Enzymatic synthesis of 4'- and 3',4 -hydroxylated flavanones and flavones with flower extracts of 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 of flavonoid biosynthesis, chalcone synthase, revealed that the 3',4'-hydroxylated flavonoids are formed by hydroxylation of flavonoid compounds rather than by incorporation of caffeoyl-CoA into the flavonoid skeleton during the condensation reaction. In fact, flavonoid 3'-hydroxylase activity could be dem onstrated 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 P H as cofactor. Besides flavanone 3'-hydroxylase a further NADPH -dependent enzyme activity (flavone 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.