ELEMENTARY CHEMICAL PHYSIOLOGY 199 



I. Note that it is precipitated with 1 per cent, acetic acid, the 



precipitate being soluble in excess of acid. 

 II. Perform the colour tests for protein, and record your results. 



III. Perform the " salting out " tests with (NH 4 ) 2 S0 4 and MgS0 4 . 



IV. Heat the solution. 



With the solid substance perform the following experiments : 



V. Heat some solid caseinogen upon a piece of broken porcelain with 

 " combustion mixture " (a mixture of sodium carbonate and 

 potassium nitrate). When cool, extract with nitric acid, filter, 

 add ammonium molybdate in nitric acid, and heat. The canary 

 yellow precipitate denotes presence of phosphorus. 



VI. Heat a little caseinogen with 1 per cent. NaOH in the incubator or 

 on a water bath at 37 C. for twenty-four hours. Phosphoric 

 acid is broken off. Precipitate the phosphoric acid, after acidify- 

 ing with acetic acid and filtering, by the addition of ammoniacal 

 magnesium citrate. Filter. Dissolve the precipitate in nitric 

 acid, and test with molybdate as above. 



In connection with the above experiments it will be found that caseinogen 

 yields all the colour tests except Molisch. It therefore contains no carbo- 

 hydrate group (see p. 210). The xanthoproteic, Millon's, and the glyoxylic 

 tests will be very well marked, showing that caseinogen is rich in tyrosin and 

 tryptophan. 



In " salting out " caseinogen behaves like a globulin, being 

 precipitated by full saturation with magnesium sulphate and half 

 saturation of ammonium sulphate. 



Caseinogen is not coagulated by heat. 



The Sclero-proteins comprise the group of proteins formerly 

 termed albuminoids. They are obtained mainly from " the hard " 

 or supporting structures of the body. Those of physiological 

 importance are Collagen, Elaslin and the Keratins. 



Collagen, the precursor of gelatin, forms the chief constituent of 

 white fibrous tissue and of the organic substance of bone. It also 

 exists in cartilage, where, how r ever, it is mixed with several other 

 bodies (see under gluco-proteins, p. 201). 



Preparation of Collagen. A piece of tendon is macerated overnight 

 in 1 per cent, caustic alkali to remove other proteins, and then 

 washed with water till alkali free. The resulting mass is collagen. 

 Place a piece of this in a flask and boil it for ten minutes with water 

 which is rendered faintly acid with acetic acid. By this treatment, 

 the collagen is transformed into gelatin and, on cooling the solution, 

 it gelatinises. Solutions containing about 1 per cent, and upwards 

 set to a jelly on cooling. If the process of heating and cooling be 

 repeated too often the solution ceases to set. 



Gelatin. This is really the hydride of collagen, the boiling with 

 acidulated water in the above experiment having caused the collagen 

 to take up a molecule of water. Conversely, the gelatin can be 

 reconverted into collagen by heating it to 130 C., whereby it loses 

 water. 



EXPERIMENT. Apply the following tests to a solution of gelatin 

 in water : (1) The Biuret reaction : a violet colour is produced. 



