In enzymology, a tetrahydroxypteridine cycloisomerase (EC 5.5.1.3) is an enzyme that catalyzes the chemical reaction

tetrahydroxypteridine ${\displaystyle \rightleftharpoons }$ xanthine-8-carboxylate

Hence, this enzyme has one substrate, tetrahydroxypteridine, and one product, xanthine-8-carboxylate.

This enzyme belongs to the family of isomerases, specifically the class of intramolecular lyases. The systematic name of this enzyme class is tetrahydroxypteridine lyase (isomerizing). It employs one cofactor, NAD+.