478 EFFECT OF ENZYME ON DIGESTION OF PROTEINS 



The effect of the peptone in Solution B might be qualitatively ex- 

 plained by the hypothesis that the peptone in the solution combines 

 with the substrate and so reduces the concentration of active sub- 

 strate molecules, thereby causing the enzyme to become "saturated" 

 with substrate. This explanation, however, fails to explain the re- 

 sults of Experiment C since the same concentration of peptone is 

 present here as in Solution B and yet in this experiment the rate is 

 proportional to the amount of enzyme taken. 



According to the hypotheses outlined above, the rate of digestion 

 is always directly proportional to the concentration of active pepsin; 

 and the apparent divergence from this relation is due to the fact that 

 the peptone combines with the pepsin and so renders it inactive. 

 The total concentration of enzyme and the active concentration are 

 then no longer equal nor directly proportional; and since the rate is 

 proportional to the active concentration, it is not proportional to 

 the total concentration. It is also assumed that the pepsin and pep- 

 tone combine according to the law of mass action. This reaction 

 may be considered to take place as follows: 



Pepsin + peptone j=^ pepsin— peptone 



and if the reaction obeys the law of mass action the following equa- 

 tion must hold. 



Concentration pepsin X concentration peptone . 



Concentration pepsin-peptone 



or 



Q-id-{E-Q) + x) 



E-Q 



= K (2) 



where E is the total enzyme concentration, Q is the concentration of 

 active (uncombined) pepsin, d is the concentration of peptone present 

 at the beginning of the reaction, and x is the amount of peptone 

 formed during the course of the reaction at the time t. K is the equi- 

 librium constant expressed in arbitrary units since it contains the 

 unit of measurement used. (For the sake of simplicity only the case 

 is considered in which the substance combined with the pepsin at 

 the beginning of the reaction is the same as that formed during the 



