The embryo of oviparous species is confronted by a highly oxidative stress generating as it grows and must rely on effective antioxidant system for protection. Proteins of avian egg albumin have been suggested to play the major redox-modulatory role during embryo development. Recently, we found that ovotransferrin (OTf) undergoes distinct thiol-linked self-cleavage in a redox-dependent process. In this study, we explore that OTf is SOD mimic protein with a potent superoxide anion (O(2)(-)) scavenging activity. The O(2)(-) scavenging activity was investigated using the natural xanthine/xanthine oxidase (X/XOD) coupling system. OTf exhibited O(2)(-) scavenging activity in a dose-dependent manner and showed remarkably higher scavenging activity than the known antioxidants, ascorbate or serum albumin. The isolated half-molecules of OTf exhibited higher scavenging activity than the intact molecule, whereas the N-lobe showed much greater activity. OTf dramatically quenched the O(2)(-) flux but had no effect on the urate production in the X/XOD system, indicating its unique specificity to scavenge O(2)(-) but not oxidase inhibition. Strikingly, metal-bound OTf exhibited greater O(2)(-) dismutation capacity than the apo-protein, ranging from moderate (Zn(2+)-OTf and Fe(2+)-OTf) to high (Mn(2+)-OTf and Cu(2+)-OTf) activity with the Cu(2+)-OTf being the most potent scavenger. In a highly sensitive fluorogenic assay, the metal-bound OTf exhibited significant increase in the rate of H(2)O(2) production in the X/XOD reaction than the apo-OTf, providing evidence that Zn(2+)-, Mn(2+)- and Cu(2+)-OTf possess SOD mimic activity. This finding is the first to describe that OTf is an O(2)(-) scavenging molecule, providing insight into its novel SOD-like biological function, and heralding a fascinating opportunity for its potential candidacy as antioxidant drug.