Membrane permeabilization induced by sphingosine: effect of negatively charged lipids

Sphingosine [(2S, 3R, 4E)-2-amino-4-octadecen-1, 3-diol] is the most common sphingoid long chain base in sphingolipids. It is the precursor of important cell signaling molecules, such as ceramides. In the last decade it has been shown to act itself as a potent metabolic signaling molecule, by activating a number of protein kinases. Moreover, sphingosine has been found to permeabilize phospholipid bilayers, giving rise to vesicle leakage. The present contribution intends to analyze the mechanism by which this bioactive lipid induces vesicle contents release, and the effect of negatively charged bilayers in the release process. Fluorescence lifetime measurements and confocal fluorescence microscopy have been applied to observe the mechanism of sphingosine efflux from large and giant unilamellar vesicles; a graded-release efflux has been detected. Additionally, stopped-flow measurements have shown that the rate of vesicle permeabilization increases with sphingosine concentration. Because at the physiological pH sphingosine has a net positive charge, its interaction with negatively charged phospholipids (e.g., bilayers containing phosphatidic acid together with sphingomyelins, phosphatidylethanolamine, and cholesterol) gives rise to a release of vesicular contents, faster than with electrically neutral bilayers. Furthermore, phosphorous 31-NMR and x-ray data show the capacity of sphingosine to facilitate the formation of nonbilayer (cubic phase) intermediates in negatively charged membranes. The data might explain the pathogenesis of Niemann-Pick type C1 disease.

Correlation between fluorescence and structure in the orange-emitting GFP-like protein, monomeric Kusabira Orange

The mKO is the monomeric version of Kusabira Orange, a GFP-like protein emitting bright orange fluorescence at 559 nm. This protein shows the characteristic β-barrel motif typical of the fluorescent protein family which it belongs to, similar spectral properties to the tetrameric form and an exceptional photo-stability to pH changes. Here, we demonstrate that mKO in solution at physiological pH exhibits a secondary structure analogue to that of the crystal. Moreover, we describe the thermal unfolding, revealing an outstanding structural stability with a denaturation temperature close to 90 °C and identifying the existence of a thermodynamic intermediate. The denaturation process of mKO results to be absolutely irreversible because of the complete lost of the native structure and the consequent aggregation, while the presence of the intermediate state is most likely due to coexistence of two different species of mKO, with protonated and deprotonated chromophore respectively, that affects the fluorescence properties and the structural stability of the protein.

Activation of protein kinase C alpha by lipid mixtures containing different proportions of diacylglycerols

Lipid activation of protein kinase C alpha (PKC alpha) was studied using a model mixture containing POPC/POPS (molar ratio 4:1) and different proportions of either DPG or POG. The lipid mixtures containing DPG were physically characterized by using different physical techniques, and a phase diagram was constructed by keeping a constant POPC/POPS molar ratio of 4:1 and changing the concentration of 1,2-DPG. The phase diagram displayed three regions delimited by two compounds: compound 1 (CO(1)) with 35 mol % of 1,2-DPG and compound 2 (CO(2)) with 65 mol % of 1,2-DPG. PKC alpha activity was assayed at increasing concentrations of 1,2-DPG, maximum activity being reached at 30 mol % 1,2-DPG, which decreased at higher concentrations. Maximum activity occurred, then, at concentrations of 1,2-DPG which corresponded to the transition from region 1 to region 2 of the phase diagram. It was interesting that this protein was maximally bound to the membrane at all DPG concentrations. Similar results were observed when the enzyme was activated by POG, when a maximum was reached at about 10 mol %. This remained practically constant up to 50 mol %, about which it decreased, the binding level remaining maximal and constant at all POG concentrations. The fact that in the assay conditions used maximal binding was already reached even in the absence of diacylglycerol was attributed to the interaction of the C2 domain with the POPS present in the membrane through the Ca(2+) ions also present. To confirm this, the isolated C2 domain was used, and it was also found to be maximally bound at all DPG concentrations and even in its absence. Since the intriguing interaction patterns observed seemed to be due then to the C1 domain, the PKC alpha mutant D246/248N was used. This mutant has a decreased Ca(2+)-binding capacity through the C2 domain and was not activated nor bound to membranes by increasing concentrations of DPG. However, POG was able to activate the mutant, which showed a similar dependence on POG concentration with respect to activity and binding to membranes. These data underline the importance of unsaturation in one of the fatty acyl chains of the diacylglycerol.

Correlation between the effect of the anti-neoplastic ether lipid 1-O-octadecyl-2-O-methyl-glycero-3-phosphocholine on the membrane and the activity of protein kinase Calpha

The antineoplastic ether phospholipid 1-O-octadecyl-2-O-methyl-sn-glycero-3-phophocholine (ET-18-OCH3) was incorporated into dimyristoylglycerophosphocholine (Myr2Gro-PCho)/dimyristoylglycerophosphoserine (Myr2Gro-PSer) (4 : 1 molar ratio) mixtures. Electron microscopy showed that the addition of ET-18-OCH3 reduced the size of the vesicles. Small vesicles could be detected even at 60 mol% ET-18-OCH3. Sedimentation studies showed the increasing presence of phospholipids in the supernatant, while turbidity measurements indicated a decrease in absorbance as the ET-18-OCH3 concentration was increased. These findings may be explained by the formation of small vesicles and/or mixed micelles. Infrared spectroscopy showed that at 60 mol% the fluidity of the membrane was considerably increased at temperatures below the phase transition, with only a small increase in the proportion of gauche isomers after the gel-to-fluid phase transition of this sample. On the other hand, protein kinase Calpha (PKCalpha) activity progressively decreased when ET-18-OCH3 was incorporated into multilamellar vesicles, reaching a minimum value at 20 mol%, this inhibition being attributed to the modification of the membrane produced by a cone-shaped molecule. At higher concentrations, however, ET-18-OCH3 activated the enzyme with a maximum being attained at 50 mol%. This activation being attributed to the formation of small vesicles and/or micelles. At still higher concentrations of ET-18-OCH3 the enzyme was once again inhibited, inhibition being almost complete at 80 mol%. When PKC was assayed using large unilamellar vesicles a slight activation was observed at very low ET-18-OCH3 concentrations.

Identification of the phosphatidylserine binding site in the C2 domain that is important for PKC alpha activation and in vivo cell localization

The C2 domain of classical PKCs binds to membranes through Ca(2+) bridging to phosphatidylserine as recently observed through X-ray diffraction of the isolated domain. Additionally, it has been proposed that N189, T251, R216, and R249A interact directly with phosphatidylserine [Verdaguer, N., et al. (1999) EMBO J. 18, 6329-6338]. When these four residues were mutated to Ala to determine their role in PKC binding to phospholipid membranes, PKC activation, and in its in vivo localization, the results revealed that they were very important for the activation of full-length PKCalpha. N189, in particular, was involved in the activation of the enzyme after its interaction with PS, since its mutation to Ala did not decrease the level of membrane binding but did prevent full enzyme activation. On the other hand, mutations R216A, R249A, and T251A affected both membrane binding and enzyme activation, although T251A had the most drastic effect, suggesting that the protein interactions with the carbonyl groups of the phospholipid are also a key event in the activation process. Taken together, these results show that the four residues located near the calcium binding site are critical in phosphatidylserine-dependent PKCalpha activation, in which N189 plays an important role, triggering the enzyme activation probably by interacting with neighboring residues of the protein when lipid binding occurs. Furthermore, these results provide strong evidence for better defining one of the two phosphatidylserine isomer models proposed in the previous crystallographic report.

Conformation of the C-terminal domain of the pro-apoptotic protein Bax and mutants and its interaction with membranes

The C-terminal domain of the pro-apoptotic protein Bax is a hydrophobic stretch which, it has been predicted, anchors this protein to the outer mitochondrial membrane when apoptosis is induced in the cell. A 21mer peptide imitating this domain has been synthesized together with two mutants, one with a S184 substituted by K and the other with the S184 deleted. When their structures were studied by infrared spectroscopy, it was seen that the three peptides formed aggregates both in solution and within lipid membranes, and that the peptide changed its secondary structure as a consequence of these two mutations. It was also observed that the wild-type peptide and the two mutants became membrane-integral molecules and changed their conformation when they were incorporated into model membranes with the same composition as the outer mitochondrial membrane. With the peptides incorporated in the membranes the location of W188 was studied by fluorescence quenching using the water soluble quencher acrylamide and different doxyl-PC located in the membrane, this residue being found at different membrane depths in each of the three peptides. The fact that the three peptides were able to perturb the motion of the fluorescent probe diphenylhexatriene confirmed their insertion in the membrane. However, whereas the wild type and the DeltaS184 mutant peptides were very efficient in releasing encapsulated carboxyfluorescein from liposomes, the mutant S184K was less efficient. Taken together, these results showed that the mutation tested changed the conformation of the C-terminal domain of Bax and the positions that they adopted when inserted in membranes, confirming the importance of S184 determining the conformation of this domain. At the same time, these results confirmed that the C-terminal domain of Bax participates in disrupting the barrier properties of biomembranes.

