JOHN H. NORTHROP 225 



a greater volume of gelatin present at this point. It will be seen, 

 however, from Table II and Fig. 3 that this is not true since the 

 figures show that there is a maximum even when the results are calcu- 

 lated to the basis of pepsin per cubic centimeter of gelatin. That 

 is, there is not only more pepsin combined with a gram of gelatin 

 at this pH but also the concentration of the pepsin in the gelatin 

 is greatest here. There is considerable uncertainty as to the pH 

 measurements since, as the table shows, the reaction of the liquid 

 was always considerably more acid than that of the gelatin. In 

 most of the experiments the difference was much more marked than 

 in the experiment given; in some cases the maximum fell at about 

 pH 2.2. This agrees much more closely with the optimum acidity 

 found for digestion and for the ionization of the protein. Owing to 

 the uncertainty of the pH measurement, however, it is probably 

 better to make no definite statement as to the exact position of the 

 optimum acidity for the combination between the gelatin and pepsin. 

 The determining factor in regulating the amount of pepsin which is 

 combined with the gelatin is the chemical condition of the gelatin 

 and pepsin and not a difference in the rate of diffusion of the pepsin 

 since the same curve is obtained irrespective of the time (after the 

 first few minutes) during which the gelatin is left in the solution. 

 The simplest explanation would seem to be that the gelatin combines 

 only with the ionized protein and the amount combined therefore is 

 dependent on the amount of ionized gelatin present. Pepsin there- 

 fore behaves just as do the inorganic anions studied by Loeb^*' as far 

 as the influence of the hydrogen ion concentration on the combination 

 is concerned. 



DISCUSSION AND SUMMARY. 



The experiments described above show that the rate of digestion 

 and the conductivity of protein solutions are very closely parallel. 

 If the isoelectric point of a protein is at a lower hydrogen ion con- 

 centration than that of another, the conductivity and also the rate 

 of digestion of the first protein extends further to the alkahne side. 

 The optimum hydrogen ion concentration for the rate of digestion 

 and the degree of ionization (conductivity) of gelatin solutions is 

 the same, and the curves for the ionization and rate of digestion as 



