DIVALENT CATION SPECIFICITY OF THE CALCIUM REQUIREMENT FOR FAST TRANSPORT OF PROTEINS IN AXONS OF DESHEATHED NERVES

The presence of a requirement for calcium during the fast transport of [³H]protein in axons was assessed in desheathed spinal nerves of bullfrog. The nerves were desheathed locally along 4 mm of their length, and desheathing was judged effective on the basis of an enhanced uptake of [³H]leucine into that region of nerve trunk. Desheathing per se had a slight inhibitory effect on transport. Incubation of desheathed nerve trunks in calcium-free medium reduced transport by 60-80% relative to that in desheathed nerves incubated in normal medium. Addition of Mg²⁺ or Sr²⁺ to the calcium-free medium allowed transport to proceed normally. Addition of Co²⁺ or Mn²⁺ to normal medium did not affect transport in desheathed isolated nerve trunks. When ganglia and nerve trunks were both incubated in medium containing 0.18 mM-CoCl₂, transport was depressed to a similar extent proximal and distal to the desheathed region. This again indicates that Co²⁺ does not inhibit transport in desheathed nerves, whereas it does inhibit transport in the ganglia. Additive inhibitory effects were observed when ganglia were incubated in medium containing 0.018 mM-CoCl₂, and desheathed nerves were incubated in calcium-free medium. Differences in the divalent cation specificities of the axonal and ganglionic calcium requirements suggest that calcium supports transport in nerves in a manner distinct from its role in maintaining transport in spinal ganglia. It is concluded that the ganglionic calcium requirement involves initiation of axonal transport in the soma rather than translocation in the intraganglionic region of axon.     

Author index for volume 93

The presence of Ca²⁺ is essential for survival in culture of fully grown oocytes isolated from mouse ovaries but not for survival of small, meiotically incompetent oocytes, metaphase II oocytes, and early embryos. Ninety percent of fully grown ovarian oocytes die within 2 hr when cultured in calcium-free medium (CFM). CFM death does not occur if other cations (1 mM La³⁺ or 10 mM Sr²⁺, but not 12 mM Mg²⁺ nor 1 mM D-600) replace Ca²⁺ in the medium. Sensitivity to CFM is progressively acquired by the oocyte during the growth phase, in parallel with the acquisition of meiotic competence, and is lost after 2 hr of culture in the presence of at least 0.5 mM Ca²⁺. The loss of sensitivity to CFM during in vitro culture is not related to the concomitant spontaneous resumption of meiosis, since the oocyte becomes resistant to CFM even if germinal vesicle breakdown is prevented by the addition of dibutyryl cAMP to the culture medium. Some hypotheses are put forward to explain the peculiar and transient high calcium requirements of fully grown oocytes.     

Effect of divalent cations on succinate transport in Rhizobium tropici,R. leguminosarum bv phaseoli and R. loti

Rhizobium tropici, R. leguminosarum bv phaseoli and R. loti each have an active C₄-dicarboxylic acid transport system dependent on an energized membrane. Free thiol groups are probably involved at the active site. Since EDTA inhibited succinate transport in R. leguminosarum bv phaseoli and R. loti, divalent cations may participate in the process; the activity was reconstituted by the addition of Ca²⁺ or Mg²⁺. However, EDTA had no effect on succinate transport in R. tropici, R. meliloti or R. trifolii strains. Ca²⁺ or Mg²⁺ had a similar effect on the growth rates of R. tropici and R. leguminosarum bv phaseoli; R. tropici did not require Ca²⁺ to grow on minimal medium supplemented with succinate but R. leguminosarum bv phaseoli required either or both of the divalent cations Ca²⁺ and Mg²⁺. A R. tropici Mu-dI (lacZ) mutant defective in dicarboxylic acid transport, was isolated and found unable to form effective bean nodules.The authors are with the Division of Biochemistry, Instituto de Investigaciones Biológicas Clemente Estable, Avda, Italia 3318, 11.600 Montevideo, Uruguay     

The essentiality of calcium ion in the enzymatic activity of Taiwan cobra phospholipase A2

