DIGESTION OF THE PROTEINS 249 



precipitated at ordinary temperatures. If a little less calcium chloride 

 be employed, it will remain in solution at ordinary temperatures, but 

 will form a curd on elevating the temperature. Even in the absence 

 of free calcium ions a solution of calcium caseinate becomes markedly 

 opalescent on heating to 45 C. The calcium salt of paracasein is, 

 however, for a like concentration of free calcium ions, clotted or curdled 

 at a lower temperature than calcium caseinate. The presence of 

 free calcium ions is therefore necessary to permit the clotting of 

 milk by gastric juice or extracts of the gastric mucosa. They are not 

 necessary, however, for the conversion of the casein into paracasein, 

 which occurs just as readily in a medium free from calcium ions as in 

 one which contains them; but visible evidence of the change which has 

 occurred is lacking until calcium salts are added. Thus if excess of 

 ammonium oxalate be added to milk, the free calcium ions are removed, 

 through the formation of calcium oxalate. The calcium combined 

 with the casein is unaffected because it is not ionized. On adding 

 Rennet (extract of gastric mucosa) or gastric juice in small amount and 

 warming the mixture to body temperature no visible change in the 

 milk occurs. The mixture may be heated to boiling to destroy the 

 enzyme without causing any precipitation or clotting of the milk, but on 

 adding soluble calcium salts, after cooling, clotting of the milk instantly 

 occurs. The calcium is necessary, therefore, merely to render the 

 product of the enzyme action insoluble; not to enable the enzyme to 

 act upon the casein. The part played by calcium in this process is 

 therefore sharply in contrast to the part which it plays in the coagu- 

 lation of the blood. 



It has long been supposed that this change in the properties of casein 

 which is brought about by gastric juice is due to a special enzyme, which 

 is termed Rennin or Chymosin. Evidence has accumulated in recent 

 years, however, tending to show that rennin is, in fact, identical 

 with Pepsin and that rennet preparations which are devoid of power 

 to digest proteins other than casein represent merely pepsin, weakened 

 so greatly as to have lost ability to hydrolyze the majority of proteins 

 at any appreciable speed. Thus, although pepsin and rennin are 

 found in a great variety of situations both in the animal and in the 

 vegetable kingdoms, yet they are invariably found to be associated 

 with cne another in the same tissue or tissue-fluid. The close relation- 

 ship of rennin action to pepsin action is also shown by the following 

 experiment of Morgenroth's. If mixtures of calcium caseinate con- 

 taining free calcium ions and rennet are kept at low temperatures 

 no coagulation occurs, but slow digestion of the casein (proteose pro- 

 duction) does occur. If, however, these mixtures be heated to 20 C. 

 they clot immediately. Thus the process which underlies the clotting, 

 has taken place during the digestion which occurs at low temperatures, 

 but it cannot be visibly evidenced by clotting until the temperature 

 is raised. 



