Purification and characterization of thermostable monomeric chloroplastic Cu/Zn superoxide dismutase from Chenopodium murale.

Superoxide dismutase is the first line of defense against oxidative stress and thus helps in maintaining the cellular integrity. Chenopodium murale, a weed species adapted to widely varying climatic conditions faces extremes of temperatures ranging from 4 °C to 45 °C (Tmax) during growth and development. From this plant, we have purified a thermostable chloroplastic Cu/Zn superoxide dismutase (Chl Cu/Zn SOD) to homogeneity using minimal steps. Incubation of lysed chloroplasts at 70 °C for 1h reduced the interference of cytosolic SOD isoforms and reduced the protein content by 75 %. Chloroplastic SOD was purified from the heat stable fraction by gel filtration chromatography. The purified enzyme had a native molecular weight of 24 kDa, a half-life of 47.9 min at 80 °C and showed a single band at 24 kDa on SDS-PAGE. The N-terminus contained the conserved amino acids of chl Cu-Zn SOD. The Chl Cu/Zn SOD protein and its activity were enhanced under very high temperatures, high light intensities and in water stress/recovered C. murale plants under controlled environment conditions. Chl Cu/Zn SOD was also one of the predominant isoforms throughout growing period in field grown plants and declined during senescence. The Chl Cu/Zn SOD activity increased with the increase in ambient temperature and peaked in April with a 45 °C Tmax. These results clearly indicate that the chloroplastic Cu/Zn SOD is stably expressed at extreme environmental conditions. The presence of stable monomeric chloroplastic Cu/Zn SOD might help the plants to maintain the cellular homeostatis against adverse environmental conditions.