Involved in the last two steps of the degradation of uric acid, i.e. the hydrolysis of 5-hydroxyisourate (HIU) to 2-oxo-4-hydroxy-4-carboxy-5-ureidoimidazoline (OHCU) and its stereoselective decarboxylation to (S)-allantoin. Might function as a negative regulator to modulate brassinosteroid-mediated plant growth. 5-hydroxyisourate + H2O = 5-hydroxy-2-oxo-4-ureido-2,5-dihydro-1H-imidazole-5-carboxylate + H(+) 5-hydroxy-2-oxo-4-ureido-2,5-dihydro-1H-imidazole-5-carboxylate + H(+) = (S)-allantoin + CO2 Purine metabolism; urate degradation; (S)-allantoin from urate: step 2/3. Purine metabolism; urate degradation; (S)-allantoin from urate: step 3/3. Interacts with BRI1. Homodimer. Expressed ubiquitously. Not regulated by plant steroids. The N-terminal 29 amino acids are essential, but not sufficient, for the interaction with BRI1. Phosphorylated by BRI1 in vitro. In the N-terminal section; belongs to the OHCU decarboxylase family. In the C-terminal section; belongs to the transthyretin family. 5-hydroxyisourate hydrolase subfamily.