JOHN H. NORTHROP 227 



Nothing has been said in regard to the quantitative agreement 

 between the increasing amounts of ionized protein found in the solu- 

 tion (as shown by the conductivity values) and the amount predicted 

 by the hydrolysis theory of the formation of salts of weak bases and 

 strong acids. There is little doubt that the values are in qualitative 

 agreement with such a theory. In order to make a quantitative 

 comparison, however, it would be necessary to know the ionization 

 constant of the protein and of the protein salt and also the number 

 of hydroxyl (or amino) groups in the protein molecule as well as the 

 molecular weight of the protein. Since these values are not known 

 with any degree of certainty there appears to be no value at present 

 in attempting to apply the hydrolysis equations to the data obtained. 



It it clear that the hypothesis as outlined above for the hydrolysis 

 of proteins by pepsin cannot be extended directly to enzymes in 

 general, since in many cases the substrate is not known to exist in 

 an ionized condition at all. It is possible, however, that ionization 

 is really present or that the equihbrium instead of being ionic is 

 between two tautomeric forms of the substrate, only one of which 

 is attacked by the enzyme. Furthermore, it is clear that even in 

 the case of proteins there are difficulties in the way since the pepsin 

 obtained from young animals, or a similar enzyme preparation from 

 yeast or other microorganisms, is said to have a different optimum 

 hydrogen ion concentration than that found for the pepsin used in 

 these experiments. The activity of these enzyme preparations there- 

 fore would not be found to depend on the ionization of the protein. 

 It is possible of course that the enzyme preparations mentioned may 

 contain several proteolytic enzymes and that the action observed is 

 a combination of the action of several enzymes. Dernby^^ has shown 

 that this is a very probable explanation of the action of the autolytic 

 enzymes. The optimum hydrogen ion concentration for the activity 

 of the pepsin used in these experiments agrees very closely with that 

 found by Ringer for pepsin prepared by him directly from gastric 

 juice and very carefully purified. Ringer's pepsin probably repre- 

 sents as pure an enzyme preparation as it is possible to prepare. 

 There is every reason to suppose therefore that the enzyme used in 

 this work was not a mixture of several enzymes. 



