The influence of mono- and divalent cations on the cardiac metabolism of arachidonic acid

Our previous study indicated that, in the isolated rabbit heart, perfusion with Ca²⁺ free Krebs Henseleit buffer (KHB) results in increased conversion of exogenous arachidonic acid to PGE₂ and 6-keto-PGF_1α, probably as the result of increased availability of substrate to cuclooxygenase. Since perfusion with Ca²⁺ free buffer is known to cause alterations in the cardiac content of various mono- and divalent cations, the present study was performed to determine: a) The relationship between the conversion of exogenous arachidonic acid to prostaglandins and cardiac content of Na⁺, K⁺, Ca²⁺ and Mg²⁺; and b) Whether enhanced arachidonic acid conversion to prostaglandins during Caa²⁺ free perfusion is due to reduced incorporation of this fatty acid into tissue lipids. Perfusion of the rabbit heart with Ca²⁺ free buffer produced a significant reduction in the tissue content of Na⁺, K⁺, Ca²⁺ and Mg²⁺. However, the production of 6-keto-PGF_1α from exogenous arachidonic acid was linearly correlated with tissue Mg²⁺. These observations, together with our finding that perfusion with Ca²⁺ free KHB reduced the incorporation of [³H] arachidonic acid into tissue lipids, suggests that Ca²⁺ free perfusion may, by reducing the activity of arachidonyl CoA synthetase (a Mg²⁺ dependent enzyme), decrease the acylation of arachidonic acid into lipids, thus increasing the availability of arachidonic acid to cyclooxygenase.     

The stability of polypurine tetraplexes in the presence of mono- and divalent cations.

As with other guanine-rich sequences, poly[d(GGA)], poly[d(GA)] and poly[d(GAA)] probably form four-stranded or tetraplex structures. Thermal denaturation profiles were measured for these polymers at pH8 in the presence of Na+, NH4+, K+, Cs+, Mg2+, Ca2+ and Ba2+. For poly[d(GA)], Na+, NH4+, K+ stabilize the tetraplex to similar extents and the Tm increases with increasing ionic strength. In contrast the Tms with Mg2+, Ca2+ and Ba2+ are significantly different and reach maxima at about 5mM of cation. The tetraplex from poly [d(GAA)] behaves in a similar manner. Thermal denaturation profiles for poly[d(GGA)] yield transitions whose hyperchromicity depends both on the concentration and nature of the ion. A reversible cooperative transition is not observed at concentrations greater than 0.15M K+, 1mM Ca2+ or 0.3 mM Ba2+ and hysteresis is evident at some concentrations. These results are consistent with the idea that K+ and ions of a similar size can form a coordination complex with the 6-Keto group of eight guanines (G8-DNA). Unlike the tetraplex polymer this G8-DNA does not melt cooperatively.     

Inhibition of protein kinase C phosphorylation by mono- and divalent cations

The effect of a matrix of concentrations of Ca2+ (0.01, 0.1, 0.5, 5 mM), Mg2+ (0.2, 0.5, 1, 2, 5, 10 mM), and Na+ (50, 100, 150 mM) on the phosphorylation of histone H-1 by protein kinase C was measured in the presence of 5 mol % diacylglycerol and Mg-ATP in both phosphatidylserine micelles and liposomes formed from a 1:4 mixture of phosphatidylserine and phosphatidylcholine. Monovalent cations (150 mM) reduced activity by 60 and 84% in the micelle and liposome assay systems, respectively. Inhibition was also observed with 5 mM Ca2+ and 10 mM Mg2+. The phosphorylating activity was compared with computer calculations of the negative electrostatic potentials (psi o) of the phospholipid membranes in the presence of the cations.     

Role of mono- and divalent metal cations in the catalysis by yeast aldolase

The rate of deuterium exchange between [1-(S)-2H]dihydroxyacetone 3-phosphate and the solvent catalyzed by native and metal-substituted yeast aldolases has been measured. In the presence of 0.1 M potassium acetate at 15 degrees C, pH 7.3, the deuterium exchange reaction catalyzed by native yeast aldolase has a kcat of 95 s-1. In contrast to the 7-fold activity enhancement by 0.1 M potassium ion (relative to 0.1 M sodium ion) of the cleavage of D-fructose 1,6-bisphosphate catalyzed by native yeast aldolase, a negligible (1.1-fold) activation by 0.1 M potassium ion is observed in the rate of dedeuteration of [1(S)-2H]dihydroxyacetone 3-phosphate. The order of reactivity of the yeast metalloaldolases in the deuterium exchange roughly parallels that seen in the fructose bisphosphate cleavage reaction. These findings suggest that the carbonyl groups of enzyme-bound D-fructose 1,6-bisphosphate and dihydroxyacetone phosphate are both polarized by the active site divalent metal cation. A mechanistic formulation consistent with the results of this and the previous paper is presented.     

