470 INACTIVATION OF PEPSIN SOLUTIONS 



to that of the solution containing the highest amount of acid. 1 cc. 

 of this diluted "solution was then added to a standard egg albumin 

 solution and the time necessary to cause a 10 per cent change in the 

 conductivity of the latter determined as described in a previous 

 paper. ^ The relative concentration of active pepsin was about the 

 same at the beginning of each experiment. This quantity was taken 

 as 10 in each case. Under the conditions of these experiments neither 

 the products of the digestion of the egg albumin nor the inactivated 

 pepsin interferes with the determination; i.e., the reciprocal of the 

 time to cause a given change is directly proportional to the total 

 quantity of active pepsin present. 



SUMMARY. 



1. Pepsin in solution at 38°C. fs most stable at a hydrogen ion con- 

 centration of about 10~^ (pH 5.0). 



2. Increasing the hydrogen ion concentration above pH 5.0 causes 

 a slow increase in the rate of destruction of pepsin. 



3. Decreasing the hydrogen ion concentration below pH 5.0 causes 

 a very rapid increase in the rate of destruction of the enzyme. 



4. Neither the purity of the enzyme solution nor the anion of the 

 acid used has any marked effect on the rate of destruction or on the 

 zone of hydrogen ion concentration in which the enzyme is most 

 stable. 



5. The existence of an optimum range of hydrogen ion concentration 

 for the digestion of proteins by pepsin cannot be explained by the 

 destruction of the enzyme by either too weak or too strong acid. 



9 Northrop, J. H., J. Gen. Physiol, 1919-20, ii, 113. 



