154 PHYSIOLOGY 



We might represent these changes as follows : 



(1) CH 3 CH 3 



I I 



CHNH 2 + = CO + NH 3 



I I 



COOH COOH 



(2) CH 3 



I CH 3 



CO +0 = 1 + C0 2 



I COOH 



COOH 



Is the reverse change ever effected in the animal body ? If it were 

 possible to replace the OH group in an oxy-fatty acid by NH 2 or the in an 

 a ketonic acid by HNH 2 , it ought also to be possible to nourish an animal 

 from a mixture of carbohydrates and ammonia, or at any rate by supplying 

 him with a mixture of the appropriate oxy-acids or ketonic acids and am- 

 monia. Until recently there was no evidence that the animal body is able to 

 utilise nitrogen, except in organic combination as amino-acids or the complex 

 aggregate of amino-acids known as proteins. In the plant the process 

 of synthesis of protein from ammonia and a carbohydrate such as hexose is 

 continuously going on, and it is probable that the formation of amino-acid 

 occurs by a process the reverse of that which we have just been studying. 

 Knoop has shown that the same reversed change may occur even in a 

 mammal, and that here again the intermediate substance is an a ketonic 

 acid. On administering benzylpyruvic acid (C 6 H 5 .CH 2 .CH 2 .CO.COOH) 

 to a dog, a certain amount of benzylalanine (CeHg.CHaCHgCHNHg.COOH) 

 appeared in the urine. The first phase of the oxidative deamination of 

 amino-acids is thus a reversible one and may be represented : 

 R R R 



I I OH ^ il 



CHNH 2 + ^ > C< < ^ CO + NH 3 



I | NH 2 | 



COOH COOH COOH 



(3) DECARBOXYLATION. Many amino-acids when subjected to the 

 agency of bacteria lose a molecule of carbon dioxide and are converted into 

 a corresponding amine. 



For instance, lysine, which is diamino-caproic acid, is converted into 

 pentamethylene diamine or cadaverine. Thus : 



CH 2 .NH 2 CH,.NH a 



I I 



CH 2 CH, 



I I 



CH 2 becomes CH 2 



I I 



CH 2 CH 2 



I I 



CH.NH, CH 2 .NH 2 



I 

 COOH 



