Effect of Ca2+ on thermotropic properties of saturated phosphatidylcholine liposomes

Lipoplex-Mediated Deintercalation of Doxorubicin from Calf Thymus DNA-Doxorubicin Complex

In this paper, we report the lipoplex-mediated deintercalation of anticancer drug doxorubicin (DOX) from the DOX-DNA complex under controlled experimental conditions. We used three zwitterionic liposomes, namely, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), and 2-oleoyl-1-palmitoyl-sn-glycero-3-phosphocholine (POPC), which are widely different in their phase transition temperatures to form a lipoplex with calf thymus DNA in the presence of Ca(2+) ions. The study revealed that DPPC being in sol-gel phase was more effective in releasing the drug from the DOX-DNA complex compared with liposomes that remain in liquid crystalline phase (DMPC and POPC). The higher extent of drug release in the case of DPPC liposomes was attributed to the stronger lipoplex formation with DNA as compared with that of other liposomes. Owing to the relatively smaller head group area, the DPPC liposomes in their sol-gel phase can absorb a larger number of Ca(2+) ions and hence offer a strong electrostatic interaction with DNA. This interaction was confirmed by time-resolved anisotropy and circular dichroism spectroscopy. Apart from the electrostatic interaction, the possible hydrophobic interaction between the liposomes and DNA was also taken into account for the observed deintercalation. The successful uptake of drug molecules by liposomes from the drug-DNA complex in the post-release period was also confirmed using confocal laser scanning microscopy (CLSM).

Critical Conditions for Adsorption of Calcium Ions onto Dipolar Lipid Membranes

Dipolar lipid membranes may adsorb multivalent ions. The binding constant depends on the type of lipid and ions. In this paper, we focus on the adsorption of calcium ions onto 1,2-dilauroylphosphatidylcholine (DLPC) membrane. Using small-angle-X-ray scattering we found that at ambient room temperature ca. 0.6 mM CaCl2 is a critical concentration at which calcium ions adsorbed to 30 mg/mL (ca. 48 mM) DLPC membrane. We then determined the structure of the lamellar phases formed at CaCl2 concentrations below and above the critical concentration and characterized the effect of temperature and incubation time on the adsorption process. Our findings suggest that calcium adsorption to DLPC membranes requires an initial nucleation phase.

Organelle-mimicking liposome dissociates G-quadruplexes and facilitates transcription

Important biological reactions involving nucleic acids occur near the surface of membranes such as the nuclear membrane (NM) and rough endoplasmic reticulum (ER); however, the interactions between biomembranes and nucleic acids are poorly understood. We report here that transcription was facilitated in solution with liposomes, which mimic a biomembrane surface, relative to the reaction in a homogeneous aqueous solution when the template was able to form a G-quadruplex. The G-quadruplex is known to be an inhibitor of transcription, but the stability of the G-quadruplex was decreased at the liposome surface because of unfavourable enthalpy. The destabilization of the G-quadruplex was greater at the surface of NM- and ER-mimicking liposomes than at the surfaces of liposomes designed to mimic other organelles. Thermodynamic analyses revealed that the G-rich oligonucleotides adopted an extended structure at the liposome surface, whereas in solution the compact G-quadruplex was formed. Our data suggest that changes in structure and stability of nucleic acids regulate biological reactions at membrane surfaces.

