778 PHYSIOLOGY OF DIGESTION AND SECRETION. 



the gastric tubules and to be present in the glands in a zymogen 

 form, the prorennin or prochymosin, which after secretion is con- 

 verted to the active enzyme. This conversion takes place very 

 readily under the influence of acid. Rennin (or its zymogen) may be 

 obtained easily from the mucous membrane of the stomach (with 

 the exception of the pyloric end) by extracting with glycerin or water 

 or by digesting with dilute acid. Good extracts of rennin cause 

 the milk to clot with great rapidity at a temperature of 40° C; the 

 milk (cows' milk), if unchsturbed, sets at first into a soUd clot, which 

 afterward shrinks and presses out a clear, yellowish liquid — the 

 whey. With human milk the curd is much less firm, and takes the 

 form of loose fiocculi. The whole process resembles much the clotting 

 of blood. The time of clotting is said to vary inversely as the 

 amount of rennin, or, in other words, the product of the amount of 

 rennin and the time necessary for clotting is a constant. The 

 curdling of the milk involves two apparently independent proc- 

 esses : First, the rennin acts upon the casein of the milk and converts 

 it into a substance known as paracasein. The paracasein then 

 reacts with the calcium salts of the milk, forming an insoluble 

 protein, which constitutes the curd or coagulum. According 

 to this view, the enzyme does not cause clotting directly.* What 

 takes place when the casein is changed to paracasein is not under- 

 stood. Hammarsten originally regarded the change as a cleavage 

 process, and this view is still supported by some authors,! although 

 denied by others. Others have supposed that a transformation or 

 rearrangement of molecular structure occurs. Indeed, the differ- 

 ences in properties between casein and paracasein are not great, 

 the most marked difference being that the latter in the presence of 

 calcium salts gives an insoluble coagulum. If soluble calcium salts 

 are removed from milk by the addition of oxalate solutions, it does 

 not curdle upon the addition of rennin. Addition of lime salts re- 

 stores this property. It should be added that casein is also pre- 

 cipitated from milk by the addition of an excess of acid. The 

 curdling of sour milk in the formation of bonnyclabber is a well- 

 known illustration of this fact. When milk stands for some time 

 the action of bacteria upon the milk-sugar leads to the formation 

 of lactic acid, and when this acid reaches a certain concentration 

 it causes the precipitation of the casein. 



So far as our positive knowledge goes, the action of rennin is 

 confined to milk. Casein is the chief protein constituent of milk, 

 and has, therefore, an important nutritive value. It is interesting 

 to find that before its peptic digestion begins the casein is acted 

 upon by an altogether different enzyme. The value of the curdling 

 action ie not at once apparent, but we may suppose that casein is 



* See Bang, " Skandinavisches Archiv f. Physiologie," 25, 105, 1911. 



t Bosworth and Van Slyke, "Journal of Biological Chemistry," 19, 67, 1914. 