In order to address the mechanism whereby Ca²⁺ wad crucial for the manifestation of the enzymatic activity of phospholipase A₂ (PLA₂), four divalent cations were used to assess their influences on the catalytic activity and the fine structures ofNaja naja atra PLA₂. It was found that substitution of Mg²⁺ or Sr²⁺ for Ca²⁺ in the substrate solution caused a decrease in the PLA₂ activity to 77.5% or 54.5%, respectively, of that in the presence of Ca²⁺. However, no PLA₂ activity was observed with the addition of Ba²⁺. With the exception of Mg²⁺, the nonpolarity of the 8-anilinonaphthalene-1-sulfonate (ANS)-binding site of PLA₂ markedly increased with the binding of cations to PLA₂. In the meantime, the accessibilities of Lys-6 (65) and Tyr-3 (63) toward trinitrobenzene sulfonate andp-nitrobenzenesulfonyl fluoride were enhanced by the addition of Ca²⁺, Sr²⁺, and Ba²⁺, but not by Mg²⁺. The order of the ability of cations to enhance the ANS fluorescence and the reactivity of Lys and Tyr residues toward modified reagents was Ba²⁺> Sr²⁺> Ca²⁺> Mg²⁺, which was the same order as the increase in their atomic radii. These results, together with the observations that the ANS molecule binds at the active site of PLA₂ and that Tyr-3, Lys-6, and Tyr-63 of PLA₂ are involved in the binding with the substrate, suggest that the binding of Ca²⁺ to PLA₂ induces conformational changes at the active site and substrate-binding site. However, the smaller atomic radius with Mg²⁺ or the bigger atomic radii with Sr²⁺ and Ba²⁺ might render the conformation improperly rearranged after their binding to PLA₂ molecule.     

Variation in selectivity of univalent cations in slime moldPhysarum polycephalum caused by reception of polyvalent cations

Summary Specificity of reception on 11 electrolytes in the slime moldPhysarum polycephalum was investigated in the presence of polyvalent cations in media. Membrane potential and motive force of tactic movement were examined with the aid of the double chamber method, and the zeta potential at the membrane surface of the slime mold was measured by electrophoretic mobility. The results obtained are summarized as follows: (1) The presence of polyvalent cations (e.g., Ca²⁺, Mg²⁺, Sr²⁺, Ba²⁺, La³⁺, Th⁴⁺) in medium led to an increase in threshold concentration,C _th , determined from the potential measurements for Na- or Li-salts, and to a decrease inC _th for K-, Rb-, or NH₄-salts,C _th for 11 electrolytes changed discontinuously when the concentration of polyvalent cations in medium exceeded their respective thresholds. (2) TheC _th determined from chemotaxis agreed with that from the potential response both in the presence and absence of polyvalent cations. (3) Sequence of selectivity of univalent cations varied extensively in the presence of polyvalent cations. (4) Changes in the zeta potential induced by NaCl reception agreed with those in the membrane potential even in the presence of Ca²⁺ in medium. (5) TheC _th for reception of NaCl changed sharply at about 12 °C in the presence of polyvalent cations, while that for KCl was independent of the temperature.Conformational changes in surface membrane of the slime mold in response to reception of polyvalent cations were then discussed in relation to the discrimination of univalent cations.     

Divalent Cation Interactions with Na,K-ATPase Cytoplasmic Cation Sites: Implications for the <Emphasis Type="Italic">para</Emphasis>-Nitrophenyl Phosphatase Reaction Mechanism

The interactions of divalent cations with the adenosine triphosphatase (ATPase) and para-nitrophenyl phosphatase (pNPPase) activity of the purified dog kidney Na pump and the fluorescence of fluorescein isothiocyanate (FITC)-labeled pump were determined. Sr²⁺ and Ba²⁺ did not compete with K⁺ for ATPase (an extracellular K⁺ effect). Sr²⁺ and Ba²⁺ did compete with Na⁺ for ATPase (an intracellular Na⁺ effect) and with K⁺ for pNPPase (an intracellular K⁺ effect). These results suggest that Ba²⁺ or Sr²⁺ can bind to the intracellular transport site, yet neither Ba²⁺ nor Sr²⁺ was able to activate pNPPase activity; we confirmed that Ca²⁺ and Mn²⁺ did activate. As another measure of cation binding, we observed that Ca²⁺ and Mn²⁺, but not Ba²⁺, decreased the fluorescence of the FITC-labeled pump; we confirmed that K⁺ substantially decreased the fluorescence. Interestingly, Ba²⁺ did shift the K⁺ dose-response curve. Ethane diamine inhibited Mn²⁺ stimulation of pNPPase (as well as K⁺ and Mg²⁺ stimulation) but did not shift the 50% inhibitory concentration (IC₅₀) for the Mn²⁺-induced fluorescence change of FITC, though it did shift the IC₅₀ for the K⁺-induced change. These results suggest that the Mn²⁺-induced fluorescence change is not due to Mn²⁺ binding at the transport site. The drawbacks of models in which Mn²⁺ stimulates pNPPase by binding solely to the catalytic site vs. those in which Mn²⁺ stimulates by binding to both the catalytic and transport sites are presented. Our results provide new insights into the pNPPase kinetic mechanism as well as how divalent cations interact with the Na pump.     

