Xanthine oxidoreductase (XOR) catalyzes the conversion of hypoxanthine to xanthine and of xanthine to uric acid. Inhibitors of XOR block conversion of the oxypurines (hypoxanthine and xanthine) to uric acid, thus decreasing serum concentration of uric acid. Crystal structures of XOR- ibnhibitor complexes reveal that each inhibitor has different inhibition mechanisms, utilizing the enzyme structure and the reaction mechanism. Such variation of the inhibition mechanism affects in vivo hypouricemic effects of the inhibitors.