PROTEINS 173 



(3) Derived proteins, decomposition products of simple pro- 

 teins. 



The first two of these classes comprise all the natural pro- 

 teins; while the third includes the artificial polypeptides and pro- 

 teins which have been modified by reagents. 



These major classes are further subdivided into the following 

 sub-classes, which depend in part upon the solubilities of the 

 individual proteins, and in part upon the nature of their products 

 of hydrolysis: 



1. The Simple Proteins 



A. Albumins soluble in water and dilute salt solutions, coagulated by 



heat. 



B. Globulins insoluble in water, soluble in dilute salt solutions, coag- 



ulated by heat. 



C. Glutelins insoluble in water or dilute salt solutions, soluble in dilute 



acids or alkalies, coagulated by heat. 



D. Prolamins insoluble in water, etc., soluble in 80 per cent alcohol. 



E. Histones soluble in water, insoluble in ammonia, not coagulated by 



heat. 



F. Protamines soluble in water and ammonia, not coagulated by heat, 



yielding large proportions of diamino-acids on hydrolysis. 



G. Albuminoids insoluble in water, salt solutions, acids, or alkalies. 



2. Conjugated Proteins 



A. Chromoproteins compounds of proteins with pigments. 



B. Glucoproteins compounds of proteins with carbohydrates. 



C. Phosphoproteins proteins of the cytoplasm, containing phosphoric 



acid. 



D. Nucleoproteins proteins of the nucleus, containing nucleic acids. 



E. Lecithoproteins compounds of proteins with phospholipins. 



F. Lipoproteins compounds of proteins with fats, existence in nature 



doubtful, artificial forms easily prepared. 



3. Derived Proteins 



A. Primary protein derivatives. 



a. Proteans first products of hydrolysis, insoluble in water. 



b. Metaproteins result from further action of acids or alkalies, 



soluble in weak acids and alkalies, but insoluble in dilute salt 

 solutions. 



c. Coagulated proteins insoluble forms produced by the action 



of heat or alcohol. 



B. Secondary protein derivatives. 



a. Proteases products of hydrolysis, soluble in water, not coag- 

 ulated by heat, precipitated by saturation of solution with 

 ammonium sulfate. 