Interactions of tamoxifen with distearoyl phosphatidylcholine multilamellar vesicles: FTIR and DSC studies

Interactions of a non-steroidal antiestrogen drug, tamoxifen (TAM), with distearoyl-sn-glycero-3-phosphatidylcholine (DSPC) multilamellar liposomes (MLVs) were investigated as a function of drug concentration (1-15 mol%) by using two noninvasive techniques, namely Fourier transform infrared (FTIR) spectroscopy and differential scanning calorimetry (DSC). FTIR spectroscopy results show that increasing TAM concentrations (except 1 mol%) increased the wavenumbers of the CH2 stretching modes, implying an disordering effect for DSPC MLVs both in the gel and liquid crystalline phases. The bandwidth values of the CH2 stretchings except for 1 mol% increased when TAM concentrations increased for DSPC liposomes, indicating an increase in the dynamics of liposomes. The CO stretching and PO2- antisymmetric double bond stretching bands were analyzed to study interactions of TAM with head groups of lipids. As the concentrations of TAM increased, dehydration occurred around these functional groups in the polar part of the lipids. The DSC studies on thermal properties of DSPC lipids indicate that TAM eliminated the pre transition, shifted the main phase transition to lower temperatures and broadened the phase transition curve of the liposomes.

Formation of divalent ion mediated anionic disc bicelle-DNA complexes

Disc-shaped bicelles are formed by mixing long-chain lipids with short-chain lipids at suitable molar ratios and they have a relatively uniform size, typically around a few tens of nanometers in diameter. Different from the typically formulated cationic or anionic liposome–DNA complexes, which are used as nonviral vectors for improving the transfection efficiency of gene therapy, a novel way of packing the DNA can be developed by using the much smaller disc-like bicelles. We demonstrate that anionic lipid bicelle-ion–DNA (AB–DNA) complexes can be formed with the help of divalent ions. Multi-stacked AB–DNA complexes can be formed with diameters of around 50–100 nm and lengths of around 50–150 nm as revealed by TEM. Using the anionic lipid–DNA complexes has the advantage of lower cytotoxicity than using cationic lipids. The interaction of DNA with anionic bicelles was investigated by SAXS. It was found that the anionic bicelle could not form stable complexes with DNA at low calcium ion concentrations, such as 1 mM. The AB–DNA complexes can be formed in the investigated range of 10 mM to 100 mM calcium ion concentrations. However, for an equal anionic lipid charge and DNA charge system, an ion-membrane phase (multilamellar vesicles) would gradually appear as the calcium ion concentration is increased above a critical concentration. It indicates that DNA could be packed closer at above the critical divalent ion concentration. If more DNA is added to such a two-phase coexistence system (originally with the total anionic lipid charge equal to that of DNA), the ion-membrane phase could be transformed into the AB–DNA complexes. As a result, more DNA can be packed in the form of AB–DNA complexes at above the critical calcium ion concentration.

Thermosensitive liposomes for localized delivery and triggered release of chemotherapy

Liposomes are a promising class of nanomedicine with the potential to provide site-specific chemotherapy, thus improving the quality of cancer patient care. First-generation liposomes have emerged as one of the first nanomedicines used clinically for localized delivery of chemotherapy. Second-generation liposomes, i.e. stimuli-responsive liposomes, have the potential to not only provide site-specific chemotherapy, but also triggered drug release and thus greater spatial and temporal control of therapy. Temperature-sensitive liposomes are an especially attractive option, as tumors can be heated in a controlled and predictable manner with external energy sources. Traditional thermosensitive liposomes are composed of lipids that undergo a gel-to-liquid phase transition at several degrees above physiological temperature. More recently, temperature-sensitization of liposomes has been demonstrated with the use of lysolipids and synthetic temperature-sensitive polymers. The design, drug release behavior, and clinical potential of various temperature-sensitive liposomes, as well as the various heating modalities used to trigger release, are discussed in this review.

