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  • Title: The enhancement of CO2 fixation in isolated chloroplasts by low sulfite concentrations and by ascorbate.
    Author: Ziegler I, Libera W.
    Journal: Z Naturforsch C Biosci; 1975; 30(5):634-7. PubMed ID: 129994.
    Abstract:
    The enhancement of CO2 fixation in isolated, intact spinach chloroplasts by ascorbate or by low sulfite concentrations (less than 1 mM) is strongly reduced or even abolished by the addition of superoxide dismutase (SOD). By the use of 35SO3(2-) it is demonstrated that the rate of sulfate formation is much lower than the sulfite induced increase in CO2 fixation. This indicates that the superoxide radical is the chain initiating event; and, in parallel to ascorbate (Elstner and Kramer, 1973), the HSO3 radical, acting as a Hill reagent for photosystem I, is reduced to sulfite in turn. The ingibitory action of sulfite at concentrations greater than 1 mM is not relieved by SOD, since this effect is mainly based on a competitive inhibition of ribulosediphosphate carboxylase. SOD itself stimulates the CO2 fixation, if the reaction is started after 3 min of pre-illumination. This effect is discussed with respect to factors linked with the isolation procedure.
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