280 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



( 'lassification of Enzymes. — Enzymes are classified according to the kind 

 of reaction they produce — namely: 



1. Proteolytic enzymes, or those acting upon proteids, converting them to a 

 soluble modification, peptone or proteose. As examples of this group we have 

 in the animal body pepsin of the gastric juice and trypsin of the pancreatic 

 juice. In plants a similar enzyme is found in the pineapple family (bromelin) 

 and in the papaw (papain). 



2. Amylolytic enzymes, or those acting upon the starches, converting them 

 to a soluble form, sugar, or sugar and dextrin. As examples of this group 

 we have in the animal body ptyalin, found in saliva, amylopsin, found in 

 pancreatic juice, and in the liver an enzyme capable of converting glycogen 

 to sugar. In the plants the best-known example is diastase, found in 

 germinating seeds. This particular enzyme has been known for a long time 

 from the use made of it in the manufacture of beer. In fact, the name " dias- 

 tase " is frequently used in a generic sense, " the diastatic enzymes/' to cha- 

 racterize the entire group of starch-destroying ferments. 



3. Fat-splitting enzymes, or those acting upon the neutral fats, breaking 

 them up into glycerin and the corresponding fatty acid. The best-known 

 example in the animal body is found in the pancreatic secretion; it is known 

 usually as steapsin, although it has been given several names. Similar 

 enzymes are known to occur in a number of seeds. 



4. Sugar-splitting enzymes, or those having the property of converting the 

 double into the single sugars — the di-saccharides, such as cane-sugar and 

 maltose, into the mono-saccharides, such as dextrose and levulose. Two 

 enzymes of this character are found in the small intestine of the animal 

 body, oik; acting upon cane-sugar and one on maltose. The one acting on 

 cane-sugar is known as invertine or invertase, while that acting on maltose 

 is designated as maltase. 



5. Coagulating enzymes, or those acting upon soluble proteids, precipitating 

 them in an insoluble form. As examples of this class we have fibrin ferment 

 {thrombin), formed in shed blood, and rennin,the milk-curdling ferment of the 

 gastric juice. An enzyme similar to rennin has been found in pineapple-juice. 



These five classes comprise the chief groups of enzymes that are known to 

 occur in the animal body. One or more examples of each group take part in 

 the digestion of food at some time during its passage through the alimentary 

 canal. From time to time other enzymes have been recognized in the liquids 

 or tissues of the body. 1 Thus in shed blood and indeed in other tissues an 

 enzyme (glycolytic enzyme) that i> capable of destroying sugar seems to be 

 present. When sugar is added to shed blood it disappears as such, although 

 the products formed have not been recognized. Similarly from many tissues 

 of the body oxidizing enzymes have been extracted that are capable of caus- 

 ing energetic oxidation of organic bodies; for instance, they can convert 

 salicylaldehyde to salicylic acid. It i> possible that these oxidizing enzymes, 



1 For :i recent summary of tacts and literature upon enzymes see < rreen : The Soluble Ferments 

 and Fermt ntation, 1897. 



