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PubMed1988

Effect of osmolar and ionic composition of the extracellular fluid on transferrin endocytosis and exocytosis and iron uptake by reticulocytes.

The effects of osmolar and ionic factors on endocytosis and exocytosis were investigated using rabbit reticulocytes and 125I-59Fe labelled transferrin. Endocytosis and exocytosis of transferrin and the uptake of iron were inhibited by increasing the osmolality or decreasing the ionic strength or pH of the cell incubation medium. However, elevation of the pH above 8.0 inhibited endocytosis but not exocytosis. Replacement of the NaCl in the incubation medium by Nal, NaF, NaSCN, NaClO4, Na2SO4, Na phosphate, or Na Hepes inhibited endocytosis and iron uptake but only Nal, NaF, and NaSCN inhibited exocytosis. Transferrin exocytosis was insensitive to inhibitors of anion or cation transport, but endocytosis and iron uptake were inhibited by several anion transport inhibitors. Overall, transferrin endocytosis was more sensitive than exocytosis to most of the factors which were investigated, and the effects on the rates of endocytosis and iron uptake were quantitatively very similar. The results provide strong support for the concept that transferrin endocytosis is a necessary step in iron uptake by reticulocytes. They do not support the chemiosmotic models of exocytosis in their present formulations, but do not rule out the possible role of an osmotic event in exocytosis.

Ammonium Chloride↗
PubMed1986

Lipid and glycolipid metabolism of cultured astrocytes: a time course study.

Primary cultures of astrocytes free of neurons and containing less than 1% of oligodendrocytes were examined for their ability to incorporate labeled precursors into lipids and glycolipids. At selected developmental stages cultures were double-labeled with either [3H]glycerol and [14C]acetate or with [3H]galactose and Na2[35SO4] for a total of 72 hr. Lipids were extracted with CHCl3/CH3OH, fractionated on a silicic acid column, and further resolved by two-dimensional thin-layer chromatography. It was found that cultured astrocytes actively incorporate acetate and glycerol into various phospholipids; they have very limited ability to utilize galactose and virtually lack the synthetic machinery to use Na2SO4 for the synthesis of sulfated sphingogalactolipids; and their overall lipid metabolism is very distinct from that of oligodendrocytes. It was also found that cultured astrocytes have low levels of 2',3' cyclic phosphodiesterase and glycerol phosphate dehydrogenase activities; the latter is less than one fifth of that in oligodendrocytes.

2',3'-Cyclic-Nucleotide Phosphodiesterases↗
PubMed1977

Sodium chloride stimulated respiration of Anacystis nidulans.

With certain salts a stimulation of respiration of the blue-green alga Anacystis nidulans was found in the dark. The stimulation was observed only at high concentrations (10(-2)M--10(-1)M). NaCl or LiCl are the most effective salts and on addition the increase of the respiration is about 2.5fold. Li is assumed to function as a substitute for Na. Potassium salts, except KCl, are ineffective. The order for the effectiveness is: NaCl greater than NaNO3, Na2SO4 greater than KCl greater than KNO3, K2SO4 (=zero). Accordingly, the cation Na+, and to a less degree the anion Cl- are responsible for the stimulatory effect. K, which is ineffective, is passively accumulated by Anacystis according to the membrane potential. Na is actively extruded. At 0.1 M external NaCl, the passive influx of Na is high, but even then it is balanced by an active efflux. This increases the energy consumption of the cells and leads to a stimulated respiration. With DCCD (N,N'-dicyclohexylcarbodiimide) or NEM (N-ethylmaleimide), the Na efflux is inhibited, simultaneously the stimulation of respiration is abolished and the passive influx of Na becomes detectable. At 0.1 M NaCl, the passive influx of Na measured in presence of DCCD is 5 x 10(-6) moles Na/min and ml packed cells. In absence of DCCD on addition of 0.1 M NaCl the extra oxygen consumption is 2 x 10(-6) moles O2/min and ml cells. This may prove that the stimulation of respiration is mainly caused by the active Na extrusion.

