JOHN H. NORTHROP 497 



zyme in the solid phase and that the kinetics of the reaction are the 

 same there as in the liquid phase. There is some experimental evi- 

 dence in favor of this point of view. It was found that the rate of 

 digestion of edestin was the same when in solution and when sus- 

 pended in the enzyme solution. ^i Dauwe^* has shown that pepsin 

 can diffuse through a membrane of solid protein. 



The hypothesis enables us to set an upper limit for the purity of 

 an enzyme preparation. It was found, for instance, in Experiment 1, 

 that the enzyme solution used contained about twenty-seven (arbi- 

 trary) units of pepsin and about thirty units of peptone. Assuming 

 that the combining weights of the substances are approximately the 

 same it is obvious that the original preparation could not have been 

 more than 50 per cent pure pepsin. It is, however, quite possible 

 that the enzyme may be combined with some substance and still 

 retain its activity (as found for invertase combined with charcoal 

 by Nelson and Griffin^^) or that impurities are present which do not 

 combine with the enzyme at all. It is not possible therefore to as- 

 sume that the active pepsin consists solely of pepsin molecules. For 

 similar reasons it is not possible to draw any definite conclusions 

 from the results of Experiment 2 in which it was found that a 1 per 

 cent egg albumin solution after complete digestion contained about 

 two arbitrary units of peptone while a 1 per cent pepsin solution 

 contained about ten units of pepsin. 



It is well known that the kinetics of enzyme reactions differ in 

 another respect from the general laws of chemical reactions in that 

 the rate of reaction in high concentration of substrate does not vary 

 directly with the total substrate concentration. This phenomenon is 

 very similar to the one discussed in the present paper and it would 

 seem that the same explanation applies to both cases; i.e., that the 

 active substrate concentration is not directly proportional to the 

 total substrate concentration. 



It may be pointed out that, according to the above mechanism of 

 the reaction, pepsin cannot be considered a catalyst in the sense of 

 the classical definition since it combines with some, at least, of the 

 products of reaction and so enters directly into the equation. Since 



2^ Dauwe, F., Beitr. Chein. Physiol, u. Path., 1905, vi, 426. 



25 Nelson, J. M., and Griffin, E. G., /. Am. Chem. Soc, 1916, xxxviii, 1109. 



