The influence of dioxan on the bilayer and monolayer phase transition of phospholipids

The influence of 1.4.-dioxan on the bilayer phase transition of various phospholipids was studied by differential scanning calorimetry and turbidity measurements. The addition of 1.4.-dioxan to lipid bilayers decreases the transition temperature T_m increases the transition enthalpy of the transition. The cooperativity of the transition is unaffected. The phospholipid monolayer transition from the liquid-condensed to the liquid-expanded phase was measured by recording area versus temperature curves at constant surface pressure (isobars). The monolayer transition temperature at constant surface pressure is increased when 1.4.-dioxan is added to the subphase. The change in molecular area becomes larger. A comparison of monolayer isobars on water and water/dioxan as subphase at constant surface tension rather than surface pressure leads to a decrease of the transition temperature on water/dioxan as subphase. This decrease as well as the larger change in molecular area at the monolayer transition can be correlated to the decrease in T_m and the increase in the transition enthalpy of the corresponding bilayer system. 1.4.-Dioxan seems to accumulate at the lipid head group/water interface, thus lowering the tension of the bilayer membrane. This cyclic ether can be used for altering the characteristics of bilayer membranes without disturbing the lipid chain organization.     

A theoretical model of the temperature- and pressure-induced phase transition of phospholipid bilayers

A statistical thermodynamic model of phospholipid bilayers is developed. In the model, a new concept of a closely packed system is applied, i.e., a system of hard cylinders of equal radii, the radius being a function of the average number of gauche rotations in a hydrocarbon chain. Using this concept of a closely packed system, reasonable values are obtained for the change in specific volume at the order-disorder transition of lecithin bilayers. In addition to interactions between the lipid matrix and water molecules, between the head groups themselves and between hydrocarbon chains, as well as the intramolecular energy associated with chain conformation, the Hamiltonian of the membrane also includes the energy of the pressure field. Thus, the phase transition of phospholipid membranes induced not only by temperature hut also by hydrostatic pressure is described by this model simultaneously. In accordance with the experimental results, a linear relationship is obtained between the phase transition temperature and phase transition pressure. The other calculated phase transition properties of lecithin homologues. e.g., changes in enthalpy, surface area. thickness and gauche number per chain are in agreement with the available experimental data. The ratio of kink to interstitial conduction of bilayers is also estimated.     

Interaction of a purified hydrophobic protein from myelin with phospholipid membranes: studies on ultrastructure, phase transitions and permeability.

A purified protein fraction from the proteolipids of human brain myelin was recombined with different lipids either in aqueous buffer or in a chloroform-methanol-water (10:5:1, v/v/v) mixture. It was found that under both conditions it binds strongly to phospholipids irrespective of surface charge, the presence of cholesterol or double bonds on the fatty acyl chains. The buoyant density of the resulting lipoprotein membranes is intermediate to that of pure lipids, and proteins. The lipoproteins formed by either of these methods were observed by either freeze-fracture or negative stain electron-microscopy. The overall morphology was similar to that of pure phospholipids, showing large closed multilamellar vesicles. The presence of the protein was detected by the appearance of intramembrane particles in freeze-fracture. The addition of the N-2 protein generally increases the permeability vesicles to 22-Na-+ by 2-3 orders of magnitude depending on the concentration. The presence of calcium in the aqueous medium further increases the Na-+ efflux through negatively charged vesicles. Changes in lipid composition, surface charge, cholesterol, etc., have no appreciable influence on the effect of the protein. Differential scanning calorimetry indicates that the presence of small amounts of N-2 have no effect on the lipid phase transition from solid to liquid crystalline. As the amount of protein bound to the phospholipid increases, the enthalpy of the transition decreases, the main endothermic peak broadens, but there is no change on the midpoint temperature. Membranes containing 50% by weight of protein still show a transition with an enthalpy approximately one half that of the original lipid.     