Chlorides↗
PubMed1985

Quantitative analysis of the interfacial barrier to membrane transport of cholesterol solubilized in a charged micellar system.

The transport of cholesterol solubilized in polyoxyethylene(10)-nonylphenol ether with benzyldimethyltetradecylammonium chloride additive (4:1, w/w) was studied in a diffusion cell separated by a silicone rubber membrane. Overall kinetics revealed the presence of an interfacial barrier. Additions of NaCl or Na2SO4 abolished the barrier to the extent that total cholesterol flux was essentially limited by membrane diffusion considerations. The results are consistent with the concept of micelles diffusing in an electrical field followed by a collision-complex transfer of cholesterol in the aqueous-membrane interfacial region. The electrostatic force of repulsion arises from the overlap of diffuse electrical double layers emanating from the charged mixed micelle and the cationic surfactant adsorbed on the membrane. The influence of surfactant concentration on cholesterol transport kinetics was consistent with electrostatic phenomena. The derived physical model focused on interfacial electrical properties in the donor chamber by maintaining a high concentration (greater than or equal to 0.1 M) of strong electrolyte in the receiver. A linear regression of the logarithm of theoretical transport resistance, total resistance less membrane and receiver boundary layer resistances, versus (ionic strength)1/2 in the donor resulted in a reasonable estimate of the total surface potential of the micelle and membrane surfaces as well as the net dispersion attraction constant.

Chemical Phenomena↗
PubMed1998

Anion binding to the ubiquitin molecule.

Effects of different salts (NaCl, MgCl2, CaCl2, GdmCl, NaBr, NaClO4, NaH2PO4, Na2SO4) on the stability of the ubiquitin molecule at pH 2.0 have been studied by differential scanning calorimetry, circular dichroism, and Tyr fluorescence spectroscopies. It is shown that all of the salts studied significantly increase the thermostability of the ubiquitin molecule, and that this stabilization can be interpreted in terms of anion binding. Estimated thermodynamic parameters of binding for Cl- show that this binding is relatively weak (Kd = 0.15 M) and is characterized by a negative enthalpy of -15 kJ/mol per site. Particularly surprising was the observed stabilizing effect of GdmCl through the entire concentration range studied (0.01-2 M), however, to a lesser extent than stabilization by NaCl. This stabilizing effect of GdmCl appears to arise from the binding of Cl- ions. Analysis of the observed changes in the stability of the ubiquitin molecule in the presence of GdmCl can be adequately described by combining the thermodynamic model of denaturant binding with Cl- binding effects.

Animals↗
PubMed2001

Effect of impurities on the matrix-assisted laser desorption/ionization mass spectra of insulin.

The effect of impurity (sodium sulfate Na2SO4; copper sulfate CuSO4; potassium ferrocyanide K4Fe(CN)6; and triammonium citrate (NH4)3C6H5O7) concentration on the positive matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) spectra of insulin is described. 2,5-Dihydroxybenzoic acid was used as a desorption matrix. The estimated maximum concentrations of tested salts for which no insulin signal was observed were determined.

Drug Contamination↗
PubMed2002

Some aspects of matrix-assisted laser desorption/ionization analysis of hemoglobin from whole human blood.

Some aspects of the matrix-assisted laser desorption/ionization (MALDI) method in the analysis of hemoglobin from whole human blood are described. These aspects are: sensitivity of this method, fragmentation of hemoglobin ions, and influence of some impurities (sodium chloride NaCl, sodium sulfate Na2SO4, and potassium ferrocyanide K4Fe(CN)6) on the detection process. As a matrix, alpha-cyano-4-hydroxycinnamic acid (CCA) was used. The estimated maximum concentrations of tested salts for which no hemoglobin signal was observed were determined.

Gas Chromatography-Mass Spectrometry↗
PubMed1997

Laser flash photolysis experiments on the effects of freezing and salt addition on intramolecular electron transfer within one-electron reduced ascorbate oxidase.

