Mechanisms of strontium uptake by laboratory and brewing strains of Saccharomyces cerevisiae.

Laboratory and brewing strains of Saccharomyces cerevisiae were compared for metabolism-independent and -dependent Sr2+ uptake. Cell surface adsorption of Sr2+ to live cells was greater in the brewing than in the laboratory strain examined. However, uptake levels were greater in denatured (dried and ground) S. cerevisiae, and the relative affinities of Sr2+ for the two strains were reversed. Results for the brewing S. cerevisiae strain were similar whether the organism was obtained fresh from brewery waste or after culturing under the same conditions as for the laboratory strain. Reciprocal Langmuir plots of uptake data for live biomass were not linear, whereas those for denatured biomass were. The more complex Sr2+ binding mechanism inferred for live S. cerevisiae was underlined by cation displacement experiments. Sr2+ adsorption to live cells resulted in release of Mg2+, Ca2+, and H+, suggesting a combination of ionic and covalent bonding of Sr2+. In contrast, Mg2+ was the predominant exchangeable cation on denatured biomass, indicating primarily electrostatic attraction of Sr2+. Incubation of live S. cerevisiae in the presence of glucose resulted in a stimulation of Sr2+ uptake. Cell fractionation revealed that this increased Sr2+ uptake was mostly due to sequestration of Sr2+ in the vacuole, although a small increase in cytoplasmic Sr2+ was also evident. No stimulation or inhibition of active H+ efflux resulted from metabolism-dependent Sr2+ accumulation. However, a decline in cytoplasmic, and particularly vacuolar, Mg2+, in comparison with that of cells incubated with Sr2+ in the absence of glucose, was apparent. This was most marked for the laboratory S. cerevisiae strain, which contained higher Mg2+ levels than the brewing strain.     

Uptake and accumulation of Mn2+ and Sr2+ in Saccharomyces cerevisiae

Initial uptake of Mn2+ and Sr2+ in the yeast Saccharomyces cerevisiae was studied in order to investigate the selectivity of the divalent cation uptake system and the possible involvement of the plasma-membrane ATPase in this uptake. The initial uptake rates of the two ions were not significantly different. This ruled out a direct role of the plasma-membrane ATPase, since this ATPase is specific for Mn2+ compared to Sr2+. After 1 h uptake, Mn2+ had accumulated 10-times more than Sr2+. Influx of Mn2+ and Sr2+ remained unchanged during that time, however. The differences in accumulation level found for Mn2+ and Sr2+ could be ascribed to a greater efflux of Sr2+ as compared with Mn2+. Probably this greater efflux of Sr2+ was only apparent, since differential extraction of the yeast cells revealed that Mn2+ is more compartmentalised than Sr2+, giving rise to a lower relative cytoplasmic Mn2+ concentration.     

Kinetics of ion translocation across charged membranes mediated by a two-site transport mechanism. Effects of polyvalent cations upon rubidium uptake into yeast cells.

(1) The effect of surface charge upon the kinetics of monovalent cation translocation via a two-site mechanism is investigated theroretically. (2) According to the model dealt with, typical relations are expected for the dependence of the kinetic parameters of the translocation process upon the concentration of a polyvalent cation, differing essentially from those derived for the case in which the membrane carries no excess charge. (3) Even when a polyvalent cation does not compete with the substrate cation for binding to the translocation sites, apparently competitive inhibition may occur when the membrane is negatively charged. (4) The model is tested experimentally by studying the effects of the polyvalent cations Mg2+, Sr2+, Ca2+, Ba2+ and Al3+ upon Rb+ uptake into yeast cells at pH 4.5 A good applicability is found. (5) Equimolar concentrations of polyvalent cations reduce the rate of the Rb+ uptake into yeast cells in the order Mg2+ less than Sr2+ less than Ca2+ less than Ba2+ less than Al3+. (6) The conclusion is reached that the reduction in the rate of Rb+ uptake caused by the polyvalent cations applied results mainly from screening of the negative fixed charges on the membrane surface and binding to these negative sites rather than competition with Rb+ for the transport sites. (7) The results of our investigation indicate the affinity of the alkaline-earth cations for the negative fixed charges on the surface to the yeast cell membrane increases in the orther Mg2+ less than Sr2 less than Ca2+ less than Ba2+. (8) Probably mainly phosphoryl groups determine the net charge on the membrane of the yeast cell at a medium pH of 4.5.     

