Inhibition of urease by Ni 2+ ions : Analysis of reaction progress curves

Abstract

The inhibition of jack bean urease by Ni 2+ ions was studied in 20 mM HEPES buffer pH 7.0. The inhibition was observed in two systems which differed in the order in which the components of the reaction mixture were mixed. In the first (unincubated), the reaction was initiated by adding urease to the mixture of urea and Ni 2+ ions, and in the second (incubated), by adding urea to the mixture of urease incubated with Ni 2+ ions prior to the reaction. It was shown that Ni 2+ ions are a competitive slow-binding inhibitor of urease. In the first system the inhibition constants are K i=0.042 mM and K i ∗=0.0028 mM, and in the second system K i ∗=0.0024 mM. The inhibition was found to involve the rapid formation of a urease-Ni 2+complex followed by its relatively slow, reversible isomerization, with forward and reverse rate constants of 0.64 and 0.045 min −1, respectively.