112 PROTEINS 



Nucleoproteins. These are basic proteins, e.g., histones, in combination 

 with nucleic acids. They are obtained most readily from thymus and 

 other glands, yeast, and wheat germ. 



Glycoproteins or Glucoproteins. The prosthetic group is carbohydrate 

 in character, and makes up a large proportion (25-35 per cent) of the 

 glycoprotein. As a consequence, the nitrogen content of glycoproteins 

 is low, 9-13 per cent. Example^ are: proteins from saliva (mucin), 

 vitreous humor, gastric mucosa, and jellyfish. 



Phosphoproteins. The prosthetic group is phosphoric acid, linked as 

 an ester to the protein through a hydroxyamino acid, e.g., serine. Two 

 of the best known proteins, casein and vitellin, belong to this class. 



Chromoproteins. The prosthetic group is colored. For example: 

 hematin of hemoglobin is red, cyanin of hemocyanin (the respiratory pig- 

 ment in the blood of the lobster and other molluscs) is blue, melanin of 

 hair proteins is black, riboflavin phosphate of flavoproteins (respiratory 

 enzymes) is red, and retinene (aldehyde of vitamin A) of rhodopsin (the 

 chromoprotein involved in vision) is yellow. -^ 



Lipoproteins. The prosthetic group is fatty acid, lecithin, or a phos- 

 pholipide other than lecithin. Lipoproteins are a poorly defined group 

 occurring in egg yolk, brain tissue, lungs, etc. 



Derived proteins 



This division includes denatured proteins and cleavage products formed 

 by partial hydrolysis of naturally occurring proteins with acids or en- 

 zymes. The hydrolysis products are polypetides of varying size, and 

 the subdivisions have little chemical justification. The principal classes 

 still in use are: 



Proteoses. Hydrolytic products of proteins that are soluble in water, 

 not coagulable by heat, and precipitated by saturating the solution con- 

 taining them with ammonium sulfate. 



Peptones. These polypeptides are probably of smaller molecular 

 weight than the proteoses since they are found in the filtrate of the 

 ammonium sulfate precipitation of proteoses. So-called "peptones," used 

 in the preparation of bacteriological media, are mixtures of polypeptides, 

 mainly proteoses. 



Peptides. Combinations of two or more amino acids. They are called 

 di-, tri-, tetra-, etc., peptides, according to whether they contain two, 

 three, four, or more amino acid residues in the niolecule. Some peptides 

 occur naturally, e.g., glutathione, pteroylglutamic acid (a vitamin), peni- 

 cillins, etc. Large numbers of peptides have been synthesized. 



