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  • sarcoplasmic reticulum  (3)
  • Ubiquinol-10  (1)
  • 1
    Electronic Resource
    Electronic Resource
    Springer
    European biophysics journal 22 (1993), S. 151-155 
    ISSN: 1432-1017
    Keywords: α-Tocopherol ; Ubiquinol-10 ; Ubiquinone-10 ; Magic angle spinning NMR
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Physics
    Notes: Abstract α-Tocopherol, ubiquinone-10 and ubiquinol-10 have been studied by high resolution magic angle samples spinning 13C-nuclear magnetic resonance in egg yolk phosphatidylcholine multilamellar vesicles model membranes in order to assess their location and the induced perturbations on this model system. α-Tocopherol is placed in such a position that it is in close contact with the head group of the phospholipid and exposed to the solvent. In this position it significantly perturbs the phospholipid head group, the 5a-CH3 and the 7a-CH3 groups being the closest parts to the membrane surface. On the other hand, ubiquinol-10 perturbs the membrane surface more than ubiquinone-10, but neither compound significantly changed the phospholipid head group conformation.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    ISSN: 1573-6881
    Keywords: H+ flux ; K+ flux ; Ca2+-pump ; sarcoplasmic reticulum ; rabbit muscle
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology , Physics
    Notes: Abstract The release of H+ during the oxalate-supported Ca2+ uptake in sarcoplasmic reticulum vesicles is kinetically coincident with the initial phase of Ca2+ accumulation. The Ca2+ uptake is increased and the H+ release is decreased in the presence of KCl and other monovalent chloride salts as expected for a H+-monovalent cation exchange. The functioning of the Ca2+-pump is disturbed by the presence of potassium gluconate and, to a lesser extent, of choline chloride. These salts do not inhibit the ATPase activity of Ca2+-permeable vesicles, suggesting a charge imbalance inhibition which is specially relevant in the case of gluconate. Therefore, K+, and also Cl−, appear to be involved in secondary fluxes during the active accumulation of Ca2+. The microsomal preparation seems homogeneous with respect to the K+-channel, showing an apparent rate constant for K+ release of approximately 25 s−1 measured with the aid of86Rb+ tracer under equilibrium conditions. A Rb+ efflux, sensitive to Ca2+-ionophore, can be also detected during the active accumulation of Ca2+. The experimental data suggest that both monovalent cations and anions are involved in a charge compensation during the Ca2+ uptake and H+ release. Fluxes of these highly permeable ions would contribute to cancel the formation of a resting membrane potential through the sarcoplasmic reticulum membrane.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    ISSN: 1573-6881
    Keywords: (Ca2+ + Mg2+)-ATPase ; sarcoplasmic reticulum ; membrane fluidity ; enzyme kinetics
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology , Physics
    Notes: Abstract (Ca2+ + Mg2+)-ATPase from sarcoplasmic reticulum has been reconstituted with dipalmitoylphosphatidylcholine, and the activating effect of ATP and Ca2+ on this enzyme has been studied at different temperatures. It has been found that two kinetic forms of the enzyme are interconverted at about 31°C, and this is possibly related to a phase change in the phospholipid which is more directly associated with the protein. Above 31°C the enzyme is less dependent on ATP activation at high ATP concentrations but shows positive cooperativity for Ca2+ activation. On the other hand, below 31°C, the reconstituted enzyme is more dependent on ATP for activation at high ATP concentrations than the purified ATPase and does not show cooperativity for Ca2+ activation.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    ISSN: 1573-6881
    Keywords: (Ca2+-Mg2+)-ATPase ; sarcoplasmic reticulum ; enzyme kinetics
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology , Physics
    Notes: Abstract The (Ca2+-Mg2+)-ATPase from sarcoplasmic reticulum presents negative cooperativity for the hydrolysis of Mg2+-ATP at different concentration ranges of this substrate. A kinetic model is proposed according to which Mg2+-ATP may bind to three different enzymatic species present during the catalytic cycle, E (K 1=1 µM), E′∼P.Ca2 (K 9=500 µM) and *EP (K 7=20 µM), accelerating the release of Pi. The fact that each of these species has a different affinity for Mg2+-ATP allows a significant enhancement of the rate of Pi release to the medium at the different ranges of Mg2+-ATP concentration where the enzyme shows a kinetic cooperativity. The kinetic analysis of this mechanism yields an equation which is a ratio of two cubic polynomials (3:3 rate equations) with respect to Mg2+-ATP and which may explain the negative cooperativity of the enzyme at different concentration ranges of Mg2+-ATP.
    Type of Medium: Electronic Resource
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