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  • 1
    ISSN: 1520-510X
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    The journal of membrane biology 125 (1992), S. 119-132 
    ISSN: 1432-1424
    Keywords: Xenopus oocyte ; Na+/K+ ATPase ; Torpedo electroplax ; expression ; current-voltage relationship ; protein kinase
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary Modulation of the current generated by the Na+/K+ pump by membrane potential and protein kinases was investigated in oocytes of Xenopus laevis. In addition to a positive slope region in the current-voltage (I–V) relationship of the Na+/K+ pump, a negative slope region has been described in these cells (Lafaire & Schwarz, 1986) and has been attributed to a voltage-dependent apparent K m value for pump stimulation by external [K+] (Rakowski et al., 1991). To study this feature in more detail. Xenopus oocytes were used for comparative analysis of the negative slope of the I–V relationship of the endogenous Na+/K+ pump and of the Na+/K+ pump of the electric organ of Torpedo californica expressed in the oocytes. The effects of stimulation of protein kinases A and C on the negative slope were also analyzed. To investigate the negative slope over a wide potential range, experiments were performed in Na+-free solution and in the presence of high concentrations of Ba2+ and tetraethylammonium, to block all nonpump related K+-sensitive currents. Pump currents and pump-mediated fluxes were determined as differences of currents or fluxes in solutions with and without extracellular K+. The voltage dependence of the K m value for stimulation of the Na+/K+ pump by external [K+] shows significant species differences. Over the entire voltage range from -140 to +20 mV, the K m value for the Na+/K+ pump of Torpedo electroplax is substantially higher than for the endogenous pump and exhibits more pronounced voltage dependence. For the Xenopus pump, the voltage dependence can be described by voltage-dependent stimulation by external [K+] and can be interpreted by voltage-dependent K+ binding, assuming that an effective charge between 0.37 and 0.56 of an elementary charge is moved in the electrical field. An analogous evaluation of the voltage dependence of the Torpedo pump requires the assumption of movement of two effective charges of 0.16 and 1.0 of an elementary charge. Application of 1,2-dioctanoyl-sn-glycerol (diC8. 10–50 μm). which is known to stimulate protein kinase C, reduces the maximum activity of the Xenopus pumps in the oocyte membrane by 40% and modulates the voltage dependence of K+ stimulation. For the endogenous Xenopus pump, the apparent effective charge increased from 0.37 to 0.51 of elementary charge and the apparent K m at 0 mV increased from 0.46 to 0.83 mm. For the Torpedo pump, one of the apparent effective charges increased from 1.0 to 2.5 of elementary charge. Injection of cAMP (final concentration 50 μm), which stimulates protein kinase A, has an effect opposite to stimulation of protein kinase C. The activity of the Xenopus Na+/K+ pump is elevated by 80%, and the voltage dependence of K+ stimulation becomes less pronounced. For the endogenous pump the apparent effective charge decreased from 0.56 to 0.38 of elementary charge and the apparent K m at 0 mV decreased from 0.78 to 0.65 mm. Also for the Torpedo pump, the effective charges and apparent affinities became reduced. The data suggest that species differences in voltage-dependent stimulation of the Na+/K+ pump by external K+ can account for differences in the steepness of the negative slope in the I–V relationships observed in different preparations. In addition, they suggest that the voltage dependence and the maximum activity of the Na+/K+ pump can be modulated by activation of protein kinases.
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Biological cybernetics 69 (1993), S. 173-182 
    ISSN: 1432-0770
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Computer Science , Physics
    Notes: Abstract Exact predictions for two-pulse visual temporal integration data are derived from the Bouman-van der Velden quantum coincidence model for threshold vision. The predictions of the model start with complete summation for superposed pulses, then pass to a transition zone of partial integration, and finally reach the level of probability summation for pulses presented with large interstimulus intervals. From our results we can clearly reject the assumption of constant integration times with the basic model. We thus generalize the coincidence model to allow for variable integration times, derive the corresponding predictions for two-pulse integration data, and compare these predictions to published data currently available. It is shown that detectors of low order of coincidence generally underestimate the actual reduction of threshold intensity (or equivalently the corresponding increase of the detection probability) for two pulses as compared to the singlepulse performance.
