Initiation of Aerobic Oxidation of Sulfite by Illuminated Spinach Chloroplasts

Spinach chloroplasts induced aerobic oxidation of sulfite on illumination. The oxidation of sulfite was not found in KCl‐ and Tris‐treated chloroplasts and was inhibited by 3(3,4‐dichloro‐phenyl)‐1,1′‐dimethylurea, suggesting that the reaction is induced through the electron transport system in chloroplasts. The oxidation rate of sulfite was much higher than the rate of the Hill reaction. In addition, inhibition by several sugars indicates that the oxidation of sulfite proceeds as a chain reaction. Although catalase did not affect the reaction, ferredoxin plus NADP, cytochrome c and superoxide dismutase prevented the oxidation of sulfite. We thus propose that a superoxide anion formed by the univalent reduction of oxygen by illuminated chloroplasts is the initiator of sulfite oxidation.