Formation of ion channels by a negatively charged analog of gramicidin A.

O-pyromellitylgramicidin is a derivative of gramicidin in which three carboxyl groups are introduced at the terminal hydroxyl end of the peptide. Experiments with artificial lipid membranes indicate that this negatively charged analog forms ion-permeable channels in a way similar to that of gramicidin. If O-pyromellitylgramicidin is added to only one aqueous solution, the membrane conductance remains small, but increases by several orders of magnitude if the same amount is also added to the other side. In accordance with the dimer model of the channel, the membrane conductance under symmetrical conditions is proportional to the square of the aqueous concentration of O-pyromellitylgramicidin over a wide range. The ratio lambdaPG/lambdaG of the single-channel conductance of O-pyromellitylgramicidin to that of gramicidin is close to unity at high ionic strength, but increases more than fivefold at smaller ionic strength (0.01 M). This observation is explained in terms of an electrostatic effect of the fixed negative charges localized near the mouth of the channel. In a mixture of O-pyromellitylgramicidin and gramicidin, unit conductance steps of intermediate size are observed in addition to the conductance steps corresponding to the pure compounds, indicating the formation of hybrid channels. Hybrid channels with preferred orientation may be formed if small amounts of gramicicin and O-pyromellitylgramicidin are added to opposite sides of the membrane. These hybrid channels show a distinct asymmetry in the current-voltage characteristic.     

Centrifugal field relaxation and ionic strength effects on calf thymus DNA gels

The gellike phase formed by DNA on sedimentation to the bottom of a centrifuge cell has been studied as a function of centrifuge speed and of solvent ionic strength. The swelling pressure of the gel was found to be logarithmically related to the DNA concentration and also to the ionic strength of the solvent. The concept of electrostatic persistence length was useful in interpreting the gel-phase volume changes occurring with ionic strength.     

Interactions of voltage-sensing dyes with membranes. III. Electrical properties induced by merocyanine 540

The effects of merocyanine 540 on the electrical properties of lipid bilayer membranes have been investigated. The alterations this dye was found to produce in the intrinsic conductances of these membranes were minimal, but it profoundly altered the conductances produced by extrinsic permeant species. These alterations were much larger for neutral membranes than for negatively charged ones. The dye increased the conductances mediated by positively charged permeant species and decreased those by negatively charged permeant species, suggesting that it produces a negative electrostatic potential on the membrane; it also altered the kinetics and the voltage dependencies of permeation by these charge carriers. The magnitudes of dye-mediated conductance changes were much larger for positively charged permeants than for negatively charged ones; also, changes in ionic strength altered these dye effects in opposite directions from those predicted by the Stern equation, and the dependence of the conductance alteration on dye concentration was steeper than that predicted by this equation. Finally, only very small changes in liposome zeta potentials were induced by the dye. Calculations show that a large fraction of these effects can be accounted for by the dipole potential produced by merocyanine at the membrane surface, but that additional effects of the dye must be postulated as well.     

Electrostatic interactions during activation of coagulation factor IX via the tissue factor pathway: effect of univalent salts

Interaction between the Gla-domain of coagulation proteins and negatively charged phospholipid membranes is essential for blood coagulation reactions. The interaction is calcium-dependent and mediated both by electrostatic and hydrophobic forces. This report focuses on the electrostatic component of factor IX activation via the extrinsic pathway. Effective charges during the reaction are measured by ionic titration of activity, according to the Debye-Huckel and Gouy-Chapman models. Rates of activation decrease with ionic strength independently of the type of monovalent salt used to control ionic strength. Moreover, the effect of ionic strength decreases at concentrations of charged phospholipid approaching saturation levels, indicating that membrane charges participate directly in the ionic interaction measured. The effective charge on calcium-bound factor IX during activation on phospholipid membranes is 0.95+/-0.1. Possible sites mediating contacts between the Gla-domain and membranes are selected by geometrical criteria in several metal-bound Gla-domain structures. A pocket with a solvent opening-pore of area 24-38 A2 is found in the Gla-domain of factors IX, VII, and prothrombin. The pocket contains atoms with negative partial charges, including carboxylate oxygens from Gla residues, and has a volume of 57-114 A3, sufficient to accommodate additional calcium atoms. These studies demonstrate that electrostatic forces modify the activity coefficient of factor IX during functional interactions and suggest a conserved pocket motif as the contact site between the calcium-bound Gla-domain and charged membranes.     

