NUCLEIN OR PURINE METABOLISM 779 



or nucleic acid 



I 



nuclease 4 mononucleotides 



! 



purine nuclease 



pentose-phosphoric acid guanine adenine pyrimidine bases 

 deaminase 

 (guanase and adenase) xanthin hypoxanthine 



oxidase 



xanthine 



oxidase 



uric acid 



The question arises whether the uric acid excreted by a man represents 

 the whole of the nucleins which have been destroyed in the body. Although 

 complete equivalence has been found between the amount of hypoxanthine 

 ingested and the amount of uric acid excreted, the same equivalence has 

 not been established in the case of nucleic acid, and the important question 

 arises whether uric acid once formed is stable or whether it may undergo 

 further changes before being excreted. In many animals, such as the dog, 

 the amount of uric acid in the urine is only minute, the chief purine 

 derivative in this fluid being allantoin. Allantoin is formed when uric 

 acid is oxidised with potassium permanganate, the following changes 

 taking place : 



NH CO NH O 



I I I 



CO C NH, + O + H 2 O = CO NH 2 + CO 2 



)CO >CO 



NH C NH/ NH-H NH 



The same transformation can be effected by extracts made from the 

 liver of the dog and probably of other animals. The ferment carrying out 

 this change is known as uricase. No such ferment is found in human liver 

 or any human organs, and, according to Jones and others, uric acid once 

 formed in the human organism is not further oxidised. The small trace 

 of allantoin which may occur in human urine is directly derived from the 

 food. Modern research does not confirm the idea which was formerly 

 held that a portion of the uric acid formed might undergo further oxidation 

 in man with the production of urea. 



On the other hand it is important to bear in mind the possibility that 

 some of the uric acid which occurs in human urine may be formed by a 

 process of synthesis. We have seen already that in the bird the greater 

 part of the uric acid is formed not from purines at all but by a process of 

 synthesis from lactic acid and ammonia, and though we have no evidence 

 of a similar change occurring in the mammal, we are not able definitely to 

 exclude its possibility. 



EXCRETION OF URIC ACID 



The complexity of these various processes in man renders it a difficult 

 task to form a clear idea of the origin of the urinary uric acid and of the 

 conditions which determine the variations in the amount excreted at 



