Cloning of active human manganese superoxide dismutase and its oxidative protection in Escherichia coli

Superoxide radical (O2^•-) is a toxic byproduct of oxidative metabolism that extensively damages cellular macromolecules and organelles. Superoxide dismutase (SOD) catalyzes the conversion of superoxide radical to hydrogen peroxide (H₂O₂) and molecular oxygen (O ₂) thus providing a biological defense against oxygen toxicity. The structural gene of human manganese superoxide dismutase (hMnSOD) was successfully cloned into the pET46Ek/LIC by using a Ligation Independent Cloning (LIC) technique. The recombinant human MnSOD was expressed in E. coli strain BL21(DE3)pLysS and purified to homogeneity by Ni²⁺-NTA. Supplementation of Mn²⁺ in the bacterial growth media was proven to be crucial for production of enzymatically active hMnSOD. The recombinant enzyme revealed a specific activity up to 2,857 U mg^-1 as measured by inhibition of photoreduction of nitroblue tetrazolium (NBT). The molecular weight of each subunit was estimated to be 22 kDa by SDS-PAGE. More interestingly, E. coli expressing hMnSOD provides resistance against oxidative stress induced by the herbicide paraquat up to 1.2 mM. These findings gain insights into the biochemical characterization and significant roles of oxidative-protection of the hMnSOD in biological systems.