Anion influence on the binding of divalent cations to phosphatidylcholine

We have used the osmotic pressure technique of Rand, Parsegian and co-workers (Nature 259 (1976) 601–603) to investigate the effect of anion species on the binding of M²⁺ to dipalmitoylphosphatidylcholine bilayers. Calcium and magnesium salts show a complex behavior which is consistent with both anion binding and screening. We observe virtually no change, within the accuracy of our experiment, in the decay of repulsive pressure with inter-bilayer separation for the acetate and nitrate salts of magnesium and calcium; however, the chloride salt does show a different pressure decay. At any given bilayer separation, , with calcium and magnesium salts present, the anions produce a decrease in the repulsive pressure in the order acetate⁻ > Cl⁻ > NO₃⁻.     

C6-oxidized cellulose: Ion interactions with mono- and divalent cations

The conformational behavior of different molecular weight fractions of a synthetic C6-oxidized derivative of cellulose were investigated by means of capillary viscometry, CD, and microcalorimetric measurements. Experiments were carried out in the presence of either monovalent or divalent counterions. The experimental data indicated that C6-oxidized cellulose can assume an ordered extended conformation at low ionic strength, induced by the intrachain repulsions of negative charges. This conformation was suggested to be very similar to the fully extended structure of cellulose. In addition to this, upon increasing the ionic strength, a conformational transition of the order-to-disorder type occurred. In fact, the screening of the electrostatic repulsions introduced a number of conformational kinks into the cellulosic backbone, which enabled the polymer to assume a more coiled conformation hence producing less viscous aqueous solutions. © 1998 John Wiley & Sons, Inc. Biopoly 45: 157–163, 1998     

The dependence of the conductance of the hemocyanin channel on applied potential and ionic concentration with mono- and divalent cations

Incorporation of Megatura crenulata hemocyanin into phosphatidylcholine black lipid membranes results in the formation of ion channels. Channel properties depend on many factors, three of which are examined in this work: type and concentration of electrolyte and applied voltage. Eight cations at different concentrations have been used. Instantaneous conductance of the channel is a saturating function of both applied voltage and ionic strength of the bathing solution with monovalent cations, but only of ionic strength with divalent cations. Steady-state voltage-conductance relations are nonlinear for both signs but show slight saturation with ionic strength. Relaxation towards the steady state can be fitted by two exponentials with different time constants. All experimental data are fitted postulating the existence of a mechanism of voltage gating of the channel, and of discrete negative charge near its mouth. Specific and nonspecific binding of cations is required.     

Transport of mono- and divalent cations across chloroplast membranes mediated by the ionophore A23187

Effects of the ionophore A23187 on isolated broken and intact chloroplasts in the pH range of 6.2 to 7.6 have been studied. In both types of chloroplasts, uncoupling of photosynthetic electron transport by A23187 (6–10 μm) was mediated either by Mg²⁺ or—in the absence of divalent cations (i.e., when EDTA was added to the medium)—by high concentrations of Na⁺, but not of K⁺ ions. At increased concentrations of the ionophore (above about 10 μm) and high pH (7.2 to 7.6), uncoupling in broken chloroplasts was also mediated by K⁺ ions. The inhibition of the energy-dependent slow decline of chlorophyll fluorescence in intact chloroplasts by the ionophore (which denotes uncoupling) is reversed by EDTA in the presence of K⁺, but not of Na⁺ ions. In 3-(3′,4′-dichlorophenyl)1,1-dimethylurea-poisoned intact chloroplasts, the yield of variable chlorophyll fluorescence is lowered by A23187 + EDTA and increased again by addition of NaCl or KCl. Chlorophyll fluorescence spectra at 77 °K of intact chloroplasts incubated with A23187 + EDTA indicated that the distribution of excitation energy had changed in favor of photosystem I, as expected from a depletion of Mg²⁺. This change was reversed by MgCl²⁺, KCl, or NaCl. From a comparison of low-temperature fluorescence spectra of broken and intact chloroplasts at different levels of Mg²⁺ in the medium, the concentration of free Mg²⁺ in the stroma of the intact chloroplasts at pH 7.6 in the dark was estimated at 1 to 4 mm. The results show that in chloroplasts the specificity of A23187 for divalent cations is limited. In the presence of EDTA, the ionophore mediates fast $\text{Na}{}^{\mathrm{+}}\text{H}{}^{\mathrm{+}}$ exchange across thylakoid membranes, whereas K⁺ is transferred much less efficiently. Both Na⁺ and K⁺ ions seem to be transported readily across the chloroplast envelope by the action of the ionophore, leading to an exchange of Mg²⁺ for monovalent cations at the thylakoid membrane surfaces in intact chloroplasts.     

