Effect of surface potential on the intramembrane electrical field measured with carotenoid spectral shift in chromatophores from Rhodopseudomonas sphaeroides

Changes in the surface potential, the electrical potential difference between the membrane surface and the bulk aqueous phase were measured with the carotenoid spectral shift which indicates the change of electrical field in the membrane. Chromatophores were prepared from a non-sulfur purple bacterium, Rhodopseudomonas sphaeroides, in a low-salt buffer. Surface potential was changed by addition of salt or by pH jump as predicted by the Gouy-Chapman diffuse double layer theory.When a salt was added at neutral pH, the shift of carotenoid spectrum to shorter wavelength, corresponding to an increase in electrical potential at the outside surface, was observed. The salts of divalent cations (MgSO₄, MgCl₂, CaCl₂) were effective at concentrations lower than those of monovalent cation salts (NaCl, KCl, Na₂SO₄) by a factor of about 50. Among the salts of monoor divalent cation used, little ionic species-dependent difference was observed in the low-concentration range except that due to the valence of cations. The pH dependence of the salt-induced carotenoid change was explained in terms of the change in surface charge density, which was about 0 at pH 5–5.5 and had negative values at higher pH values. The dependence of the pH jump-induced absorbance change on the salt concentration was also consistent with the change in the charge density. The surface potential change by the salt addition, which was calibrated by H⁺ diffusion potential, was about 90 mV at the maximum. From the difference between the effective concentrations with salts of mono- and divalent cations at pH 7.8, the surface charge density of (?1.9 ± 0.5) · 10^?3 elementary charge per Ų, and the surface potential of about ?100 mV in the presence of about 0.1 mM divalent cation or 5 mM monovalent cation were calculated.     

Cation-dependent changes in the thylakoid membrane appression of the diatom Thalassiosira pseudonana

Diatoms show a special organisation of their plastid membranes, such that their thylakoids span the entire plastid in bands of three. While in higher plants the interaction of the light harvesting complex II and photosystem II with divalent cations (especially Mg²⁺) was found to take part in the interplay of electrostatic attraction and repulsion in grana membrane appression, for diatoms the key players in maintaining proper membrane distances were not identified so far. In this work, we investigated the changes in the thylakoid architecture of Thalassiosira pseudonana in reaction to different salts by using circular dichroism and fluorescence spectroscopy in combination with other techniques. We show that divalent cations have an important influence on optimal pigment organisation and thus also on maintaining membrane appression. Thereby, monovalent cations are far less effective. The concentration needed is in a physiological range and fits well with the values obtained for higher plant grana stacking, despite the fact that strict protein segregation as seen in higher plant grana is missing.     

Induction of Myxospore Formation in Stigmatella aurantiaca (Myxobacterales) by Monovalent Cations

Suspension cultures of Stigmatella aurantiaca can be induced to form myxospores by addition of the monovalent cations Li⁺, Na⁺, NH₄⁺, K⁺, or Rb⁺.     

Diffusion NMR-based comparison of electrostatic influences of DNA on various monovalent cations

Counterions are important constituents for the structure and function of nucleic acids. Using ⁷Li and ¹³³Cs nuclear magnetic resonance (NMR) spectroscopy, we investigated how ionic radii affect the behavior of counterions around DNA through diffusion measurements of Li⁺ and Cs⁺ ions around a 15-bp DNA duplex. Together with our previous data on ²³Na⁺ and ¹⁵NH₄⁺ ions around the same DNA under the same conditions, we were able to compare the dynamics of four different monovalent ions around DNA. From the apparent diffusion coefficients at varied concentrations of DNA, we determined the diffusion coefficients of these cations inside and outside the ion atmosphere around DNA (D_b and D_f, respectively). We also analyzed ionic competition with K⁺ ions for the ion atmosphere and assessed the relative affinities of these cations for DNA. Interestingly, all cations (i.e., Li⁺, Na⁺, NH₄⁺, and Cs⁺) analyzed by diffusion NMR spectroscopy exhibited nearly identical D_b/D_f ratios despite the differences in their ionic radii, relative affinities, and diffusion coefficients. These results, along with the theoretical relationship between diffusion and entropy, suggest that the entropy change due to the release of counterions from the ion atmosphere around DNA is also similar regardless of the monovalent ion types. These findings and the experimental diffusion data on the monovalent ions are useful for examination of computational models for electrostatic interactions or ion solvation.     

