CHEMISTRY OF DIGESTION AND NUTRITION. 281 



or oxidases, form u group that plays an important part in the functional 

 metabolism of the tissues, but at present our knowledge of their existence 

 and functional value in the living organism is very uncertain. 



A great number of general reactions have been discovered, applicable, with 

 an exception here and there, to the whole group of enzymes. Among these 

 reactions the following are the most useful or significant : 



1. Solubility. — The enzymes are soluble in water. They are also solu- 

 ble in glycerin, this being the most generally useful solvent for obtaining 

 extracts of the enzymes from the organs in which they are formed. 



2. Effect of Temperature. — In a moist condition they are destroyed by 

 temperatures below the boiling-point ; 60° to 80° C. are the limits actually 

 observed. Very low temperatures retard or even suspend entirely (0° C.) their 

 action, without, however, destroying the enzyme. For each enzyme there is 

 a temperature at which its action is greatest. 



3. Incompleteness of Action. — With the exception perhaps of the coagulat- 

 ing enzymes, they are characterized by the fact that in any given solution they 

 never completely destroy the substance upon which they act. It seems that 

 the products of their activity, as they accumulate, finally prevent the enzymes 

 from acting further; when these products are removed the action of the enzyme 

 begins again. The most familiar example of this very striking peculiarity is 

 found in the action of pepsin on proteids. The products of digestion in this 

 case are peptones and proteoses, and when they have reached a certain concen- 

 tration they prevent any further proteolysis on the part of the pepsin. 



4. Relation of the Amount of Enzyme to the Effect it Produces. — With most 

 substances the extent of the chemical change produced is proportional to the 

 amount of the substance entering into the reaction. With the enzymes this is 

 not so. Except for very small quantities, it may be said that the amount of 

 change caused is independent of the amount of enzyme present, or, to state the 

 matter more accurately, " with increasing amounts of enzymes the extent of 

 action also increases, reaching a maximum with a certain percentage of enzyme; 

 increase of enzyme beyond this has no effect." 1 This fact was formerly inter- 

 preted to mean that the enzyme is not used up — that is, not permanently altered 

 — by the reaction that it causes. This belief, indeed, must be true substan- 

 tially, but it has been found practically that a given solution of enzyme cannot 

 be used over and over again indefinitely. It is generally believed now that, 

 although an enzyme causes an amount of change in the substance it acta upon 

 altogether out of proportion to the amount of its own substance, neverthe- 

 less it is eventually destroyed; its action is not unlimited. Whether this using 

 up of the enzyme is a necessary result of its activity, or is, as it were, an acci- 

 dental effect from spontaneous changes in its own molecule, remains unde- 

 termined. 



Theories of the Manner of Action of the Enzymes. — It is now believed 

 that the action of many of the body enzymes, especially the digestive 

 enzymes, is that of hydrating agents; they produce their effeel by what is 



1 Tummumi : Zeitechrift fur physiologi&che Chemie, 1892, I'.d. xvi. S. '_'7I. 