The effect of Mg2+ on hepatic microsomal Ca2+ and Sr2+ transport

The ATP-dependent uptake of Ca2+ by rat liver microsomal fraction is dependent upon Mg2+. Studies of the Mg2+ requirement of the underlying microsomal Ca2+-ATPase have been hampered by the presence of a large basal Mg2+-ATPase activity. We have examined the effect of various Mg2+ concentrations on Mg2+-ATPase activity, Ca2+ uptake, Ca2+-ATPase activity and microsomal phosphoprotein formation. Both Mg2+-ATPase activity and Ca2+ uptake were markedly stimulated by increasing Mg2+ concentration. However, the Ca2+-ATPase activity, measured concomitantly with Ca2+ uptake, was apparently unaffected by changes in the Mg2+ concentration. In order to examine the apparent paradox of Mg2+ stimulation of Ca2+ uptake but not of Ca2+-ATPase activity, we examined the formation of the Ca2+-ATPase phosphoenzyme intermediate and formation of a Mg2+-dependent phosphoprotein, which we have proposed to be an attribute of the Mg2+-ATPase activity. We found that Ca2+ apparently inhibited formation of the Mg2+-dependent phosphoprotein both in the absence and presence of exogenous Mg2+. This suggests that Ca2+ may inhibit (at least partially) the Mg2+-ATPase activity. However, inclusion of the Ca2+ inhibition of Mg2+-ATPase activity in the calculation of Ca2+-ATPase activity reveals that this effect is insufficient to totally account for the stimulation of Ca2+ uptake by Mg2+. This suggests that Mg2+, in addition to stimulation of Ca2+-ATPase activity, may have a direct stimulatory effect on Ca2+ uptake in an as yet undefined fashion. In an effort to further examine the effect of Mg2+ on the microsomal Ca2+ transport system of rat liver, the interaction of this system with Sr2+ was examined. Sr2+ was sequestered into an A23187-releasable space in an ATP-dependent manner by rat liver microsomal fraction. The uptake of Sr2+ was similar to that of Ca2+ in terms of both rate and extent. A Sr2+-dependent ATPase activity was associated with the Sr2+ uptake. Sr2+ promoted formation of a phosphoprotein which was hydroxylamine-labile and base-labile. This phosphoprotein was indistinguishable from the Ca2+-dependent ATPase phosphoenzyme intermediate. Sr2+ uptake was markedly stimulated by exogenous Mg2+, but the Sr2+-dependent ATPase activity was unaffected by increasing Mg2+ concentrations. Sr2+ uptake and Sr2+-dependent ATPase activity were concomitantly inhibited by sodium vanadate. In contrast to Ca2+, Sr2+ had no effect on Mg2+-dependent phosphoprotein formation. Taken together, these data indicate that Mg2+ stimulated Ca2+ and Sr2+ transport by increasing the Ca2+ (Sr2+)/ATP ratio.(ABSTRACT TRUNCATED AT 400 WORDS)     

Chromaffin cell calcium channel kinetics measured isotopically through fast calcium, strontium, and barium fluxes

