GASTRIC DIGESTION. 1 2 I 



the action of the enzyme trypsin (see p. 141 ) . The relative amounts 

 of proteoses, peptones and crystalline substances formed depends 

 to a great extent upon the character of the protein undergoing 

 digestion, e. g., a greater proportion of proteoses results from the 

 digestion of fibrin than from the digestion of coagulated egg-white. 

 We must not be led into the error of thinking that the large num- 

 ber of protein cleavage products just mentioned are formed in the 

 course of normal gastric digestion within the animal organism. 

 They are formed only after comparatively long-continued hydroly- 

 sis. In pancreatic digestion, however, there are formed even under 

 normal conditions, the large number of cleavage products to which 

 reference has been made. Peptic proteolysis therefore, within the 

 animal organism differs from tryptic proteolysis (see page 141) in 

 that the former yields larger amounts of proteoses, smaller amounts 

 of peptones and no considerable quantity of crystalline bodies as 

 end-products in the brief period during which proteins are ordinarily 

 subjected to gastric digestion. Prolonged hydrolysis with gastric 

 juice does, hov/ever, yield considerable quantities of the non-pro- 

 tein end-products. 



Gastric rennin, the second enzyme of the gastric juice, is what is 

 known as a milk curdling or protein coagulating enzyme. Rennin 

 acts upon the caseinogen of the milk, splitting it into a proteose- 

 like body and soluble casein. This soluble body, in the presence 

 of calcium salts, combines with calcium, forming calcium casein 

 or true casein which is insoluble and precipitates. There is some 

 uncertainty regarding the reaction to litmus in which gastric 

 rennin shows the greatest activity. It is, however, said to be active 

 in neutral, alkaline or acid solution. However, it probably pos- 

 sesses its greatest activity in the presence of a slight acid reaction, as 

 would naturally be expected. It is especially abundant in the 

 gastric mucosa of the calf, and is used to curdle the milk used in 

 cheese making. Gastric rennin is always present normally in the 

 gastric juice but in certain pathological conditions such as atrophy 

 of the mucosa, chronic catarrh of the stomach or in carcinoma it 

 may be absent. 



The theory that the proteolytic activity and the milk curdling 

 property of the gastric juice reside in a single molecule is causing 

 much controversy at the present time. The theory was originally 

 advanced by the Pawlow school. 



Gastric lipase, the third enzyme of the gastric juice, is a fat- 

 splitting enzyme. It possesses but slight activity when the gastric 



