Maize superoxide dismutase has been resolved into five major electrophoretic forms by starch gel electrophoresis (Fig. 1). These have been tentatively labeled SOD-1, SOD-2, SOD-3, SOD-4, and SOD-5 in order of their migration to the anode, in accordance with the recommendations of the Subcommittee on Multiple Molecular Forms of Enzymes of the IUPAC-IUB (1971). All five isozymes are present in the liquid endosperm, scutellum, pericarp, root, primary leaf, coleoptile sheath, and mesocotyl, and show quantitative differences in their expression among these various tissues. Cell fractionation studies using sucrose gradient centrifugation have demonstrated the presence of a cyanide-resistant isozyme (SOD-3) in the mitochondrial fractions of the scutellum and epicotyl. In addition, SOD-1 (a cyanide-sensitive isozyme) was found to be associated with the chloroplasts and the etioplasts of developing maize leaves. Contrary to what has been previously reported (Giannopolitis and Ries, Pl. Physiol. 59:309, 1977), cyanide-sensitive superoxide dismutases were not found to be associated with the mitochondria. Furthermore, only a single electrophoretic form (SOD-1) was observed in the plastid fraction.
Further research with this system will be devoted in part to the characterization of the biochemical and genetic bases of these enzymes. In addition, experiments are being planned to study the physiological relationship between superoxide dismutase, catalase, and O2-generating systems within the cell.
James A. Baum and John G. Scandalios
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