Properties of ionic transport through liquid rat brain lipid artificial membranes

Summary In this paper we describe some electrical- and ionic-permeability characteristics of liquid-lipid artificial membranes (model membranes made from millipore filters impregnated with phospholipids extracted from rat brain membranes), as well as the effect of monovalent cations, potassium and sodium, on the conductance of these membranes. Model membranes showed a sub-Nerstian cationic response to the cations, according to the series K⁺>Na⁺. Their conductance showed an almost exponential dependence on salt concentration and the current had Ohmic behaviour up to 180 mV, tending to saturation with voltage in higher range. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Biophysical changes induced by cholesterol on phosphatidylcholine artificial biomembranes containing alamethicin oligomers

Cholesterol is an important constituent of eukaryotic cell membranes, whose interaction with phospholipids leads to a broad range of biological roles, such as: maintenance of proper fluidity, formation of raft domains, reduction of passive permeability of various chemical species through the bilayer (e.g., glucose, glycerol, K⁺, Na⁺ and Cl⁻ ions), and increased mechanical strength of the membrane. In this work we studied an interesting paradigm, as to whether cholesterol-containing phosphatidylcholine biomembranes influence the kinetics and transport features of alamethicin oligomers embedded into it. We demonstrate that moderate relative amounts of cholesterol increase the electrical conductance of various sub-conductance states of the alamethicin oligomer, caused probably by a non-monotonic change in the lumped dipole moment of the biomembrane. Our data suggest that biomembrane stiffness caused by cholesterol, visibly modifies the association-dissociation rates of alamethicin oligomerization in the biomembrane. Moreover, increasing concentrations of cholesterol seem to lead to more stable intermediate alamethicin oligomers. We show that in the presence of cholesterol, as the diameter of the alamethicin oligomer increases, so does the time of another monomer to get picked up. These results brings into focus the interesting issue of how oligomerization of proteins affects their interaction affinities for membrane-based lipids.     

Ultrasonic studies of membrane suspensions

Summary The mechanical and the dynamic properties of various membranes were studied by the ultrasonic-resonance method. The compressibility of the membranes was in the range from 2.5·10⁻¹¹ to 5·10⁻¹¹ cm²/dyn and influenced by several factors such as phase transition, incorporation of cholesterol or proteins and configuration of proteins. The ultrasonic relaxation was measured for two membrane systems, dipalmitoylphosphatidylcholine bilayer and sarcoplasmic-reticulum membrane. The results indicated that the ultrasonic measurements are very sensitive to the co-operative transition in lipid bilayers as well as the relaxation of biological membranes containing intrinsic proteins. Paper presented at the ?Meeting on Lyotropics and Related Fields?, held in Rende, Cosenza, September 13–18, 1982.     

Membrane-peptide interaction studied by PELDOR and CW ESR: Peptide conformations and cholesterol effect on the spatial peptide distribution in the membrane

Pulsed electron-electron double resonance (PELDOR) combined with continuous-wave electron paramagnetic resonance was used to study inter- and intramolecular dipole-dipole interactions between spin labels for spin-labeled analogs of trichogin GA IV bound to multilamellar membranes of egg L-α-phosphatidylcholine (ePC) and in ePC membranes containing cholesterol. All samples were frozen to 77 K. For mono-labeled peptide concentrations in lipid over the range between 0.5 to 2.2 mol%, it is shown that in these membranes trichogin molecules are distributed homogeneously and are likely to be located on or near the inner and outer membrane surfaces. Addition of cholesterol to a final concentration of 16.5 mol% leads to an increase of the local concentration of trichogin molecules in the membranes. For the double-labeled trichogin, a distribution of the intramolecular distance between the two spin labels was observed. The distribution function is characterized by two main maxima located at distances of 1.3 and 1.8 nm. The distance of 1.3 nm is close to that expected for the α-helix structure of the peptide chain. The distance of 1.8 nm corresponds to a mixed structure in which a 3₁₀-helix is combined with a set of even more elongated conformations.     