Cation Penetration through Isolated Leaf Cuticles

The rates of penetration of various cations through isolated apricot Prunus armeniaca L. leaf cuticles were determined. Steady state rates were measured by using a specially constructed flow-through diffusion cell. The penetration rates of the monovalent cations in group IA followed a normal lyotropic series, i.e., CS⁺ ≥ Rb⁺ > K⁺ > Na⁺ > Li⁺. The divalent cations all penetrated through the cuticle more slowly than the monovalent cations. Comparison of the relative values of k (permeability coefficient) and D (diffusion coefficient) indicates that the penetration of ions through isolated cuticles took place by diffusion and was impeded by charge interactions between the solute and charge sites in the penetration pathway. Cuticular penetration rates of K⁺ and H₂O at pH above 9 were of similar magnitude. At pH 5.5 H₂O penetration was not affected but that of K⁺ was greatly reduced. From this observation and from data on cuticle titration and ion adsorption studies, we hypothesize that cuticular pores are lined with a substance (perhaps a protein) which has exposed positively charged sites.     

Effects of monovalent cations on Ca uptake by skeletal and cardiac muscle sarcoplasmic reticulum

Ca²⁺ transport by the sarcoplasmic/endoplasmic reticulum Ca²⁺ ATPase (SERCA) is sensitive to monovalent cations. Possible K⁺ binding sites have been identified in both the cytoplasmic P-domain and the transmembrane transport-domain of the protein. We measured Ca²⁺ transport into SR vesicles and SERCA ATPase activity in the presence of different monovalent cations. We found that the effects of monovalent cations on Ca²⁺ transport correlated in most cases with their direct effects on SERCA. Choline⁺, however, inhibited uptake to a greater extent than could be accounted for by its direct effect on SERCA suggesting a possible effect of choline on compensatory charge movement during Ca²⁺ transport. Of the monovalent cations tested, only Cs⁺ significantly affected the Hill coefficient of Ca²⁺ transport (n_H). An increase in n_H from ∼2 in K⁺ to ∼3 in Cs⁺ was seen in all of the forms of SERCA examined. The effects of Cs⁺ on the maximum velocity of Ca²⁺ uptake were also different for different forms of SERCA but these differences could not be attributed to differences in the putative K⁺ binding sites of the different forms of the protein.     

Schistosoma mansoni: effect of some cations on the proteolytic enzymes of cercariae

The effects of various salts on the proteolytic activity of extracts from Schistosoma mansoni cercariae were tested. Using an Azocoll substrate, stimulation (2 to 2.5-fold) of activity by the monovalent cations Na⁺ and K⁺ was demonstrated, with maximum stimulation at 20–40 mM concentrations. The divalent cations Mg²⁺ and Ca²⁺ stimulated proteolytic activity at low concentrations (between 0 and 10 mM) but inhibited activity at higher concentrations. The divalent cations Zn²⁺, Cu²⁺, Fe²⁺, and Co²⁺ were inhibitory even at very low concentrations. The results presented here are discussed in relation to previously described ion effects on cercarial infectivity.     

Effects of monovalent cations on the catalytic activity of pig liver phosphofructokinase.

The monovalent cations NH₄⁺, K⁺, and Rb⁺ activate pig liver phosphofructokinase by increasing the maximal velocity. In the presence of these cations the enzyme retains sigmoid kinetics with respect to fructose-6-phosphate. However, these cations bring about a decrease in the [S]_0.5 for fructose-6-phosphate to an extent directly proportional to their ionic volumes. The apparent dissociation constants of NH₄⁺, K⁺, and Rb⁺ for the enzyme at 0.5 mM ATP and 4 mM Fru6P are 0.2 mM, 8 mM, and 15 mM, respectively. The maximal velocity of the enzyme in the presence of saturating concentrations of Rb⁺ is about 70% of that seen with NH₄⁺ or K⁺. The monovalent cations Li⁺, Na⁺, and Cs⁺ inhibit the enzyme at high concentrations (> 50 mM) by decreasing the maximal velocity. Although the efficiency of inhibition by these cations qualitatively increases with decreasing size, there is no obvious quantitative relationship between efficiency of inhibition and any parameter of ionic size.     

