Xanthine oxidase (XO) generates reactive oxygen species, including hydrogen peroxide (H2O2), as it oxidizes specific substrates in the presence of water and oxygen. 6-Formylpterin is an oxidized pterin produced by photolytic breakdown of folic acid. It binds to one of two active sites on XO nearly quantitatively and irreversibly and prevents the metabolism of other substrates at the second site, resulting in "hetero-substrate" inhibition at nanomolar concentrations. However, 6-formylpterin itself is converted by XO to 6-carboxylpterin and H2O2 and the turnover rate of this reaction can actually be accelerated by prior binding of a hetero-substrate to XO. In this way, 6-formylpterin acts as an intracellular generator of H2O2 in cells expressing XO, altering cellular function.