JOHN H. NORTHROP 473 



of the molecule and the rate of reaction therefore varies directly with 

 the hydrogen ion concentration and not with the total acid concen- 

 tration. In sufficiently dilute solutions the two of course become 

 practically identical since the acid is then completely dissociated. It 

 will be shown in the succeeding part of this paper that pepsin solutions 

 obey the same laws as weak acid solutions in regard to the relation 

 between the total concentration and the rate of hydrolysis; and that 

 the divergence from the law of mass action is not due to any peculi- 

 arity of the enzyme reaction itself, but to the fact that the active 

 enzyme concentration is not always directly proportional to the total 

 enzyme concentration. 



Experimental Procedure and Results of the Present Investigation. 



In a former paper^ a method was described for determining the 

 rate of pepsin digestion by means of changes in the conductivity of 

 an egg albumin solution to which the pepsin had been added. From 

 these results the time necessary to cause a given change in the con- 

 ductivity of the solution was determined by graphic interpolation. 

 In the experiments reported in this paper the time in hours necessary 

 to cause the first 10 per cent change was taken as the standard. The 



reciprocal of this time then ( ^rr ) is proportional to the mean 



rate of digestion for the first 10 per cent of the reaction. For con- 

 venience this value will be spoken of as the amount of "active pep- 

 sin." The volume noted in the tables is considered in every case 

 as the number of cc. of diluted enzyme solution containing 1 cc. of 

 the original enzyme solution. It is therefore a measure of the dilu- 

 tion of the pepsin before adding to the egg albumin solution. Since 

 1 cc. of this diluted solution was added to 25 cc. of egg albumin 

 in order to make a determination, the concentration of the pepsin 

 during the actual digestion was 26 of that shown in Tables I, II, III, 

 and V. The conductivity and pH of all solutions were kept equal 

 as nearly as possible. It was pointed out that this change in con- 

 ductivity did not exactly parallel the change in amino nitrogen of the 

 solution, and so cannot be considered as representing the true course 



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