Changes in growth and water-soluble solute concentrations in Sorghum bicolor stressed with sodium and potassium salts

Sorghum bicolor L. Moench, RS 610, was grown in liquid media salinized with NaCl, KCl, Na₂SO₄, K₂SO₄ or with variable mixtures of either NaCl/KCl or Na₂SO₄/K₂SO₄ at osmotic potentials ranging from 0 to -0.8 MPa. The purpose was to study the effects of different types and degrees of salinity in growth media on growth and solute accumulation. In 14-day-old plants the severity of leaf growth inhibition at any one level of osmotic potential in the medium increased according to the following order: NaCl < Na₂SO₄ < KCl = K₂SO₄. Inhibition of growth by mixtures of Na⁺ and K⁺ salts was the same as by K⁺ salts alone. Roots responded differently. Root growth was not affected by Na⁺ salts in the range of 0 to -0.2 MPa while it was stimulated by K⁺ salts. The major cation of leaves was K⁺ because S. bicolor is a Na⁺-excluder, while Na⁺ was the major cation in roots except at low Na⁺/K⁺ ratios in media. Anions increased in tissues linearly in relation to total monovalent cation, but not with a constant anion/cation ratio. This ratio increased as the cation concentrations in tissues increased. Sucrose in leaf tissue increased 75 fold in Chloride-plants (plants growing in media in which the only anion of the salinizing salts was Cl^?) and 50 fold in Sulphate-plants (the only anion of the salinizing salts was SO₄^2-). Proline increased 60 and 18 fold in Chloride- and Sulphate-plants, respectively, as growth media potentials decreased from 0 to -0.8 MPa. The concentrations of both sucrose and proline were directly proportional to the amount of total monovalent cation in the tissue. Sucrose concentrations began increasing when total monovalent cations exceeded 100 μmol (g fresh weight)^?1 (the monovalent cation level in non-stressed plants), but proline did not start accumulating until monovalent cation concentrations exceeded 200 μmol (g fresh weight)^?1. Therefore, sucrose seemed to be the solute used for osmotic adjustment under mild conditions of saline stress while proline was involved in osmotic adjustment under more severe conditions of stress. Concentrations of inorganic phosphate, glucose, fructose, total amino acids and malic acid fluctuated in both roots and leaves in patterns that could be somewhat correlated with saline stress and, sometimes, with particular salts in growth media. However, the changes measured were too small (at most a 2–3 fold increase) to be of importance in osmotic adjustment.     

Effects of monovalent cations on phosphate accumulation and storage of the ectomycorrhizal fungus Suillus bovinus

Comparative in vivo ³¹P-NMR studies of the fungus Suillus bovinus (L.: Fr.) O. Kuntze in pure culture have produced interesting new data. To investigate the response of phosphate metabolism to a change in external monovalent cations, samples were exposed to a Hoagland solution containing different monovalent cations Li⁺, Na⁺, K⁺, or Rb⁺ at 10 mM concentration. A method of nutrient cycling during analysis where the cation was changed and the phosphate kept constant allowed us to determine the kinetics of phosphate accumulation, storage and incorporation into polyphosphate following exposure to the range of test cations. Different external monovalent cations had different effects upon changes in the content of both phosphate and polyphosphate. Treatment with Li⁺, Na⁺, or Rb⁺ resulted in a change in phosphate accumulation to 60, 73, and 107% and in content of the intracellular mobile polyphosphate (polyP) to 119, 112, and 94%, respectively, compared with the control taken as 100%. The effect of each cation is related to its position in the periodic table. Reversing this process, i.e., exchanging with K⁺, returned phosphate metabolism to normal. Although, the increase in depolarization of the cell membrane should affect the internal pH, fungal metabolism using energy requiring mechanisms appeared necessary to maintain the intracellular pH. Thus, increasing contents of mobile polyP were the consequence of an increasing energy demand. On the other hand, the increasing depolarization of the cell membrane following the sequence Rb⁺ < K⁺ < Na⁺ < Li⁺ inhibited the net P_i accumulation. Furthermore, it is postulated that the P_i accumulation was also regulated by the intracellular content in polyP.     

