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At least 19 recordsLinked to original sources
PubMed1992

Dynamic ion-ion and water-ion interactions in ion channels.

The dynamic interactions among ions and water molecules in ion channels are treated based on an assumption that ions at binding sites can be knocked off by both transient entering ions and local water molecules. The theory, when applied to a single-site model K+ channel, provides solutions for super- and subsaturations, flux-ratio exponent (n') greater than 1, osmotic streaming current, activity-dependent reversal potentials, and anomalous mole-fraction behavior. The analysis predicts that: (a) the saturation may but, in general, does not follow the Michaelis-Menten relation; (b) streaming current results from imbalanced water-ion knock-off interactions; (c) n' greater than 1 even for single-site channels, but it is unlikely to exceed 1.4 unless the pore is occupied by one or more ion(s); (d) in the calculation involving two permeant ion species with similar radii, the heavier ions show higher affinity; the ion-ion knock-off dissociation from the site is more effective when two interacting ions are identical. Therefore, the "multi-ion behaviors" found in most ion channels are the consequences of dynamic ion-ion and water-ion interactions. The presence of these interactions does not require two or more binding sites in channels.

Binding Sites↗
PubMed1992

The ATP4- receptor-operated ion channel of human lymphocytes: inhibition of ion fluxes by amiloride analogs and by extracellular sodium ions.

Extracellular ATP is known to increase the membrane permeability of a variety of cells. Addition of ATP to human leukemic lymphocytes loaded with the Ca2+ indicator, fura-2, induced a rise in cytosolic Ca2+ concentration which was attenuated or absent in NaCl media compared with KCl, choline Cl, or NMG Cl media. In contrast, anti-immunoglobulin antibody gave similar Ca2+ transients in NaCl and KCl media. A half-maximal inhibition of peak ATP-induced Ca2+ response was observed at 10-16 mM extracellular Na+. Basal 45Ca2+ influx into lymphocytes was stimulated 9.6-fold by ATP added to cells in KCl media, but the effect of ATP was greatly reduced for cells in NaCl media. Hexamethylene amiloride blocked 74% of the ATP-stimulated Ca45 uptake of cells in KCl media. Flow cytometry measurements of fluo-3-loaded cells confirmed that the ATP-induced rise in cytosolic Ca2+ was inhibited either by extracellular Na+ or by addition of hexamethylene amiloride. Extracellular ATP stimulated 86Rb efflux from lymphocytes 10-fold and this increment was inhibited by the amiloride analogs in a rank order of potency 5-(N-methyl-N-isobutyl)amiloride greater than 5-(N,N-hexamethylene)amiloride greater than 5-(N-ethyl-N-isopropyl)amiloride greater than amiloride. ATP-induced 86Rb efflux showed a sigmoid dependence on the concentration of ATP and Hill analysis gave K1/2 of 90 and 130 microM and n values of 2.5 and 2.5 for KCl and NaCl media, respectively. However, the maximal ATP-induced 86Rb efflux was 3-fold greater in KCl than in NaCl media. Raising extracellular Na+ from 10 to 100 mM increased ATP-induced Na+ influx from a mean of 2.0 to 3.7 nEq/10(7) cells/min, suggesting either saturability or self-inhibition by Na+ of its own influx. These data suggest that ATP opens a receptor-operated ion channel which allows increased Ca2+ and Na+ influx and Rb+ efflux and these fluxes are inhibited by extracellular Na+ ions as well as by the amiloride analogs.

Adenosine↗
PubMed1992

Secondary-ion and electron production from surfaces bombarded by large polyatomic ions.

