Iron- and Manganese-Containing Superoxide Dismutases: Structure, Distribution, and Evolutionary Relationships
In 1969, a well-studied family of mammalian copper-containing proteins was discovered to possess superoxide dismutase activity (1). This rather bizarre activity was postulated to play a protective role for the oxygen-metabolizing organism. Its ubiquity among the various mammalian tissues led us to attempt isolating the activity from an evolutionarily distant species, Escherichia coli (2). Although cell-free extracts of E. coli contained roughly the same amount of superoxide dismutase activity as mammalian tissue extracts, the enzyme’s behaviour during purification bore no resemblance to that of the bovine enzyme. When the enzyme was purified to homogeniety and concentrated, we did not see the familiar blue-green color of the mammalian copper-containing superoxide dismutases. The enzyme was pink. Undaunted, and perhaps comforted by the fact that some copper proteins are pink (3), we set out to determine the copper content of the new superoxide dismutase. No copper could be detected, either by a colorimetric method or by electron paramagnetic resonance (EPR). In fact, no EPR signal at all could be detected in the native protein. Upon denaturation by boiling in 0.1 N HCl, however, the characteristic six-pronged spectrum of manganese(II) appeared.
KeywordsElectron Paramagnetic Resonance Superoxide Dismutase Electron Paramagnetic Resonance Signal Manganese Superoxide Dismutase Iron Superoxide Dismutases
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