Catalyzes the oxidation of uric acid to 5-hydroxyisourate, which is further processed to form (S)-allantoin. H2O + O2 + urate = 5-hydroxyisourate + H2O2 Competitively inhibited by xanthine. Purine metabolism; urate degradation; (S)-allantoin from urate: step 1/3. Expressed in liver. Not detected in other tissues tested. Belongs to the uricase family.