In chloroplasts O−2 is photoproduced via the univalent reduction of O2 in PSI even under conditions that are favorable for photosynthesis. The photogenerated O−2 is disproportionated to H2O2 and O2 in a reaction that is catalyzed by superoxide dismutase (SOD). The H2O2-scavenging ascorbate peroxidase is bound to the thylakoid membranes at or near the PSI reaction center [Miyake and Asada (1992) Plant Cell Physiol. 33: 541], and the primary product of oxidation in the peroxidase-catalyzed reaction, the monodehydroascorbate radical, is photoreduced to ascorbate in PSI in a reaction mediated by ferredoxin [Miyake and Asada (1994) Plant Cell Physiol. 35: 539]. Therefore, SOD should be localized at or near the PSI complex. We report here the microcompartmentalization of the chloroplastic CuZn-SOD on the stromal-faces of thylakoid membranes where the PSI-complex is located. Spinach leaves were fixed and substituted by a rapid freezing and substitution method that allows visualization of intact chloroplasts. The embedded sections were immuno labeled with the antibody against CuZn-SOD by the immunogold method. About 70% of the immunogold particles were found within 5 nm from the surface of the stromal-faces of thylakoid membranes. Of these particles, about 40% were found at the ends and margins of the grana thylakoids and 60% were found on the stromal side of the stromal thylakoids. From these results, the local concentration of CuZn-SOD on the stroma-facing surfaces of the thylakoid membranes was estimated to be about 1 mM. The effect of the microcompartmentalization of CuZn-SOD on the scavenging of superoxide radicals is discussed.
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Ogawa et al. (1995) studied this question.