Characterization of an iron-containing superoxide dismutase from a higher plant, Citrus limonum

An iron-containing superoxide dismutase (SOD, EC 1.15.1.1) was fully characterized from leaves of the higher plant Citrus limonum R. cv. Verna. This enzyme is the first iron-containing SOD to be characterized in the plant family Rutaceae . The purified Fe-SOD has a molecular mass of about 47 kDa and is composed of two non-covalently joined equal subunits. The amino acid composition determined for the enzyme was compared with that of a wide range of SODs and had highest degree of homology with the Fe-SODs from Brassica campestris and Nuphar luteum . The enzyme was more labile at high temperatures than some eucaryotic and procaryotic Fe-SODs. It showed a maximum stability at pH 7.8. The sensitivity of the enzyme to cyanide, hydrogen peroxide and o -phenanthroline was similar to those reported for other Fe-SODs. but the lemon enzyme was comparatively resistant to H₂O₂. By kinetic competition experiments, the rate constant for the disproportionation of superoxide radicals by lemon Fe-SOD was found to be 1.9 × 10⁹ M ^?1 s^?1 at pH 7.8 and 25°C. A comparative study between the molecular properties of this higher plant Fe-SOD and SODs from different origins is presented.