On the formation of calcium(II) taurocholate aggregate species in aqueous solution

Abstract

The influence of the presence of calcium(II) ions in solutions containing sodium and taurocholate ions at 25°C and in 0.5 mol dm^?3 N(CH₃)₄Cl as the constant ionic medium was studied. The composition and existence range of aggregates formed by taurocholate sodium and calcium(II) were investigated by means of two different procedures. First, the increasing calcium oxalate solubility due to the presence of taurocholate ions was studied as a function of the taurocholate, sodium and hydrogen ions. The free concentration of sodium and hydrogen ions was determined in solutions equilibrated with solid calcium(II) oxalate. After filtration, the concentration of calcium(II) (by atomic absorption spectro-photometry) and that of oxalate were also determined. In the second approach, electromotive force measurements carried out in solutions containing taurocholate, sodium and calcium(II) ions provided hydrogen and sodium ions free concentrations. The results from both procedures could be explained by assuming the presence of aggregates of different composition with the participation of sodium, calcium(II) and taurocholate ions, depending on the concentration of the reagents. No protonated species were present in appreciable concentrations. All the species found have even anion aggregation numbers. A strong analogy with the composition of sodium taurocholate and glycocholate is observed, while a comparison with sodium deoxycholate, glycodeoxycholate and taurodeoxycholate shows wide differences.     

Mechanisms of neurotransmitter release facilitation in strontium solutions

Mechanisms of neurotransmitter release facilitation were studied using electrophysiological recording of end-plate currents (EPC) and nerve ending (NE) responses after substitution of extracellular Ca ions with Sr ions at the frog neuromuscular junction. The solutions with 0.5 mM concentration of Ca ions (calcium solution) or 1 mM concentration of Sr ions (strontium solution) were used where baseline neurotransmitter release (at low-frequency stimulation) is equal. Decay of paired-pulse facilitation of EPC at calcium solutions with increase of interpulse interval from 5 to 500 ms was well described by three-exponential function consisting of early, first and second components. Facilitation at strontium solutions was significantly diminished due mainly to decrease of early and first components. At the same time, EPC facilitation with rhythmic stimulation (10 or 50 imp/s) at strontium solutions was significantly increased. Also more pronounced decrease of NE response 3rd phase, reflecting potassium currents was detected under rhythmic stimulation of 50 imp/s at strontium solutions comparing to calcium solutions. It was concluded that facilitation sites underlying first and early components had lower affinity to Sr ions than to Ca ions. The enhancement of frequency facilitation at strontium solutions is mediated by two mechanisms: more pronounced broadening of NE action potential and increase of bivalent cation influx due to feebly marked activation of Ca(2+)-dependent potassium current by Sr ions, and slower dynamics of Sr(2+) removal from NE axoplasm comparing to Ca(2+).     

Influence of Prolactin and Calcium Gluconate Concentration on Permeation and Intestinal Absorption of Ca(II) Ions

The in vitro permeation and absorption of calcium ions across the small intestine were measured at different concentrations of calcium gluconate solutions (1.0, 10.0 and 20.0 mM) with or without prolactin. The calcium ions permeated through the small intestine from a donor environment to an acceptor environment that mimicked the conditions in the stomach to ileum segment of the digestive tract. The permeation and absorption of calcium were directly dependent on the calcium concentration of the solutions. At 10 and 20 mM permeation was significantly higher than that at 1.0 mM (p < 0.05). In the presence of prolactin both permeation and absorption increase considerably. At the lowest concentration (1.0 mM) simulating calcium deficiency, there was compensation by the small intestine, suggesting that such deficiency stimulates its mobilization from intestinal tissue. Prolactin enhances the calcium mobilization process even at sufficient calcium intakes. It is suggested that prolactin takes part in regulation of calcium homeostasis in the organism.     

Rheological evidence of the gelation behavior of hyaluronan-gellan mixtures

It was found that solutions of calcium hyaluronate (CaHA) (0.1 to approximately 0.5 wt%) could form a gel by mixing with solutions of sodium type gellan (0.1 to approximately 0.5 wt%), although neither polymer by itself forms a gel at low concentrations (0.1 to approximately 0.5 wt% in this experiment). The rheological properties of CaHA-gellan mixtures were investigated by dynamic and steady shear measurements. Both storage shear modulus G' and loss shear modulus G' for CaHA-gellan mixtures increased with increasing time, and tended to an equilibrium value after 1 h. After reaching steady values of G' and G", the frequency dependence of G' and G' was observed. G' was always larger than G' in the accessible frequency range from 10(-2) to 10(2) rad/s. The effects of pH and calcium ions were examined. Gel formation of the mixtures was promoted by decreasing pH and adding from 0.01 to 0.1 M calcium ions, but excessive calcium ions weakened the gel.     

