The object of this paper is the comparative study of three oxidizing enzymes—uricase, amino acid oxidase, and xanthine oxidase. We shall describe first the main properties of uricase and amino acid oxidase, laying special stress on characters which have not been sufficiently investigated by previous workers. This will include the study of the effect of various factors on the activity of these enzymes, the reaction between these enzymes and their substrates, the activation of the substrate molecules, their reaction with the molecular oxygen, and the reduction of the latter to hydrogen peroxide. We shall then examine briefly the main characters of xanthine oxidase, and this will enable us to compare the properties of these three enzymes and to determine some of the characters they have in common. II−Uricase or Urico-oxidase 1−
Previous Work
That uricase or urico-oxidase catalyses the oxidation of uric acid to allantoin has been known since the work of Schittenhelm (1905), Wiechowski and Wiener (1909), and others, but its main properties have been established only by Battelli and Stern (1909, 1912) in their important investigation on this subject. According to these authors, for the oxidation of a molecule of uric acid to allantoin one atom of oxygen and one molecule of water are taken up while one molecule of CO
2
is given off. The reaction consists, therefore, in oxidation, hydration, and decarboxylation, and the R. Q. of the reaction is usually equal to 2. It varies slightly, however, according to the age of the enzyme preparation. The relationship which these authors have established between the amount of uric acid disappearing, the oxygen taken up, and the CO
2
given off, has made possible the study of the reaction by the estimation of either oxygen or CO
2
. The velocity of oxidation of uric acid was found to depend on the oxygen tension, being, for instance, at least twice as great in pure oxygen as in air. The main results obtained by Battelli and Stern have been recently confirmed by other workers, who have, however, paid special attention to the study of the kinetics of this reaction (Felix, Scheel, and Schuler, 1929; Schuler, 1932; Rô, 1931; Grynberg, 1931). The velocity of the reaction was measured by these authors in terms of the amount of uric acid oxidized, of oxygen absorbed, and of CO
2
liberated; and these reactions were studied at various hydrogen ion concentrations, at different tensions of oxygen, and at different concentrations of enzyme and substrate. The oxidation of uric acid catalysed by the enzyme was also compared with that obtained by permanganate and by hydrogen peroxide. One of the important conclusions which resulted from this work was that the enzyme does not catalyse directly the oxidation of uric acid to allantoin, but that the reaction takes place in two steps: (1) the catalytic oxidation and hydration of uric acid by uricase to an oxyacetylene-diurein carboxylic acid, and (2) the decarboxylation of this unstable compound to allantoin, which is independent of the enzyme (Biltz and Schauder, 1923; Felix and his co-workers, 1929; Grynberg, 1931; and Schuler, 1932).
Xanthine Oxidase as a Source of Hydrogen Peroxide for the Lactoperoxidase System in Milk