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 x 10(9) M-1 s-1 at pH 7.8 and 25-degrees-C. A comparative study between the molecular properties of this higher plant Fe-SOD and SODs from different origins is presented.