The structure of ions and zwitterionic lipids regulates the charge of dipolar membranes

In pure water, zwitterionic lipids form lamellar phases with an equilibrium water gap on the order of 2 to 3 nm as a result of the dominating van der Waals attraction between dipolar bilayers. Monovalent ions can swell those neutral lamellae by a small amount. Divalent ions can adsorb onto dipolar membranes and charge them. Using solution X-ray scattering, we studied how the structure of ions and zwitterionic lipids regulates the charge of dipolar membranes. We found that unlike monovalent ions that weakly interact with all of the examined dipolar membranes, divalent and trivalent ions adsorb onto membranes containing lipids with saturated tails, with an association constant on the order of ∼10 M(-1). One double bond in the lipid tail is sufficient to prevent divalent ion adsorption. We suggest that this behavior is due to the relatively loose packing of lipids with unsaturated tails that increases the area per lipid headgroup, enabling their free rotation. Divalent ion adsorption links two lipids and limits their free rotation. The ion-dipole interaction gained by the adsorption of the ions onto unsaturated membranes is insufficient to compensate for the loss of headgroup free-rotational entropy. The ion-dipole interaction is stronger for cations with a higher valence. Nevertheless, polyamines behave as monovalent ions near dipolar interfaces in the sense that they interact weakly with the membrane surface, whereas in the bulk their behavior is similar to that of multivalent cations. Advanced data analysis and comparison with theory provide insight into the structure and interactions between ion-induced regulated charged interfaces. This study models biologically relevant interactions between cell membranes and various ions and the manner in which the lipid structure governs those interactions. The ability to monitor these interactions creates a tool for probing systems that are more complex and forms the basis for controlling the interactions between dipolar membranes and charged proteins or biopolymers for encapsulation and delivery applications.

Polymersomes containing iron sulfide (FeS) as primordial cell model : for the investigation of energy providing redox reactions

According to Wächtershäuser's "Iron-Sulfur-World" one major requirement for the development of life on the prebiotic Earth is compartmentalization. Vesicles spontaneously formed from amphiphilic components containing a specific set of molecules including sulfide minerals may have lead to the first autotrophic prebiotic units. The iron sulfide minerals may have been formed by geological conversions in the environment of deep-sea volcanos (black smokers), which can be observed even today. Wächtershäuser postulated the evolution of chemical pathways as fundamentals of the origin of life on earth. In contrast to the classical Miller-Urey experiment, depending on external energy sources, the "Iron-Sulfur-World" is based on the catalytic and energy reproducing redox system FeS+H2S-->FeS2+H2. The energy release out of this redox reaction (∆RG°=-38 kJ/mol, pH 0) could be the cause for the subsequent synthesis of complex organic molecules and the precondition for the development of more complex units similar to cells known today. Here we show the possibility for precipitating iron sulfide inside vesicles composed of amphiphilic block-copolymers as a model system for a first prebiotic unit. Our findings could be an indication for a chemoautotrophic FeS based origin of life.

Study of the effect of Na+ and Ca2+ ion concentration on the structure of an asymmetric DPPC/DPPC + DPPS lipid bilayer by molecular dynamics simulation

A molecular dynamics simulation study of the steady and dynamic properties of an asymmetric phospholipid bilayer was carried out in the presence of sodium or calcium ions. The asymmetric lipid bilayer was seen to resemble a cellular membrane of an eukaryotic cell, which was modeled by dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylserine (DPPS), placing the DPPS in one of the two leaflets of the lipid bilayer. From a numerical analysis of the simulated trajectories, information was obtained with atomic resolution for both membrane leaflet concerning the effect of bilayer asymmetry on different properties of the lipid/water interface, such as the translational diffusion coefficient and rotational relaxation time of the water molecules, lipid hydration, and residence time of water around different lipid atoms. In addition, information related to lipid conformation, and lipid-lipid interactions was also analyzed.

Competitive effect of vitamin D2 and Ca2+ on phospholipid model membranes: an FTIR study

The interaction of Ca(2+), with dipalmitoyl phosphatidylcholine (DPPC) model membranes was studied in the presence and absence of vitamin D(2) by using Fourier transform infrared spectroscopy. Addition of vitamin D(2) and/or Ca(2+) into pure DPPC liposomes shifts the phase transition to higher temperature, orders and decreases the dynamics of the acyl chains in both phases and does not induce hydrogen bond formation in the interfacial region. Moreover, the dynamics of the head group of the phospholipid decreases in both phases. The addition of vitamin D(2) into DPPC liposomes containing Ca(2+), decreases the effect of Ca(2+) at all the functional groups under investigation. Similarly, the effect of vitamin D(2) also decreases in the presence of Ca(2+). This behavior is dominant at high Ca(2+) concentrations. Our results show how simultaneous presence of vitamin D(2) and Ca(2+) alter the behavior of each other, which is reflected as a decrease in the interactions between the ions and vitamin D(2) within the membrane.