Structural characterization of the C2 domain of novel protein kinase Cepsilon

Infrared spectroscopy (IR) and differential scanning calorimetry (DSC) were used to study the biophysical properties of the PKCepsilon-C2 domain, a C2 domain that possess special characteristics as it binds to acidic phospholipids in a Ca2+-independent manner and no structural information about it is available to date. When the secondary structure was determined by IR spectroscopy in H2O and D2O buffers, beta sheet was seen to be the major structural component. Spectroscopic studies of the thermal denaturation in D2O showed a broadening in the amide I' band starting at 45 degrees C. Curve fitting analysis of the spectra demonstrated that two components appear upon thermal denaturation, one at 1623 cm(-1) which was assigned to aggregation and a second one at 1645 cm(-1), which was assigned to unordered or open loop structures. A lipid binding assay has demonstrated that PKCepsilon-C2 domain has preferential affinity for PIP2 although it exhibits maximal binding activity for phosphatidic acid when 100 mol% of this negatively charged phospholipid was used. Thus, phosphatidic acid containing vesicles were used to characterize the effect of lipid binding on the secondary structure and thermal stability. These experiments showed that the secondary structure did not change upon lipid binding and the thermal stability was very high with no significant changes occurring in the secondary structure after heating. DSC experiments demonstrated that when the C2-protein was scanned alone, it showed a Tm of 49 degrees C and a calorimetric denaturation enthalpy of 144.318 kJ x mol(-1). However, when phoshatidic acid vesicles were included in the mixture, the transition disappeared and further IR experiments demonstrated that the protein structure was not modified under these conditions.

Aggregational behavior of aqueous dispersions of the antifungal lipopeptide iturin A

Iturin A, a lipopeptide isolated from Bacillus subtilis, possesses a strong antifungal activity, and has been devoted to a great deal of attention. Since iturin is an amphiphilic compound with a great propensity to self-associate in solution as well as inside the membrane, the question arises to whether its aggregational behavior is dependent on the concentration of the lipopeptide. In order to test this, the ability of iturin suspensions to encapsulate water-soluble molecules has been examined. Iturin was dispersed at different concentrations above its critical micellar concentration, in a buffer containing the water-soluble dye 5,6-carboxyfluorescein. For iturin A micelles, a Stokes radius of 1.3 nm and an aggregational number of 7 was obtained. The results shown in this work clearly demonstrate that iturin dispersions in water, at concentrations of 0.7, 1.4 and 3 mM, i.e. far above the critical micellar concentration (40 microM), are capable of encapsulating carboxyfluorescein, probably by adopting a type of aggregate different from the micelle. Negative-staining electron microscopy shows the presence of vesicles with an average size of 150 nm. By using (14)C-iturin, it is shown that, at 3 mM concentration, 40 % of the iturin molecules adopt this vesicular state. It is proposed that iturin molecules form a fully interdigitated bilayer, where each hydrocarbon tail span the entire hydrocarbon width of the bilayer, resulting in multilamellar vesicles capable of encapsulating an aqueous compartment. The possible implications of these results to the membrane destabilizing effect of iturin A, are discussed according to the dynamic cone-shape of the iturin molecule.

The C2 domain of protein kinase calpha is directly involved in the diacylglycerol-dependent binding of the C1 domain to the membrane

Protein kinase Calpha (PKCalpha), which is known to be critical for the control of many cellular processes, was submitted to site-directed mutagenesis in order to test the functionality of several amino acidic residues. Thus, D187, D246 and D248, all of which are located at the Ca(2+) binding site of the C2 domain, were substituted by N. Subcellular fractionation experiments demonstrated that these mutations are important for both Ca(2+)-dependent and diacylglycerol-dependent membrane binding. The mutants are not able to phosphorylate typical PKC substrates, such as histone and myelin basic protein. Furthermore, using increasing concentrations of dioleylglycerol, one of the mutants (D246/248N) was able to recover total activity although the amounts of dioleylglycerol it required were larger than those required by wild type protein. On the other hand, the other mutants (D187N and D187/246/248) only recovered 50% of their activity. These data suggest that there is a relationship between the C1 domain, where dioleylglycerol binds, and the C2 domain, and that this relationship is very important for enzyme activation. These findings led us to propose a mechanism for PKCalpha activation, where C1 and C2 domains cannot be considered independent membrane binding modules.

Study of the secondary structure of the C-terminal domain of the antiapoptotic protein bcl-2 and its interaction with model membranes

Bcl-2 is a protein which inhibits programmed cell death. It is associated to many cell membranes such as mitochondrial outer membrane, endoplasmic reticulum, and nuclear envelope, apparently through a C-terminal hydrophobic domain. We have used infrared spectroscopy to study the secondary structure of a synthetic peptide (a 23mer) with the same sequence as this C-terminal domain (residues 217-239) of Bcl-2. The spectrum of this peptide in D(2)O buffer shows an amide I' band with a maximum at 1622 cm(-1), which clearly indicates its tendency to aggregate in aqueous solvent. However, the peptide incorporated in multilamellar phosphatidylcholine membranes shows a totally different spectrum of the amide I' band, with a maximum at 1655 cm(-)(1), indicating a predominantly alpha-helical structure. Addition of the peptide to unilamellar vesicles destabilized them and released encapsulated carboxyfluorescein. Differential scanning calorimetry of dimyristoylphosphatidylcholine multilamellar vesicles in which the peptide was incorporated revealed that increasing concentrations of the peptide progressively broadened the pretransition and the main transition, as is to be expected for a membrane integral molecule. Fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene in fluid phosphatidylcholine vesicles showed that increasing concentrations of the peptide produced increased polarization values, pointing to an increase in the apparent order of the membrane and indicating that high concentrations of the peptide considerably broaden the phase transition of dimyristoylphosphatidylcholine multilamellar vesicles. Quenching the intrinsic fluorescence of the Tyr-235 of the peptide, by KI, indicated that this aminoacyl residue is highly exposed to aqueous solvent when incorporated in phospholipid vesicles. The results are discussed in terms of their relevance to the proposed topology of insertion of Bcl-2 into biological membranes.

A biophysical study of the interaction of the lipopeptide antibiotic iturin A with aqueous phospholipid bilayers

Iturin A is a lipopeptide extracted from the culture media of Bacillus subtilis which shows a strong antifungal action. The interaction of iturin A with multilamellar vesicles of dimyristoylphosphatidylcholine (DMPC) induced structures which did not sediment during centrifugation. Electron microscopy after negative staining showed that, at 30 mol%, iturin A/DMPC vesicles were visible but smaller than those formed by pure DMPC. Thermograms of DMPC/iturinA obtained after differential scanning calorimetry, at low concentrations of iturin A, were interpreted as indicating the presence of two laterally separated phases, one formed by pure phospholipid and the other by lipopeptide-phospholipid complexes, these two separated phases being already detected even at low concentrations such as 2 mol%. Fluorescence quenching experiments showed that the D-Tyr residue of the lipopeptide was fully accessible to the aqueous medium, indicating that the polar part of iturin A is located outside of the membrane hydrophobic palisade. It was concluded that the membrane barrier properties are likely to be damaged in the area where the lipid complexes are accumulated, due to structural fluctuations, and this may be one of the bases of its biological activity. Iturin-A was also able to greatly destabilize dielaidoylphosphatidylethanolamine (DEPE) membranes in the fluid form, producing a new structure which had a poor correlation in X-ray diffraction, and in 31P NMR spectroscopy gave rise to a spectrum containing a double isotropic signal. Iturin A was shown to induce DEPE to adopt phases other than H(II) inverted hexagonal, underlining that this lipopeptide is capable of modifying the curvature of the membrane, which may also be important in explaining the tendency of iturin A to create small vesicles and which may be another of the bases of its biological activity.

The cancer chemopreventive agent resveratrol is incorporated into model membranes and inhibits protein kinase C alpha activity

Resveratrol is a phytoalexin found in grapes and other foods that cancer chemopreventive and other biological activities have been attributed recently. We report that resveratrol is able to incorporate itself into model membranes in a location that is inaccessible to the fluorescence quencher, acrylamide. Differential scanning calorimetry revealed that resveratrol considerably affected the gel to liquid-crystalline phase transition of multilamellar vesicles made of phosphatidylcholine/phosphatidylserine and increased the temperature at which the fluid lamellar to H(II) inverted hexagonal transition took place in multilamellar vesicles made of 1,2-dielaidoyl-sn-phosphatidylethanolamine. Such a transition totally disappeared at 2.5 mM of resveratrol (resveratrol/lipid molar ratio of 2:1). This effect on 1, 2-dielaidoyl-sn-phosphatidylethanolamine polymorphism was confirmed through (31)P-NMR, which showed that an isotropic peak appeared at high temperature instead of the H(II)-characteristic peak of 42 mM of resveratrol (resveratrol/lipid molar ratio of 1.5:1). Finally, resveratrol inhibited PKCalpha when activated by phosphatidylcholine/phosphatidylserine vesicles with an IC(50) of 30 microM, whereas when the enzyme was activated by Triton X-100 micelles the IC(50) was 300 microM. These results indicate that the inhibition of PKCalpha by resveratrol can be mediated, at least partially, by membrane effects exerted near the lipid-water interface.

