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Environmental and Stress Physiology

Oxidation and Reduction of Sulfite by Chloroplasts and Formation of Sulfite Addition Compounds ¹

Julius-von-Sachs-Institut für Biowissenschaften der Universität Würzburg, Mittlerer Dallenbergweg 64, D-8700 Würzburg, Federal Republic of Germany

After exposing intact chloroplasts isolated from spinach (Spinacia oleracea L. cv Yates) and capable of photoreducing CO₂ at high rates to different concentrations of radioactive sulfite in the light or in the dark, ³⁵SO₂ and H₂³⁵S were removed from the acidified suspensions in a stream of nitrogen. Remaining activity could be fractionated into sulfate, organic sulfides, and sulfite addition compounds. When chloroplast suspensions contained catalase, superoxide dismutase and O-acetylserine, the oxidation of sulfite to sulfate was slower in the light than the reductive formation of sulfides that exhibited a maximum rate of about 2 micromoles per milligram chlorophyll per hour, equivalent to about 1% of maximum carbon assimilation. Botht the oxidative and the reductive detoxification of sulfite were very slow in the dark. Oxidation was somewhat, but not much, accelerated in the light in the absence of O-acetylserine, which caused a dramatic decrease in the formation of organic sulfides and an equally dramatic increase in the concentration of sulfite addition compounds whose formation was light-dependent. The sulfite addition compounds were not identified. Addition compounds did not accumulate in the dark. In the light, the electron transport inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea, diuron, decreased not only the reduction, but also the oxidation of sulfite and the formation of addition compounds.

¹ This work was supported by the Projektgruppe Bayern zur Erforschung der Wirkung von Umweltschadstoffen (PBWU). It is also part of the research performed within the Sonderforschungsbereich 251 of the University of Würzburg.