Effects of lipid phase transition and membrane surface charge on the interfacial activation of phospholipase A2

Phospholipase A2 (PLA2) enzymes act at the membrane-water interface to access their phospholipid substrate from the membrane. They are regulated by diverse factors, including the membrane charge, fluidity, mode of membrane binding (insertion, orientation), and allosteric conformational effects. Relative contributions of these factors to the complex kinetics of PLA2 activation are not well understood. Here we examine the effects of thermal phase transitions and the surface charge of phospholipid membranes on the activation of human pancreatic PLA2. The temperature dependence of the initial catalytic rate of PLA2 peaks around the lipid phase transition temperature (Tm) when Tm is not too far from physiological temperatures (30-40 degrees C), and the peak is higher in the presence of anionic membranes. High PLA2 activity can be induced by thermal perturbations of the membrane. Temperature-dependent fluorescence quenching experiments show that despite dramatic effects of the lipid phase transition on PLA2 activity, the membrane insertion depth of PLA2 increases only modestly above Tm. The data show that membrane structural disorder, and not the depth of membrane insertion, plays a major role in PLA2 activity.     

Novicidin's membrane permeabilizing activity is driven by membrane partitioning but not by helicity: A biophysical study of the impact of lipid charge and cholesterol

We have investigated the interactions between the antimicrobial peptide Novicidin (Nc) and vesicles containing the phospholipid DOPC, with various amounts of DOPG and cholesterol using circular dichroism spectroscopy, calcein release, equilibrium dialysis and isothermal titration calorimetry. Nc adopts a random coil structure in the absence of lipids and in the presence of vesicles containing 100% DOPC. Lipids with 25–40% DOPG induce the highest level of helicity in Nc; higher DOPG levels lead to lower helicity levels and an altered tertiary arrangement of the peptide. However, the ability of Nc to permeabilize vesicles correlates not with helicity but rather with its overall membrane affinity, which is enthalpically favorable but opposed by entropy. Permeabilization declines with increasing mole percentage PG. Changes in helicity correlate with changes in enthalpy, reflecting the enthalpy of helix formation, but not with affinity. There is also a large favorable enthalpic interaction between Nc and lipids in the absence of negative charge and structural changes. Cholesterol slightly reduces membrane permeabilization but has little effect on Nc affinity and secondary structure, and probably protects the membrane by inducing the liquid ordered state. We conclude that helicity is not a prerequisite for activity, and charge–charge interactions are not the only major driving force for AMP interactions with membranes. Our data are compatible with a model in which a superficial binding mode with a large membrane surface binding area per peptide is more efficient than a more intimate embedding within the membrane environment.     

Interactions of the Australian tree frog antimicrobial peptides aurein 1.2, citropin 1.1 and maculatin 1.1 with lipid model membranes: differential scanning calorimetric and Fourier transform infrared spectroscopic studies

The interactions of the antimicrobial peptides aurein 1.2, citropin 1.1 and maculatin 1.1 with dimyristoylphosphatidylcholine (DMPC), dimyristoylphosphatidylglycerol (DMPG) and dimyristoylphosphatidylethanolamine (DMPE) were studied by differential scanning calorimetry (DSC) and Fourier-transform infrared (FTIR) spectroscopy. The effects of these peptides on the thermotropic phase behavior of DMPC and DMPG are qualitatively similar and manifested by the suppression of the pretransition, and by peptide concentration-dependent decreases in the temperature, cooperativity and enthalpy of the gel/liquid-crystalline phase transition. However, at all peptide concentrations, anionic DMPG bilayers are more strongly perturbed than zwitterionic DMPC bilayers, consistent with membrane surface charge being an important aspect of the interactions of these peptides with phospholipids. However, at all peptide concentrations, the perturbation of the thermotropic phase behavior of zwitterionic DMPE bilayers is weak and discernable only when samples are exposed to high temperatures. FTIR spectroscopy indicates that these peptides are unstructured in aqueous solution and that they fold into alpha-helices when incorporated into lipid membranes. All three peptides undergo rapid and extensive H-D exchange when incorporated into D(2)O-hydrated phospholipid bilayers, suggesting that they are located in solvent-accessible environments, most probably in the polar/apolar interfacial regions of phospholipid bilayers. The perturbation of model lipid membranes by these peptides decreases in magnitude in the order maculatin 1.1>aurein 1.2>citropin 1.1, whereas the capacity to inhibit Acholeplasma laidlawii B growth decreases in the order maculatin 1.1>aurein 1.2 congruent with citropin 1.1. The higher efficacy of maculatin 1.1 in disrupting model and biological membranes can be rationalized by its larger size and higher net charge. However, despite its smaller size and lower net charge, aurein 1.2 is more disruptive of model lipid membranes than citropin 1.1 and exhibits comparable antimicrobial activity, probably because aurein 1.2 has a higher propensity for partitioning into phospholipid membranes.     

