Flavonoid Biosynthesis in Flowers of Verbena hybrida

Extracts from flowers of different strains of Verbena hybrida were found to contain flavones, flavonols, and anthocyanins. Furthermore, the presence of flavanones and dihydroflavonols was demonstrated. With regard to the B-ring substitution pattern, flavonoids with hydroxy groups at the 4'-, the 3',4'-, and the 3',4',5'-positions were found to occur. Red flowering strains contain 4'- and 3',4'-hydroxylated flavonoids in varying amounts. Enzymic studies revealed that the compounds with the 3',4'-hydroxylation pattern can be established by two different reactions. A small part of them is formed by incorporation of caffeic acid beside p-coumaric acid into the flavonoid skeleton during the condensation reaction catalysed by chalcone synthase. This basic level can be considerably enhanced by hydroxylation of 4'-hydroxylated flavonoids in the 3'-position. The reaction is catalysed by the well-known enzyme flavonoid 3'-hydroxylase. Enzyme activity was found to be correlated with the presence of enhanced amounts of 3',4'-hydroxylated flavonoid compounds in the flowers. Beside chalcone synthase and 3'-hydroxylase a further enzyme activity could be demonstrated in enzyme preparations from flowers of Verbena hybrida which catalyses the oxidation of naringenin to apigenin and of eriodictyol to luteolin. The reaction required NADPH as cofactor and had a pH optimum of about 6.5. Enzyme activity was found to be localized in the microsomal fraction.