68 PLANT PHYSIOLOGY 



maltose only by diastase and cellulose only by cytase, and that 

 the hydrolysis of two disaccharides, maltose and sucrose, was 

 brought about, respectively, by maltase and sucrase. The 

 classification and nomenclature of enzymes are based on this 

 selective action. In giving the enzyme a name, the name of the 

 substance it hydrolyzes is taken with the addition of the suffix 

 •'-ase." Some of the enzymes found and described in earUer 

 times retain their historical names, e.g., diastase, invertase, pepsin, 

 and several others. 



The specificity of enzymatic action should not be understood 

 in the sense that each disaccharide or polysaccharide, each 

 protein, each fat, etc., can be hydrolyzed only by a special 

 enzyme. Quite the contrary; it is established beyond doubt 

 that there are few^er enzymes than are often described, and that 

 various proteins, for example, are acted upon by one and the 

 same enzyme. Their specific action consists largely in the fact 

 that each enzyme possesses the ability to break down any 

 linkage of a certain character in the complex molecule of organic 

 compounds and that in the presence of two optical isomers, each 

 enzyme may possess the ability to act only upon one of them. 



Since little is known of the chemical composition of enzymes 

 and since their presence is attested to only by their activity, 

 the classification of enzymes is thus far based on the classification 

 of enzymatic reactions. Up to the present time, there is no 

 well-established, unified classification of enzymes, and different 

 authors adhere to different systems. The most consecutive 

 classification of enzymes is the one suggested recently by Neuberg 

 and Oppenheimer. According to this classification, all the 

 enzymes are first of all divided into two groups, hydrolases and 

 desmolases. Hydrolases catalyze hydrolytic processes, i.e., 

 processes where there is a rupture of the linkages between atoms 

 of carbon on the one side and either oxygen or nitrogen on the 

 other with the introduction of w^ater at the point of the break. 

 Desmolases represent enzymes that break the linkages of the 

 chains of carbon atoms, i.e., cause the so-called ''desmolysis." 



Each of these two groups is in its turn divided into two sub- 

 groups. The group of hydrolases includes all the enzymes that 

 have thus far been examined, viz.: (1) carbohydrases disintegrating 

 compUcated carbohydrates, disaccharides (invertase, maltase), 

 and polysaccharides (diastase, cytase, and others); (2) amidases 