Protein and lipid structural transitions in cytochrome c oxidase-dimyristoylphosphatidylcholine reconstitutions

The thermotropic behavior of the mitochondrial enzyme cytochrome c oxidase (EC 1.9.3.1) reconstituted in dimyristoylphosphatidylcholine (DMPC) vesicles has been studied by using high-sensitivity differential scanning calorimetry and fluorescence spectroscopy. The incorporation of cytochrome c oxidase into the phospholipid bilayer perturbs the thermodynamic parameters associated with the lipid phase transition in a manner analogous to other integral membrane proteins: it reduces the enthalpy change, lowers the transition temperature, and reduces the cooperative behavior of the phospholipid molecules. Analysis of the dependence of the enthalpy change on the protein:lipid molar ratio indicates that cytochrome c oxidase prevents 99 +/- 5 lipid molecules from participating in the main gel-liquid-crystalline transition. These phospholipid molecules presumably remain in the same physical state below and above the transition temperature of the bulk lipid, thus providing a more or less constant microenvironment to the protein molecule. The effect of the phospholipid bilayer matrix on the thermodynamic stability of the cytochrome c oxidase complex was examined by high-sensitivity differential scanning calorimetry. Detergent (Tween 80)-solubilized cytochrome c oxidase undergoes a complex, irreversible thermal denaturation process centered at 56 degrees C and characterized by an enthalpy change of 550 +/- 50 kcal/mol of enzyme complex. Reconstitution of the cytochrome c oxidase complex into DMPC vesicles shifts the transition temperature upward to 63 degrees C, indicating that the phospholipid bilayer moiety stabilizes the native conformation of the enzyme. The lipid bilayer environment contributes approximately 10 kcal/mol to the free energy of stabilization of the enzyme complex. The thermal unfolding of cytochrome c oxidase is not a two-state process.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of amphotericin B on membrane permeability--kinetics of spin probe reduction

The effect of the polyene antibiotic amphotericin B on the permeability of both unilamellar and multilamellar model membranes is investigated. The method measures the loss of the electron paramagnetic resonance signal of a spin probe, trapped in the aqueous compartment of a lipid dispersion, upon addition of ascorbate ions to the bulk aqueous phase. Amphotericin B causes large increases in the permeability of cholesterol-containing egg phosphatidylcholine membranes, whereas the effects are small in the absence of sterol and do not depend on surface charge. The effect of amphotericin depends upon the antibiotic:sterol mole ratio. The antibiotic appears to be unable to cross the membrane, acting only on the outermost bilayer of a multibilayer dispersion. When a phospholipid in the gel phase is used, amphotericin B causes large increases in permeability, independently of the presence or absence of sterol. It is suggested that the mechanism of action of amphotericin B is different for lipids in the liquid crystalline or gel states.     

Use of laurdan fluorescence intensity and polarization to distinguish between changes in membrane fluidity and phospholipid order

Laurdan is a fluorescent probe that detects changes in membrane phase properties through its sensitivity to the polarity of its environment in the bilayer. Variations in membrane water content cause shifts in the laurdan emission spectrum, which are quantified by calculating the generalized polarization (GP). We tested whether laurdan fluorescence could be used to distinguish differences in phospholipid order from changes in membrane fluidity by examining the temperature dependence of laurdan GP and fluorescence anisotropy in dipalmitoylphosphatidylcholine (DPPC) vesicles. The phase transition from the solid ordered phase to the liquid disordered phase was observed as a decrease in laurdan GP values from 0.7 to −0.14 and a reduction in anisotropy from 0.25 to 0.12. Inclusion of various amounts of cholesterol in the membranes to generate a liquid ordered phase caused an increase in the apparent melting temperature detected by laurdan GP. In contrast, cholesterol decreased the apparent melting temperature estimated from anisotropy measurements. Based on these results, it appeared that laurdan anisotropy detected changes in membrane fluidity while laurdan GP sensed changes in phospholipid order. Thus, the same fluorescent probe can be used to distinguish effects of perturbations on membrane order and fluidity by comparing the results of fluorescence emission and anisotropy measurements.     

