122 



COMBINATION OF ENZYME AND SUBSTRATE 



left in contact. It seems probable that this is due to a change in 

 the condition of the protein rather than to a change in the enz3ane, 

 since, according to Ringer,!^ the optimum reaction is different for 

 different proteins. 



The simplest explanation of the above facts would seem to be 

 that the quantity of ionized protein present determines the amount 

 of pepsin which combines with the protein, and hence also deter- 

 mines the rate of digestion. Some direct evidence confirming this 

 hypothesis has been obtained and will be discussed fully later. 



pH 



Fig. 3. 



SUMMARY. 



1. A quantitative method for the determination of pepsin is de- 

 scribed depending on the change in conductivity of a digesting egg 

 albumin solution. 



2. The combination of pepsin with an insoluble substrate has been 

 followed by this method. 



3. The amount of pepsin removed from solution by a given weight 

 of substrate is independent of the size of the particles of the substrate. 



4. There is an optimum zone of hydrogen ion concentration for 

 the combination of enzyme and substrate corresponding to the opti- 

 mum for digestion. 



5. It is suggested that the pepsin combines largely or entirely with 

 the ionized protein. 



Ringer, W. E., Kolloid-Z., 1916, xix, 253. 