Some characteristics of Ca2+ uptake by yeast cells

Summary Experiments were performed to obtain information on: (i) the specific properties of Ca²⁺ binding and transport in yeast (ii) the relationship between both parameters; (iii) similarities to or differences from other biological systems as measured by the effects of inhibitors; and (iv) the effects of mono and divalent cations, in order to get some insight on the specificity and some characteristics of the mechanism of the transport system for divalent cations in yeast.The results obtained gave some kinetic parameters for a high affinity system involved in the transport of Ca²⁺ in yeast. These were obtained mainly by considering actual concentrations of Ca²⁺ in the medium after substracting the amounts bound to the cell. Ak _m of 1.9 m and aV _max of 1.2 nmol (100 mg·3 min)^–1 were calculated.The effects of some inhibitors and other cations on Ca²⁺ uptake allow one to postulate some independence between binding and transport for this divalent cation.Of the inhibitors tested, only lanthanum seems to be a potent inhibitor of Ca²⁺ uptake in yeast.The effects of Mg²⁺ on the uptake of Ca²⁺ agree with the existence of a single transport system for both divalent cations.The actions of Na⁺ and K⁺ on the transport of Ca²⁺ offer interesting possibilities to study further some of the mechanistic properties of this transport system for divalent cations.     

ATP-dependent strontium uptake by basolateral membrane vesicles from rat renal cortex in the absence or presence of calcium

ATP-dependent Sr²⁺ transport was examined in vitro using basolateral membrane (BLM) vesicles isolated from rat renal cortex to clarify the discrimination mechanisms between strontium (Sr) and calcium (Ca) in renal tubules during reabsorption. ATP-dependent Sr²⁺ uptake and Ca²⁺ uptake were observed in renal BLM vesicles and were inhibited by vanadate. Hill plots indicate similar kinetic behavior for Ca²⁺ and Sr²⁺ uptake. The apparentK _m andV _max of ATP-dependent Sr²⁺ uptake were both higher than those for Ca²⁺ uptake. ATP-dependent Sr²⁺ uptake by BLM vesicles diminished in the presence of 0.1 μM Ca²⁺ and was more markedly inhibited by 1 μM Ca²⁺. Hill plots of Sr²⁺ uptake data with and without 0.1 μM Ca²⁺ showed that the cooperative behavior of Sr²⁺ uptake was not changed by Ca²⁺. In the presence of 0.1 μM Ca²⁺, the affinity of the transport system for Sr²⁺ and the velocity of Sr²⁺ uptake in the BLM were both decreased. However, the rate of Ca²⁺ uptake was not diminished by Sr²⁺ concentrations of <1.6 μM. These results suggest that Ca²⁺ is preferentially transported in the renal cortex BLM when Ca²⁺ and Sr²⁺ are present at the same time.     

Mechanisms of cation permeation across apical cell membrane ofNecturus gallbladder: Effects of luminal pH and divalent cations on K+ and Na+ permeability