Heavy molecular ions with energies in the range 10-20 keV and masses from 276 u to 132,000 u, produced by matrix-assisted laser desorption, were used as primary projectiles to produce secondary-ion spectra from a variety of surfaces in a tandem time-of-flight mass spectrometer. In the negative mode the ratio of electron emission to secondary-ion emission was found to decrease rapidly with increasing projectile mass. Ion emission was found to dominate for primary ions larger than approximately 10,000 u. Positive or negative molecular ions and cations were observed from several organic targets of masses up to 1140 u (gramicidin S) for incident projectiles up to mass 132,000 u, i.e., for projectile speeds down to approximately 7000 m/s. Other ions characteristic of the target were also observed for these projectiles. Thus, large polyatomic ions can cause secondary-ion desorption even at very low velocity. The background ions of both polarities are similar to those found in keV particle bombardment by monatomic projectiles. The same ions are observed for all the projectiles; most can be identified with hydrocarbon background. The relative intensities of the background positive ions are largely independent of projectile, and for both polarities the ratio of the ions characterizing the target to those forming the background is approximately constant for all the projectiles. These results strongly suggest that the background ions come from the usual layer of organic impurities attached to the target surface. No direct evidence for surface-induced dissociation was observed in this mass and energy range.

Mass Spectrometry↗
PubMed1975

Genetic determinant of lithium ion metabolism. II. An in vivo study of lithium ion distribution across erythrocyte membranes.

A study was conducted to determine if membrane factors, known to influence the distribution of sodium ion (Na) and potassium ion (K), also influence lithium ion distribution. Two groups of sheep with genetically determined differences in their cation concentrations were administered lithium chloride for ten days. The low red blood cell (RBC) potassium ion sheep (LK) had a greater RBC lithium ion concentration than the high RBC potassium ion sheep (HK). In vitro incubation of erythrocytes with lithium chloride also produced substantially different RBC lithium ion: plasma lithium ion ratios similar to those seen in the vivo study. Distribution of lithium ion was generally similar to that of Na ion. It seems that lithium ion distribution may be controlled by the same genetic factors that regulate Na ion distribution.

Animals↗
PubMed1991

The binding of terbium ions to gelsolin reveals two classes of metal ion binding sites.

Spectroscopically active terbium ions have been used to probe the Ca2+ ion-binding sites on human plasma gelsolin. The luminescence of Tb3+ ions bound to gelsolin is markedly enhanced when excited indirectly at 295 nm due to Förster type dipole-dipole energy transfer from neighboring tryptophan residues. Titration of this luminescence with increasing concentrations of Tb3+ ions was saturable although the shape of this titration curve was complex indicating the involvement of multiple classes of sites. Luminescence lifetime measurements (obtained by indirect excitation at 295 nm) demonstrate the presence of two classes of sites characterized by a major lifetime of 1.0-1.1 ms and a minor lifetime of 0.7-0.8 ms. However, while the amplitude of the minor lifetime showed a hyperbolic dependence on the Tb3+ ion concentration, the amplitude of the major lifetime showed a strongly sigmoidal dependence. Different classes of Tb3+ ion binding sites can also be distinguished by the different Ca2+ ion concentrations needed to displace Tb3+ ions from these sites on gelsolin. It is proposed that the occupancy of one class of Tb3+ ion binding sites on gelsolin causes a conformational change in gelsolin which then allows a second class of cryptic Tb3+ ion binding sites to be expressed. The implications of these results in terms of the binding of Ca2+ ions to gelsolin and the regulation of the activities of gelsolin by calcium are discussed.

Binding Sites↗
PubMed1975

Spectator-ion effect on the passage of ions through membranes.

In this paper, we investigate the interplay between geometric and dielectric factors in influencing the image force acting on an ion passing through a membrane, for a system having the approximate dimensions of Escherichia coli. We also study the effect of one ion in a membrane on the passage of a second ion through the membrane, by calculating the radial and angular forces experienced by the second ion due to the presence of the "spectator ion." Our conclusions follow from numerical studies on expressions obtained by solving (exactly) Laplace's equation for the model assumed in this paper. The conclusions are: (i) small changes in the dielectric constant of the membrane are far more significant in determining the image force acting on an ion in a membrane than dramatic changes in the dielectric character of the regions interior and/or exterior to the cell; (ii) a spectator ion in a membrane situated near a boundary may influence in a significant way the passage of a second ion through the middle third of the membrane. We suggest that this latter result should be taken into account in discussing the mechanism of ion migration across membranes.