Some physicochemical properties of plasma membrane vesicles.

The plasma membranes of many animal cells can be disrupted into small sealed vesicles that can be purified centrifugally and utilized for studies on membrane transport. The vesicles behave as micro-osmometers. However, the presence of charges fixed at the internal and external surfaces of the membrane walls produce pH levels at these surfaces that deviate considerably from bulk pH. Transverse symmetry of charge distribution further leads to transverse asymmetry of surface pH. Finally, charges fixed at the internal membrane surface produced significant Donnan osmotic effects that depend upon membrane composition and ionic environment.     

Interaction between filamentous actin and lipid bilayer causes the increase of syringomycin E channel-forming activity

The effect of filamentous (F) actin on the channel-forming activity of syringomycin E (SRE) in negatively charged and uncharged bilayer lipid membranes (BLM) was studied. F-actin did not affect the membrane conductance in the absence of SRE. No changes in SRE-induced membrane conductance were observed when the above agents were added to the same side of BLM. However, the opposite side addition of F-actin and SRE provokes a multiple increase in membrane conductance. The similar voltage dependence of membrane conductance, equal values of single channel conductance and the effective gating charge of the channels upon F-actin action suggests that the actin-dependent increase in BLM conductance may result from an increase in the number of opened SRE-channels. BLM conductance kinetics depends on the sequence of SRE and F-actin addition, suggesting that actin-dependent rise of conductance may be induced by BLM structural changes that follow F-actin adsorption. F-actin exerted similar effect on membrane conductance of both negatively charged and uncharged bilayers, as well as on conductance of BLM with high ionic strength bathing solution, suggesting the major role for hydrophobic interactions in F-actin adsorption on lipid bilayer.     

The interaction of leucocidin with the cell membrane of the polymorphonuclear leucocyte

1. When leucocidin is incubated with leucocytes it is inactivated in solution and only a little adsorption takes place. This reaction has been used to purify the cell membrane. 2. The interaction of the membrane with leucocidin is very complex and at least three phenomena occur: (a) An inactivation of leucocidin in solution by large amounts of membrane which is synergistic between the two components of leucocidin, is thermolabile and is not inhibited by electrolyte. (b) An adsorption of leucocidin which is synergistic between the two components of leucocidin, does not proceed to the same extent as the inactivation in solution and is a function of the phospholipid components. Phospholipids isolated from the membrane adsorb leucocidin but the adsorption requires the presence of several molecular species. (c) Polymerization of leucocidin induced by tenfold smaller amounts of membrane than are required to bring about the first two interactions. The polymerization is reversed by adjustment of the ionic strength. It is due to the presence of the lipid components of the membrane. Different lipids are equally effective in inducing the polymerization. 3. Each component of leucocidin will polymerize in the absence of membranes and lose biological activity at low ionic strength. This is reversed by electrolyte and it does not proceed to the same extent as in the presence of membranes. 4. The nature of the interaction of leucocidin with cells, membranes and lipids and the spontaneous polymerization indicate that each component of leucocidin can adopt different isomeric forms. 5. The relationship of the interaction with the membrane to the cytotoxic effect of leucocidin is discussed.     

