Biological role of xanthine oxidase and tetrazolium-reductase inhibitor

The biochemical properties of tetrazolium reductase inhibitor (“reductase inhibitor”) from beef brain and liver were compared with superoxide dismutase (erythrocuprein) from beef erythrocytes, using xanthine oxidase and xanthine as a model system. In all assays the behavior of erythrocuprein and the reductase inhibitor was identical; it is concluded that the tetrazolium reductase inhibitor is a member of the superoxide dismutase class of enzymes. Its possible identity with other proteins of this group remains to be established. Xanthine dehydrogenase is activated by natural or synthetic detergents. These compounds eliminate the enzymatic action of both erythrocuprein and reductase inhibitor. Low levels of cyanide, which do not inhibit xanthine dehydrogenase block superoxide dismutase. The biological role of xanthine oxidase is discussed. We propose that an important function of xanthine oxidase is to provide an ubiquitous source of hydrogen peroxide and superoxide radicals which serve as oxidants for coupled biological oxidations.