The Inhibition of Reduction of Tetrazolium by a Protein Liver

1 Xanthine oxidase has been partially purified from mammalian liver by a simple procedure. 2 The inhibition of cream and liver xanthine oxidase by several drugs was studied in (a) spectrophotometric assay of uric acid formation and (b) tetrazolium reduction. 3 Inhibitors tested caused different response, which depended on the assay type. 4 A protein was isolated from normal mammalian liver, which selectively inhibited tetrazolium reduction; the effect was observed with the xanthine/xanthine oxidase and ethanol/alcohol dehydrogenase systems. 5 Oxygen consumption by xanthine oxidase was prevented by tetrazolium salt; this effect was blocked by the inhibitor. 6 Some properties of the inhibitor are described.