Laser flash photolysis has been used to investigate the effects of freezing protein solutions and of adding various salts on the kinetics of one-electron photoreduction by 5-deazariboflavin semiquinone (5-DRFH.) of oxidized ascorbate oxidase (AO) from zucchini in 100 mM phosphate buffer (pH 7.0). The initial reaction between oxidized AO and 5-DRFH. is quite rapid (k approximately 10(8) M-1 s-1) and occurs at the blue Type I Cu center. Subsequent to this, a slower, protein concentration-independent intramolecular reoxidation of the Type I Cu is observed, with kET approximately 150 s-1, resulting in 40-50% reoxidation of the blue Cu center and the establishment of an electron transfer (ET) equilibrium between the various Cu centers in AO. When such a sample of AO was frozen overnight at -30 degrees C, flash photolysis of the thawed sample showed no effect on the kinetics of reduction of the Type I Cu by 5-DRFH. However, the rate constant for intramolecular ET decreased to a value of 2.7 s-1, with only 20% reoxidation of the Type I center. Reduction of the enzyme with ascorbic acid, followed by O2 oxidation, resulted in restoration of rapid intramolecular reoxidation (kET = 130 s-1), with 33% of the Type I Cu reduced by 5-DRFH. being reoxidized. These results are consistent with previous work which showed that samples of AO with initially low activity can be reactivated by ascorbic acid turnover in the presence of O2. When AO was frozen in the presence of ascorbic acid, similar inhibition of intramolecular ET was obtained, whereas upon turnover of this sample by further addition of ascorbic acid and exposure to O2, activity was not restored. The effects of addition of (NH4)2SO4, Na2SO4, NH4Cl, NaCl, KCl, and KF on the kinetics of Type I Cu reduction by 5-deazariboflavin semiquinone and on the subsequent intramolecular ET were also examined. A twofold increase in the bimolecular rate constant for reduction of the Type I Cu was observed for the two sodium salts at high concentrations (500 mM). Intramolecular ET was also significantly affected upon addition of all three chloride salts. Although the intramolecular ET rate constant was not altered, the fraction of reduced Type I Cu reoxidized by the trinuclear cluster decreased with increasing Cl- concentration, regardless of the cation. Total inhibition of intramolecular ET was observed at a significantly lower concentration of KF than observed with the Cl- salts. Sulfate ion had no effect on either parameter. These changes are thus ion specific, suggesting that they are related to ion binding by the protein, possibly at one of the coppers of the trinuclear cluster.

Ascorbate Oxidase↗
PubMed1998

Purification, properties, and multiple forms of a manganese-activated inorganic pyrophosphatase from Bacillus subtilis.

The hydrolysis of magnesium pyrophosphate by inorganic pyrophosphatase from Bacillus subtilis required its specific, time-dependent, and prior activation by Mn2+ ions. This was reversed when Mn2+ ions were removed with EDTA. Free Mn2+ ions were not required for catalysis. Pyrophosphatase purified to near homogeneity gave a single main band of apparent M(r) 36,000 by SDS-PAGE, but of M(r) 34,000 by matrix-assisted laser desorption ionization-mass spectrometry. The native enzyme equilibrated at pH 7 between three distinct molecular forms. Exposure to Mn2+ generated a catalytically active trimer of specific activity about 5000 mumol pyrophosphate hydrolyzed/min/mg protein. Exposure to EDTA generated two catalytically inactive forms, a dimer at low ionic strength and a separate form, of uncharacterized multimeric nature, at molar concentrations of Na2SO4 or Li2SO4. The latter form was an intermediate in the dimer-trimer transition caused by addition or removal of manganese ions. Mn2+ reacted with this "intermediate" form, apparently by reversible association with two noninteracting binding sites of Kd approximately 0.005 and 0.35 microM, respectively. The properties of this enzyme may account in part for the unusual manganese requirements of B. subtilis and related species.

Bacillus subtilis↗
PubMed2000

Anion-induced folding of rabbit muscle pyruvate kinase: existence of multiple intermediate conformations at low pH.

