Colorimetry of diaphorase in commercial preparations and clinical chemical reagents by use of tetrazolium salts.

We describe a procedure for assay of diaphorase activity in commercial purified preparations and in clinical chemical reagents by use of iodonitrotetrazolium chloride or other tetrazolium salts. The method is based on measurement of the formazan produced by enzymic reduction of tetrazolium salts in the presence of NADH. The assay procedure has been optimized for linear kinetics, simplicity of operation, nondetectable blank rates, and extended activity/enzyme concentration proportionality. The proposed method has several advantages over the older assay by use of dichlorophenolindophenol.