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Divalent Cations (divalent + cation)
Selected AbstractsEffect of Illite Clay and Divalent Cations on Bitumen RecoveryTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 6 2006Xinlin Ding Abstract The adverse effect of illite clay on bitumen recovery was found to be related to its acidity. The addition of calcium or magnesium ions to the flotation deionized water had a marginal effect on bitumen recovery when measured using a Denver flotation cell. However, the co-addition of illite clay and divalent cations caused a significant reduction in bitumen recovery. The effect was found to be compounded at a lower process temperature and low pH values. Zeta potential distributions of illite suspensions and bitumen emulsions were measured individually and as a mixture to investigate the effect of divalent cations on the interaction between bitumen and illite clay. The presence of 1 mM calcium or magnesium ions in deionized water had a significant effect on the interactions between bitumen and illite clay. Slime coating of illite onto bitumen was not observed in zeta potential distribution measurements performed in alkaline tailings water. On a trouvé que l'effet adverse de l'argile d'illite sur la récupération de bitume était relié à son acidité. L'ajout d'ions de calcium ou de magnésium à l'eau déionisée de flottation a un effet marginal sur la récupération de bitume lorsqu'on la mesure avec une cellule de flottation de Denver. Toutefois, l'ajout combiné d'argile d'illite et de cations divalents entraîne une réduction significative de la récupération de bitume. On a trouvé que les effets étaient combinés à une faible température de procédé et de faibles valeurs de pH. Les distributions de potentiel zéta des suspensions d'illite et des émulsions de bitume ont été mesurées individuellement et dans le mélange afin d'étudier les effets des cations divalents sur l'interaction entre le bitume et l'argile d'illite. La présence de 1 mM d'ions de calcium ou de magnésium dans l'eau déionisée a un effet significatif sur les interactions entre le bitume et l'argile d'illite. On n'a pas observé de couche de boues d'illite sur le bitume dans les mesures de distributions de potentiel zéta obtenues dans de l'eau de rejets alcaline. [source] DNA Compaction by Divalent Cations: Structural Specificity Revealed by the Potentiality of Designed Quaternary Diammonium SaltsCHEMBIOCHEM, Issue 3 2004Anatoly A. Zinchenko Abstract DNA interaction with quaternary diammonium dications, R(CH3)2N+(CH2)nN+(CH3)2R, having various intercharge distances, lengths, and branching, and the chemical nature of the hydrophobic substituents were investigated by fluorescent microscopy and circular dichroism (CD) spectroscopy to reveal their structural specificity for binding to DNA. The conformational behavior of DNA was found to be highly sensitive to the structure of the dications with separated charges. The distance between two ammonium groups greatly influences the compaction activity of the dications. To explain this situation, we proposed a model that demonstrates that the charge density of the dication and the geometric fit between DNA phosphates and the ammonium groups in the dications play an important role in providing efficient DNA collapse. Elongation of the alkyl substituents (R) in the diammonium salts from ethyl to hexyl did not generate any significant alterations in the compaction activities, whereas the branching of substituents caused a drastic decrease in their compaction ability. Based on the results of CD spectroscopy, it was found that the ability of the dications to provoke a DNA transition from the B-form to A-form was also specific: it depended on their intercharge distances and was independent of the length of alkyl substituents. [source] Purification and Characterization of an ,-L-Rhamnosidase from Aspergillus terreus of Interest in WinemakingJOURNAL OF FOOD SCIENCE, Issue 2 2001M.V. Gallego ABSTRACT: An enzyme with ,-L-rhamnosidase activity was purified to homogeneity from a culture filtrate of Aspergillus terreus after growth in a medium containing L-rhamnose as the sole carbon source. The biosynthesis of this enzyme was repressed by glucose. The enzyme had a molecular mass of 96 kDa on sodium dodecyl sulfate-polyacrylamide gel electrophoresis and an isoelectric point of 4.6 as determined by analytical isoelectric focusing. The pH and temperature optima for the enzyme were found to be 4.0 and 44 °C, respectively. Using p-nitrophenyl-,-L-rhamnopyranoside as a substrate, the enzyme exhibited Michaelis-Menten kinetics with KM and Vmax values of 0.17 mM and 84 U/mg, respectively. The enzyme was inhibited competitively by L-rhamnose (K1 2.5 mM). Divalent cations such as Ca2+ Mg2+ Zn2+ and Co2+ stimulated the a-L-rhamnosidase activity, whereas this was inhibited by Hg2+ and Cd2+. Ethanol (12% v/v) and glucose (21% w/v) decreased enzyme activity by approximately 20%, while this was not affected by SO2. [source] Binding of ciprofloxacin by humic substances: A molecular dynamics studyENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 1 2010Ludmilla Aristilde Abstract A comprehensive assessment of the potential impacts of antimicrobials released into the environment requires an understanding of their sequestration by natural particles. Of particular interest are the strong interactions of antimicrobials with natural organic matter (NOM), which are believed to reduce their bioavailability, retard their abiotic and biotic degradation, and facilitate their persistence in soils and aquatic sediments. Molecular dynamics (MD) relaxation studies of a widely used fluoroquinolone antibiotic, ciprofloxacin (Cipro), interacting with a model humic substance (HS) in a hydrated environment, were performed to elucidate the mechanisms of these interactions. Specifically, a zwitterionic Cipro molecule, the predominant species at circumneutral pH, was reacted either with protonated HS or deprotonated HS bearing Ca, Mg, or Fe(II) cations. The HS underwent conformational changes