Leucocyte locomotion and chemotaxis : The influence of divalent cations and cation ionophores

Human blood neutrophil leucocytes and monocytes incubated in the absence of Ca²⁺ and Mg²⁺ showed reduced, but still substantial migration into micropore filters towards chemotactic agents, compared with cells migrating in a divalent cation-rich medium. This reduction in migration could be reversed by adding low doses of divalent cation ionophores (X537A or A23187) to the Ca²⁺- and Mg²⁺-free medium which suggests that migrating leucocytes in media depleted of extracellular divalent cations can make use of intracellular divalent cations and that the intracellular cation exchange necessary for locomotion is facilitated by the ionophores. At higher doses, the ionophores inhibited locomotion, as did procaine which reduces membrane permeability to cations. Little effect of K⁺ depletion or of ouabain on leucocyte locomotion was noted.     

(Na+ + K+)-ATPase in artificial lipid vesicles: influence of the concentration of mono- and divalent cations on the pumping rate

(Na+ + K+)-ATPase from kidney outer medulla was incorporated into artificial dioleoylphosphatidylcholine vesicles. Transport activity was induced by adding ATP to the external medium. A voltage-sensitive dye was used to detect the ATP-driven potassium extrusion in the presence of valinomycin. The observed substrate-protein interactions of the reconstituted (Na+ + K+)-ATPase largely agree with that from native tissues. An agreement between ATP hydrolysis and transport activity is given for concentration dependences of sodium, potassium, magnesium and calcium ions. The only significant deviations were observed in the influence of pH. Protons were found to have different influence on transport, enzymatic activity and phosphorylation of the enzyme. The transport studies showed a twofold interaction of protons with the protein: competition with sodium at the cytoplasmic ion binding sites, a non competitive inhibition of transport which is not correlated with protein phosphorylation.     

The effect of mono- and divalent cations on Tetrahymena thermophila telomeric repeat fragment. A photon correlation spectroscopy study

The structure of the Tetrahymena thermophila telomeric sequence d(TGGGGT)(4) was studied by photon correlation spectroscopy (PCS) in aqueous solution in the presence of NaCl, KCl and SrCl(2). The sample studied was polydisperse in all conditions studied. Translational diffusion coefficients D(T) describing the diffusion modes observed were determined. On the basis of a comparison between the experimental D(T) values with those calculated assuming the bead model, two forms were identified as telomeric quadruplex structures: monomer and tetramer. In the presence of SrCl(2) formation of aggregates was observed, with a size that reached several micrometres. The relative weighted concentrations of the structures observed for different concentrations of a salt and DNA were determined. The results obtained in the presence of monovalent ions were qualitatively similar and could be presented in a coherent plot in which the concentration of salt was expressed by the number of ions per DNA molecule. A large number of ions per DNA molecule favoured tetramer formation while a small number favoured the monomer form. A structural phase transition from the monomer to the tetramer induced by a change in the number of ions per DNA molecule was observed. The main difference between the results for Na(+) and K(+) was a greater effectiveness of the K(+) ions in formation of tetramers. The effect of Sr(2+) ions on the structures formed was different than that of the monovalent ions. The results obtained in the presence of Sr(2+) could not be described as a function of the number of ions per DNA molecule.     

Hyaluronic acid: The role of divalent cations in conformation and packing

X-ray diffraction data typical of helical structures have been obtained from strontium and calcium salts of hyaluronic acid. The data indicate three disaccharides in each helix repeat with an average pitch of 2.84 nm and therefore suggest a conformational similarity with other highly extended hyaluronate polymorphs, the packing of which in crystalline arrays is influenced both by the particular cation involved and by the extent of hydration.Intensity data from a high humidity calcium salt were used in a detailed structure refinement. Six chains were found to pack in a trigonal unit cell with symmetry P3₂12 and dimensions a = b = 2.093 nm, c = 2.830 nm. The polyanion conformation is stabilized by O(3)_AO(5)_B and O(4)_BO(5)_A hydrogen bonds across the (1 → 4) and (1 → 3) linkages, respectively. Both crystallographic and steric considerations imply a non-equivalence of the three disaccharide residues in each helix turn.Adjacent antiparallel chains are tied together through COO^?Ca²⁺^?OOC bridges while the co-ordination of each Ca²⁺ ion is completed by three pairs of dyadically related water molecules. These water molecules are also extensively hydrogen-bonded to the polyanions. Sensitivity of the a and b unit cell dimensions to the ambient relative humidity further supports the conclusion that water of hydration surrounds the polyanions.Consideration of isolation and purification procedures together with elemental analysis for a large number of hyaluronate samples demonstrates the importance of divalent cations, even in small quantities, in inducing extended 3-fold helical conformations. If interactions between chain segments have a role in determining the properties of hyaluronate-containing tissues and fluids then it is likely, because of the abundant calcium which is also present, that any polymer secondary structures usually will be similar to the conformer described in this study.     

