Catalyzes the hydroxylation of uric acid to 5-hydroxyisourate. H(+) + NADH + O2 + urate = 5-hydroxyisourate + H2O + NAD(+) Purine metabolism; urate degradation. Mutants fail to grow with hypoxanthine, xanthine or urate as the sole nitrogen source, but can grow with allantoin and allantoate. Belongs to the FAD-dependent urate hydroxylase family.