= Xanthine dehydrogenase
{title2=EC 1.17.1.4}
<Xanthine dehydrogenase> is an <enzyme> that oxidizes <xanthine> to <urate> while reducing the oxidized form of <nicotinamide adenine dinucleotide>. The IUBMB classifies this activity as EC 1.17.1.4 and gives the overall reaction
$$
\mathrm{xanthine}+\mathrm{NAD}^{+}+\mathrm{H_2O}\longrightarrow\mathrm{urate}+\mathrm{NADH}+\mathrm{H}^{+}.
$$
The conversion therefore couples a change in the <purine> substrate to transfer of reducing equivalents to a second molecule.<Xanthine dehydrogenase in IUBMB Enzyme Nomenclature>[EC 1.17.1.4, accepted name and Reaction]{ref}
A constructed stoichiometric example starts with $5\,\mu\mathrm{mol}$ of <xanthine> and $3\,\mu\mathrm{mol}$ of oxidized <nicotinamide adenine dinucleotide>, with sufficient <water> and no regeneration of the electron acceptor. At most $3\,\mu\mathrm{mol}$ can undergo the stated conversion. That would produce $3\,\mu\mathrm{mol}$ each of <urate> and reduced <nicotinamide adenine dinucleotide>, leaving $2\,\mu\mathrm{mol}$ <xanthine>. The smaller available stoichiometric amount limits this idealized reaction extent; more enzyme cannot supply the missing electron acceptor.
The electron acceptor also distinguishes the named activity from <xanthine oxidase>, which uses oxygen. The mammalian protein can convert between dehydrogenase and oxidase forms under particular conditions, including changes during purification; the names therefore describe experimentally distinguishable catalytic behavior rather than two entirely unrelated substrate names.<Xanthine dehydrogenase in IUBMB Enzyme Nomenclature>[Comments, mammalian dehydrogenase and oxygen-dependent oxidase conversion]{ref}
A meaningful activity comparison must consequently specify both substrate and acceptor conditions. Loss of the NAD-dependent readout could reflect altered reaction conditions or enzyme form, rather than absence of the protein. Conversely, detecting a protein alone does not demonstrate a particular turnover rate. The chemical equation identifies what must be supplied and measured; <enzyme kinetics> determines how quickly the conversion proceeds.
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