2l6 COMPOSITION OF THE DIGESTIVE JUICES. [CH. VIIl. 



(2). Thus, suppose that just before the end point is reached 4-5 cc. 

 of the 0-2 N. soda have been added to (3). Add 4-5 cc. of water to 

 (i) and to (2), and then complete the titration till exact equality of 

 tint is obtained. 



Calculation. 



1000 cc. N. soda =* 14 grams, of amino-acid nitrogen, 



i cc. of 0-2 N. soda =2-8 mgm. amino-acid nitrogen. 



If (a) = amount of 0-2 N. soda required for 20 cc. of the 

 digestion mixture immediate!}' after the addition of the enzyme, 

 and (b) the amount after an interval of (t) minutes, then 

 [(b) (a)] x 5 x 2-8 = mgms. of amino-acid nitrogen liberated in 

 100 cc. of the digestion mixture in (t) minutes. 



It will be found that the rate of digestion, i.e. the mgms. of 

 amino-acid nitrogen liberated in i minute, is greatest at the com- 

 mencement of the digestion, falling off as the concentration of the 

 substrate decreases. 



NOTES. i. If a parallel experiment be conducted with chloroform 

 instead of toluol as the antiseptic, it will be found that the digestion rate is 

 considerably less. For that reason it is preferable to use toluol instead of 

 chloroform for the preservation of trypsin solutions and as an antiseptic in 

 digestions. 



2. The percentage amount of the casein digested is much greater in 

 dilute than in strong solution. The addition of an equal volume of water will 

 induce a greater degree of hydrolysis in the above experiment than doubling 

 the amount of enzyme. 



261. The products of tryptic digestion of casein. 



The final products are given on pages 67 to 69. The methods 

 of separation are discussed on p. 70, and the details of the special 

 methods required for the isolation of tryptophane, tyrosine and 

 leucine are given in Exs. 87, 89 and 93 respectively. The following 

 exercise is inserted for the benefit of junior students as illustrating 

 the principles of the methods used. 



Digestion. The solution and digestion is carried out as de- 

 scribed in Ex. 87 A (i.) to (x.), but the digestion period may with 

 advantage be extended to about 2 weeks. The bottle is removed 

 from the incubator, and the contents transferred to a 3 litre flask, 

 and heated on a boiling water bath or in a steamer for about i hour. 

 The mixture is then filtered hot and a portion evaporated in the 



