URIC ACID AND THE PURINE BODIES 641 



The importance of the above described results rests in the fact that 

 from them we may hope to be able, ultimately, to state exactly in what 

 organs and tissues the intermediary metabolic processes concerned in 

 nucleic acid metabolism occur. The work at the present time is of spe- 

 cial significance, since it represents one type of evidence which we must 

 have before we can trace exactly every step in the metabolism of any 

 other biochemical substance. 



The absence of uricase from the tissues of man places him in a unique 

 position with regard to the metabolism of nucleic acid, and renders the 

 investigation of the problem particularly difficult, since animal experi- 

 mentation is useless. Recently, however, S. R. Benedict has discovered 

 that the Dalmatian breed of dog also known as the carriage dog, and 

 having a spotted or mottled skin has a purine metabolism like that of 

 man. 4 When fed on food containing no purine substances, he excretes 

 large quantities of uric acid, and when the latter substance is injected 

 subcutaneously, it is eliminated quantitatively as such in the urine. We 

 shall see later how experiments on this animal have been made use of 

 in the investigation of problems of purine metabolism as applied to man. 

 In all other animals most of the uric acid is oxidized to allantoine before 

 being excreted. The degree to which this occurs varies between 79 

 and 98 per cent of the uric acid in different species. This has been 

 called the uricolytic index (Hunter and Givens). 



The Balance between Intake and Output of Purine Substances under 

 Various Physiologic and Pathologic Conditions. The main purine ex- 

 cretory product in man is uric acid, but there is also a certain amount 

 of purine bases. The presence of uric acid has attracted attention for 

 a great many decades in medical investigation, because of the relative 

 ease with which it can approximately be determined quantitatively, and 

 because of the well-known fact that it may be responsible for certain 

 diseases, such as gout, when it accumulates in the tissues in an insoluble 

 form. On a diet containing meat, or more particularly on one con- 

 taining glandular substances, the total daily excretion of uric .acid is 

 very considerably greater than when the diet contains no such food 

 stuffs. The conclusion which Burian and Schur 43 drew from this ob- 

 servation is that purine must be partly of exogenous and partly of 

 endogenous origin. In other words, some* of it is derived more or less 

 directly from performed purine substances in the food, and the remain- 

 der from the purine constituents of the animal's own tissues. 



Endogenous Purines. It was thought that a definite proportion of 

 each of the administered purines could be invariably recovered from 

 the urine. Although this has not been found to be exactly true, there 

 is nevertheless a certain constancy in the proportion of administered 