Adsorption of polyvalent cations to bilayer membranes from negatively charged lipid: estimating the lipid accessibility in the case of complete binding

The effect of trivalent (Gd(3+) and Yb(3+)) and divalent (Be(2+) and Ca(2+)) cations on suspensions of multilamellar liposomes formed from brain PS and DMPS has been studied using microelectrophoresis and DSC techniques, respectively. The zeta potential values have been shown to strongly depend on the total lipid concentration in the suspension. At moderate concentrations of the polyvalent cations, the total cation concentration exceeds the bulk one several times due to adsorption of cations to the liposomes. A modification of the Gouy-Chapman-Stern theory in the case of unknown bulk concentration of the polyvalent cation is presented. An intrinsic association constant for Be(2+) ions was evaluated to be about K(2) approximately 50 M(-1). The algorithm for estimating the concentrations of the accessible (to exogenously added polyvalent cations) lipid-binding sites is described. These values are consistent with the subsurface concentrations of the polyvalent cations, which monotonously increase with the total concentration of the polyvalent cations. The calculated lipid accessibilities are shown to be in accordance with the DSC data.

Phases and phase transitions of the phosphatidylcholines

LIPIDAT (http://www.lipidat.chemistry.ohio-state.edu) is an Internet accessible, computerized relational database providing access to the wealth of information scattered throughout the literature concerning synthetic and biologically derived polar lipid polymorphic and mesomorphic phase behavior and molecular structures. Here, a review of the data subset referring to phosphatidylcholines is presented together with an analysis of these data. This subset represents ca. 60% of all LIPIDAT records. It includes data collected over a 43-year period and consists of 12,208 records obtained from 1573 articles in 106 different journals. An analysis of the data in the subset identifies trends in phosphatidylcholine phase behavior reflecting changes in lipid chain length, unsaturation (number, isomeric type and position of double bonds), asymmetry and branching, type of chain-glycerol linkage (ester, ether, amide), position of chain attachment to the glycerol backbone (1,2- vs. 1,3-) and head group modification. Also included is a summary of the data concerning the effect of pressure, pH, stereochemical purity, and different additives such as salts, saccharides, amino acids and alcohols, on phosphatidylcholine phase behavior. Information on the phase behavior of biologically derived phosphatidylcholines is also presented. This review includes 651 references.

Solute-induced shift of phase transition temperature in Di-saturated PC liposomes: adoption of ripple phase creates osmotic stress

We have examined the calorimetric behavior of large liposomes consisting of symmetric saturated chain phosphatidylcholines. Most notably, for systems made in solutions containing solute (e.g., NaCl, glucose, etc.) there was an additional major endotherm just below the main phase transition temperature. The new endotherm was found to represent a population of lipid whose main phase transition was shifted to lower temperature due to an induced osmotic stress across the membrane. Absent for isoosmotic systems, the osmotic stress was created when the liposome internal volume decreased, a consequence of the Lbeta' (gel) to Pbeta' (rippled) phase transition. That is, rippling of the membrane caused vesicle volume to decrease (> or = 28%) and because the free flow of water outward was restricted by solute, an osmotic gradient was created where none had existed before. The distribution of enthalpy between the new shifted Tm and the expected Tm correlated with the percent of lipid in the outer bilayer and it was concluded that only the outer bilayer sensed the induced stress. Internalized liposome structures were shielded, thus explaining the persistence of the expected Tm in preparations made in solute. The shift in Tm (deltaTm) was discrete and linearly dependent upon lipid chain length for the PC series di-17:0 (deltaTm approximately 1.4 degrees C) through di-20:0 (deltaTm approximately 0.6 degrees C), suggesting a structural change (i.e., lipid packing/orientation) was involved. Although freeze-fracture electron microscopy of stressed and unstressed bilayers revealed no differences in ripple periodicity there were differences in surface features and in vesicle shape. The fact that this phenomenon has gone unnoticed for MLVs is probably due to the fact that these systems are known to exclude solute and thus exist under osmotic compression.