Biological Transport, Active↗
PubMed1978

Acetylcholine-receptor-mediated ion flux in electroplax membrane microsacs (vesicles): change in mechanism produced by asymmetrical distribution of sodium and potassium ions.

The kinetics of acetylcholine-receptor-mediated efflux of inorganic ions from electroplax microsacs of Electrophorus electricus in the presence of varying alkali metal ion concentrations on both sides of the membrane have been investigated. The efflux, a monophasic process when the ion distribution is symmetrical (the same concentrations and types of ions on both sides of the membrane), becomes a biphasic process, consisting of a very rapid initial release of ions followed by a slower first-order process, under conditions that resemble the physiological state of the neural membrane (potassium ions inside the microsacs and sodium ions on the outside). The initial phase of the efflux discriminates between calcium and sodium ions and is inhibited by potassium ions in the external solution. The rate constant associated with this phase is at least 40 times larger than the rate constant associated with the slower efflux. Both phases depend on the concentration of acetylcholine or carbamoylcholine, and are inhibited by receptor inhibitors (d-tubocurarine and alpha-bungarotoxin).A simple model is proposed which relates the kinetics of the flux to ligand-induced conformational changes in the receptor. We also indicate the relationship between the biphasic kinetics of the flux observed in microsacs to "desensitization," the phenomenon in which, on addition of acetylcholine, the transmembrane voltage of muscle and nerve cells first increases and then decreases to its resting value within a few seconds.

Animals↗
PubMed1976

Thermodynamic and kinetic parameters of ion condensation to polynucleotides. Outer sphere complex formed by Mg++ions.

The coupling of ion binding to the single strand helix-coil transition in poly (A) and poly (C) is used to obtain information about both processes by ion titration and field-jump relaxation methods. Characterisation of the field-jump relaxation in poly (C) at various concentrations of monovalent ions leads to the evaluation of a stability constant K = 71 M(-1) for the ion binding to the polymer. the rate constant of helix formation is found to be 1.3 X 10(7)s(-1), whereas the dissociation rate is 1.0 X 10(6)s(-1). Similar data are preseented for poly (A) and poly (dA). The interaction of Mg(++) and Ca(++) with poly (A) and poly (C) is measured by a titration method using the polymer absorbance for the indication of binding. The data can be represented by a model with independent binding "sites". The stability constants increase with decreasing salt concentration from 2.7 X 10(4) M(-1) at medium ionic strengths up to 2.7 X 10(7) M(-1) at low ionic strength. The number of ions bound per nucleotide residue is in the range 0.2 to 0.3. Relaxation time constants associated with Mg(++) binding are characterised over a broad range of Mg(++) concentrations form 5 muM to 500 muM. The observed concentration dependence supports the conclusion on the number of binding places inferred from equilibrium titrations. The rate of Mg(++) and Ca(++) association to the polymer is close to the limit of diffusion control kR =1 X 10(10) to 2 X 10(10) M(-1) s(-1)). This high rate demonstrates that Mg(++) and Ca(++) ions do not form inner-sphere complexes with the polynucleotides. Apparently the distance between two adjacent phosphates is too large for a simultaneous site binding of Mg(++) or Ca(++), and inner sphere complexation at a single phosphate seems to be too weak. The data support the view that the ions like Mg(++) and Ca(++) surround the polynucleotides in the form of a mobile ion cloud without site binding.

Ions↗
PubMed1975

Negative surface charge near sodium channels of nerve: divalent ions, monovalent ions, and pH.

