Purification and Subunit Structure of Extracellular Superoxide Dismutase from Mouse Lung Tissue

The first purification of mouse extracellular superoxide dismutase (EC-SOD) and the analysis of the native enzyme are described. Mouse EC-SOD was purified from lung tissues with a high recovery (41%) and a specific polyclonal antibody against the purified enzyme was obtained. The purified enzyme had a strong affinity for heparin and a molecular mass of 150 kDa (estimated by a gel filtration chromatography). The native mouse EC-SOD was composed of two different sizes of subunits, aMrof 33 and 35 kDa (determined by SDS–PAGE). The 35-kDa subunit had an interchain disulfide bond at the C-terminus and existed as a covalent dimer in the molecule, whereas the 33-kDa subunit resulted from the 35-kDa subunit by truncating its C-terminus as a posttranslational modification, with resultant loss of the interchain disulfide bond. These results suggest that the native mouse EC-SOD is a heterotetramer composed of two different dimers, with or without a covalent bond.