In enzymology, an allantoinase () is an enzyme that catalyzes the chemical reaction (S)-allantoin + H2O allantoate Thus, the two substrates of this enzyme are (S)-allantoin and H2O, whereas its product is allantoate. This enzyme belongs to the family of hydrolases, those acting on carbon-nitrogen bonds other than peptide bonds, specifically in cyclic amides. The systematic name of this enzyme class is (S)-allantoin amidohydrolase. This enzyme participates in purine metabolism. References EC 3.5.2 Enzymes of unknown structure