Isolation of phospholipase A2 from sheep erythrocyte membranes in the presence of detergents

Isolation of phospholipase A₂ (EC 3.1.1.4) from sheep erythrocyte membranes was carried out by a combination of (1) extraction of membranes at low ionic strength, (2) solubilization of extracted membranes with sodium dodecyl sulfate, (3) replacement of dodecyl sulfate with cholate by means of gel exclusion chromatography and (4) affinity chromatography on dialkyl-phosphatidylcholine-Sepharose in the presence of cholate. The phospholipase was prepared with good yield and purified to near homogeneity, as judged by sodium dodecyl sulfate gel electrophoresis. The protein is a minor component of the sheep erythrocyte membrane and has an apparent molecular weight of 18 500.     

A theory for the effects of neutral carriers such as the macrotetralide actin antibiotics on the electric properties of bilayer membranes

Summary To develop a quantitiative theoretical treatment for the effects of neutral macrocyclic antibiotics on the electrical properties of phospholipid bilayer membranes, this paper proceeds from the known ability of such molecules to form stoichiometric, lipid-soluble complexes with cations and deduces the electrical properties that a simple organic solvent phase would have if it were made into a membrane of the thinness of the phospholipid bilayer. In effect, we postulate that the essential barrier to ion movement across a bilayer membrane is its liquid-like hydrocarbon interior and that the neutral macrocyclic antibiotics bind monovalent cations and solubilize them in the membrane as mobile positively charged complexes. Using the Poisson-Boltzmann equation to describe the equilibrium profile of the electrical potential, it is shown that an excess of the positive complexes over all the other ions is expected in the membrane as a net space charge for appropriate conditions of membrane thickness and values of the partition coefficients of the various ionic species and without requiring the presence of fixed charges. Describing the fluxes of these complexes by the Nernst-Planck equation and neglecting the contribution to the electric current of uncomplexed ions, theoretical expressions are derived for the membrane potential in ionic mixtures, as well as for the limiting value of the membrane conductance at zero current when the membrane is interposed between identical solutions. The expressions are given in terms of the ionic activities and antibiotic concentrations in the aqueous solutions so as to be accessible to direct experimental test. Under suitable experimental conditions, the membrane potential is described by an equation recognizible as the Goldman-Hodgkin-Katz equation, in which the permeability ratios are combinations of parameters predicted from the present theory to be independently determinable from the ratio of membrane conductances in single salt solutions. Since this identity between permeability and conductance ratios is expected also for systems obeying the Independence Principle of Hodgkin and Huxley, the applicability of this principle to membranes exposed to antibiotics is discussed, and it is shown that this principle is compatible with the permeation mechanism proposed here.     

Asymmetric electrostatic effects on the gating of rat brain sodium channels in planar lipid membranes.

The effects of ionic strength (10-1,000 mM) on the gating of batrachotoxin-activated rat brain sodium channels were studied in neutral and in negatively charged lipid bilayers. In neutral bilayers, increasing the ionic strength of the extracellular solution, shifted the voltage dependence of the open probability (gating curve) of the sodium channel to more positive membrane potentials. On the other hand, increasing the intracellular ionic strength shifted the gating curve to more negative membrane potentials. Ionic strength shifted the voltage dependence of both opening and closing rate constants of the channel in analogous ways to its effects on gating curves. The voltage sensitivities of the rate constants were not affected by ionic strength. The effects of ionic strength on the gating of sodium channels reconstituted in negatively charged bilayers were qualitatively the same as in neutral bilayers. However, important quantitative differences were noticed: in low ionic strength conditions (10-150 mM), the presence of negative charges on the membrane surface induced an extra voltage shift on the gating curve of sodium channels in relation to neutral bilayers. It is concluded that: (a) asymmetric negative surface charge densities in the extracellular (1e-/533A2) and intracellular (1e-/1,231A2) sides of the sodium channel could explain the voltage shifts caused by ionic strength on the gating curve of the channel in neutral bilayers. These surface charges create negative electric fields in both the extracellular and intracellular sides of the channel. Said electric fields interfere with gating charge movements that occur during the opening and closing of sodium channels; (b) the voltage shifts caused by ionic strength on the gating curve of sodium channels can be accounted by voltage shifts in both the opening and closing rate constants; (c) net negative surface charges on the channel's molecule do not affect the intrinsic gating properties of sodium channels but are essential in determining the relative position of the channel's gating curve; (d) provided the ionic strength is below 150 mM, the gating machinery of the sodium channel molecule is able to sense the electric field created by surface changes on the lipid membrane. I propose that during the opening and closing of sodium channels, the gating charges involved in this process are asymmetrically displaced in relation to the plane of the bilayer. Simple electrostatic calculations suggest that gating charge movements are influenced by membrane electrostatic potentials at distances of 48 and 28 A away from the plane of the membrane in the extracellular sides of the channel, respectively.     

