1 62 NUTRITION OF FARM ANIMALS 



Katabolism 



228. Nitrogenous end products. The total katabolism of 

 the proteins results in the elimination of all their nitrogen 

 through the kidneys in the form of the various relatively simple 

 crystalline products found in the urine. Of the nitrogenous 

 excretory products of man and the carnivora, urea is the most 

 prominent, while others, such as uric acid, creatin, creatinin, 

 ammonia, etc., are of subordinate importance quantitatively. 

 Traces of hippuric acid are also found in the urine of man and 

 carnivora, while it is present in relatively large amounts in that 

 of herbivora along with considerable quantities of ammonia 

 and apparently but little urea. 



The nitrogenous ingredients of the urine of mammals other 

 than those just mentioned are either derived chiefly from the 

 nucleoproteins, whose metabolism will be considered later, 

 or are present in such small amounts as to call for no special 

 consideration from the present very general point of view. 

 Finally, it should be noted for completeness that a small amount 

 of nitrogenous products is eliminated in the perspiration and 

 also that from one point of view the incompletely katabolized 

 nitrogenous excretory products of the feces (154) may also be 

 regarded as products of protein katabolism. 



Urea, or dicarbamid, CO (NH 2 )2, is the chief nitrogenous product 

 of the katabolism of the simple proteins in carnivora and omnivora. 

 In human urine from 80 to 90 per cent of the nitrogen is ordinarily 

 present in this form, although the proportion may be considerably 

 diminished under special conditions, notably on a low protein diet. 

 Urea, however, is not simply split off as such from the proteins as 

 some earlier schematic statements have sometimes been taken to 

 imply. The immediate antecedent of urea is ammonium carbonate, 

 which undergoes a dehydration in the liver or elsewhere, while there 

 is evidence in favor of the view that the ammonia is brought to the 

 liver in the form of ammonium lactate. At any rate it is an accepted 

 fact that most, if not all, of the nitrogen of the simple proteins passes 

 through the ammonia stage on its way to excretion as urea. 1 That 

 the formation of urea from ammonia is not exclusively a function of 

 the liver is shown by the fact that it still continues when this organ 

 is excluded from the circulation by means of an Eck fistula. 



1 It has been shown that the liver, kidneys and other organs contain an enzym 

 which splits off the guanidin group from arginin (47) producing urea and ornithin. 



