298 URINE. [CH. XII. 



G. Purine bases, other than uric acid. 



The most important of these found in normal urine 

 are hypoxanthine, xanthine and adenine (see p. 62), 

 derived from the metabolism of food and tissue nucleins : 

 heteroxanthine (/-methyl-xanthine) and paraxanthine 

 (i, 7-dimethyl-xanthine) derived from the breakdown of 

 caffeine (i, 3, /-trimethyl-xanthine) and theobromine 

 (3, 7-dimethyl-xanthine) of the coffee, tea and cocoa 

 ingested. 



In man the methylated xanthines constitute the 

 greater part of these purine bases. But it is interesting 

 to note that the non-methylated ones are much increased 

 in fever. Also during severe muscular exercise there is an 

 increase, accompanied by a decrease of uric acid. After 

 the exercise there is an increase of uric acid, and a decrease 

 of the other purines. 



The simplest method of estimation is to determine 

 uric acid nitrogen by the method in Exs. 394-397, and 

 the total purine nitrogen by applying Kjeldahl's method 

 to the total purines precipitated by ammoniacal silver 

 nitrate (Ex. 357). The difference is the nitrogen of the 

 purine bases. 



H. Creatinine and Creatine. 



The chemical relationships of these bodies are de- 

 scribed on p. 178. In normal human urine creatinine is 

 always present, but creatine only after a meat diet, being 

 derived from that of the food. Creatine, however, is a 

 normal constituent of the urine of children. 



Creatine seems to be a product of tissue metabolism, 

 and the amount excreted is regarded by Folin as a measure 

 of endogenous metabolism. (See tables B and C, p. 270.) 

 There is an increase in complete starvation and in fevers, 

 due to the increased tissue breakdown. E. Mellanby 

 has drawn attention to the fact that the liver is probably 

 the seat of formation of creatinine. Thus in most diseases 

 of the liver there is a decreased excretion, an important 