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  • 4
    ISSN: 1432-1017
    Keywords: Na-Pump current-voltage relationship ; Na dependence ; Access channel ; N-Terminus truncation ; (Xenopus oocyte)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Physics
    Notes: Abstract Currents generated by the Na+/K+ ATPase were measured under voltage clamp in oocytes of Xenopus laevis. The dependence of pump current on external [Na+] was investigated for the endogenous Xenopus pump as well as for wild-type and mutated pumps of electroplax of Torpedo californica expressed in the oocytes. The mutants had α-subunits truncated before position Lys28 (αΔK28) or Thr29 (αΔT29) of the N-terminus. The currents generated by all variants of pump molecules in the presence of 5 mM K+ show voltage-dependent inhibition by external [Na+]. The apparent K1 values increase with membrane depolarisation, and the potential dependence can be described by the movement of effective charges in the electrical potential gradient across the membrane. Taking into account Na+-K+ competition for external binding to the E2P form, apparent K1 values and effective charges for the interaction of the Na+ ions with the E2P form can be estimated. For the Xenopus pump the effective charge amounts to 1.1 of an elementary charge and the K1 value at 0 mV to 44 mM. For the wild-type Torpedo pump, the analysis yields values of 0.73 of an elementary charge and 133 mM, respectively. Truncation at the N-terminus removing a lysinerich cluster of the a-subunit of the Torpedo pump leads to an increase of the effective charge and decrease of the K1 value. For αΔK28, values of 0.83 of an elementary charge and 117 mM are obtained, respectively. If LyS28 is included in the truncation (α·T29), the effective charge increases to 1.5 of an elementary charge and the apparent K1 value is reduced to 107 mM. The K, values for pump inhibition by external Na+, calculated by taking into account Na+-K+ competition, are smaller than the K/12 values determined in the presence of 5 mM [K+]. The difference is more pronounced for those pump variants that have higher Km, values. The variations of the parameters describing inhibition by external [Na+] are qualitatively similar to those described for the stimulation of the pumps by external [K+] in the absence of extracellular [Na+]. The observations may be explained by an acess channel within the membrane dielectric that has to be passed by the external Na+ and K+ ions to reach or leave their binding sites. The potential-dependent access and/or the interaction with the binding sites shows species differences and is affected by cytoplasmic lysine residues in the N-terminus.
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  • 5
    Electronic Resource
    Electronic Resource
    Springer
    Archiv der Mathematik 60 (1993), S. 154-156 
    ISSN: 1420-8938
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Type of Medium: Electronic Resource
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  • 6
    ISSN: 1617-4623
    Keywords: DNA sequence ; Cellulase ; Endoglucanase ; Clostridium stercorarium ; Avocado
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary The nucleotide sequence of the celZ gene coding for a thermostable endo-β-1,4-glucanase (Avicelase I) of Clostridium stercorarium was determined. The structural gene consists of an open reading frame of 2958 by which encodes a preprotein of 986 amino acids with an Mr of 109000. The signal peptide cleavage site was identified by comparison with the N-terminal amino acid sequence of Avicelase I purified from C. stercorarium culture supernatants. The recombinant protein expressed in Escherichia coli is proteolytically cleaved into catalytic and cellulose-binding fragments of about 50 kDa each. Sequence comparison revealed that the N-terminal half of Avicelase I is closely related to avocado (Persea americana) cellulase. Homology is also observed with Clostridium thermocellum endoglucanase D and Pseudomonas fuorescens cellulase. The cellulose-binding region was located in the C-terminal half of Avicelase I. It consists of a reiterated domain of 88 amino acids flanked by a repeated sequence about 140 amino acids in length. The C-terminal flanking sequence is highly homologous to the non-catalytic domain of Bacillus subtilis endoglucanase and Caldocellum saccharolyticum endoglucanase B. It is proposed that the enhanced cellulolytic activity of Avicelase I is due to the presence of multiple cellulose-binding sites.
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  • 7
    ISSN: 0044-8249
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 1 Ill.
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  • 8
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Zeitschrift für die chemische Industrie 106 (1994), S. 721-723 
    ISSN: 0044-8249
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 2 Ill.
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  • 9
    ISSN: 0044-2313
    Keywords: Benzamidinates ; Strontium-bis[N,N′-bis(trimethylsilyl)benzamidinate] ; Barium-bis[N,N′-bis(trimethylsilyl)benzamidinate] ; X-ray Structures ; Spectroscopic Data ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Strontium and Barium Bis[N,N′-bis(trimethylsilyl)benzamidinates] from the Addition Reaction of the Alkaline Earth Metal Bis[bis(trimethylsilyl)amides] and BenzonitrileThe reaction of strontium bis[bis trimethylsilyl)amide] with benzonitrile yields strontium bis[N,N′- bis(trimethylsilyl)benzamidinate] · 2THF, which crystallizes in the orthorhombic space group Pbcn (a = 1845.4(3); b = 131 1,3(2); c = 1838,(3) pm; Z = 4). During the similar reaction of barium bis[bis(trimethylsilyl)amide] with benzonitrile the benzonitrile adduct barium bis[N,N′-bis(trimethylsilyl)benzamidinate] · 2 THF · benzonitrile is formed. After the addition of diphenylacetylene to the strontium di(benzamidinate) in diglyme a clathrate of the composition strontium bis[N,N′-bis(trimethylsilyl)benzamidinate] · diglyme · diphenylacetylene could be isolated; the spectroscopic data as well as the X-ray structure (monoclinic, C2/c, a = 1492.2(2); b = 1539.1(2); c = 2337.8(3)pm; Z = 4) confirm the isolated appearance of the acetylene molecule without interaction to the metal center in solution and in the solid state, respectively.
