David W. Deamer

Permeability of lipid bilayers to water and ionic solutes

The lipid bilayer moiety of biological membranes is considered to be the primary barrier to free diffusion of water and solutes. This conclusion arises from observations of lipid bilayer model membrane systems, which are generally less permeable than biological membranes. However, the nature of the permeability barrier remains unclear, particularly with respect to ionic solutes. For instance, anion permeability is significantly greater than cation permeability, and permeability to proton-hydroxide is orders of magnitude greater than to other monovalent inorganic ions. In this review, we first consider bilayer permeability to water and discuss proposed permeation mechanisms which involve transient defects arising from thermal fluctuations. We next consider whether such defects can account for ion permeation, including proton-hydroxide flux. We conclude that at least two varieties of transient defects are required to explain permeation of water and ionic solutes.

Lamellar and hexagonal lipid phases visualized by freeze-etching

Abstract 1. 1. Rapid freezing of lipid-water preparations preserves the structure of the high temperature phases. Both lamellar and hexagonal phases can be readily demonstrated by electron microscopy and X-ray observations. 2. 2. Without careful controls, contaminants are readily deposited on fresh fracture faces. The contamination can take the form of particles resembling those found on natural membranes. 3. 3. Neither degree of saturation, degree of hydration, nor cholesterol admixture significantly affects the appearance of lamellar fracture faces which appear uniformly smooth. On uncontaminated specimens, no structures were found which resembled the particulate material of natural membranes.

Oxidative stress impairs the function of sarcoplasmic reticulum by oxidation of sulfhydryl groups in the Ca2+-ATPase

Sarcoplasmic reticulum (SR) microsomes were oxidized by exposure to peroxydisulfate, hydrogen peroxide, or iron/ascorbate or by extended storage. The decline in Ca2+-ATPase activity, Ca2+ transport, and the increase in Ca2+ permeability which occurred under these conditions did not appear to result from lipid oxidation because these functional changes were not correlated with the amount of thiobarbituric acid-reactive lipid. Consistent with this interpretation, lipid antioxidants did not prevent the decline in SR function. This suggests that inhibition was independent of lipid oxidation. Instead, oxidation directly inhibited the Ca2+-ATPase. The decline in enzyme activity may be due to oxidation of SH groups, as suggested by the ability of reducing agents to prevent inhibition, the decline in sulfhydryl content of oxidized SR, and the ability of sulfhydryl-binding agents to inhibit Ca2+-ATPase. Inhibition was not primarily due to crosslinking of the Ca2+-ATPase, because sodium dodecyl sulfate-polyacrylamide gels of normal and oxidized SR showed that the area of the Ca2+-ATPase band was not correlated with the Ca2+-ATPase activity. Inhibition of the Ca2+-ATPase by oxidative stress is relevant to models of cellular dysfunction in which toxicity is caused by a rise in intracellular calcium.

Amphiphilic components of the murchison carbonaceous chondrite: Surface properties and membrane formation

We have investigated physicochemical properties of amphiphilic compounds in carbonaceous meteorites. The primary aim was to determine whether such materials represent plausible sources of lipid-like compounds that could have been involved as membrane components in primitive cells. Samples of the Murchison CM2 chondrite were extracted with chloroform-methanol, and the chloroform-soluble material was separated by two-dimensional thin layer chromatography. Fluorescnece, iodine stains and charring were used to identify major components on the plates. These were than scraped and eluted as specific fractions which were investigated by fluorescence and absorption spectra, surface chemical methods, gas chromatography-mass spectrometry, and electron microscopy. Fraction 5 was strongly fluorescent, and contained pyrene and fluoranthene, the major polycyclic aromatic hydrocarbons of the Murchison chondrite. This fraction was also present in extracts from the Murray and Mighei CM2 chondrites. Fraction 3 was surface active, forming apparent monomolecular films at air-water interfaces. Surface force measurements suggested that fraction 3 contained acidic groups. Fraction 1 was also surface active, and certain components could self-assemble into membranous vesicles which encapsulated polar solutes. The observations reported here demonstrate that organic compounds plausibly available on the primitive Earth through meteoritic infall are surface active, and have the ability to self-assemble into membranes.

The chemical logic of a minimum protocell

Traditional schemes for the origin of cellular life on earth generally suppose that the chance assembly of polymer synthesis systems was the initial event, followed by incorporation into a membrane-enclosed volume to form the earliest cells. Here we discuss an alternative system consisting of replicating membrane vesicles, which we define as minimum protocells. These consist of vesicular bilayer membranes that self-assemble from relatively rare organic amphiphiles present in the prebiotic environment. If some of the amphiphiles are primitive pigment molecules asymmetrically oriented in the bilayer, light energy can be captured in the form of electrochemical ion gradients. This energy could then be used to convert relatively common precursor molecules into membrane amphiphiles, thereby providing an initial photosynthetic growth process, as well as an appropriate microenvironment for incorporation and evolution of polymer synthesis systems.

