162 INTERMEDIATE METABOLISM 



sulphate to precipitate it. It is now known as a primary 

 proteose or alburaose. 



In the third stage it is precipitated only on full satura- 

 tion with ammonium sulphate. This is a secondary proteose 

 or albumose. 



In the fourth stage the molecule is sufficiently small to 

 diffuse through an animal membrane. It cannot be pre- 

 cipitated. This is a peptone. 



In the fifth stage diifusibility has increased. The molecule 

 is now a polypeptide. 



The final stage is the separation into individual amino- 

 acids. 



It must be realised that, notwithstanding these stages, 

 the process is essentially a continuous one, involving a gradual 

 disintegration of the protein molecule. Further, the pro- 

 cess takes place irregularly, so that at any stage molecules 

 of different sixe are present. 



The importance of recognising the above stages Ues in 

 the light thus thrown upon the action of the different pro- 

 teolytic ferments. 



Pepsin, acting only in presence of free hydrochloric acid, 

 converts protein into a mixture of proteoses and peptones. 

 Trypsin, acting in an alkahne medium, converts protein 

 through all its stages into polypeptides and amino-acids, 

 but it appears to be incapable of breaking down all poly- 

 peptides into amino-acids. Erepsin, also alkahne, has no 

 action upon proteins, but converts peptones and poly- 

 peptides completely into amino-acids. 



The succession of an acid by an alkahne digestion occurs 

 not only in all animals, including even Amceba, but also 

 in insectivorous plants. It appears that certain protein 

 hnkages are more readily sundered by an alkahne ferment 

 after other linkages have been broken by an acid ferment. 



* Absorption of Proteins 



The proteins found in the various tissues differ from 

 one another not in containing different amino-acids, but 



