Catalyzes the stereoselective decarboxylation of 2-oxo-4-hydroxy-4-carboxy-5-ureidoimidazoline (OHCU) to (S)-allantoin. 5-hydroxy-2-oxo-4-ureido-2,5-dihydro-1H-imidazole-5-carboxylate + H(+) = (S)-allantoin + CO2 Purine metabolism; urate degradation; (S)-allantoin from urate: step 3/3. Homodimer. Belongs to the OHCU decarboxylase family.