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. 5-hydroxy-2-oxo-4-ureido-2,5-dihydro-1H-imidazole-5-carboxylate + H(+) = (S)-allantoin + CO2 H2O + O2 + urate = 5-hydroxyisourate + H2O2 Optimum pH is 8. Optimum temperature is 37 degrees Celsius. Retains 90% of its activity after 72 hours of incubation at 20 degrees Celsius or -4 degrees Celsius, but loses 50% of its activity after 12 hours of incubation at 37 degrees Celsius. Purine metabolism; urate degradation; (S)-allantoin from urate: step 1/3. Purine metabolism; urate degradation; (S)-allantoin from urate: step 3/3. Expression is very low in excess nitrogen (glutamate plus ammonia) and is induced by TnrA during imiting-nitrogen conditions (glutamate). Expression is further induced when allantoin or uric acid are added during limiting-nitrogen conditions. 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.