126 



Studies on Enzyme Action. 



It is necessary to account, on this assumption, for the effects produced 

 by salts and non-electrolytes. The marked inhibiting effect of ammonia and 

 the slight effect produced by ammonium chloride in hastening hydrolysis 

 are hardly to be accounted for on the assumption that these compounds 

 directly affect the stability of the urea : if we are not mistaken, according 

 to current views, it is rather to be supposed that the ammonium salt would 

 exercise the greater effect in repressing hydrolysis and the alkali ammonia 

 should tend to promote hydrolysis. 



We are inclined to think that an explanation is to be found by regarding 

 the enzyme as a feebly acidic substance and that, in order that it may 

 condition the change, it must unite with the feebly basic substance urea — 

 the effect of a more basic substance such as ammonia would be to interfere 

 with such union and consequently to retard change. Carbonic acid, by 

 fixing the ammonia, would tend to promote the action of the enzyme from 

 this point of view by leaving it free to act as hydrolyst. To account for 

 the extraordinary retardation effected by urea itself, it must be supposed 

 that the alterations in the osmotic conditions which are the consequence 

 of an increase in the concentration of the solution in some way affect the 

 rate at which change can take place ; it may be that the proportion of urea 

 effectively hydrated diminishes as the concentration is increased — that is to 

 say that the compound C(OH) 2 (lSrH 2 )2 is formed in diminishing proportion 

 as the concentration is increased. The influence of non-electrolytes is at 

 least in part to be accounted for on this assumption. 



Urea is to be regarded as a compound which has very little tendency 

 under ordinary conditions to become carbonate. Apparently it is protected 

 from decomposition by its tendency to become cyanate ; on the other hand, 

 the cyanic acid which is formed from it when the change occurs is protected 

 by the presence of an equivalent proportion of ammonia and is protected 

 also by its tendency to revert to urea. In other words, both urea and 

 cyanate are protected from further change by the tendency each has to 

 become the other. The production of a relatively large amount of carbonate 

 when amnionic cyanate is taken instead of urea is perhaps due to the fact 

 that the solution then contains a relatively much larger amount of free cyanic 

 acid than is present in a solution of urea, cyanic acid (in absence of ammonia) 

 being an unstable substance which is readily hydrolysed to carbonic acid. 

 That is to say, carbonic acid may be formed in two ways, from cyanic acid 

 and from urea : under normal conditions, in a solution of urea neither the 

 cyanic acid nor the urea-hydrate which are contained in the solution undergoes 

 decomposition into carbonic acid to an extent which is appreciable, being 