Summary Conventional microelectrode techniques were combined with unilateral mucosal ionic substitutions to determine the effects of luminal pH and luminal alkali-earth cation concentrations on apical membrane cation permeability inNecturus gallbladder epithelium. Acidification of the mucosal solution caused reversible depolarization of both cell membranes and increase of transepithelial resistance. Low pH media also caused: (a) reduction of the apical membrane depolarization induced by high K, and (b) increase of the apical membrane hyperpolarization produced by Na replacement with Li or N-Methyl-d-glucamine. These results, in conjunction with estimates of cell membrane conductances, indicate that acidification of the luminal solution produces a reduction of apical membrane K permeability (P _K). Addition of alkali earth cations (Mg²⁺, Ca²⁺, Sr²⁺, or Ba²⁺) produced cell membrane depolarization, increase of relative resistance of the luminal membrane and reduction of the apical membrane potential change produced by a high-K mucosal medium. These results, as those produced by low pH, can be explained by a reduction of apical membraneP _K. The effects of Ba²⁺ on membrane potential and relative apical membraneP _K were larger than those of all other four cations at all concentrations tested (1–10mm). The effect of Sr²⁺ was significantly larger than those of Mg²⁺ and Ca²⁺ at 10mm, but not different at 5mm. The reduction ofP _K produced by mucosal acidification appears to be mediated by: (a) nonspecific titration of membrane fixed negative charges, and (b) an effect of luminal proton activity on the apical K channel. Divalent cations reduce apical membraneP _K probably by screening negative surface charges. The larger magnitude of the effects of Ba²⁺ and Sr²⁺ can be explained by binding to membrane sites, in the surface or in the K channel, in addition to their screening effect. We suggest that the action of luminal pH on K secretion in some segments of the renal tubule could be mediated in part by this pH-dependent K permeability of the luminal membrane.     

Kinetics of Ca2+ and Sr2+ uptake by yeast. Effects of pH,cations and phosphate

The uptake of Ca²⁺ and Sr²⁺ by the yeast Saccharomyces cerevisiae is energy dependent, and shows a deviation from simple Michaelis-Menten kinetics. A model is discussed that takes into account the effect of the surface potential and the membrane potential on uptake kinetics.The rate of Ca²⁺ and Sr²⁺ uptake is influenced by the cell pH and by the medium pH. The inhibition of uptake at low concentrations of Ca²⁺ and Sr²⁺ at low pH may be explained by a decrease of the surface potential.The inhibition of Ca²⁺ and Sr²⁺ uptake by monovalent cations is independent of the divalent cation concentration. The inhibition shows saturation kinetics, and the concentration of monovalent cation at which half-maximal inhibition is observed, is equal to the affinity constant of this ion for the monovalent cation transport system. The inhibition of divalent cation uptake by monovalent cations appears to be related to depolarization of the cell membrane.Phosphate exerts a dual effect on uptake of divalent cations: and initial inhibition and a secondary stimulation. The inhibition shows saturation kinetics, and the inhibition constant is equal to the affinity constant of phosphate for its transport mechanism. The secondary stimulation can only partly be explained by a decrease of the cell pH, suggesting interaction of intracellular phosphate, or a phosphorylated compound, with the translocation mechanism.     

The essentiality of calcium ion in the enzymatic activity of Taiwan cobra phospholipase A2

In order to address the mechanism whereby Ca²⁺ wad crucial for the manifestation of the enzymatic activity of phospholipase A₂ (PLA₂), four divalent cations were used to assess their influences on the catalytic activity and the fine structures ofNaja naja atra PLA₂. It was found that substitution of Mg²⁺ or Sr²⁺ for Ca²⁺ in the substrate solution caused a decrease in the PLA₂ activity to 77.5% or 54.5%, respectively, of that in the presence of Ca²⁺. However, no PLA₂ activity was observed with the addition of Ba²⁺. With the exception of Mg²⁺, the nonpolarity of the 8-anilinonaphthalene-1-sulfonate (ANS)-binding site of PLA₂ markedly increased with the binding of cations to PLA₂. In the meantime, the accessibilities of Lys-6 (65) and Tyr-3 (63) toward trinitrobenzene sulfonate andp-nitrobenzenesulfonyl fluoride were enhanced by the addition of Ca²⁺, Sr²⁺, and Ba²⁺, but not by Mg²⁺. The order of the ability of cations to enhance the ANS fluorescence and the reactivity of Lys and Tyr residues toward modified reagents was Ba²⁺> Sr²⁺> Ca²⁺> Mg²⁺, which was the same order as the increase in their atomic radii. These results, together with the observations that the ANS molecule binds at the active site of PLA₂ and that Tyr-3, Lys-6, and Tyr-63 of PLA₂ are involved in the binding with the substrate, suggest that the binding of Ca²⁺ to PLA₂ induces conformational changes at the active site and substrate-binding site. However, the smaller atomic radius with Mg²⁺ or the bigger atomic radii with Sr²⁺ and Ba²⁺ might render the conformation improperly rearranged after their binding to PLA₂ molecule.     

Fluxes of Ca2+, Sr2+ and Mg2+ in synaptosomes.

