408 THE METABOLISM OF THE PURIN BODIES 



also as uric acid, and of caffein-N, about one - third as basic 

 purin-N. 



The amount of this exogenous urinary purin remains constant 

 for any given dietetic purin, and a very important fact it is the 

 same in all individuals of the same species. (To obviate subsequent 

 confusion, it may not be out of place to mention here that, 

 although constant for all individuals of the same species, it varies 

 enormously for different species. For example, as we have seen 

 above, of hypoxanthin man re-excretes one-half as uric acid, a 

 dog on the other hand excretes only one-twentieth.) Burian and 

 Schur have collected together a large number of experiments, 

 in which hypoxanthin and hypoxanthin-containing organs were 

 fed to different individuals, and have shown that the varia- 

 tions in the percentage of purin which passes into the urine 

 lies between 63'2 per cent, as a maximum, and 46*2 per cent, 

 as a minimum. These differences are probably due to errors 

 in estimation and technique, the most carefully conducted estima- 

 tions lying near 50 per cent. 



We have seen so far: (1) that, by placing a person on a 

 purin-free diet, the endogenous moiety of urinary purins can be 

 directly estimated ; (2) that any given purin body, when given 

 in the food, raises the purin excretion always to the same 

 extent; (3) that the exogenous moiety depends solely on the 

 chemical nature of the purin given in the food. From these facts 

 it follows, then, that the endogenous moiety can l>e calculated in- 

 directly. To do this we must know the exact amount of purin- 

 containing food ingested and the nature of the purins which it 

 contains. By deducting from the total purin excretion the 

 amount of purin which we know by applying the results detailed 

 above must have been derived from the food, we obtain the 

 endogenous purin excretion. The accuracy of this method can, 

 of course, very easily be put to the test. A person is placed on 

 a diet containing an accurately known amount of purins ; after 

 a few days on this diet, the purins are removed and a purin- 

 free diet is given. By subtracting from the purin excretion 

 of the first period the amount of exogenous purins which it 

 ought to yield, a product is obtained which should be identical 

 with the directly estimated endogenous moiety of the second 

 period. Is this the case ? A table arranged on this plan, and 

 compiled from various sources, by Burian and Schur, shows 



