The A-ring specific hydroxylation of flavonols in position 6 in Tagetes sp. is catalyzed by a cytochrome P450 dependent monooxygenase

Yellow flavonols represent important plant pigments in Asteraceae. In contrast to common flavonols, they show additional hydroxyl groups in position 6 and/or 8 of the aromatic A-ring in addition to the basic 5,7-hydroxylation pattern. A novel flavonol 6-hydroxylase (F6H) introducing a hydroxyl group in position 6 of quercetin was demonstrated in enzyme preparations from petals of Tagetes patula and Tagetes erecta. The enzyme was classified as cytochrome P450 dependent monooxygenase by photoreversible carbon monoxide inhibition, inhibition by NADPH-cytochrome P450 reductase specific antibodies and cytochrome P450 specific inhibitors. The flavone luteolin was barely accepted as a substrate. Methylated flavonols, quercetin 7-O-glucoside, flavanones, and dihydroflavonols were not accepted as substrates. The presence of the enzyme was demonstrated in various varieties showing different coloration.