Monocarboxylic acid permeation through lipid bilayer membranes

Summary The membrane permeability coefficients for the homologous monocarboxylic acids, formic through hexanoic, as well as benzoic and salicylic, were determined for egg phosphatidylcholine-decane planar bilayer membranes. The permeabilities of formic, acetic and propionic acid were also determined for solvent-free phosphatidylethanolamine bilayers. Permeability coefficients were calculated from tracer fluxes measured under otherwise symmetrical conditions, and precautions were taken to ensure that the values were not underestimated due to unstirred layer effects. The relation between the nonionic (HA) permeability (P ^m ) and the hexadecane/water partition coefficient (K _p ) was: log ^m =0.90 log K_p+0.87 (correlation coefficient=0.996). Formic acid was excluded from the analysis because its permeability was sixfold higher than predicted by the other acids. The permeabilities for solvent-free membranes were similar to those for decanecontaining membranes. The exceptionally high permeability of formic acid and the high correlation of the other permeabilities to the hexadecane/water partition coefficient is a pattern that conforms with other nonelectrolyte permeabilities through bilayers. Similarly, the mean incremental free energy change per methylene group (G-CH₂-) was –764 cal mol^–1, similar to other homologous solutes in other membrane systems. However, much less negative G values (–120, to –400 cal mol^–1) were previously reported for fatty acids permeating bilayers and biological membranes. These values are due primarily to unstirred layer effects, metabolism and binding to membranes and other cell components.     

Valinomycin-mediated ion transport through neutral lipid membranes: Influence of hydrocarbon chain length and temperature

Summary Stationary electrical conductance experiments together with nonstationary relaxation experiments allow a quantitative determination of rate constants describing carrier-mediated ion transport. Valinomycin-induced ion transport across neutral lipid membranes was studied. The dependence of the transport parameters on the chain length of the lipid molecules, on the kind of alkali ion, and on the temperature was determined. The relaxation time the current following a voltage jump shows a marked increase with decreasing temperature or with increasing chain length of the lipid molecules. This variation of is interpreted on the basis of a varying membrane fluidity. It is shown that under favorable circumstances the equilibrium constant of complex formation in the aqueous phase may be obtained from membrane experiments. Furthermore, the kinetics of exchange of valinomycin between membrane and water was studied. We found a marked influence of the totus surrounding the black film on the kinetics as well as on the total amount of valinomycin molecules in the membrane. The problem of location of the free carrier molecules inside the membrane is discussed.     

Mechanism of proton permeation through chloroplast lipid membranes.

Electrical measurements were carried out to investigate the contribution of chloroplast lipids to the passive proton permeability of both the thylakoid and inner-envelope membranes. Permeability coefficient and conductance to protons were measured for solvent-free bilayers made from monogalactosyldiglyceride:digalactosyldiglycerid: sulfoquinovosyldiglyceride:phosphatidylglycerol (2:1:0.5:0.5, w/w) in the presence of a pH gradient of 7.4/8.1. The permeability coefficient for protons in glycolipids was 5.5 +/- 1.1 x 10(-4) cm s-1 (n = 14). To determine whether this high H+ permeability could be explained by the presence of lipid contaminants such as weak acids, we investigated the effects of (a) bovine serum albumin, which can remove some amphiphilic molecules such as free fatty acids, (b) 6-ketocholestanol, which increases the membrane dipole potential, (c) oleic acid, and (d) chlorodecane, which increases the dielectric constant of the lipid bilayer. Our results show that free fatty acids are inefficient protonophores, as compared with carbonylcyanide-m-chlorphenythydrazone, and that the hypothesis of a weak acid mechanism is not valid with glycolipid bilayers. In the presence of deuterium oxide the H+ conductane was reduced significantly, indicating that proton transport through the glycolipid matrix could occur directly by a hydrogen bond process. The passive transport of H+ through the glycolipid matrix is discussed with regard to the activity of the thylakoid ATP synthase and the inner-envelope H(+)-ATPase.     

