PURINES 175 



3. Diet. — A meal rich in proteins, though free from 

 purines, leads to an increase in the excretion of uric acid 

 which precedes the rise in urea excretion. Its causation is 

 not clear. It may be derived from the digestive glands 

 owing to their increased activity. It may be due to the 

 metaboHsm of leucocytes, the numbers of which in the 

 circulation are increased during digestion. The latter view 

 is supported by the fact that in leucocythsemia, a patho- 

 logical condition associated with a high leucocytosis, there 

 is a considerable rise in purine excretion. On the other 

 hand there is no quantitative relationship between the 

 rise in uric acid excretion and the degree of leucocytosis. 



Purine Metabolism in Animals other than Man 



Man is almost unique among mammals in excreting uric 

 acid as the principal end-product of purine metaboUsm. 

 Other mammals, with the curious exception of the Dal- 

 matian breed of dogs, carry purine metabolism one stage 

 further — to allantoine : — 



HN— CO HN— CO NH, 



2 



I 



OC C— NH. OC CO 



I il >co I 



HN— C— NH^ HN— CH— NH 



Uric acid. Allantoine. 



or 

 C^H.N^Oa -f H^O -f = C.HgN.Og -f CO^ 



the allantoine being excreted by the kidneys. 



The conversion of uric acid to allantoine is effected by 

 the enzyme uricase or uricolytic ferment, which is found 

 chiefly in the kidney and hver. This ferment is not 

 present in man. 



In birds uric acid forms the chief end-product not only 

 of purine metabohsm but also of protein metabolism in 