Synaptosomes isolated from sheep brain cortex accumulate Ca²⁺, Sr²⁺ and Mg²⁺ when incubated in isosmotic sucrose media containing 5 mM of either of these cations. The maximal levels of cations retained per mg of protein are 100 nmol of Ca²⁺, 85 nmol of Mg²⁺ and 80 nmol of Sr²⁺. The loss of Ca²⁺ or Sr²⁺ from the preloaded synaptosomes is increased by monovalent cations in the following order: Na⁺> K⁺ > Li⁺> choline, whereas for the loss of Mg²⁺ this order is different: K⁺ > Na⁺ > Li ～ choline. The efflux of Ca²⁺ or Sr²⁺ induced by monovalent cations decreases as the temperature is lowered and it is nearly abolished at 0°C, whereas the efflux of Mg²⁺ is much less influenced by temperature. The results suggest that the mechanism of exchange of Ca²⁺ for Na⁺ in synaptosomes operates similarly for Sr²⁺, but not for Mg²⁺.     

Gating and Conduction Properties of a Sodium-activated Cation Channel from Lobster Olfactory Receptor Neurons

The gating and conduction properties of a channel activated by intracellular Na⁺ were studied by recording unitary currents in inside-out patches excised from lobster olfactory receptor neurons. Channel openings to a single conductance level of 104 pS occurred in bursts. The open probability of the channel increased with increasing concentrations of Na⁺. At 210 mm Na⁺, membrane depolarization increased the open probability e-fold per 36.6 mV. The distribution of channel open times could be fit by a single exponential with a time constant of 4.09 msec at −60 mV and 90 mm Na⁺. The open time constant was not affected by the concentration of Na⁺, but was increased by membrane depolarization. At 180 mm Na⁺ and −60 mV, the distribution of channel closed times could be fit by the sum of four exponentials with time constants of 0.20, 1.46, 8.92 and 69.9 msec, respectively. The three longer time constants decreased, while the shortest time constant did not vary with the concentration of Na⁺. Membrane depolarization decreased all four closed time constants. Burst duration was unaffected by the concentration of Na⁺, but was increased by membrane depolarization. Permeability for monovalent cations relative to that of Na⁺ (P _X /P _Na ), calculated from the reversal potential, was: Li⁺ (1.11) > Na⁺ (1.0) > K⁺ (0.54) > Rb⁺ (0.36) > Cs⁺ (0.20). Extracellular divalent cations (10 mm) blocked the inward Na⁺ current at −60 mV according to the following sequence: Mn²⁺ > Ca²⁺ > Sr²⁺ > Mg²⁺ > Ba²⁺. Relative permeabilities for divalent cations (P _Y /P _Na ) were Ca²⁺ (39.0) > Mg²⁺ (34.1) > Mn²⁺ (15.5) > Ba²⁺ (13.8) > Na⁺ (1.0). Both the reversal potential and the conductance determined in divalent cation-free mixtures of Na⁺ and Cs⁺ or Li⁺ were monotonic functions of the mole fraction, suggesting that the channel is a single-ion pore that behaves as a multi-ion pore when the current is carried exclusively by divalent cations. The properties of the channel are consistent with the channel playing a role in odor activation of these primary receptor neurons. Received: 17 September 1996/Revised: 15 November 1996     

Salinomycin: a new monovalent cation ionophore.

The cation discriminations of salinomycin and its derivatives have been studied by measuring complexability with cations and transport rate of them across organic phase. Salinomycin exhibited a great preference for K⁺ over other monovalent and divalent cations in migrating cations into organic phase in two phase systems. The antibiotic mediated the transport of Na⁺ and Rb⁺ as effectively as that of K⁺ across CCl₄ bulk phase, but not those of Cs⁺, Mg²⁺, Ca²⁺, Sr²⁺. From the above results, salinomycin is concluded to act as an alkali ion carrier. The OH-acylated salinomycins retained the activity of parent compound, but the COOH-esterified salinomycins lost the activity.     