Direct measurement of the energetics of association between myelin basic protein and phosphatidylserine vesicles

A newly designed high-sensitivity isothermal reaction calorimetry system has been used to investigate the thermodynamics of the association between myelin basic protein and phosphatidylserine vesicles. This instrument has allowed us to measure directly the energetics of the protein-lipid interaction under various conditions. Above the phospholipid phase transition temperature the enthalpy of association is highly exothermic amounting to -160 kcal/mol of protein. Below the phospholipid phase transition temperature the enthalpy of association is exothermic at protein/lipid ratios smaller than 1/50 and endothermic at higher protein/lipid ratios. These studies indicate that the association of myelin basic protein to phosphatidylserine vesicles consists of at least two stages involving different types of binding. The first stage, at low protein/lipid ratios, involves a strong exothermic association of the protein to the membrane and the second, at high protein/lipid ratios, a weaker association probably involving attachment of the protein to the membrane surface only. In the gel phase the second binding stage is endothermic and appears to be correlated with the formation of large vesicle aggregates. This vesicle aggregation is a reversible process dependent upon the physical state of the membrane. The isothermal titration studies have been complemented with high-sensitivity differential scanning calorimetry experiments. It is shown that the dependence of the phospholipid transition enthalpy on the protein/lipid molar ratio can be expressed in terms of the different protein-membrane association enthalpies in the gel and fluid phases of the membrane.     

Profile of changes in lipid bilayer structure caused by beta-amyloid peptide.

beta-Amyloid peptide (A beta) is the primary constituent of senile plaques, a defining feature of Alzheimer's disease. Aggregated A beta is toxic to neurons, but the mechanism of toxicity is uncertain. One hypothesis is that interactions between A beta aggregates and cell membranes mediate A beta toxicity. Previously, we described a positive correlation between the A beta aggregation state and surface hydrophobicity, and the ability of the peptide to decrease fluidity in the center of the membrane bilayer [Kremer, J. J., et al. (2000) Biochemistry 39, 10309--10318]. In this work, we report that A beta aggregates increased the steady-state anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH) embedded in the hydrophobic center of the membrane in phospholipids with anionic, cationic, and zwitterionic headgroups, suggesting that specific charge--charge interactions are not required for A beta--membrane interactions. A beta did not affect the fluorescence lifetime of DPH, indicating that the increase in anisotropy is due to increased ordering of the phospholipid acyl chains rather than changes in water penetration into the bilayer interior. A beta aggregates affected membrane fluidity above, but not below, the lipid phase-transition temperature and did not alter the temperature or enthalpy of the phospholipid phase transition. A beta induced little to no change in membrane structure or water penetration near the bilayer surface. Overall, these results suggest that exposed hydrophobic patches on the A beta aggregates interact with the hydrophobic core of the lipid bilayer, leading to a reduction in membrane fluidity. Decreases in membrane fluidity could hamper functioning of cell surface receptors and ion channel proteins; such decreases have been associated with cellular toxicity.     

Effects of cholesterol on lipid organization in human erythrocyte membrane

The molar ratio of cholesterol to phospholipid (C/P) in human erythrocyte membrane is modified by incubating the cells with liposomes of various C/P ratios. The observed increase in cell surface area may be accounted for by the addition of cholesterol molecules. Fusion between liposomes and cells or attachment of liposomes to cells is not a significant factor in the alteration of C/P ratio. Onset temperatures for lipid phase separation in modified membranes are measured by electron diffraction. The onset temperature increases with decreasing C/P ration from 2 degrees C at C/P = 0.95 to 20 degrees C at C/P = 0.5. Redistribution of intramembrane particles is observed in membranes freeze-quenched from temperatures below the onset temperature. The heterogeneous distribution of intramembrane particles below the onset temperature suggests phase separation of lipid, with concomitant segregation of intramembrane protein into domains, even in the presence of an intact spectrin network.     

Regulation of transbilayer distribution of a fluorescent sterol in tumor cell plasma membranes