Sodium-dependent succinate uptake in purple bacterium Ectothiorhodospira shaposhnikovii

Succinate, malate and fumarate uptake in purple sulfur bacterium Ectothiorhodospira shaposhnikovii, strain 1 K MSU, obligatorily depends on the presence of Na⁺. Other monovalent cations such as K⁺, Li⁺, NH₄⁺ could not replace Na⁺. Experiments with energy-depleted cells have shown that succinate uptake against its concentration gradient can be energized by artificially imposed sodium gradients (ΔpNa).An artificial membrane potential (inside negative) inhibited ΔpNa-driven succinate uptake at pH 7.0 but stimulated it at pH 9.0.The results confirm the suggestion that succinate uptake in E. shaposhnikovii is carried out in symport with Na⁺.     

Osmotic swelling of heart mitochondria in acetate and chloride salts. Evidence for two pathways for cation uptake.

Swelling of nonenergized heart mitochondria suspended in acetate salts appears to depend on the activity of an endogenous cation/H⁺ exchanger. Passive swelling in acetate shows a characteristic cation selectivity sequence of Na⁺ >Li⁺ >K⁺, Rb⁺, Cs⁺, or tetramethylammonium, a sharp optimum at pH 7.2–7.3, activation by Ca²⁺, and loss of activity on aging which can be related to loss of endogenous K⁺. The reaction is nearly insensitive to either addition of exogenous Mg²⁺ or removal of membrane Mg²⁺ with EDTA. Each of these characteristics of passive swelling in acetate salts is duplicated in chloride media when tripropyltin is added to induce Cl^?/OH^? exchange. In contrast to nonenergized mitochondria, swelling of respiring mitochondria has been postulated to depend on electrophoretic uptake of cations in response to an interior negative membrane potential. Respiration-dependent swelling in acetate shows an indistinct cation selectivity sequence with Li⁺ and Na⁺ supporting higher rates of swelling at higher efficiency than K⁺, Rb⁺, and Cs⁺. The high rates of respiration-dependent swelling in Li⁺ and Na⁺ are inhibited by low levels of exogenous Mg²⁺ (K_i of 5–10 μm), but a significant swelling with almost no cation selectivity persists in the presences of 2 mm Mg²⁺. Removal of membrane Mg²⁺ by addition of EDTA strongly activates the rate of respiration-dependent swelling and converts a sigmoid dependency of swelling rate on Li⁺ concentration to a hyperbolic one with a Km of about 14 mm Li⁺. The cation selectivity and Mg²⁺ dependence of the reaction induced in chloride salts by tripropyltin are identical to these properties in acetate. Energy-dependent swelling in acetate shows optimum activity at pH 6.5 which appears related to the availability of free acetic acid, since the corresponding reaction induced in chloride shows a broad optimum at about pH 7.5. These studies support the concept that monovalent cations enter nonenergized mitochondria by electroneutral exchange with protons but penetrate respiring mitochondria by electrophoretic movement through one or more uniport pathways. They further suggest that both a Mg²⁺-sensitive uniport with high activity for Na⁺ and Li⁺ and a Mg²⁺-insensitive pathway with little cation discrimination are available in the membrane.     

Determination of cation exchange capacity of ryegrass roots by summing exchangeable cations

Cations were desorbed from root exchange sites of ryegrass (Lolium multiflorum Lam. cvs. Gulf, Marshall, Urbana, and Wilo) using BaCl₂, BaCl₂-triethanolamine, NH₄OAc, and KCl. Results were analysed using multivariate analysis of variance. Ba²⁺-containing desorbents displaced more Ca²⁺ while monovalent desorbents displaced more exchangeable monovalent cations. The sum of adsorbed cations was significantly correlated with root exchange capacity (CEC) as determined by the H⁺ titration procedure, although slightly larger values were obtained with all desorbents. Lower CEC values were obtained for ryegrass cultivars less sensitive to Al.     

