JOHN H. NORTHROP 599 



to explain many of the facts but in the absence of direct evidence 

 it can hardly be considered proved. 



According to this theory a certain amount of enzyme can act only 

 on a limited amount of substrate; after this quantity is reached any 

 excess of substrate has no effect on the reaction. It is clear that 

 according to this mechanism it is the ratio of the concentration of 

 substrate to that of the enzyme which causes the relative decrease 

 in the rate of digestion of the substrate as the concentration of sub- 

 strate increases, and not the actual concentration of substrate present 

 in the solution. If the effect, however, is due to the fact that the 

 active concentration of substrate is not directly proportional to the 

 total concentration then the falling off of the rate of reaction with 

 increasing substrate concentration is independent of the ratio of sub- 

 strate to enzyme and depends only on the actual concentration of 

 substrate. 



It occurred to the writer that this question might be tested experi- 

 mentally by comparing the rate of digestion of dift'erent substrate 

 concentrations when hydrolyzed with different enzyme concentrations. 

 Assume, for instance, that the substrate at concentration 10 6* is 

 found to hydrolyze five times as rapidly as the substrate at concen- 

 tration S, when enzyme concentration E is used. According to the 

 monomolecular formula the substrate at concentration 10 S should 

 digest ten times as rapidly as the substrate at concentration 6*. The 

 saturation hypothesis would explain this divergence by the assumption 

 that the enzyme becomes saturated with substrate at a concentration 

 of the latter of less than 10 S. In concentration 10 S, therefore, much 

 of the substrate takes no part in the reaction and the rate of reaction 

 is less than the expected. It would be predicted further that in- 

 creasing the substrate concentration from 10 5 to 20 5 would have 

 relatively less effect on the rate of reaction than increasing the sub- 

 strate concentration from 5 to 2 .S*. This is true. It follows also 

 on the saturation hypothesis that increasing the enzyme concentration 

 from £ to 10 £ should have a relatively greater effect on the rate of 

 digestion of substrate 10 S than on the rate of digestion of substrate 

 at concentration S; since it was assumed in accounting for the eff'ect of 

 increasing the substrate concentration that the enzyme (at concen- 

 tration E) was more saturated with substrate at (substrate) concen- 



