574 Dr. Armstrong and Prof. Armstrong. Studies on [June 13, 



any further amount be required and the presence of acid (or alkali) in excess 

 serve to accelerate the action of the enzyme, it is to be supposed that the 

 acid (or alkali) neutralises some product or products of the change. This 

 has been shown to be true in the case of urease, as ammonia retards the 

 hydrolysis of urea by the enzyme whilst weak acids accelerate the change. 



Assuming that it is a derivative of arginine, the enzyme urease may be 

 represented by the following diagram, the section which the urea molecule 

 would fit being that within the figure : — 



The carboxyl group in such a compound would tend to unite with one of 

 the contiguous basic NH groups, thus forming an internal salt. It would 

 remain free if sufficient acid were present to hinder the formation of the salt 

 but neutralisation would set in as ammonia was liberated from the urea : 

 the enzyme would not be entirely thrown out of action, however, as the 

 neutralisation would be more or less incomplete, as the ammonium salt 

 would be dissociated by the water : its activity would be in great measure 

 if not entirely preserved if an acid were present which neutralised the 

 ammonia, so long as the effective acidity of the solution did not exceed a certain 

 maximum — beyond which the acid would itself interfere by combining with 

 the enzyme and perhaps also with the hydrolyte. 



Apparently a not inconsiderable degree of acidity is essential in the case 

 of peptic digestion. It is therefore not improbable that the active radicle in 

 the enzyme is stronger than carboxyl and that it may even be a phosphoric 



Fig. 4. — Urease. 



