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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Pharmaceutical research 12 (1995), S. 1614-1617 
    ISSN: 1573-904X
    Keywords: pKa ; stratum corneum ; fatty acid ; ionization
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Purpose. The apparent pKa of the fatty acids within hydrated (30 % w/w) model human stratum corneum (SC) lipid mixtures should be measured. Methods. The degree of ionisation of the fatty acids was calculated as a function of pH using Fourier transform infra-red spectroscopy. The relative intensity of the stretching bands of the unionized and ionized carboxylic groups was determined and fitted to the relevant expression for ionic equilibrium of a monoprotic acid. The pKa was then calculated for increasing proportion of unsaturated fatty acid in the lipid mixture. Results. Values for pKa in the range 6.2-7.3 were found, increasing with greater proportion of oleic acid. These are some 1.5-3 pH units higher than the pKas of fatty acids in molecular solution. Conclusions. As there exists a pH-gradient across the SC, the degree of ionisation will also vary. In the innermost SC layers, a pH of 7 will produce 90% ionization of the fatty acids and head-group repulsion will be great. At the SC surface, the pH of 5 will cause almost minimal head-group repulsion, tending to increase crystallinity and promote a bilayer structure.
    Type of Medium: Electronic Resource
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  • 2
    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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  • 3
    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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  • 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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