674 METABOLISM 



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 42 ) . 



The Balance between Intake and Output of Purine Substances under 

 Various Physiological and Pathological 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 in urine has attracted at- 

 tention for 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 w r hen 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 preformed 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 

 purine that is excreted. Thus, Mendel and Lyman have found recently 

 ithat about 60 per cent of injected hypoxanthine, 50 per cent of xan- 

 thine, 19-30 per cent of guanosine, and 30-37 per cent of adenine are 

 eliminated in the urine as uric acid. When combined purines i. e., nu- 

 clear material are given, only a small proportion of the purine thus 

 reappears. There is, therefore, a general parallelism between the purine 

 content of the food and that of the urine, w r hich indicates that purine- 

 rich food ought to be eliminated from the diet of patients who are 

 suffering from deposition of insoluble urate in the tissues, as in gout. 

 The fate of the purine that disappears in the body is unknown; some of 

 it may be decomposed in the intestine, but why so much of the remainder 

 should disappear, after absorption by the blood, is a mystery, since no 

 uricase can be discovered in any of the organs or tissues. The destroyed 

 purines can not be shown to influence any of the other well-known 

 nitrogenous metabolites of the urine. 



The following table of experiments by Taylor and Eose 45 may serve 



/to illustrate these points. The subject was placed on a purine-free diet 



consisting of milk, eggs, starch and sugar, for three days. After this 



