derivatives to clarify the relationship of these effects with the inhibitory effects of the compounds on aggregation. Among the phenolic compounds tested, monohydric phenols (phenol and two monosubstituted derivatives) decreased the fluorescence anisotropy of DPH, which is thought to be located within the hydrophobic core of the membrane, in concentration ranges in which they inhibited platelet aggregation. On the other hand, they had little or no effects on the fluorescence anisotropies of the ionic derivatives of DPH, which are thought to be located in the interfacial region of the lipid bilayer. Consistent with their effects on the fluorescence anisotropy of DPH, these monohydric phenols increased the intracellular cAMP concentration. Thus, these monohydric phenols may inhibit platelet function by stimulation of adenylate cyclase mediated by perturbation of the central region of the membrane lipid bilayer. On the other hand, pyrocatechol and pyrogallol, which have two and three phenolic hydroxyl groups and have much larger electron donor activities than the monohydric phenols tested, inhibited platelet function by a different mechanism, because they did not cause increase in either membrane fluidity or the cAMP concentration of platelets.

Fluorescence studies of a local anesthetic-phospholipid interaction

Steady-state and time-resolved fluorescence data are reported for the local anesthetic dibucaine in the absence and presence of phospholipid vesicles. These vesicles were comprised of dimyristylphosphatidyl choline and approximately 10% dimyristylphosphatidyl glycerol. Solute quenching studies show the bound drug to be protected from collision with iodide ion. The fluorescence lifetime of dibucaine is not significantly changed upon binding to vesicles. The fluorescence anisotropy of dibucaine increases upon association with the vesicles. Anisotropy decay measurements show that the rotational correlation time, phi, of bound dibucaine is increased about one hundred fold over that for free dibucaine. This indicates that the rotational motion of bound dibucaine is slowed by its interaction with the phospholipids. However, we find no evidence that the rotational motion of bound dibucaine is anisotropic.

Local anesthetic-phospholipid interactions. Effects of ionic strength, temperature, and phospholipid mixtures on the binding of dibucaine to phospholipids

The nature of the interaction of amphipathic drugs, such as dibucaine, with phospholipid bilayer membranes was investigated using equilibrium dialysis. Profiles for the binding of cationic dibucaine to unilamellar vesicles were obtained at different temperature and ionic strengths, and for mixtures of neutral phospholipid dimyristylphosphatidylcholine (DMPC) with negatively charged dimyristylphosphatidylglycerol (DMPG). The degree of binding of the cationic drug at pH 5 was found to be higher at temperatures above the Tm of DMPC (24 degrees C) than below Tm. Also enhanced drug binding was found to occur as the concentration of monovalent salt was increased (0.01-0.85 M) and as the percentage of DMPG was increased. Using the Stern and Guoy-Chapman model, which takes into consideration electrostatic effects, we were able to simultaneously fit all our binding data with a minimum of fitting parameters. These parameters (for data at 45 degrees C) are an association constant, K, of 330 M-1, a maximum possible number of drug molecules absorbed per unit surface of vesicle, sigma m+, of 1.70 x 10(-2) A2, and a surface area per bound drug, gamma D, of 48 A2. The data were fitted equally well by an alternate model in which binding of the drug is described as a partitioning equilibrium, with factors included for electrostatic effects and surface expansion caused by drug intercalation between the fatty acid chains.

Conformations of dibucaine and tetracaine in small unilamellar phosphatidylcholine vesicles as studied by nuclear Overhauser effects in 1H nuclear magnetic resonance spectroscopy

Conformations of dibucaine and tetracaine in small unilamellar phosphatidylcholine vesicles have been investigated by nuclear Overhauser effects (NOEs) in 1H nuclear magnetic resonance spectroscopy. Two-dimensional NOE and chemical exchange correlated spectroscopy (NOESY) and rotating frame NOE spectroscopy (ROESY) methods have been applied for obtaining the NOEs. In the NOESY spectra, NOEs between protons within the drug were overwhelmed by spin diffusion even at a short mixing time. This observation reduced the usefulness of the NOESY method on the one hand, however, on the other hand it facilitated remarkably in revealing signals due to the drug, hidden in the broad resonances of the membranes. In the ROESY spectra, the spin diffusion phenomena were less effective; accordingly the conformations of the drugs interacting with membranes were determined by the ROESY method. The observed NOE data showed that dibucaine takes more than two conformations and that both dibucaine and tetracaine are present as a dimer in the membranes. Molecular dynamics calculations supported these findings.

