= Xanthine oxidase
<Xanthine oxidase> is an <enzyme> that oxidizes <xanthine> while using <molecular oxygen> as an electron acceptor. Its activity is classified as EC1.17.3.2. The IUBMB gives the overall reaction
$$
\mathrm{xanthine}+\mathrm{H_2O}+\mathrm{O_2}\longrightarrow\mathrm{urate}+\mathrm{H_2O_2}.
$$
The products are <urate> and <hydrogen peroxide> in this stated reaction. Other conditions can favor <superoxide> formation, so the equation does not specify every oxygen-reduction product in every assay.<Xanthine oxidase in IUBMB Enzyme Nomenclature>[Reaction and Comments, peroxide and superoxide routes]{ref}
For an idealized peroxide-route inventory, four micromoles of <xanthine> and ten micromoles of <molecular oxygen>, with sufficient <water> and no competing reactions, can yield four micromoles each of <urate> and <hydrogen peroxide>. Six micromoles of <molecular oxygen> remain. This is a stoichiometric limit; it does not predict the reaction rate or completion under a particular set of conditions.
<Xanthine dehydrogenase> instead couples the oxidation to reduction of <nicotinamide adenine dinucleotide>. Mammalian enzyme preparations can convert between the two forms. A 2000 comparison determined bovine dehydrogenase and oxidase structures at $2.1\,\text{Å}$ and $2.5\,\text{Å}$, respectively. Changes near the <flavin adenine dinucleotide> region helped explain altered access of the NAD substrate and electron-acceptor preference.<Crystal structures of bovine milk xanthine dehydrogenase and xanthine oxidase: Structure-based mechanism of conversion by C. Enroth and others>[Abstract and primary indexed structural comparison]{ref} Relating structure to both donor and acceptor chemistry is more informative than identifying the purine substrate alone.
Back to article page