through rearrangements of its hydrophobic and hydrophilic regions and disruption of its intramolecular H-bonds to facilitate favorable intermolecular H-bonding interactions with Cipro. Complexation of the metal cations with HS carboxylates appeared to impede binding of the positively charged amino group of Cipro with these negatively charged HS complexation sites. On the other hand, an outer-sphere complex between Cipro and the HS-bound cation led to ternary Cipro,metal,HS complexes in the case of Mg,HS and Fe(II),HS, but no such bridging interaction occurred with Ca,HS. The results suggested that the ionic potential (valence/ionic radius) of the divalent cation may be a determining factor in the formation of the ternary complex, with high ionic potential favoring the bridging interaction. Environ. Toxicol. Chem. 2010;29:90,98. © 2009 SETAC [source] Molecular dynamics simulations of solvated UDP,glucose in interaction with Mg2+ cationsFEBS JOURNAL, Issue 20 2001Pavla Petrová ,Glycosyltransferases are key enzymes involved in biosynthesis of oligosaccharides. Nucleotide-sugars, the glycosyltransferase substrates, serve as activated donors of sugar residues during the enzymatic reaction Although very little is known about the catalytic mechanism of these enzymes, it appears that the catalytic activity in most glycosyltransferases is dependent upon the presence of a divalent cation, for example Mn2+ or Mg2+. It is not known whether the ion is bound to the enzyme before its interaction with the substrate, or if it binds the substrate before the enzymatic reaction to modify its conformation to fit better the active site of the enzyme. We have inspected the latter possibility by running four 2-ns molecular dynamics trajectories on fully solvated UDP-glucose in the presence of Mg2+ ions. Our results indicate that the divalent cation interacts strongly with the nucleotide-sugar in solution, and that it can alter its conformational behavior. It is also shown that a conformation of the pyrophosphate moiety that results in an eclipsed or almost eclipsed orientation of two of the oxygen atoms, and which is found in protein interacting with a nucleotide di- or tri-phosphate X-ray data, is energetically favored. The results are also discussed in light of existing NMR data, and are found to be in a good agreement with them. [source] Pharmacological "cross-inhibition" of connexin hemichannels and swelling activated anion channelsGLIA, Issue 3 2009Zu-Cheng Ye Abstract The study of ion channels has relied heavily on the use of pharmacological blocking agents. However, many of these agents have multiple effects, which may compromise interpretation of results when the affected mechanisms/pathways mediate similar functions. Volume regulated anion channels (VRAC) and connexin hemichannels can both mediate the release of glutamate and taurine, although these channels have distinct activation stimuli and hemichannels, but not VRAC, are permeable to Lucifer Yellow (LY). It has been reported that some anion channel blockers may inhibit connexin hemichannels. We further examined the effects of classic gap junction/hemichannel blockers and anion channel blockers on these channels. The typical VRAC blockers, NPPB, IAA-94, and tamoxifen blocked low divalent cation-induced glutamate and taurine release and LY loading, presumed due to hemichannel opening. The blocking action of these compounds on hemichannels was concentration dependent and fell within the same range where the drugs classically block VRACs. Conversely, carbenoxolone (CBX), the most widely used gap junction/hemichannel blocker, was an effective blocker of VRAC-mediated glutamate and taurine release, and blocked these channels at similar concentrations at which it blocked hemichannels. The CBX effect on VRACs was verified using astrocytes from connexin 43 knock out (Cx43 KO) animals. In these cells, the hypotonic induced amino acid flux was retained whereas the low divalent cation solution-induced flux was lost. These results extend our knowledge about "cross-inhibition" of VRACs and gap junctions/hemichannels by certain pharmacological agents. Given the overlap in function of these two types of channels, great care must be exerted in using pharmacological blockers to identify one channel from the other. © 2008 Wiley-Liss, Inc. [source] Conformation-dependent intermolecular interaction energies of the triphosphate anion with divalent metal cations.JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 2 2004Application to the ATP-binding site of a binuclear bacterial enzyme. Abstract We have explored the conformation-dependent interaction energy of the triphosphate moiety, a key constituent of ATP and GTP, with a closed-shell divalent cation, Zn2+, used as a probe. This was done using the SIBFA polarizable molecular mechanics procedure. We have resorted to a previously developed approach in which triphosphate is built out from its elementary constitutive fragments, and the intramolecular, interfragment, interaction energies are computed simultaneously with their intermolecular interactions with the divalent cation. This approach has enabled reproduction of the values of the intermolecular interaction energies from ab initio quantum-chemistry with relative errors <3%. It was extended to the complex of a nonhydrolyzable analog of ATP with the active site of a bacterial enzyme having two Mg2+ cations as cofactors. We obtained following energy-minimization a very close overlap of the ATP analog over its position from X-ray crystallography. For models of the ATP analog,enzyme complex encompassing up to 169 atoms, the values of the SIBFA interaction energies were found to match their DFT counterparts with relative errors of <2%. © 2003 Wiley Periodicals, Inc. J Comput Chem 25: 160,168, 2004 [source] Crystal structures and cation ordering in Cs2MgSi5O12, Rb2MgSi5O12 and Cs2ZnSi5O12 leucitesACTA CRYSTALLOGRAPHICA SECTION B, Issue 4 2009A. M. T. Bell The crystal structures of the leucite analogues Cs2MgSi5O12, Cs2ZnSi5O12 and Rb2MgSi5O12 have been determined by synchrotron X-ray powder diffraction using Rietveld refinement in conjunction with 29Si MAS NMR spectroscopy. These leucites are framework structures with distinct tetrahedral sites (T sites) occupied by Si and a divalent cation (either Mg or Zn in these samples); there is also a monovalent extra-framework cation (either Cs or Rb in these samples). The refined crystal structures were based on the Pbca leucite structure of Cs2CdSi5O12, thus a framework with five ordered Si T sites and one ordered Cd T site was used as the starting model for refinement. 