Kinetic characterization of the normal and procaine-perturbed reaction cycles of the sarcoplasmic reticulum calcium pump

We investigated the effect of the local anesthetic procaine on the activity of the calcium pump protein of sarcoplasmic reticulum (SR) vesicles. Procaine slowed down the rate of calcium uptake by SR vesicles without enhancing the vesicles' passive permeability. This slowing of the unidirectional pumping rate was reflected by the inhibition of the maximal rate of the transport-coupled Ca(2+)-ATPase activity. The inhibition was dependent on Mg2+ concentration; at optimal (i.e. low) concentrations of magnesium, half-maximal inhibition occurred with procaine concentrations close to 15-20 mM. Inhibition of ATPase was not mediated by a change in the properties of the bulk lipid phase. Procaine moderately reduced the true affinity of ATPase for ATP, whereas equilibrium binding of calcium to ATPase in the absence of ATP was virtually not modified by procaine. In fast-kinetics studies, we explored the various intermediate steps in the ATPase catalytic cycle, in order to determine which of them were targets for inhibition by procaine. We found that procaine slowed down ATPase dephosphorylation, an effect which is at least partly responsible for the observed inhibition of overall ATPase activity. In contrast, procaine accelerated the calcium-induced transconformation of unphosphorylated ATPase in the absence of ATP, and altered neither the rate of the Ca(2+)-dependent phosphorylation of ATPase, nor the rate of the dissociation of Ca2+ from phosphorylated ATPase towards the SR lumen, a critical step, the rate of which was measured by a novel fast-filtration method. These results are discussed with respect to the possible site(s) of binding of this amphiphile on the ATPase, and in relation to the contribution of individual steps in the catalytic cycle to the rate limitation of unperturbed SR ATPase activity.

The effects of density fluctuations on the partitioning of foreign molecules into lipid bilayers: application to anaesthetics and insecticides

An extensive computer-simulation study is performed on a simple but general molecular model recently proposed (Jørgensen et al. (1991) Biochem. Biophys. Acta 1062, 277-238) to describe foreign molecules interacting with lipid bilayers. The model is a multi-state lattice model of the main bilayer transition in which the foreign molecules are assumed to intercalate at interstitial lattice positions. Specific as well as non-specific interactions between the foreign molecules and the lipid acyl chains are considered. Particular attention is paid to the fluctuating properties of the membrane and how the presence of the foreign molecules modulates these fluctuations in the transition region. By means of computer-stimulation techniques, a detailed account is given of the macroscopic as well as microscopic consequences of the fluctuations. The macroscopic consequences of the fluctuations are seen in the thermal anomalies of the specific heat and the passive trans-membrane permeability. Microscopically, the fluctuations manifest themselves in lipid-domain formation in the transition region which implies an effective dynamic membrane heterogeneity. Within the model it is found that certain anaesthetics and insecticides which are characterised by specific interactions with the lipids have a strong effect on the heterogeneity of the membrane inducing regions of locally very high concentration of the foreign molecules. This leads to a broadening of the specific heat peak and a maximum in the membrane/water partition coefficient. These results are in accordance with available experimental data for volatile general anaesthetics like halothane, local anaesthetics like cocain derivatives, and insecticides like lindane.

On the use of deuterium nuclear magnetic resonance as a probe of chain packing in lipid bilayers

