INFLUENCE UPON HYDROLYSES BY ENZYMES 217 



also able to inactivate enzymes provided oxygen be present. Evi- 

 dently two different types of change in enzymes may be brought about 

 by light, involving different parts of the spectrum. 



THE INFLUENCE OF REACTION UPON HYDROLYSES BY 

 ENZYMES. 



The great majority of the enzymes are very decidedly influenced by 

 the reaction, or H + or OH" ion concentration of the medium in which 

 they act. For each enzyme or group of enzymes there is a certain 

 range of H + or OH~ concentrations within which they work best and 

 below or above which their activity is impeded. The upper limit of 

 H+ or OH~ concentrations is set by the destruction of the enzyme 

 which rapidly occurs in solutions which are too acid or alkaline. The 

 factor which sets the lower limit is not so easy to perceive since the 

 stability of the enzymes in neutral solutions is often greater than it is 

 in the faintly acid or alkaline solutions in which their hydrolyzing 

 activity is most favorably displayed. 



The most striking dependence upon reaction is shown by the Proteo- 

 lytic Enzymes, Pepsin and Trypsin. Pepsin acts best in a faintly acid, 

 while trypsin acts best in a faintly alkaline medium. A slight excess 

 of alkali rapidly destroys pepsin, while an even slighter acidity inacti- 

 vates trypsin. Both of these enzymes will hydrolyze proteins in 

 neutral solutions, but their activity is much inferior to that which they 

 will display in a medium of favorable reaction. 



So far as pepsin is concerned it is not difficult to infer that the need 

 for a slight acidity of the medium arises from the fact that a compound 

 of the pepsin with the acid is formed which possesses much greater 

 proteolytic power than the uncombined pepsin. This is evidenced 

 by the fact that not all acids are equally efficient in promoting the 

 hydrolysis of protein by pepsin, there being a marked specificity in the 

 relationship of pepsin to Hydrochloric Acid. While other acids will 

 accelerate the hydrolysis of proteins by pepsin, their accelerative 

 influence is far inferior to that of hydrochloric acid, and the favoring 

 action of different acids, instead of running parallel to their degree of 

 electrolytic dissociation, as we should expect if it were an effect purely 

 due to hydrogen ions, bears, in fact, no relationship to the "strength," 

 i. e., dissociation of the acid. Lactic Acid, for example, in equimolecular 

 solutions, has a more favorable effect than sulphuric acid. 



In the case of Trypsin the accelerative action of alkalies runs strictly 

 parallel to their dissociation, so that here we are left in doubt as to 

 whether the effect is due to the formation of a compound of the trypsin 

 with the base employed, or whether the alkali does not, on the con- 

 trary, act as an accessory catalyzer, so altering the substrate as to 

 render it more susceptible to attack by the enzyme. 



There are certain observations, however, which seem to show that 

 in this case also a compound is formed of the trypsin and the added 



