250 PHYSIOLOGICAL CHEMISTRY 



consists of the globin. This may be filtered off, and it will be 

 found to give the usual protein color tests. 



v. Hemoglobin, although crystallizing readily, differs from 

 other crystalloids in not diffusing through an animal membrane. 



3. Phosphoproteins. The phosphoproteins are compounds 

 consisting of a protein combined with some phosphorus-contain- 

 ing substance other than nucleic acid or lecithin. These proteins 

 often are called nucleoalbumins ; they differ from the nucleopro- 

 teins by containing no purine bases. Two members of the group 

 will be studied, casein from milk and vitellin from egg yolk. In 

 connection with these two substances, some other constituents of 

 milk and egg yolk will be considered. 



i. Caseinogen. 



(a) Test with litmus the reaction of the milk furnished. 

 Fresh milk is neutral or faintly alkaline. 



(b) Boil a few cubic centimeters ^ of milk. Observe that the 

 caseinogen does not coagulate. In this respect caseinogen differs 

 from most other proteins. If a skin forms over the surface of the 

 milk, it is due partly to evaporation from the surface of the 

 liquid, partly, perhaps, to coagulation of other milk proteins. 



(c) To a few cubic centimeters of milk add an equal volume 

 of saturated ammonium sulphate. The precipitate consists main- 

 ly of caseinogen. In this respect caseinogen resembles the glob- 

 ulins. The member of this group occurring in milk also is pre- 

 cipitated with caseinogen, but its amount "is extremely small. 

 Filter the mixture. Divide the filtrate from the caseinogen into 

 two portions. Heat one to boiling, acidifying slightly if neces- 

 sary. The slight precipitate is lactalbumin. Saturate the sec- 

 ond portion with ammonium sulphate. Albumin is precipitated. 



(d) The usual method for the preparation of caseinogen con- 

 sists in precipitating with dilute acetic acid. This process is 

 analogous to the clotting of sour milk ; by the action of bacteria 

 milk sugar is fermented. The resulting lactic acid, when present 

 in sufficient concentration, causes the caseinogen to precipitate. 

 Dilute 25 c.c. of milk with three times its volume of water, warm 

 slightly (about to body temperature) and add 1% acetic acid 



