JOHN H. NORTHROP 



483 



the active pepsin. Table V summarizes the result of an experiment 

 similar to Experiment 2 but covering a wider range. In this experi- 

 ment an impure solution of pepsin (the same as used in Experiment 

 1) was diluted, A, with acid of the same hydrogen ion concentration, 

 and, B, with the same solution which had been previously inactivated 

 by alkali and then brought back to the same pH as the original. The 



03 



a. 



;> 



e2 



0.4 0.8 1.2 1.5 2.0 2.4 2.8 

 Units of active pepsin added 



3.2 3.6 



Fig. 3. Curves showing efifect of addition of "pure" pepsin to pepsin solution 

 containing peptone (c/. Table IV). 



activity of the solution diluted with acid is not directly proportional 

 to its concentration. When the same solution is diluted with an 

 inactivated portion of itself, the activity of the resultant solution is 

 directly proportional to its concentration. The figures show again 

 that the result is predicted quantitatively by the hypothesis. Pep- 

 sin inactivated by alkali therefore retains the ability to combine with 

 peptone exactly as does the active pepsin. It has, however, lost the 

 power to hydrolyze protein. A very similar phenomenon is known 

 in immunology^ — the so called toxoids; i.e., toxins which are no longer 

 injurious but are able to bind antibody in the same way as true toxin. 



