5-Hydroxy-2-pyridone (2,5-DHP) is a central metabolic intermediate in catabolism of many pyridine derivatives, and has been suggested as a potential carcinogen. 2,5-DHP is frequently transformed to N-formylmaleamic acid (NFM) by a 2,5-DHP dioxygenase. Three hypotheses were formerly discussed for conversion of 2,5-DHP to maleamate. Based on enzymatic reactions of dioxygenase (Hpo) and N-formylamide deformylase (Nfo), we demonstrated that the dioxygenase does not catalyze the hydrolysis of NFM but rather that this activity is brought about by a separate deformylase. We report that the deformylase acts both on NFM and its trans-isomer, N-formylfumaramic acid (NFF), but the catalytic efficiency of Nfo for NFM is about 1,400 times greater than that for NFF. In addition, we uncover catalytic and structural characteristics of the new family that the Hpo belongs to, and support a potential 2-His-1-carboxylate motif (HX52HXD) by three-dimensional modeling and site-directed mutagenesis. This study provides a better understanding of 2,5-DHP catabolism.