218 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



not justified in making any general application of this fact to the whole 

 group. 



Classification 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. Inverting 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 have been found in the animal body, one acting upon cane-sugar and 

 one on maltose. They are usually spoken of as invertin or inverting enzymes. 

 A similar enzyme may be obtained from the yeast-plant. 



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 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. 

 Two other important groups of enzymes which are not formed in the animal 

 body may be mentioned briefly in this connection for the sake of completeness : 



6. Glucoside-splitting enzymes, or those acting upon the glucosides, giving a 

 carbohydrate as one of the products of decomposition. Examples : emulsin, 

 found in bitter almonds; myrosin, in mustard-seeds. 



7. Urea-splitting enzymes, or those acting upon urea, converting it to ammo- 

 nium carbonate ; found in many bacteria, especially in those normally occur- 

 ring in the urine. 



A great number of general reactions have been discovered, applicable, with 