Characterization of phenylmaleimide inhibition of the Ca(2+)-ATPase from skeletal-muscle sarcoplasmic reticulum

The Ca(2+)-ATPase from sarcoplasmic reticulum reacts with phenylmaleimide, producing the inhibition of the ATPase activity following a pseudo-first-order kinetic with a rate constant of 19 M(-1) s(-1). Calcium and ATP binding are not altered upon phenylmaleimide inhibition. However, the presence of millimolar calcium, and to a lesser extent magnesium, in the inhibition medium enhances the effect of phenylmaleimide, causing a higher degree of inhibition. Solubilization with C(12)E(8) does not affect the ATPase inhibition, excluding any kind of participation of the lipid bilayer. Phosphorylation with ATP in steady-state conditions as well as phosphorylation with inorganic phosphate in equilibrium conditions were strongly inhibited. Conversely, we have found that the occupancy of the phosphorylation site by ortovanadate fully protects against the inhibitory effect of phenylmaleimide, indicating a conformational transition associated with the phosphorylation reaction.

Ca(2+) bridges the C2 membrane-binding domain of protein kinase Calpha directly to phosphatidylserine

The C2 domain acts as a membrane-targeting module in a diverse group of proteins including classical protein kinase Cs (PKCs), where it plays an essential role in activation via calcium-dependent interactions with phosphatidylserine. The three-dimensional structures of the Ca(2+)-bound forms of the PKCalpha-C2 domain both in the absence and presence of 1, 2-dicaproyl-sn-phosphatidyl-L-serine have now been determined by X-ray crystallography at 2.4 and 2.6 A resolution, respectively. In the structure of the C2 ternary complex, the glycerophosphoserine moiety of the phospholipid adopts a quasi-cyclic conformation, with the phosphoryl group directly coordinated to one of the Ca(2+) ions. Specific recognition of the phosphatidylserine is reinforced by additional hydrogen bonds and hydrophobic interactions with protein residues in the vicinity of the Ca(2+) binding region. The central feature of the PKCalpha-C2 domain structure is an eight-stranded, anti-parallel beta-barrel with a molecular topology and organization of the Ca(2+) binding region closely related to that found in PKCbeta-C2, although only two Ca(2+) ions have been located bound to the PKCalpha-C2 domain. The structural information provided by these results suggests a membrane binding mechanism of the PKCalpha-C2 domain in which calcium ions directly mediate the phosphatidylserine recognition while the calcium binding region 3 might penetrate into the phospholipid bilayer.

Structure of the Alzheimer beta-amyloid peptide (25-35) and its interaction with negatively charged phospholipid vesicles

The secondary structure of amyloid betaAP(25-35) peptide was studied in pure form and in the presence of different phospholipid vesicles, by using Fourier transform infrared spectroscopy (FT-IR). Pure peptide aggregated with time, forming fibrils with beta-structure. Phospholipid vesicles formed by negatively charged phospholipids such as 1,2-dimyristoyl-sn-glycerol-3-phospho-L-serine (Myr2PtdSer), 1,2-dimyristoyl-sn-glycerol-3-phospho-rac-1-glycerol (Myr2PtdGro) and 1,2-dimyristoyl-sn-glycerol 3-phosphate (Myr2PtdH), greatly accelerated the aggregation of the peptide. However, the presence of vesicles formed by the zwitterionic phospholipid, 1, 2-dimyristoyl-sn-glycerol-3-phosphocholine (Myr2PtdCho), slowed down the aggregation process. Differential scanning calorimetry (DSC) measurements showed that the effect of betaAP(25-35) on the gel to crystal liquid phase transition was small at neutral pH for negatively charged phospholipids and practically nil for Myr2PtdCho. In the case of Myr2PtdSer the effect was also zero at pH 9 but the effect was large at pH 3. The effect on Myr2PtdH was not, however, very dependent on pH. These results were fully confirmed by the observation through FT-IR of the change with temperature of the CH2 antisymmetric stretching vibration. The case of Myr2PtdGro was special as this phospholipid presents polymorphism giving solid quasicrystalline phases when it is not sufficiently hydrated, and it is remarkable that betaAP(25-35) was able to induce the formation of crystalline phases in samples prepared through a method which ensure a good hydration of phospholipid. These results show that the interaction of amyloid betaAP(25-35) peptide with phospholipids is based on electrostatic interactions, that these interactions favour the aggregation of the peptides, and that the presence of the aggregates may disturb the lipid-water interphase of the membrane.

Effect of calcium and phosphatidic acid binding on the C2 domain of PKC alpha as studied by Fourier transform infrared spectroscopy

Fourier transform infrared (FTIR) spectroscopy was used to investigate the structural and thermal denaturation of the C2 domain of PKC alpha (PKC-C2) and its complexes with Ca(2+) and phosphatidic acid vesicles. The amide I regions in the original spectra of PKC-C2 in the Ca(2+)-free and Ca(2+)-bound states are both consistent with a predominantly beta-sheet secondary structure below the denaturation temperatures. Spectroscopic studies of the thermal denaturation revealed that for the PKC-C2 domain alone the secondary structure abruptly changed at 50 degrees C. While in the presence of 2 and 12.5 mM Ca(2+), the thermal stability of the protein increased to 60 and 70 degrees C, respectively. Further studies using a mutant lacking two important amino acids involved in Ca(2+) binding (PKC-C2D246/248N) demonstrated that these mutations were inherently more stable to thermal denaturation than the wild-type protein. Phosphatidic acid binding to the PKC-C2 domain was characterized, and the lipid-protein binding became Ca(2+)-independent when 100 mol% phosphatidic acid vesicles were used. The mutant lacking two Ca(2+) binding sites was also able to bind to phosphatidic acid vesicles. The effect of lipid binding on secondary structure and thermal stability was also studied. Beta-sheet was the predominant structure observed in the lipid-bound state, although the percentage represented by this structure in the total area of the amide I band significantly decreased from 60% in the lipid-free state to 47% in the lipid-bound state. This decrease in the beta-sheet component of the lipid-bound complex correlates well with the significant increase observed in the 1644 cm(-1) band which can be assigned to loops and disordered structure. Thermal stability after lipid binding was very high, and no sign of thermal denaturation was observed in the presence of lipids under the conditions that were studied.

Influence of the physical state of the membrane on the enzymatic activity and energy of activation of protein kinase C alpha

The activation of protein kinase C alpha was studied by using a lipid system consisting of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoserine (POPS) (molar ratio 4:1) and different proportions of 1-palmitoyl-2-oleoyl-sn-glycerol (POG). The phase behavior of the lipidic system was characterized by using differential scanning calorimetry and 31P NMR, and a phase diagram was elaborated. The results suggested the formation of two diacylglycerol/phospholipid complexes, one at 15 mol % of POG and the second at 30 mol % of POG. These two complexes would define the three regions of the phase diagram: in the first region (concentrations of POG lower than 15 mol %) there is gel-gel immiscibility at temperatures below that of the phase transition between C1 and pure phospholipid, and a fluid lamellar phase above of the phase transition. In the second region (between 15 and 30 mol % of POG), gel-gel immiscibility between C1 and C2 with fluid-fluid immiscibility was observed, while inverted hexagonal HII and isotropic phases were detected by 31P NMR. In the third region (concentrations of POG higher than 30 mol %), gel-gel immiscibility seemed to occur between C2 and pure POG along with fluid-fluid immiscibility, while an isotropic phase was detected by 31P NMR. When PKC alpha activity was measured, as a function of POG concentration, maximum activity was found at POG concentrations as low as 5-10 mol %; the activity slightly decreased as POG concentration was increased to 45 mol % at 32 degrees C (above Tc) whereas activity did not change with increasing concentrations of POG at 5 degrees C (below Tc). When the activity was studied as a function of temperature, at different POG concentrations, and depicted as Arrhenius plots, it was found that the activity increased with increasing temperatures, showing a discontinuity at a temperature very close to the phase transition of the system and a lower activation energy at the upper slope of the graph, indicating that the physical state of the membrane affected the interaction of PKC alpha with the membrane.

Modulation of polymorphic properties of dielaidoylphosphatidylethanolamine by the antineoplastic ether lipid 1-O-octadecyl-2-O-methyl-glycero-3-phosphocholine

The capacity of the antineoplastic ether lipid 1-O-octadecyl-2-O-methyl-glycero-3-phosphocholine (ET-18-OCH3) to modulate the polymorphic properties of dielaidoylphosphatidylethanolamine has been studied using biophysical techniques. Differential scanning calorimetry showed that ET-18-OCH3 depresses the onset of the Lbeta to Lalpha phase transition, decreasing also DeltaH of the transition. At the same time, the onset of the transition from Lalpha to inverted hexagonal HII phase was gradually increased as the ether lipid concentration was increased, totally disappearing at concentrations higher than 5 mol%. Small-angle X-ray diffraction and 31P-NMR confirmed that ET-18-OCH3 induced that the appearance of the inverted hexagonal HII phase was shifted towards higher temperatures completely disappearing at concentrations higher than 5 mol%. These results were used to elaborate a partial phase diagram and they were discussed as a function of the molecular action of ET-18-OCH3.

