PROTEINS 251 



drop by drop, stirring and allowing a short interval to elapse 

 between the addition of successive drops. Caseinogen flocks out 

 as a heavy white precipitate. Add acid until the supernatant 

 liquid is clear. Filter and save both the filtrate (x) for use in 

 (e) and the precipitate. Dissolve the precipitate in 2% sodium 

 carbonate, and reprecipitate with acetic acid. This reprecipita- 

 tion is for the purpose of freeing the caseinogen from fat, which 

 is carried down mechanically. It may be repeated several times 

 if a purer product is desired. Redissolve the caseinogen in 2% 

 sodium carbonate, filter through a wet filter to remove more of 

 the fat, and with this solution perform the biuret, Millon, and 

 xanthoproteic tests, and the test for loosely combined sulphur. 

 All should be positive. The presence of phosphorus in caseino- 

 gen already has been observed. 



(e) The filtrate (x) from the caseinogen prepared in (d) con- 

 tains lactalbumin and lactoglobulin, whose presence in milk was 

 demonstrated in (c). The filtrate (x) is already acid. Boil it 

 and add 2% sodium carbonate drop by drop until nearly neutral. 

 If the liquid becomes alkaline, reacidify with a few drops of 

 acetic acid. This treatment removes lactalbumin and lactoglobu- 

 lin. Filter from the precipitate and test the filtrate with Feh- 

 ling's solution for sugar, and for phosphates. Casein is one of 

 the chief constituents of cheese, which also contains much fat. 



(f ) Recall that unboiled milk gives a positive guaiac test. 



ii. Vitellin. This phosphoprotein is found in egg yolk. Ex- 

 tracting the yolk with ether removes the greater part of the 

 lecithin and cholesterol. Vitellin is soluble in 10% NaCl. If the 

 solution is poured into a large beaker of distilled water the vitel- 

 lin precipitates. Vitellin responds to the protein color tests and 

 it may be shown to contain phosphorus ; on digestion with arti- 

 ficial gastric juice, the phosphoproteins leave an insoluble residue. 

 This residue consists of pseudonuclein, which contains phos- 

 phorus. 



4. Nucleoproteins. Nucleoproteins are present in all cells. 

 They may be prepared from various organs and tissues. 



i. Preparation of nucleoprotein from the pancreas. 