Effects of calcium ions on electrical responses of gastric gland cells to histamine and pentagastrin

The effect of Ca ions on electrical responses of gastric gland cells on histamine and pentagastrin was investigated using intracellular glass microelectrodes. It was established that in low-calcium solutions hyperpolarization induced by these secretagogues was diminished. In calcium-free solutions and in solutions with blockers of the calcium current hyperpolarization induced by histamine and pentagastrin was not observed. It was suggested that external calcium ions are necessary for hyperpolarization responses to histamine and pentagastrin actions on gastric gland cells to occur.     

The activity of calcium ions in aqueous solutions of the lower calcium oligogalacturonates

The activity of calcium ions in aqueous solutions of calcium mono-, di-, tri-, and tetra-galacturonates was determined by a spectrophotometric method in which tetramethylmurexide was used as an auxiliary ligand. The concentration of the solutions was 3.00 mequiv./l with respect to [-COOCa_0.5]. In this system, the activity of calcium ions in the solution of calcium galacturonate was very close to that of calcium chloride, while the activity of the calcium ions decreased in a continuous manner with increasing degree of polymerisation of the oligogalacturonate anion. The results are interpreted as evidence that, in calcium pectate, an intramolecular chelate bond of calcium involving two consecutive galacturonic acid units is not very probable.     

An investigation on calcium taurodeoxycholate micellar aggregates

Abstract

The formation of micellar aggregates in the presence of calcium(II) ions in solutions containing sodium and taurodeoxycholate ions and their composition at 25°C and in 0.5 mol dm^?3 N(CH₃)₄Cl as constant ionic medium was studied. The study was carried out by means of two different procedures. In the first one, solid calcium oxalate was equilibrated with taurodeoxycholate, sodium and hydrogen ions and the free concentration of sodium and hydrogen ions was determined. After filtration, the calcium(II) (by atomic absorption spectrophotometry) and oxalate concentration were also determined. In the second approach, hydrogen and sodium ions free concentrations were obtained by electromotive force measurements carried out in solutions containing taurodeoxycholate. The results of both procedures could be explained by assuming the presence of aggregates of different composition with the participation of sodium, calcium(II) and taurodeoxycholate ions, depending on the concentration of the reagents. Protonated species were even present in appreciable concentrations. All the found species have taurodeoxycholate aggregation numbers in multiples of three. A mechanism for the micellar aggregates containing calcium and sodium is proposed. Sodium taurodeoxycholate in the presence of calcium(II) forms larger aggregates than does taurocholate in the presence of calcium(II); the building block of the former is a trimer whereas the latter system has lower aggregation numbers and its building block is a dimer or an octamer.     

Galvanic Stimulation of Volvox globator II. Mechanism of Galvanic Response, an Effect of Calcium Displacement

Slight increases or decreases in calcium ions in solutions which supported the growth of Volvox globator colonies caused the colonies to fall to the bottoms of their containers. High speed cinematography (600 frames/sec) showed that the flagella beat normally (21/sec) in balanced electrolyte solutions which have calcium concentrations between 0.5 and 1.0 mM. When colonies were placed in 10.0 or 0.0 mM CaCl₂ solutions, flagellar beating disappeared within 1 hr. The cessation of flagellar beating was reversible when colonies were replaced in the balanced solution. The Volvox cell wall has been shown to be a fairly good cation-exchanger with calcium ions acting as the counterion to the fixed negative change. Colonies that were photopositive and gave a cathodal galvanotaxis responded to DC electrical potentials by producing solution patterns that were indicators of colony electronegativity. Colony resistance to electroosmotic flow was compared in potassium and calcium solutions. When colonies were placed in darkness for 24 hr and stimulated by DC electrical potentials, their cation-exchange properties became reduced and the cell walls appeared thinner. Application of a high DC electrical potential to dark-adapted colonies caused the colonies to shrink on their anode sides (anodal contraction). Other workers have found that the flagella on the anodal sides of dark-adapted colonies ceased beating during DC electrical stimulation. It is hypothesized that the electric current caused an increase of calcium ions on the anodal side of the colony that inhibited the flagellar mechanism of beating on that side. It is also hypothesized that the galvanotaxis associated with light-adapted (photopositive) colonies was due to calcium displacements in the colony cell walls that affected the flagellar beating on both sides of the colony.     

