214 HYDROGEN ION CONCENTRATION AND PEPSIN DIGESTION 



protein determines the ease with which it is attacked by the enzyme. 

 The viscosity is assumed to be a measure of the hydration. The 

 same explanation has been proposed by Briicke/^ by Pfliederer/' 

 and recently by Traube.^^ The writer has been able to show,i^ how- 

 ever, that gelatin digests at the same rate in sulfuric or hydrochloric 

 acid solution (provided the pH is the same) although the swelHng, 

 which Ringer considers a measure of the hydration, is much greater 

 in hydrochloric than in sulfuric acid. It was also found^^ that the 

 rate of digestion of egg albumin solutions decreased as the viscosity 

 increased with the age of the solution instead of increasing as would 

 be expected if the rate of digestion was determined by the hydration 

 of the protein (as shown by the viscosity). Loeb^"* has shown that 

 the ionization of the protein and the viscosity and swelling are all 

 approximately proportional for a small range of acidity to the acid 

 side of the isoelectric point. The maximum for the swelling and 

 viscosity, however, occurs at about pH 3.4 whereas that for the 

 ionization is much further to the acid side and agrees very well for 

 that of the rate of digestion. This question will be discussed more 

 fully below. It is clear, however, that in certain cases the swelling 

 or viscosity and the iom'zation and rate of digestion may all be pro- 

 portional. It would seem from the experiments described here that 

 the determining factor for the rate of digestion is the ionization of 

 the protein, and the swelling and viscosity are secondary character- 

 istics which are probably also connected with the ionization. 



It is known that, with most proteins, pepsin becomes inactive at 

 a pH of about 4.5. This cannot be ascribed to the destruction of the 

 enzyme since the author^ found pepsin to be more stable in this 

 range of acidity than at any other. The ionization of most proteins 

 is very sHght at this pH, however, so that it would be expected (from 

 the hypothesis that it is the protein ion which is attacked by the 

 enzyme) that little or no hydrolysis should occur at this point. Oxy- 



^^Briicke, E., Sitzungsh. k. Akad. Wissensch. Math-naturw. CI., Wien., 1859, 

 xxxvii, 131. 



^^ Pfliederer, R., Arch. ges. Physiol., 1897, Ixvi, 605. 

 ^^Traube, M., Deutsch. med. Woch., 1919, xxvii, 

 1^ Northrop, J. H., /. Gen. Physiol., 1918-19, i, 607. 

 ^OLoeb, J., /. Gen. Physiol., 1918-19, i, 39; 1920-21, iii, 85. 



