128 GENERAL BIOCHEMISTRY 



adverse conditions. Certain en/ymes seem to be retained without much 

 diminution by starving cells. Others decrease drastically long before 

 the cells themselves die. Thus the organisms exist at least temporarily 

 without a full complement of these latter dispensable enzymes. Al- 

 though the persistent group may or may not actually be critical to the 

 cells, they seemingly cannot be broken down to any great extent. This 

 discovery may apply to other proteins as well as enzymes and be a 

 general phenomenon. Or it may transpire that inducible and dispensa- 

 ble enzymes are basically the same, appearing and disappearing as 

 the critical molecules are added to or disappear from the environ- 

 ment. 



Work on the mechanism of food digestion, the transport of sub- 

 stances across cell walls, and the like has led to the classification of 

 enzymes according to the site of action. The two major groups are 

 intracellular and extracelhdar enzymes, respectively, occurring and 

 functioning inside or outside the cells synthesizing them. In addition, 

 enzymes of the cell siuface are occasionally considered separately. 



All the above classifications have important applications but mostly 

 for rather specialized purposes. However, enzymes as a group are 

 most noted for their catalytic function. Since they are distinguished 

 from proteins in this particular regard, a natural system of classifica- 

 tion might be based on differences in catalytic action. 



Several possibilities exist for classification along this general line. 

 The mechanism of enzyme action might be chosen, for example. How- 

 ever, as yet too little is known to make this basis a useful one, although 

 ultimately it may become important. Secondly, enzymes might be 

 grouped according to the extent of their catalytic effects, in other 

 words according to the degree to which they accelerate the reactions 

 affected. This procedure would automatically require comparisons 

 with equal quantities of other enzymes, requiring in turn knowledge 

 of the concentration and purity of the enzymes investigated. Since the 

 purity of proteins and enzymes is difficult to ascertain, this second basis 

 of classification is seldom used. 



Finally, enzymes may be classified according to the kinds of other 

 compounds involved in the reactions, either the reactants or the 

 products. Usually the reactants are chosen for the purpose. When 

 more than one kind of molecule is involved, only one is selected in 

 order to simplify the nomenclature. The choice seems to be a matter 

 of custom, convenience, or personal preference. In a number of cases 

 the names of classes of enzymes denote the type of reaction catalyzed 

 instead of the reactants. Depending upon the circumstances of classifi- 

 cation, a particular enzyme may fall into one category or another or 



