Opens the cyclic ring of dihydroxy-phenylalanine (DOPA) between carbons 4 and 5, thus producing an unstable seco-DOPA that rearranges nonenzymatically to betalamic acid. Produces mainly (S)-betalamic acid. Required for the coloration of flowers. L-dopa + O2 = 4-(L-alanin-3-yl)-2-hydroxy-cis,cis-muconate 6-semialdehyde + H(+) Binds 1 zinc ion per subunit. Pigment biosynthesis; betalain biosynthesis. Expressed in petals. Not detected in leaves, stems and roots. Requires ascorbic acid for iron to be in the proper redox state (PubMed:19366710). For the same enzyme from B. vulgaris (AC Q70FG7), presence of Fe(2+) is not necessary to detect activity. Belongs to the DODA-type extradiol aromatic ring-opening dioxygenase family.