68 STORAGE AND DIGESTIOxN OF FOODS 



acids and other compounds soluble and therefore usable. 



II. The oxidizing cnzyrns (oxidases) which are con- 

 cerned hi the intracellular oxidations which release the 

 energy required for work by or in living organisms. 



III. The de-aminases, which break down amino-acids 

 and similar substances to ammonia and carbohydrates. 



IV. Ureases, capable of breaking urea down into am- 

 monia (NH^OH) and carbon dioxid (COo). (See Chap. 

 XXIV.) 



V. Coagulating enzyms, such as cause milk to clot. 



VI. The fermentation enzyms, which convert sugar, 

 for example, into alcohol and carbon dioxid, which sour 

 milk, spoil butter, raise bread, etc. 



This classification includes enzyms not in any way 

 connected with digestion either in animals (see Chap. X) 

 or in plants, but which carry on fermentations and putre- 

 factions (see Chap. XXIII), or are concerned in main- 

 taining the fertility of soil (see Chap. XXIV), or are 

 involved in the metabolic processes which supply the 

 living cells with energy. We may mention some of these 

 enzyms here, in addition to the descriptions in the places 

 referred to. 



The oxidases. — These are enzyms of various sorts 

 which accelerate the oxidations going on in living cells 

 and furnishing the cells with required energy. They are 

 concerned in the physiological utilization, as sources of 

 energy, of the carbohydrates, especially the sugars. They 

 may also be concerned in fermentations, and they are 

 undoubtedly concerned in some of the oxidations involved 

 in maintaining soil fertility. 



The de-amina.'^es take part in breaking down amino- 

 acids and similar complex nitrogenous compounds to the 

 ammonia and carbohydrate out of which they were 

 formed, making the carbohydrate available for oxidation 

 in the cell, by which energy is released for use. 



The coagulating enzyms are represented by rennet, ob- 

 tainable in commerce in the form of " junket tablets," 



