THE METABOLISM OF THE PURIN BODIES 423 



We have seen that by multiplying the endogenous urinary 

 purins by a certain factor, varying in different species of animals, 

 an estimate may be made of the purins actually set free in the 

 tissues. So far as is possible from the small amount of work 

 which has been done since the dual origin of urinary purins was 

 clearly indicated, let us inquire into the exact source of the 

 endogenous purins : and how their excretion behaves under various 

 physiological and pathological conditions. 



First of all let us see whether any of the endogenous moiety of 

 urinary purins in mammals is produced by a synthetic process 

 analogous with that occurring in birds. The fact that the endo- 

 genous moiety remains constant, even when the diet (purin-free) 

 undergoes enormous variations, would tend to show that the 

 synthetic production, if it does exist, can be only of very secondary 

 importance. On the other hand, there are certain chemical 

 substances which are not purins, but which, when given to 

 animals (mammals) on a purin-free diet, can cause a slight increase 

 in the endogenous purin excretion. These same substances cause 

 a very marked increase in the purin excretion of birds where 

 uric acid synthesis undoubtedly obtains. It is obvious, therefore, 

 that before going farther, we must study the synthetic process 

 in birds, and then, after we are familiar with this, see in how 

 far the knowledge can be applied to what occurs in mammals. 



That uric acid in birds is produced by a synthetic process is 

 shown by the fact that if urea be given them in their food it 

 all reappears as uric acid in the urine. The same thing results 

 if these substances which cause increased urea excretion in 

 mammals, such as ammonia salts and amido acids, are given to 

 birds, an immediate increase of uric acid results. Chemical 

 investigation has shown us that uric acid consists of two urea 

 molecules linked together by a central chain of carbon atoms. 

 What substance in the organism furnishes this central carbon 

 chain, and in what organ does the synthesis ensue? To 

 answer these questions Minkowski ( 3 ) extirpated the liver in 

 geese. 1 This operation in birds, and especially in large birds, 

 is, technically, not very difficult, because no artificial anasta- 

 mosis of portal vein and vena cava has to be created, as in 



1 That it is in the liver that this synthesis occurs in birds has been shown by 

 T. H. Milroy, who found that galvanic stimulation of the liver of geese caused 

 an increase in the uric acid excretion ( 37 ), 



