Superoxide dismutase (SOD; EC 1.15.1.1) activity was assayed by its ability to inhibit the photochemical reduction of nitrotetrazolium blue chloride (NBT) at 560 nm (Beuchamp & Fridovich, 1971). The assays were performed at 25 C. The reaction mixture (3 ml) contained 33 lM NBT, 10 mM l-methionine, 0.66 mM EDTANa
and 0.0033 mM riboflavin in 0.05 M sodium phosphate buffer (pH 7.8). The reaction was started by adding riboflavin and placing the tubes containing the reaction mixture under 300 lmol m
irradiance at 25 C for 10 min. The absorbance was recorded at 560 nm. One unit of SOD enzyme activity was defined as the quantity of enzyme that reduced the absorbance reading of samples to 50% in comparison with tubes lacking enzymes, and causes changes in substrate (on a molecular weight basis). The specific SOD activity was expressed as mol/g and per g is on the protein basis.