In vitro, 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. The physiological substrate is unknown. L-dopa + O2 = 4-(L-alanin-3-yl)-2-hydroxy-cis,cis-muconate 6-semialdehyde + H(+) kcat is 0.17 min(-1) with L-DOPA. Optimum pH is 8.0. Monomer. Betalamic acid is the structural unit of the betalains, natural nitrogen-containing water-soluble pigments with high colorant and bioactive properties, characteristic of plants of the order Caryophyllales. Belongs to the DODA-type extradiol aromatic ring-opening dioxygenase family. It is uncertain whether Met-1 or Met-10 is the initiator. Truncated N-terminus.