Catalyzes the oxidation of uric acid to 5-hydroxyisourate, which is further processed to form (S)-allantoin. H2O + O2 + urate = 5-hydroxyisourate + H2O2 Purine metabolism; urate degradation; (S)-allantoin from urate: step 1/3. Homotetramer. Expressed predominantly in the uninfected cells of the central tissue of the root nodule. During nodulation in legume roots after Rhizobium infection. The N-terminus is blocked. Belongs to the uricase family.