328 FOOD AND DIGESTION 



Enzymes are more or less specific in their action. That is, each enzyme 

 is supposed to produce its change in only one particular substance, as in 

 starch, maltose, protein, fat, etc. An enzyme that can cause cleavage of the 

 starch molecule will not act on fat or protein or even on other members of 

 the starch group. This specific action is doubtless expressive of a definite 

 relation between the structure of the enzyme and the substance acted on. 



An interesting fact as to enzyme action is its reversibility a phenomenon 

 now well known and well established for carbohydrates and fats. Kastle 

 and Lowenhart have shown that lipase, which acts to split neutral fats into 

 fatty acid and glycerin, will also produce a synthesis, at least of butyric 

 acid and alcohol into ethylbutyrate. Taylor and Robertson in independent 

 papers have recently made the far-reaching discovery that the protein 

 molecule can be synthesized by the agency (apparent reversible action) of 

 enzymes. 



Enzymes are classified either according to the chemical nature of their 

 action or according to the class of substances on \vhich they act; the former 

 classification is more logical, but the latter is more convenient and more 

 generally used. 



TABLE OF DIGESTIVE ENZYMES. 



Amylolytic. 



Ptyalin of saliva, and amylopsin of pancreatic juice, change starch to mal- 

 tose. Maltase in the saliva, and pancreatic juice in the small intestine, 

 change maltose to dextrose. Lactase splits lactose to galactose and dextrose, 

 and invertase splits cane-sugar to levulose and dextrose in the small intestine. 

 Lipolytic. 



Steapsin or lipase, found in the pancreatic juice, splits neutral fats into 

 glycerin and fatty acid. 

 Proteolytic. 



Pepsin of the gastric secretion, and trypsin of the pancreatic secretion, 

 change proteins to proteoses and peptones, trypsin breaking the protein down 

 to simpler nitrogenous products. Erepsin of the intestine splits peptones to 

 simpler products. 

 Coagulating. 



Rennin of the gastric juice coagulates milk. 

 Activating. 



Enterokinase of the intestinal juice converts trypsinogen to trypsin. 

 (Thrombokinase of the blood is of this class.) 



DIGESTION IN THE MOUTH. 



The food is received into the mouth and is subjected to the action of the 

 teeth and tongue, being at the same time mixed with the first of the digestive 

 juices, the saliva. It is then swallowed, and, passing through the pharynx 

 and esophagus into the stomach, is subjected to the action of the gastric 

 juice, the second digestive juice. Thence it passes into the small intestines, 

 where it meets with the bile, the pancreatic juice, and the intestinal juices, all 