Interaction of a G-DNA quadruplex with mono- and divalent cations. A force field calculation

The formation and stability of four-stranded DNA in solution is specifically dependent on the type of cations present. The interaction potential of a model quadruplex structure with different mono- and divalent ions was determined by force field calculations. Though the electrostatic contribution to the total energy is mainly responsible for the stabilisation of the cations within the quadruplex channel, it is the van der Waals interaction at short distances that determines the specific characteristics of the different cations. An explicit consideration of the solvent indicates that the position of water molecules in close proximity to the DNA channel have a strong influence on the form of the potential, and hence on the capability of the cations for leaving and re-entering the cavity. The effect of cation size, as expressed through their Lennard-Jones parameters, is discussed.     

Effect of mono- and divalent cations on polyp morphogenesis in isolated tentacles of Aurelia aurita(Scyphozoa)

Tentacles excised from syphistoma polyps of Aurelia aurita undergo rapid regeneration to form whole polyps following exposure to an excess or absence of specific ions. It has been shown that a 12–18 h exposure of isolated tentacles to 58 mM excess of Cs⁺ results in a rapid firing of nematocysts, followed by an accelerated, synchronous polyp morphogenesis. Absence of Mgt²⁺ from the culture solution for 4–24 h also led to an accelerated, synchronous polyp regeneration. In either experimental set-up, incubation in 5–10 mM hydroxyurea effectively halted regeneration. Exposure to an excess of Li⁺ (50–200 mm) or K⁺ (10–50 mM) caused no firing of nematocysts and a percentage of polyp regeneration only slightly higher than control tentacles. Use of the K⁺ channel blocker tetraethylammonium (TEA; 100–300 mM) lead to similar levels of regeneration. A Ca²⁺ or K⁺-reduced artificial culture solution did not enhance regeneration. Ouabain (1 mM) dampened the Cs⁺ induced acceleration of polyp morphogenesis, and when given without Cs⁺, elicited a control level response.     

The effect of acid water on the loss of divalent cations and primary amines from natural membranes

Losses of Ca2+, Mg2+ and primary amines into waters between pH = 5 and 3 from eyed Chinook salmon eggs, Oncorhynchus tshawytscha , and gills excised from the freshwater, bivalve mollusc Anodonta californiensis were measured and compared to effluxes into distilled water. Sulfuric, nitric and hydrochloric acids were used. Even at pH = 5 losses of Ca2+ and Mg2+ from both biological systems occur at short times, minutes, and can exceed those found in water of higher pH (non-acid waters). Increasing acidity increases short term primary amine loss from both systems. For both divalent cation and amino acid losses gills of A. californiensis are more sensitive to acidity than eggs of O. tshawytscha .     

Interactions of divalent cations and nucleotides with solubilized cardiac guanylate cyclase.

Some properties of guanylate cyclase, which was solubilized from the rabbit heart washed particles by the treatment with Triton X-100, were investigated. The solubilized enzyme activity was stimulated by Mg2+ in the presence of low (subsaturating) Mn2+ (GTP is greater than Mn2+); under these conditions, Ga2+ was inhibitory. At subsaturating MnGTP and free Mn2+, the solubilized enzyme was markedly stimulated by MnGDP and MnATP; CaGTP on the other hand, was inhibitory. These results are consistent with the view that the particulate guanylate cyclase may exist in the cell as a metalloenzyme with tightly bound Mn2+ and that Mg2+ supports its catalysis while Ca2+ as well as nucleotides may exert regulatory effects on its activity.     

The influence of divalent cations on the dynamic properties of actin filaments: a spectroscopic study.

Cells can be transiently permeabilized by a membrane potential difference increase induced by the application of high electric pulses. This was shown to be under the control of the pulsing buffer osmotic pressure, when short pulses were applied. In this paper, the effects of buffer osmotic pressure during electric treatment and during the following 10 min were investigated in Chinese hamster ovary cells subjected to long (ms) square wave pulses, a condition needed to mediate gene transfer. No effect on cell permeabilization for a small molecule such as propidium iodide was observed. The use of a hypoosmolar buffer during pulsation allows more efficient loading of cells with beta-galactosidase, a tetrameric protein, but no effect of the postpulse buffer osmolarity was observed. The resulting expression of plasmid coding for beta-galactosidase was strongly controlled by buffer osmolarity during as well as after the pulse. The results, tentatively explained in terms of the effect of osmotic pressure on cell swelling, membrane organization, and interaction between molecules and membrane, support the existence of key steps in plasmid-membrane interaction in the mechanism of cell electrically mediated gene transfer.