PROTEOLYTIC ENZYMES AS CATALYSORS 397 



themselves, which are too frequently overlooked, and which 

 doubtless play a part in determining the complexity of the 

 phenomena which are experimentally observed. 



In the first place, we have seen that certain of the proteins, 

 or protein salts, undergo hydrolysis at a measureable rate in 

 the absence of proteolytic enzymes. Now, from the principle of 

 the mutual independence of different reactions (21) (53) (82) 

 it follows that when the proteins are acted upon by proteolytic 

 enzymes both the catalysed and the uncatalysed reactions must 

 be proceeding side by side, albeit the latter, possibly, at a re- 

 duced velocity.* Hence if the catalysed reaction is not over- 

 whelmingly more rapid than the uncatalysed reaction the prog- 

 ress of the latter must disturb the time- and mass-relations of the 

 former. The velocity-constant which is actually measured will 

 be the sum of the constants for the catalysed and the uncatalysed 

 reactions; only the former constant will bear a specific relation 

 to the mass of the catalysor; the sum of the constants, which 

 would be the quantity actually measured, would not, therefore 

 bear this relation to the mass of the catalysor, but exhibit de- 

 partures from it of greater or less magnitude according to the 

 magnitude of the constant for the uncatalysed reaction. 



In the second place, we have seen, in discussing the hydrolysis 

 of the polypeptids, that a tripeptid may undergo hydrolysis by 

 splitting at either linkage, and that the hydrolysis due to the 

 splitting of the one linkage may be accelerated by one proteo- 

 lytic enzyme, and that due to the splitting of the other linkage 

 by another enzyme. In so complex a peptid as a protein many 

 linkages vulnerable to the enzyme under consideration must 

 exist and it is entirely within the bounds of probability that any 

 given enzyme shares its attack between two or more vulnerable 

 linkages and that, consequently, what we directly observe is not 

 the progress of one definite reaction, or even a catenary series 

 of reactions, but a number of parallel chains of successive hydrol- 

 yses. On the other hand, we know that the vulnerability of the 

 various linkages in the protein molecule to various proteoclastic 

 enzymes, differs so that one group of linkages will be preferen- 

 tially attacked by one enzyme, and another group by yet another 

 enzyme, while no enzyme or group of enzymes will open all of the 



* Owing to reduction of the active mass of the free protein through the 

 formation of a protein-ferment compound. 



