Showing papers on "Membrane published in 1974"

Biological Membranes as Bilayer Couples. A Molecular Mechanism of Drug-Erythrocyte Interactions

Abstract: We propose that membranes whose proteins and polar lipids are distributed asymmetrically in the two halves of the membrane bilayer can act as bilayer couples, i.e., the two halves can respond differently to a perturbation. This hypothesis is applied to the interactions of amphipathic drugs with human erythrocytes. It is proposed that anionic drugs intercalate mainly into the lipid in the exterior half of the bilayer, expand that layer relative to the cytoplasmic half, and thereby induce the cell to crenate, while permeable cationic drugs do the opposite and cause the cell to form cup-shapes. This differential distribution of the drugs is attributed to interactions with the phosphatidylserine that is concentrated in the cytoplasmic half of the membrane. Impermeable amphipathic drugs intercalate only into the exterior half of the bilayer, and therefore are crenators of the intact cell. Several predictions of this hypothesis have been confirmed experimentally with erythrocytes and erythrocyte ghosts. The bilayer couple hypothesis may contribute to the explanation of many membrane-mediated phenomena in cell biology.

Purification of a plasma membrane-bound adenosine triphosphatase from plant roots.

Abstract: Publisher Summary The purification of a specific membrane-bound ATPase is complicated by the presence of several membrane-associated ATPases in plant cells. In situ cytochemical studies have shown ATPase activity to be associated with mitochondria, plastids, plasma membranes, tonoplast, Golgi apparatus, and the endoplasmic reticulum of plant root cells. Using continuous gradient centrifugation and taking advantage of differing pH and ion-sensitivities of the various ATPases, the chapter has distinguished 5 different membrane-associated ATPases in oat root extracts. To facilitate identification of these membranes, a discontinuous gradient was devised, based on the membrane densities found from isopycnic centrifugations, which provided membrane fractions enriched in the various ATPases.

Lateral diffusion of rhodopsin in the photoreceptor membrane

Abstract: Rhodopsin undergoes rapid lateral diffusion in the disk membranes of isolated frog and mudpuppy rods. The rate of lateral diffusion is consistent with the rapid rotational diffusion of rhodopsin: both indicate the disk membrane is highly fluid with a viscosity of ∼1P.

The interrelationship between sodium and calcium fluxes across cell membranes

Abstract: III. Calcium Fluxes in Nerve . . . . . . . . . . . . . . . . . . . . . . . . . . 41 A. Invertebrate Nervous Tissue . . . . . . . . . . . . . . . . . . . . . . . . 41 1. The Squid Axon . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 a) Calcium Efflux . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 b) Calcium Influx . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 c) External Calcinm-Dependent Sodium Efflux . . . . . . . . . . . . . . 48 d) Do Cardiac Glycosides Directly Affect the Calcium Fluxes ? . . . . . . . 49 e) A Model for the Na-Ca Exchange Mechanism in Squid Axons . . . . . . 50 2. Nerves of Other Invertebrates . . . . . . . . . . . . . . . . . . . . . . 53

Preparation and Use of Liposomes as Models of Biological Membranes

Abstract: The recognition that biological cells exploit the water-oil interfacial activity of certain lipids to define anatomical boundaries has, in recent years, encouraged many workers in this laboratory* and others to develop and study protein-free model membrane systems prepared with such compounds A considerable technical advance was made more than 10 years ago, when Mueller et al (1962a,b) first reported a method for preparing usable preparations of bimolecular membrane from membrane molecules, the so-called black lipid membranes or BLMs The merits of this powerful technique were quickly realized, and, as with Aladdin’s lamp, a property requested was a property acquired! Indeed, it might be said that the skeptics themselves were the holders of the lamp, because it was they who, by dismissing the model as unrealistic for a succession of reasons, indicated explicitly the characteristic implicit (passive) properties of a biological membrane: before very long, the earlier convictions of the Langmuir-Hardy-Rideal schools of surface chemistry, relating the role of amphiphilic molecules to biological membrane structure, were vindicated

