The enhancement of CO2 fixation in isolated chloroplasts by low sulfite concentrations and by ascorbate

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.