29Si MAS NMR shows five distinct Si T sites for Cs2MgSi5O12 and Rb2MgSi5O12, but six Si T sites for Cs2ZnSi5O12. The refined structures for Cs2MgSi5O12 and Rb2MgSi5O12 were determined with complete T -site ordering, but the refined structure for Cs2ZnSi5O12 was determined with partial disorder of Mg and Si over two of the T sites. [source] Structure of spinach acetohydroxyacid isomeroreductase complexed with its reaction product dihydroxymethylvalerate, manganese and (phospho)-ADP-riboseACTA CRYSTALLOGRAPHICA SECTION D, Issue 4 2000Karine Thomazeau Acetohydroxyacid isomeroreductase catalyses a two-step reaction composed of an alkyl migration followed by an NADPH-dependent reduction. Both steps require a divalent cation and the first step has a strong preference for magnesium. Manganese ions are highly unfavourable to the reaction: only 3% residual activity is observed in the presence of this cation. Acetohydroxyacid isomeroreductase has been crystallized with its substrate, 2-aceto-2-hydroxybutyrate (AHB), Mn2+ and NADPH. The 1.6,Å resolution electron-density map showed the reaction product (2,3-dihydroxy-3-methylvalerate, DHMV) and a density corresponding to (phospho)-ADP-ribose instead of the whole NADP+. This is one of the few structures of an enzyme complexed with its reaction product. The structure of this complex was refined to an R factor of 19.3% and an Rfree of 22.5%. The overall structure of the enzyme is very similar to that of the complex with the reaction-intermediate analogue IpOHA [N -hydroxy- N -isopropyloxamate; Biou et al. (1997), EMBO J.16, 3405,3415]. However, the active site shows some differences: the nicotinamide is cleaved and the surrounding amino acids have rearranged accordingly. Comparison between the structures corresponding to the reaction intermediate and to the end of the reaction allowed the proposal of a reaction scheme. Taking this result into account, the enzyme was crystallized with Ni2+ and Zn2+, for which only 0.02% residual activity were measured; however, the crystals of AHB/Zn/NADPH and of AHB/Ni/NADPH also contain the reaction product. Moreover, mass-spectrometry measurements confirmed the cleavage of nicotinamide. [source] Electrophysiological classification of P2X7 receptors in rat cultured neocortical astrogliaBRITISH JOURNAL OF PHARMACOLOGY, Issue 8 2010W Nörenberg Background and purpose:, P2X7 receptors are ATP-gated cation channels mediating important functions in microglial cells, such as the release of cytokines and phagocytosis. Electrophysiological evidence that these receptors also occur in CNS astroglia is rare and rather incomplete. Experimental approach:, We used whole-cell patch-clamp recordings to search for P2X7 receptors in astroglial,neuronal co-cultures prepared from the cerebral cortex of rats. Key results:, All the astroglial cells investigated responded to ATP with membrane currents, reversing around 0 mV. These currents could be also detected in isolated outside-out patch vesicles. The results of the experiments with the P2X [,,,-methylene ATP and 2,-3,-O-(4-benzoyl) ATP] and P2Y receptor agonists [adenosine 5,-O-(2-thiodiphosphate), uridine 5,-diphosphate, uridine 5,-triphosphate (UTP) and UDP-glucose] suggested the involvement of P2X receptors in this response. The potentiation of ATP responses in a low divalent cation or alkaline bath, but not by ivermectin, made it likely that a P2X7 receptor is operational. Blockade of the ATP effect by the P2X7 antagonists Brilliant Blue G, calmidazolium and oxidized ATP corroborated this assumption. Conclusions and implications:, Rat cultured cortical astroglia possesses functional P2X7 receptors. It is suggested that astrocytic P2X7 receptors respond to high local ATP concentrations during neuronal injury. [source] Anisotropic contraction in forisomes: Simple models won't fitCYTOSKELETON, Issue 5 2008Winfried S. Peters Abstract Forisomes are ATP-independent, Ca2+ -driven contractile protein bodies acting as reversible valves in the phloem of plants of the legume family. Forisome contraction is anisotropic, as shrinkage in length is associated with radial expansion and vice versa. To test the hypothesis that changes in length and width are causally related, we monitored Ca2+ - and pH-dependent deformations in the exceptionally large forisomes of Canavalia gladiata by high-speed photography, and computed time-courses of derived geometric parameters (including volume and surface area). Soybean forisomes, which in the resting state resemble those of Canavalia geometrically but have less than 2% of the volume, were also studied to identify size effects. Calcium induced sixfold volume increases in forisomes of both species; in soybean, responses were completed in 0.15 s, compared to about 0.5 s required for a rapid response in Canavalia followed by slow swelling for several minutes. This size-dependent behavior supports the idea that forisome contractility might rest on similar mechanisms as those of polyelectrolyte gels, a class of artificial "smart" materials. In both species, time-courses of forisome length and diameter were variable and lacked correlation, arguing against a simple causal relationship between changes in length and width. Moreover, changes in the geometry of soybean forisomes differed qualitatively between Ca2+ - and pH-responses, suggesting that divalent cations and protons target different sites on the forisome proteins. Cell Motil. Cytoskeleton 2008. © 2008 Wiley-Liss, Inc. [source] Interaction study of a lysozyme-binding aptamer