Swelling of the Transverse Tubular System in Frog Sartorius

Electron microscopy shows that the transverse tubular system of frog sartorius swells in Ringer fluid in which NaCl is partially replaced by sucrose (sucrose isotonic solutions). At constant tonicity, the degree of swelling is roughly proportional to the decrease in ionic strength and to the sucrose concentration of the bathing solution. Swelling is time-dependent and reversible within 2 hr. The late after potential which follows a train of impulses is prolonged with swelling, but not to the extent expected from the model of Adrian and Freygang. This discrepancy remains unexplained, as does the mechanism of swelling of the transverse tubular system, although some suggestions are offered. One is that the transverse tubular system contains fixed charges and swells like a fixed charge gel.     

A horizontal apparatus for isoelectric protein purification in a segmented immobilized pH gradient

A modification of the previously described apparatus (Faupel et al. (1987) J. Biochem. Biophys. Methods 15, 147-162), for recycling isoelectric focusing in a segmented immobilized pH gradient, is here reported. The most important improvements are: (1) a horizontal, vs. the previously vertical assembly; (2) a reduction of the thickness of the central flow chamber to 6 mm, vs. the previous 3 cm length and (3) the introduction, at both gel extremities of each Immobiline segment, of polypropylene filters, thus efficiently blocking the gel in situ. The advantages are: (i) the spontaneous removal of air bubbles, which in the vertical apparatus tend to accumulate in the ceiling of the flow chamber and to obstruct the flow of electric current; (ii) a more efficient hydraulic flow with a reduced chance of heating the liquid stream in the flow chamber, due to its reduced length along the separation path and (iii) a reduced risk of gel detachment from the tube walls, due to osmotic swelling caused by focused protein zones in the gel phase and by the fixed Immobiline charges in the polyacrylamide matrix.     

Membrane and intracellular modes of sugar-dependent increments in red cell stability.

Sugar-dependent increments in red cell stability under osmotic stress can be ascribed to changes either in the membrane or in the intracellular matrix. These two possible modes of action have been tested and characterized. Rheological investigation of membrane-free haemoglobin solutions has shown that D-glucose, but not D-fructose, promotes the formation of a visco-plastic gel structure. Gel strength is a function of glucose concentration, haemoglobin concentration and temperature. The ability of various sugars to promote gel formation correlates with their solution properties. The existence of gel structure reduces K+ and haemoglobin leak from red cells whose membranes were partially destroyed by gamma-radiation. Reduced osmotic swelling in the presence of glucose is also due to gel formation since the glucose effect is lost in resealed red cell ghosts. D-Fructose does not protect red cells against radiation damage; its mode of action in increasing red cell stability under osmotic stress is a membrane effect. Cell sizing using the Coulter Counter has shown that fructose, but not glucose, can increase the maximal volume at lysis. At 50 mM, D-fructose expands the red cell ghost volume by 11.2%; this represents a 7.2% increase in membrane area. Ghost expansion by fructose is fructose concentration dependent (0-100 mM) and is insensitive to temperature variation (0-37 degrees C).     

