title: The enhancement of CO2 fixation in isolated chloroplasts by low sulfite concentrations and by ascorbate.
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.
mesh_terms: D001205:Ascorbic Acid / Q000494:pharmacology*; D002245:Carbon Dioxide / Q000378:metabolism*; D002734:Chlorophyll / Q000378:metabolism; D002736:Chloroplasts / Q000187:drug effects / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D010944:Plants / Q000378:metabolism*; D013431:Sulfates / Q000378:metabolism; D013447:Sulfites / Q000494:pharmacology*; D013482:Superoxide Dismutase / Q000494:pharmacology
chemical_list: D013431:Sulfates; D013447:Sulfites; D002734:Chlorophyll; D002245:Carbon Dioxide; D013482:Superoxide Dismutase; D001205:Ascorbic Acid