391 THE METABOLISM OF THE PURIN BODIES 



man about one-half) undergoes decomposition ; the purin ring be- 

 comes disrupted, the urea molecules are liberated, and the connecting 

 carbon chain becomes oxidised into carbon dioxide and water. The 

 process is, therefore, exactly the reverse of that which obtains in 

 birds. Urea in birds is a precursor of uric acid ; in mammals, urea 

 is an end product never forming uric acid but being in small part 

 derived from uric acid. 



If these facts be remembered, it will at once become evident 

 that a knowledge of the metabolism of uric acid in birds will be 

 absolutely valueless in explaining its metabolism in mammals ; 

 indeed, to try and do so would lead, and has in the past led, 

 to very serious misconceptions. Let us confine our studies for the 

 present entirely to mammals. 



Our chemical introduction has shown us that uric acid is the 

 oxidation product of xanthin and hypoxanthin; as we would 

 expect, therefore, these two bodies may appear in the urine along 

 with uric acid. To form a true estimate of purin metabolism we 

 must, therefore, measure the total purin excretion, and, in order 

 to investigate further those purins which are derived from the 

 tissues (endogenous purins), we must eliminate the purins taken 

 into the body in the food (exogenous purins). This could, of 

 course, easily be done by giving food which contained no purin. 

 Such a precaution would, however, be unnecessary did we know 

 exactly how much purin we were giving in the diet and what 

 proportion of this reappeared as purin in the urine. 1 



From these preliminary remarks we see that the questions to 

 be considered first of all are these : what food-stuffs contain sub- 

 stances capable of influencing the urinary purin excretion and 

 what is the nature of these substances ? what proportion of these 

 substances reappears in the urine and in what form ? (that is, as 

 uric acid or some other purin). In answering these questions, it 

 will be necessary to consider a method for estimating endogenous 

 purins. We will then be in a position to see whether or not the 

 amount of endogenous purins excreted in the urine bears any con- 



1 In fact, to be certain of maintaining everything in a perfectly physiological 

 state, the latter diet would probably be preferable, at least in the case of 

 carnivora, since these animals are habituated to a diet containing purin bodies, 

 and it might conceivably be subjecting them to abnormal conditions, were 

 we to place them on one devoid of purins. As we shall see later, this hypo- 

 thetical purin starvation is of no account, since nucleins are built up out of 

 purin-free bodies and not out of purins themselves. 