Fast Ca2+ uptake into K+-depolarized cultured bovine adrenal chromaffin cells has been isotopically measured in a time scale of 1-10 s. Depolarized cells retained as much as 80-fold 45Ca2+ taken up by resting cells; Ca2+ was not taken up by fibroblasts or endothelial-like cells. Because Ca2+ entry was inhibited by inorganic (La3+, Co2+, Mg2+) and organic (nifedipine) Ca2+ channel antagonists and enhanced by the Ca2+ channel activator Bay-K-8644, it seems clear that Ca2+ gains access to the chromaffin cell cytosol mainly through specific voltage-dependent Ca2+ channels. Ca2+ uptake evoked by 59 mM K+ was linear during the first 5 s of stimulation and continued to rise at a much slower rate up to 60 s. The rate of Ca2+ entry became steeper as the external [Ca2+] increased; initial rates of Ca2+ uptake varied from 0.06 fmol/cells . s at 0.125 mM Ca2+ to 2.85 fmol/cell . s at 7.5 mM Ca2+. The early 90Sr2+ uptake was linear but faster than Ca2+ uptake and later on was also saturated; 133Ba2+ was taken up still at a much faster rate and was linear for the entire depolarization period (2-60 s). Increased [K+] gradually depolarized chromaffin cells; Ca2+ and Sr2+ uptakes were not apparent below 30 mM K+ but were linear for 30 to 60 mM K+. In contrast, substantial Ba2+ uptake was seen even in K+-free solutions; and in 5.9 mM K+, Ba2+ uptake was as high as Ca2+ uptake obtained in 60 mM K+. Five to ten-second pulses of 45Ca2+, 90Sr2+, or 133Ba2+ given at different times after pre-depolarization of chromaffin cells served to analyze the kinetics of inactivation of the rates of entry of each divalent cation. Inactivation of Ca2+ uptake was faster than Sr2+, and Ba2+ uptake inactivated very little. Neither voltage changes nor Ca2+ ions passing through the channels seems to cause their inactivation; however, experiments aimed to manipulate the levels of internal Ca2+ using the cell-permeable chelator Quin-2 or the ionophore A23187 strongly suggest that intracellular Ca2+ levels determine the rates of inactivation of these channels.     

Mechanism of adsorption of hard and soft metal ions to Saccharomyces cerevisiae and influence of hard and soft anions.

The applicability of the hard-and-soft principle of acids and bases in predicting metal adsorption characteristics in a biological context was investigated for metabolism-independent uptake of the metal ions Sr2+, Mn2+, Zn2+, Cu2+, Cd2+, and Tl+ by Saccharomyces cerevisiae. Metal adsorption increased with external metal concentration (5 to 50 microM), although some saturation of uptake of the harder ions examined, Sr2+, Mn2+, and Zn2+, was evident at the higher metal concentrations. Cation displacement experiments indicated that, with the exception of Tl+, relative covalent bonding (H+ displacement) of the metals was greater at low metal concentrations, while weaker electrostatic interactions (Mg2+ plus Ca2+ displacement) became increasingly important at higher concentrations. These results were correlated with curved Scatchard and reciprocal Langmuir plots of metal uptake data. Saturation of covalent binding sites was most marked for the hard metals, and consequently, although no relationship between metal hardness and ionic/covalent bonding ratios was evident at 10 microM metal, at 50 microM the ratio was generally higher for harder metals. Increasing inhibition of metal uptake at increasing external anion concentrations was partially attributed to the formation of metal-anion complexes. Inhibitory effects of the hard anion SO42(-) were most marked for uptake of the hard metals Sr2+ and Mn2+, whereas greater relative effects on adsorption of the softer cations Cu2+ and Cd2+ were correlated with complexation by the soft anion S2O32(-). Inhibition of uptake of the borderline metal Zn2+ by SO42(-) and that by S2O32(-) were approximately equal.(ABSTRACT TRUNCATED AT 250 WORDS)     

Two Ca2+ functions are demonstrated by the substitution of specific divalent and lanthanide cations for the Ca2+ required by the aggregation factor complex from the marine sponge, Microciona prolifera

