Studies on xanthine oxidase

Abstract

Inosine, inosinic acid, adenosine, muscle adenylic acid and cozymase (but not xanthosine or guanosine) were oxidized, though at a lower rate than with the free purine bases, by milk and by crude xanthine oxidase preparations. The xanthine oxidase acted by oxidizing hypoxanthine, which was liberated from the various substances by other enzymes present; e.g., in the case of inosine, by nucleosidase. When xanthine oxidase was obtained free from other enzymes, it was incapable of oxidizing any of the substances. If the inosine was incubated anaerobically with the enzyme preparation before the oxidation was allowed to take place, the rate of oxidation was increased ca. 40 times, becoming equal to that of free hypoxanthine (incubation with purified or boiled crude xanthine oxidase had no effect). The nucleosidase was extremely specific towards the purine part of the molecule and was unable to hydrolyze even such a similar substance as xanthosine. Previous incubation of adenosine with milk or with crude oxidase preparations also greatly increased the rate of oxidation, which became equal to that of free hypoxanthine, which was much higher than that of free adenine.[long dash]Adenosine is evidently first deaminated by a 3d enzyme, giving ino-sine, which is then hydrolyzed and oxidized as before. Incubation of free adenine with the oxidase preparations seems to have little or no effect, showing that this enzyme is an "adenosine deaminase'' which can act only on combined adenine and not on adenine itself. The deamination must therefore precede the hydrolysis.[long dash]Inosinic acid and adenylic acid behaved exactly like inosine and adenosine respectively, but it was possible that this was due to the presence of phosphatase, which would convert them into the nucleosides. Improved methods for the preparation of the nucleosides are given.