384 PHYSIOLOGY CHAP. 



sented different stages in this oxidation, and as such were the 

 precursors of urea. This theory was inspired by the undeniable 

 fact that the oxidising process is the most important in the 

 animal body, as also by the fact that in many reactions uric acid 

 and creatinine give rise to urea by oxidation. 



Later work, however, has shown that urea cannot be looked 

 on merely as an oxidation product of protein, although it un- 

 doubtedly comes indirectly from the cleavage of the protein 

 molecule. 



We saw in Chapter IV. (p. 211) that protein, when acted 

 upon by the tryptic enzyme, gives rise by simple hydrolytic 

 cleavage to large quantities of peptone, leuciue, and tyrosine 

 (Kiihne). Other researches have shown that the same process 

 gives rise to aspartic acid (Salkowski, Knieriem) as well as to large 

 quantities of glycocoll, on the tryptic digestion of gelatin and 

 collagens in general (Nencki). These same cleavage products 

 (which are known as ammo-acids) are obtained from proteins by 

 long boiling with strong acids or alkalies. According to the most 

 recent work, a series of basic compounds is also formed by the 

 same means, i.e. lysine, arginine and histidine (Hedin). 



All these nitrogenous compounds into which the complex 

 protein molecule breaks up may perhaps be regarded as precursors 

 of urea, the more so since some of them, particularly leucine and 

 tyrosiue, if not normally present in the urine, are found in certain 

 tissues, e.g. spleen and pancreas. 



Experimental evidence shows that some of these substances are 

 converted into urea in the body. On injecting leucine or glycocoll 

 into dogs, they are not found in the urine, but, on the other hand, 

 the urea increases (Schultzen and Nencki, Salkowski). Aspartic 

 acid, again, if given to the animal, reappears in the urine only in 

 the form of urea (Knieriem). 



But the transformation of this and other amino-acids into urea 

 can only be chemically explained by synthetic processes. Leucine, 

 glycocoll, and aspartic acid contain only one atom of nitrogen, 

 while urea contains two. It is, therefore, probable that amino-acids 

 previous to their conversion into urea undergo a further decom- 

 ppsition in the body, leading to the formation of ammonia (NH 3 ). 

 The ammonia combines with the carbonic acid, forming 

 ammonium carbonate, which is again converted, with the loss of 

 one molecule of water, into ammonium carbamate, as shown by the 

 equation : 



C0< -H.,0 = CO 



^ONH 4 



The ammonium carbamate is then converted with loss of a second 

 molecule of. water into urea : 