Lanthanide ion probes of calcium-binding sites on cellular membranes

The chemical basis for the similarity between the lanthanide series of ions and calcium is outlined together with the experimental difficulties associated with the use of these ions. A number of properties of the lanthanide ions are highlighted which make them potentially valuable probe elements. In this context the use of lanthanum and the lanthanide ions in probing calcium sites on cellular membranes is reviewed. In most instances, the lanthanide ions displace membranebound Ca and inhibit Ca-mediated membrane function, but, unlike Ca, these ions do not appear to be transported across cellular membranes (mitochondria may be an exception). Generally two relationships can be demonstrated between the inhibition of the Ca-mediated function and the atomic number of the lanthanide ion. Extracellular membranes appear to respond selectively to Tm(III) while intracellular membranes lack the Tm peak and instead exhibit a broad trend in which the smaller ions are the least effective inhibitors.     

Multispectral imaging of ion transport in neutral carrier-based cation-selective membranes.

BACKGROUND: High-resolution spectroscopic imaging of the cross section of ion-selective membranes during real-time electrochemical measurements is termed spectroelectrochemical microscopy (SpECM). SpECM is aimed for optimizing the experimental conditions in mass transport controlled ion-selective electrode (ISE) membranes for improved detection limit. METHODS: The SpECM measurements are performed in a thin layer electrochemical cell. The key element of the cell is a membrane strip spacer ring assembly which forms a two compartment electrochemical cell. The cell is placed onto the stage of a microscope and the membrane strip is positioned in the center of the field of view. A slice of the image is focused onto the entrance slit of the imaging spectrometer. RESULTS: SpECM has been used for the determination of the diffusion coefficients of different membrane ingredients and for the quantitative assessment of the charged site concentrations in ISE membranes and membrane plasticizers. In addition, changes in the concentration profiles of the ionophore (free and complexed) and charged mobile sites inside the ISE membranes are documented upon the application of large external voltages. CONCLUSIONS: This account demonstrates the power and advantages of SpECM, a multispectral imaging method for investigations of mass transport processes in ISE membranes during electrochemical measurements.     

A neutral theory of biogenesis

The selective Darwinian theory of chemical evolution is critically reviewed and the tentative conclusion is reached that neither the theoretical analyses nor the experiments with phages can really prove it. An alternative proposal is put forth which considers the possibility that the biogenetic process has been driven by stochastic forces, e.g. it took place in the absence of Darwinian selection which, in turn, started only when the first protocells came into existence. The dynamics of the early self-organization of living structures should be understood in terms of self-assembly. The complexification of living matter is thus not represented as a gradual phenomenon but as a series of abrupt and relatively fast transitions consisting in the aggregation of pre-systems which had evolved by their own. The shift towards new and variegated states proposed by the bifurcation theory are not considered particularly relevant for reasons reported in the text, nor is it believed that dissipation can entirely account for the order observed in living cells.     

Sodium ion diffusion through liposome membranes containing cerebroside

Na⁺ efflux from liposomes (small unilamellar vesicles, SUV) of various compositions was studied, using ²²Na⁺ and ³H-labelled stachyose in simultaneous dual isotope measurements, stachyose being used as a measure of liposome disintegration. Dialysis was utilised to separate liposomes from extra-liposomal activity.Liposomes were made from egg lecithin and sphingomyelin and from mixtures of egg lecithin, sphingomyelin, cerebroside, sulphatide and cholesterol. All mixtures produced more leaky and less stable SUVs than pure lecithin and pure sphingomyelin. The incorporation of cerebroside is significantly smaller than that of the phospholipids including sphingomyelin. It was found that membranes containing cerebroside had a significantly higher Na⁺ permeability than membranes without cerebroside.     