Utilization of an exopolysaccharide produced by Chryseomonas luteola TEM05 in alginate beads for adsorption of cadmium and cobalt ions

Cadmium and cobalt adsorption from aqueous solution onto calcium alginate, sodium alginate with an extracellular polysaccharide (EPS) produced by the activated sludge bacterium Chryseomonas luteola TEM05 and immobilized C. luteola TEM05 was studied. In addition, solutions containing both of these ions were prepared and partial competitive adsorption of these mixtures was investigated. Metal adsorption onto gel beads was carried out at pH 6.0 and 25 degrees C. The maximum adsorption capacities determined by fitting Langmuir isotherms to the data for calcium alginate, calcium alginate+EPS, calcium alginate + C. luteola TEM05 and calcium alginate + EPS + C. luteola TEM05 were 45.87, 55.25, 49.26, 51.81 mg g(-1) for Co(II) and 52.91, 64.10, 62.5, 61.73 mg g(-1) for Cd(II), respectively. The biosorption capacity of the carrier for both metal ions together in competition was lower than those obtained when each was present alone.     

Calcium ion binding by isolated tobacco mosaic virus coat protein

Calcium ion titrations were performed on solutions of tobacco mosaic virus coat protein using a calcium-specific ion-exchange electrode. Isolated coat protein was found incapable of binding calcium ions under equilibrium conditions at pH values above its iso-ionic point (pH 4.3 to 4.6). However, calcium ions were found to bind to coat protein under non-equilibrium conditions, which suggests that the isolated coat protein has the proper conformation to bind calcium ions at the iso-ionic point.     

Optical anisotropy of calcium-induced alginate gels

Alginate gels formed by diffusion of calcium ions into solutions of sodium alginate were found to exhibit optical anisotropy depending on preparation conditions. When observed under crossed nicols, the anisotropic alginate gels showed a birefringence pattern which is characteristic of radial orientation of polymer chains. Calcium alginate gels were prepared from different concentrations of sodium alginate and calcium ion, and the conditions for formation of the anisotropic gels were determined. The gel-formation process was measured by monitoring the development of the birefringent layer and was compared with the model in which the diffusion of calcium ions dominates gel formation.     

The influence of calcium ions on fibrin polymerization

The influence of calcium ions on the polymerization induced in fibrinogen solutions by thrombin and by Reptilase has been investigated by meansof static and dynamic light scattering in combination with measurements of the release of the fibrinopeptide A. The calcium concentration was varied in the range between 0.3 and 103 calcium ions per fibrinogen molecule. The enzyme concentration was chosen sufficiently low so that it was possible to make quantitative observations as a function of time, in particular, beforethe onset of gelation. Likewise, the influence of calcium ions on the enzymatically induced polymerization of fragment X was studied. The results indicate that there are at least three mechanisms by which calcium can influence the evolution of the polymer system on the path to gelation and clotting. Which mechanism dominates depends upon the calcium concentration.     

Caclium uptake and associated adenosine triphosphatase activity in fragmented sarcoplasmic reticulum. Requirement for potassium ions.

The effects of monovalent cations on calcium uptake by fragmented sarcoplasmic reticulum have been clarified. Homogenization of muscle tissue in salt-containing solutions leads to contamination of this subcellular fraction with actomyosin and mitochondrial membranes. When, in addition, inorganic cations are contributed by the microsomal suspension and in association with nucleotide triphosphate substrates there is an apparent inhibition of the calcium transport system by potassium and other cations. However, when purified preparations were obtained after homogenization in sucrose medium followed by centrifugation on a sucrose density gradient in a zonal rotor, calcium uptake and the associated adenosine triphosphatase activity were considerably activated by potassium and other univalent cations. When plotted against the log of the free calcium concentration there was only a slight increase in calcium uptake and ATPase activity in the absence of potassium ions but sigmoid-shaped curves were obtained in 100 mM K+ with half-maximal stimulation occurring at 2 muM Ca2+ for both calcium uptake and ATPase activity. The augmentation in calcium uptake was not due to an ionic strength effect as Tris cation at pH 6.6 was shown to be inactive in this respect. Other monovalent cations were effective in the order K+ greater than Na+ greater than NH4+=Rb+=Cs+ greater than Li+ with half-maximal stimulation in 11 mM K+, 16 mM Na+, 25 mM NH4+, Rb+, and Cs+ and in 50 mM Li+. There was nos synergistic action between K+ AND Na+ ions and both calcium uptak and associated ATPase were insensitive to ouabain. Thallous ions stimulate many K+-requiring enzymes and at one-tenth the concentration were nearly as effective as K+ ions in promoting calcium uptake. The ratio of Ca2+ ions transported to P1 released remained unchanged at 2 after addition of K+ ions indicating an effect on the rate of calcium uptake rather than an increased efficiency of uptake. In support of this it was found that during the stimulation of calcium uptake by Na+ ions there was a reduction in the steady state concentration of phosphorylated intermediate formed from [gamma-32P]ATP. It is considered that there is a physiological requirement for potassium ions in the relaxation process.     

