Catalysis of the reaction: 5-hydroxy-2-oxo-4-ureido-2,5-dihydro-1H imidazole-5-carboxylate + H+ = S-allantoin + CO2.
4-(carbamoylamino)-5-hydroxy-2-oxo-2,5-dihydro-1H-imidazole-5-carboxylate decarboxylase activity