EXCKETIONS 497 



UN -CO HISF-CO 



OC C NH 



:CO 



OC 



\ 



CO 



/ 



-C--NH- HN-CH-NH 



Uric Acid Allantoin 



The difference in the fate of uric acid in man, on the one hand, and in 

 the dog, rabbit, etc., on the other, is probably a quantitative one. Qualita- 

 tively there is no dissimilarity, for traces of allantoin do appear in human 

 urine, and the urines of the lower animals do contain small amounts of 

 purins (Hunter and Givens(c)). It is especially significant from the 

 standpoint of comparative physiology to learn that as far as their purin 

 metabolism is concerned, the monkey ranks with the lower animals rather 

 than with man. The purin metabolism of man, then, is unique in that uric 

 acid represents the principal excretory product. It is of further interest 

 to note that human blood contains from 10 to 60 times as much uric acid 

 as the blood of the rabbit, cat and sheep. Whereas the blood of these ani- 

 mals contains from 0.05 to 0.2 mg. of (free) uric acid per 100 c.c. of blood, 

 normal human blood contains 2 to 3 mg. A similar difference has been 

 found in the tissues of man and animals (Fine). This furnishes addi- 

 tional evidence pointing to the relative indestructibility of uric acid in 

 man. 



From the fact that in birds the end product of nitrogenous metabo- 

 lism in general is uric acid, apparently of synthetic origin, the attempt 

 has been made to demonstrate a similar formation in man, but without 

 conspicuous success. For the present, uric acid must be regarded as aris- 

 ing solely from the oxidative transformations of the purin bases, whether 

 they already exist in the body or have been introduced from without. 



The precursors of uric acid, nucleoprotein and purin bases, may be 

 present in the food or disintegrating cellular material of the body. In the 

 former case, the uric acid is said to be of "exogenous origin," in the latter, 

 of "endogenous origin." The output of endogenous uric acid will be de- 

 termined by the extent of the body cell activity. During starvation, for 

 example, the 24 hr. uric acid elimination may vary from 0.1 to 0.2 gram, 

 which may be increased to 0.2 to 0.4 gram on a purin-free diet. This 

 diet contains no uric acid precursors, and could, therefore, cause the in- 

 creased uric acid output only indirectly. It. is quite generally accepted 

 that the augmented output of uric acid following the ingestion of a purin- 

 free diet is due to the necessarily increased activity of the digestive glands, 

 thus raising the level of endogenous purin metabolism (Mares, Mendel and 

 Stehle). The administration of drugs, such as pilocarpin, which stimu- 

 lates glandular activity, also increases the uric acid output, while atrophin, 



