L-Aromatic amino acid decarboxylase (dopa decarboxylase; DDC) is a pyridoxal 5'-phosphate (PLP)-dependent homodimeric enzyme that catalyses the decarboxylation of L-dopa and other L-aromatic amino acids. To advance structure-function studies with the enzyme, a cDNA that codes for the protein from pig kidney has been cloned by joining a partial cDNA obtained by library screening with a synthetic portion constructed by the annealing and extension of long oligonucleotides. The hybrid cDNA was then expressed in Escherichia coli to produce recombinant protein. During characterization of the recombinant enzyme it was unexpectedly observed that it possesses certain differences from the enzyme purified from pig kidney. Whereas the later protein binds 1 molecule of PLP per dimer, the recombinant enzyme was found to bind two molecules of coenzyme per dimer. Moreover, the Vmax was twice that of the protein purified from tissue. On addition of substrate, the absorbance changes accompanying transaldimination were likewise 2-fold greater in the recombinant enzyme. Examination of the respective apoenzymes by absorbance, CD and fluorescence spectroscopy revealed distinct differences. The recombinant apoprotein has no significant absorbance at 335 nm, unlike the pig kidney apoenzyme; in the latter case this residual absorbance is associated with a positive dichroic signal. When excited at 335 nm the pig kidney apoenzyme has a pronounced emission maximum at 385 nm, in contrast with its recombinant counterpart, which shows a weak broad emission at about 400 nm. However, the holoenzyme-apoenzyme transition did not markedly alter the respective fluorescence properties of either recombinant or pig kidney DDC when excited at 335 nm. Taken together, these findings indicate that recombinant pig kidney DDC has two active-site PLP molecules and therefore displays structural characteristics typical of PLP-dependent homodimeric enzymes. The natural enzyme contains one active-site PLP molecule whereas the remaining PLP binding site is most probably occupied by an inactive covalently bound coenzyme derivative; some speculations are made about its origin. The coenzyme absorbing bands of recombinant DDC show a modest pH dependence at 335 and 425 nm. A putative working model is presented to explain this behaviour.