CH. VIII.] RENNIN. 



has found that certain of the preparations sold recently 

 contain pepsin rather than rennin, due to the fact that the 

 shortage of calves' stomachs has necessitated the use of 

 pigs'. 



253. Measure 5 cc. of fresh milk into a tube and place it in a 

 water bath at 38 C. for a few minutes. Add I cc. of an active 

 preparation of rennet ; mix, replace in the bath, and observe from 

 time to time. Note that the milk sets to a solid mass, so that the 

 tube can be inverted without spilling the contents. Replace the 

 tube in the bath and examine it again after about an hour. Note 

 that the clot has contracted, and that a nearly clear " whey " has 

 been expressed. 



254. Repeat the experiment with I cc. of a 0-5 solution of 

 commercial pepsin in water. A similar result is obtained. 



255. To 10 cc. of milk add 3 cc. of 0-2 N. ammonium oxalate 

 to remove the calcium ions. Mix, and divide into three equal 

 portions, and place these in three tubes, labelled A, B and C. 



To A add i cc. of N. calcium chloride and i cc, of rennin. 



To B add i cc. of rennin. 



To C add i cc. of boiled rennin. 



Place the three tubes in the water bath for 10 minutes. Note 

 that A clots and that B and C do not. 



Boil B (to destroy the rennin) and cool under the tap. 



To B and C add i cc. of N. calcium chloride. A flocculent pre- 

 cipitate of calcium paracaseinate is produced in B, indicating that 

 the enzyme has acted on the casein in the absence of the calcium 

 salts, but that calcium is necessary for the formation of the pre- 

 cipitate. 



NOTE. If the experiment be repeated with pepsin it will be found that on 

 boiling B and adding CaClg little or no effect is produced. This is due to the 

 fact that pepsin only seems to act on casein In the presence of calcium salts. 



256. Distinction between Rennin and Pepsin. 



To 10 cc. of 0-5 per cent, pepsin in water add 5 cc. of 0-2 M. acid 

 potassium phosphate, and make the volume up to TOO cc. with 

 distilled water. Titrate 20 cc. of the mixture with o-i N. soda to a 



p 



