THE INFLUENCE OF HYDROGEN ION CONCENTRATION 

 ON THE INACTIVATION OF PEPSIN SOLUTIONS. 



By JOHN H. NORTHROP. 

 (From the Laboratories of The Rockefeller Institute for Medical Research.) 



(Received for publication, February 14, 1920.) 



One of the many factors which must be taken into consideration in 

 any experiments with enzymes is the possible inactivation of the 

 enzyme during the course of the reaction. This factor in the case of 

 pepsin has been suggested by Sorensen^ as the cause of the displace- 

 ment of the optimum acidity for the digestion of protein to the acid 

 side during the course of the digestion. He considers that the enzyme 

 is more rapidly destroyed by the weak than by the strong acid. 

 Arrhenius,^ on the other hand, considers that the decrease in the rate 

 of digestion on the acid side of the optimum hydrogen ion concentra- 

 tion for digestion is due to the more rapid destruction of the enzyme 

 by the strong acid. If this explanation is correct the optimum phe- 

 nomenon loses much of its significance and becomes a secondary char- 

 acteristic of enzyme activity comparable to the optimum temperature. 

 The possibility also arises that the peculiar falling off of the rate of 

 digestion during the course of the reaction, at any hydrogen ion con- 

 centration, is also due to the destruction of the enzyme. 



Several investigations^ have been made on the stability of pepsin 

 in acid solutions from various points of view but the results are not at 

 all concordant. Much of this variation in results is probably due to 

 the failure to realize the importance of the hydrogen ion concentration 

 rather than the total acid concentration. 



■^ Sorensen, S. P. L., Compt. rend. trav. Lab. Carlsberg, 190$, viii, 162. Sorensen's 

 experiments were made at 52°. They are therefore not strictly comparable with 

 the present results. 



2 Arrhenius, S., Quantitative laws in biological chemistry, London, 1915, 44, 

 ^Biernacki, E., Z. Biol., 1891, xxviii, 49. Grober, J. A., Arch. Exp. Path. u. 

 Pharmacol., 1904, li, 103. Liebmann, P., and Johannesen, L., Ugesk. Lager, 1911, 

 Ixxiii, 902. Ramsay, C. F., /. Am. Pharm. Assn., 1917, vi, 1047. 



465 



