ADVANCED PHYSIOLOGICAL CHEMISTRY 451 



Separation and Properties of the Chief Products of Tryptic Digestion 

 of Proteids. 200-250 gr. fibrin, or the whites of four eggs, are digested 

 in alkaline reaction for 48 hours with a minced pancreas, several 

 crystals of thymol being added to prevent excessive bacterial growth. 

 The digest is made very faintly acid, and boiled to remove any native 

 proteid or alkali albumin which it may contain. A sample of the 

 filtrate is removed and tested for proteose. A negative result is usually 

 obtained, since no primary proteoses are developed during tryptic 

 digestion. 



1. Separation of Tyrosin. The remainder is evaporated on the 

 water-bath to a thin syrup. This is allowed to stand on ice or in 

 a cold place for several days. White flocculi of tyrosin separate out. 

 These are filtered through fine muslin, and removed to a beaker by 

 means of a jet of cold distilled water and washed several times with 

 distilled water by decantation. They are then dissolved by boiling 

 with water made alkaline by the addition of a few drops of ammonia, 

 and the resulting solution is quickly filtered hot. The filtrate is 

 heated till all the ammonia is expelled; it is then cooled, when the 

 tyrosin separates out as a white precipitate. This is collected on a 

 filter paper, washed, and dried. The following reactions may be 

 applied to the resulting powder : 



(1) Tyrosin is insoluble in cold water, slightly soluble in hot water, 

 and very soluble in dilute alkali. 



(2) A solution in hot water gives a red colour on the addition of 

 Millon's reagent. This is because tyrosin contains an aromatic radicle 



(3) Piria's Test. Place some of the powder in a dried test-tube, add 

 about 2 c.c. concentrated sulphuric acid, and place the test-tube in 

 a boiling water-bath for half an hour. Now cool and dilute with 

 water, transfer to an evaporating basin, and remove the sulphuric acid 

 by adding powdered barium carbonate ; filter off the barium sulphate, 

 evaporate the filtrate to small bulk, and add a drop or two of very 

 weak ferric chloride solution. A violet colour results. This reaction 

 is due to the formation of ty rosin-sulphuric acid. 



2. Separation of Leucin. The tyrosin-free filtrate is evaporated till 

 a skin of leucin forms on the surface. It is then mixed with several 

 times its bulk of alcohol, whereby the antipepton (see p. 449) which 

 it contains is precipitated, the leucin remaining in solution. The 

 precipitate is removed by filtration, and the filtrate evaporated to 

 dryness : the residue is mainly Leucin. 



Reactions of Leucin. (1) It is much more soluble in water than is 

 tyrosin ; it is soluble also in alcohol. 



