Purification and Partial Characterization of a Genetically-Defined Superoxide Dismutase (SOD-1) Associated with Maize Chloroplasts

The chloroplast-associated form of superoxide dismutase from maize (Zea mays L.) (SOD-1) has been purified by a stepwise procedure consisting of (NH₄)₂SO₄ fractionation, G-100 Sephadex gel filtration, DEAE-Sephacel chromatography, and hydroxylapatite chromatography. This procedure resulted in a single band on sodium dodecyl sulfate-polyacrylamide gels indicating that the preparation is homogeneous. The holoenzyme molecular weight was estimated at 31,000 to 33,000 by gel filtration. The subunit molecular weight of this dimeric protein was estimated at 14,500 on sodium dodecyl sulfate-polyacrylamide gels. Studies involving amino acid composition analysis, immunological cross-reactivity, in vitro subunit hybridizations, and H₂O₂ sensitivity indicate that SOD-1 differs significantly from SOD-2 and SOD-4, the other cupro-zinc forms of SOD from maize. The possible physiological role of SOD-1 within the chloroplast is discussed.