The lipid composition and transbilayer distribution of plasma membrane isolated from primary tumor (L-929, LM, A-9 and C3H) and nine metastatic cell lines cultured under identical conditions was examined. Cultured primary tumor and metastatic cells differed two-fold in sterol/phospholipid molar ratios. There was a direct correlation between plasma membrane anionic phospholipid (phosphatidylinositol and phosphatidylserine) content and plasma membrane sterol/phospholipid ratio. This finding may bear on the possible link between oncogenes and inositol lipids. The fluorescent sterol, dehydroergosterol, was incorporated into primary tumor and metastatic cell lines. Selective quenching of outer monolayer fluorescence by covalently linked trinitrophenyl groups demonstrated an asymmetric transbilayer distribution of sterol in the plasma membranes. The inner monolayer of the plasma membranes from both cultured primary and metastatic tumor cells was enriched in sterol as compared with the outer monolayer. Consistent with this, the inner monolayer was distinctly more rigid as determined by the limiting anisotropy of 1,6-diphenyl-1,3,5-hexatriene. Dehydroergosterol fluorescence was temperature dependent and sensitive to lateral phase separations in phosphatidylcholine vesicles and in LM cell plasma membranes. Dehydroergosterol detected phase separations near 24 degrees C in the outer monolayer and at 21 degrees C and 37 degrees C in the inner monolayer of LM plasma membranes. Yet, no change in transbilayer sterol distribution was detected in ascending or descending temperature scans between 4 and 45 degrees C. Alterations in plasma membrane phospholipid polar head group composition by choline analogues (N,N-dimethylethanolamine, N-methylethanolamine, and ethanolamine) also did not perturb transbilayer sterol asymmetry. Treatment with phenobarbital or prilocaine, drugs that selectively fluidize the outer and inner monolayer of LM plasma membranes, respectively, did not change dehydroergosterol transbilayer distribution.     

Influence of surface potentials on the mitochondrial H+ pump and on lipid-phase transitions.

It has been shown that the surface potential of lipid membranes, as well as of mitochondria, can be shifted more positive by absorption of alkylbiguanides. Both phospholipid vesicles and natural membranes respond in an analogous way to this shift. Ion activities at the immediate membrane surface are influenced by sign and magnitude of the surface charge. Corresponding effects on ion transport and on fluorescence-probe binding can be observed. The mitochondrial H+ pump is inhibited when the surface charge is shifted more positive. In contrast,the absolute charge density determines the temperature of the ordered-fluid transition. The latter is increased by biguanides, suggesting that the membrane is rendered more rigid. The experiments make obvious that physical relations derived from model systems apply equally well to lipid-containing natural membranes.     

The effects of cotyledon senescence on the composition and physical properties of membrane lipid

The phospholipid content of rough and smooth microsomal fractions from cotyledons of germinating bean declines as the tissue becomes senescent. Both types of membrane contain comparable proportions of three major phospholipids, phosphatidylcholine, phosphatidylethanolamine and phosphatidylinositol, which collectively comprise about 90% of the total. This proportionality does not change appreciably during senescence. Only small quantities of lysophosphatides were noted at all stages of senescence. The unsaturated:saturated fatty acid ratio for total extracted lipid declined only slightly in both membrane systems, but pronounced differences in this ratio were observed among the major phospholipids of the membranes. The most striking alteration in lipid composition with advancing senescence was an increase in the sterol:phospholipid ratio; this rose by about 50% for rough microsomes and 400% for smooth microsomes. For both types of membrane the patterns of change in this ratio correlated with previously reported changes in bulk lipid transition temperature, suggesting that the increase in sterol level may contribute to changes in phase behaviour of the membranes during senescence. Arrhenius plots of rotational correlation times for the electron spin label 2,2-dimethyl-5-dodecyl-5-methyloxazolidine-N-oxide (2N14) partitioned into the membrane lipid showed an increase in viscosity with advancing senescence and a corresponding increase in activation energy for both types of membrane. These changes in activation energy and viscosity correlated closely with the increase in sterol:phospholipid ratio. However, no phase transitions were detectable between temperatures of 2 and 55 degrees C despite the fact that transitions from a lipid-crystalline to gel state are detectable within this temperature range by wide angle X-ray diffraction.     

Interactions of the Australian tree frog antimicrobial peptides aurein 1.2, citropin 1.1 and maculatin 1.1 with lipid model membranes: Differential scanning calorimetric and Fourier transform infrared spectroscopic studies

