ENZYMES AND ENZYMATIC ACTION 143 



the lock. This is known as the lock-and-key theory. This 

 specific action in the case of the hydrolytic enzymes is distinctly 

 different from the action of acids in producing hydrolysis. 

 While the hydrolysis of the different carbohydrates, fats and 

 proteins may all be produced by boiling with acids or alkalies, 

 an individual enzyme is necessary for each particular compound 

 hydrolyzed. 



Reversible Nature of Enzyme Action. — In discussing the 

 reactions of esterification and hydrolysis in connection with 

 fats (p. 58) we referred to the fact that these two reactions 

 represent the two directions of one of the most common rever- 

 sible reactions. It is a striking fact that the fat-hydrolyzing 

 enzymes especially have been found to possess the property of 

 reversibility, i.e. they are able either to cause the hydrolysis 

 of a fat or to esterify glycerol and a fatty acid, depending upon 

 conditions, e.g. the excess of end products of the first reaction. 

 The first enzyme with which this property was established was 

 the one which hydrolyzes maltose sugar to glucose. Proteolytic 

 enzymes have also been claimed to act reversibly. The im- 

 portance of this property will be better realized when we study 

 the digestion, absorption and metabolism of food. 



Zymogens 



We have referred to enzymes as occurring in plant and animal 

 organisms. In some cases the enzyme does not occur as such 

 and is probably not elaborated as such by the organism but in 

 the form of a mother substance known as a zymogen. The 

 zymogen of an enzyme is itself inactive but when acted upon 

 by another substance called an activator or a kinase it is con- 

 verted into the active enzyme. The best examples of this are 

 the enzymes, pepsin of the gastric juice and trypsin of the pan- 

 creatic juice. In the stomach cells the pepsin is not active, but 

 is in the form of its zymogen known as pepsinogen. This pep- 

 sinogen becomes activated by hydrochloric acid, which is a normal 

 constituent of the stomach, and pepsin, the active enzyme, is the 

 result. Trypsin occurs in the pancreatic juice as the zymogen, 



