Characterization of new maize chloroplastic copper/zinc superoxide dismutase isoforms by high resolution native two-dimensional polyacrylamide gel electrophoresis. Identification of chilling responsive chloroplastic superoxide dismutase isoforms

The response of superoxide dismutases (SOD, EC1.15.1.1) to chilling-induced oxidative stress in differentially sensitive maize genotypes (Zea mays L) was examined. A native 2D-PAGE system that resolves the maize leaf SOD isoforms has been developed. The chloroplastic SOD activity was resolved into four Cu/Zn SOD isoforms designated SOD1a -> d with pI values of 3.9, 4.0, 4.5 and 5.6, respectively. These SODs are located in the stroma and display a higher resistance to hydrogen peroxide inactivation than the cytosol Cu/ZnSODs. They operate as 32 kDa homodimers and have an AT motif at the NH2-terminal, which characterizes the chloroplastic SODs of most species. A light chilling treatment resulted in a rapid increase in the activity of SOD1a and SOD1b. Because this increase was observed in the presence of the protein synthesis inhibitor cycloheximide, it is suggested that short-term regulation of chloroplastic SODs occurs at a post-translational level.