Evidence is given for a high density of negative surface charge near the sodium channel of myelinated nerve fibres. The voltage dependence of peak sodium permeability is measured in a voltage clamp. The object is to measure voltage shifts in sodium activation as the following external variables are varied: divalent cation concentration and type, monovalent concentration, and pH. With equimolar substitution of divalent ions the order of effectiveness for giving a positive shift is: Ba equals Sr less than Mg less than Ca less than Co approximately equal to Mn less than Ni less than Zn. A tenfold increase of concentration of any of these ions gives a shift of +20 to +25 mV. At low pH, the shift with a tenfold increase in Ca-2+ is much less than at normal pH, and conversely for high pH. Soulutions with no added divalent ions give a shift of minus 18 mV relative to 2 mM Ca-2+. Removal of 7/8 of the cations from the calcium-free solution gives a further shift of minue 35 mV. All shifts are explained quantitatively by assuming that changes in an external surface potential set up by fixed charges near the sodium channel produce the shifts. The model involves a diffuse double layer of counterions at the nerve surface and some binding of H+ions and divalent ions to the fixed charges. Three types of surface groups are postulated: (1) an acid pKa equals 2.88 charge density minus 0.9 nm- minus 2; (i) an acid pKa equals 4.58, charge density minus 0.58 nm- minus 2; (3) a base pKa equals 6.28, charge density +0.33 nm- minus 2. The two acid groups also bind Ca-2+ ions with a dissociation constant K equals 28 M. Reasonable agreement can also be obtained with a lower net surface charge density and stronger binding of divalent ions and H+ ions.

Animals↗
PubMed1992

Synthetic metal-binding protein surface domains for metal ion-dependent interaction chromatography. II. Immobilization of synthetic metal-binding peptides from metal ion transport proteins as model bioactive protein surface domains.

This preliminary investigation tests the premise that biologically relevant (1) peptide-metal ion interactions, and (2) metal ion-dependent macromolecular recognition events (e.g., peptide-peptide interactions) may be modeled by biomimetic affinity chromatography. Divinylsulfone-activated agarose (6%) was used to immobilize three different synthetic peptides representing metal-binding protein surface domains from the human plasma metal transport protein histidine-rich glycoprotein (HRG). The synthetic peptides represented 1-3 multiple repeat units of the 5-residue sequence (Gly-His-His-Pro-His) found in the C-terminal of HRG. By frontal analyses, immobilized HRG peptides of the type (GHHPH)nG, where n = 1-3, were each found to have a similar binding capacity for both Cu(II) ions and Zn(II) ions (31-38 mumol/ml gel). The metal ion-dependent interaction of a variety of model peptides with each of the immobilized HRG peptide affinity columns demonstrated differences in selectivity despite the similar internal sequence homology and metal ion binding capacity. The immobilized 11-residue HRG peptide was loaded with Cu(II) ions and used to demonstrate selective adsorption and isolation of proteins from human plasma. These results suggest that immobilized metal-binding peptides selected from known solvent-exposed protein surface metal-binding domains may be useful model systems to evaluate the specificity of biologically relevant metal ion-dependent interaction and transfer events in vitro.

Amino Acid Sequence↗
PubMed1992

Modeling ion permeation through batrachotoxin-modified Na+ channels from rat skeletal muscle with a multi-ion pore.

The mechanism of ion permeation through Na+ channels that have been modified by batrachotoxin (BTX) and inserted into planar bilayers has been generally described by models based on single-ion occupancy, with or without an influence of negative surface charge, depending on the tissue source. For native Na+ channels there is evidence suggestive of a multi-ion conduction mechanism. To explore the question of ion occupancy, we have reexamined permeation of Na+, Li+, and K+ through BTX-modified Na+ channels from rat skeletal muscle. Single-channel current-voltage (I-V) behavior was studied in neutral lipid bilayers in the presence of symmetrical Na+ concentrations ranging from 0.5 to 3,000 mM. The dependence of unitary current on the mole fraction of Na+ was also examined in symmetrical mixtures of Na(+)-Li+ and Na(+)-K+ at a constant total ionic strength of 206 and 2,006 mM. The dependence of unitary conductance on symmetrical Na+ concentration does not exhibit Michaelis-Menten behavior characteristic of single-ion occupancy but can be simulated by an Eyring-type model with three barriers and two sites (3B2S) that includes double occupancy and ion-ion repulsion. Best-fit energy barrier profiles for Na+, Li+, and K+ were obtained by nonlinear curve fitting of I-V data using the 3B2S model. The Na(+)-Li+ and Na(+)-K+ mole-fraction experiments do not exhibit an anomalous mole-fraction effect. However, the 3B2S model is able to account for the biphasic dependence of unitary conductance on symmetrical [Na+] that is suggestive of multiple occupancy and the monotonic dependence of unitary current on the mole fraction of Na+ that is compatible with single or multiple occupancy. The best-fit 3B2S barrier profiles also successfully predict bi-ionic reversal potentials for Na(+)-Li+ and Na(+)-K+ in both orientations across the channel. Our experimental and modeling results reconcile the dual personality of ion permeation through Na+ channels, which can display features of single or multiple occupancy under various conditions. To a first approximation, the 3B2S model developed for this channel does not require corrections for vestibule surface charge. However, if negative surface charges of the protein do influence conduction, the conductance behavior in the limit of low [Na+] does not correspond to a Gouy-Chapman model of planar surface charge.

