ENZYMES AND THEIR ACTION 185 



When an enzyme has once been inactivated by heating, or " killed," 

 it can never be restored to activity again. 



Enzymes are extremely sensitive to acids, bases, or salts, their 

 activity being often enormously enhanced or, in other cases, 

 entirely inhibited, by the presence in the reacting medium of very 

 small amounts of free acids, or bases, or even of certain neutral 

 salts. For example, pepsin, the enzyme of the stomach will act 

 only in the presence of a slightly acid medium and is wholly inactive 

 in a mixture which contains even the slightest amount of free 

 alkaline material; while trypsin, the similar enzyme of the intes- 

 tine, acts only under alkaline conditions. Practically all enzymes 

 are rendered inactive, but not destroyed, by the presence of either 

 acid or alkali in excess of N/10 strength. Many will act only in 

 the presence of small quantities of certain specific neutral salts; 

 while, on the other hand, other salts are powerful inhibitors of 

 enzyme action. Enzymes often differ from the protoplasm which 

 secretes them in their response to antiseptics, such as toluene, 

 xylene, etc., which inhibit the activity or growth of the cell, but 

 have no effect upon the activity of the enzymes which it contains. 



THE CHEMICAL NATURE OF ENZYMES 



Nothing is known with certainty concerning the chemical 

 nature of enzymes. Being colloidal in nature, they adsorb carbo- 

 hydrates, proteins, fats, etc., so that active enzyme preparations 

 often respond to the characteristic tests for these groups of sub- 

 stances; and many investigators have reported what has, at first, 

 seemed to be conclusive evidence that some particular enzyme 

 which they have studied is either a carbohydrate, a protein, or 

 some other type of organic compound. Later investigations have 

 always shown, however, that if the preparation in question be 

 submitted to the digestive action of the enzymes which hydrolyze 

 the particular type of substances to which it is supposed to belong, 

 the material will lose its characteristic protein, or carbohydrate, 

 etc., properties, without losing its specific activity, thus clearly 

 indicating that the substance which responds to the characteristic 

 tests for some well-known type of organic compounds is present as 

 an impurity and is not the enzyme itself. 



The present state of knowledge concerning the' nature of 

 enzymes seems to indicate that, like the inorganic catalysts, they 



