Compound as reactive (1)

H⁺ + (S)-2-oxo-4-hydroxy-4-carboxy-5-ureidoimidazoline → CO₂ + (S)-(+)-allantoin

Compound as product (1)

H₂O + (S)-5-hydroxyisourate → H⁺ + (S)-2-oxo-4-hydroxy-4-carboxy-5-ureidoimidazoline

Reversible reaction (1)

(S)-2-oxo-4-hydroxy-4-carboxy-5-ureidoimidazoline ↔ allantoin + CO2