Induction of catalytic activity of plasminogen by monoclonal antibody IV-Ic in the presence of divalent metal cations and α2-antiplasmin

Investigation of the influence of divalent metal cations on the induction of plasminogen catalytic activity by monoclonal antibody IV-Ic showed that the presence of metal cations in the reaction medium changes the induction by slowing down or accelerating the process. Ions of Zn²⁺, Mn²⁺, and Cu²⁺ completely inhibit activation. Ions of Co²⁺ and Ni²⁺ decrease the rate of the first and second phases of the reaction more than 2 times. Ca²⁺ ions do not have any effect on the activation rate. Ions of Mg²⁺, Ba²⁺, and Sr²⁺ increase the rate of the first phase of the reaction by 1.5, 2.0, and 2.0 times and the rate of the second phase by 2.0, 3.8, and 4.7 times, correspondingly. Sr²⁺ ions have the strongest stimulating effect on plasminogen activation by monoclonal antibody IV-Ic. Investigation of the dose dependent effect of Sr²⁺ on the rate of plasminogen activation by monoclonal antibody IV-Ic showed stimulating effect of Sr²⁺ at concentrations from 0.1 to 1.0 mM with half maximum at 0.6 mM. However, Sr²⁺ ions do not affect amidolytic activity of plasmin and activation of plasminogen by streptokinase. Sr²⁺ ions also do not affect monoclonal antibody IV-Ic binding to plasminogen. The effect of Sr²⁺ is specific and mediated by the IV-Ic component. The presence of metal cations affects conformational changes in the process of active site formation. Metal cations also affect structure of the plasminogen molecule active site in the complex with monoclonal antibody IV-Ic and enzyme-substrate interaction. The effect of α₂-antiplasmin on the induction of plasminogen catalytic activity by monoclonal antibody IV-Ic in range of concentrations from 5 to 30 nM has been studied. α₂-Antiplasmin at concentration 30 nM almost completely inhibits induction of plasminogen catalytic activity by monoclonal antibody IV-Ic at the ratio plasminogen/α₂-antiplasmin of 3:1. This can be explained by competition of α₂-antiplasmin and monoclonal antibody IV-Ic for the lysine-binding sites of plasminogen and inhibition of the active center in activated complex plasminogen*—mAB IV-Ic. Divalent metal cations and α₂-antiplasmin are important factors in induction of plasminogen catalytic activity by monoclonal antibody IV-Ic. Published in Russian in Biokhimiya, 2006, Vol. 71, No. 6, pp. 778–785.     

Al3+-Ca2+ interactions in aluminum rhizotoxicity

Several mineral rhizotoxicities, including those induced by Al³⁺, H⁺, and Na⁺, can be relieved by elevated Ca²⁺ in the rooting medium. This leads to the hypothesis that the toxic cations displace Ca²⁺ from transport channels or surface ligands that must be occupied by Ca²⁺ in order for root elongation to occur. In this study with wheat (Triticum aestivum L.) seedlings, we have determined, in the case of Al³⁺, that (i) Ca²⁺, Mg²⁺, and Sr²⁺ are equally ameliorative, (ii) that root elongation does not increase as Ca²⁺ replaces Mg²⁺ or Sr²⁺ in the rooting media, and (iii) that rhizotoxicity is a function solely of Al³⁺ activity at the root-cell membrane surface as computed by a Gouy-Chapman-Stern model. The rhizotoxicity was indifferent to the computed membrane-surface Ca²⁺ activity. The rhizotoxicity induced by high levels of tris(ethylenediamine)cobaltic ion (TEC³⁺), in contrast to Al³⁺, was specifically relieved by Ca²⁺ at the membrane surface. The rhizotoxicity induced by H⁺ exhibited a weak specific response to Ca²⁺ at the membrane surface. We conclude that the Ca²⁺-displacement hypothesis fails in the case of Al³⁺ rhizotoxicity and that amelioration by cations (including monovalent cations) occurs because of decreased membrane-surface negativity and the consequent decrease in the membrane-surface activity of Al³⁺. However, TEC³⁺, but not Al³⁺, may be toxic because it inhibits Ca²⁺ uptake. The nature of the specific H⁺-Ca²⁺ interaction is uncertain.Abbreviations {Al³⁺ }₀ chemical activity of Al³⁺ at the root-cell membrane surface - {Al³⁺ }_E chemical activity of Al³⁺ in the external rooting medium - E₀ electrical potential at the root-cell membrane surface - HXM²⁺ hexamethonium ion - TEC³⁺ tris(ethylenediamine)cobaltic ion     

