Kinetic Mechanism and Cofactor Content of Soybean Root Nodule Urate Oxidase

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Abstract
The kinetic mechanism of urate oxidase isolated from soybean root nodules has been determined by initial velocity kinetic studies monitoring oxygen uptake, in order to avoid potential artifacts in the spectrophotometric assay which arise from absorbance due to unidentified products of the enzymatic reaction. Urate and O2 bind to the enzyme sequentially; xanthine is a competitive inhibitor versus urate and a noncompetitive inhibitor versus O2, which suggests that urate binds to the enzyme before O2. This kinetic mechanism was confirmed by an 18O isotope-trapping experiment, which demonstrated that O2 does not bind productively to the enzyme in the absence of urate. The pH dependence of V and (V/K)urate reveal the presence of an ionizable residue on the enzyme with a pK of ∼6.2, which must be unprotonated for the catalytic reaction to occur. The (V/K)O2 profile is pH independent; these data are accomodated by a model in which a unimolecular step intervenes between the binding of urate and O2. The pKi profile for 9-methylurate, a competitive inhibitor versus urate, is pH independent, confirming that the protonation state of the ionizable residue is not important for binding. The pKi profile for xanthine defines a pK of 7.4, which demonstrates that the monoanion of xanthine binds to the enzyme; by analogy, the monoanion of urate is predicted to be the substrate. The four isomeric N-methylurates were examined as potential inhibitors of urate oxidase. Only 9-methylurate showed significant inhibition suggesting that ionization at N9 of urate is not required for binding; it is proposed that the N3-deprotonated urate monoanion is the species which binds to urate oxidase. The gene encoding urate oxidase was cloned from soybeans and expressed in Escherichia coli. The metal content of the recombinant enzyme was examined by inductively coupled argon plasma emission spectroscopy, and only trace quantities of copper were found. The molecular mass of the protein was determined by MALDI-TOF mass spectrometry and found to be 35 059.8 Da. The calculated molecular mass of urate oxidase is 35 052 Da; therefore, these data suggest that there is no covalently bound cofactor in urate oxidase.

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