In enzymology, a creatinine deaminase (EC 3.5.4.21) is an enzyme that catalyzes the chemical reaction: creatinine + H2O ↔ N-methylhydantoin + NH3. Thus, the two substrates of this enzyme are creatinine and H2O, whereas its two products are N-methylhydantoin and NH3. This enzyme belongs to the family of hydrolases, those acting on carbon-nitrogen bonds other than peptide bonds, specifically in cyclic amidines. The systematic name of this enzyme class is creatinine iminohydrolase.