Influence of Ca2+ and ethanol on the aggregation and thermal phase behaviour of L-dihexadecylphosphatidylcholine liposomes

The influence of Ca2+ and ethanol on vesicle aggregation and thermal phase behaviour of the diether lipid 1,2-dihexadecylphosphatidylcholine (DHPC) was studied by light absorbance and DSC. At temperatures below the pretransition the ethanol-injected vesicles of L-DHPC were rapidly aggregated by Ca2+. Upon raising the cation concentration a biphasic increase in aggregation saturating at an approximate [Ca2+]/[lipid] ratio of 1.5:1 was observed. Further increase in [Ca2+] up to [Ca2+]/[lipid] stoichiometries exceeding 2.5:1 led to the loss of aggregation. Removal of ethanol by dialysis abolished Ca(2+)-induced aggregation. Ethanol-injected vesicles of the ester-linked L-dipalmitoylphosphatidylcholine (L-DPPC) or the racemic DL-DHPC were not aggregated by Ca2+ thus indicating the importance of the absence of ester carbonyls as well as the stereochemical configuration of the lipid in determining the mode of interaction of DHPC with Ca2+. Differential scanning calorimetry of multilamellar liposomes of L-DHPC showed an increase by 8 degrees in the pretransition temperature Tp in the presence of 250 mM ethanol. Both with and without ethanol, increasing concentrations of Ca2+ corresponding to [Ca2+]/[lipid] ratios of 1:1 to 20:1 caused a gradual decrease in Tp and finally the disappearance of the pretransition. Concomitantly a slight elevation in Tm occurred. No principal differences were observed in the thermal phase behaviour of the L-isomer and racemic DL-DHPC.

Lipid-alginate interactions render changes in phospholipid bilayer permeability

Lipid vesicles, e.g. liposomes, generally release their contents in a continuous manner. However, when these vesicles are entrapped in Ca-alginate and coated with poly(L-lysine), they release their contents in an unusual fashion, in 'bursts'. Molecular-level studies indicated that lipid-alginate interactions are responsible for changes in the barrier properties of lipid vesicles. Differential scanning calorimetry revealed that exposure of liposomes to alginate resulted in a 4-fold reduction in the phase transition enthalpy, with no change in the melting temperature. Size-exclusion chromatography of liposomes-in-alginate gave an additional liposomal peak with a smaller elution volume. These studies suggested that alginate is inserted into the lipid bilayer of vesicles. Lipid-alginate interactions were highly dependent on phospholipid head group charge and the phase transition temperature of the phospholipid. Based on these interactions, a mechanism to explain the 'burst' from these entrapped liposomes is suggested.

Pulse NMR study of the interaction of calcium ion with dipalmitoylphosphatidylcholine lamellae

Molecular motion of dipalmitoylphosphatidylcholine (DPPC)/CaCl2 lamellae in a gel phase was studied by pulse NMR. Proton 1/T1 for DPPC in a gel phase showed that the rate of reorientation about the long axis of the lipid molecule decreased gradually from 0 to 500 mM CaCl2. At 10-50 mM CaCl2 the correlation time reached the value of the inverse Larmor frequency (approx. 2.6 ns). A proton NMR absorption spectrum and a spin-pair-dipolar-echo (SPDE) decay showed that the second moment in the hydrocarbon chain region decreased below about 1 mM CaCl2 and increased from 1 to 500 mM CaCl2. The second moment in the polar head group increased gradually with an increase in the CaCl2 concentration. The increase in the second moment at the high CaCl2 concentrations was attributed to an increase in the order parameters of the segments both in the polar head group and in the hydrocarbon chain region. At the lower CaCl2 concentrations, however, calcium ion possibly induced disorder in the lamellae which led to a decrease in the order parameter in the hydrocarbon chain region.