Structural and functional characteristics of rabbit muscle pyruvate kinase (PK), a tetrameric enzyme having identical subunits, were investigated under neutral as well as acidic conditions by using enzymatic activity measurements and a combination of optical methods, such as circular dichroism, fluorescence, and ANS binding. At low pH and low ionic strength, pyruvate kinase exists in a partially unfolded state (UA state) retaining half of the secondary structure and no tertiary interactions along with a strong binding to the hydrophobic dye, ANS. Addition of anions, like NaCl, KCl, and Na2SO4, to the acid-unfolded state induces refolding, resulting structural propensities similar to that of native tetramer. When anion concentration exceeds a critical limit (0.7 M KCl), a sudden loss of secondary structure and decrease in fluorescence intensity with a redshift in the emission maximum are seen which may be due to the aggregation of the protein, probably due to the intermolecular association. The anion-refolded state is more stable than the UA state, and its stability is nearly equal to that of native protein toward chemical-induced unfolding by Gu-HCl and urea. Moreover, at low concentrations, Gu-HCl behaves like an anion, by inducing refolding of the acid-unfolded state with structural features equivalent to that of native molecule. These observations support a model of protein folding where certain conformations of low free energy prevail and are populated under non-native conditions with different stability.

Anilino Naphthalenesulfonates↗
PubMed2001

Reaction of oxygen with 6-hydroxydopamine catalyzed by Cu, Fe, Mn, and V complexes: identification of a thermodynamic window for effective metal catalysis.

In the autoxidation of 6-hydroxydopamine, we investigated the reactivity of metals and metal complexes with a range of abilities to catalyse the reaction with oxygen. Comparing the catalytic effectiveness of aquo metals at pH 7.4, copper accelerated autoxidation 61-fold, iron 24-fold, manganese 7.3-fold, and vanadium 5.7-fold. Copper was thus the most effective catalyst despite being the weakest oxidant, indicating reduction of oxygen as rate limiting. EDTA, which decreases the reduction potential of Fe(III)/Fe(II), increased catalysis by iron 74% to almost that of aquo copper. Conversely, EDTA inhibited catalysis by copper, manganese, and vanadium. Desferrioxamine strongly inhibited catalysis by all of the metals. Histidine prevented catalysis by copper, accelerated catalysis by iron (43%), and had little effect on catalysis by manganese or vanadium. ADP and phytate inhibited catalysis by iron and manganese (50% or more), accelerated catalysis by vanadium (10-27%), and had no effect on catalysis by copper. The effects of the ligands largely reflected their influence on the reduction potential of the metal. Accordingly, addition of NaBr, which increases the reduction potential of Cu(II)/Cu(I), inhibited by 50%. In contrast, Na2SO4 augmented catalysis by copper 3-fold. Consistent with effects of OH- on reduction potentials and on metal coordination to 6-hydroxydopamine, an increase in pH to 8.0 decreased catalysis by copper and iron, but increased that of manganese 10-fold. In conclusion, the catalytic effectiveness of the metal-ligand complexes are largely attributable to their reduction potential, with steric accessibility playing secondary roles. The results delineate a window of catalytically effective potentials suitable for facile reduction and reoxidation by oxygen. By extension the results identify factors determining the pro- and antioxidant roles of ligands in metal mediated reduction of oxygen.

Adenosine Diphosphate↗
PubMed1997

Expression of perlecan, a proteoglycan that binds myogenic inhibitory basic fibroblast growth factor, is down regulated during skeletal muscle differentiation.

Heparan sulfate proteoglycans (HSPG) have been shown to be involved in the activation of tyrosine kinase receptors by basic fibroblasts growth factor (bFGF), a strong inhibitor of skeletal muscle differentiation. Skeletal muscle fibers contact extracellular matrix (ECM) that surrounds individual fibers (endomysium) and bundles of several fibers (perimysium). Perlecan is a HSPG present in the majority of basement membranes. In this study we evaluated the expression and localization of perlecan during differentiation of C2C12 skeletal muscle cells. C2C12 myoblasts incubated with [35S]Na2SO4 synthesize a HSPG that can be specifically immunoprecipitated with antibodies against murine perlecan. The immunoprecipitated HSPG eluted from a Sepharose CL-4B with a Kav of 0.44. Analysis of the core protein of the HSPG immunoprecipitated from [35S]methionine-labeled C2C12 after treatment with heparitinase revealed two polypeptides of 170 and over 300 kDa. The amount of polypeptides immunoprecipitated decreased with muscle differentiation. Immunocytolocalization studies indicate that perlecan is localized on the myoblast surface and by immunogold staining we have demonstrated that it is associated with patches of incipient extracellular matrix. The expression of perlecan mRNA decreased substantially during skeletal muscle differentiation, in contrast to the increase in transcripts for specific skeletal muscle proteins such as myogenin and creatine kinase. By immunofluorescence microscopy almost no perlecan staining associated with the surface of myotubes was observed. All these results suggests that perlecan, a HSPG that binds myogenic inhibitory bFGF, normally associated with basement membranes in adult tissues is present on the surface of myoblasts and its expression is down regulated during skeletal muscle differentiation.

