Superoxide dismutase (SOD) activity was determined with a commercially available kit (Sigma, St. Louis, MO), according to the manufacturer's instructions. Briefly, the assay is based on a tetrazolium salt that is reduced by superoxide anions (O2 - ) to form a formazan dye. SOD activityis determined indirectly as the inhibition of the production rate of O2 - by xanthine oxidase by easuring the decrease in absorbance of light at 440 nm. SOD activity was measured against an SOD standard curve and expressed as units (U)/mg protein, where one unit is defined as the amount of SOD that leads to 50% inhibition at 37ºC.