THE CLASSIFICATION OF ENZYMES 463 



For example, the enzyme lipase cannot be detected in dormant castor bean 

 seeds, but as soon as they begin to germinate its presence can be readily 

 demonstrated. It is therefore believed that the enzyme is present in the 

 ungerminated seeds as a zymogen and that, upon germination, it is rapidly 

 converted into the enzyme proper. Zymogens may be either intermediate 

 stages in the synthesis of enzymes, or may be compounds in which the enzyme 

 is combined with some component which results in its inactivation. 



It is customary to distinguish between intracellular or endoenzymes and 

 extracellular or exoenzymes. Endoenzymes are those which operate within 

 the cells in which they are produced, while the exoenzymes are secreted from 

 the cells and accomplish their effects in some outside medium. In the higher 

 animals enzymes are secreted from specialized cells or glands into the various 

 organs of the alimentary tract where they effect the digestive processes. 

 "Ptyalin" (an amylase), the starch hydrolyzing enzyme secreted into the saliva, 

 and pepsin, a protein splitting enzyme secreted into the stomach, are familiar 

 examples of exoenzymes produced in the bodies of the higher animals. Most 

 of the enzymes in the higher plants are endoenzymes while extracellular diges- 

 tion is a regular feature of the metabolism of many bacteria and fungi. The 

 secretion of exoenzymes by many such species can be readily demonstrated. 

 When, for example, colonies of certain species of bacteria are grown on cul- 

 ture media containing starch, gradual digestion of the starch in a concentric 

 zone around each colony occurs. 



The Classification of Enzymes. — Numerous investigations have revealed 

 the presence of a large number of enzymes in living organisms and probably 

 many more remain to be discovered. Since, with a few exceptions, nothing 

 is known of the chemical nature of enzymes themselves, their presence is rec- 

 ognized by the chemical transformations which they catalyze, and such 

 chemical changes provide a basis for their classification. The compound 

 which undergoes chemical transformations under the influence of an enzyme 

 is called the substrate; the results of the reaction the end products. 



The outline classification of the hydrolytic enzymes presented in Table 

 43 includes all of the better known enzymes in this group. Certain of the 

 so-called enzymes listed in this table are definitely known to be mixtures of 

 two or more enzymes and it is probable that this is also true for some of the 

 others. 



With rare exceptions enzymes are named for the substrates upon which 

 they act, the name of the enzyme being derived by appending the suffix ase to 

 the name of the substrate, e.ff. maltose, maltase; cellulose, cellulase ; etc. 



A classification of the other important group of enzymes found in plants, 

 the oxidizing-reducing enzymes, will be found in Chap. XXX. 