Non-covalent (iso)guanosine-based ionophores for alkali(ne earth) cations

Different (iso)guanosine-based self-assembled ionophores give distinctly different results in extraction experiments with alkali(ne earth) cations. A lipophilic guanosine derivative gives good extraction results for K⁺, Rb⁺, Ca²⁺, Sr²⁺, and Ba²⁺ and in competition experiments it clearly favors the divalent Sr²⁺ (and Ba²⁺) cations. 1,3-Alternate calix[4]arene tetraguanosine hardly shows any improvement in the extraction percentages compared to its reference compound 1,3-alternate calix[4]arene tetraamide. This indicates that one G-quartet does not provide efficient cation complexation under these conditions. In the case of the lipophilic isoguanosine derivative there is a cation size dependent affinity for the monovalent cations (Cs⁺ ? Rb⁺ ? K⁺), but not for the divalent cations (Ca²⁺ > Ba²⁺ > Sr²⁺ > Mg²⁺). In competition experiments the isoguanosine derivative, unlike guanosine, does not discriminate between monovalent and divalent cations, giving an almost equal extraction of Cs⁺ and Ba²⁺.     

Metal ion binding to anticoagulation factor II from the venom of <Emphasis Type="Italic">Agkistrodon acutus</Emphasis>: stabilization of the structure and regulation of the binding affinity to activated coagulation factor X

Anticoagulation factor II (ACF II) isolated from the venom of Agkistrodon acutus is an activated coagulation factor X (FXa)-binding protein with both anticoagulant and hypotensive activities. The thermodynamics of the binding of alkaline earth metal ions to ACF II and their effects on the stability of ACF II and the binding of ACF II to FXa were investigated by isothermal titration calorimetry, fluorescence, differential scanning calorimetry, and surface plasmon resonance. The binding of ACF II to FXa does not have an absolute requirement for Ca²⁺. Mg²⁺, Sr²⁺, and Ba²⁺ can induce the binding of ACF II to FXa. The radii of the cations bound in ACF II crucially affect the binding affinity of ACF II for cations and the structural stability of ACF II against guanidine hydrochloride and thermal denaturation, whereas the radii of cations bound in FXa markedly affect the binding affinity between ACF II and FXa. The binding affinities of ACF II for cations and the capacities of metal-induced stabilization of ACF II follow the same trend: Ca²⁺ > Sr²⁺ > Ba²⁺. The metal-induced binding affinities of ACF II for FXa follow the trend Mg²⁺ > Ca²⁺ > Sr²⁺ > Ba²⁺. Although Mg²⁺ shows significantly low binding affinity with ACF II, Mg²⁺ is the most effective to induce the binding of ACF II with FXa. Our observations suggest that in blood the bindings of Ca²⁺ in two sites of ACF II increase the structural stability of ACF II, but these bindings are not essential for the binding of ACF II with FXa, and that the binding of Mg²⁺ and Ca²⁺ to FXa may be essential for the recognition between FXa and ACF II. Like Ca²⁺, the abundant Mg²⁺ in blood also plays an important role in the anticoagulation of ACF II.     

Molecular dynamics simulation study for ion mobility of alkali earth metal cations in water at 25°C

We present the results of molecular dynamics simulations for alkali earth metal cations (Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺) in an aqueous solution at 25°C using the extended simple point charge water potential with Ewald summation. The ion mobilities (defined by u_i = D_iz_iF/RT) obtained from the simulations are in good accord with the experimental measures. The strong divalent cation–water interactions explain well the static and dynamic properties of the alkali earth metal cations. The classical solvent-berg picture describes the ion mobilities of these cations in water adequately.     

Divalent cations stabilize GroEL under conditions of oxidative stress

The divalent cations Mg²⁺, Mn²⁺, Zn²⁺, Ca²⁺, and Ni²⁺ were found to protect against proteolysis a form of GroEL (ox-GroEL) prepared by exposing GroEL for 16 h to 6 mM hydrogen peroxide (H₂O₂). K⁺ and other monovalent cations did not have any effect. Divalent cations also induced a conformational change of ox-GroEL that led to the decrease of its large exposed hydrophobic surfaces (exposed with H₂O₂). Ox-GroEL incubated with a divalent cation behaved like N-GroEL in that it could transiently interact with H₂O₂-inactivated rhodanese (ox-rhodanese), whereas ox-GroEL alone could strongly interact with ox-rhodanese. Although, ox-GroEL incubated with a divalent cation could not recover the ATPase activity (66%) lost with H₂O₂, it could facilitate the reactivation of ox-rhodanese (>86% of active rhodanese recovered), without requiring ATP or the co-chaperonin, GroES. This is the first report to demonstrate a role for the divalent cations on the structure and function of ox-GroEL.