Xanthine Oxidase: Purification and Properties

The oxidation of hypoxanthine and xanthine to uric acid in the presence of tissue brei and oxygen was first recognized in 1899 by Spitzer (11) as an enzymatic process.The enzyme involved was named xanthine oxidase by Burian (3).In 1902 Schardinger (10) described a reaction in which the reduction of methylene blue by aldehyde was catalyzed by the presence of fresh milk, the reaction and enzyme involved being named after the discoverer.These two enzymatic processes remained unrelated until some 20 years later when Morgan, Stewart, and Hopkins (9) showed that milk was a rich source of xanthine oxidase.This finding immediately raised the question of the identity of xanthine oxidase and the Schardinger enzyme.From a study of the various factors affecting these two reactions most workers (cf.Dixon and Thurlow (7), Booth (2)) h ave reached the conclusion that the same enzyme is involved in both.It is the purpose of this paper to describe the isolation of an enzyme preparation from whole milk which catalyzes both of these reactions and which is 500 times more active per unit of dry weight than the starting material.Evidence will be presented which indicates that this enzyme is a flavoprotein.The properties of this preparation and those of its flavin prosthetic group will be described.