The interaction of coenzyme Q with phosphatidylethanolamine membranes

Coenzyme Q (CoQ) is a component of the mitochondrial respiratory chain which carries out additional membrane functions, such as acting as an antioxidant. The location of CoQ in the membrane and the interaction with the phospholipid bilayer is still a subject of debate. The interaction of CoQ in the oxidized (ubiquinone-10) and reduced (ubiquinol-10) state with membrane model systems of 1,2-dielaidoyl-sn-glycero-3-phosphoethanolamine (Ela2Gro-P-Etn) has been studied by means of differential scanning calorimetry (DSC), 31P-nuclear magnetic resonance (31P-NMR) and small angle X-ray diffraction (SAXD). Ubiquinone-10 did not visibly affect the lamellar gel to lamellar liquid-crystalline phase transition of Ela2Gro-P-Etn, but it clearly perturbed the multicomponent lamellar liquid-crystalline to lamellar gel phase transition of the phospholipid. The perturbation of both transitions was more effective in the presence of ubiquinol-10. A location of CoQ forming head to head aggregates in the center of the Ela2Gro-P-Etn bilayer with the polar rings protruding toward the phospholipid acyl chains is suggested. The formation of such aggregates are compatible with the strong hexagonal HII phase promotion ability found for CoQ. This ability was evidenced by the shifting of the lamellar to hexagonal HII phase transition to lower temperatures and by the appearance of the characteristic hexagonal HII 31P-NMR resonance and SAXD pattern at temperatures at which the pure Ela2Gro-P-Etn is still organized in extended bilayer structures. The influence of CoQ on the thermotropic properties and phase behavior of Ela2Gro-P-Etn is discussed in relation to the role of CoQ in the membrane.

A comparative study of the activation of protein kinase C alpha by different diacylglycerol isomers

The lipid activation of protein kinase C alpha (PKC alpha) has been studied by comparing the activation capacity of different 1, 2-diacylglycerols and 1,3-diacylglycerols incorporated into mixed micelles or vesicles. Unsaturated 1,2-diacylglycerols were, in general, more potent activators than saturated ones when 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoserine (POPS)/Triton X-100 mixed micelles and pure POPS vesicles were used. In contrast, these differences were not observed when 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/POPS (4:1, molar ratio) vesicles were used. Diacylglycerols bearing short fatty acyl chains showed a very high activation capacity, however, the capacity was less in mixed micelles. Furthermore, 1, 2-diacylglycerols had a considerably higher activating capacity than 1,3-diacylglycerols in POPS/Triton X-100 mixed micelles and in POPC/POPS vesicles. However, the differences between the two types of diacylglycerols were smaller when pure POPS vesicles were used. Differential scanning calorimetry (DSC) showed that POPC/POPS membrane samples containing diacylglycerols had endothermic transitions in the presence of 200 microM Ca2+ and 5 mM Mg2+. Transitions were not detected when using pure POPS vesicles due to the formation of dehydrated phases as demonstrated by FTIR (Fourier-transform infrared) spectroscopy. PKC alpha binding studies, performed by differential centrifugation in the presence of 200 microM Ca2+ and 5 mM Mg2+, showed that 1,2-sn-dioleoylglycerol (1, 2-DOG) was more effective than 1,3-dioleoylglycerol (1,3-DOG) in promoting binding to POPC/POPS vesicles. However, when pure POPS vesicles were used, PKC alpha was able to bind to membranes containing either 1,2-DOG or 1,3-DOG to the same extent.

Correlation between protein kinase C alpha activity and membrane phase behavior

Lipid activation of protein kinase C alpha (PKC alpha) was studied by using a model mixture containing 1, 2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), 1, 2-dimyristoyl-sn-glycero-3-phosphoserine (DMPS), and 1, 2-dimyristoyl-sn-glycerol (1,2-DMG). This lipid mixture was physically characterized by differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), and 31P-nuclear magnetic resonance (31P-NMR). Based on these techniques, a phase diagram was constructed by keeping a constant DMPC/DMPS molar ratio of 4:1 and changing the concentration of 1,2-DMG. This phase diagram displayed three regions and two compounds: compound 1 (C1), with 45 mol% 1,2-DMG, and compound 2 (C2), with 60 mol% 1,2-DMG. When the phase diagram was elaborated in the presence of Ca2+ and Mg2+, at concentrations similar to those used in the PKC alpha activity assay, the boundaries between the regions changed slightly and C1 had 35 mol% 1,2-DMG. The activity of PKC alpha was studied at several temperatures and at different concentrations of 1,2-DMG, with a maximum of activity reached at 30 mol% 1,2-DMG and lower values at higher concentrations. In the presence of Ca2+ and Mg2+, maximum PKC alpha activity occurred at concentrations of 1,2-DMG that were close to the boundary in the phase diagram between region 1, where compound C1 and the pure phospholipid coexisted in the gel phase, and region 2, where compounds C1 and C2 coexisted. These results suggest that the membrane structure corresponding to a mixture of 1,2-DMG/phospholipid complex and free phospholipid is better able to support the activity of PKC alpha than the 1,2-DMG/phospholipid complex alone.

Determination of the calcium-binding sites of the C2 domain of protein kinase Calpha that are critical for its translocation to the plasma membrane

The C2 domain is a conserved protein module present in various signal-transducing proteins. To investigate the function of the C2 domain of protein kinase Calpha (PKCalpha), we have generated a recombinant glutathione S-transferase-fused C2 domain from rat PKCalpha, PKC-C2. We found that PKC-C2 binds with high affinity (half-maximal binding at 0.6 microM) to lipid vesicles containing the negatively charged phospholipid phosphatidylserine. When expressed into COS and HeLa cells, most of the PKC-C2 was found at the plasma membrane, whereas when the cells were depleted of Ca2+ by incubation with EGTA and ionophore, the C2 domain was localized preferentially in the cytosol. Ca2+ titration was performed in vivo and the critical Ca2+ concentration ranged from 0.1 to 0.32 microM. We also identified, by site-directed mutagenesis, three aspartic residues critical for that Ca2+ interaction, namely Asp-187, Asp-246 and Asp-248. Mutation of these residues to asparagine, to abolish their negative charge, resulted in a domain expressed as the same extension as wild-type protein that could interact in vitro with neither Ca2+ nor phosphatidylserine. Overexpression of these mutants into COS and HeLa cells also showed that they cannot localize at the plasma membrane, as demonstrated by immunofluorescence staining and subcellular fractionation. These results suggest that the Ca2+-binding site might be involved in promoting the interaction of the C2 domain of PKCalpha with the plasma membrane in vivo.

The use of FT-IR for quantitative studies of the apparent pKa of lipid carboxyl groups and the dehydration degree of the phosphate group of phospholipids

Fourier-transform infrared spectroscopy (FT-IR) has been applied to the quantitative study of the dehydration of the phosphatidylserine phosphate group in the presence of Ca2+ exerted by different molecules, such as diacylglycerol, sphingosine and stearylarnine, by using a partial least-squares statistical procedure. By using this method it was observed that diacylglycerol enhanced the dehydration of this PO2- group produced by Ca2+ whereas the amino-bases sphingosine and stearylamine protected the phosphate group from the dehydration produced by Ca2+ due to the very strong electrostatic interaction established. The apparent pKa of lipid carboxyl groups can also be estimated by using FTIR. The method consisted in quantifying the absorbance intensities due to the protonated and the unprotonated forms of the specific group being studied. The pKa of the carboxyl group of [1-13C]-palmitic acid included in dipalmitoylphosphatidylcholine membranes was found to be 8.7, a value much higher than that estimated from a molecular solution of the fatty acid. It was observed using the same method that the pKa of free fatty acids in model stratum corneum lipid mixtures was in the range 6.2-7.3 increasing with the preponderance of oleic acid over palmitic acid. Finally the pKa of the carboxyl group of phosphatidylserine was shifted from 4.6 in the pure phospholipid to 2.1 and 2.2 in the presence of equimolar sphingosine and stearylamine, respectively, as a consequence of electrostatic interactions.