Conditions for initiation of fertilization of eggs in the copulating elkhorn sculpin

The reason for failure to initiate fertilization internally was examined in a cottid fish, the elkhorn sculpin, Alcichthys alcicornis which has internal gametic association and external fertilization. While eggs could be activated in calcium free hypertonic media but not be fertilized, fertilization occurred in isotouic media rich in calcium ions. The rate of fertilization was dependent on calcium concentration, and eggs were not fertilized in solutions with a calcium ion concentration of less than 0·57 mmol kg⁻¹. Calcium ions could be replaced to some extent by magnesium ions, but the former were the more effective in fertilization. Since calcium ion concentration of ovarian fluid of A. alcicornis was 0·41 mmol kg⁻¹, it was inferred that low calcium concentration in the ovarian fluid was the cause of the failure of A. alcicornis eggs to fertilize internally.     

On the effects of divalent cations and ethylene glycol-bis-(beta- aminoethyl ether) N,N,N'',N''-tetraacetate on action potential duration in frog heart

Resting and action potentials were recorded from superfused strips of frog ventricle. Reducing the bathing calcium concentration ([Ca2+]0) with or without ethylene glycol-bis(beta-aminoethyl ether)N,N,N',N'-tetraacetate (EGTA) prolongs the action potential (AP). The change in the duration of the AP extends over many minutes, but is rapidly reversed by restoring calcium ions. Other changes (e.g., in resting potential and overshoot) are, however, only more slowly reversed. Reducing [Ca2+]0 with 0.2, 2, or 5 mM EGTA produces progressively greater prolongation of AP; maximum values were well in excess of 1 min. This prolongation can be reversed by other divalent cations in EGTA (Mg2+, Sr2+) or Ca-free (Mn2+) solutions, or by acetylcholine. Barium ions increase AP duration in keeping with their known effect on potassium conductance. D600, which blocks the slow inward current in cardiac muscle, is without effect on the action potentials recorded in EGTA solutions, or on the time course and extent of the recovery to normal duration upon restoring calcium ions. It is concluded that divalent cations exert an influence on membrane potassium conductance extracellularly in frog heart. The cell membrane does not become excessively "leaky" in EGTA solutions.     

Investigation of the TEA-resistant outward current in the somatic membrane of perfused nerve cells

Outward currents remaining after addition of 20–50 mM of tetraethylammonium (TEA) ions to the extracellular or intracellular solution, were investigated in perfused isolatedHelix neurons. After this addition, the inactivated inward current carried by potassium ions, the potential-dependent and kinetic characteristics of which differ from those of potassium outward currents suppressed by TEA, is preserved in the membrane. A component dependent on the inward calcium current was found in this TEA-resistant outward current; it was abolished by replacement of the extra-cellular calcium ions by magnesium ions, by blocking of the calcium channels by extracellular cadmium ions, and by their destruction by intracellular fluoride ions. Increasing the intracellular concentration of free calcium ions by perfusing the cell with solutions containing calcium-EGTA buffer potentiated the TEA-resistant component of the outward current, whereas removal of these ions with EGTA weakened it. It is concluded that a system of outward current channels whose activation depends on the presence of calcium ions near the inner surface of the membrane is present in the somatic membrane. It is suggested that to keep these channels capable of being activated, calcium ions must bind with the structures forming their internal opening.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 11, No. 5, pp. 460–468, September–October, 1979.     

