208 NUTRITION AND METABOLISM 



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oxidations and many reducing processes have been recognized as caused 

 by enzymes, and it is quite probable that the whole process of intra- 

 cellular food decomposition in all organisms is accomplished entirely 

 by means of enzymes. 



CLASSIFICATION OF ENZYMES 



Since the chemical nature of enzymes and of their action is largely 

 unknown, they can be arranged for convenience only according to the 

 compounds they act upon. It is possible, however, to distinguish 

 between the following four groups: Hydrolyzingj zymatic, oxidizing, 

 reducing enzymes. This definition is not quite exact, since the urea 

 fermenting enzyme is also a hydrolyzing enzyme, and the acetic fer- 

 mentation is caused by an oxidizing enzyme. The distinction between 

 endo-enzymes (intra-cellular) and exo-enzymes (secreted) is not exact, 

 either, since invertase and lactase are retained in the cells of some 

 organisms and secreted by others. 



The following classification is used in the further discussions: 



I. Hydrolytic Enzymes. 



1. of carbohydrates: cellulase (cytase), diastase (ptyalin, amylase), invertase, 

 lactase, maltase. 



2. of fats: lipase (steapsin). 



3. of proteins: 



(a) proteolytic (proteases): pepsin (peptase), trypsin (tryptase), erep- 



sin (ereptase). 



(&) coagulating (coagulases) : thrombase, rennet (chymosin). 

 II. Zymases. 



1. of carbohydrates: alcoholase, lactacidase. 



2. of other nitrogen-free bodies: vinegar-oxidase. 



3. of proteins: endo-tryptase, autolytic enzymes, amidase, urease. 



III. Oxidizing Enzymes. 

 Vinegar-oxidase, tyrosinase. 



IV. Reducing Enzymes. 



Katalase, reductases of nitrates, sulphur, sulphites, telluric salts, methylene 

 blue, litmus. 



Several different names have been given to some of the enzymes; 

 these are found in parenthesis in the above classification. 



The general action of enzymes being explained in the preceding 

 pages, it remains to describe more in detail the different enzymes of 

 microbial origin. 