A study of tetraphenylborate transport through rat brain lipids impregnated filter-membranes

Summary The electrical properties of rat brain lipids impregnated filtermembranes exposed to the lipophilic ion tetraphenylborate (TPhB⁻) have been studied, using electrical relaxation techniques. The state of TPhB⁻ transport system depends on the applied voltage. It was also found that the initial membrane conductance depends on the applied voltage and changes with the concentration of TPhB⁻. Our results are presented in comparison with the electrical properties of BLM exposed to TPhB⁻. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Cd-binding to model membranes

The binding of Cd²⁺ to the model membranes Di-myristoyl L-α-phosphatidic acid (DMPA) and Di-myristoyl L-α-phosphatidylcholine (DMPC) was studied by time differential perturbed angular correlation (TDPAC) on^111mCd, via its nuclear quadrupole interaction. Whereas Cd²⁺ does not bind to the neutral DMPC, it binds to charged DMPA up to a 0.8∶1 Cd/lipid ratio.     

Spin-label HF-EPR of lipid ordering in cholesterol-containing membranes

Conventional oxazolidine spin-labelled lipids have the axial¹⁴N-hyperfine tensorz-axis directed along the long axis of the lipid chain. Investigation of lateral ordering of the lipids in membranes requires measurement of thex-y Zeeman anisotropy of the nonaxialg-tensor at high fields. Both the lateral and transverse ordering of the lipid chains in membranes of dimyristoyl phosphatidylcholine containing 40 mol% cholesterol in the liquid-ordered phase have been studied with 94 GHz electron paramagnetic resonance spectroscopy. This has been done by using probe amounts of phosphatidylcholine systematically spin-labelled at positionsn along the length of thesn- 2 chain [n-PCSL, 1-acyl-2-(n-(4,4-dimethyloxazolidine-N-oxyl) stearoyl)-sn-glycero-3-phosphocholine]. Nonaxial (g_xx?g_yy) anisotropy of the spin-labelled lipid chains is detected over a wide range of temperature throughout the liquid-ordered phase. The transverse profile of lateral ordering with position,n, of chain labelling follows the profile of the rigid steroid nucleus of cholesterol. It becomes progressively averaged towards the terminal methyl group of thesn- 2 chain, in the region of the flexible hydrocarbon chain of cholesterol. The nonaxial lipid ordering may be related to lipid domain formation in membranes containing cholesterol and saturated-chain lipids.     

Toy model that explains the regulation of cholesterol on lipid rafts

Cholesterol, as a common lipid on mammalian cell membranes, plays an important role in the formation of lipid rafts. Recent experiments suggest that the strength of cholesterol’s regulation on lipid rafts can be affected by the length of the unsaturated phospholipid acyl chain on the membrane. In order to understand this observation, a simplified toy model containing three different molecules is proposed in this paper, where the tail length of phospholipids is considered. This model shows the regulation of membrane cholesterol on the phase separation of the lipid mixture and the formation of nano-domains, and also suggests that the configuration entropy of phospholipid tails is an essential factor.     

Cholesterol addition and removal in pacific oyster oocytes does not improve cryopreservation success

The optimal cholesterol content in cells could provide the benefit of lowering or eliminating the lipid phase transition temperature, while maintaining membrane fluidity and strength; thus, making cells less sensitive to chilling injury and more amenable to cryopreservation. Such effects were shown in some gametes and embryos of certain mammalian species, however, some other cell types, benefited from cholesterol removal. The experiments developed in this study aimed to determine the effect of incubating Pacific oyster (Crassostrea gigas) oocytes in cholesterol-addition or removal solutions prior to cryopreservation on their post-thaw fertilization ability. The results showed a positive association of cholesterol with the oocytes when assessed by fluorescent microscopy. However, this uptake was not reflected by an increase in cholesterol as determined by colorimetric analysis or in the post-thaw fertilization rate of treated oocytes. It is presumed either that oyster oocytes already contain a substantial amount of cholesterol or other lipids in their plasma membranes and do not benefit from any additional cholesterol or there is no lipid phase transition temperature in oyster oocytes.     

Superlinearity of current-voltage characteristics of oxidized cholesterol bilayer membranes

Current-voltage characteristics of bilayer oxidized cholesterol membranes were measured in 0·1 M KCl solution. The ohmic conductance was (5±3) × 10^–4–1 m^–2. To explain the superlinearity of the characteristics, a model of the ion transport through transient pores was used. Alternative diffusion barrier models could also explain the nonlinearity, but the distribution coefficients of permeable ions between the membrane hydrocarbon phase and the aqueous phase had to be taken unreasonably high.