468 



INACTIVATION OF PEPSIN SOLUTIONS 



This is shown by Curve T>, Fig. 2, which is taken from Sorensen's 

 paper and which represents the rate of digestion of egg albumin by 

 pepsin at various hydrogen ion concentrations. If the decline in the 

 rate of digestion on the acid side of pH 2.0 was due to the increased 

 destruction of the pepsin by the acid in greater concentration than 

 this, the same drop should be noticed in Curves A, B, and C as in 

 Curve D, since these curves represent the actual amount of destruction 

 of the enzyme at the acid concentration in question. Fig. 2 shows 

 that this is not the case. But little more enzyme was destroyed at a 



Fig. 2. Relative amount of active pepsin in various solutions at different 

 hydrogen ion concentrations after 24 hours at 38°C. 



pH of 1.0 than at a pH of 2.0 or 3.0. The rate of destruction is in 

 any case much too slow to account for the rapid drop in the rate of 

 the digestion curve. This drop is noticeable in the first few minutes of 

 the reaction, while, as the figures show, only 10 to 20 per cent of the 

 enzyme is destroyed in 48 hours at this hydrogen ion concentration. 

 The fact that the action of the acid on the enzyme is nearly the 

 same across the whole range of hydrogen ion concentration in which 

 the enz5rme is active may be considered as indirect evidence that the 

 optimum phenomenon is connected with changes in the substrate 

 rather than in the enzyme. It is apparent from the figures that the 

 enzyme is most stable at a pH of about 5.0; i.e., the same as that for 

 the isoelectric point of many proteins. There is no evidence, however, 

 that pepsin is isoelectric at this point. A series of migration experi- 

 ments made by the writer confirmed those of Michaelis and David- 

 sohn* (except that the enzyme was never found to migrate to both 



* Michaelis, L., and Davidsohn, H., Biochem. Z., 1910, xxviii, 1. 



