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Xanthine is a purine nucleobase with two oxo groups at positions2 and6 of the purine framework. The neutral compound has formula ${\mathrm{C}}_5{\mathrm{H}}_4{\mathrm{N}}_4{\mathrm{O}}_2$ and can occur in different forms related by tautomerism. The general chemical name therefore need not select the same hydrogen placement as a particular structural drawing.^[ref]

A metabolic example is the oxidation of hypoxanthine through xanthine toward uric acid and urate. The accepted xanthine dehydrogenase reaction uses NAD+ as an electron acceptor; the related xanthine oxidase form uses oxygen. The identity of the electron acceptor matters even though both names involve xanthine oxidation.^[ref]

For an atom-and-charge bookkeeping example, writing the product in its neutral uric acid form gives the formal oxidation half-reactionBoth sides contain five carbons, four nitrogens, three oxygens and six hydrogens, with total charge zero. This represents a two-electron oxidation and addition of oxygen to the framework. It is a balanced half-reaction, not a claim that the enzyme transfers two free electrons directly into solution. Subsequent uric acid/urate protonation is a distinct equilibrium.

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 reactionThe conversion therefore couples a change in the purine substrate to transfer of reducing equivalents to a second molecule.^[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.^[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.

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 reactionThe 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.^[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 $2.1\text{A˚}$ and $2.5\text{A˚}$, respectively. Changes near the flavin adenine dinucleotide region helped explain altered access of the NAD substrate and electron-acceptor preference.^[ref] Relating structure to both donor and acceptor chemistry is more informative than identifying the purine substrate alone.