Catalyzes two steps in the degradation of uric acid, i.e. the oxidation of uric acid to 5-hydroxyisourate (HIU) and the stereoselective decarboxylation of 2-oxo-4-hydroxy-4-carboxy-5-ureidoimidazoline (OHCU) to (S)-allantoin (By similarity). 5-hydroxy-2-oxo-4-ureido-2,5-dihydro-1H-imidazole-5-carboxylate + H(+) = (S)-allantoin + CO2 H2O + O2 + urate = 5-hydroxyisourate + H2O2 Purine metabolism; urate degradation; (S)-allantoin from urate: step 1/3. Purine metabolism; urate degradation; (S)-allantoin from urate: step 3/3. HIU and OHCU are unstable, they spontaneously decompose to form a racemic mixture of allantoin. In the N-terminal section; belongs to the OHCU decarboxylase family. In the C-terminal section; belongs to the uricase family.