The packing of hydrocarbon chains in the bilayers of lamellar (L alpha) phases of soap/water and phospholipid/water mixtures has been studied by deuterium NMR spectroscopy and X-ray diffraction. A universal correlation is shown to exist between the average C-D bond order parameter SCD of hydrocarbon chains and the average area per chain ach, irrespective of the chemical structure of the surfactant (hydrophilic group, number of chains per molecule, and chain length), composition, and temperature. The practical utility of the correlation is illustrated by its application to the characterization of the distribution of various hydrophobic and amphiphilic solutes in bilayers. The distribution of hydrocarbons within a bilayer is shown to depend upon their molecular structure in a manner which highlights the nature of the molecular interactions involved. For example, benzene is shown to be fairly uniformly distributed across the bilayer with an increasing tendency to distribute into the center at high concentrations. In contrast, the more complex hydrocarbon tetradecane preferentially distributes into the center of the bilayer at low concentrations, while at higher concentrations it intercalates between the surfactant chains. Alcohols such as benzyl alcohol, octanol, and decanol all interact similarly with the bilayer in so far as they are pinned to the polar/apolar interface, presumably by involvement of the hydroxyl group in a hydrogen bond. But the response of the surfactant chains to the void volume created in the center of the bilayer is dependent upon the distance of penetration of the alcohol into the bilayer. For benzyl alcohol, the shortest molecule, this void volume is taken up by the disordering of the chains, while for decanol, the longest molecule, it is absorbed by interdigitation of the chains of apposing monolayers. For octanol, the chain interdigitation mechanism is dominant at low concentrations, but there is a transition to chain disordering at high concentrations. Finally, it is shown that the correlation provides a useful test for statistical mechanical models of chain ordering in lipid bilayers.

Studies of conformation and interaction of the cyclohexenone and acetyl group of progesterone with liposomes

The conformations of the A-ring and the 17-acetyl groups of progesterone were examined within liposomes, which were prepared from L-alpha-phosphatidylcholine in the presence or absence of cholesterol in the buffer, using qualitative nuclear magnetic resonance and circular dichroism of the progesterone spectra in the wavelength regions of 260-360 nm. The preferred conformational assignments, in the rotational conformations of the 17-acetyl group and invertible conformations of the cyclohexenone of progesterone were discussed on the basis of the elliptical strength of the Cotton effect and an energy estimation of the preferred conformers. Energetically unstable conformers of the acetyl group and alpha,beta-unsaturated cyclohexenone of progesterone remarkably increased with an increase in the concentration of the liposomes. The liposomes containing 10% cholesterol were similar to the effect of the liposomes lacking cholesterol on the 17-acetyl group and the alpha,beta-unsaturated cyclohexenone but those containing 50% cholesterol showed an increase in the number of energetically stable conformers of the alpha,beta-unsaturated cyclohexenone. The nuclear magnetic resonance signal from liposomes together with the progesterone indicated the existence of the progesterone adjacent to a double bond or ester moiety in the lipid molecule. Therefore, it was apparent that the liposomes and the cholesterol within the liposomes regulated the conformational populations of both the cyclohexone and acetyl groups of the progesterone molecule.

Selective effects of some anesthetics and detergents on lipid peroxidation of mouse heart homogenates

1. The effects of some anesthetics and detergents on the Fe2+/ascorbate-stimulated non-enzymatic lipid peroxidation potential and on the NADPH-dependent enzymatic lipid peroxidation capacity were characterized in mouse heart homogenates. 2. Chlorpromazine turned out to be the most efficient inhibitor, causing a 50% inhibition at a concentration of 0.03 mM in the non-enzymatic assay, and at a concentration of 0.02 mM in the enzymatic assay. 3. Tetracaine was about a 10-times weaker inhibitor with IC50-values of 0.25 mM. High concentration of dibucaine (1 mM) exerted a 60% inhibition in the non-enzymatic assay, but lidocaine and procaine had no prominent effect with the concentrations used. 4. In the non-enzymatic, Fe(2+)-stimulated system, a 50% inhibition was obtained by using SDS, Triton X-100, and deoxycholic acid at concentrations of 0.004, 0.03, and 0.15%, respectively. 5. In the NADPH-dependent enzymatic lipid peroxidation system, corresponding concentrations were 0.02, 0.04 and 0.1%. Deoxycholate and Triton X-100 even stimulated (10-20%) the enzymatic lipid peroxidation at the lowest concentrations (0.005-0.01%). Saponin was the least effective of these detergents. 6. It is suggested that anesthetics and detergents induce structural rearrangements in the myocardiac membranes which result in the unavailability of phospholipid substrates to lipid peroxidation.

