DIGESTION AND ABSORPTION IN THE STOMACH. 789 



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

 more easily digested under the conditions that exist in the body 

 after it has been brought into a solid form, or, perhaps, the coagu- 

 lation of the casein ensures that it will be retained in the stomach 

 and be submitted to gastric digestion, instead of being ejected 

 promptly into the duodenum, as happens with liquid material. 

 The action of rennin goes no further than the curdling; the diges- 

 tion of the curd is carried on by the pepsin, and later, in the intes- 

 tines, by the trypsin, as in the case of other proteins.^ 



Rennin is found elsewhere than in the gastric mucosa. It has been 

 described in the pancreatic juice, in the testis, and in many other organs 

 as well as in the tissues of many plants. In fact, wherever proteolytic enzymes 

 are found there also some evidence of a curdling action on milk may be ob- 

 tained. For this reason some observers have taken the view that the milk 

 coagulation is not due to a specific ferment, but is an action of the pepsin itself. 

 That is, the proteolytic enzyme is capable of causing the change from casein 

 to paracasein as well as the hydrolysis of the protein. This view is opposed to 

 the prevalent opinion regarding the specificity of enzyme actions, and is con- 



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

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

 j For references to very abundant literature, consult Oppenheimer, loc. cit. 

 See Pawlow and Parastschuk, "Zeitschrift f. physiol. Chemie," 42 415 

 1904: Burge, "American Journal of Physiology," 29, 330, 1912. 



