424 PROTEINS 



ether is added in order to make the precipitation more com- 

 plete and also to hold any fats which may have been ex- 

 tracted by the alcohol. 



CLASSIFICATION OF PROTEINS. 



The classification of the proteins was originally, for want 

 of chemical knowledge, based on their different physical pro- 

 perties, such as solubilities, coagulation by heat, precipitation 

 by neutral salts, etc. 



Now that, from a study of their products of hydrolysis, a 

 little more is known of the chemistry of the proteins, it is 

 found that, on the whole, the physical method of classification 

 is more or less in accordance with the chemical evidence. 



Appended is the scheme of classification generally adopted 

 in this country : — 



Protamines. — These are the simplest proteins known, and 

 are represented by such substances as salmine, sturine, 

 cyclopterine, etc., which have been isolated from fish 

 sperm.* 



They usually occur associated with nucleic acid in the 

 form of salts. 



No compounds resembling the protamines have as 

 yet been isolated from plants, although they may possibly 

 occur in pollen. 



Histones. — The histones, of which the best known one is that 

 obtained from blood corpuscles, are characterized by being 

 precipitated from solution by ammonia ; they are related 

 to the protamines, but are more complex than these 

 substances. 



Albumins. — This group includes egg-albumin, serum-albumin ; 

 and such vegetable albumins as legumelin of the pea and 

 leucosin of wheat and other cereals. 



The albumins are typically soluble in water and are 

 coagulated by heat. They are not precipitated by satura- 

 tion with sodium chloride or magnesium sulphate, nor 

 by half saturation with ammonium sulphate, but, like 



* Kossel : " Bull. soc. chim. Paris," 1903, [23], 29, i-xviii. 