Calcium ion influence on thermotropic behaviour of dipalmitoylphosphatidylcholine-vitamin D3 systems

An analysis of Ca2+ influence on the thermotropic behaviour of different phosphatidylcholine-vitamin D3 mixtures was carried out by differential scanning calorimetry technique (DSC). A competitive effect between Ca2+ and vitamin D3 on the membrane fluidity was detected. The observed shifts of the gel-liquid crystal transition temperature were correlated with the mole fraction of vitamin D3 dissolved within the lipid bilayer as well as with the Ca2+ concentration in the surrounding medium. These shifts were rationalized on the ground of a simple microscopic model through the calculation of the internal pressure exerted by the adsorbed Ca2+ on the lipid matrix by the Clapeyron equation. The experimental results and the obtained equations accord with each other and support the idea of micro-domain formation richer in one lipid component. The extent of such lateral phase separation of the lipid components seems to be favoured by the adsorption of Ca2+ at the membrane-water interface.

Thiocyanate and bromide ions influence the bilayer structural parameters of phosphatidylcholine bilayers

The influence of monovalent cations and anions on the structural parameters of dipalmitoylphosphatidylcholine (DPPC) bilayers was examined at 25 degrees C using X-ray diffraction. It was shown that monovalent salts, in general, have little effect on lipid packing within the bilayer. However, fully hydrated DPPC bilayers in 1 M KSCN pack in an interdigitated acyl chain phase. This is the first observation of an ion-induced interdigitated bilayer phase in a zwitterionic lipid. In addition, gel state DPPC bilayers in 1 M KBr imbibe approx. 10 A more solvent than bilayers in water. The influence of these same salts on the phase transitions of DPPC bilayers was also examined using high-resolution differential scanning calorimetry. These results are discussed in terms of ion-induced changes in solvent and solvent/bilayer structure.

The influence of ion species on phosphatidylcholine bilayer structure and packing

The effects of various monovalent cations and anions on the bilayer packing and structure of dipalmitoylphosphatidylcholine were studied using X-ray diffraction and differential scanning calorimetry. It was observed from the X-ray diffraction studies that monovalent salts, in general, have no effect on bilayer packing. The results of DSC studies on metal chloride systems are consistent with the interpretation that cations in general and Li+ in particular bind to DPPC bilayers. The effect of potassium salts on pre- and main-transition temperatures suggest that anions, such as Acetate-, also significantly bind to DPPC head groups.

Interaction of calcium and neomycin with anionic phospholipid-lecithin liposomes. A differential scanning calorimetry study

The interactions of calcium and neomycin with liposomes of various anionic phospholipids plus lecithin were studied by differential scanning calorimetry. Phosphatidylinositol bisphosphate differed from other acidic phospholipids in its interactions with both calcium and neomycin. Calcium, at concentrations as low as 1 mM, induced the appearance of a second transition peak in phosphatidylinositol bisphosphate-enriched liposomes only. Neomycin acted antagonistically and precluded this phase separation. In addition, neomycin lowered the phase transition temperature of phosphatidylinositol bisphosphate-lecithin liposomes while it raised the transition temperature of all other anionic phospholipid-lecithin liposomes tested. This fluidizing effect of neomycin and the antagonism to calcium may induce critical alterations of properties of biological membranes. The study supports and extends our previous findings and conclusions that phosphatidylinositol bisphosphate may play a crucial role in the expression of aminoglycoside toxicity.