The phase behavior of aqueous dispersions of unsaturated mixtures of diacylglycerols and phospholipids

The phase behavior of mixtures of 1-palmitoyl-2-oleoyl-sn-glycerol (1,2-POG) with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoserine (POPS) was studied by using DSC, small-angle X-ray diffraction and 31P-NMR. The results have been used to construct phase diagrams for both type of mixtures, in the 0-45 degreesC range. It is concluded that 1, 2-POG form complexes in the gel phases with both POPC and POPS. In the case of POPC, two complexes are postulated, the first one at a 1, 2-POG/POPC molar ratio of 40:60, and the second one at 70:30, defining three different regions in the phase diagram. Two eutectic points are proposed to occur: one at a very low 1,2-POG concentration and the other at a 1,2-POG concentration slightly lower than 70%. In the case of the 1,2-POG/POPS mixtures, the pattern was similar, but the first complex was seen to happen at a higher concentration, about 50 mol% of 1,2-POG, whereas the second was found at 80 mol% of 1,2-POG. This indicated a bigger presence of 1,2-POG in the complexes with POPS than with POPC. In the first region of the phase diagram, i.e. at concentrations of 1,2-POG lower than that required for the formation of the first complex, and at temperatures above the phase transition, lamellar phases were seen in all the cases. In region 2 of the phase diagram, i.e. at concentrations where the first and the second complexes coexist, a mixture of lamellar and non-lamellar phases was observed. Finally, at high concentrations of 1,2-POG, non-lamellar phases were detected as predominant, these phases being of an isotropic nature, according to 31P-NMR. An important conclusion of this study is that, using unsaturated lipids, similar to those found in biological membranes, it has been shown that diacylglycerols are found separated in domains, and that this process starts at very low concentrations of diacylglycerols. The formation of separated domains enriched in diacylglycerol is biologically relevant as it will allow them to have important effects on the membrane structure besides the fact that their concentration in the biomembrane is relatively low.

Location of N-cyclohexyl-N'-(4-dimethyl-amino-alpha-naphthyl)carbodiimide- binding site in sarcoplasmic reticulum Ca2+-transporting ATPase

The Ca2+-transporting ATPase has been labeled with N-cyclohexyl-N'-(4-dimethyl-amino-alpha-naphthyl)carbodiimide (NCD-4), a fluorescent carbodiimide which reacts with carboxyl groups of acidic residues. It has been reported that NCD-4 labels a transmembrane portion of the protein at the high-affinity calcium-binding sites. We have determined the depth of the calcium-sensitive probe by quenching the fluorescence by nitroxide-substituted fatty acids with its spin probe located at different carbons of the fatty acid chain (5, 7, 10, 12 and 16-nitroxide derivatives). We have found that all the calcium-sensitive fluorescence is quenched and that the efficiency of quenching decreases as the n-(4,4-dimethyl-3-oxazolinyloxy) (Doxyl) group is deeper in the membrane. We conclude that the NCD-4 label which is involved in the high-affinity calcium-binding site is located near the water/lipid interface. The fluorescence of the NCD-4 bound to that site can be quenched by acrylamide and Cu2+ but not by iodide, probably due to its anionic nature which will be repulsed by the abundance of negative charges of Glu and Asp residues of NCD-4 located at this site. The hydrophobic location of NCD-4 was confirmed by the fact that its fluorescence could be quenched by the spin label 2,2,6,6-tetramethyl-1-piperidine-N-oxyl but not by 4-hydroxy-2,2,6,6-tetramethyl-1-piperidine-N-oxyl which is much less hydrophobic.

Influence of alpha-tocopherol incorporation on Ca(2+)-induced fusion of phosphatidylserine vesicles

The effect of alpha-tocopherol on the Ca(2+)-induced fusion of large unilamellar phosphatidylserine vesicles has been investigated. Mixing of aqueous vesicle contents was followed continually with the terbium-dipicolinic acid (Tb-DPA) assay, while the dissociation of preencapsulated Tb-DPA complex was taken as a measure of the release of vesicle contents. Vesicles consisting of pure phosphatidylserine and phosphatidylserine containing 2, 5, and 10 mol% of alpha-tocopherol were employed at different Ca2+ concentrations. The presence of low amounts of alpha-tocopherol decreased the initial rate of fusion without changing the Ca2+ threshold concentration. The reduction of the initial rate of fusion was proportional to the amount of alpha-tocopherol present in the bilayer. An alpha-tocopherol concentration-dependent decrease of both the initial rate and the final extent of release of vesicle contents was also observed. This effect was more evident as more alpha-tocopherol was incorporated in the bilayer, so that in the presence of 10 mol% of alpha-tocopherol no significant release was observed after 5 min. The stabilization of the vesicular structure exerted by alpha-tocopherol was responsible for the apparent increase of the fluorescence intensity of the Tb-DPA complex at later stages of the process. The results reflect a perturbation of the membrane by low concentrations of alpha-tocopherol which may account for a number of biological effects of this vitamin, not related to its antioxidant role.

Involvement of an arginyl residue in the nucleotide-binding site of Ca(2+)-ATPase from sarcoplasmic reticulum as seen by reaction with phenylglyoxal

1. Chemical modification of the Ca(2+)-ATPase with phenylglyoxal, as a modifier of arginine residues, leads to an almost total loss of the ATPase activity. The presence of nucleotides in the reaction medium protects against the binding of 18 nmol of phenylglyoxal/mg of protein and this reduction in the binding of phenylglyoxal is accompanied by a substantial retention of ATPase activity. The incorporation of phenylglyoxal to the protein alters neither calcium binding nor phosphorylation from inorganic phosphate. Nevertheless the binding of nucleotides is dramatically inhibited and, consequently, so is phosphorylation from ATP. Fluorescein 5'-isothiocyanate labelling of the phenylglyoxal-modified ATPase is not affected but, on the other hand, phenylglyoxal is not able to modify the fluorescein 5'-isothiocyanate-prelabelled ATPase. The way in which ATPase inhibition depends on the presence of phenylglyoxal indicates that this process occurs in a pseudo-first-order reaction. However, the dependence of the apparent first-order rate constant on phenylglyoxal concentration appears to be more complex and an inhibition mechanism of two steps, with phenylglyoxal binding, has to be taken into account. 2. We have found that phenylglyoxal labels both A and B tryptic fragments, but only B fragment labelling is prevented by ATP. The sequencing of peptides from mild acid hydrolysis of phenylglyoxal-labelled ATPase shows that phenylglyoxal is located in the Ala506-Gly595 peptide that is a part of the B fragment. 3. We conclude that phenylglyoxal inactivates the calcium pump in a two-step mechanism in which the second step is irreversible. Phenylglyoxal labels an arginyl residue in the Ala506-Gly595 peptide that can be protected by the binding of ATP to its site.

The interaction of alpha-tocopherol with phosphatidylserine vesicles and calcium

The interaction of alpha-tocopherol with dimyristoylphosphatidylserine (DMPS) has been studied in the presence and in the absence of Ca2+ by using differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FT-IR) and 45Ca2+-binding. In the absence of Ca2+, DSC showed that alpha-tocopherol decreases the temperature of the lamellar gel to lamellar liquid crystalline phase transition as well as it decreases delta H of this transition. Two different peaks were detected at 10 mol% of alpha-tocopherol and probably one of the peaks correspond to pure or nearly pure DMPS and the other to DMPS incorporating most of the alpha-tocopherol. The phase transition was totally abolished at 30 mol% of alpha-tocopherol. In the presence of Ca2+ this L(beta) to L(alpha) phase transition of DMPS was even more perturbed by alpha-tocopherol, so that it was totally abolished by only 7 mol% of alpha-tocopherol, at Ca2+ concentrations which were clearly non-saturating, like those giving DMPS/Ca2+ molar ratio of 4:1 and 10:1. Furthermore, the transition of the DMPS/Ca2+ complex observed at 91.6 degrees C was perturbed by the presence of alpha-tocopherol, indicating a change in the structure of the crystalline complex. The FT-IR analysis of the effect of alpha-tocopherol on DPMS phase transition confirmed the decrease in the phase transition temperature of the phospholipid, and also that alpha-tocopherol increases the number of gauche isomers in the gel state but has no effect in the liquid crystalline state. The binding of 45Ca2+ was also affected by the presence of alpha-tocopherol, so that the number of binding sites was decreased, and this may be interesting for situations in which phosphatidylserine and Ca2+ are simultaneously implicated in biological functions, such as membrane fusion and enzyme activation.

Interaction between alpha-tocopherol and heteroacid phosphatidylcholines with different amounts of unsaturation

Differential scanning calorimetry was used to study the influence of a alpha-tocopherol on the thermotropic properties of model membranes composed by a series of heteroacid phosphatidylcholines with different amount of unsaturation in the sn-2 chain. The effect of alpha-tocopherol on 1,2-distearoylglycerophosphocholine (18:0,18:0), 1-stearoyl-2-oleoylgylcerophosphocholine (18:0,18:1), 1-stearoyl-2-linoleoylglycerophosphocholine (18:0,18:2), 1-stearoyl-2-linolenoylglycerophosphocholine (18:0,18:3), and 1-stearoyl-2-arachidonoylglycerophosphocholine (18:0,20:4) was determined. The addition of alpha-tocopherol perturbed the thermotropic gel to liquid-crystalline phase transition of these phospholipids. alpha-Tocopherol broadened the endotherm, lowered the transition temperature and decreased the associated enthalpy change. Partial phase diagrams showed the presence of fluid immiscibilities giving rise to lateral phase separation of domains containing different amounts of alpha-tocopherol. We suggest that, in these alpha-tocopherol-rich domains, the influence exerted by the vitamin on the phospholipids is strong enough to alter their thermotropic properties such that an additional endotherm appears in the thermogram, a characteristic not observed in homoacid phosphatidylcholines. alpha-Tocopherol caused a concentration-dependent removal of the detectable phase transition in all cases. The magnitude of the influence of alpha-tocopherol on phospholipid was dependent on the degree of unsaturation of the sn-2 acyl chain. These results are explained on the basis of the effect of alpha-tocopherol which will reduce the differences between gel and liquid crystalline states, the magnitude of these differences depending on the type of phospholipid considered, which are probably related to the change of molecular shape of phosphatidylcholines containing a polyunsaturated acyl chain.