Lipid modulation of ion channels through specific binding sites

Ion channel conformational changes within the lipid membrane are a key requirement to control ion passage. Thus, it seems reasonable to assume that lipid composition should modulate ion channel function. There is increasing evidence that this implicates not just an indirect consequence of the lipid influence on the physical properties of the membrane, but also specific binding of selected lipids to certain protein domains. The result is that channel function and its consequences on excitability, contractility, intracellular signaling or any other process mediated by such channel proteins, could be subjected to modulation by membrane lipids. From this it follows that development, age, diet or diseases that alter lipid composition should also have an influence on those cellular properties. The wealth of data on the non-annular lipid binding sites in potassium channel from Streptomyces lividans (KcsA) makes this protein a good model to study the modulation of ion channel structure and function by lipids. The fact that this protein is able to assemble into clusters through the same non-annular sites, resulting in large changes in channel activity, makes these sites even more interesting as a potential target to develop lead compounds able to disrupt such interactions and hopefully, to modulate ion channel function. This Article is Part of a Special Issue Entitled: Membrane Structure and Function: Relevance in the Cell's Physiology, Pathology and Therapy.     

Understanding how biodiversity unfolds through time under neutral theory

Theoretical predictions for biodiversity patterns are typically derived under the assumption that ecological systems have reached a dynamic equilibrium. Yet, there is increasing evidence that various aspects of ecological systems, including (but not limited to) species richness, are not at equilibrium. Here, we use simulations to analyse how biodiversity patterns unfold through time. In particular, we focus on the relative time required for various biodiversity patterns (macroecological or phylogenetic) to reach equilibrium. We simulate spatially explicit metacommunities according to the Neutral Theory of Biodiversity (NTB) under three modes of speciation, which differ in how evenly a parent species is split between its two daughter species. We find that species richness stabilizes first, followed by species area relationships (SAR) and finally species abundance distributions (SAD). The difference in timing of equilibrium between these different macroecological patterns is the largest when the split of individuals between sibling species at speciation is the most uneven. Phylogenetic patterns of biodiversity take even longer to stabilize (tens to hundreds of times longer than species richness) so that equilibrium predictions from neutral theory for these patterns are unlikely to be relevant. Our results suggest that it may be unwise to assume that biodiversity patterns are at equilibrium and provide a first step in studying how these patterns unfold through time.     

High conversion in asymmetric hydrolysis during permeation through enzyme-multilayered porous hollow-fiber membranes

We describe a novel porous hollow-fiber support for immobilizing aminoacylase in multilayers. Epoxy-group-containing polymer chains were grafted onto a porous hollow-fiber membrane by radiation-induced graft polymerization of glycidyl methacrylate, and subsequently a diethylamino group as an anion-exchange group was introduced into the graft chain. Aminoacylase was adsorbed in multilayers by allowing the amioacylase buffer solution to permeate through the pores across the hollow fiber; the graft chains provided three-dimensional space for the enzymes because of their electrostatic repulsion. The adsorbed enzyme at a degree of multilayer binding of 15 was cross-linked with glutaraldehyde to prevent leakage. An acetyl-DL-methionine solution was allowed to permeate through the pores surrounded by the aminoacylase-immobilized graft chain. Production of L-methionine was observed at a 4.1 mol/h per L of the fiber for a space velocity of 200 h(-1), defined as the flow rate of the effluent penetrating the outside surface of the hollow fiber divided by the membrane volume including the lumen.     

Cadmium and thallous ion permeabilities through lipid bilayer membranes

Cadmium (Cd2+) and thallous ion (Tl+) permeabilities were measured in planar (Mueller-Rudin) lipid bilayer membranes made from diphytanoylphosphatidylcholine in decane. Permeabilities of the electroneutral Cl- complexes, measured with tracers (109Cd and 204Tl), were about 10(-8) cm X s-1 for CdCl2 and 10(-6) cm X s-1 for TlCl. Electrical conductance measurements showed that permeabilities to Cd2+ and Tl+ were approx. 10(-11) cm X s-1, similar to the Na+ permeability. The low permeabilities to both Cd2+ and CdCl2 are consistent with biological studies which suggest that Cd transport and toxicity are protein mediated and correlated with Cd2+, not CdCl2, concentration. However, the low bilayer permeability to Tl+ raises questions about recent reports that Tl+ is a lipid permeable cation in biological membranes and liposomes. An alternative explanation for the lipid permeable behavior of Tl+ is presented, based on the diffusion of TlCl and other complexes of Tl+ with inorganic and organic anions.