Animals↗
PubMed1997

Spatial Distribution of Chemical Components in Aerosol Particles as Determined from Secondary Electron Yield Measurements: Implications for Mechanisms of Multicomponent Aerosol Crystallization

Secondary electron yield measurements were used to determine the distribution of chemical components in multicomponent aerosol particles formed by crystallization from aqueous solution droplets. Yield measurements were made by measuring the charge acquired by a beam of particles as they passed through an electron beam (100-600 eV energy) inside a high-vacuum apparatus. Yields were sufficiently different for certain compounds that measurements made on two-component particles could be used to obtain information on the spatial distribution of components. Variations in chemical composition as a function of depth beneath the particle surface were ascertained from the energy dependence of the measured yields, since the electron penetration depth, and therefore the probe depth, increases with electron energy. The results for mixed NaCl-NH4Cl particles indicate that the distribution of components within these particles is relatively homogeneous, while measurements made on mixed NaCl-NaNO3 particles are indicative of particles having a heterogeneous core-shell morphology. Results for mixed Na2SO4-(NH4)2SO4 particles are ambiguous, probably because of the complexity of the phase diagram of this system. It appears that the mechanism by which aerosol particles crystallize, and therefore the resulting distribution of chemical components within particles, is strongly dependent on particle composition and environmental variables. Copyright 1997 Academic Press. Copyright 1997Academic Press

Journal Article↗
PubMed1999

Volume Changes Due to SO2-4, SeO2-4, and H2PO-4 Adsorption on Amorphous Iron(III) Hydroxide in an Aqueous Suspension.

Volume changes due to SO2-4, SeO2-4, and H2PO-4 adsorption on amorphous iron(III) hydroxide were determined by dilatometry in a mixture comprising an aqueous solution of Na2SO4, Na2SeO4, or NaH2PO4 and amorphous iron(III) hydroxide suspended in 0.1 mol dm-3 NaClO4 at an initial pH ranged from 4.50 to 6.50. System volumes increased during SO2-4, SeO2-4, and H2PO-4 adsorption on amorphous iron(III) hydroxide, suggesting that some hydrated water around aqueous SO2-4, SeO2-4, or H2PO-4 ions and amorphous iron(III) hydroxide were released to the bulk. The volume changes due to SO2-4, SeO2-4, and H2PO-4 adsorption at initial pH 4.50 were +18, +18, and +14 cm3 mol-1, respectively. Phosphate, which adsorbed as an inner-sphere complex, released most of its hydration water to the bulk. The volume changes due to SO2-4 and SeO2-4 adsorption indicated that they were dehydrated at the water/amorphous iron(III) hydroxide interface. The smaller changes in volume for H2PO-4 compared to SO2-4 and SeO2-4 may be explained by differences of charges and adsorption mechanisms of these oxoanions. Copyright 1999 Academic Press.

Journal Article↗
PubMed2002

Adsorption of nonionic mixtures at the air-water interface: effects of temperature and electrolyte.

