43 



COOH 



From the fact that these three bodies (especially arg'iiin) occur 

 largely in the simplest proteins known (protamines), and appear to 

 be among the very last bodies split off by hydrolysis from more 

 complex proteins, it has been thought that they form the central 

 nucleus of protein. 



Cystin is notable for the amount of sulphur which it contains. It 

 is di-amino-di-thio-lactylic acid : 



HNH 2 CHNH 2 



COOH COOH 



It is found largely in the keratin of hair, horn, nails, and hoofs. It 

 crystallizes in colourless hexagonal plates. 



The pyrimidin and purin bases are obtained chiefly from the group 

 of proteins known as nucleoproteins, and are more fully discussed later 



When protein is subjected to hydrolysis by the digestive juices, 

 it is partly converted by the action of the acid or alkali present into 

 a derivative of protein known as metaprotein. A considerable amount 

 of ammonia is also split off. The hydrolysis of protein may be repre- 

 sented as follows : 



Protein - * Metaprotein 



