226 HYDROGEN ION CONCENTRATION AND PEPSIN DIGESTION 



plotted against the pH are nearly parallel throughout. The addition 

 of a salt with the same anion as the acid to a solution of protein 

 already containing the optimum amount of the acid has the same 

 depressing effect on the digestion as has the addition of the equivalent 

 amount of acid. These facts are in quantitative agreement with the 

 hypothesis that the determining factor in the digestion of proteins 

 by pepsin is the amount of ionized protein present in the solution. 

 It was shown in a previous paper^^ that this would also account for 

 the peculiar relation between the rate of digestion and the concen- 

 tration of protein. The amount of ionized protein in the solution 

 depends on the amount of salt formed between the protein (a weak 

 base) and the acid. This quantity, in turn, according to the hydro- 

 lysis theory of the salts of weak bases and strong acids, is a function 

 of the hydrogen ion concentration, up to the point at which all the 

 protein is combined with the acid as a salt. This point is the optimum 

 hydrogen ion concentration for digestion, since the solution now 

 contains the maximum concentration of protein ions. The hydrogen 

 ion concentration in this range therefore is merely a convenient 

 indicator of the amount of ionized protein present in the solution 

 and takes no active part in the hydrolysis. After sufficient acid 

 has been added to combine with all the protein, i.e. at pH of about 

 2.0, the further addition of acid serves to depress the ionization of 

 the protein salt by increasing the concentration of the common 

 anion. The hydrogen ion concentration is, therefore, no longer an 

 indicator of the amount of ionized protein present, since this quantity is 

 now determined by the anion concentration. Hence on the acid side 

 of the optimum the addition of the same concentration of anion should 

 have the same influence on the rate of digestion irrespective of whether 

 it is combined with hydrogen or some other ion (provided, of course, 

 that there is no other secondary effect of the other ion). The pro- 

 posed mechanism is very similar to that suggested by Stieglitz and 

 his coworkers^^ for the hydrolysis of the imido esters. 



Pekelharing and Ringer^ have shown that pure pepsin in acid 

 solution is always negatively charged; i.e., it is an anion. The 

 experiments described above show further that it behaves just as 

 would be expected of any anion in the presence of a salt containing 

 the protein ion as the cation and as has been shown by Loeb-" to be 

 the case with inorganic anions. 



29 Stieglitz, J., and collaborators, Am. Chem. J., 1908, xxxix, 29, 164, 402, 437, 

 586, 719. 