Multivalent cations were tested for their ability to replace the Ca2+ requirements of aggregation factor (AF) complex in activity, stability, and integrity assays. The ability of each cation to replace the Ca2+ required for the cell aggregation-enhancing activity of AF was examined by replacing the usual 10 mM Ca2+ with the test cation at various concentrations in the serial dilution assay of the AF. The other alkaline earth cations, Mg2+, Sr2+, and Ba2+, could not replace Ca2+; two transition elements, Mn2+ and Cd2+, partially replaced calcium. All 15 of the available lanthanides (including La3+ and Y3+) produced normal activity but only at 10-400-fold lower cation concentrations than Ca2+. An AF preparation is stable and remains active for months in 1 mM Ca2+ but decays rapidly when Ca2+ is lowered. Sr2+ and Ba2+ at 20 mM but not at 1 mM could replace 1 mM Ca2+ and give long term stability. AF was not stable in the presence of Mg2+, even at 100 mM. High Mn2+ concentrations did not stabilize AF even though AF was partially active in Mn2+. Cd2+ gave full stability at 75 mM and La3+ at about 0.1 mM. When Ca2+ is chelated, the macromolecular subunits of the AF slowly dissociate. Permeation chromatography and analytical ultracentrifugation showed that the cations that stabilized activity maintained the integrity of AF complex while those that failed to stabilize activity allowed the complex to dissociate into subunits, indicating that these two Ca2+ requirements are related. The cation specificities for activity and for stability-integrity are different indicating that these are separate Ca2+-dependent functions.     

A cation channel for K+ and Ca2+ from Tetrahymena cilia in planar lipid bilayers

The single-channel properties for monovalent and divalent cations of a voltage-independent cation channel from Tetrahymena cilia were studied in planar lipid bilayers. The single-channel conductance reached a maximum value as the K+ concentration was increased in symmetrical solutions of K+. The concentration dependence of the conductance was approximated to a simple saturation curve (a single-ion channel model) with an apparent Michaelis constant of 16.3 mM and a maximum conductance of 354 pS. Divalent cations (Ca2+, Ba2+, Sr2+, and Mg2+) also permeated this channel. The sequence of permeability determined by zero current potentials at high ionic concentrations was Ba2+ greater than or equal to K+ greater than or equal to Sr2+ greater than Mg2+ greater than Ca2+. Single-channel conductances for Ca2+ were nearly constant (13.9 pS-20.5 pS) in the concentrations between 0.5 mM and 50 mM Ca-gluconate. In the experiments with mixed solutions of K+ and Ca2+, a maximum conductance of Ca2+ (gamma Camax) and an apparent Michaelis constant of Ca2+ (K Cam) were obtained by assuming a simple competitive relation between the cations. Gamma Camax and K Cam were 14.0 pS and 0.160 mM, respectively. Single-channel conductances in mixed solutions were well-fitted to this competitive model supporting that this cation channel behaves as a single-ion channel. This channel had relatively high-affinity Ca2+-binding sites.     

The effect of ions on the adhesion and internalization of Chlamydia trachomatis by HeLa cells

The adhesion and internalization of Chlamydia trachomatis by HeLa cells was unaffected by removal of K+, Mg2+, or glucose from the incubation medium, slightly reduced by removal of Na+, and significantly reduced by omission of Ca2+, Sr2+, Mg2+, and Mn2+ could replace Ca2+ in the adhesion but only Sr2+ supported internalization, and La3+, Co2+, Fe3+, Ba2+, and Zn2+ all reduced internalization more than adhesion. During initial infection there was no measurable difference in the uptake or release of 45Ca2+ or 86Rb+ between infected and noninfected HeLa monolayers. Infection was not prevented by pretreatment of the monolayers with the calcium channel blockers, verapamil, D600, and nitrendipine, or the calmodulin inhibitors, TMB-8 or trifluperazine. The results suggest that divalent cations are not essential for chlamydial infection but that the process of internalization is facilitated by the presence of cations, particularly Na+ and Ca2+.     

