JOHN H. NORTHROP 213 



tration of the substrate and the rate of hydrolysis. Experiments 

 described in a former paper" show that the rate of hydrolysis of 

 protein solutions of var3ring concentration but the' same pH was 

 directly proportional to the amount of ionized protein present in 

 the solution but not to the total concentration of protein. They 

 agree therefore with the hypothesis that the ionized protein is the 

 form which takes part in the reaction. If this explanation is correct 

 it follows that the optimum hydrogen ion concentration for the activ- 

 ity of pepsin is also due to the increased ionization of the protein 

 and must coincide with the hydrogen ion concentration at which the 

 protein solution contains the greatest number of protein ions. (It 

 was first suggested by Pauli^^ that the ionized protein was the form 

 which was attacked. Euler^^ and Arrhenius^* have made a similar 

 suggestion. Ringer^^ considers also that the ionization of the sub- 

 strate has an influence on the rate of digestion at least in the later 

 stages.) It should be possible therefore to determine the optimum 

 degree of acidity for pepsin digestion by measuring the conductivity 

 of the protein solution. It will be shown below that this is true. 

 It will further be shown that the range of hydrogen ion concentration 

 in which the enzyme is active shifts in the same sense as the con- 

 ductivity of the protein solution when a protein of different isoelectric 

 point is used, and also that when the protein is insoluble the enzyme 

 combines with it only over that range of hydrogen ion concentration 

 in which the enzyme is active and in which the protein is ionized. 



The Influence of the Isoelectric Point of the Protein on the Activity of 

 Pepsin at Di^erent Hydrogen Ion Concentrations. 



Ringer^'' has already shown that the optimum hydrogen ion con- 

 centration for the digestion of proteins by pepsin varies somewhat 

 with the protein hydrolyzed and with the acid used. He accounts 

 for this phenomenon by the assumption that the hydration of the 



11 Northrop, J. H., /. Gen. Physiol., 1919-20, ii, 595. 



12 Pauli, W., Arch. ges. Physiol., 1910, cxxxvi, 483. 



1^ Euler, H., Allgemeine Chemie der Enzymes, Wiesbaden, 1910. 

 i^Arrhenius, S., Quantitative laws in biological chemistry, London, 1915, 44. 

 15 Ringer, W. E., Arch. Neerl. Physiol., 1917-18, ii, 571. 



