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Studies on Enzyme Action. XIX. — Urease: a Selective Enzyme. 

 II. — Observations on Accelerative and Inhibitive Agents. 

 By H. E. Armstrong, F.R.S., M. S. Benjamin and Edward Horton. 



(Received January 1, — Read February 20, 1913.) 



In the previous communication* experiments were described which had been 

 made with the Urease present in the Soja bean proving that the enzyme is 

 strictly selective in its action and that whilst its activity is much reduced by 

 ammonia it is increased, in a remarkable manner, by the presence of carbonic 

 acid : in other words, the two products of change affect the activity of the 

 enzyme in opposite ways — a result altogether without precedent. In 

 explanation of these results, the suggestion was made that Urease is a feebly 

 acidic substance. 



Though it was obvious that the results were not to be harmonised with the 

 views that were current as to the manner in which enzymes act, we refrained 

 from comment, deeming it desirable to obtain more information before 

 discussing the new situation that was created. In the interval, the behaviour 

 of other enzymes has been under observation by Dr. E. F. Armstrong and 

 ourselves and it is proposed to discuss the general outcome of the work, in a 

 comprehensive communication, at an early date. Meanwhile, we desire to 

 bring forward an account of further observations on Urease carried out with 

 the object of ascertaining the manner in which the activity of the enzyme is 

 affected by the presence of various substances together with the urea. 



Experimental Method. — In cases in which the substance to be added was 

 easily soluble in water, solutions were prepared containing either one-half or 

 one-tenth of a molecular proportion of the substance per litre. Having 

 measured out 50 c.c. of a half-molecular solution of urea into each of two 

 200 c.c. Jena flasks fitted with indiarubber stoppers, 50 c.c. of water were added 

 to the one and to the other 50 c.c. of the M/2 (or M/10) solution of the 

 substance of which the effect was to be determined ; each flask received also 

 25 c.c. of Soja extract (prepared as described in our former communication) ; 

 all operations were carried out as near as possible at 25°. 



As soon as the two flasks were charged, they were placed in an incubator 

 which was maintained at 25°. After intervals of 5, 10, 15, 30, 45, 60, 75, 90 

 and 120 minutes, samples (10 c.c.) were withdrawn from each flask by 

 means of pipettes previously warmed to 25° ; each sample was run into a 



* " Studies on Enzyme Action. XV. — Urease : a Selective Enzyme," ' Eoy. Soc. Proc.,' 

 1912, B, vol. 85, p. 109. 



