AMMONIA 167 



While the process of deaminisation seems to occur in all 

 hving cells, the formation of urea occurs pre-eminently in the 

 hver. Ammonium carbonate perfused through the hver 

 is converted into urea. When in the living animal the 

 hver is short-circuited by leading blood direct from the 

 portal to the hepatic vein (Eck fistula) ammonia accumulates 

 in the blood. But even under these circumstances urea 

 formation does not cease. The hver, therefore, though the 

 principal, is not the sole seat of the change. 



A small amount of urea may be derived from arginine, 

 the amino-acid which contains the guanidine group. 

 Several tissues contain an enzyme, arginase, which has the 

 power of splitting arginine into urea and ornithine. 



NH.— C— NH— CH2— CH2— CH2CHNH2COOH 



11 Arginine. 



NH 

 NH2— CO + NHo— CH2— CH2CH2CHNH2COOH 



I Ornithine. 



Urea. 



The Excretion of Ammonia 



When ammonia, spht off from amino-acids, combines with 

 an acid radicle other than CO2 it is excreted as an ammonium 

 salt. If it combines, for instance, with chlorine it is 

 excreted as ammonium chloride. When abnormal acids 

 accumulate in the blood as /3 -hydroxy butyric acid in 

 diabetes, ammonium salts of these acids are formed and 

 excreted. The ammonia may be said to be diverted from 

 its normal metabolic path in order to neutralise the acids. 



Synthesis and Inter-conversion of Amino-acids 



Can the body synthesise amino-acids from ammonia and 

 a non-nitrogenous group, and can it transform one amino- 

 acid into another ? These questions are of fundamental 

 importance, for upon the answers to them depends the 



