JOHN H. NORTHROP 



477 



sin to form a rather highly dissociated compound and that the pepsin 

 so combined is inactive. The concentration of active pepsin would 

 therefore be decreased by the peptone and the decrease would be 

 greater in concentrated than in dilute solution. This hypothesis also 

 accounts for the results of Experiment C in which the solution is 

 diluted with an inactivated portion of the same solution. If the in- 

 activated pepsin enters into equilibrium in the same way as the 



10 



12.5 25 50 06 100 



Concentration pepsin 



Fig. 2. Curves showing efifect of peptone on activity of diluted pepsin solutions 

 (c/. Table II). 



active pepsin, the concentration of active pepsin in a solution, diluted 

 with an inactivated portion, should decrease in direct proportion to 

 the total concentration. The experiment shows that this is the case. 

 (This question will be taken up more fully later.) The results of this 

 experiment show also that in order to determine the total amount of 

 pepsin present in solution it is necessary to use a dilution such that 

 the rate of digestion is directly proportional to the amount of enzyme 

 solution taken. If this is done the value for the total amount of 

 enzyme, found at dilutions where this value has become constant, is 

 an experimental determination of the total amount of enzyme present, 

 expressed, however, in arbitrary units. 



