34 THE PROTEINS. [CH. II. 



1. Protamines. Basic substances forming stable salts with 

 mineral acids, and containing a high percentage of nitrogen. On 

 hydrolysis they yield only a few of the amino acids, and these are 

 mainly the bases. They occur in the heads of ripe spermatozoa and 

 in ova. 



2. Histones. Similar to the protamines, but less rich in 

 nitrogen and the basic amino acids. They are, however, more 

 basic than the majority of the proteins, and are precipitated by 

 ammonia. They are found in unripe spermatozoa, the stroma of 

 red blood corpuscles, and in lymphoid tissue. 



3. Albumins. Soluble in distilled water and coagulated by 

 boiling. 



4. Globulins. Insoluble in distilled water, soluble in dilute 

 salt solutions. Coagulated by boiling. 



Groups 3 and 4 are sometimes known as " native " proteins. 



5. Glutelins. Found in abundance in vegetables. Insoluble 

 in neutral solvents, but soluble in acids and alkalies. 



6. Prolamines [Gliadins (B)]. Also found in vegetables, but 

 distinguished from the glutelins by their solubility in 75 per cent, 

 alcohol. 



7. Albuminoids [Scleroproteins (B)]. Found in the skeletal 

 and connective tissues of animals. They are characterised by their 

 insolubility in most reagents. Examples are keratin, elastin, and 

 collagen (the anhydride of gelatine). 



8. Phospho-proteins. Rich in phosphorus. They must be 

 carefully distinguished from the nucleoproteins. Examples are the 

 casein [caseinogen (B)] of milk and vitellin of egg yolk. 



9. Conjugated Proteins. Proteins united to a non-protein 

 group. 



(i.) Chromoproteins. Protein -f pigment molecule, e.g. haemo- 

 globin. 



(ii.) Glycoproteins [Glucoproteins (B)]. Protein + carbohy- 

 drate group, e.g. mucin. 



(iii.) Nucleoproteins. Protein + nucleic acid. These are prob- 

 ably indefinite salts of nucleic acid with proteins (see 

 p. 60). 



