JOHN H. NORTHROP 505 



{b) The casein and gelatin are hydrolyzed by different enz3rmes 

 acting entirely independently. 



The amount digested at any time in the mixture must then be 

 equal to the sum of the amounts digested at the same time when 

 the casein and gelatin are hydrolyzed separately. The same result 

 would be predicted if the rate of hydrolysis depended on the concen- 

 tration of free enzymes. This is also contradicted by the experiment. 



(c) The casein and gelatin are acted on by two different enzymes, 

 but the products formed by either enzyme inhibit the action of the 

 other. 



The rate of hydrolysis of the mixture, according to this mechanism, 

 will equal the sum of the rates of hydrolysis of the two separate 

 solutions but the amount of hydrolysis at any given time will be 

 slightly less in the mixture than the sum of the two separate solutions. 

 The same result would be predicted if the rate of hydrolysis were 

 proportional to the concentration of free enzymes. 



This is the experimental result. 



II. Rate of Hydrolysis is Proportional to the Concentration of Free 



Enzyme. 



(a) The same enzyme acts on both the casein and gelatin. This 

 assumption predicts that the rate of hydrolysis of the mixture will 

 be equal to the sum of the rates of the two solutions but the amount 

 of hydrolysis of the mixture will be less than the sum of the two sepa- 

 rate solutions. This is the experimental result. 



We are, therefore, bound to conclude either, first, that the rate 

 of hydrolysis is proportional to the concentration of free enzyme 

 (i.e. that the amount combined is negligibly small), or second, that 

 there are two enzymes at work, each of which is inhibited by the 

 products of hydrolysis formed by the other. This latter assumption 

 is gratuitous unless some independent evidence can be found for the 

 existence of two such enzymes. Many experiments were made from 

 this point of view but no evidence could be found for the existence 

 of two enzymes. The ratio of the rate of hydrolysis of gelatin and 

 casein was always the same within the experimental error of about 

 1 per cent, no matter how the trypsin preparation was treated. 



