COMPOSITION OF FOOD AND ACTION OF ENZYMES. 753 



animal physiology, the following classification will be used in the 

 treatment of the subjects of digestion and nutrition: 



1. The proteolytic or protein-splitting enzymes. Examples: Pepsin of gastric 



juice, trypsin of pancreatic juice. They cause a hydrolytic cleavage of 

 the protein molecule. 



2. The amylolytic or starch-splitting enzymes. Examples: Ptyalin or sali- 



vary diastase, amylase, or pancreatic diastase. Their action is closely 

 similar to that of the classical enzyme of this groups-diastase found 

 in germinating barley grains. They cause a hydrolytic cleavage of the 

 starch molecule. 



3. The- lipolytic, or fat-splitting enzymes. Example: The lipase found in 



the pancreatic secretion, in the liver, connective tissues, blood, etc. 

 They cause a hydrolytic cleavage of the fat molecule. 



4. The sugar-splitting enzymes. These again fall into two subgroups: (a) 



The inverting enzymes, which convert the double sugars or disaccharids 

 into the monosaccharids. Examples: Maltase, which splits maltose to 

 dextrose; invertase, which splits cane-sugar to dextrose and levulose; 

 and lactase, which splits milk-sugar (lactose) to dextrose and galactose. 

 (6) The enzymes, which split the monosaccharids. There is evidence of 

 the presence in the tissues of an enzyme or enzymes capable of splitting 

 the sugar of the blood and tissues (dextrose) into lactic acid. 



5. The coagulating enzymes, which convert soluble to insoluble proteins. 



Example: The coagulation of the casein of milk by rennin. 



6. The oxidizing enzymes, or oxidases. A group of enzymes which set up 



oxidation processes. Some of the details of the activity of these enzymes 

 are considered in the discussion of physiological oxidations (see p. 973). 



7. The deaminizing enzymes, which by hydrolytic cleavage split off an NH 2 



group as ammonia. Thus alanin (aminopropionic acid) by hydrolysis 

 loses its NH 2 group as ammonia and passes into lactic acid. 



CH 3 CHNH 2 COOH + H 2 O = NH 3 + CH 3 CHOHCOOH. 



The enzymes contained in the first, second, third, and fourth (a) 

 of these groups are the ones that play the chief roles in the digestive 

 processes, and it will be noticed that they all act by hydrolysis, 

 that is, they cause the molecules of the substance to undergo de- 

 composition or cleavage by a reaction with water. Thus, in the 

 conversion of maltose to dextrose by the action of maltase the re- 

 action may be expressed so : 



CufiaOu + H 2 = C 6 H ]2 6 + C 6 H 12 O a . 



Maltose. Dextrose. Dextrose. 



And the hydrolysis of the neutral fats by lipase may be expressed 

 so: 



VfU&fLpOJt + 3H 2 = OH^OH), + 3(C M H,,p,). 



Tristearin. Glycerin. Stearic acid. 



Protective Enzymes. Experimental work in recent years has 

 brought out the interesting and important fact that when foreign 

 proteins, carbohydrates, or fats are introduced into the blood of a 

 living animal corresponding enzymes are formed which are adapted 

 to break down the foreign material by a process of digestion.* 



* Consult Abderhalden, "Schutzfermente des tierischen organismus," 

 1912; also Vaughan, "Protein Split Products in Relation to Immunity and 

 Disease," 1913, and "Journal of the Amer. Med. Assoc.," August 1, 1914. 



48 