Animals↗
PubMed1990

Chloride ion increases [3H]dopamine accumulation by synaptic vesicles purified from rat striatum: inhibition by thiocyanate ion.

The effect of chloride ion on the transport of [3H]dopamine into synaptic vesicles purified from rat striatum has been evaluated. The inclusion of 10 mM chloride ion in the incubation medium produced a 100% increase in temperature-sensitive [3H]dopamine uptake into synaptic vesicles from approximately 1800 pmol/mg to 3600 pmol/mg of protein. Half-maximal effects were observed with chloride ion at 4 mM concentration. The anion selectivity of stimulation supports the presence of anion channels within the membranes of dopaminergic storage organelles. Low concentrations of thiocyanate ion (less than 10 mM), 4-acetamido-4'-isothiocyano-2-2'-disulfonic acid stilbene (100 microM), and duramycin (5 micrograms/ml) selectively blocked the chloride ion stimulated accumulation of [3H]dopamine. Higher concentrations of these agents are required to significantly reduce [3H]dopamine uptake in the absence of chloride ion. These results suggest that both components of the proton electrochemical gradient (delta psi and delta pH) are important for dopamine uptake by brain vesicles. This article presents the first demonstration that chloride ion plays a role in the transport of dopamine into vesicles isolated from the CNS.

Animals↗
PubMed1978

[Accumulation of phosphate ions in biological materials, phosphate ion diffusion and electron microscopical investigations of phosphohydrolases (author's transl)].

Isolated unfixed nuclei of mouse liver accumulate different amounts of phosphate ions dependent on pH, temperature and concentration of phosphate ions. At 37 degrees C and pH = 7.2, 8.5 X 10(9) binding places and a stability constant of 2.0 +/- 0,5 . 10(4) l/mol were calculated, the data at 30 degrees C and pH = 6.5 are 4 X 10(9) Nucleus-1 and 6 X 10(3) l/mol. Nuclei fixed by formaldehyde-ethanol or glutaraldehyde do not accumulated phosphate ions. Under conditions of nearly undisturbed diffusion phosphate ions taken up by the nuclei and precipitated by the help of lead ions are detected electron microscopically in the nuclear envelope, preferently in intracisternal space. Nucleoprotein structures do not show enrichments of crystals. These structures are also not stained after precipitation of phosphate ions under conditions of diminuation of diffusion. A possible mode of phosphate binding in cell nuclei and the influence of adsorbed phosphate ions on localization of enzymatic activities are discussed.

Animals↗
PubMed1992

Synthetic metal-binding protein surface domains for metal ion-dependent interaction chromatography. I. Analysis of bound metal ions by matrix-assisted UV laser desorption time-of-flight mass spectrometry.

