Spectrophotometric assay for superoxide dismutase based on tetrazolium salt 3'--1--(phenylamino)-carbonyl--3, 4-tetrazolium]-bis(4-methoxy-6-nitro)benzenesulfonic acid hydrate reduction by xanthine-xanthine oxidase.

XTT (3'-{1-[(phenylamino)-carbonyl]-3, 4-tetrazolium}bis(4-methoxy-6-nitro)benzenesulfonic acid hydrate) was reduced to a water-soluble product with an absorbance maximum at about 470 nm by superoxide anion generated by xanthine-xanthine oxidase (XO). The rate of XTT reduction was linearly related to XO activity and the reduction was inhibited by superoxide dismutase (SOD). A perfect inhibition of the reduction of XTT by SOD was achieved, suggesting that XTT does not interact with XO. The present XTT-based assay had a higher sensitivity than a conventional nitroblue tetrazolium-based assay by a factor of 2.5 at pH 10.2. This method was applicable to the SOD assay in the pH range 8.0-10.2.