The enthalpimetric determination of inhibition constants for the inhibition of urease by acetohydroxamic acid

The effect of concentration of acetohydroxamic acid (AHA) on inhibition of jack bean urease in phosphate buffer, pH 7.0, at 25°C, was studied. The meas­urements were performed at urease concentration of 2.5 mg/100 cm3 for con­centrations of urea and AHA ranging in the range of 2-50 mmol dm-3 and 0.25-10 mmol dm-3, respectively. The reactions were monitored by two techniques: analytical and enthalpimetric. For the analytical technique the growth of ammonia concentration in the course of the reaction was determined. From the recorded progress curves the following parameters were calculated for each inhibitor concentration: the initial reaction rate, the steady-state rate and the inversion constant. From these parameters the inhibition constants of the initial and steady-state stages of the reaction, K\ and K\y were calculated. The former constant did not change whereas the latter one proved to decrease quickly with an increase in inhibitor concentration. This behaviour resulted from the fact that the inactive complex EI* was not a product of internal inversion but was formed in the reaction: — 2/3I + EI —> (EI • — 2/3I)*. The disso3ciation constant of this complex is equal to about 0.3 x 10-3 (mol dm-3)2/3.