THE CLASSIFICATION OF THE PROTEINS 125 



The classification adopted by the British Medical Association is the 

 following : 



I. SIMPLE PROTEINS: 



Protamines. 



Histones. 



Albumins. 



Globulins. 



Glutelins. 



Alcohol-soluble Proteins. 



Scleroproteins. +- 



Phosphoproteins. V 

 H. CONJUGATED PROTEINS: 



Glucoproteinsr 



Nucleoproteins M 



Chromoproteins. * 

 m. PRODUCTS OF PROTEIN HYDROLYSIS: 



Infraproteins. 



Proteoses. 



Peptones. 



Polypeptides. 



. 



Neither of these systems of classification is free from objection. 

 To them both the general objection applies, that the distinctions 

 drawn are largely based upon variations in physical behavior which 

 do not necessarily correspond to fundamental differences of chemical 

 architecture while, on the other hand, many protein or protein-like 

 substances are known which display intermediate characteristics, or 

 individual peculiarities which render their inclusion in any of the classes 

 enumerated, a matter of more or less arbitrary opinion. In particular 

 the defect of the American system lies in the rather intangible dis- 

 tinctions which are made between various classes of primary protein 

 derivatives, and the inclusion of coagulated proteins which are almost 

 certainly derived from native proteins by abstraction of water from 

 the molecule, in the same class with proteans and metaproteins which 

 are derived from native proteins by partial hydrolysis, is unfortunate. 

 The English classification has the merit of simplicity, but it would be 

 more advisable to include the phosphoproteins among the conjugated 

 proteins, as in the American classification, and to add a fourth group 

 to accommodate the coagulated proteins. The term prolamine is also 

 preferable to the designation a alcohol-soluble proteins," because it 

 draws attention to the high content of the amino-acid proline which 

 characterizes these proteins more fundamentally than the physical 

 property of solubility in eighty per cent, alcohol. 



The salient characteristics of these various classes of protein sub- 

 stances are as follows: 



