DIGESTION 427 



Normal gastric juice in man and dogs contains a variety of inorganic 

 salts, mucins (glycoproteins), enzymes, and hydrochloric acid. The 

 salts have no known role in gastric digestion, and the mucins are 

 supposed to serve only as lubricants. 



Pepsin is the most important enzyme of the gastric juice. It is not 

 secreted as active pepsin but is first produced in an inactive form 

 called pepsinogen, the precursor of pepsin. When pepsinogen comes 

 in contact with hydrochloric acid, active pepsin is formed. Pepsin 

 is most active in acid solutions, and it has been shown that the pro- 

 teolytic activit) of pepsin is not due to hydrochloric acid per se, for 

 many acids, at proper concentrations, can produce satisfactory restdts. 

 The optimum concentration of hydrochloric acid for the action of 

 pepsin is in the range of pH 1.5 to 2.0, while the enzyme is inactivated 

 above pH 6. Like many enzymes of animal origin, pepsin is very 

 active at temperatures from 38 to 40°C. 



A second gastric enzyme called rennin is active in curdling milk by 

 catalyzing the conversion of casein to paracasein, which combines with 

 calcium ions to yield insoluble calcium paracaseinate. Removal of 

 calcium salts from the milk prevents coagulation. Such precipitation 

 may aid digestion in young animals by prolonging the exposure to 

 hydrolysis by pepsin. Both pepsin and hydrochloric acid also curdle 

 milk thoizgh not so powerfully as rennin. Although rennin is foinid 

 in the stomachs of infants and young animals, such as the calf, little, 

 if any, rennin is secreted by adtdt human beings or mature animals. 

 Hydrochloric acid and pepsin probably play the major role in gastric 

 coagulation of milk so far as adult human beings are concerned. 

 Rennin acts best in slightly acid solutions (pH 6.0 to 6.5), although 

 it will coagtdate casein in neutral and even slightly alkaline solutions. 

 Commercial rennin, known as rennet, is obtained from the calf stomach 

 and is used in the manufacture of cheese. When casein is made for 

 industrial purposes, it is precipitated from skim milk by adding acid. 

 At the isoelectric point (pH 4.6 to 4.8) casein precipitates, differing 

 from calcium paracaseinate, which is of smaller molecular size. 



The existence of a final gastric enzyme called lipase has been 

 debated vigorously. Apparently it does occur in small amounts, but 

 its role is uncertain. Perhaps this lipase hydrolyzes glycerides to a 

 small extent, but the low pH of the stomach prevents large-scale 

 activity. 



Gastric juice is secreted by the numerous glands of the stomach 

 lining (gastric mucosa). These glands differ somewhat in the various 

 regions of the stomach and hence supply different relative quantities 

 of the three fluids concerned. Mucus is secreted by the mucous cells. 



