ABSTRACT
Xanthine oxidase is widely distributed in mammals, especially in cow's milk and calf liver. The enzyme from milk, being easy to purify, has been used in most studies of this enzyme. Xanthine oxidase is a rather nonspecific enzyme and a great variety of compounds serve as substrates. The rates of oxidation vary quite markedly among the different types of substrates, with hypoxanthine and xanthine being the best substrates. To demonstrate the rate of formation and decay of signals due to molybdenum, flavin, and iron, reactions were performed in which the xanthine concentration was essentially identical to that of the enzyme and oxygen was present in excess. Additional evidence that the mechanism of xanthine oxidase is a Ping-Pong Bi Bi mechanism comes from varying the concentration of one substrate while holding the other substrate concentration constant. A number of methods are available for assaying xanthine oxidase activity.