Metastability of dimiristoylphosphatidylethanolamine as studied by FT-IR and the effect of alpha-tocopherol

The metastability of dimiristoylphosphatidylethanolamine (DMPE) has been studied by means of Fourier transform infrared spectroscopy (FT-IR), both in the absence and in the presence of alpha-tocopherol. Two different methods of hydration were used to prepare the samples, poorly hydrated and well hydrated, and the results have been compared with anhydrous DMPE. Poorly hydrated DMPE gave place to a high-melting phase formed upon melting from gel to L alpha at approx. 49 degrees C, with a new transition to L alpha at approx. 55 degrees C. However, well hydrated DMPE incubated at 4 degrees C for 49 days gave place to a subgel phase which was transformed by heating into a L beta phase at about 40 degrees C and this into a L alpha phase after further heating at 52 degrees C. The subgel phase was more hydrated and less rigid than the high-melting phase. On the other hand, alpha-tocopherol, when included in poorly hydrated DMPE, stabilized a high-melting phase, which was transformed by heating, directly into a L alpha. However, when a sample of DMPE containing alpha-tocopherol was incubated for 49 days at 4 degrees C a dehydrated solid phase different from the subgel and the high-melting phases was formed.

Apparent pKa of the fatty acids within ordered mixtures of model human stratum corneum lipids

PURPOSE

The apparent pKa of the fatty acids within hydrated (30% w/w) model human stratum corneum (SC) lipid mixtures should be measured.

METHODS

The degree of ionisation of the fatty acids was calculated as a function of pH using Fourier transform infra-red spectroscopy. The relative intensity of the stretching bands of the unionized and ionized carboxylic groups was determined and fitted to the relevant expression for ionic equilibrium of a monoprotic acid. The pKa was then calculated for increasing proportion of unsaturated fatty acid in the lipid mixture.

RESULTS

Values for pKa in the range 6.2-7.3 were found, increasing with greater proportion of oleic acid. These are some 1.5-3 pH units higher than the pKas of fatty acids in molecular solution.

CONCLUSIONS

As there exists a pH-gradient across the SC, the degree of ionisation will also vary. In the innermost SC layers, a pH of 7 will produce 90% ionization of the fatty acids and head-group repulsion will be great. At the SC surface, the pH of 5 will cause almost minimal head-group repulsion, tending to increase crystallinity and promote a bilayer structure.

Effect of sphingosine and stearylamine on the interaction of phosphatidylserine with calcium. A study using DSC, FT-IR and 45Ca(2+)-binding

The lamellar gel to lamellar liquid-crystalline phase transition of dipalmitoylphosphatidylserine (DPPS) multilamellar membranes is abolished by the presence of Ca2+ at DPPS/Ca2+ molar ratios of 2:1 or lower. However, when equimolar sphingosine (SPH) or stearylamine (SA), which are positively charged at the pH studied in this work, were included in DPPS vesicles, the phase transition of DPPS was still observed by differential scanning calorimetry, even in the presence of very high Ca2+ concentrations such as a DPPS/Ca2+ molar ratio of 1:10. According to that, delta H was similar for samples formed by equimolar DPPS and SPH and SA, either in the presence or in the absence of Ca2+, whereas no phase transition was observed for the pure phospholipid in the presence of Ca2+ at molar ratios lower than DPPS/Ca2+ 2:1. 45Ca(2+)-binding experiments showed that for DPPS/SPH or DPPS/SA molar ratios of 2:1, only half of the Ca2+ was bound to DPPS with respect to pure DPPS, i.e., in the absence of SPH or SA. At concentrations of SPH or SA equimolar with DPPS, the Ca2+ binding was nearly abolished. The effect of SPH and SA on the the apparent pKapp of the carboxyl group of DPPS was also studied in the presence and in the absence of Ca2+ by using Fourier transform infrared spectroscopy. The dehydration of the phosphate group of DPPS induced by the binding of Ca2+ was followed through the observation of the PO2- antisymmetric stretching, and the percentage of dehydrated PO2- groups quantitatively assayed. It was again confirmed that, in the presence of equimolar concentrations of SPH or SA, Ca2+, at concentrations which are saturating for pure DPPS, was not bound at all to DPPS. It was also found that the pKapp was considerably shifted to lower values in the presence of the amino bases, decreasing from 4.6 in pure DPPS to 2.1 and 2.2 for the equimolar mixtures of DPPS with SPH and SA, respectively. These results show that SPH and SA, being positively charged molecules anchored in the membrane, are able of preventing the binding of positively charged ions such as Ca2+ through an electrostatic charge neutralization.

Capsaicin affects the structure and phase organization of phospholipid membranes

Capsaicin is a natural compound with pharmacological and toxicological effects, which given its hydrophobicity, can influence the structure of membranes. The interaction of capsaicin with model membranes of dipalmitoylphosphatidylcholine and dielaidoylphosphatidylethanolamine has been studied by using differential scanning calorimetry, fluorescent probe spectroscopy and 31P-nuclear magnetic resonance. Capsaicin remarkably affects the phase transition of dipalmitoylphosphatidylcholine, shifting the transition temperature to lower values, and giving rise, at relatively high capsaicin concentrations, to the appearance of two peaks in the thermogram. These peaks may correspond to separated phases as indicated by the partial phase diagram. Whereas capsaicin did not affect the fluorescence polarization of the probes diphenylhexatriene and trimethylammonium-diphenylhexatriene, it clearly affected that of the probe 2-anthroyloxystearic acid, indicating that the perturbation produced by capsaicin on the membrane would be mainly at the position where this fluorophore is located. On the other hand, capsaicin, at relatively low concentrations, gives rise to immiscible phases in the presence of dielaidoylphosphatidylethanolamine and decrease the temperature of the lamellar to hexagonal HII phase transition. At concentrations of capsaicin higher than 0.3 mol fraction, isotropic phases were detected. The possible implications of the effects of capsaicin on biological membranes are discussed.

The dissimilar effect of diacylglycerols on Ca(2+)-induced phosphatidylserine vesicle fusion

We have studied the effect of physiological concentrations of different diacylglycerols on Ca(2+)-induced fusion between phosphatidylserine vesicles. We monitored vesicle fusion as mixing of membrane lipids under conditions where the limiting factor was the aggregation and also in conditions where this aggregation was not the limiting factor. We found that diacylglycerols have a different modulating effect on the Ca(2+)-induced fusion: i) depending on their interfacial conformation, so that 1,2-isomers of diacylglycerols containing unsaturated or short saturated acyl chains stimulated fusion and their 1,3-isomers did not, and ii) depending on their specific type of bilayer interior perturbation, so that diacylglycerols containing unsaturated or short chain saturated acyl chains stimulated fusion but those containing long-chain saturated acyl chains did not. These requirements resembled those required for the diacylglycerol activation of protein kinase C, suggesting that diacylglycerol acts in both the specific activation of this enzyme and the induction of membrane fusion through the same perturbation of lipid structure. We found that polylysine affected the stimulatory role of 1,2-dioleoylglycerol differently, depending on whether aggregation was the limiting factor of fusion. When we studied the effect of very low concentrations of diacylglycerols on the bulk structural properties of phosphatidylserine, we found that they neither significantly perturbed the thermotropic transitions of phosphatidylserine nor affected the interaction of Ca2+ with the phosphate group of phosphatidylserine. The underlying mechanism of fusion between phosphatidylserine vesicles is discussed.

Role of phosphatidylserine and diacylglycerol in the fusion of chromaffin granules with target membranes

The role of phosphatidylserine and diacylglycerol in the fusion of chromaffin granules with target membranes was investigated in vitro, monitoring the mixing of membrane lipids with the octadecyl rhodamine B fluorescence dequenching assay. Polylysine was able to induce fusion of chromaffin granule ghosts with plasma membrane vesicles in the absence of Ca2+. This fusion was maximal at 1.6 microM polylysine and the kinetics were dependent on the amount of plasma membrane added. Polylysine lowered the Ca2+ threshold concentration for inducing fusion, increased the extent of fusion at a given Ca2+ concentration and acted synergistically with Ca2+ when added prior to the cation. Removal of membrane proteins by trypsinization of chromaffin granule ghost and/or plasma membranes increased the extent of polylysine induced fusion. Incorporation of diacylglycerol into the plasma membrane promoted Ca(2+)-induced fusion. Chromaffin granule ghosts were induced to fuse with model membranes of different complexities from one resembling the inner monolayer of the erythrocyte membrane to one composed solely of pure phosphatidylserine. The characteristics of the fusion with model membranes were qualitatively similar to that observed with target plasma membrane, although differences in kinetics and stoichiometries were found. Interestingly, considerably low (microM) Ca2+ concentrations were able to induce fusion of chromaffin granule ghosts with diacylglycerol containing phosphatidylserine vesicles in the presence of polylysine. Our results indicate that acidic phospholipid-like phosphatidylserine may have an important role in the process of fusion and suggest that diacylglycerol, as a destabilizing lipid, may have a role by itself in promoting exocytosis in chromaffin cells.