    Notes: Bei der Umsetzung des Strontiumbis[bis(trimethylsilyl)amids] mit Benzonitril in THF bildet sich Stronitium-bis[N,N′-bis(trimethylsilyl)benzamidinat] · 2THF, das in der orthorhombischen Raumgruppe Pbcn mit {a = 1 845,4(3); b = 1311,4(2); c = 1838,8(3) pm, Z = 4} kristallisiert. Die entsprechende Reaktion des Barium-bis[bis(trimethylsilyl)amids] mit Benzonitril ergibt das Benzonitril-Addukt Barium bis[N,N′ -bis(trimethylsilyl)benzamidinat] · 2 THF · Benzonitril. Nach der versuchsweisen Umsetzung des Strontium-di(benzamidinats) mit Diphenylacetylen in Diglyme läßt sich ein Clathrat der Zusammensetzung Strontiumbis[N,N′-bis(trimethylsilyl)benzamidinat] · Diglyme · Diphenylacetylen erhalten, dessen spektroskopische und strukturanalytische Untersuchungen (monoklin, C2/c, a = 1492,2(2); b = 1539,1(2); c = 2337,8(3) pm; β = 100,74(1)°; Z = 4) das Vorliegen eines unkomplexierten Diphenylacetylen-Moleküls sowohl in Lösung als auch im Festkörper bestätigen.
    Additional Material: 4 Ill.
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  • 10
    ISSN: 0044-2313
    Keywords: Tris(trimethylsilyl)silyltriflate ; Tris(trimethylsilyl)silylamine ; Lithium Amide ; X-Ray Structure ; Spectroscopic Data ; Agostic Interactions ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Tris(trimethylsilyl)silylamine and the lithiated and silylated Derivatives  -  X-Ray Structure of the dimeric Lithium Trimethylsilyl-[tris(trimethylsilyl)silyl]amideThe ammonolysis of the chlor, brom or trifluormethanesulfonyl tris(trimethylsilyl)silane yields the colorless tris(trimethylsilyl)silylamine, destillable at 51°C and 0.02 Torr. The subsequent lithiation, reaction with chlor trimethylsilane and repeated lithiation lead to the formation of lithium tris(trimethylsilyl)silylamide, trimethylsilyl-[tris(trimethylsilyl)silyl]amine and finally lithium trimethylsilyl-[tris(trimethylsilyl)silyl]amide, which crystallizes in the monoclinic space group P21/n with a = 1 386.7(2); b = 2 040.2(3); c = 1 609.6(2) pm; β = 96.95(1)° and Z = 4 dimeric molecules. The cyclic Li2N2 moiety with Li—N bond distances displays a short transannular Li … Li contact of 229 pm. The dimeric molecule shows nearly C2-symmetry, so that one lithium atom forms agostic bonds to both the trimethylsilyl groups, the other one to the tris(trimethylsilyl)silyl substituents. However, the 7Li{1H}-NMR spectrum displays a high field shifted singlet at  - 1.71 ppm. The lithiation of trimethylsilyl-[tris(trimethylsilyl)silyl]amine leads to a high field shift of the 29Si{1H} resonance of about 12 ppm for the Me3SiN group, whereas the parameters of the tris(trimethylsilyl)silyl ligand remain nearly unaffected.
    Notes: Die Ammonolyse von Chlor-, Brom- oder Trifluormethansulfonyl-tris(trimethylsilyl)silan ergibt das bei 51°C/0,02 Torr zu destillierende, farblose Tris(trimethylsilyl)silylamin. Durch Lithiierung erhält man das in Benzol dimer vorliegende Lithium-tris(trimethylsilyl)silylamid; die anschließende Umsetzung mit Chlortrimethylsilan und abermalige Lithiierung führen zu dem in Benzol ebenfalls dimer auftretenden Lithiumtrimethylsilyl-[tris(trimethylsilyl)silyl]amid, das in der monoklinen Raumgruppe P21/n mit a = 1 386,7(2); b = 2 040,2(3); c = 1 609,6(2) pm; β = 96,95(1)° und Z = 4 Dimeren kristallisiert. Das zentrale Strukturelement bildet der Li2N2-Cyclus mit Li—N-Abständen um 202 pm und einem kurzen Li … Li-Kontakt von 229 pm. Das dimere Molekül weist nahezu C2-Symmetrie auf, so daß das eine Lithiumatom agostische Bindungen zu den zwei Trimethylsilyl-Gruppen, das andere hingegen zu den Tris(trimethylsilyl)silyl-Substituenten aufweist, allerdings läßt sich in benzolischer Lösung 7Li{1H}-NMR-spektroskopisch nur ein Hochfeld-verschobenes Singulett bei  -  1,71 ppm beobachten. Im 29Si{1H}-NMR-Spektrum läßt sich durch die Lithiierung des Trimethylsilyl-[tris(trimethylsilyl)silyl]amins eine Hochfeldverschiebung um 12 ppm für den Me3SiN-Rest registrieren, während die NMR-Parameter des Tris(trimethylsilyl)silyl-Liganden eine nur geringe Verschiebung erfahren.
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