Catalyzes the hydroxylation of urate to 5-hydroxyisourate (HIU). Is likely to be involved in the urate degradation pathway to allantoin. Prefers NADH over NADPH as the electron donor. H(+) + NADH + O2 + urate = 5-hydroxyisourate + H2O + NAD(+) kcat is 56.0 sec(-1) for the NADH-dependent oxidation of urate. Exhibits a Michaelian behavior only toward urate and a cooperative behavior toward NADH and NADPH. Purine metabolism; urate degradation. Monomer. Belongs to the FAD-dependent urate hydroxylase family.