Permeability of lipid bilayers to amino acids and phosphate

Permeability coefficients for amino acid classes, including neutral, polar, hydrophobic, and charged species, were measured and compared with values for other ionic solutes such as phosphate. The rates of efflux of glycine, lysine, phenylalanine, serine and tryptophan were determined after they were passively entrapped in large unilamellar vesicles (LUVs) composed of egg phosphatidylcholine (EPC) or dimyristoylphosphatidylcholine (DMPC). The following permeability coefficients were obtained for: glycine, 5.7 x 10(-12) cm s-1 (EPC), 2.0 x 10(-11) cm s-1 (DMPC); serine, 5.5 x 10(-12) cm s-1 (EPC), 1.6 x 10(-11) cm s-1 (DMPC); lysine, 5.1 x 10(-12) cm s-1 (EPC), 1.9 x 10(-11) cm s-1 (DMPC); tryptophan, 4.1 x 10(-10) cm s-1 (EPC); and phenylalanine, 2.5 x 10(-10) cm s-1 (EPC). Decreasing lipid chain length increased permeability slightly, while variations in pH had only minor effects on the permeability coefficients of the amino acids tested. Phosphate permeability was in the range of 10(-12)-10(-13) cm s-1 depending on the pH of the medium. The values for the polar and charged amino acids were surprisingly similar to those previously measured for monovalent cations such as sodium and potassium, which are in the range of 10(-12)-10(-13) cm s-1, depending on conditions and the lipid species used. This observation suggests that the permeation rates for the neutral, polar and charged amino acids are controlled by bilayer fluctuations and transient defects, rather than partition coefficients and Born energy barriers. The results are relevant to the permeation of certain peptides into lipid bilayers during protein translocation and membrane biogenesis.

Catecholamine uptake and concentration by liposomes maintaining pH gradients

Abstract Liposomes were prepared with pH gradients across their membranes (acidic interiors with respect to the external buffer). These liposomes efficiently concentrated several catecholamines (dopamine, norepinephrine, and epinephrine) added to the external buffer. Our observations support a mechanism which suggests that pH gradients may contribute to uptake of catecholamines by sub-cellular storage sites.

Proton permeation of lipid bilayers

Proton permeation of the lipid bilayer barrier has two unique features. First, permeability coefficients measured at neutral pH ranges are six to seven orders of magnitude greater than expected from knowledge of other monovalent cations. Second, proton conductance across planar lipid bilayers varies at most by a factor of 10 when pH is varied from near 1 to near 11. Two mechanisms have been proposed to account for this anomalous behavior: proton conductance related to contaminants of lipid bilayers, and proton translocation along transient hydrogen-bonded chains (tHBC) of associated water molecules in the membrane. The weight of evidence suggests that trace contaminants may contribute to proton conductance across planar lipid membranes at certain pH ranges, but cannot account for the anomalous proton flux in liposome systems.Two new results will be reported here which were designed to test the tHBC model. These include measurements of relative proton/potassium permeability in the gramicidin channel, and plots of proton flux against the magnitude of pH gradients. (1) The relative permeabilities of protons and potassium through the gramicidin channel, which contains a single strand of hydrogenbonded water molecules, were found to differ by at least four orders of magnitude when measured at neutral pH ranges. This result demonstrates that a hydrogen-bonded chain of water molecules can provide substantial discrimination between protons and other cations. It was also possible to calculate that if approximately 7% of bilayer water was present in a transient configuration similar to that of the gramicidin channel, it could account for the measured proton flux. (2) The plot of proton conductance against pH gradient across liposome membranes was superlinear, a result that is consistent with one of three alternative tHBC models for proton conductance described by Nagle elsewhere in this volume.

Encapsulation of macromolecules by lipid vesicles under simulated prebiotic conditions.

SummaryPhospholipid vesicles (liposomes) were subjected to dehydration-hydration cycles in the presence of 6-carboxyfluorescein or salmon sperm DNA. We found that the vesicles fused into multilamellar structures during dehydration with solutes trapped between the lamellae. Upon rehydration the lamellae swelled and formed large vesicular structures containing solute. This model can be used to study encapsulation of macromolecules by lipid membranes to form protocellular structures under prebiotic conditions.

A novel method for encapsulation of macromolecules in liposomes

Hemoglobin and alkaline phosphatase were each encapsulated in phosphatidylcholine liposomes using a dehydration-rehydration cycle for liposome formation. In this method, liposomes prepared by sonication are mixed in aqueous solution with the solute desired to be encapsulated and the mixture is dried under nitrogen in a rotating flask. As the sample is dehydrated, the liposomes fuse to form a multilamellar film that effectively sandwiches the solute molecules. Upon rehydration, large liposomes are produced which have encapsulated a significant fraction of the solute. The optimal mass ratio of lipid to solute is approx. 1:2 to 1:3. This method has potential application in large-scale liposome production, since it depends only on a controlled drying and rehydration process, and does not require extensive use of organic solvents, detergents, or dialysis systems.