Stringent requirement for Ca2+ in the removal of Z-lines and alpha-actinin from isolated myofibrils by Ca2+-activated neutral proteinase

Treatment of isolated myofibrils with Ca2+-activated neutral proteinase (CANP) results in specific removal of Z-line and of alpha-actinin. To investigate the ionic requirement for these processes, we measured Z-line removal by phase-contrast and interference microscopy and alpha-actinin removal by sodium dodecyl sulphate/polyacrylamide-gel electrophoretic analysis of myofibrillar proteins. The proteolytic digestion of native purified proteins was measured directly on polyacrylamide gels and by the fluorescamine technique. We found that the removal of Z-line and alpha-actinin as well as the release of proteolytic degradation products from isolated myofibrils by CANP occur only in the presence of Ca2+; Sr2+, Ba2+, Mn2+, Mg2+, Co2+ and Zn2+ are all ineffective. In contrast with this stringent requirement for Ca2+, the proteolytic activity of CANP measured with denatured casein, native and denatured haemoglobin, native actin and tropomyosin also occurs in the presence of other bivalent cations, in the following order: Ca2+ greater than Sr2+ greater than Ba2+. These data suggest that only Ca2+ can produce the conformational change in myofibrils that renders them susceptible to the action of CANP, whereas its proteolytic activity is stimulated by several bivalent ions.     

Divalent cation transport systems of Rhodopseudomonas capsulata.

Separate divalent cation transport systems for energy-dependent uptake of Mg2+ and Mn2+ were found both with aerobically and heterotrophically grown and with photosynthetically grown cells of Rhodopseudomonas capsulata. The maximum rate of Mg2+ uptake differed between photosynthetic and aerobic cells, while the Km for the Mg2+ transport system was constant. Photosynthetic midlog-phase cells exhibited Km's for uptake of about 55 micrometer Mg2+ and 0.5 micrometer Mn2+. The Vmax's also differed between the two systems: 0.6 to 1.8 mumol/min per g (dry weight) of cells for Mg2+, but only 0.020 mumol/min per g for Mn2+, making the distinction between a "macro-requirement" system and a system functioning at trace nutrient levels. Calcium was not normally taken up by intact cells of R. capsulata. However, chromatophore membranes isolated from photosynthetic cells took up Ca2+ by an energy-dependent process.     

Activation of three types of membrane currents by various divalent cations in identified molluscan pacemaker neurons

We investigated membrane currents activated by intracellular divalent cations in two types of molluscan pacemaker neurons. A fast and quantitative pressure injection technique was used to apply Ca2+ and other divalent cations. Ca2+ was most effective in activating a nonspecific cation current and two types of K+ currents found in these cells. One type of outward current was quickly activated following injections with increasing effectiveness for divalent cations of ionic radii that were closer to the radius of Ca2+ (Ca2+ greater than Cd2+ greater than Hg2+ greater than Mn2+ greater than Zn2+ greater than Co2+ greater than Ni2+ greater than Pb2+ greater than Sr2+ greater than Mg2+ greater than Ba2+). The other type of outward current was activated with a delay by Ca2+ greater than Sr2+ greater than Hg2+ greater than Pb2+. Mg2+, Ba2+, Zn2+, Cd2+, Mn2+, Co2+, and Ni2+ were ineffective in concentrations up to 5 mM. Comparison with properties of Ca2(+)-sensitive proteins related to the binding of divalent cations suggests that a Ca2(+)-binding protein of the calmodulin/troponin C type is involved in Ca2(+)-dependent activation of the fast-activated type of K+ current. Th sequence obtained for the slowly activated type is compatible with the effectiveness of different divalent cations in activating protein kinase C. The nonspecific cation current was activated by Ca2+ greater than Hg2+ greater than Ba2+ greater than Pb2+ greater than Sr2+, a sequence unlike sequences for known Ca2(+)-binding proteins.     

