194 NUTRITION AND METABOLISM 



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. 



Classipication of Enzymes 



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

 unknown, they can be classified only according to the compounds 

 they act upon. It is possible, however, to distinguish between the 

 following four groups: Hydrolyzing, zymotic, oxidizing, reducing en- 

 zymes. This definition is not quite exact, since the urea fermenting 

 enzyme is also a hydrolyzing enzyme, and the acetic fermentation 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. HydrolyHc Enzymes. 



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

 lactase, maltase. 



2. of fats: lipase (steapsin). 



3. of proteins: 



(fl) proteolytic (proteases); pepsin (peptase), trypsin (tryptase), erep- 



sin (ereptase). 

 (6) 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.l!lf| 



The general action of enzymes being explained in the preceding 

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

 microbial origin. 



