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 OF FERMENTATION. It has been demonstrated in the 

 above paragraph that food is prepared for digestion or fermentation by 

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

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

 years have demonstrated that these processes also are caused by 

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

 acetic and urea fermentations the fermentation process may continue 

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

 tion, the fermenting agent has been separated from the lacerated cells and 

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

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

 being formed, remains and acts independent of the cell. These enzymes 

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

 They are much more sensitive to injurious influences than the above 

 mentioned food-preparing enzymes. Much skill and patience was 

 required to demonstrate their independence of the living cell. After 

 these enzymes were found in microorganisms, similar enzymes were 

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

 chemical changes taking place in the final dissociation of food within the 

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

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

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

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

 possible that the whole process of intracellular food decomposition 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 classified only according to the compounds they 

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

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

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

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

 ing enzyme. The distinction between endo-enzymes (infra-cellular) and 



