Studies on Columnidin Biosynthesis with Flower Extracts from Columnea hybrida

Columnidin, the characteristic 3-deoxyanthocyanidin of some Columnea species, possesses the 3′,4′-B-ring hydroxylation pattern of luteolinidin and an additional hydroxyl group at the A-ring, most likely in the 8-position. Studies on substrate specificity of chalcone synthase and flavanone 4-reductase and the demonstration of high flavonoid 3′-hydroxylase activity revealed that the 3′-hydroxyl group of the B-ring of columnidin is introduced at the flavanone stage by hydroxylation of naringenin to eriodictyol. Enzymatic hydroxylation of the A-ring, however, could neither be observed with soluble enzyme preparation nor with microsomal fraction. Most probably this step first occurs at the anthocyanidin level. Besides flavonoid 3′-hydroxylase the microsomal fraction of Columnea flower extracts contains flavone synthase II activity catalysing desaturation of flavanones to flavones with NADPH as co-factor. The presence of some apigenin, appreciable amounts of luteolin and of the 3′,4′-hydroxylated flavan-4-ol luteoforol in the flowers confirm the enzymatic data.