DIGESTION AND ABSORPTION 173 



Gastric Rennin. — This enzyme is not a protein-hydrolyzing 

 enzyme, but a prottin-coagulating enzyme. It acts upon milk 

 caseinogen, the form of casein present in milk, and converts 

 it into soluble casein and a proteose body. The soluble casein 

 reacts with calcium salts forming calcium casein or true casein, 

 which is insoluble and is precipitated as a curd or coagulum. 

 Gastric rennin acts best in acid media equal to the acidity of 

 gastric juice. 



Pepsin. — The most important enzyme of the gastric juice 

 is pepsin^ which is a true protein-hydrolyzing or proteolytic 

 enzyme. The enzyme does not occur as such in the cells 

 where it is secreted, but is present there in the form of its 

 mother substance or zymogen, known as pepsinogen. Pepsin- 

 ogen is activated or converted into pepsin enzyme by the action 

 of the free hydrochloric acid present in the gastric juice. Pepsin 

 acts best in acid media, the amount of acidity varying with the 

 character of the protein to be hydrolyzed. Fibrin protein is 

 digested better by pepsin when the acidity is 0.08 to o.io per 

 cent, while coagulated egg albumen requires an acidity of 0.25 

 per cent. The hydrolysis of protein by means of the enzyme 

 pepsin yields as products not only the derived proteins, pro- 

 teoses and peptones, but also amino-acids. It is probable that 

 in the stomach, however, the derived proteins, proteoses and 

 peptones are the chief hydrolytic products varying more or 

 less with the character of the protein food. Therefore, when 

 the protein food has passed through the stomach, it is more or 

 less completely converted into proteoses and peptones with some 

 amino-acids and some unhydrolyzed protein. 



Hydrochloric Acid. — As has been stated the distinctive 

 character of gastric juice is its relatively strong acidity, 0.2 to 

 0.3 per cent, due to the presence of free hydrochloric acid. 

 The origin of this hydrochloric acid is not well established, but 

 it is probably derived from the chlorides, principally sodium 

 chloride, present in the blood and may be produced by elec- 

 trolytic action or by the action of the lactic acid which is also 

 present. 