Phospholipids and plant membrane permeability

Abstract: SUMMARY Current views on membrane construction are presented as a basis for discussing certain modifications of structure that may render membranes leaky, allowing solutes to diffuse from them. Biophysical experiments on dehydrated membranes indicate that, when their water content is reduced below 20%, they no longer adopt the lipid bilayer conformation. When dry seeds, spores or lichens are placed in water, soluble cell constituents leak from them for a few seconds or minutes until membrane integrity is re-established; the same may apply also to the liberation of solutes from pollen grains. Moreover, there are indications that ‘dry’ in this context means having less than about 20% water. Some plants are sensitive to chilling, their tissues becoming leaky when exposed to temperatures between 0 and 10°C. This chilling-induced leakage, attributed to a phase change in all the phospholipids of which the membrane is composed occurs at a slightly lower temperature than the change in the activation energy of membrane-bound enzymes. At senescence, leaf tissues become leaky and may eventually dry out. So much phospholipid disappears from senescing cotyledons of cucumber that there is no longer sufficient to fabricate complete and intact membranes around the cells. Treating cucumber cotyledons with iodoacetate also causes a loss of phospholipids, allowing solutes to leak out of the cells. It is possible that exposing seeds or mature plant tissues to oxygen at pressures of 1 or more atm renders them leaky because of lipid peroxidation.

Shape of the hydrophobic barrier of phospholipid bilayers (Evidence for water penetration in biological membranes)

Abstract: Previous work has shown that there is a small solvent effect on the electron spin resonance spectra of nitroxide spin labels. The aim of this paper is to develop a semiquantitative treatment of the solvent effects and to use this treatment to estimate the shape of the hydrophobic barrier (i.e., polarity profile) of lipid bilayers. In this semiquantitative treatment of solvent effects, the total effect on the isotropic14N coupling constant, ΔA, is expressed as a sum of terms associated with van der Waals interactions, hydrogen bonding, the charged double layer of phospholipid polar groups and the membrane potential. The magnitude of ΔA as a function of the electric field is estimated with Huckel molecular orbital theory and, independently, using the Onsager model. With these relations, estimates of the relative importance of the various solvent effect terms are obtained from accurate ESR measurements on dilute solutions of di-t-butyl nitroxide in thirty-three solvents and calculations of the electric field produced by the charged double layer and the membrane potential. To estimate the shape of the hydrophobic barrier of lipid bilayers, fatty acids andl-α-lecithins with doxyl (i.e., 4′, 4′-dimethyloxazolidine-N-oxyl) labels bonded to various positions along the lipid chains were diffused into phospholipid vesicles and membranes (from the calf liver microsomal fraction). The shape of the hydrophobic barrier is plotted as a polarity index operationally defined in terms of the ΔA of the lipid spin labels. The effect of the charged double layers is less important than water penetration except when the spin label is within a few Angstroms of the charged groups. Any effects of a membrane potential on ΔA are insignificant. A comparison of ESR spectra indicates that significant water penetration into the bilayer occurs in both the pure lipid bilayers and in the membrane preparations.

Methods in membrane biology

Abstract: 1 Electron Microscopic Methods in Membrane Biology.- 2 The Use of Phospholipases in the Determination of Asymmetric Phospholipid Distribution in Membranes.- 3 External Labeling of Cell Surface Carbohydrates.- 4 Phospholipid Exchange between Membranes.

A molecular model of membrane excitability.

Abstract: Alamethicin, monazomycin, or EIM induce electrical excitability in lipid bilayers. The voltage-dependent gating displays all the characteristics observed in excitable cells and its basic features can be quantitatively described by the Hodgkin-Huxley equations. A common molecular mechanism of membrane excitation has been postulated. It assumes that in the absence of an electrical field the channel-forming molecules lie at the surface of the membrane. An applied potential tilts them from the surface into the hydrocarbon region of the bilayer. Once in this position the molecules diffuse laterally and form aggregates which act as channels for the flow of ions. In the case of alamethicin we assume that the molecule forms an elongated ellipsoid with two glutamic residues at one end, and a metal ion in four- or five-fold coordination with peptide carbonyl oxygens at the other. An applied field pulls the cationic end through the membrane to the other side, while the glutamic residues hold the other end attached to the original surface. The molecules now span the membrane and aggregate, forming oligomeric channels in which most of the peptide carbonyls face toward the center, and the methyl groups outward. Monomers and dimers do not conduct and an individual channel can have different conductance values depending on the number of monomers in the aggregate and the resulting channel diameter. A quantitative description of this process matches observed gating kinetics, gating currents, and the single channel conductance increments. Without additional assumptions, inactivation follows directly from the aggregation process because with proper rate constants, the average degree of polymerization and therefore number of open channels goes through a maximum in time. The model may also apply to the excitation process of higher cells.

