Allantoinase () is an enzyme characterised from Escherichia coli that catalyzes the hydrolysis of (S)-allantoin to allantoic acid:
Since allantoin is product of purine metabolism and a component in soil, it can be a source of nitrogen for plants that contain the enzyme. It can be used to measure oxidative stress.
This enzyme is a hydrolase, one acting on carbon-nitrogen bonds other than peptide bonds, specifically in cyclic amides. The systematic name of this enzyme class is (S)-allantoin amidohydrolase.
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