Activation by divalent cations of a Ca2+-activated K+ channel from skeletal muscle membrane

Several divalent cations were studied as agonists of a Ca2+-activated K+ channel obtained from rat muscle membranes and incorporated into planar lipid bilayers. The effect of these agonists on single-channel currents was tested in the absence and in the presence of Ca2+. Among the divalent cations that activate the channel, Ca2+ is the most effective, followed by Cd2+, Sr2+, Mn2+, Fe2+, and Co2+. Mg2+, Ni2+, Ba2+, Cu2+, Zn2+, Hg2+, and Sn2+ are ineffective. The voltage dependence of channel activation is the same for all the divalent cations. The time-averaged probability of the open state is a sigmoidal function of the divalent cation concentration. The sigmoidal curves are described by a dissociation constant K and a Hill coefficient N. The values of these parameters, measured at 80 mV are: N = 2.1, K = 4 X 10(-7) mMN for Ca2+; N = 3.0, K = 0.02 mMN for Cd2+; N = 1.45, K = 0.63 mMN for Sr2+; N = 1.7, K = 0.94 mMN for Mn2+; N = 1.1, K = 3.0 mMN for Fe2+; and N = 1.1 K = 4.35 mMN for Co2+. In the presence of Ca2+, the divalent cations Cd2+, Co2+, Mn2+, Ni2+, and Mg2+ are able to increase the apparent affinity of the channel for Ca2+ and they increase the Hill coefficient in a concentration-dependent fashion. These divalent cations are only effective when added to the cytoplasmic side of the channel. We suggest that these divalent cations can bind to the channel, unmasking new Ca2+ sites.     

Interaction of calcium with bovine plasma protein C

The binding of 45Ca2+ to bovine plasma protein C (PC) and to activated bovine plasma protein C (APC) has been examined by equilibrium ultrafiltration at pH 7.4 and 25 degrees C. Under these conditions, PC possesses 16.0 plus or minus 2.0 equivalent Ca2+ binding sites, of average KD (8.7 plus or minus 1.5) x 10(-4) M, and APC contains 9.0 plus or minus 1.0 equivalent Ca2+ binding sites, with an average KD of (4.3 plus or minus 1.1) x 10(-4) M. Both Mn2+ and Sr2+ were capable of ready displacement of Ca2+ from a Ca2+-PC complex, while Mg2+ was less effective in this regard. The alpha-thrombin-catalyzed activation of PC was inhibited by the presence of Ca2+. A kinetic analysis of this effect demonstrated that it was, in large part, due to an increase in the Km of the reaction. Addition of other divalent cations, e.g. Mn2+, Sr2+, and Mg2+, in place of Ca2+ also resulted in inhibition of the alpha-thrombin-catalyzed activation of PC in a manner which paralleled their ability to displace Ca2+ from a Ca2+-PC complex. On the other hand, the activation of PC by the coagulant protein from Russell's Viper venom was augmented by the presence of Ca2+. Other divalent metal ions, such as Sr2+ and Mn2+, in the absence of Ca2+, also weakly stimulated this reaction. Mg2+ was without notable effect.     

Divalent cation sensitivity of BK channel activation supports the existence of three distinct binding sites

