124 THE PROTEINS AND THE AMINO-ACIDS 



these radicals are lacking. The Hopkins-Cole reaction is attributable 



to the indole linkage: 



c 



C 6 H 4 CH 



\ / 



NH 



which is present in the tryptophane (indole aminopropionic acid) 

 radical of protein. It is therefore not given by proteins from which 

 this radical is absent, such as, for example, zein, a protein obtained 

 from corn (maize). 



The biuret-reaction, on the contrary, is yielded by a variety of sub- 

 stances such as oxamide, biuret, etc., which are not proteins. The 

 Deaminized Proteins which are laboratory-products formed by acting 

 upon proteins with nitrous acid, and in which the NH 2 groups have 

 been replaced by hydroxyl-groups, no longer give the biuret-reaction, 

 although they otherwise resemble the natural proteins in physical and 

 chemical behavior. 



THE CLASSIFICATION OF THE PROTEINS. 



American and English biochemists have unfortunately adopted 

 slightly different systems of classification of the proteins. The Ameri- 

 can classification, adopted by the American Physiological Society 

 and the American Society of Biological Chemists, is as follows: 

 I. SIMPLE PROTEINS: 

 Albumins. 

 Globulins. 

 Glutelins. 



Prolamins (alcohol-soluble proteins). 

 Albuminoids. 

 Histones. 

 Prot amines. 



H. CONJUGATED PROTEINS: 

 Nucleoproteins. 

 Glycoproteins. 

 Phosphoproteins. 

 Hemoglobins. 

 Lecithoproteins. 

 HI. DERIVED PROTEINS: 



1. Primary Protein Derivatives: 



Proteans. 

 Met a proteins. 

 Coagulated Proteins. 



2. Secondary Protein Derivatives: 



Proteoses. 

 Peptones. 

 Peptides. 



