NUTRITION 271 



N=CH 



I I 



HC C NH X 



\CH. 



N .c isr 



Uric acid is 



HN CO 



OC C NH X 



>co, 



-L, 



and is described as 2-6-8 trioxy-purine. A large number of important derivatives 

 are known, in which amino-, oxy-, or methyl groups occur in various positions. 



The substances called nucleins are compounds of a protein with nucleic acid. 

 This latter is itself a compound of phosphoric acid with a pentose (five carbon 

 sugar) and a purine or pyrimidine derivative. There is a whole series of enzymes 

 concerned in the metabolism of nucleins, according to the nature of the particular 

 purine derivative present (see the monograph by Walter Jones, 1914). 



Puriiie Metabolism. Like that of proteins, is exogenous and endogenous. 

 If we look at Folin's table reproduced on p. 94 of Cathcart's monograph (1912), 

 we -may note that, although the excretion of uric acid, taken as representing 

 the purine metabolism, increases somewhat on a diet rich in nitrogen, the 

 relative increase is much less than that of urea. Thus, while urea rises 

 from 2-2 g. to 14'7 g., uric acid only rises from 0'09 to 0'18 g. This indicates, 

 as Folin points out, that the chief source of uric acid is endogenous. In 

 these experiments, purines were, of course, excluded from the diet, as far as 

 possible. At the same time, if purine derivatives are given in the food, in 

 excess of the amount required for maintenance, they are excreted. The data 

 of Hamill and Schryver (1906) show that, on ordinary diet, there is a constant 

 ratio between the uric acid and total nitrogen output. The organism can also 

 form purines from ordinary proteins, as shown by their increase in the developing 

 chick ; before incubation, there are practically no purines in the egg. Although 

 we have no evidence of such synthesis in the adult mammal, it cannot be excluded 

 as a possibility. Moreover, the question is complicated by the fact that there are 

 oxidising enzymes in various tissues, whose action results finally in the conversion 

 of uric acid into urea and oxalic acid. One of the intermediate substances formed 

 is alloxan, whose possible intervention in the process of de-amination we have seen 

 above (page 266). Prof. Hopkins informs me that he has obtained evidence that 

 arginine and histidine together serve as sources of the purine ring. 



As regards endogenous uric acid, there are two states in which increased 

 excretion occurs, fever and severe muscular work. Both are associated with 

 breakdown of muscular tissue, so that the uric acid seems to be chiefly derived 

 from this tissue. 



For further information, the reader is referred to Starling's book (1912, pp. 

 874-883). 



The increased production of uric acid in severe muscular exertion leads 

 us next to consider the question of protein metabolism in work. 



NITROGEN METABOLISM IN MUSCULAR WORK 



Since the endogenous output of nitrogen is to be regarded as the expression 

 of wear and tear of the tissues, it would naturally be expected that muscular 

 work would lead to a marked increase. 



But it is a remarkable fact that, so long as the work is not excessive and does 

 not lead to pathological conditions, there is practically no change in the nitrogen 

 output, assuming also that the supply of carbohydrate and of oxygen are in 

 sufficient amount. 



For the various evidence bearing on thia point, the reader is referred to 

 Cathcart's monograph (1912, pp. 109-121). The work of Higgins and Benedict 

 (1911) on the urine of the runners in one of the Marathon races may be added. 

 They were unable to determine the absolute amounts of the various constituents, 



