Inhibition of jack bean urease by N-(n-butyl) thiophosphorictriamide and N-(n-butyl) phosphorictriamide: determination of the inhibition mechanism.

N-(n-butyl)thiophosphorictriamide (NBPT) and its oxygen analogue N-(n-butyl)phosphorictriamide (NBPTO) were studied as inhibitors of jack bean urease. NBPTO was obtained by spontaneous conversion of NBPT into NBPTO. The conversion under laboratory conditions was slow and did not affect NBPT studies. The mechanisms of NBPT and NBPTO inhibition were determined by analysis of the reaction progress curves in the presence of different inhibitor concentrations. The obtained plots were time-dependent and characteristic of slow-binding inhibition. The effects of different concentration of NBPT and NBPTO on the initial and steady-state velocities as well as the apparent first-order velocity constants obeyed the relationships for a one-step enzyme-inhibitor interaction, qualified as mechanism A. The inhibition constants of urease by NBPT and NBPTO were found to be 0.15 microM and 2.1 nM, respectively. The inhibition constant for NBPT was also calculated by steady-state analysis and was found to be 0.13 microM. NBPTO was found to be a very strong inhibitor of urease in contrast to NBPT.