172 INTERMEDIATE METABOLISM 



Physiological Synthesis of Purines 



This is abundantly proved. 



1, Salmon during the breeding season form large quan- 

 tities of nucleic acid in the sexual organs, the heads of 

 spermatozoa consisting almost entirely of this substance. 

 Since during this period the fish take no food, the nucleic 

 acid must be formed from the tissue proteins, chiefly the 

 muscles. 



2, Purines, absent from the newly laid egg, develop during 

 incubation. 



3, Mammals, both growing and adult, produce and 

 excrete purines indefinitely when fed on milk or other 

 purine-free diet. 



Exogenous and Endogenous Purine 



The amount of purine excreted depends upon the amount 

 ingested. In man the urinary uric acid is increased after 

 feeding with substances such as thymus which are rich 

 in purine. ^Alien uric acid itself is administered it can be 

 recovered in the urine, sometimes almost completely. From 

 hypoxanthine and xanthine there is a yield of uric acid 

 corresponding to about 50 per cent., and from adenine 

 and guanine a smaller yield. 



When no purines are present in the diet, uric acid con- 

 tinues to be excreted, being derived evidently from the 

 purines of the body. The source of the uric acid excreted 

 is therefore twofold, exogenous and endogenous. 



Two questions now have to be considered. 



1. How does the body transform the purines, whether 

 from the food or from the tissues, into uric acid ? 



2. What conditions determine the conversion and 

 excretion of body purines ? 



The Formation of Uric Acid from Nucleic Acid 



Our knowledge of this subject has been obtained by 

 studying the chemical changes which occur when nucleic 



