PHYSIOLOGY 169 



materials in solution. By enzyme action, proteins are converted to 

 soluble amino acids, starches and sugars to maltose and finally glucose, 

 and fats to fatty acids and glycerin. 



In general, an enzyme is an organic substance which by its presence 

 under certain conditions will cause or hasten chemical reaction be- 

 tween other substances without itself being consumed. The enzymes 

 are formed in the protoplasm of cells, and their action is similar to 

 that of a catalyst, since they accelerate chemical action. There are 

 different types of enzymes each capable of producing specific kinds of 

 reactions. There are oxidizing enzymes (oxidases) capable of bring- 

 ing about oxidation; reducing enzymes (reductases), which produce 

 reduction in tissues; coagulating enzymes (coagulases), which cause 

 clotting or coagulation; and hydrolyzing enzymes (hydrolases), which 

 act by causing a reaction between a substance and water. Most of the 

 digestive enzymes fall in this latter class. Most enzymes consist of a 

 parent substance or precursor, zymogen, which becomes active only 

 in the presence of a certain other substance, termed activating agent 

 or co-enzyme. As an example, the precursor of pepsin is pepsinogen 

 which becomes activated in the presence of dilute acid. 



Classes of Digestive Enzymes 



1. Diastases or di-astatic enzymes — split carbohydrates 



(a) Ptyaliii in saliva 



(b) Amylase in pancreatic juice 



(c) Glycogenases — liver and muscles 

 Converts glycogen to glucose 



2. Lipase or lipolytic enzyme — splits fats 

 (a) Steapsin in pancreatic juice 



3. Inverting enzymes — convert disaccharids to the less complex monosaccharids 

 (simple sugars) — intestinal juice 



(a) Maltase 



(b) Lactase 



(c) Sucrase (invertase) 



4. Proteases w proteolytic enzymes — split complex proteins 



(a) Pepsin in gastric juice 



(b) Trypsin in pancreatic juice — functions in small intestine 



(c) Erepsin in intestinal juice 



5. Clotting or coagulating enzyme 

 (a) Eennin in gastric juice 



In higher Metazoa digestion is accomplished principally extra- 

 cellularly through secretion of enzymes by certain groups of cells. 

 Such systems consist of: (1) an alimentary canal proper; and (2) 