Mutational analyses have suggested that BK channels are regulated by three distinct divalent cation-dependent regulatory mechanisms arising from the cytosolic COOH terminus of the pore-forming alpha subunit. Two mechanisms account for physiological regulation of BK channels by microM Ca2+. The third may mediate physiological regulation by mM Mg2+. Mutation of five aspartate residues (5D5N) within the so-called Ca2+ bowl removes a portion of a higher affinity Ca2+ dependence, while mutation of D362A/D367A in the first RCK domain also removes some higher affinity Ca2+ dependence. Together, 5D5N and D362A/D367A remove all effects of Ca2+ up through 1 mM while E399A removes a portion of low affinity regulation by Ca2+/Mg2+. If each proposed regulatory effect involves a distinct divalent cation binding site, the divalent cation selectivity of the actual site that defines each mechanism might differ. By examination of the ability of various divalent cations to activate currents in constructs with mutationally altered regulatory mechanisms, here we show that each putative regulatory mechanism exhibits a unique sensitivity to divalent cations. Regulation mediated by the Ca2+ bowl can be activated by Ca2+ and Sr2+, while regulation defined by D362/D367 can be activated by Ca2+, Sr2+, and Cd2+. Mn2+, Co2+, and Ni2+ produce little observable effect through the high affinity regulatory mechanisms, while all six divalent cations enhance activation through the low affinity mechanism defined by residue E399. Furthermore, each type of mutation affects kinetic properties of BK channels in distinct ways. The Ca2+ bowl mainly accelerates activation of BK channels at low [Ca2+], while the D362/D367-related high affinity site influences both activation and deactivation over the range of 10-300 microM Ca2+. The major kinetic effect of the E399-related low affinity mechanism is to slow deactivation at mM Mg2+ or Ca2+. The results support the view that three distinct divalent-cation binding sites mediate regulation of BK channels.     

Formation of a hexagonal lattice structure by an R-form lipopolysaccharide of Klebsiella: effect of various divalent cations on the lattice formation

The R-form lipopolysaccharide from Klebsiella pneumoniae strain LEN-111 (O3-:K1-), from which cationic material had been removed by electrodialysis, was previously shown to form a hexagonal lattice structure with the lattice constant of 14 to 15 nm when suspended in 50 mM tris(hydroxymethyl)aminomethane buffer at pH 8.5 containing 10 mM Mg2+. Under this experimental condition, effects of other divalent metal cations on the hexagonal assembly of the electrodialyzed LPS were compared with that of Mg2+. The Zn2+, Hg2+, Cu2+, and Ni2+ could produce essentially the same hexagonal lattice structure with the lattice constant of 14.5 to 15.0 nm as that formed with Mg2+. The Cd2+, Co2+, and Fe2+ produced the hexagonal lattice structure with the lattice constant of 15.5 to 16.0 nm, and Ba2+, Sr2+, and Ca2+ produced that with the lattice constant of 18 to 19 nm. In addition, the hexagonal lattice structures formed with the latter three cations were less orderly than those formed with the other cations. When the higher concentrations of Ba2+, Sr2+, and Ca2+ were used, the lattice constants were not shortened. The length of lattice constants of the hexagonal lattice structures formed with the divalent cations did not relate to the quantity of the cations bound to the LPS. Among the divalent cations tested, Hg2+ was bound to the LPS in the smallest amount (its atomic ratio to P, 0.07), and Zn2+ and Fe2+ were bound in very large amounts (their atomic ratios to P, 2.94 and 8.28, respectively).     

Enhancement by Ca2+ or Mg2+ of catalytic activity of the superoxide-producing NADPH oxidase in membrane fractions of human neutrophils and monocytes

To examine the role of divalent cations in the generation of superoxide anion (O2-) by the NADPH oxidase system of phagocytic cells, membrane-rich fractions were prepared from human neutrophils and monocytes. O2- generation by the fractions in sucrose was enhanced by addition of Ca2+ or Mg2+. EDTA inhibited most of the O2- generation; Ca2+ or Mg2+ reversed the inhibition. Zn2+, Mn2+, or Cu2+ completely inhibited O2- production. Neutrophil membrane fraction solubilized with Triton X-100, then passed through a chelating column, lost 80% of its oxidase activity; the loss could be reversed by addition of Ca2+ or Mg2+. Addition of 0.3 mM Ca2+ or Mg2+ protected against thermal instability of the enzyme. Kinetic analysis of the neutrophil oxidase activity as a function of NADPH and Ca2+ or Mg2+ concentrations showed that cation did not interact with NADPH in solution or affect the binding of NADPH to the oxidase; rather, cation bound directly to the oxidase, or to some associated regulatory component, to activate the enzyme. For the neutrophil oxidase, the Km for NADPH was 51 +/- 6 (S.D.) microM. Hyperbolic saturation was observed with Ca2+ and Mg2+, and the Kd values were 1.9 +/- 0.3 and 2.9 +/- 0.3 microM, respectively, suggesting that the oxidase, or some associated component, has a relatively high-affinity binding site for Ca2+ and Mg2+.     

