16 



VETERINARY PHYSIOLOGY 



certain bacteria other lines of breaking down may occur, but 

 these have thrown less important light upon the constitution 

 of the proteins. 



As the molecules become smaller the colloidal characters 

 decrease, dittusion through animal membranes becomes more 

 marked, and the tendency to precipitate is lessened. This 

 latter property has been used to classify the proteins into 

 three main groups — 



Precipitated by — 



1. Native Proteins boiling and by saturation with (NH4)2S04. 



2. Proteoses. . not by boiling but by saturation with 



(NHJpSO, 



3. Peptones . . not by boilina;, nor by saturation with 



(NH,)2S0',. 



Products of Disintegration. — Under the influence of a 

 prolonged action of acids and of certain enzymes such as 

 the erepsin of the intestine (p. 328) the splitting of the 

 molecule is carried further, till finally the greater part of 

 the nitrogen of the protein comes to be distributed in bodies 

 of three different classes — 



1. Mon-amino acids .... 



2. Di-amino acids .... 



3. Amides and ammonia compounds . 



1. Mon-amino Acids are thus the bodies from which the 

 proteins are chiefly formed, and the character of the proteins 

 of different animals and of different tissues depends largely 

 upon the mon-amino acids which predominate. 



The different proportions of these which occur in 

 different proteins is indicated by the following table : — 



