Superoxide dismutase activities in the midgut of Helicoverpa armigera larvae: identification and biochemical properties of a manganese superoxide dismutase

Insects possess a complex network of enzymatic antioxidant systems protecting against reactive oxygen species generated during stress. Superoxide dismutases (SODs) are vital antioxidant enzymes converting superoxide into oxygen and hydrogen peroxide. Helicoverpa armigera is a polyphagous insect and has developed resistance to various phytochemicals and pesticides. Although SODs are studied in several insect species, their characterization has been reported in only a few insect species. Here, we attempted to identify and characterize SOD activity from H. armigera. Electrophoretic separation followed by nitroblue tetrazolium staining revealed the presence of five SOD activity isoforms in the midgut of H. armigera larvae. Total SOD activity was measured at the different larval developmental stages and found maximum at fourth instar and further declined at sixth instar. Total SOD activity was significantly increased by metal ions Mn, Fe, Zn, and Cu and reduced by Ca and Pb. The isoform with highest activity was identified as manganese SOD (MnSOD) because it was found insensitive to [H.SUB.2][O.SUB.2]. The identified MnSOD was partially purified and characterized. After gel filtration chromatography, specific activity of the purified enzyme was found to be 6,348.0 U/mg/min with 26.61% yield. The molecular mass of purified enzyme was calculated to be ~29.3 kDa by sodium dodecyl sulfate polyacrylamide gel electrophoresis. The optimum pH and temperature for the SOD activity were found to be around 11 and 60°C, respectively. The study could be useful to further understand the functional significance of SODs in the antioxidant mechanism of H. armigera. Keywords: antioxidant mechanism, H. armigera, superoxide dismutases