Catalyzes the stereoselective decarboxylation of 2-oxo-4- hydroxy-4-carboxy-5-ureidoimidazoline (OHCU) to (S)-allantoin, a step in the uric acid degradation pathway following urate oxidation (By similarity). In humans, the protein appears to be poorly or not expressed, suggesting that this activity is likely not operative (Probable). {ECO:0000250|UniProtKB:Q283N4, ECO:0000305|PubMed:16462750}. This is the function of ENSG00000183463 (URAD, ureidoimidazoline (2-oxo-4-hydroxy-4-carboxy-5-) decarboxylase).