JOHN H. NORTHROP 605 



centration is due to some equilibrium in the protein solution itself 

 and is independent of the enzyme concentration. 



It is well known that in acid solution protein exists in an ionized 

 condition. The concentration of ionized protein is not directly pro- 

 portional to the total concentration but will increase more slowly 

 than the total concentration. It is obvious, therefore, that the rate 

 of digestion will be more nearly proportional to the concentration of 

 ionized protein than to the total concentration of protein. The 

 hypothesis, then, that the ionized protein is the form which takes 

 part in the reaction, will allow a nearer approach to the predicted 

 rate of reaction. Pauli^^ has suggested that the enzyme attacks 

 the ionized protein; there seems, however, to be no direct evidence 

 for this view. It can be tested experimentally by comparing the 

 rate of digestion with the degree of ionization of the protein. 



The concentration of ionized protein can be determined approximately from the 

 pH and conductivity measurements. If the total conductivity and the hydrogen 

 ion concentration of a solution are accurately known, the conductivitA'^ due to the 

 protein-salt ions can be determined by subtracting the conductivit}'' of the free 

 HCl from that of the solution. The validity of this method rests on three condi- 

 tions: (1) the conductivity of the free HCI in the solution is the same as that of 

 the same concentration of acid in water solution; (2) the Cci- is equal to or greater 

 than the Ch; and (3) the hydrogen ion concentration as determined by the e.m.f. 

 method must agree with that found by the conductivity method. -** The first 

 assumption cannot be tested directly but it has been shown by Hardv"^ and by 

 Loeb^^ that the viscosity of the solution has no significant effect on the conduc- 

 tivity since the viscosity may increase till the solution is nearly solid without an 

 appreciable change in the conductivity. This experiment was repeated and con- 

 firmed. The second condition can be shown to hold also by direct measurements 

 of the chlorine ion concentration by means of concentration cells as was done by 

 Manabe and Matula.^^ Many measurements of this kind were made and con- 

 firmed those of the above mentioned authors; namely, the chlorine ion concen- 

 tration is always equal to or greater than the hydrogen ion concentration. It was 



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



^^ For the purpose of these experiments it is only necessary that the conduc- 

 tivity and E. M. F. methods should agree. The absolute value for the Ch is 

 immaterial. 



21 Hardy, \V. B., J. Physiol, 1905, xxxiii, 251. 



22Loeb, J., J. Gen. Physiol, 1918-19, i, 559. 



-^ Manabe, K., and ]\Iatula, J., Biochem. Z., 1913, iii, 369, 