Formation of ion channels by a negatively charged analog of gramicidin a

Summary O-pyromellitylgramicidin is a derivative of gramicidin in which three carboxyl groups are introduced at the terminal hydroxyl end of the peptide. Experiments with artificial lipid membranes indicate that this negatively charged analog forms ion-permeable channels in a way similar to that of gramicidin. If O-pyromellitylgramicidin is added to only one aqueous solution, the membrane conductance remains small, but increases by several orders of magnitude if the same amount is also added to the other side. In accordance with the dimer model of the channel, the membrane conductance under symmetrical conditions is proportional to the square of the aqueous concentration of O-pyromellitylgramicidin over a wide range. The ratio _PG/ _G of the single-channel conductance of O-pyromellitylgramicidin to that of gramicidin is close to unity at high ionic strength, but increases more than fivefold at smaller ionic strength (0.01m). This observation is explained in terms of an electrostatic effect of the fixed negative charges localized near the mouth of the channel. In a mixture of O-pyromellitylgramicidin and gramicidin, unit conductance steps of intermediate size are observed in addition to the conductance steps corresponding to the pure compounds, indicating the formation of hybrid channels. Hybrid channels with preferred orientation may be formed if small amounts of gramicidin and O-pyromellitylgramicidin are added to opposite sides of the membrane. These hybrid channels show a distinct asymmetry in the current-voltage characteristic.     

Modeling of discrete currents of single ion channels of cell membranes using synthetic nanometer pores in polyethylene terephthalate films

Study of the conductivity of single supernarrow pores (1–15 nm in diameter) formed in thin membranes (10–12 μm in thickness) from polyethylene terephthalate (PETP) has revealed discrete changes in the currents passing through such pores when applied from an external source of potential difference of 200–1000 mV. Based on several characteristics, such discrete currents (discrete conductivity changes) appeared to be identical to the so-called currents of single ionic channels in cell membranes. The supernarrow pores whose properties are described in the present work were obtained by alkaline etching of tracks in thin PETP membranes (a variant of the so-called nuclear filters). On the walls of the pores, carboxyl groups, i.e., negative fixed charges, and their compensating counterion (cation) layer are formed. Upon setting the potential difference onto the PETP membrane, this cation layer is able to transfer current, through a process called surface conductance. In the case of nanometer-sized diameters of the pores, such surface conductance can turn out to be dominating. We have shown that these discrete changes of currents passing through the nanometer pores are associated with metastability of their surface conductance. In the highly cation-selective channels in the cell membranes, there should inevitably exist an area with dominating cation surface conductance and, hence, conductance metastability. Therefore, a new explanation is proposed of the characteristic discreteness of the currents of single cation-specific ionic channels in cell membranes. Such an explanation does not rule out the existence of any other traditional explanation of the discreteness of ion channel currents.     

Fatty acid transfer from intestinal fatty acid binding protein to membranes: electrostatic and hydrophobic interactions

Intestinal fatty acid binding protein (IFABP) is thought to participate in the intracellular transport of fatty acids (FAs). Fatty acid transfer from IFABP to phospholipid membranes is proposed to occur during protein-membrane collisional interactions. In this study, we analyzed the participation of electrostatic and hydrophobic interactions in the collisional mechanism of FA transfer from IFABP to membranes. Using a fluorescence resonance energy transfer assay, we examined the rate and mechanism of transfer of anthroyloxy-fatty acid analogs a) from IFABP to phospholipid membranes of different composition; b) from chemically modified IFABPs, in which the acetylation of surface lysine residues eliminated positive surface charges; and c) as a function of ionic strength. The results show clearly that negative charges on the membrane surface and positive charges on the protein surface are important for establishing the "collisional complex", during which fatty acid transfer occurs. In addition, changes in the hydrophobicity of the protein surface, as well as the hydrophobic volume of the acceptor vesicles, also influenced the rate of fatty acid transfer. Thus, ionic interactions between IFABP and membranes appear to play a primary role in the process of fatty acid transfer to membranes, and hydrophobic interactions can also modulate the rates of ligand transfer.     

