In the animal body, nucleic acids are constantly being degraded and re synthesized. The degradation process follows the route:

The degradation of dietary (exogenous) nucleic acids to nucleosides and bases occurs mainly in the small intestinal lumen.

The purine nucleosides and free bases could then be deaminated to form hypoxanthine and xanthine by various specific deaminases as shown in FIG 1. These enzymes include AMP aminohydrolase (adenylic deaminase) (EC. 3.5.4.6), adenosine deaminase (EC. 3.5.4.4 ), adenine deaminase (EC. 3.5.4.2) and guanine deaminase (EC. 3.5.4.3). Among which only adenine deaminase is present in trace amounts in animal tissues. The others are widely distributed in animal tissues.

FIG 1

In the presence of xanthine oxidase (EC. 1.2.3.2), hypoxanthine and xanthine are converted to uric acid, which is subsequently converted to allantoin by uricase ( EC. 1.7.3.3) . Xanthine oxidase i s present in most animal tissues, but its distribution varies between tissues and between animal species [1]. However, it appears to be present at high levels in the liver of all animals. Reports in the literature on the distribution of uricase are limited.

More about xanthine oxidase and uricase

In most mammals, allantoin in the last product of the purine degradation chain and is excreted in the urine as the major component of the purine end products. Hypoxanthine, xanthine and uric acid are also excreted probably due to the high clearance rate in the blood. Man and other primates excrete uric acid as the final product of purine catabolism as the result of lacking uricase. Bird also lack uricase and thus excrete uric acid, the advantage of do so is saving water. However, uric acid can also be produced from urea in bird. Other species of less developed animals (e.g. fish) have enzyme to further degrade allantoin to allantoic acid and glyoxylic acid and finally to urea.

Compare with the pathway of purine catabolism for birds and man.
Inter-relationship between purine degradation, de novo synthesis and salvage of preformed purines

References

[1] AL-KHALIDI, U. A. S., CHAGLASSIAN, T. H., The species distribution of xanthine oxidase. Biochem. J. 97 (1965) 318-320.