272 PHYSIOLOGICAL CHEMISTRY. 



of one of the older methods for the quantitative determination of 

 uric acid (Heintz Method, p. 373). 



Uric acid is very closely related to the purine bases as may be 

 seen from a comparison of its structural formula with those of the 

 purine bases given on page 241. According to the purine nomen- 

 clature it is designated 2-6-8-trioxypurine. Uric acid forms the 

 principal end-product of the nitrogenous metabolism of birds and 

 scaly amphibians ; in the human organism it occupies the fourth 

 position inasmuch as here urea, ammonia and creatinine are the 

 chief end-products of nitrogenous metabolism. It is generally said 

 that the relation existing between uric acid and urea in human urine 

 under normal conditions varies on the average from 1 140 to i :ioo 

 and is subject to wider variations under pathological conditions; 

 and further that because of the high content of uric acid in the urine 

 of new-born infants the ratio may be reduced to i :io or even 

 lower. We now know that this ratio of uric acid to urea is' of 

 little significance under any conditions. 



In man, uric acid probably results principally from the destruc- 

 tion of nuclein material. It may arise from nuclein or other purine 

 material ingested as food or from the disintegrating cellular matter 

 of the organism. The uric acid resulting from the first process is said 

 to be of exogenous origin, whereas the product of the second form 

 of activity is said to be of endogenous origin. As a result of experi- 

 mentation, Siven, and Burian and Schur, and Rockwood claim that 

 the amount of endogenous uric acicl formed in any given period 

 is fairly constant for each individual under normal conditions, and 

 that it is entirely independent of the total amount of nitrogen elimi- 

 nated. Recently Folin has taken exception to the statements of 

 these investigators and claims that, following a pronounced decrease 

 in the amount of protein metabolized, the absolute quantity of uric 

 acid is decreased but that this decrease is relatively smaller than the 

 decrease in the total nitrogen excretion and that the per cent of the 

 uric acid nitrogen, in terms of the total nitrogen, is therefore de- 

 cidedly increased. 



In birds and scaly amphibians the formation of uric acid is anal- 

 ogous to the formation of urea in man. In these organisms it is 

 derived principally from the protein material of the tissues and the 

 food and is formed through a process of synthesis which occurs for 

 the most part in the liver; a comparatively small fraction of the 

 total uric acid excretion of birds and scaly amphibians may result 

 from nuclein material. 



' 