Isolation and partial characterization of rat brain synaptic plasma membranes.

Abstract: — Synaptic plasma membranes from the cortices of adult rat brain were isolated from synaptosomes prepared by flotation of a washed mitochondrial pellet (P2) in a discontinuous Ficoll-sucrose gradient. Contamination of the synaptosome fraction by microsomes was estimated by enzymic and chemical analysis to be less than 15 per cent. (2) The purified synaptosome fraction was subjected to osmotic shock, subfractionated on a discontinuous sucrose gradient and the distribution of enzymic and chemical markers for synaptic plasma membranes, microsomal membranes and mitochondria was determined. (3) Comparison of synaptosome subfractions prepared in the presence and absence of 1 mM NaH2 PO4/0.1 mM EDTA buffer pH 7.5, indicated that the ionic composition of the isolation medium markedly affected the distribution and enzymic composition of the subfractions. (4) Synaptic plasma membranes prepared in the presence of PO4/EDTA exhibited a 10-fold enrichment in [Na++ K+] ATPase and were characterized by less than 15 and 10 per cent contamination by microsomes and mitochondria respectively. (5) The polypeptide composition of the purified synaptic plasma membranes was compared with the microsomes and mitochondria by polyacrylamide gel electrophoresis in sodium dodecyl sulphate. No differences between the protein and glycoprotein composition of the synaptic plasma membranes and microsomes were detected. The mitochondria, in contrast, possessed a unique protein composition.

Statistical analysis of alamethicin channels in black lipid membranes.

Abstract: Pore formation of alamethicin has been studied by the analysis of steadystate fluctuations of single-pore conductances. An aggregation model is proposed where transitions to the next higher or lower pore state occur by uptake or release of one monomer. It is assumed that alamethicin forms an elongated loop in the bilayer. The main voltage-dependent step is the insertion of this monomer into the membrane after complexation with a cation. This mechanism is equivalent to dipole orientation in an electric field. Pore formation is restricted by the energy required to enlarge the channel in the membrane.

[26] Isolation of (Na+ + K+)-ATPase

Abstract: Publisher Summary the discovery of (Na + + K + )-ATPase, intensive studies have been carried out to elucidate its rote in the active transport of sodium and potassium. The enzyme has not been purified to a homogeneous state, but recently some progress in this direction has been made and information about the molecular components of the sodium pump is beginning to emerge. The difficulties encountered in isolation of (Na + + K + )-ATPase are mainly explained by the firm association of the enzyme with cell membranes. The membranes are sheared into fragments and vesicles of varying sizes during homogenization. This explains that the (Na + + K + )-ATPase is found in all sediments after conventional differential centrifugations and that a major fraction of the activity in the subcellular fractions is latent a owing to limited access of substrate or activators to the inside of the membrane vesicles. Furthermore, it is difficult to separate the enzyme from other proteins associated with the cell membranes.

Alterations in the Outer Membrane of the Cell Envelope of Heptose-Deficient Mutants of Escherichia coli

Abstract: The composition of the cell envelope of a heptose-deficient lipopolysaccharide mutant of Escherichia coli, GR467, was studied after fractionation into its outer and cytoplasmic membrane components by means of sucrose density gradient centrifugation. The outer membrane of GR467 had a lower density than that of its parent strain, CR34. Analysis of the fractionated membranes of GR467 indicated that the phospholipid-to-protein ratio had increased 2.4-fold in the outer membrane. The ratio in the mutant cytoplasmic membrane was also increased, although to a lesser extent. By employing a third parameter, the lipid A content of the outer membrane, it was found that the observed phospholipid-to-protein change in the outer membrane was due predominantly to a decrease in the relative amount of protein. This decrease in protein was particularly significant, since it was concomitant with a 68% decrease in the lipid A recovered in the outer membrane of GR467 relative to the lipid A recovered in the outer membrane of CR34. Similar findings were observed in a second heptose-deficient mutant of E. coli, RC-59. The apparent protein deficiency in GR467 was further studied by subjecting solubilized envelope proteins to sodium dodecyl sulfate-polyacrylamide gel electrophoresis. It was found that major envelope proteins which were localized in the outer membrane were greatly diminished in GR467. Two revertants of GR467 with the wild-type amounts of heptose had wild-type relative levels of protein in their outer membranes. A partial heptose revertant had a relative level of protein in its outer membrane between those of the mutant and wild type. Images