Specular neutron reflection has been used to investigate the effects of temperature and added electrolyte on the adsorption of nonionic surfactants and nonionic surfactant mixtures at the air-water interface. For the alkyl poly-oxyethylene oxide nonionic surfactants, C(n)EO(m), the adsorption at the air-water interface is independent of temperature for surfactants with shorter ethylene oxide groups, whereas there is an increasing tendency for increased adsorption with temperature for surfactants with longer ethylene oxide groups. The addition of "salting in" (sodium thiocyanate, NaSCN) and "salting out" (sodium chloride, NaCl, sodium sulphate, Na2SO4) electrolyte results in reduced and enhanced adsorption, respectively, for C12EO8, whereas both types of electrolyte result in enhanced adsorption for C12EO12. The addition of electrolyte does not substantially alter the temperature dependence of the adsorption of the pure monolayers. For the nonionic mixtures of C12EO3/C12EO8 increasing temperature results in a surface richer in the least surface-active component, C12EO8. For the same nonionic mixture, the addition of "salting in" and "salting out" electrolyte results in an reduced and increased adsorption, respectively. The addition of "salting in" electrolyte results in a surface more rich in C12EO3, whereas for the addition of both "salting in" and "salting out" electrolyte the surface composition is essentially unaltered.

Journal Article↗
PubMed2002

Surface layers of 6-thioguanine on the mercury electrode.

The adsorption behavior of 6-thioguanine (6TG) on a hanging mercury drop electrode has been studied with ac and cyclic voltammetry in 0.1 M Na2SO4 and 0.01 M sodium acetate solutions at pH 4.3. Several condensed phases of chemically adsorbed 6TG as well as one phase of physically adsorbed 6TG have been characterized. Under total coverage conditions, the films of chemiadsorbed molecules inhibit rather efficiently the electrode reaction of mercury oxide formation.

Adsorption↗
PubMed1999

The Greek key protein apo-pseudoazurin folds through an obligate on-pathway intermediate.

Folding of the 123 amino acid residue Greek key protein apo-pseudo azurin from Thiosphaera pantotropha has been examined using stopped-flow circular dichroism in 0.5 M Na2SO4 at pH 7.0 and 15 degrees C. The data show that the protein folds from the unfolded state with all eight proline residues in their native isomers (seven trans and one cis) to an intermediate within the dead-time of the stopped-flow mixing (50 ms). The urea dependence of the rates of folding and unfolding of the protein were also determined. The ratio of the folding rate to the unfolding rate (extrapolated into water) is several orders of magnitude too small to account for the equilibrium stability of the protein, consistent with the population of an intermediate. Despite this, the logarithm of the rate of folding versus denaturant concentration is linear. These data can be rationalised by the population of an intermediate under all refolding conditions. Accordingly, kinetic and equilibrium measurements were combined to fit the chevron plot to an on-pathway model (U <==> I <==> N). The fit shows that apo-pseudoazurin rapidly forms a compact species that is stabilised by 25 kJ/mol before folding to the native state at a rate of 2 s-1. Although the data can also be fitted to an off-pathway model (I <==> U <==> N), the resulting kinetic parameters indicate that the protein would have to fold to the native state at a rate of 86,000 s-1 (a time constant of only 12 microseconds). Similarly, models in which this intermediate is bypassed also lead to unreasonably fast refolding rates. Thus, the intermediate populated during the refolding of apo-pseudoazurin appears to be obligate and on the folding pathway. We suggest, based on this study and others, that some intermediates play a critical role in limiting the search to the native state.

Apoproteins↗
PubMed2001

Folding of circular permutants with decreased contact order: general trend balanced by protein stability.

To examine the influence of contact order and stability on the refolding rate constant for two-state proteins, we have analysed the folding kinetics of the small beta-alpha-beta protein S6 and two of its circular permutants with relative contact orders of 0.19, 0.15 and 0.12. Data reveal a small but significant increase of the refolding rate constant (log k(f)) with decreasing contact order. At the same time, the decreased contact order is correlated to losses in global stability and alterations of the folding nucleus. When the differences in stability are accounted for by addition of Na2SO4 or by comparison of the folding kinetics at the transition mid-point, the dependence between log k(f) and contact order becomes stronger and follows the general correlation for two-state proteins. The observation emphasizes the combined action of topology and stability in controlling the rate constant of protein folding.

Binding Sites↗