36 PHYSIOLOGY. 



If serum-albumin, egg-albumin, or washed muscle is acted on by 

 an alkali, instead of an acid, the proteid undergoes changes similar to 

 those produced by the acid, except that the product formed is an 

 alkali-albumin instead of an acid one. 



3. Compound Proteids, Conjugated Protein, or Proteides of Germans. 



These are native proteids with another organic substance, in 

 contrast to albuminates, which are compounds of native proteids 

 with inorganic substances. The compound proteins include (1) 

 glucoproteins, like mucin, consisting of a proteid combined with a 

 carbohydrate group; (2) nucleoproteins are built up of albumins, 

 nucleic acid, and always contain iron. They exist in the cell-nu- 

 cleus; (3) phosphoproteins, like casein of milk and vitellin of yelk 

 of eggs; (4) chromoproteins, like haemoglobin. 



Tests for Proteids. (A) COLOR TESTS. (1) The biuret test 

 of Eose and Wiedemann, when a solution of albumin is made 

 strongly alkaline with caustic potash, and a solution of copper sulphate 

 is added drop by drop, then a pink-violet color is produced. 



(2) The xanthoproteic test of Fourcroy and Vauquelin. Add 

 nitric acid, and a white precipitate ensues, which, on being boiled, 

 turns yellow ; on cooling, add ammonia ; the yellow-colored precipitate 

 becomes orange. This reaction depends upon the presence of the 

 benzol ring in the proteid molecule (phenylalanin, tyrosin, indol). 



(3) Millon's reagent. This reagent is a solution of mercuric 

 nitrate in water containing free nitrous acid. On adding it to a 

 solution of albumin, a whitish precipitate ensues, which becomes a 

 brick-red on boiling. This reaction indicates the presence of oxy- 

 phenol group (tyrosin). 



(B) PRECIPITATION OF PROTEIDS BY NEUTRAL SALTS. (1) 

 Saturation with ammonium sulphate precipitates all proteids except 

 peptones. 



(C) PRECIPITATION OF PROTEIDS BY ACIDS. (1) Add a drop 

 or two of strong nitric acid, a white precipitate ensues. 



4. Globulins. 



The globulins are quite abundant. The globulins differ from 

 the albumins in that they are not soluble in distilled water. There 

 must be present an appreciable amount of sodium chloride or mag- 

 nesium sulphate. 



Globulins are insoluble in saturated solution of all the neutral 



