JOHN H. NORTHROP 609 



the enzpne rather than to a change in the resistance of the protein. 

 This is borne out by the fact that pepsin diffuses only very slowly 

 through coagulated protein {cf. for instance Dauwe).^" Reformatsky" 

 has shown that the rate of hydrolysis of methyl acetate by acids is 

 identical in water solution and in a solid agar gel. In this case the 

 rate of diffusion of the H+ is also independent of the viscosity (Voigt- 

 lander).-* 



Ringer^^ has pointed out that the optimum pH for the digestion of 

 protein coincides approximately with the maximum viscosity and has 

 suggested that the rate of digestion is dependent on the degree of 

 hydration of the protein; the viscosity of the solution is also assumed 

 to be a measure of the degree of hydration. It would seem from the 

 experiment just described that an increase in viscosity decreases the 

 rate of digestion instead of increasing it, as supposed by Ringer. If 

 the protein ion is the active form of the protein the optimum pH 

 should depend on the maximum degree of ionization. According to 

 Pauli^^ the maximum viscosity also depends on the ionization. Loeb^- 

 has shown, however, that this is not true. The hypothesis outlined 

 above requires that the rate of digestion of a protein solution at dif- 

 ferent pH should be directly proportional to the amount of protein 

 ionized. Preliminary experiments show that this is true, qualitatively 

 at least. Unfortunately the change in conductivity (as pointed out 

 above) cannot be used to follow the rate of digestion at lower Ch so 

 that the experimental difhculties are much greater. 

 . The results of the present paper may be considered in qualitative 

 agreement at least with the mechanism of pepsin digestion as out- 

 lined in the preceding paper. ^^ The hypothesis advanced considers 

 that there is an equilibrium in the pepsin solution between pepsin 

 and peptone (substances combining with pepsin and so rendering it 

 inactive) . There is also an equilibrium between ionized and unionized 

 protein in the protein solution. The reaction takes place according 



^® Dauwe, F., Beitr. chem. Physiol, ii. Path., 1905, vi, 426. 



^"^ Reformatsky, S., Z. physik. Chem., 1891, vii, 34. 



^* \'oigtlander, F., Z. physik. Chem., 1889, iii, 316. 



23 Ringer, W. E., Arch. Need. Phys., 1918, ii, 571; Z. physiol. Chem., 1915, 

 xcv, 195. Ringer considers that the charge on the protein is also of importance. 

 "This agrees with the present experiments. 



