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'magnesium' in keywords Facet   Publication Year 1979  [X]
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1979[X]
1Author    B. Demmig, H. GimmlerRequires cookie*
 Title    Effect of Divalent Cations on Cation Fluxes Across the Chloroplast Envelope and on Photosynthesis of Intact Chloroplasts  
 Abstract    The effect of divalent cations on cation fluxes across the chloroplast envelope and on photo­ synthetic reactions of intact spinach chloroplasts was investigated. In the absence of EDTA, divalent cations inhibited photosynthetic C 0 2-fixation and PGA-reduction at low PGA concentrations, but had almost no effect on the reduction of OAA, BQ, and on PGA-reduction at high PGA concentrations. The inhibitory effect of Ca2+ was greater than that of M g2+. Inhibition of photosynthesis was greater when the divalent cations were added in the dark than when added in the light. In spite of its inhibitory effect, Mg2+ partially restored the Ca2+ inhibited photosynthesis, indicating the involvement of a Mg2+ /Ca2+ antagonism in the inhibitory effect. The inhibitory effect of divalent cations is stronger in a medium with low concentrations of K + than in the presence of 20 — 50 m M KC1. Mg2+ induced a release of plastidal K + and an in­ crease of stromal H + concentration. The results indicate that external Mg2+ in the absence of EDTA does not influence neither photosynthetic electron transport nor photophosphorylation, but inhibits the light activation of some enzymes of the carbon reduction cycle. The latter is assumed to be due to an acidification of the stroma pH and the decrease of endogenous K + level. Since the chloroplast envelope has only a very low permeability towards Mg2+, possible mechanisms are discussed by which M g2+ changes the properties of the chloroplast envelope and thus secondarily induces the observed effects. 
  Reference    Z. Naturforsch. 34c, 233—241 (1979); received December 28 1978 
  Published    1979 
  Keywords    Intact Chloroplasts, Chloroplast Envelope, Assimilation of C 0 2, Magnesium, Light Activation 
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 TEI-XML for    default:Reihe_C/34/ZNC-1979-34c-0233.pdf 
 Identifier    ZNC-1979-34c-0233 
 Volume    34 
2Author    HorstW. AlterRequires cookie*
 Title    Permeability of Plasma Membrane Vesicles to Ouabain and Mg2+ as a Factor Determining Rate of Binding of Ouabain to Na+ and K+ Dependent ATPase  
 Abstract    N a+, K+-ATPase of the plasma membrane isolated from sheep kidney medulla exhibits functio­ nal asymmetry for the cardiac glycoside ouabain. In this vesicular membrane preparation the rate of binding o f ouabain was slow (time constant > 60 min) when the vesicles were incubated in the presence o f isotonic sucrose. Upon treatment o f the preparation with hypoosmotic shock or phos-pholipase A the initial rate of ouabain binding was enhanced at least 3 fold. In equilibrium a concentration of the ouabain-enzyme-complex was obtained which was about twofold that of the untreated vesicles. This result suggests two types o f ouabain binding sites with an approximate stoichiometry of 1 to 1. The stoichiometry seems to be maintained at high concentrations of ouabain where binding curves show a biphasic time course. Additional information about hetero­ geneity of binding sites comes through experiments in which the vesicles were treated with Mg2+ prior to the addition o f ouabain. A minor fraction of the binding sites were occupied by ouabain only after longtime incubation with Mg2+. 
  Reference    Z. Naturforsch. 34c, 1224 (1979); received July 13/August 18 1979 
  Published    1979 
  Keywords    N a+, K+-ATPase, Plasma Membrane Vesicles, Cardiac Glycoside, Magnesium, Tightness 
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 TEI-XML for    default:Reihe_C/34/ZNC-1979-34c-1224.pdf 
 Identifier    ZNC-1979-34c-1224 
 Volume    34