Decyclization of 6-oxo-1,4,5,6-tetrahydronicotinate to form 2-(enamine)glutarate, followed by hydrolysis to form (S)-2-formylglutarate. 1,4,5,6-tetrahydro-6-oxonicotinate + 2 H2O = 2-formylglutarate + NH4(+) Binds 1 Fe cation per subunit. Binds 1 zinc ion per subunit. Cofactor degradation; nicotinate degradation; propanoate and pyruvate from 6-hydroxynicotinate: step 2/8. Homotetramer. Dimer of dimers. Belongs to the metallo-dependent hydrolases superfamily. Adenine deaminase family.