Modulatory effects of different temperatures and Ca2+ concentrations on gangliosides and phospholipids in monolayers at air/water interfaces and their possible functional role

Gangliosides are neuraminic acid-containing glycolipids preferently localized in nervous membranes and showing physicochemical peculiarities, e.g., drastically changing amphiphilic properties by Ca2+ binding. On account of this they are favorite compounds to act as modulators of membraneous organization and functions during synaptic transmission. Lipid monolayers are suitable experimental systems for the study of the surface behavior of amphipatic molecules and therefore are useful to interpret membraneous organization. The surface pressure/area isotherms of monolayers of different individual gangliosides (GM1, GD1a, GD1b, GT1b) of an artificial reconstituted and a natural ganglioside mixture from bovine brain and of ganglioside mixtures from different brain parts of summer- and winter-adapted dsungarian hamsters were compared at three temperatures (11, 20, and 37 degrees C) with egg phosphatidylcholine (PC) and phosphatidylserine (PS) monolayers. The monolayers were formed in a Teflon trough on a triethanolamine/HCl-buffered (pH 7.4) subphase, in some cases containing different amounts of CaCl2. The surface pressure/area isotherms of ganglioside monolayers, in contrast to phospholipids, generally showed slowly rising slopes, with transitions from the liquid-expanded to the liquid-condensed state at a surface pressure of 20-30 mN/m. Ganglioside monolayers, in particular from GD1a or GT1b versus GD1b or from mixtures from summer- versus winter-adapted hamster brain, were differently affected by temperature and/or by Ca2+. PS monolayers were slightly condensed only by Ca2+. PC monolayers, however, were influenced neither by temperature nor by Ca2+. In mixed monolayers of the unpolar natural lipid cholesterol (Ch) and the disialoganglioside GD1a, intermolecular interactions were indicated. Ganglioside monolayers, in contrast to phospholipids, were shown to be easily modulated by temperature and/or Ca2+ ions, thus enabling gangliosides to act as possible membrane modulators, e.g., during synaptic transmission. In particular, the differences concerning the influences of temperature and/or Ca2+ on the surface behavior of ganglioside mixtures from the brain of summer- compared with winter-adapted hamsters are correlated with other physiologically relevant data.

Perturbations of liver plasma membranes induced by Ca2+ are detected using a fatty acid spin label and adenylate cyclase as membrane probes

Ca2+ decreased the lipid fluidity of rat liver plasma membranes labeled with 5-nitroxide stearate, I(12,3), as indicated by the order parameter (S). These effects form a reversible, saturable process with an association constant of 1 x 10(3) M-1. Arrhenius-type plots of S indicated that the lipid phase separation, present in the external leaflet of native membranes between 28 and 19 degrees C, is perturbed by mM Ca2+ such that the high temperature onset is elevated to 32-34 degrees C. Fluoride-stimulated adenylate cyclase was similarly inhibited by Ca2+ (ID50 = 1 mM) for the enzyme in membrane-bound or solubilized states. The glucagon-stimulated activity was more sensitive to Ca2+ inhibition with an ID50 of 0.2 mM. These inhibitory effects are due neither to perturbations of glucagon binding to its receptor nor to fluidity changes, but are instead attributed to direct Ca2+-enzyme interactions. Such binding desensitizes the enzyme to fluidity alterations induced by temperature elevation or benzyl alcohol addition. With Ca2+, Arrhenius plots of glucagon-stimulated activity indicated breaks at 32 and 16 degrees C, whereas those of fluoride-stimulated activity showed one break at 17 degrees C. Without Ca2+, Arrhenius plots exhibited one break at 28 degrees C for glucagon-stimulated activity, whereas fluoride-stimulated plots were linear. We propose that Ca2+ achieves these effects through asymmetric perturbations of the membrane lipid structure.