METAPROTEINS 95 



Glycoproteins. The glycoproteins are to be considered as compounds 

 of protein and a carbohydrate complex. The carbohydrate group can be 

 split from the protein by boiling with mineral acids or by the action of alka- 

 lies. The group of glycoproteins includes a number of proteins, of which the 

 mucines and mucoids are the most important. 



Mucines are very widely distributed. They give the mucilaginous 

 character to many secretions and are formed and discharged through the 

 respiratory, digestive, and other tracts, partly by mucous cells and in part by 

 the mucous glands, especially by the submaxillary and sublingual sali- 

 vary glands, and in the bile passages. On hydrolysis the mucines are split, 

 the carbohydrate moiety yielding glucose amine or galactose amine. 



Mucoids occur in the connecting tissues along with the albuminoids. 

 They are found especially in the tendon, bone, and cartilage. They are 

 combinations of protein and a carbohydrate containing ethereal sulphuric 

 acid known as chondroitin sulphuric acid. On cleavage, besides the products 

 formed from the protein, they yield sulphates and a reducing substance. 



Phosphoproteins. The phosphoproteins, sometimes called nucleo- 

 albumins, are compounds of the protein molecule with some as yet unde- 

 fined phosphorus-containing substance other than a nucleic acid or lecithin. 

 While the phosphorus content of these substances is quite similar to that 

 of the nucleoproteins, they do not yield any purine or pyrimidine bases on 

 hydrolytic cleavage. Two of the best known phosphoproteins are the 

 casein of milk, and vitellin of the egg yolk. The phosphorus is apparently 

 present as a phosphoric acid ester. 



Hemoglobins. These are compounds of the simple protein histone, 

 with an iron- or copper-containing pigment substance. The hemoglobins 

 are more fully discussed in the chapter on the Blood. 



Lecithoproteins. These are combinations of proteins and a fat-like 

 substance, lecithin. Lecithin is a compound of fatty acids, glycerin, phos- 

 phoric acid, and an ammonium-like organic base, choline. The combination 

 of lecithin and protein is apparently a loose one: the lecithin ordinarily can 

 be split off by boiling alcohol. The lecithoproteins include substances 

 commonly termed lecithans and phosphatids. 



The derived proteins are formed as intermediate products in the hydro- 

 lytic cleavage of the original protein molecule. The primary protein de- 

 rivatives are (l apparently formed through hydrolytic changes which involve 

 only slight alteration of the protein molecule." 



Metaproteins. These are formed from the simple proteins by the 

 action of weak acids and alkalies. This class comprises what have com- 

 monly been termed acid and alkali albuminates. The metaproteins are 

 soluble in acid or alkaline solution, but are insoluble in neutral solutions. In 

 the formation of alkali metaproteins, the sulphur in organic combination 