The interactions of the antimicrobial peptides aurein 1.2, citropin 1.1 and maculatin 1.1 with dimyristoylphosphatidylcholine (DMPC), dimyristoylphosphatidylglycerol (DMPG) and dimyristoylphosphatidylethanolamine (DMPE) were studied by differential scanning calorimetry (DSC) and Fourier-transform infrared (FTIR) spectroscopy. The effects of these peptides on the thermotropic phase behavior of DMPC and DMPG are qualitatively similar and manifested by the suppression of the pretransition, and by peptide concentration-dependent decreases in the temperature, cooperativity and enthalpy of the gel/liquid-crystalline phase transition. However, at all peptide concentrations, anionic DMPG bilayers are more strongly perturbed than zwitterionic DMPC bilayers, consistent with membrane surface charge being an important aspect of the interactions of these peptides with phospholipids. However, at all peptide concentrations, the perturbation of the thermotropic phase behavior of zwitterionic DMPE bilayers is weak and discernable only when samples are exposed to high temperatures. FTIR spectroscopy indicates that these peptides are unstructured in aqueous solution and that they fold into α-helices when incorporated into lipid membranes. All three peptides undergo rapid and extensive H-D exchange when incorporated into D₂O-hydrated phospholipid bilayers, suggesting that they are located in solvent-accessible environments, most probably in the polar/apolar interfacial regions of phospholipid bilayers. The perturbation of model lipid membranes by these peptides decreases in magnitude in the order maculatin 1.1 > aurein 1.2 > citropin 1.1, whereas the capacity to inhibit Acholeplasma laidlawii B growth decreases in the order maculatin 1.1 > aurein 1.2 ≅ citropin 1.1. The higher efficacy of maculatin 1.1 in disrupting model and biological membranes can be rationalized by its larger size and higher net charge. However, despite its smaller size and lower net charge, aurein 1.2 is more disruptive of model lipid membranes than citropin 1.1 and exhibits comparable antimicrobial activity, probably because aurein 1.2 has a higher propensity for partitioning into phospholipid membranes.     

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

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

Influence of surface potentials on the mitochondrial H+ pump and on lipid-phase transitions

It has been shown that the surface potential of lipid membranes, as well as of mitochondria, can be shifted more positive by absorption of alkylbiguanides. Both phospholipid vesicles and natural membranes respond in an analogous way to this shift. Ion activities at the immediate membrane surface are influenced by sign and magnitude of the surface charge. Corresponding effects on ion transport and on fluorescence-probe binding can be observed. The mitochondrial H⁺ pump is inhibited when the surface charge is shifted more positive. In contrast, the absolute charge density determines the temperature of the ordered-fluid transition. The latter is increased by biguanides, suggesting that the membrane is rendered more rigid. The experiments make obvious that physical relations derived from model systems apply equally well to lipid-containing natural membranes.     

Protective effect of blackcurrant on liver cell membrane of rats intoxicated with ethanol

Chronic ethanol intoxication oxidative stress participates in the development of many diseases. Nutrition and the interaction of food nutrients with ethanol metabolism may modulate alcohol toxicity. One such compound is blackcurrant, which also has antioxidant abilities. We investigated the effect of blackcurrant as an antioxidant on the composition and electrical charge of liver cell membranes in ethanol-intoxicated rats. Qualitative and quantitative phospholipid composition and the presence of integral membrane proteins were determined by high-performance liquid chromatography. Electrophoresis was used to determine the surface charge density of the rat liver cell membranes. Ethanol intoxication is characterized by changes in cell metabolism that alter the structure and function of cell membrane components. Ethanol increased phospholipid levels and altered the level of integral proteins as determined by decreased phenylalanine, cysteine, and lysine. Ethanol significantly enhanced changes in the surface charge density of the liver cell membranes. Administration of blackcurrant to rats intoxicated with ethanol significantly protected lipids and proteins against oxidative modifications. It is possible that the beneficial effect of blackcurrant is connected with its abilities to scavenge free radicals and to chelate metal ions.     

Organotin compounds alter the physical organization of phosphatidylcholine membranes

Organotin compounds have a broad range of biological activities and are ubiquitous contaminants in the environment. Their toxicity mainly lies in their action on the membrane. In this contribution we study the interaction of tributyltin and triphenyltin with model membranes composed of phosphatidylcholines of different acyl chain lengths using differential scanning calorimetry, (31)P-nuclear magnetic resonance, X-ray diffraction and infrared spectroscopy. Organotin compounds broaden the main gel to liquid-crystalline phase transition, shift the transition temperature to lower values and induce the appearance of a new peak below the main transition peak. These effects are more pronounced in the case of tributyltin and are quantitatively larger as the phosphatidylcholine acyl chain length decreases. Both tributyltin and triphenyltin increase the enthalpy change of the transition in all the phosphatidylcholine systems studied except in dilauroylphosphatidylcholine. Organotin compounds do not affect the macroscopic bilayer organization of the phospholipid but do affect the degree of hydration of its carbonyl moiety. The above evidence supports the idea that organotin compounds are located in the upper part of the phospholipid palisade near the lipid/water interface.