= Xanthine
\b[Xanthine] is a <purine> <nucleobase> with two oxo groups at positions2 and6 of the purine framework. The neutral compound has formula $\mathrm{C_5H_4N_4O_2}$ 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.<Xanthine in ChEBI>[CHEBI:15318 general record and CHEBI:17712 specific9H form]{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.<Xanthine dehydrogenase in IUBMB Enzyme Nomenclature>[EC1.17.1.4, Reaction and Comments]{ref}
For an atom-and-charge bookkeeping example, writing the product in its neutral <uric acid> form gives the formal oxidation half-reaction
$$
\mathrm{C_5H_4N_4O_2+H_2O\longrightarrow C_5H_4N_4O_3+2H^++2e^-}.
$$
Both 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.
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