with mono- and divalent cations by ACEELECTROPHORESIS, Issue 3 2010Marie Girardot Abstract Binding between an aptamer and its target is highly dependent on the conformation of the aptamer molecule, this latter seeming to be affected by a variety of cations. As only a few studies have reported on the interactions of monovalent or divalent cations with aptamers, we describe herein the use of ACE in its mobility shift format for investigating interactions between various monovalent (Na+, K+, Cs+) or divalent (Mg2+, Ca2+, Ba2+) cations and a 30-mer lysozyme-binding aptamer. This study was performed in BGEs of different natures (phosphate and MOPS buffers) and ionic strengths. First, the effective charges of the aptamer in 30,mM ionic strength phosphate and MOPS (pH 7.0) were estimated to be 7.4 and 3.6, respectively. Then, corrections for ionic strength and counterion condensation effects were performed for all studies. The effective mobility shift was attributed not only to these effects, but also to a possible interaction with the buffer components (binary or ternary complexes) as well as possible conformational changes of the aptamer. Finally, apparent binding constants were calculated for divalent cations with mathematical linearization methods, and the influence of the nature of the BGE was evidenced. [source] Electrophoretic behaviors of human hepatoma HepG2 cellsELECTROPHORESIS, Issue 9 2009Jyh-Ping Hsu Abstract The electrophoretic mobility of HepG2 cells was measured and a charge-regulated model was proposed to simulate the results obtained. Here, a cell was simulated by a rigid core and an ion-penetrable membrane layer containing both acidic and basic functional groups. The influences of the key parameters, including the pH, the ionic strength, the thickness of the membrane layer of a cell, the density and the dissociation constant of the dissociable functional groups in the membrane layer, and the binding constant of divalent cations on the electrophoretic mobility of a cell were investigated. In particular, the role of the buffer used in the experiment was discussed; this effect was neglected in almost all the relevant theoretical analyses in the literature. We showed that the binding ability of divalent cations to the dissociated functional groups in the membrane layer of a cell ranks as Ca2+>Mg2+>hexamethonium. [source] Temperature and Ca2+ ion as modulators in cellular immunity of the Sunn pest Eurygaster integriceps Puton (Heteroptera: Scutelleridae)ENTOMOLOGICAL RESEARCH, Issue 6 2009Arash ZIBAEE Abstract Environmental conditions in addition to divalent cations may affect the interactions between pathogens and insects. Elucidation of factors which modulate insect immune responses could be a significant part of investigations in this area. In this study, adults of Eurygaster integriceps, as the destructive pest of wheat, were kept at different temperatures in addition to injection with different concentrations of Ca2+ to find the effect on cellular immune reactions against Beauveria bassiana. Results showed that total and differentiate hemocyte numbers, nodule formation and phenoloxidase activity increased with elevation of temperature so that the higher values were obtained at 30 and 40°C at various intervals. Higher concentrations of Ca2+ ion (5 mM) caused an increase in plasmatocyte length and width especially after 60 min. Similar results were observed for nodule formation and phenoloxidase activity of E. integriceps adults after injection by B. bassiana spores and phenoloxidase activity. It is clear from the current study that thermoregulation and Ca2+ ion can positively affect the hemocyte numbers especially plasmatocytes and granulocytes, nodule formation and phenoloxidase activity in E. integriceps. The understanding of modulators of the insect immune response may directly influence novel approaches to obtain safe and effective biological control agents. [source] Determination of the metal ion dependence and substrate specificity of a hydratase involved in the degradation pathway of biphenyl/chlorobiphenylFEBS JOURNAL, Issue 4 2005Pan Wang BphH is a divalent metal ion-dependent hydratase that catalyzes the formation of 2-keto-4-hydroxypentanoate from 2-hydroxypent-2,4-dienoate (HPDA). This reaction lies on the catabolic pathway of numerous aromatics, including the significant environmental pollutant, polychlorinated biphenyls (PCBs). BphH from the PCB degrading bacterium, Burkholderia xenoverans LB400, was overexpressed and purified to homogeneity. Atomic absorption spectroscopy and Scatchard analysis reveal that only one divalent metal ion is bound to each enzyme subunit. The enzyme exhibits the highest activity when Mg2+ was used as cofactor. Other divalent cations activate the enzyme in the following order of effectiveness: Mg2+ > Mn2+ > Co2+ > Zn2+ > Ca2+. This differs from the metal activation profile of the homologous hydratase, MhpD. UV-visible spectroscopy of the Co2+,BphH complex indicates that the divalent metal ion is hexa-coordinated in the enzyme. The nature of the metal ion affected only the kcat and not the Km values in the BphH hydration of HPDA, suggesting that cation has a catalytic rather than just a substrate binding role. BphH is able to transform alternative substrates substituted with methyl- and chlorine groups at the 5-position of HPDA. The specificity constants (kcat/Km) for 5-methyl and 5-chloro substrates are, however, lowered by eight- and 67-fold compared with the unsubstituted substrate. Significantly, kcat for the chloro-substituted substrate is eightfold lower compared with the methyl-substituted substrate, showing that electron withdrawing substituent at the 5-position of the substrate has a negative influence on enzyme catalysis. [source] Microfibril-associated glycoprotein-1 binding to tropoelastinFEBS JOURNAL, Issue 14 2004Multiple binding sites, the role of divalent cations Microfibrils and elastin are major constituents of elastic fibers, the assembly of which is dictated by multimolecular interactions. Microfibril-associated