Urate

Urate denotes an anion obtained by removing one or more protons from uric acid; the charge state should be specified when needed. The commonly discussed monoanion has formula ${\mathrm{C}}_5{\mathrm{H}}_3{\mathrm{N}}_4{\mathrm{O}}_3^{-}$, whereas neutral uric acid has one additional hydrogen. A urate salt includes charge-balancing cations as well as the urate ion.^[ref]

Writing neutral uric acid as ${\mathrm{H}}_2\mathrm{U}$ and its monoanion as $\mathrm{H}{\mathrm{U}}^{-}$ gives the acid–base equilibriumThis changes protonation and charge without changing the carbon, nitrogen or oxygen framework. In a simplified mixture containing only these two forms, a concentration ratio $[\mathrm{H}{\mathrm{U}}^{-}]/[{\mathrm{H}}_2\mathrm{U}]=4$ means that four fifths of their combined amount is monoanion. A total of1millimole would then comprise0.8millimole monoanion and0.2millimole neutral acid; the ratio alone does not specify the total amount.

The formation of urate in purine metabolism is a separate chemical transformation: xanthine dehydrogenase oxidizes xanthine toward the uric-acid/urate product family. That reaction changes the oxidation state and oxygen content, whereas the equilibrium above describes loss or gain of a proton after a particular framework has been specified.^[ref]