I´m following this protocol:

Superoxide radicals are generated in PMS-NADH systems by oxidation of NADH and assayed by the reduction of NBT. In these experiments, the superoxide radicals were generated in 3 ml of Tris-HCl buffer (16 mM, pH 8.0) containing 1 ml of NBT (50 mM) solution, 1 ml NADH (78 mM) solution and sample solution of fraction (25 – 500 mg/ml) in ethanol. The reaction started by adding 1 ml of PMS solution (10 mM) to the mixture. The reaction mixture was incubated at 25 °C for 5 min, the absorbance was read at 560 nm by spectrophotometer (Schimadzu UV-Vis 1700) against blank samples using ascorbic acid as a control. Decreased absorbance of the reaction mixture indicated the increasing of superoxide anion scavenging activity. The percentage inhibition of superoxide anion generation was calculated using the following formula:

% inhibition = [(A0-A1)/A0] x 100

where A0 was the absorbance of the control , and A1 was the absorbance in the presence of fraction or standards.