Cyclic amidohydrolase that catalyzes the reversible conversion of creatinine to creatine. Is also active toward glycocyamidine, though the reaction rate is very low, but it is completely inert toward hydantoin and its derivatives. creatinine + H2O = creatine Binds 2 Zn(2+) ions per subunit. The Zn(2+) in the metal 1 binding site can be replaced with Mn(2+); however, the second zinc in metal binding site 2 is much more tightly bound and cannot be replaced. The enzyme with one zinc and one manganese ion is more active than that with two zinc ions. Is markedly inactivated in vitro by heavy metal ions, N-bromosuccinimide, ethoxyformic anhydride, and dye-sensitized photooxidation. Optimum pH is 7-9 for the forward and reverse reactions. Retains 75% of the activity after incubation at 75 degrees Celsius for 30 minutes. Amine and polyamine degradation; creatinine degradation. Homohexamer; trimer of dimers. The proposed catalytic mechanism involves two water molecules. The first molecule is a hydroxide ion that is bound as a bridge between the two metal ions and attacks the carbonyl carbon of the substrate. The second water molecule, that is bound to the carboxyl group of Glu-122 and to the metal 1, functions as a proton donor in catalysis. Belongs to the creatininase superfamily.