150 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



the case of arginine the exact chemical decomposition which takes 

 place is known. We have already seen that arginine is a compound 

 of a urea radical and a substance called ornithine (di-amino-valeric 

 acid, see p. 32) ; the liver is able to hydrolyse arginine, and so urea 

 is liberated. This power is due to the action of a special ferment 

 called arginase, which, although it is also found in other organs, is 

 specially abundant in the liver. It is, however, possible that the 

 ornithine itself may be further broken up, and so an extra quantity of 

 urea formed. On the other hand there are some amino-acids (e.y. 

 tyrosine) which on injection do not lead to any increase in urea 

 formation. 



If, however, we glance at the formula of ornithine, we see that it 

 has one point in common with other amino-acids, such as glycine and 

 leucine, to take simple examples : 



Ornithine 5 H 12 N 2 02 

 Glycine C 2 H 5 NO 2 

 Leucine C 6 H 13 N0 2 



This is, that in all cases the carbon atoms are more numerous than 

 the nitrogen atoms. In urea, CON 2 H 4 , the reverse is the case. The 

 amino-acids must therefore be split into simpler compounds, which 

 unite with one another to form urea. Urea formation is thus in 

 part synthetic. These simpler compounds are ammonium salts. 

 Schroder's work proves that ammonium carbonate is one of the 

 urea precursors, if not the principal one. The equation which 

 represents the reaction is as follows : 



(NH 4 ) 2 C0 3 =CON 2 H 4 + 2H 2 



[ammonium [urea] [water] 



carbonate] 



Schroder's principal experiment was this : a mixture of defibrinated 

 blood and ammonium carbonate was injected into the liver by the 

 portal vein ; the blood leaving the liver by the hepatic vein was found 

 to contain urea in great abundance. This does not occur when the 

 same experiment is performed with any other organ of the body, so 

 that Schroder's experiments also prove the great importance of the 

 liver in urea formation. Similar results were obtained by Nencki 

 with ammonium carbamate. 



We must further remember that ammonia itself is one of the 

 products of digestion of protein in the intestine, and it may possibly, 

 to a small extent, be a result of tissue katabolism. This ammonia 

 passes into the blood, where it unites with carbonic acid to form either 

 the carbamate or carbonate of ammonium. Thus ammonia, whether 