A phase behavior study of mixtures of sphingosine with zwitterionic phospholipids

The interactions of sphingosine (SPH) with dipalmitoylphosphatidylcholine (DPPC) and dielaidoylphosphatidylethanolamine (DEPE) have been studied by means of differential scanning calorimetry (DSC) and 31P-nuclear magnetic resonance (31P-NMR). Experiments were carried out with the fully protonated form of SPH, at pH 6.0. DSC studies showed that the main Tc transition temperature of DPPC was perturbed by the presence of SPH so that Tc of the mixture was higher than those of pure components at concentrations of SPH up to 50 mol%, with an azeotropic point at 30 mol% of SPH. At higher concentrations solid phase separations were observed from 70 to 95 mol% of SPH with an eutectic point at 90 mol% of SPH. 31P-NMR showed lamellar phases in DPPC/SPH mixtures, at all the range of concentrations. The behavior of DEPE/SPH mixtures was somewhat different since no azeotropic point was detected, the gel to liquid-crystalline transition being depressed by the presence of SPH, and an eutectic point was detected at 60 mol%. Solid phase immiscibilities were present between 50 mol% and 85 mol% of SPH. It is also remarkable that the liquid-crystalline to hexagonal HII phase transition of DEPE was only slightly shifted to lower temperatures at concentrations of SPH lower than 33 mol% of SPH but, this transition disappeared at concentrations of SPH higher than 33 mol% of SPH, so that isotropic phases were formed instead, as seen through 31P-NMR. The present results show the importance of taking into account the effects appearing in mixtures of SPH with zwitterionic phospholipids when considering their influence on the organization of biomembranes.

Extensive proteolytic digestion of the (Ca2+ + Mg2+)-ATPase from sarcoplasmic reticulum leads to a highly hydrophobic proteinaceous residue with a mainly alpha-helical structure

The purified (Ca2+ + Mg2+)-ATPase from sarcoplasmic reticulum was subjected to extensive proteolysis by using trypsin and proteinase K. This digestion led to the elimination of a considerable portion of the protein, so that the lipid to protein weight ratio was increased from 0.44 in the purified ATPase to 1.20 after extensive proteolysis. After the digestion, the residue was found to be considerably enriched in hydrophobic amino acids. FT-IR spectroscopic studies indicated that the secondary structure of the proteolytic residue was enriched in alpha-helix with 75%, compared with 48% in the intact purified ATPase. FT-IR studies using ATR polarization showed that the alpha-helical part of the residue of proteolytic digestion was considerably more polarized than the purified ATPase, indicating that, on average, the alpha-helices of the residual protein should lie with an orientation closer to the normal to the plane of the membrane. Thermal denaturation studies showed that the residue of proteolysis was considerably more stable than the intact purified ATPase. This would be compatible with the residue being protected from denaturation by its hydrophobic location within the membrane. This study is experimental evidence of the alpha-helical structure of the membrane part of this protein, as suggested by predictions made from its known primary structure (Brandl et al., 1986).

The phase behavior of mixed aqueous dispersions of dipalmitoyl derivatives of phosphatidylcholine and diacylglycerol

The phases and transition sequences for aqueous dispersions of mixtures of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dipalmitoyl-sn-glycerol (1,2-DPG) have been studied by differential scanning calorimetry, dynamic x-ray diffraction, freeze-fracture electron microscopy, 31P-nuclear magnetic resonance spectroscopy, and Fourier-transform infrared spectroscopy. The results have been used to construct a dynamic phase diagram of the binary mixture as a function of temperature over the range 20 degrees-90 degrees C. It is concluded that DPPC and 1,2-DPG form two complexes in the gel phase, the first one with a DPPC/1,2-DPG molar ratio of 55:45 and the second one at a molar ratio of approximately 1:2, defining three different regions in the phase diagram. Two eutectic points are postulated to occur: one at a very low 1,2-DPG concentration and the other at a 1,2-DPG concentration slightly higher than 66 mol%. At temperatures higher than the transition temperature, lamellar phases were predominant at low 1,2-DPG concentrations, but nonlamellar phases were found to be predominant at high proportions of 1,2-DPG. A very important aspect of these DPPC/1,2-DPG mixtures was that, in the gel phase, they showed a ripple structure, as seen by freeze-fracture electron microscopy and consistent with the high lamellar repeat spacings seen by x-ray diffraction. Ripple phase characteristics were also found in the fluid lamellar phases occurring at concentrations up to 35.6 mol% of 1,2-DPG. Evidence was obtained by Fourier transform infrared spectroscopy of the dehydration of the lipid-water interface induced by the presence of 1,2-DPG. The biological significance of the presence of diacylglycerol in membrane lipid domains is discussed.

Diacylglycerol, phosphatidylserine and Ca2+: a phase behavior study

The interaction of 1,2-dipalmitoylglycerol (1,2-DPG) with dipalmitoylphosphatidylserine (DPPS) has been studied in the presence and in the absence of Ca2+ by using differential scanning calorimetry (DSC) and 31P-nuclear magnetic resonance (31P-NMR). In the absence of Ca2+, DSC showed that 1,2-DPG increased the phase transition of DPPS, effect already noticed at very low 1,2-DPG concentrations, whereas lipid immiscibilities were detected at concentrations of 1,2-DPG higher than about 30 mol%. 31P-NMR indicated that lamellar phases were always present at concentration of 1,2-DPG lower than about 35 mol%, but at higher concentrations non-lamellar phases may be present in the fluid phase. As observed by DSC, the apparent pKa of the carboxyl group of DPPS was increased slightly in the presence of 1,2-DPG. In the presence of Ca2+, the effect of 1,2-DPG was to further increase the temperature of the onset of the phase transition, indicating an stabilization of the most rigid phase in the DPPS/1,2-DPG/Ca2+ samples. Even concentrations of 1,2-DPG as low as 1 mol% of the total lipid already produced a noticeable effect. Moreover, lipid immiscibilities were apparent at concentrations of 1,2-DPG higher than 20 mol%. Furthermore, the transition of the DPPS/Ca2+ complex observed by DSC at 155 degrees C was perturbed by the presence of 1,2-DPG, indicating a change in the structure of the crystalline complex. Interestingly, the effect of non-saturating Ca2+ concentrations on the DPPS phase transition was enhanced by the presence of 1,2-DPG. The effect reported here may be significant for a number of situations where Ca2+, phosphatidylserine and diacylglycerols are involved, such as fusion of membranes, where diacylglycerol may facilitate Ca(2+)-induced fusion, or the activation of enzymes such as protein kinase C and phospholipases.

Fluorescence study of a derivatized diacylglycerol incorporated in model membranes

A fluorescence study of a diacylglycerol derivatized with the n-(7-nitrobenz-2-oxa-1,3-diazol-4-yl) chromophore (NBD) was carried out. Fluorescence self-quenching was observed for this probe in lecithin model membranes due to collisional interaction rather than to an aggregational behaviour of the probe. The efficient energy migration (Ro = 28 A) of the NBD fluorophore was studied via the fluorescence depolarization upon increase of probe concentration in membranes, and the results are compared with a model where a random distribution of the probes is assumed. A surface location of the chromophore was concluded for the NBD derivative of diacylglycerol, both from the fluorescence parameters and from the study of its fluorescence quenching by spin label probes. Very high lateral diffusion coefficients were obtained for these probes, both from the self-quenching (D = 2-6 x 10(-6) cm2 s-1) and from the spin probe quenching (D = 3.5 x 10(-6) cm2 s-1) studies. A concomitant fluorescence study of the related probe NBD-phosphatidylcholine revealed that its photophysical behaviour is similar to the derivatized diacylglycerol.

Interaction of sphingosine and stearylamine with phosphatidylserine as studied by DSC and NMR

The interaction of sphingosine (SP) and stearylamine (SA) with dipalmitoylphosphatidylserine (DPPS) has been studied by using differential scanning calorimetry (DSC) and phosphorus nuclear magnetic resonance (31P-NMR). DSC showed that SP and SA rigidified the membranes, forming an azeotropic mixture with DPPS. The azeotropic mixture which was formed between DPPS and SP was found at a DPPS/SP molar ratio of 2:1 whereas SA and DPPS formed an azeotropic mixture at a DPPS/SA molar ratio of 1:1. An eutectic point was observed at 85 mol% of SP and 90 mol% of SA in DPPS. 31P-NMR showed the presence of a lamellar phase at DPPS/SP and DPPS/SA molar ratios lower than 1:1, whereas at higher molar ratios and at high temperatures, besides the lamellar phase, an isotropic component was detected. It was found that, at physiological pH, both SP and SA were protonated in a large extent, i.e., positively charged, since their apparent pK in the membrane were 9.1 and 8.9, respectively. The results reported in this work may be relevant to understand a number of biological effects produced by these positively charged molecules, due to their electrostatic interaction with negatively charged phospholipids.