glycoprotein-1 (MAGP-1) is a microfibrillar component that interacts with the soluble elastin precursor, tropoelastin. We describe here the adaptation of a solid-phase binding assay that defines the effect of divalent cations on the interactions between MAGP-1 and tropoelastin. Using this assay, a strong calcium-dependent interaction was demonstrated, with a dissociation constant of 2.8 ± 0.3 nm, which fits a single-site binding model. Manganese and magnesium bestowed a weaker association, and copper did not facilitate the protein interactions. Three constructs spanning tropoelastin were used to quantify their relative contributions to calcium-dependent MAGP-1 binding. Binding to a construct spanning a region from the N-terminus to domain 18 followed a single-site binding model with a dissociation constant of 12.0 ± 2.2 nm, which contrasted with the complex binding behavior observed for fragments spanning domains 17,27 and domain 27 to the C-terminus. To further elucidate binding sites around the kallikrein cleavage site of domains 25/26, MAGP-1 was presented with constructs containing C-terminal deletions within the region. Construct M1659, which spans a region from the N-terminus of tropoelastin to domain 26, inclusive, bound MAGP-1 with a dissociation constant of 9.7 ± 2.0 nm, which decreased to 4.9 ± 1.0 nm following the removal of domain 26 (M155n), thus displaying only half the total capacity to bind MAGP-1. These results demonstrate that MAGP-1 is capable of cumulative binding to distinct regions on tropoelastin, with different apparent dissociation constants and different amounts of bound protein. [source] Divalent metal cation binding properties of human prothymosin ,FEBS JOURNAL, Issue 15 2000Nina V. Chichkova The divalent cation binding properties of human prothymosin ,, an abundant nuclear protein involved in cell proliferation, were evaluated. By using prothymosin , retardation on a weak cation chelating resin charged with various divalent cations, specific binding of Zn2+ ions by prothymosin , was observed. This finding was further confirmed by the equilibrium dialysis analysis which demonstrated that, within the micromolar range of Zn2+ concentrations, prothymosin , could bind up to three zinc ions in the presence of 100 mm NaCl and up to 13 zinc ions in the absence of NaCl. Equilibrium dialysis analysis also revealed that prothymosin , could bind Ca2+, although the parameters of Ca2+ binding by prothymosin , were less pronounced than those of Zn2+ binding in terms of the number of metal ions bound, the KD values, and the resistance of the bound metal ions to 100 mm NaCl. The effects of Zn2+ and Ca2+ on the interaction of prothymosin , with its putative partners, Rev of HIV type 1 and histone H1, were examined. We demonstrated that Rev binds prothymosin ,, and that prothymosin , binding to Rev but not to histone H1 was significantly enhanced in the presence of zinc and calcium ions. Our data suggest that the modes of prothymosin , interaction with Rev and histone H1 are distinct and that the observed zinc and calcium-binding properties of prothymosin , might be functionally relevant. [source] Isolation of the dxr gene of Zymomonas mobilis and characterization of the 1-deoxy- D -xylulose 5-phosphate reductoisomeraseFEMS MICROBIOLOGY LETTERS, Issue 1 2000Sigrid Grolle Abstract The gene encoding the second enzyme of the 2C -methyl- D -erythritol 4-phosphate (MEP) pathway for isopentenyl diphosphate biosynthesis, 1-deoxy- D -xylulose 5-phosphate (DXP) reductoisomerase, was cloned and sequenced from Zymomonas mobilis. The deduced amino acid sequence showed the highest identity (48.2%) to the DXP reductoisomerase of Escherichia coli. Biochemical characterization of the purified DXP reductoisomerase showed a strict dependence of the enzyme on NADPH and divalent cations (Mn2+, Co2+ or Mg2+). The enzyme is a dimer with a molecular mass of 39 kDa per subunit and has a specific activity of 19.5 U mg protein,1. Catalysis of the intramolecular rearrangement and reduction of DXP to MEP is competitively inhibited by the antibiotic fosmidomycin with a Ki of 0.6 ,M. [source] Distribution of benthic diatoms in U.S. rivers in relation to conductivity and ionic compositionFRESHWATER BIOLOGY, Issue 8 2003Marina Potapova Summary 1We quantified the relationships between diatom relative abundance and water conductivity and ionic composition, using a dataset of 3239 benthic diatom samples collected from 1109 river sites throughout the U.S.A. [U.S. Geological Survey National Water-Quality Assessment (NAWQA) Program dataset]. This dataset provided a unique opportunity to explore the autecology of freshwater diatoms over a broad range of environmental conditions. 2Conductivity ranged from 10 to 14 500 ,S cm,1, but most of the rivers had moderate conductivity (interquartile range 180,618 ,S cm,1). Calcium and bicarbonate were the dominant ions. Ionic composition, however, varied greatly because of the influence of natural and anthropogenic factors. 3Canonical correspondence analysis (CCA) and Monte Carlo permutation tests showed that conductivity and abundances of major ions (HCO + CO, Cl,, SO, Ca2+, Mg2+, Na+, K+) all explained a statistically significant amount of the variation in assemblage composition of benthic diatoms. Concentrations of HCO + CO and Ca2+ were the most significant sources of environmental variance. 4The CCA showed that the gradient of ionic composition explaining most variation in diatom assemblage structure ranged from waters dominated by Ca2+ and HCO + CO to waters with higher proportions of Na+, K+, and Cl,. The CCA also revealed that the distributions of some diatoms correlated strongly with proportions of individual cations and anions, and with the ratio of monovalent to divalent cations. 