Preparative isoelectric focusing in immobilized pH gradients. II. A case report

The preparative aspects of isoelectric focusing (IEF) in immobilized pH gradients (IPG) have been investigated as a function of the following parameters: environmental ionic strength (I), gel geometry and shape of pH gradient. As model proteins, hemoglobin (Hb) A and a minor, glycosylated component (HbA1c), with a delta pI = 0.04 pH units, have been selected. The load capacity increases almost linearly, as a function of progressively higher I values, from 0.5 X up to 2 X molarity of buffering Immobiline (pK 7.0) to abruptly reach a plateau at 3 X concentration of buffering ion. The load capacity also increases almost linearly as a function of gel thickness from 1 to 5 mm, without apparently levelling off. When decreasing the pH interval from 1 pH unit (pH 6.8-7.8) to 1/2 pH unit (pH 7.05-7.55) the amount of protein loaded in the HbA zone could be increased by 40%. In 5 mm thick gels, at 2 X pK 7.0 Immobiline concentration, over a 1/2 pH unit span, up to 350 mg HbA (in a 12.5 X 11 cm gel) could be loaded in a single zone, the load limit of the system being around 45 mg protein/ml gel volume.     

Surface potential and reaction of the membrane-bound electron transfer components. II. Integrity of the chloroplast membrane and reaction of P-700

Electrostatic characteristics of the membrane in the vicinity of P-700 were estimated by analyzing the salt and detergent effects on its reaction rate with ionic reagents using the Gouy-Chapman diffuse double layer theory in various preparations of chloroplasts. Upon disruption of thylakoid membranes by sonic treatment or by treatment with digitonin, the reaction rate markedly increased, while the estimated surface charge density became smaller. It was concluded that the membrane surface which determines the reaction rate between P-700 and the ionic reagents changed as the disruption of thylakoid structure. The outer thylakoid surface had more negative charges than the inner one. Changes in the electrical potential profile across the thylakoid membrane during the illumination were also discussed from these results.     

Modeling of discrete currents of single ion channels of cell membranes using synthetic nanometer pores in polyethylene terephthalate films

In studying of conductivity of single supernarrow pores (varying 1 to 15 nm in diameter), formed in thin membranes (10-12 microm in the thickness) from polyethylene terephthalate (PETP), there were revealed discrete changes of currents passing through such pores when applied from external source of potential difference from 200 to 1000 mV. By several characteristics, such discrete currents (discrete conductivity changes) appeared to be identical the so-cold current of single ionic channels in the cell membranes. Supernarrow pores which properties are describes in the present work were obtained as a result of alkaline etching of tracks in thin PETP membranes (a variant of the so-called nuclear filters). Alkaline etching leads to formation of negative fixed charges on the walls of the pores compensated by positive counterions. When setting potential difference onto the PETP membrane, the latter cation layer is able to transfer the current and this transfer was called the surface conductance. In the case of nanometer pores, such surface conductance may be dominating. We have shown that these discrete changes of currents passing through nanometer pores are associated with metastability of the surface conductance. In the case of highly cation-selective channels in the cell membranes it is inevitable, that at least a part of these channels should have dominating cation surface conductance and mentioned above conductance metastability as well. Our findings allow us to propose a new explanation of the origin of the characteristic discreteness of the currents of cation-selective ionic channels in the cell membranes.     

On the voltage-dependent action of tetrodotoxin.

The use of the maximum rate-of-rise of the action potential (Vmax) as a measure of the sodium conductance in excitable membranes is invalid. In the case of membrane action potentials, Vmax depends on the total ionic current across the membrane; drugs or conditions that alter the potassium or leak conductances will also affect Vmax. Likewise, long-term depolarization of the membrane lessens the fraction of total ionic current that passes through the sodium channels by increasing potassium conductance and inactivating the sodium conductance, and thereby reduces the effect of Vmax of drugs that specifically block sodium channels. The resultant artifact, an apparent voltage-dependent potency of such drugs, is theoretically simulated for the effects of tetrodotoxin on the Hodgkin-Huxley squid axon.