106 GENERAL ZOOLOGY 



Zymogen has been identified with certain granules in the cyto- 

 plasm of the cell producing it. To bring about its normal reac- 

 tion, the zymogen must be activated by some agent encountered 

 after it leaves the cell. Thus trypsinogen, the inactive form of 

 trypsin, is a product of the pancreas that is converted into 

 active trypsin by enter vkinase, an organic substance secreted by 

 the small intestine. 



The enzymes concerned in the digestion of protein, starches, 

 sugars, and fats all act by hydrolysis. Other kinds of enzymes 

 are: (1) coagulating enzymes, such as the rennin of the stomach 

 of young mammals, that produces the coagulation of casein in 

 milk; (2) oxidizing enzymes, that bring about oxidizing processes 

 in the body; and (3) deaminizing enzymes, that produce the 

 separation of the NH 2 group from amino acids, of which an 

 example is the formation of ammonia and lactic acid from alanine 

 (CH 3 CH(NH 2 )-COOH). 



Enzymes are sensitive to the hydrogen-ion content of the 

 medium. Thus an enzyme normally acting in a neutral or 

 alkafine medium becomes inactive in the presence of acid. 



Digestive Enzymes. — The enzymes concerned in digestion and 

 nutrition in the human body include the following: 



1. Proteolytic enzymes, acting on proteins. 



a. Pepsin, secreted by the glands of the stomach, acts in 

 an acid medium ; it converts proteins into peptones and proteoses. 

 (In the frog pepsin is also secreted by the esophagus, but becomes 

 active only after passing into the stomach.) 



b. Trypsin, secreted in an inactive form as trypsinogen by 

 the pancreas and activated by the enterokinase in the intestine; 

 it acts in an alkaline, neutral, or slightly acid medium; converts 

 proteins into polypeptides. 



c. Erepsin, secreted principally by the mucosa of the small 

 intestine and in small amounts by the pancreas. It also is 

 found in the tissues generally. It acts in an alkaline or neutral 

 medium and splits the products formed by pepsin and trypsin 

 into amino acids. (In the frog no erepsin is produced by the 

 small intestine.) 



2. Diastatic enzymes, act on starches in an alkaline or neutral 

 medium. 



a. Ptyalin, secreted by the salivary glands, converts 

 starch into sugar (maltose). 