5We present species indicator values (optima) for conductivity, major ions and proportions of those ions. We also identify diatom taxa characteristic of specific major-ion chemistries. These species optima may be useful in future interpretations of diatom ecology and as indicator values in water-quality assessment. [source] Geochemical weathering at the bed of Haut Glacier d'Arolla, Switzerland,a new modelHYDROLOGICAL PROCESSES, Issue 5 2002M. Tranter Waters were sampled from 17 boreholes at Haut Glacier d'Arolla during the 1993 and 1994 ablation seasons. Three types of concentrated subglacial water were identified, based on the relative proportions of Ca2+, HCO3, and SO42, to Si. Type A waters are the most solute rich and have the lowest relative proportion of Si. They are believed to form in hydrologically inefficient areas of a distributed drainage system. Most solute is obtained from coupled sulphide oxidation and carbonate dissolution (SO,CD). It is possible that there is a subglacial source of O2, perhaps from gas bubbles released during regelation, because the high SO42, levels found (up to 1200 µeq/L) are greater than could be achieved if sulphides are oxidized by oxygen in saturated water at 0 °C (c.414 µeq/L). A more likely alternative is that sulphide is oxidized by Fe3+ in anoxic environments. If this is the case, exchange reactions involving FeIII and FeII from silicates are possible. These have the potential to generate relatively high concentrations of HCO3, with respect to SO42,. Formation of secondary weathering products, such as clays, may explain the low Si concentrations of Type A waters. Type B waters were the most frequently sampled subglacial water. They are believed to be representative of waters flowing in more efficient parts of a distributed drainage system. Residence time and reaction kinetics help determine the solute composition of these waters. The initial water,rock reactions are carbonate and silicate hydrolysis, and there is exchange of divalent cations from solution for monovalent cations held on surface exchange sites. Hydrolysis is followed by SO,CD. The SO42, concentrations usually are <414 µeq/L, although some range up to 580 µeq/L, which suggests that elements of the distributed drainage system may become anoxic. Type C waters were the most dilute, yet they were very turbid. Their chemical composition is characterized by low SO42, : HCO3, ratios and high pH. Type C waters were usually artefacts of the borehole chemical weathering environment. True Type C waters are believed to flow through sulphide-poor basal debris, particularly in the channel marginal zone. The composition of bulk runoff was most similar to diluted Type B waters at high discharge, and was similar to a mixture of Type B and C waters at lower discharge. These observations suggest that some supraglacial meltwaters input to the bed are stored temporarily in the channel marginal zone during rising discharge and are released during declining flow. Little of the subglacial chemical weathering we infer is associated with the sequestration of atmospheric CO2. The progression of reactions is from carbonate and silicate hydrolysis, through sulphide oxidation by first oxygen and then FeIII, which drives further carbonate and silicate weathering. A crude estimate of the ratio of carbonate to silicate weathering following hydrolysis is 4 : 1. We speculate that microbial oxidation of organic carbon also may occur. Both sulphide oxidation and microbial oxidation of organic carbon are likely to drive the bed towards suboxic conditions. Hence, we believe that subglacial chemical weathering does not sequester significant quantities of atmospheric CO2 and that one of the key controls on the rate and magnitude of solute acquisition is microbial activity, which catalyses the reduction of FeIII and the oxidation of FeS2. Copyright © 2002 John Wiley & Sons, Ltd. [source] Theoretical study of acetonitrile-exchange reactions on hexasolvated divalent cations in the first transition series elementsINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 10 2009Hiroaki Wasada Abstract Penta-, hexa-, and heptaacetonitrile complexes of divalent cations of the first transition series are studied by ab initio molecular orbital calculations. The factors that determine the structural stability and the reaction mechanism of solvent-exchange reactions are discussed. All the penta- and hexaacetonitrile species are at local minima, whereas the geometrical stability of the heptacoordinated species depends on the 3d electron configurations. The structural stability of heptaacetonitrile species is intermediate between those of hydrogen cyanide complexes and hydrates. Acetonitrile exchange reactions have more dissociative character than hydrogen cyanide exchange reactions because the inductive effect of the methyl group in CH3CN destabilizes the heptacoordinated structures. The successive binding energies show that associative mechanisms are favorable for acetonitrile exchange with earlier members of the first transition series, whereas dissociative mechanisms become favorable for later members. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [source] Mouse spermatozoa contain a nuclease that is activated by pretreatment with EGTA and subsequent calcium incubationJOURNAL OF CELLULAR BIOCHEMISTRY, Issue 5 2008Segal M. Boaz Abstract We demonstrated that mouse spermatozoa cleave their DNA into ,50 kb loop-sized fragments with topoisomerase IIB when treated with MnCl2 and CaCl2 in a process we term sperm chromatin fragmentation (SCF). SCF can be reversed by EDTA. A nuclease then further degrades the DNA in a process we term sperm DNA degradation (SDD). MnCl2 alone could elicit this activity, but CaCl2 had no effect. Here, we demonstrate the existence of a nuclease in the vas deferens that can be activated by ethylene glycol tetraacetic acid (EGTA) to digest the sperm DNA by SDD. Spermatozoa were extracted with salt and dithiothreitol to remove protamines and then incubated with EGTA. Next, the EGTA was removed and divalent cations were added. We found that Mn2+, Ca2+, or Zn2+ could each activate SDD in spermatozoa but Mg2+ could not. When the reaction was slowed by incubation on ice, EGTA pretreatment followed by incubation in Ca2+ elicited the reversible fragmentation of sperm DNA evident in SCF. When the reactions were then incubated at 37°C they progressed to the more complete degradation of DNA by SDD. EDTA could also be used to activate the nuclease, but required a higher concentration than EGTA. This EGTA-activatable nuclease activity was found in each fraction of the vas deferens plasma: in the spermatozoa, in the surrounding fluid, and in the insoluble components in the fluid. These results suggest that this sperm nuclease is regulated by a mechanism that is sensitive to EGTA, possibly by removing inhibition of a calcium binding protein. J. Cell. Biochem. 