JOHN H. NORTHROP 



467 



in the conductivity of an egg albumin solution under constant 

 conditions is considered as inversely proportional to the amount of 

 active enzyme present and was used as a measure of the enzyme 

 concentration. The experiments with different enzyme solutions 

 were not done at the same time and are not strictly comparable. The 

 experiments with the various acids, however, are comparable. 



It will be seen that in all the experiments the enzyme is most 

 stable at a pH of about 5.0, irrespective of the anion of the acid and 

 of the purity of the solution. Increasing the alkalinity of the solu- 

 tion causes a very great increase in the destruction of the enzyme. 

 There is some indication that the impure solutions are inactivated 

 more slowly under these conditions than the purer ones. 



TABLE II. 



Influence of Various Acids on the Destruction of Pepsin at Various Hydrogen Ion 



Concentrations. 



Increasing the acidity of the solution above pH 5.0 causes a very 

 slow increase in the amount of pepsin destroyed, and the quantity 

 inactivated is not influenced either by the purity of the solution or 

 by the anion of the acid. It would seem necessary to conclude from 

 the marked asymmetry of the curve for the destruction of the enz3nTie, 

 as plotted against the hydrogen ion concentration, that the process 

 of inactivation of the enzyme on the acid side of pH 5.0 differs from 

 the process of inactivation on the alkaline side of pH 5.0. 



Fig. 2 shows that the amount of pepsin remaining in solution after 

 24 hours at 38°C. is about the same throughout the range of acidity 

 in which the enzyme is active. The rate of destruction of the enzyme 

 therefore differs very little at a pH of 1.0 and a pH of 3.0. As is 

 well known, the activity of the enzyme varies greatly within this range. 



