218 HYDROGEN ION CONCENTRATION AND PEPSIN DIGESTION 



however, that the chemical changes followed by the increase in amino 

 nitrogen represent much more complete hydrolysis than those fol- 

 lowed by the other authors. ' The curve therefore probably does not 

 represent the correct optimum for the digestion of the protein itself 

 but probably also the digestion of some of the primary products. 

 The careful work of Ringer has shown that the optimum zone for 

 the digestion of these products extends further to the acid side than 

 the zone for the digestion of the protein itself. This probably accounts 

 for the difference in the optimum found by the different methods 

 and agrees with the results of Sorensen^ who found that the optimum 

 shifts to the acid side with more complete digestion. 



The Effect of Adding Salt with a Common Ion to a Solution Already 

 Containing the Optimum Amount of Acid. 



It will be noted from the curve (Fig. 2) that the amount of digestion 

 increases with increasing amounts of acid in the solution until the 

 hydrogen ion concentration is about 0.1 N and then decreases. Ac- 

 cording to the hypothesis that it is the ionized protein which is hydro- 

 lyzed by the pepsin, the increase in digestion from pH 4.0 to 1.0 is 

 due to the fact that as acid is added to the albumin more protein salt 

 and hence more protein ions are formed in the solution, until all the 

 albumin is present as salt. The addition of a further amount of acid 

 serves to depress the concentration of protein ions again due to the 

 effect of the common ion. According to this mechanism the hydrogen 

 ion concentration is the determining factor on the alkaline side of 

 the optimum while on the acid side the concentration of the anion 

 is the determining factor. It can be predicted therefore that if a 

 solution of a salt (having the same anion as the acid) is added to a 

 solution of the protein which already contains the optimum amount 

 of acid, the depressing effect of the salt on the digestion should be 

 the same as if excess acid had been added, provided the final anion 

 concentration is the same. The conductivity of the albumin salt 

 should also be diminished. In the case of egg albumin this cannot 

 be experimentally verified owing to the fact that the albumin pre- 

 cipitates under these conditions, and also since the conductivity of 

 the protein in such strongly acid solutions is so small, compared to 