Divalent cation conduction in the ryanodine receptor channel of sheep cardiac muscle sarcoplasmic reticulum

The conduction properties of the alkaline earth divalent cations were determined in the purified sheep cardiac sarcoplasmic reticulum ryanodine receptor channel after reconstitution into planar phospholipid bilayers. Under bi-ionic conditions there was little difference in permeability among Ba2+, Ca2+, Sr2+, and Mg2+. However, there was a significant difference between the divalent cations and K+, with the divalent cations between 5.8- and 6.7-fold more permeant. Single-channel conductances were determined under symmetrical ionic conditions with 210 mM Ba2+ and Sr2+ and from the single-channel current-voltage relationship under bi-ionic conditions with 210 mM divalent cations and 210 mM K+. Single-channel conductance ranged from 202 pS for Ba2+ to 89 pS for Mg2+ and fell in the sequence Ba2+ greater than Sr2+ greater than Ca2+ greater than Mg2+. Near-maximal single-channel conductance is observed at concentrations as low as 2 mM Ba2+. Single-channel conductance and current measurements in mixtures of Ba(2+)-Mg2+ and Ba(2+)-Ca2+ reveal no anomalous behavior as the mole fraction of the ions is varied. The Ca(2+)-K+ reversal potential determined under bi-ionic conditions was independent of the absolute value of the ion concentrations. The data are compatible with the ryanodine receptor channel acting as a high conductance channel displaying moderate discrimination between divalent and monovalent cations. The channel behaves as though ion translocation occurs in single file with at most one ion able to occupy the conduction pathway at a time.     

Regulation of intracellular magnesium by Mg2+ efflux

Chicken erythrocytes were loaded with Mg2+ by incubation with the cation ionophore A 23187 in the presence of Mg2+. After removing A 23187 by intensive washing with serum albumin and reincubating the Mg2+-loaded cells, Mg2+ was transported out of the cells until the original Mg2+ content was achieved. The net Mg2+ efflux followed Michaelis-Menten-kinetics and was independent of extracellular and intracellular Ca2+ and calmodulin. The net Mg2+ efflux was not affected by adrenalin, isoproterenol, p-chloromercuribenzenesulfonate, ouabain and tetrodotoxin, but was inhibited by dicyclohexylcarbodiimide, KCN, iodoacetate, high extracellular concentrations of Mg2+, Mn2+ and when extracellular Na+ was substituted by choline or K+. The efflux of 1 Mg2+ was coupled with the uptake of 2 Na+. It is concluded that there exists an additional gating process at the inner cell surface becoming active only at increased concentrations of intracellular free Mg2+ regulating the exit of Mg2+ by the efflux system.     

Investigating the interaction mechanism between zinc and Saccharomyces cerevisiae using combined SEM-EDX and XAFS

The interaction mechanism between zinc and the intact yeast cells of Saccharomyces cerevisiae was investigated by using the scanning electron microscopy with energy-dispersive X-ray analysis, as well as X-ray absorption fine structure spectroscopy (XAFS). Displacement of H+, K+, Mg2+, and Na+ during zinc uptake confirmed the existence of both covalent interactions and ionic interactions between Zn2+ and the microbe. Ion exchange mechanism played a role in zinc uptake. The local environment of Zn accumulated in the intact yeast cells was determined by XAFS, which suggests that the nearest neighboring atom of the bound zinc ion on the biomass is oxygen atom. The adsorbed zinc ion on the intact cells of S. cerevisiae is a tetrahedron structure, with the Zn-O bond length of 1.97 A, and the coordination number is only 3.2 of Zn-O structure in the first shell.