THE ENTHALPIMETRIC DETERMINATION OF INHIBITION CONSTANTS FOR THE INHIBITION OF UREASE BY ACETOHYDROXAMIC ACID

The effect of concentration of acetohydroxamic acid (AHA) on inhibitio n of jack bean urease in phosphate buffer, pH 7.0, at 25 degrees C, wa s studied. The measurements were performed at urease concentration of 2.5 mg/100 cm(3) for concentrations of urea and AHA ranging in the ran ge of 2-50 mmol dm(-3) and 0.25-10 mmol dm(-3), respectively. The reac tions were monitored by two techniques: analytical and enthalpimetric. For the analytical technique the growth of ammonia concentration in t he course of the reaction was determined. From the recorded progress c urves the following parameters were calculated for each inhibitor conc entration: the initial reaction rate, the steady-state rate and the in version constant. From these parameters the inhibition constants of th e initial and steady-state stages of the reaction, K-i and K-i, 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 dissociation constant of this comple x is equal to about 0.3 x 10(-3) (mol dm(-3))(2/3).