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

The inhibition of jack bean urease by Ni²⁺ 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 Ni²⁺ ions, and in the second (incubated), by adding urea to the mixture of urease incubated with Ni²⁺ ions prior to the reaction. It was shown that Ni²⁺ 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-Ni²⁺complex followed by its relatively slow, reversible isomerization, with forward and reverse rate constants of 0.64 and 0.045 min^?1, respectively.