JOHN H. NORTHROP 



217 



solution was about 1 X 10~^ reciprocal ohms. Increasing amounts of HCl were 

 added to 50 cc. portions of this solution and the total volume made up to 100 

 cc. The conductivity and digestion of the solution were then determined as 

 described for the egg albumin. 



The Optimum Hydrogen Ion Concentration for Pepsin Digestion. 



The optimum hydrogen ion concentration for the activity of pepsin 

 has been determined many times. All the methods used for following 

 the digestion, however, have depended on the change in some physical 

 property of the protein. It seemed of interest therefore to determine 

 the optimum degree of acidity for the reaction when the hydrolysis 

 was followed by means of the increase in amino nitrogen, which 



.S C: 

 c: CD 



•s I 0.1 



-o 



o \~ 



pH 



2.2 20 1.8 1.6 14 1.2 1.0 0.8 0.6 04 0.2 



Fig. 2. Influence of pH on the rate of digestion of egg albumin. 



probably represents correctly the actual course of the digestion. 

 The method has the disadvantage, however, that only comparatively 

 large changes can be followed. The results of an experiment made 

 with egg albumin solutions of different pH (adjusted with HCl) are 

 given in Fig. 2. 



The time of digestion was 4 hours. The figure shows that the opti- 

 mum acidity for the digestion as determined by the increase in amino 

 nitrogen is at about pH 1.0 (0.1 n). This is sHghtly more acid than 

 that found by Sorensen,i MichaeKs and Mendelssohn, ^^ or Okada,^^ and 

 much more acid than that found byRinger.^^ It must be remembered, 



-^Michaelis, L., and Mendelssohn, A., Biochem. Z., 1914, Ixv, 1. 

 24 0kada, S., Biochem. J., 1916, x, 126. 



