Xanthine and hypoxanthine are, therefore, now to hand, and given the presence of oxygen, their oxidation to _uric acid_ ensues:— C₅H₄N₄O + O---------------------> C₅H₄N₄O₂ Hypoxanthine. Hypoxanthine-oxidase. Xanthine. C₅H₄N₄O₂ + O----------------->C₅H₄N₄O₃ Xanthine. Xanthine-oxidase. Uric acid. Now, in man and the anthropoid apes, _uric acid_ is the end-product of _purin_ catabolism. In contrast therewith in most mammals only a minimal amount of the exogenous or endogenous purins escapes in the urine as uric acid. Most of it undergoes further oxidation into _allantoin_,[15] this change taking place in most mammals chiefly in the _liver_. According to Schittenhelm, if nucleic acid be given to dogs, pigs or rabbits, from 93-95 per cent. thereof appears in the urine as allantoin, and only 3-6 per cent. as uric acid, and 1-2 per cent. as purin bases. [Illustration: DISRUPTION OF NUCLEIC ACID MOLECULE (AMBERG AND JONES).] In man, as in most mammals, uric acid is formed chiefly in the _liver_ from purins, and in the preceding table Amberg and Walter Jones schematically represent the various steps by which disruption of the nucleic acid molecule is attained, and uric acid formed. DESTRUCTION OF URIC ACID _Uricolysis_, or the destruction of uric acid, is, in most mammals, achieved through the agency of the oxidising enzyme _uricase_, which oxidises uric acid to _allantoin_. Consequently, in their instance, purin bases, ingested as such or set free in the tissues, appear in the urine, not as uric acid, but in the form of _allantoin_. On the other hand, both in man and in the anthropoid apes, this particular enzyme uricase is absent. In accordance therewith, only a trace of allantoin is to be found in the urine of man and the higher apes, while in the lower animals, _e.g._, dogs, pigs, and rabbits, a large proportion of the purin excretion assumes this form. Now, the absence of _uricase_, in _man_, is held to be proved by the fact established by Wiechowski and others, viz., that uric acid, if _injected subcutaneously_, may be almost wholly recovered in the urine, and moreover, unchanged. On the other hand, the total excretion of uric acid and the other purin bodies by no means tallies exactly with the amount of the uric acid ingested as purin bases in the food and that produced from the tissues; in other words, it has been found that, when given by the mouth, _nucleic acid_ or _purins_ are by no means _quantitatively_ excreted in the urine, even though not only _uric acid_, but also _allantoin_ and the _purin bases_, are included within the estimate. According to most experiments, a considerable proportion of the purin-nitrogen intake, about 50 per cent., is excreted as _urea_.