The local anaesthetic tetracaine as a quencher of perylene fluorescence in micelles

At neutral pH the local anaesthetic tetracaine hydrochloride quenches the fluorescence of the lipophilic dye perylene incorporated into non-ionic micelles. The process follows the Stern-Volmer equation, suggesting that quenching occurs through encounter of fluorophore and quencher. As the pH is lowered from 5 to 1, the apparent quenching constant decreases sigmoidally, the midpoint of the curve being at pH 2.3, close to the pK value characterizing the ionization of the anaesthetic aromatic butylamino group. Quenching is completely reversed below pH 1. These results show that the ability of tetracaine to quench the fluorescence of perylene incorporated into micelles depends on the absence of charge on its aromatic amine. Quenching was also studied in homogeneous dioxane-water solution. In this system the quenching constant also decreases sigmoidally as the pH is lowered. The infection point of the curve is nearly coincident with the pK of tetracaine butylamino group in the same partially non-aqueous medium. Protonation of this group induces 60% reversal of the quenching, suggesting that the main mechanism of fluorescence extinction could be the electron transfer from unprotonated tetracaine aromatic amine to perylene in the excited state. However, an additional process which remains operative even when such an amino group is positively charged must also be involved. It can be concluded that the complete reversal of tetracaine quenching of perylene fluorescence in micelles induced by low pH is due to the inability of the anaesthetic to become partitioned into micelles upon protonation of its aromatic amine. In contrast, at neutral pH the local anaesthetic is able to reach the micelle non-polar core where perylene is located. This is consistent with the models, suggesting that the membrane-bound tetracaine assumes a rod-like configuration parallel to the surface normal with the aromatic butylamino group located into a highly hydrophobic region.

Dibucaine interaction with phospholipid vesicles. A resonance energy-transfer study

Resonance energy transfer between a local anaesthetic, dibucaine (donor) and a set of functionalized probes [n-(9-anthroyloxy)stearic acids, n = 2, 3, 6, 7, 9 and 12 and 16-(9-anthroyloxy)palmitic acid] (acceptors) was found to be an efficient process with a critical radius of transfer Ro = 2.1 nm, this interaction being used to locate the drug in a model membrane system, small unilamellar vesicles of dipalmitoylglycerophosphocholine, both above and below the temperature of the gel-to-the-liquid-crystal transition of the phospholipid. From the sequence of relative quenching efficiencies of dibucaine fluorescence upon incorporation of the probes, it was concluded that the drug intercalates in the membrane near the glycerol backbone of the lipid. In addition, it was found that dibucaine location is not significantly affected upon crossing the phase-transition temperature of the phospholipid. Dibucaine photophysics was also studied and the short lifetime of the neutral form of the anaesthetic with respect to that of the monoprotonated species was attributed to an intramolecular charge-transfer interaction. From the study of its partition coefficient between the membrane and the aqueous phase, it was concluded that the only significant species present in the membrane is the charged one.

Modulation of the Ca2+, Mg2(+)-ATPase activity of synaptosomal plasma membrane by the local anesthetics dibucaine and lidocaine

It has been previously shown that local anesthetics inhibit the total Ca2+, Mg2(+)-ATPase activity of synaptosomal plasma membranes. We have carried out kinetic studies to quantify the effects of these drugs on the different Ca2(+)-dependent and Mg2(+)-dependent ATPase activities of these membranes. As a result we have found that this inhibition is not altered by washing the membranes with EDTA or EGTA. We have also found that the Ca2(+)-dependent ATPase activity is not significantly inhibited in the concentration range of these local anesthetics and under the experimental conditions used in this study. The inhibition of the Mg2(+)-dependent ATPase activities of these membranes was found to be of a noncompetitive type with respect to the substrate ATP-Mg2+, did not significantly shift the Ca2+ dependence of the Ca2+, Mg2(+)-ATPase activity, and occurred in a concentration range of local anesthetics that does not significantly alter the order parameter (fluidity) of these membranes. Modulation of this activity by the changes of the membrane potential that are associated with the adsorption of local anesthetics on the synaptosomal plasma membrane is unlikely, on the basis of the weak effect of membrane potential changes on the Ca2+,Mg2(+)-ATPase activity. It is suggested that the local anesthetics lidocaine and dibucaine inhibit the Ca2+, Mg2(+)-ATPase of the synaptosomal plasma membrane by disruption of the lipid annulus.

