CH. VIII.] EREPSIN. 219 



gives a more active preparation. Toluol should be added to the flask in 

 which the nitrate is collected. The various enzymes in the fluid are not very 

 stable, so that the material must be obtained and the extract made not more 

 than two days before it is required. 



262. Action of erepsin on peptone. Label two 150 cc. 

 flasks A and B, and into each place 100 cc. of a 2 per cent, solution 

 of commercial peptone. To A add 10 cc. of the filtered extract and 

 2 or 3 cc. of toluol, shake well, and stopper with a cork. Boil 

 another 10 cc. of the extract to destroy the enzymes, cool, and add to 

 B. Add tolnol and stopper. Incubate the flasks at 38 to 40 C. 

 for 24 to 48 hours or longer. 



(i.) To 5 cc. of each add two or three drops of strong acetic 

 acid and then bromine water, drop by drop (see Ex. 261). A pink 

 colour is obtained in A, but not in B, showing that the tryptophane 

 bound in the peptones has been set free by the action of a ferment. 



(ii.) Titrate 20 cc. of A and B according to the directions given 

 in Ex. 260. A considerable increase in amino-acid nitrogen results, 

 owing to the splitting of the peptide linkages of the peptones by the 

 action of erepsin. 



(iii.) To i cc. of A and B add 4 or 5 cc. of water, 2 drops of i 

 per cent, copper sulphate, and i or 2 cc. of 5 per cent. soda. A 

 strong biuret reaction (Ex. 24) is given by B, whereas A may give 

 none, or only a feeble reaction. 



NOTE. It is not always possible to obtain a solution that fails to give 

 the biuret test. It seems to depend on the quality of the peptone 

 employed. 



263. Action of erepsin on casein. Prepare a 2-5 per cent, 

 solution of casein in dilute alkali by diluting i part of the solution 

 described in Ex. 87 with 3 parts of water. To 50 cc. add 10 cc. 

 of the intestinal extract and toluol and label A. To another 50 cc. 

 add 10 cc. of water (or of a boiled intestinal extract), tolvol, and 

 label B. Incubate the stoppered flasks for 2 to 7 days at 38 to 40. 



To portions of the resulting solutions carefully add dilute acetic 

 acid, drop by drop. A precipitate of unchanged casein is pro- 

 duced in B. In A the precipitate is much less or may be absent. 



To the acidified solutions thus obtained add bromine water, 

 drop by drop. A reaction for free tryptophane is produced in A, 

 but not in B. 



