2C>8 COMPOSITION OF THE DIGESTIVE JUICES. [CH. VIII. 



The main differences between the pepsin and rennin 

 as enzymes that can clot milk are : 



(1) Pepsin is more stimulated by increasing the 

 concentration of calcium chloride than is rennin. 



(2) Pepsin is almost completely destroyed by heating 

 for 10 minutes at 38 C. at P H = 7-25. Rennin 

 only loses a small fraction of its activity. 



(3) Heating for 2 minutes at 70 C. at P H = 5 com- 

 pletely destroys rennin, but has no effect on 

 pepsin. Since the clotting power and ordinary 

 peptic action of such an heated solution is the 

 same as the unheated solution, it must be con- 

 cluded that the clotting action of pepsin is due to 

 this enzyme, and not to another enzyme associated 

 with pepsin. This is in opposition to the view of 

 Bang, who claims that a special clotting enzyme, 

 which he calls parachymosin, is found in pigs' 

 stomach. 



The remarkable fact that pepsin can hydrolyse casein to 

 paracasein at a reaction of about PH = 6-7, whilst at this 

 reaction it does not act on any other protein, has not been 

 satisfactorily explained. 



The first action of proteolytic enzymes on casein is to 

 hydrolyse it to paracasein. If there is a sufficient con- 

 centration of calcium ions and the reaction of the medium is 

 neither too acid nor too alkaline, the paracasein is pre- 

 cipitated as the insoluble calcium paracaseinate. Pepsin 

 in markedly acid solution, erepsin in solutions that are 

 about neutral and trypsin in neutral and faintly alkaline 

 solutions can hydrolyse this further to substances allied to 

 the proteoses and peptones (" caseoses M ), and, in the case 

 of erepsin and trypsin, to ammo-acids. 



Rennin is prepared by extracting the mucous mem- 

 brane of the fourth stomach of a sucking calf with brine. 

 It can be obtained commercially in the solid or liquid form, 

 being extensively used in the cheese industry, and also in 

 the kitchen for the preparation of " junket." The author 



