PROTEINS 99 



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 

 character. In their solubilities and precipitation properties they 

 resemble both the globulins and the nucleoproteins. They may 

 be distinguished from the former by their phosphorus content, 

 and from the latter by the fact that they yield no purine bases 

 on hydrolysis. Cow's milk contains about 3-4% casein, human 

 milk about 0.5-1.5%. Casein is not coagulated by boiling. It is 

 insoluble in water, but dissolves readily in dilute alkalies. From 

 such a solution, and from milk, casein is precipitated by the 

 addition of a small amount of acid. This occurs also in the 

 souring of milk. Bacteria decompose the lactose, forming or- 

 ganic acids. From this acid solution the casein is precipitated, 

 causing the characteristic clotting of sour milk. It may be pre- 

 pared by diluting milk, and adding a small amount of acetic 

 acid. To free the casein from fat it may be redissolved in sodium 

 carbonate and reprecipitated with acid. This process should 

 be repeated several times if a pure product is desired. Casein 

 contains no glycocoll, but little cystine and relatively much tryp- 

 tophane. On hydrolysis it yields also several acids which are 

 not obtained from other proteins. Casein is clotted by a fer- 

 ment, rennin, which occurs in digesive juices, particularly the 

 gastric juice. It may be well to state in this connection that the 

 nomenclature of casein and its allied substances is somewhat 

 confused. By many investigators the substance as it occurs in 

 milk is called caseinogen. This is converted by rennin into 

 another substance, paracasein. Paracasein differs from casein- 

 ogen at least in one respect, the insolubility of its calcium salt. 

 The calcium salt of caseinogen is soluble, that of paracasein in- 

 soluble. Since calcium salts are found normally in milk, para- 

 casein is precipitated as the familiar curdy material. If calcium 



