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. Apparently not expressed. Belongs to the OHCU decarboxylase family. Could be the product of a pseudogene. In primates the genes coding for the enzymes for the degradation of uric acid were inactivated and converted to pseudogenes (PubMed:16462750).