A kinetic study of an unstable enzyme measured through coupling reactions. Application to the self-inactivation of detergent-solubilized Ca(2+)-ATPase from sarcoplasmic reticulum

A methodology for the kinetic study of the self-inactivation of an unstable enzyme has been developed by using the transient-phase approach when the enzymatic activity is measured through a coupled enzyme system. An experimental design has been developed and applied to the inactivation of the Ca(2+)-ATPase from sarcoplasmic reticulum solubilized in the monomeric state. The catalytic activity of the ATP hydrolysis is determined in the presence of pyruvate kinase and lactate dehydrogenase as auxiliary enzymes, and the oxidation of the last substrate, NADH, is continuously monitored. The experimental results show that both substrates, ATP and calcium, protect against enzyme inactivation. This enzyme, the monomeric ATPase, fulfills the catalytic cycle of the native ATPase, and free enzyme and first-calcium bound enzyme are proposed as the intermediates which are being inactivated.

Alpha-tocopherol interacts with natural micelle-forming single-chain phospholipids stabilizing the bilayer phase

The effect of the association of alpha-tocopherol (vitamin E) with single chain or very asymmetric phospholipids has been studied by 31P nuclear magnetic resonance, Fourier transform infrared spectroscopy, and light microscopy. Our results demonstrate that alpha-tocopherol stabilizes and forms bilayer structures in systems composed of either 1-palmitoyl-sn-glycero-3-phosphocholine, 1-O-hexadecyl-2-acetyl-sn-glycero-3-phosphocholine, 1-O-hexadecyl-sn-glycero-3-phosphocholine, or 1,2-dipalmitoyl-sn-glycero-3- phosphocholine/1-palmitoyl-sn-glycero-3-phosphocholine, whereas in the absence of alpha-tocopherol all these systems produce micellar structures. The free hydroxyl group of alpha-tocopherol is shown to be important in stabilizing the bilayer structure by comparing the effects produced by an analog of alpha-tocopherol, alpha-tocopheryl acetate, which has the hydroxyl group blocked by an acetyl group. It is suggested that the complementary shapes of alpha-tocopherol and the asymmetric phospholipids may be the reason for the stabilization of the bilayer structure.

Infrared spectroscopic study of the interaction of diacylglycerol with phosphatidylserine in the presence of calcium

The interaction of 1,2-dipalmitoylglycerol (DPG) with dipalmitoylphosphatidylserine (DPPS) has been studied in aqueous dispersion in the presence and in the absence of Ca2+ by using Fourier transform infrared spectroscopy (FT-IR) and 45Ca(2+)-binding. FT-IR showed that DPG increased the phase transition of DPPS and induced a rigidification of the DPPS/DPG-Ca2+ complex. In the absence of Ca2+, the incorporation of DPG produced an increase in the proportion of dehydrated carbonyl groups in the mixture of DPPS plus DPG whereas, in the presence of Ca2+, DPG suppressed the solid-solid phase transition of phosphatidylserine-Ca2+ complexes. The phosphate band of DPPS was analyzed using a multivariate statistical analysis, indicating that DPG induced a higher dehydration of the PO2- group in the presence of subsaturating Ca2+ concentrations. Even very low concentrations of DPG, such as 2 mol%, already produced a significant effect. In the presence of both DPG and Ca2+, dehydration of DPPS increased, so that full dehydration was reached at a DPPS/Ca2+ molar ratio of 2.94 instead of 2.04 as observed for pure DPPS. However, the stoichiometry of the binding of Ca2+ to DPPS was not significantly altered by the inclusion of DPG as revealed by 45Ca(2+)-binding experiments, indicating that, in this situation, full dehydration of the PO2- groups of DPPS was reached when approx. 2 out of every 3 molecules of DPPS were binding Ca2+. The effects reported here for the interaction of DPG with DPPS may be significant for a number of biological situations where Ca2+, phosphatidylserine and diacylglycerols are involved, such as fusion of membranes or the activation of protein kinase C, where the dehydration effect produced by diacylglycerols may explain, at least in part, their effects.

Intra-articular therapy of experimental arthritis with a derivative of triamcinolone acetonide incorporated in liposomes

Triamcinolone acetonide-21-palmitate was synthesized and incorporated into liposomes for intra-articular treatment of an experimentally-induced arthritis in the knee joints of rabbits. The liposomal formulation was more efficient than free triamcinolone acetonide in solution in suppressing the arthritis. Using radioactive tracers, it was found that triamcinolone acetonide-21-palmitate incorporated into liposomes was retained in the articular cavity, together with the liposomal lipids, for a much longer period than free triamcinolone acetonide, and this correlated with its anti-inflammatory effect.

Influence of liposome charge and composition on their interaction with human blood serum proteins

Lipid composition and specially their electrostatic properties, were found to greatly influence the stability of liposomes in human blood serum. The amount and type of serum proteins bound to the liposomes were also clearly influenced by lipid composition and charge of liposomes. A good correlation was found between the amount of serum proteins adsorbed to a given type of liposome and its instability as measured by the release of an encapsulated fluorescent probe. Liposomes that bind the highest amount of protein were the least stable, except for the case of liposomes containing gangliosides, which were fairly stable even at a high amount of bound protein. Liposomes with neutral charge containing phosphatidylcholine were the most stable and bound the lowest amount of protein. Liposomes with positive charge behaved similarly to those with neutral charge. However, the stability of negatively charged liposomes was very dependent on their composition. Those liposomes containing only one class of negatively charged phospholipids bound a great amount of protein and were very unstable. However, those liposomes containing also phosphatidylcholine bound less protein and were more stable. The examination of the electrophoresis patterns of serum proteins bound to the different types of liposomes indicated the presence of specific proteins which correlated with liposome instability.

Effects of platelet-activating factor and related lipids on dielaidoylphosphatidylethanolamine by DSC, FTIR and NMR

The effect of platelet-activating factor (1-O-hexadecyl-2-acetyl-sn- glycero-3-phosphocholine, PAF) and two related molecules, 1-O-hexadecyl-sn-glycero-3-phosphocholine (LPAF) and 1-palmitoyl-sn-glycero-3- phosphocholine (LPC) on dielaidoylphosphatidylethanolamine (DEPE) lipid structure and polymorphism has been studied by differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) and 31P nuclear magnetic resonance (31P-NMR) spectroscopies. From the interaction of these molecules with DEPE it is concluded that all of them stabilize the lamellar phase with respect to the hexagonal HII phase and this effect is clear even at concentrations of these compounds as low as 1 mol%. It is also shown that, although they perturb the gel to liquid-crystalline phase transition of DEPE up to a similar extent, fluidizing the membrane, PAF but not LPAF or LPC, induces the presence of more than one peak in the calorimetric profile. Moreover, FTIR data indicate that lateral phase separations formed by PAF-rich phases are taking place. Remarkably, delta H of the main transition decreases at concentrations lower than 2 mol% but remains nearly constant up to 30 mol%. 31P-NMR measurements showed that all these molecules were capable of inducing isotropic signals in the spectra produced by molecules associated to membranes before micellization of the vesicles.

Fusion of chromaffin granules with cardiolipin-containing phospholipid vesicles

Fusion of chromaffin granule ghosts with model phospholipid vesicles is dependent on the composition of the vesicle membrane. Cardiolipin was found to make possible a process of fusion in the absence of calcium. This calcium-independent fusion appears to be partially protein-dependent. Upon interaction with pure cardiolipin vesicles calcium stimulates both fusion of chromaffin granule ghosts and release of catecholamines from intact chromaffin granules. We suggest that the release of catecholamines is not only a consequence of the fusion process. The relevance of protein-lipid interaction and the importance of the formation of HII phases or other non-lamellar phases, on the fusion of chromaffin granules are discussed.

Influence of retinoids on phosphatidylethanolamine lipid polymorphism

The interaction of all-trans-retinoic acid and all-trans-retinol with dielaidoylphosphatidylethanolamine has been studied by differential scanning calorimetry and 31P-NMR spectroscopy. Increasing concentrations of all-trans-retinoic acid up to a mol fraction of 0.09 were found to induce shifts to lower temperatures of both the L beta to L alpha and L alpha to hexagonal-HII phase transitions, with a slight decrease in the enthalpy change of the transitions. At higher concentrations no further effects on the transitions were observed, and this is interpreted as indicative of a limited miscibility of retinoic acid with the phospholipid. 31P-NMR spectroscopy confirmed that the L alpha to hexagonal-HII phase transition was shifted to lower temperatures in the presence of retinoic acid. On the other hand increasing concentrations of all-trans-retinol up to a mol fraction of 0.166, induced a progressive shift of the L beta to L alpha and the L alpha to hexagonal-HII phase transitions to lower temperatures. At higher concentrations the main gel to liquid-crystalline phase transition was further displaced to lower temperatures and the lamellar to hexagonal-HII phase transition was not observed in the thermograms. 31P-NMR spectroscopy indicated that retinol was able of inducing the phospholipid to adopt the hexagonal-HII phase at temperatures even below the main gel to liquid-crystalline phase transition temperature of the pure phospholipid.