JOHN H. NORTHROP 611 



in such cases, it is extremely difficult to obtain any independent 

 measurement of the equilibrium, there seems to be no way to test the 

 proposed mechanism. 



SUMMARY. 



1 . It is pointed out that the apparent exceptions to the law of mass 

 action found in enzyme reactions may be found in catalytic reactions 

 in strictly homogeneous solutions. 



2. These deviations in the rate of reaction from the law of mass 

 action may be explained by the hypothesis that the active mass of 

 the reacting substances is not directly proportional to the total 

 concentration of substance taken. 



3. In support of this suggestion it is shown that for any given con- 

 centration of pepsin the relative rate of digestion of concentrated and 

 of dilute protein solutions is always the same. If the rate of digestion 

 depended on the saturation of the surface of the enzyme by substrate 

 the relative rate of digestion of concentrated protein solutions should 

 increase more rapidly with the concentration of enzyme than that of 

 dilute solutions. This was found not to be true, even when the enzyme 

 could not be considered saturated in the dilute protein solutions. 



4. The rate of digestion and the conductivity of egg albumin so- 

 lutions of different concentration were found to be approximately 

 proportional at the same pH. This agrees with the hypothesis first 

 expressed by Pauli that the ionized protein is largely or entirely the 

 form which is attacked by the enzyme. 



5. The rate of digestion is diminished by a very large increase in 

 the viscosity of the protein solution. This effect is probably a me- 

 chanical one due to the retardation of the diffusion of the enzyme. 