To extend the analytical capabilities of immobilized metal ion affinity chromatography (IMAC) for evaluation of biologically relevant peptide-metal ion interactions, we have prepared synthetic peptides representing metal-binding protein surface domains from the human plasma metal transport protein known as histidine-rich glycoprotein (HRG). Three synthetic peptides, representing multiples of a 5-residue repeat sequence (Gly-His-His-Pro-His) from within the histidine- and proline-rich region of the C-terminal domain were prepared. Prior to immobilization, the synthetic peptides were evaluated for identity and sample homogeneity by matrix-assisted UV laser desorption time-of-flight mass spectrometry (LDTOF-MS), a method developed recently for the mass determination of high-molecular-mass biopolymers. 2,5-Dihydroxybenzoic acid was evaluated as a matrix to facilitate the laser desorption and ionization of intact peptides and was found to be ideally suited for determinations of mass within the low-mass region of interest (641.7 to 1772.8 dalton). We observed minimal chemical noise from photochemically generated peptide-matrix adduct signals, clustering, and multiply-charged peptide species. Peptides with bound sodium and potassium ions were observed; however, these signal intensities were reduced by immersion of the sample probe tip in water. Mixtures of the three different synthetic peptides were also evaluated by LDTOF-MS after their elution through a special immobilized peptide-metal ion column designed to investigate metal ion transfer. We found LDTOF-MS to be a useful new method to verify the presence of peptide-bound metal ions.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗
PubMed1978

Electrostatic calculations for an ion channel. I. Energy and potential profiles and interactions between ions.

The electrostatic energy profile of one, two, or three ions in an aqueous channel through a lipid membrane is calculated. It is shown that the previous solution to this problem (based on the assumption that the channel is infinitely long) significantly overestimates the electrostatic energy barrier. For example, for a 3-A radius pore, the energy is 16 kT for the infinite channel and 6.7 kT for an ion in the center of a channel 25 A long. The energy as a function of the position of the ion is also determined. With this energy profile, the rate of crossing the membrane (using the Nernst-Planck equation) was estimated and found to be compatible with the maximum conductance observed for the gramicidin A channel. The total electrostatic energy (as a function of position) required to place two or three ions in the channel is also calculated. The electrostatic interaction is small for two ions at opposite ends of the channel and large for any positioning of the three ions. Finally, the gradient through the channel of an applied potential is calculated. The solution to these problems is based on solving an equivalent problem in which an appropriate surface charge is placed on the boundary between the lipid and aqueous regions. The magnitude of the surface charge is obtained from the numerical solution for a system of coupled integral equations.

Biological Transport↗
PubMed1991

The effect of permeant ions on single calcium channel activation in mouse neuroblastoma cells: ion-channel interaction.

1. Single low-threshold inactivating (LTI or T-type) Ca2+ channels of undifferentiated neuroblastoma cells (clone N1E-115) were investigated using the patch-clamp technique. 2. Single-channel conductance, gi, for Ca2+, Sr2+ or Ba2+ as a permeant cation was similar (7.2 pS). Mean channel open time, tau op, was also practically independent of the divalent ion species; it decreased from 0.7 to 0.3 ms between -40 and 0 mV. 3. Modification of the calcium channel selectivity by lowering the external Ca2+ concentration to 10(-8) M produced an increase in gi for Na+ and Li+ ions and a shift of potential-dependent characteristics in the hyperpolarizing direction. Voltage sensitivity and absolute values of tau op were also changed. These changes were dependent on both permeant monovalent ion type and concentration. 4. At high [Na+]o, tau op was almost potential independent (congruent to 0.3 ms). Decrease in [Na+]o and substitution of Li+ for Na+ increased tau op and the steepness of its potential dependency. 5. The divalent and monovalent cations that were tested had much smaller effect on the mean intraburst shut time, tau cl(f), which was nearly independent of membrane potential (congruent to 0.6 ms). By contrast, mean burst duration was strongly potential dependent and noticeably affected by permeant ion type. 6. All kinetic changes were analysed in terms of a four-state sequential model for channel activation. According to this model the channel enters the open state through three closed states. Transitions between closed states can be formally related to the transmembrane movement of two charged gating particles (m2 process). The interaction between ion flux and a sterical region of the Ca2+ channel selectivity filter may, depending on ion transfer rate and ionic radius, lead to a local increase of the dielectric constant, resulting in redistribution of the electric field and changes in potential dependency of tau op.

Animals↗