The tree legume Leucaena leucocephala contains a large amount of a toxic nonprotein aromatic amino acid, mimosine, and also an enzyme, mimosinase, for mimosine degradation. In this study, we isolated a 1,520-bp complementary DNA (cDNA) for mimosinase from L. leucocephala and characterized the encoded enzyme for mimosine-degrading activity. The deduced amino acid sequence of the coding region of the cDNA was predicted to have a chloroplast transit peptide. The nucleotide sequence, excluding the sequence for the chloroplast transit peptide, was codon optimized and expressed in Escherichia coli. The purified recombinant enzyme was used in mimosine degradation assays, and the chromatogram of the major product was found to be identical to that of 3-hydroxy-4-pyridone (3H4P), which was further verified by electrospray ionization-tandem mass spectrometry. The enzyme activity requires pyridoxal 5'-phosphate but not α-keto acid; therefore, the enzyme is not an aminotransferase. In addition to 3H4P, we also identified pyruvate and ammonia as other degradation products. The dependence of the enzyme on pyridoxal 5'-phosphate and the production of 3H4P with the release of ammonia indicate that it is a carbon-nitrogen lyase. It was found to be highly efficient and specific in catalyzing mimosine degradation, with apparent Km and Vmax values of 1.16×10(-4) m and 5.05×10(-5) mol s(-1) mg(-1), respectively. The presence of other aromatic amino acids, including l-tyrosine, l-phenylalanine, and l-tryptophan, in the reaction did not show any competitive inhibition. The isolation of the mimosinase cDNA and the biochemical characterization of the recombinant enzyme will be useful in developing transgenic L. leucocephala with reduced mimosine content in the future.