Determination of superoxide dismutase activity (SOD) blood level. SOD Assay Kit-WST allows very conveniwater-soluble tetrazolium salt, WST-1 (2-(4-Iodophenyl)-3-(4-nitrophenyl)-5-(2,4-disulfophenyl)-2H-tetrazolium,monosodium salt) that produces a water-soluble formazan dye upon reduction with a superoxide anion. The rate of the reduction with O₂ are linearly related to the xanthine oxidase (XO) activity and is inhibited by SOD. Therefore, the 50% inhibition activity of SOD or SOD-like material can be determined by a colorimetric method (Ukwda et al., 1997).