JOHN H. NORTHROP 469 



poles at the same pH) and gave a change in the direction of migra- 

 tion at about pH 3.0. There is no relation between this point and 

 either the resistance of the enzyme to acid or the rate of its action on 

 proteins. It is probable that this is not the isoelectric point of pep- 

 sin itself but that of a compound formed between pepsin and some 

 other substance in the solution, since Peckelharing and Ringer^ found 

 that very pure pepsin solutions showed no isoelectric point. 



No evidence was found that the inactivation of the enzyme was 

 reversible under the conditions of these experiments although many 

 experiments were made with this point in view.^ 



The results show that digestion experiments with pepsin cannot be 

 carried out at 38° for longer than 24 hours without being complicated 

 by the fact that the enzyme concentration is lower at the end of the 

 experiment than at the beginning. They also show that in experi- 

 ments on the decomposition temperature it is necessary to consider 

 the reaction of the medium. 



The general effect of the hydrogen ion concentration on the sta- 

 bility of the enzyme resembles that described by Falk^ for lipase, and 

 by FrankeP for papain. In the case of papain, however, the influ- 

 ence of the reaction is reversed; i.e., papain is more sensitive 1o acid 

 than to alkali. 



Experimental Procedure. 



Pepsin Preparations Used. — Active: Fairchild's pepsin u. s. P. 1:19,500 

 Weak: Pepsin u. s. p. 1:3,000. 



Hydrogen Ion Determinations. — All determinations were made by the E. M. F. 

 method. 



Determination of the Relative Amount of Pepsin in Solution. 



The enzyme solution was made up as shown in the tables and 

 placed in a water bath at 38 ± 0.1°C. 5 cc. of the solution were 

 pipetted out for analysis and 5 cc. of an acid solution added of such 

 strength as to make the final acid concentration in each case equal 



^ Peckelharing, C. A., and Ringer, W. E., Z. physiol. Chem., 1911, Ixxv, 282. 

 8 Tichomirow, N. P., Z. physiol. Chem., 1908, Iv, 107. 

 7 Falk, K. G., /. Biol. Chem., 1917, xxxi, 97. 

 ''Frankel, E. M., /. Biol. Chem., 1917, xxxi, 201. 



