THE INFLUENCE OF THE SUBSTRATE CONCENTRATION 



ON THE RATE OF HYDROLYSIS OF PROTEINS 



BY PEPSIN. 



By JOHN H, NORTHROP. 



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



(Received for publication, May 18, 1920.) 



In contrast to the numerous papers on the influence of changes in 

 the pepsin concentration, the influence of varying the protein con- 

 centration on the rate of digestion of protein has been but little 

 studied. Weis^ found that the rate was nearly directly proportional 

 to the protein concentration in low concentration but increased more 

 slowly than the latter in concentrations of more than 2 to 3 per cent. 

 The experiments were made with a crude enzyme preparation which 

 contained several proteolytic enzymes, and were made in such a way 

 as to compare the changes in diiTerent solutions after the same time 

 interval, instead of comparing the times required to cause an equal 

 change. They are therefore difficult to interpret. 



Preliminary experiments made with a purified pepsin and purified 

 egg albumin showed in general the same results as those found by 

 Weis. In concentrations of more than 2 to 3 per cent the rate of 

 digestion increases more slowly than the protein concentration and 

 finally becomes nearly independent of it. This phenomenon is a 

 very general one in enzyme reactions and many explanations have 

 been offered to account for it. Brown^ suggested that the relative 

 decrease in the rate of digestion with increasing substrate concentra- 

 tion was due to the fact that the enzyme remained combined with 

 the substrate for a period of time large compared with the time neces- 

 sary for combination to take place. The enzyme therefore becomes 

 more and more "saturated" with substrate as the relative concen- 

 tration of substrate to enzyme increases. Van Slyke and Cullen^ 



^ Weis, Med. Carlsherg Lab., 1903, v, 127. 



^ Brown, A. J., /. Chem. Sac, 1902, Ixxxi, 373. 



3 Van Slyke, D. D., and Cullen, G. E., /. Biol. Chem., 1914, xix, 141. 



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