114 PHYSIOLOGICAL CHEMISTRY 



thetic group. Among the conjugated proteins are substances of 

 great biological importance. 



Glycoproteins. The glycoproteins are compounds which on 

 breaking down yield a protein and a relatively high percentage 

 of a carbohydrate or carbohydrate derivative. Other proteins 

 also yield carbohydrates on hydrolysis, but in smaller amounts 

 than the glycoproteins. The group is divided into two divisions, 

 mucins and mucoids. They are very difficult to purify, espe- 

 cially the mucins, since they are slimy in character, and as a 

 result there is much disagreement as to their composition and 

 properties. Mucin is secreted by the salivary glands, by certain 

 mucous membranes and elsewhere. The skin of some of the 

 lower animals secretes large quantities of mucin. The mucins are 

 distinguished from the mucoids by their slimy character and by 

 the fact that on precipitation with acetic acid, they do not re- 

 dissolve in excess of acid. The mucoids are found in tendons, in 

 cartilage, in the cornea and crystalline lens, in egg white and 

 various other places. Some authors divide this group again into 

 mucoids and chondroproteins. The mucoid of tendon or carti- 

 lage may be extracted with lime water. On acidifying with 

 acetic acid, the mucoid precipitates, but it will redissolve in ex- 

 cess. Both the mucins and the mucoids contain a relatively high 

 amount of sulphur, a part of which, at least, is in the form of 

 oxidized sulphur. On hydrolysis the mucoids yield a substance 

 chondroitic acid or chondroitin sulphuric acid, which has been 

 the subject of much study. The carbohydrate radicle in the 

 glycoproteins is usually glucosamine or galactosamine. 



Phosphoproteins. The phosphoproteins ( are compounds of 

 importance because they furnish a large share of the protein 

 nourishment of the growing young of animals and birds. There 

 are two important members of the group, the casein of milk and 

 the vitellin of egg yolk. They are characterized by containing a 

 relatively high amount of phosphorus (about 1%) which is 

 present in some form neither lecithin nor nucleic acid. On 

 digestion, the phosphoproteins leave a difficultly digestible resi- 

 due known as pseudonuclein. The phosphoproteins are acid in 