Two Open States with Progressive Proton Selectivities in the Branched Channelrhodopsin-2 Photocycle

Channelrhodopsins are light-gated ion channels that mediate vision in phototactic green algae like Chlamydomonas. In neurosciences, channelrhodopsins are widely used to light-trigger action potentials in transfected cells. All known channelrhodopsins preferentially conduct H⁺. Previous studies have indicated the existence of an early and a late conducting state within the channelrhodopsin photocycle. Here, we show that for channelrhodopsin-2 expressed in Xenopus oocytes and HEK cells, the two open states have different ion selectivities that cause changes in the channelrhodopsin-2 reversal voltage during a light pulse. An enzyme kinetic algorithm was applied to convert the reversal voltages in various ionic conditions to conductance ratios for H⁺ and divalent cations (Ca²⁺ and/or Mg²⁺), as compared to monovalent cations (Na⁺ and/or K⁺). Compared to monovalent cation conductance, the H⁺ conductance, α, is ∼3 × 10⁶ and the divalent cation conductance, β, is ∼0.01 in the early conducting state. In the stationary mixture of the early and late states, α is larger and β smaller, both by a factor of ∼2. The results suggest that the ionic basis of light perception in Chlamydomonas is relatively nonspecific in the beginning of a light pulse but becomes more selective for protons during longer light exposures.     

Low field electric birefringence study of monovalent cation-DNA interactions

The effects of monovalent cations on DNA have been studied using static and dynamic electric birefringence. Kerr's law is obeyed in a limited E range (<30 Vcm_?1) and the steady state birefringence values are close for the different cations. The birefringence kinetics have been analysed in terms of three relaxation times. On a semilogarithmice plot of Δn(t), the tail of the curve is linear over a wide range of time for Na⁺, K⁺, NH₄⁺ and Li⁺. Only for Cs⁺ solution is no linear part found and a much longer relaxation time is determined. This only contributes a small part of the total birefringence. With Cs⁺ this contribution is more field-dependent than for the other cations and we observe a larger molecular flexibility. On the other hand, with Li⁺ a greater stiffness of the DNA molecule appears. The electrical polarizabilities anisotropies decrease in the order: Cs⁺ >NH⁺₄ >K⁺ >Na⁺ >Li⁺. There are no significant differences in the optical anisotropy factors.     

Inactivation and reactivation of ribosomal subunits: amino acyl-transfer RNA binding activity of the 30 s subunit of Escherichia coli

The ability of the 30 s ribosomal subunit to bind phenylalanyl-transfer RNA in the cold in response to polyuridylic acid is lost if the subunit is subjected, even transiently, to either of two treatments: (a) the removal of certain specific monovalent cations (NH⁺₄, K⁺, Rb⁺orCs⁺), or (b) the reduction of the Mg²⁺ concentration below a critical concentration of about 2 mm. If the depleted cation is restored, the subunit reverts to an active form in a process that is greatly enhanced by heat. Thermally reactivated subunits retain full activity when rechilled, showing that the inactivation and reactivation processes involve changes, presumably conformational, in the subunit itself. Reactivation follows first-order kinetics with respect to the appearance of active subunits, with an Arrhenius activation energy of 26 kcal./mole between 30 °C and 40 °C.     

