134 NUTRITION AND METABOLISM. 



substances. Mineral salts seem essential for their action, because con- 

 tinued dialyzing weakens the activity which can be restored only by adding 

 salts. 



MECHANISM or FERMENTATION. It has been demonstrated in the 

 ibove paragraph that food is prepared for digestion or fermentation by 

 mzymes. The final decomposition, the process which yields the energy 

 "or cell life, must take place within the cell. The investigations of recent 

 fears have demonstrated that these processes also are caused by 

 mzymes. It has been proved beyond doubt that in the alcoholic, lactic, 

 icetic and urea fermentations the fermentation process may continue 

 ifter the death of the fermenting cells. In the case of alcoholic fermenta- 

 ;ion, the fermenting agent has been separated from the lacerated cells and 

 las been filtered through porcelain filters without losing its ability to act. 

 rhis proves the enzyme-nature of the fermenting agent which after once 

 :>eing formed, remains and acts independent of the cell. These enzymes 

 ire called zymases. They remain within the cell as long as it is alive, 

 rhey are much more sensitive to injurious influences than the above 

 nentioned food-preparing enzymes. Much skill and patience was 

 equired to demonstrate their independence of the living cell. After 

 :hese enzymes were found in microorganisms, similar enzymes were 

 liscovered in the cells of higher plants and animals. Many of the bio- 

 :hemical changes taking place in the final dissociation of food within the 

 :ell are now known to be the result of enzymic action; heretofore these 

 eactions were believed to be a part of the life-processes, inseparable 

 rom the living cell. Even some of the oxidations and many reducing 

 >rocesses have been recognized as caused by enzymes, and it is quite 

 >ossible that the whole process of intracellular food decomposition is 

 iccomplished entirely by means of enzymes. 



CLASSIFICATION or ENZYMES. 



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

 mknown, they can be classified only according to the compounds they 

 ict upon. It is possible, however, to distinguish between the following 

 our groups: Hydrolyzing, zymatic, oxidizing, reducing enzymes. This 

 lefinition is not quite exact, since the urea fermenting enzyme is also a 

 lydrolyzing enzyme, and the acetic fermentation is caused by an oxidiz- 

 ng enzyme. The distinction between endo-enzymes (intr a- cellular) and 