Calcium-ion-induced stabilization of the protease from Bacillus cereus WQ9-2 in aqueous hydrophilic solvents: effect of calcium ion binding on the hydration shell and intramolecular interactions

The neutral protease WQ from Bacillus cereus is stable in various aqueous organic mixtures, with the exception of those containing acetonitrile (ACN) and dimethylformamide (DMF). The stability of the enzyme in aqueous hydrophilic solvents was dramatically enhanced with the addition of calcium ions, with the degree of improvement in the half-life relative to different solutions ranging from fourfold to more than 70-fold. Studies of the kinetic constants showed that calcium ions induced slight conformational changes in the active site of the enzyme in aqueous ACN. We investigated the molecular mechanisms underlying this stabilizing effect by employing a combination of biophysical techniques and molecular dynamics simulation. In aqueous ACN, the intrinsic fluorescence and circular dichroism analysis demonstrated that the addition of calcium ions induced a relatively compact conformation and maintained both the native-like microenvironment near the tryptophan residues and the secondary structure. Alternatively, homology modeling confirmed the location of four calcium-ion-binding sites in the enzyme, and molecular dynamics simulation revealed that three other calcium ions were bound to the surface of the enzyme. Calcium ions, known as a type of kosmotrope, can strongly bond with water molecules, thus aiding in the formation of the regional hydration shell required for the maintenance of enzyme activity. In addition, the introduction of calcium ions resulted in the formation of additional ionic interactions, providing propitious means for protein stabilization. Thus, the stronger intramolecular interactions were also expected to contribute partially to the enhanced stability of the enzyme in an aqueous organic solvent.     

Development of electrochemical calcium sensors by using silicon nanowires modified with phosphotyrosine

This paper reports the electrical detection of calcium ions by using silicon nanowires (SiNWs) as channels in a chemically gated field-effect-transistor (FET) configuration. To obtain a selective and sensitive layer for calcium sensing, the SiNWs are modified with a biologically relevant amino acid phosphotyrosine (p-Tyr), which is able to complex calcium ions with high affinity. It is found that when the p-Tyr modified SiNWs are exposed to aqueous solutions containing calcium ions, their conductances increase with the increasing of calcium concentration up to 10microM. In contrast, when the SiNWs are exposed to sodium or potassium, or when they are modified with tyrosine (Tyr), no significant increase in the conductance is observed. This finding suggests that the calcium ions complexed with the phosphate group of p-Tyr can act as a positive gate voltage on the FET device comprising of n-type SiNWs, and leads to an increase in their conductances. The FET device is also sensitive to magnesium ions. However, the response is 10 times lower than that of calcium at the same concentration. The study reported here may pave the way for designing an intracellular calcium sensor which permits the monitoring of calcium concentration in real time.     

Calcium ions are required for cell fusion mediated by the CD4-human immunodeficiency virus type 1 envelope glycoprotein interaction.

Calcium ions are required for fusion of a wide variety of artificial and biological membranes. To examine the role of calcium ions for cell fusion mediated by interactions between CD4 and the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (gp120-gp41), we used two experimental systems: (i) cells expressing gp120-gp41 and its receptor CD4, both encoded by recombinant vaccinia viruses, and (ii) chronically infected cells producing low levels of HIV-1. Fusion was measured by counting the number of syncytia and by monitoring the redistribution of fluorescence dyes by video microscopy. Syncytia did not form in solutions without calcium ions. Addition of calcium ions partially restored the formation of syncytia. EDTA and EGTA [ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid] blocked syncytium formation in culture media containing calcium ions. Membrane fusion as monitored by fluorescence dye redistribution also required calcium ions. Cell fusion increased with an increase in calcium ion concentration from 100 microM to 10 mM but was not affected by magnesium ions in the concentration range from 0 to 30 mM. Fibrinogen and fibronectin did not promote fusion in the absence or presence of Ca2+. Binding of soluble CD4 to gp120-gp41-expressing cells was not affected by Ca2+ and Mg2+. We conclude that Ca2+ is involved in postbinding steps in cell fusion mediated by the CD4-HIV-1 envelope glycoprotein interaction.     

Utilization in alginate beads for Cu(II) and Ni(II) adsorption of an exopolysaccharide produced by <Emphasis Type="Italic">Chryseomonas luteola</Emphasis> TEM05

Copper and nickel adsorption onto calcium alginate, sodium alginate with an extracellular polysaccharide (EPS) produced by the activated sludge bacterium Chryseomonas luteola TEM05 and the immobilized C. luteola TEM05 from aqueous solutions were studied. After that, the multi metal ions containing these ions together were prepared and partial competitive adsorptions of these mixtures were also investigated. The metal adsorption of gel beads were carried out at pH 6.0, 25 °C. The maximum adsorption capacities in Langmuir isotherm for calcium alginate, calcium alginate + EPS, calcium alginate + C. luteola TEM05 and calcium alginate + EPS + C. luteola TEM05 were 1.505, 1.989, 1.976, 1.937 mmol/g dry weight for Cu(II) and 0.996, 1.224, 1.078, 1.219 mol/g dry weight for Ni(II), respectively.The competitive biosorption capacities of the carrier for all metal ions were lower than single conditions.