Catalyzes the hydroxylation of urate to 5-hydroxyisourate (HIU). Is likely to be involved in the urate degradation pathway to allantoin. Is slightly more efficient (about 2.6 times) with NADPH than NADH as the electron donor. H(+) + NADH + O2 + urate = 5-hydroxyisourate + H2O + NAD(+) H(+) + NADPH + O2 + urate = 5-hydroxyisourate + H2O + NADP(+) FAD cannot be replaced by FMN or riboflavin. kcat is 1.06 sec(-1) for the NADPH-dependent oxidation of urate. Purine metabolism; urate degradation. Homodimer. Belongs to the HpyO family.