Local anesthetic-divalent cation binding center interaction

This communication explicitly considers the possibility that local anesthetics interact with divalent cation binding centers, such as chlortetracycline, quin 2, ethyleneglycol bis (B-aminoethyl ether)-N-N,N',N'-tetraacetic acid (EGTA), ethylenediamine tetraacetic acid (EDTA) and ATP. Alterations of local anesthetic fluorescence spectra have been found in the presence of EGTA, EDTA and ATP. On the other hand, the fluorescence of chlortetracycline is enhanced and that of quin 2 is quenched by local anesthetics. The spectrofluorometric evidence presented in this paper clearly indicates that local anesthetics and these divalent cation chelators interact in solution. The fluorescence alterations observed do not derive from parallel changes of their respective absorption spectra, thus, they appear to be due to quantum yield changes. On the basis of the spectral perturbations observed, it is likely that local anesthetics interact with M2+ binding centers via their electron defective aromatic ring. From the association constants obtained in this study, we make an estimation of the free energy of this interaction ranging from -2.8 to -4.0 kcal/mole in the following experimental conditions: pH 7.4 at an ionic strength of 0.1 at 25 degrees. The relevance of these results to define the physical-chemical characteristics of the local anesthetic receptor site is briefly discussed. It is suggested that local anesthetics can bind strongly to Ca2+ and Mg2+ binding centers, provided that a hydrophobic region is located nearby.

Interaction of the local anesthetics dibucaine and tetracaine with sarcoplasmic reticulum membranes. Differential scanning calorimetry and fluorescence studies

The local anesthetics dibucaine and tetracaine inhibit the (Ca2+ + Mg2+)-ATPase from skeletal muscle sarcoplasmic reticulum [DeBoland, A. R., Jilka, R. L., & Martonosi, A. N. (1975) J. Biol. Chem. 250, 7501-7510; Suko, J., Winkler, F., Scharinger, B., & Hellmann, G. (1976) Biochim. Biophys. Acta 443, 571-586]. We have carried out differential scanning calorimetry and fluorescence measurements to study the interaction of these drugs with sarcoplasmic reticulum membranes and with purified (Ca2+ + Mg2+)-ATPase. The temperature range of denaturation of the (Ca2+ + Mg2+)-ATPase in the sarcoplasmic reticulum membrane, determined from our scanning calorimetry experiments, is ca. 45-55 degrees C and for the purified enzyme ca. 40-50 degrees C. Millimolar concentrations of dibucaine and tetracaine, and ethanol at concentrations higher than 1% v/v, lower a few degrees (degrees C) the denaturation temperature of the (Ca2+ + Mg2+)-ATPase. Other local anesthetics reported to have no effect on the ATPase activity, such as lidocaine and procaine, did not significantly alter the differential scanning calorimetry pattern of these membranes up to a concentration of 10 mM. The order parameter of the sarcoplasmic reticulum membranes, calculated from measurements of the polarization of the fluorescence of diphenylhexatriene, is not significantly altered at the local anesthetic concentrations that shift the denaturation temperature of the (Ca2+ + Mg2+)-ATPase.(ABSTRACT TRUNCATED AT 250 WORDS)

Nuclear magnetic resonance spectroscopic studies of the interaction of methyl methacrylate and ethylene dimethacrylate with phosphatidylcholine liposomes as a model for biomembranes

The interaction of methyl methacrylate (MMA) and ethylene dimethacrylate (EDMA) with dipalmitoyl phosphatidylcholine (DPPC) liposomes was studied by 1H and 13C nuclear magnetic resonance spectroscopy (NMR). It was found that the changes in the 1H chemical shift of EDMA were larger than those of MMA when comparing membrane-bound state with free state and that the amount of EDMA incorporated into DPPC liposomes was approximately 74%, whilst MMA was approximately 41%. The major changes in chemical shifts of EDMA appeared to be due to its interaction with the acyl chains of DPPC liposomes.

Biphasic nature of the binding of cationic amphipaths with artificial and biological membranes

We have studied the interaction with liposomes and red cell membrane of various cationic amphipaths, chlorpromazine, methochlorpromazine, imipramine and propranolol. At low concentrations the interaction is a partition of the molecule between the lipid hydrophobic phase and the aqueous medium. The extent of the partition is dependent on the membrane composition or physical properties, on the incubation conditions (pH, ions) and on the amphipath used. After a given amount of amphipath has entered in the membrane, a new type of interaction appears which leads to an apparent saturable association. This association, which probably involves the anionic groups of the membrane components, might result from structural or/and electrical membrane perturbations induced by the presence of drug molecules between the phospholipids. Thus the interaction of a molecule of cationic amphipath with a membrane varies according to the amount of drug present.