Biogenesis of betalaine. Biotransformation of dopa and tyrosine in betalaminic acid part of the betanins

During studies on the biogenesis of betalains (I) in cactus fruits (Opuntia sp.). DL -dopa-1-[¹⁴C] and -2-[¹⁴C] were incorporated into betanin (III) which was obtained radiopure after crystallization. The specific activity remained constant after conversion to betanidin (IV) and to a neobetanidin derivative (IX). Reaction of radiobetanin with proline afforded indicaxanthin (V) carrying more than 90% of the radioactivity. Dopa (VI) is thus an efficient precursor for betalamic acid (VIII) but not for cyclodopa (VII). Decarboxylation of radiobetanidin and radioindicaxanthin showed that the carboxyl group of dopa remained a carboxyl group in the biotransformation to betalamic acid. It is concluded that the aromatic ring of dopa is cleaved and that re-cyclization involving the nitrogen generates the dihydropyridine moiety. Under the same conditions mevalonic acid, aspartic acid and phenylalanine showed low incorporations. Studies with beet seedlings and DL -dopa-1-[¹⁴C], -2-[¹⁴C] and DL -tyrosine-1-[¹⁴C] afforded similar results but with low incorporations.