Catalyzes the hydrolysis of 5-hydroxyisourate (HIU) to 2-oxo-4-hydroxy-4-carboxy-5-ureidoimidazoline (OHCU). 5-hydroxyisourate + H2O = 5-hydroxy-2-oxo-4-ureido-2,5-dihydro-1H-imidazole-5-carboxylate + H(+) Purine metabolism; urate degradation; (S)-allantoin from urate: step 2/3. Homotetramer. HIU hydrolysis also occurs spontaneously, but more slowly. Belongs to the transthyretin family. 5-hydroxyisourate hydrolase subfamily.