604 SUBSTRATE CONCENTRATION AND HYDROLYSIS 



It seems necessary to conclude from these experiments that the 

 relative decrease in the rate of digestion of protein solutions of in- 

 creasing concentration is independent (within the limits of error of 

 these experiments) of the enzyme concentration used. 



There does not appear to be any direct experimental evidence on 

 the above point in connection with other enzymes. It is frequently 

 stated, however, (Nelson and Vosburgh,i^ Van Slyke and Cullen') 

 that the velocity of reaction is directly proportional to the enzyme 

 concentration under all conditions and irrespective of the substrate 

 concentration. If this is true it follows necessarily that the relative 

 rate of digestion of various substrate concentrations, when hydro- 

 lyzed with any given enzyme concentration, is independent of the 

 enzyme concentration used. 



It appears to the writer that this is contrary to the result predicted 

 by the saturation theory. According to this theory it would be pre- 

 dicted that the falling off in the increase in the rate of digestion as 

 compared to the increase in concentration of a protein solution 

 (above a certain low concentration) is due to the fact that at 

 this concentration the enzyme begins to become saturated with 

 substrate; i.e., the time necessary for the enzyme to combine 

 with the substrate becomes small compared with the time during 

 which it remains combined. If this saturation effect becomes notice- 

 able at a concentration of protein of 2 per cent with 0.08 per cent pep- 

 sin it should become noticeable at a lower protein concentration with 

 0.008 per cent pepsin. The experiment shows this is not the case. 

 If anything, the figures show that the rate of digestion of the substrate 

 falls off more rapidly (as compared to the concentration) with the 

 higher pepsin concentration than with the lower. In Table I, which 

 is more reliable owing to the larger number of determinations there 

 is less than 1 per cent difference. ^^ 



It seems necessary to conclude therefore that the relative decrease 

 in the rate of digestion as compared with the increase in protein con- 



1^ Nelson, J. M., and Vosburgh, W. C, /. Am. Chan. Soc. 1917, xxxix, 790. 



^* It probably cannot be assumed that, according to the saturation theory, 

 the rate of digestion (caused by increasing the enzyme concentration ten times) 

 should be increased ten times as much in the concentrated as in the dilute sub- 

 strate concentration. The increase should be large enough to detect, however. 



