Urease Catalysis: III. Stoichiometry, Kinetics, and Inhibitory Properties of a Third Substrate: Dihydroxyurea

Abstract

Abstract Urease catalyzes the degradation of dihydroxyurea (HOHN-CO-NHOH) in aqueous solution. It was shown that the decline in dihydroxyurea was paralleled by the appropriate stoichiometric appearance of hydroxylamine and carbon dioxide, thus eliminating adsorption and contaminant substrates as the responsible factors. The reaction was not elicited by equivalent concentrations of bovine serum albumin, and the dihydroxyurea/urea specific activity ratio was maintained over a 70-fold range of urease purity. The catalysis is thus urease-specific and not due to proteins in general, nor to another enzyme contaminating urease preparations. In Tris-maleate buffers the pH activity curves for dihydroxyurea, (mono)hydroxyurea, and urea all had maxima at pH 7.0, with progressive narrowing of the activity range as hydroxylamine groups replaced amine groups in the substrates. Despite a high Km = 12.5 mm, dihydroxyurea, which has no primary amide bond, was hydrolyzed more rapidly than (mono)hydroxyurea, which has. Like the latter compound, dihydroxyurea also produced marked inhibition of urease. The inhibition was less powerful, however, than that generated by hydroxyurea and the aliphatic hydroxamates, and was kinetically reversible and noncompetitive with Ki = 0.14 mm.