Modification of foliar solute concentrations by calcium in two species of wheat stressed with sodium chloride and/or potassium chloride

The effects of saline-stresses due to different salts on growth and on foliar solute concentrations in seedlings of two species of wheat that differed in salt tolerance. Triticum aestivum L. cv. Probred and Triticum turgidum L. (Durum group) cv. Aldura, were studied. Triticum aestivum is the more salt tolerant species. The salts used were NaCl, KCI, a 1:1 mixture of NaCI and KCI, and these same monovalent cation salts but mixed with CaCI₂ at a ratio of 2:1 on a molar basis of monovalent to divalent cation salts. Growth inhibition of both species was a function of media osmotic potentials. There was a small additional inhibition of growth if KCI replaced NaCI as the salinizing salt. CaCI₂ had little or no effect on growth inhibition beyond an osmotic effect except at the most severe stress level, i.e. when Ca²⁺ concentrations may be excessive. The amounts of water-soluble Ca²⁺ were about 10 times higher in leaves of plants grown in the presence of CaCI₂ than in its absence, but its concentrations even then were approximately 10% or less of those of the monovalent cations. Including CaCI₂ in growth media resulted in a reduction in the amount of Na⁺ in leaves compared to the amounts in plants grown at the same osmotic potential but in the absence of CaCI₂. Triticum aestivum was a better Na⁺-excluder than T. turgidum. With CaCI₂ in media, (Na⁺+ K⁺) remained relatively constant or increased by small amounts as media osmotic potentials décreased. In the absence of CaCI₂₊ (Na⁺+ K⁺) increased by large amounts when media osmotic potentials were at ?0.6 and ?0.8 MPa. It is concluded that the accumulation system in leaves for monovalent cations was under feed-back control, and that this control mechanism was inhibited by high media concentrations of Na⁺ and/or K⁺. Sucrose was present at a constant amount under all growth conditions. Proline started accumulating when (Na⁺+ K⁺) exceeded a threshold value of 200 μmol (g fresh weight)^?1. Its concentration was 5 to 13% of that portion of (Na⁺+ K⁺) that exceeded the threshold value.     

Nonselective cation channel activated by patch excision from lobster olfactory receptor neurons

Summary A nonselective cation channel activated by patch excision was characterized in inside-out patches from spiny lobster olfactory receptor neurons. The channel, which was permeable to Na⁺, K⁺ and Cs⁺, had a conductance of 320 pS and was weakly voltage dependent in the presence of micromolar divalent cations. Millimolar internal divalent cations caused a voltage-and concentration-dependent block of Na⁺ permeation. Analysis of the voltage dependence indicated that the proportion of the membrane's electric field sensed by Mg²⁺ was >1, suggesting that the channel contains a multi-ion pore. Internal divalent cations also reduced the frequency of channel opening in a concentration-dependent, but not voltage-dependent, manner, indicating that different cation binding sites affect gating and conductance. While block of gating prevented determining if internal divalent cations permeate the channel, a channel highly permeable to external divalent cations was observed upon patch excision to the inside-out configuration. The monovalent and divalent cation conductances shared activation by patch excision, weak voltage dependence, and steady-state activity, suggesting that they are the same channel. These data extend our understanding of this type of channel by demonstrating permeation by monovalent cations, detailing Mg²⁺ block of Na^– permeation, and demonstrating the channel's presence in arthropods.     

Correlation between cation-induced formation of heavy subchloroplast fractions and cation-induced increase in chlorophyll a fluorescence yield in Tricine-washed chloroplasts

A good correlation exists between the extent of thylakoid aggregation (grana reconstitution) and the increase in the chlorophyll a fluorescence yield (F_DCMU; DCMU = 3-(3′,4′-dichlorophenyl)-1, 1-dimethyl urea) caused by the addition of monovalent or divalent cations to low-salt disorganized (agranal) chloroplasts. The extent of grana stacking was monitored by the yield of heavy subchloroplast fractions after digitonin disruption of chloroplasts. A good correlation of the cation effect on both parameters was also found in light subchloroplast fractions (10,000g supernatants) obtained from sonicated “low-salt” Tricine-suspended pea chloroplasts. Addition of cations to the agranal protochloroplasts of etiolated pea or bean leaves exposed to periodic light-dark cycles, suspended in low-salt Tricine buffer, does not affect formation of heavy subchloroplast fractions, nor does it affect their chlorophyll a fluorescence yield level (F_DCMU). The cation effect on the increase of the chlorophyll a fluorescence yield level seems to be due to the cation-induced thylakoid structural changes leading to grana stacking.