Enzymatic Synthesis of 4'- and 3 ' ,4 -Hydroxylated Flavanones and Flavones with Flower Extracts of Sinningia cardinalis

Flavonoid Biosynthesis, Chalcone Synthase, Flavonoid 3'-Hydroxylase, Flavone Synthase II, Sinningia cardinalis Flowers of Sinningia (syn. Rechsteineria) cardinalis contain glycosides of the flavones apigenin (4'-O H ) and luteolin (3',4'-O H ) respectively, and of the related 3-deoxyanthocyanidins apigeninidin and luteolinidin. Studies on substrate specificity of the key enzyme o f flavonoid biosynthesis, chalcone synthase, revealed that the 3',4'-hydroxylated flavonoids are formed by hydroxylation of flavonoid com pounds rather than by incorporation of caffeoyl-CoA into the flavonoid skeleton during the condensation reaction. In fact, flavonoid 3'-hydroxylase activity could be dem onstrat­ ed in the microsomal fraction of the flower extracts. The enzyme catalyses hydroxylation of naringenin and apigenin in the 3'-position to eriodictyol and luteolin, respectively, with N A D PH as cofactor. Besides flavanone 3'-hydroxylase a further NADPH -dependent enzyme activity (fla­ vone synthase II) was observed in the microsomal fraction catalysing the oxidation of naringenin to apigenin and of eriodictyol to luteolin. The Cytochrome P-450 inhibitor ancymidol was found to abolish com pletely flavone synthase II activity, whereas flavonoid 3'-hydroxylase activity was not impaired.