103: 1636,1645, 2008. © 2007 Wiley-Liss, Inc. [source] Critical amino acid residues of the ,4 subunit for ,4,7 integrin functionJOURNAL OF CELLULAR BIOCHEMISTRY, Issue 2 2001Yvonka Zeller Abstract A characteristic feature of integrin,ligand interactions is the requirement for divalent cations. Putative cation binding sites have been identified in the , and , subunit of the ,4 integrins, ,4,1 and ,4,7, and within their ligands which display the tripeptide LDV in fibronectin and homologous motifs in VCAM-1 and MAdCAM-1. The extracellular domain of the murine and human ,4-subunit contains three conserved LDV motifs, designated LDV-1 to -3. Using site directed mutagenesis and transfection studies, we now examined the functional relevance of the LDV motifs for ,4,7 integrins. We present evidence that LDV-1 mutants (D489N) behave like ,4 wt cells, but LDV-3 mutants (D811N) are impaired in ,4,7 integrin-triggered homotypic cell aggregation and in adhesion and spreading on ,4 specific ligands. Further characterization of LDV-3 mutants revealed a defect in mAb-induced ,4,7-cell surface cluster formation. Mutation of the LDV-2 motif (D698N) caused loss of ,4,7 integrin cell surface expression. Our results indicate: (i) that LDV-3, located proximal to the cell membrane, is important for ,4,7 integrin-triggered functions and for lateral clustering and (ii) that LDV-2 affects ,4,7 heterodimer stability. J. Cell. Biochem. 83: 304,319, 2001. © 2001 Wiley-Liss, Inc. [source] Adsorption of fulvic acids from aqueous solutions by carbon nanotubesJOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 8 2007Shu-Guang Wang Abstract Carbon nanotubes (CNTs) were used as adsorbent to remove fulvic acids (FA) from aqueous solutions. The adsorption capacity of CNTs for FA can reach 24 mg g,1 at 5 °C and equilibrium concentration of 18 mg dm,3. The kinetic and thermodynamic parameters, such as rate of adsorption, standard free energy changes (,G0), standard enthalpy change (,H0) and standard entropy change (,S0), have been obtained. Acidic conditions (pH = 2,5) favor FA removal. An increase in the ionic strength or the addition of divalent cations increase the adsorption of FA dramatically (FA = 60 mg dm,3). An increase in the maximum adsorbed amount of FA was observed when treating FA in synthetic seawater. Desorption studies reveal that FA can be easily and quickly removed from CNTs by altering the pH values of the solution. Good adsorption capacity and quick desorption indicate that CNTs are a promising adsorbent to remove FA from aqueous solutions. Copyright © 2007 Society of Chemical Industry [source] The Green Oat Story: Possible Mechanisms of Green Color Formation in Oat Products during CookingJOURNAL OF FOOD SCIENCE, Issue 6 2009D.C. Doehlert ABSTRACT:, Consumers occasionally report greenish colors generated in their oat products when cooking in tap water. Here we have investigated pH and ferrous (Fe2+) ion as possible mechanisms for this color change. Steel-cut oat groats can turn brown-green color when cooked in alkaline conditions (pHs 9 to 12). Extraction of this color with methanol, and high-pressure liquid chromatography indicated a direct association of this color with the phenolic acid or avenanthramide content of the oat. The presence of 50 mM NaHCO3 in water will cause oat/water mixtures to turn alkaline when cooked as CO2 is driven off, generating OH, ion. Although tap water rarely, if ever, contains so much bicarbonate, bicarbonate is used as a leavening agent in baking applications. Industrial interests using baking soda or alkaline conditions during oat processing should be aware of possible off color generation. We have also found that as little as 10 ppm Fe2+ will turn oat products gray-green when cooked. The aleurone stained darker than the starchy endosperm. Other divalent cations, such as Ca2+ or Mg2+ had no effect on cooked oat color. As much as 50 ppm Fe2+ may be found in freshly pumped well water, but Fe2+ reacts quickly with oxygen and precipitates as Fe(OH)3. Thus, some freshly pumped well water may turn oats green when cooked, but if the water is left under atmospheric conditions for several hours, no discoloration will appear in the cooked oats. [source] Evidence for two conductive pathways in P2X7 receptor: differences in modulation and selectivityJOURNAL OF NEUROCHEMISTRY, Issue 3 2010Susanna Alloisio J. Neurochem. (2010) 113, 796,806. Abstract The P2X7 receptor (P2X7R) is an ATP-gated cation channel whose biophysical properties remain to be unravelled unequivocally. Its activity is modulated by divalent cations and organic messengers such as arachidonic acid (AA). In this study, we analysed the differential modulation of magnesium (Mg2+) and AA on P2X7R by measuring whole-cell currents and intracellular Ca2+ ([Ca2+]i) and Na+ ([Na+]i) dynamics in HEK293 cells stably expressing full-length P2X7R and in cells endowed with the P2X7R variant lacking the entire C-terminus tail (trP2X7R), which is thought to control the pore activation. AA induced a robust potentiation of the P2X7R- and trP2X7R-mediated [Ca2+]i rise but did not affect the ionic currents in both conditions. Extracellular Mg2+ reduced the P2X7R- and trP2X7R-mediated [Ca2+]i rise in a dose-dependent manner through a competitive mechanism. The modulation of the magnitude of the P2X7R-mediated ionic current and [Na+]i rise were strongly dependent on Mg2+ concentration but occurred in a non-competitive manner. In contrast, in cells expressing the trP2X7R, the small ionic currents and [Na+]i signals were totally insensitive to Mg2+. Collectively, these results support the tenet of a functional structure of P2X7R possessing at least two distinct conductive pathways one for Ca2+ and another for monovalent ions, with the latter which depends on the presence of the receptor C-terminus. [source] Environmentally responsive micelles from polystyrene,poly[bis(potassium carboxylatophenoxy)phosphazene] block copolymersJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 13 2005Youngkyu Chang Abstract Amphiphilic diblock copolymers that contained hydrophilic poly[bis(potassium carboxylatophenoxy)phosphazene] segments and hydrophobic polystyrene sections were synthesized via the controlled cationic polymerization of Cl3PNSiMe3 with a polystyrenyl,phosphoranimine as a macromolecular terminator. These block copolymers self-associated in aqueous media to form micellar structures which were investigated by fluorescence spectroscopy, dynamic light scattering, and transmission electron microscopy. The size and shape of the micelles were not affected by the introduction of different monovalent cations (Li+, K+, Na+, and Cs+) into the stable micellar solutions. However, exposure to divalent cations induced intermicellar crosslinking through carboxylate groups, which caused precipitation of the ionically crosslinked aggregates from solution. This micelle-coupling behavior was reversible: the subsequent addition of monovalent cations caused the redispersion of the polystyrene- block -poly[bis(potassium carboxylatophenoxy)phosphazene] (PS,KPCPP) block copolymers into a stable micellar solution. Aqueous micellar solutions of PS,KPCPP copolymers also showed pH-dependent behavior. These attributes make PS,KPCPP block copolymers suitable for studies of guest retention and release in response to ion charge and pH. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2912,2920, 2005 [source] Effect of Divalent Cation Additives on the ,-Al2O3 -to-,-Al2O3 Phase TransitionJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 4 2000Kiyoshi Okada The effect on the ,-Al2O3 -to-,-Al2O3 phase transition of adding divalent cations was investigated by differential thermal analysis, X-ray diffractometry, and surface-area measurements. The cations, Cu2+, Mn2+, Co2+, Ni2+, Mg2+, Ca2+, Sr2+, and Ba2+, were added by impregnation, using the appropriate nitrate solution. These additives were classified into three groups, according to their effect: (1) those with an accelerating effect (Cu2+ and Mn2+), (2) those with little or no effect (Co2+, Ni2+, and Mg2+), and (3) those with a retarding effect (Ca2+, Sr2+, and Ba2+). The crystalline phase formed by reaction of the additive with ,-Al2O3 at high temperature was a spinel-type structure in groups (1) and (2) and a magnetoplumbite-type structure in group (3). In groups (2) and (3), a clear relationship was found between the transition temperature and the difference in ionic radius of Al3+ and the additive (,r): The transition temperature increased as ,r increased. This result indicates that additives with larger ionic radii are more effective in suppressing the diffusion of Al3+ and O2, in ,-Al2O3, suppressing the grain growth of ,-Al2O3, and retarding the transformation into ,-Al2O3. [source] Inhibiting 3-phosphoglycerate kinase by EDTA stimulates the development of the cleavage stage mouse embryoMOLECULAR REPRODUCTION & DEVELOPMENT, Issue 2 2001Michelle Lane Abstract Addition of EDTA to the medium significantly enhances mouse embryo development in culture. Embryos cultured in the absence of EDTA exhibit abnormal increases in glycolytic activity that result in reduced development. Culture with EDTA was able to prevent this increase in glycolysis and, therefore, maintain developmental competence. EDTA was shown to inhibit the activity of the glycolytic enzyme, 3-phosphoglycerate kinase. Additionally, the effect of EDTA on maintaining high rates of embryo development in culture could be mimicked by the addition of Cibacron blue, an inhibitor of 3-phosphoglycerate kinase. The inhibition of 3-phosphoglycerate kinase by EDTA could be overcome by the addition of exogenous magnesium, indicating that the effect of EDTA was to reduce the availability of this co-factor to the glycolytic kinases. Embryos cultured with EDTA had significantly lower levels of intracellular magnesium compared to embryos cultured without EDTA. Therefore, the effect of EDTA appears to be as a chelator of divalent cations such as magnesium, that are required for normal activity of kinases such as 3-phosphoglycerate kinase. Mol. Reprod. Dev. 60: 233,240, 2001. © 2001 Wiley-Liss, Inc. [source] Identification of hyperpolarization-activated calcium channels in apical pollen tubes of Pyrus pyrifoliaNEW PHYTOLOGIST, Issue 3 2007Hai-Yong Qu Summary ,,The pollen tube has been widely used to study the mechanisms underlying polarized tip growth in plants. A steep tip-to-base gradient of free cytosolic calcium ([Ca2+]cyt) is essential for pollen-tube growth. Local Ca2+ influx mediated by Ca2+ -permeable channels plays a key role in maintaining this [Ca2+]cyt gradient. ,,Here, we developed a protocol for successful isolation of spheroplasts from pollen tubes of Pyrus pyrifolia and identified a hyperpolarization-activated cation channel using the patch-clamp technique. ,,We showed that the cation channel conductance displayed a strong selectivity for divalent cations, with a relative permeability sequence of barium (Ba2+) , Ca2+ > magnesium (Mg2+) > strontium (Sr2+) > manganese (Mn2+). This channel conductance was selective for Ca2+ over chlorine (Cl,) (relative permeability PCa/PCl = 14 in 10 mm extracellular Ca2+). We also showed that the channel was inhibited by the Ca2+ channel blockers lanthanum (La3+) and gadolinium (Gd3+). Furthermore, channel activity depended on extracellular pH and pollen viability. ,,We propose that the Ca2+ -permeable channel is likely to play a role in mediating Ca2+ influx into the growing pollen tubes to maintain the [Ca2+]cyt gradient. [source] | ||||||||||||||||||||||||||||||||||