Chalcone synthesis and hydroxylation of flavonoids in 3′-position with enzyme preparations from flowers of Dianthus caryophyllus L. (carnation)

Abstract

Chalcone synthase activity was demonstrated in enzyme preparations from flowers of defined genotypes of Dianthus caryophyllus L. (carnation). In the absence of chalcone isomerase activity, which could be completely excluded by genetic methods, the first product formed from malonyl-CoA and 4-coumaroyl-CoA proved to be naringenin chalcone, followed by formation of naringenin as a result of chemical cyclization. In the presence of chalcone isomerase activity, however, naringenin was the only product of the synthase reaction. In vitro, both 4-coumaryl-CoA and caffeoyl-CoA were found to be used as substrates for the condensation reaction with respective pH optima of 8.0 and 7.0. The results of chemogenetic and enzymatic studies, however, showed that in vivo only 4-coumaroyl-CoA serves as substrate for the formation of the flavonoid skeleton. In confirmation of these results, an NADPH-dependent microsomal 3′-hydroxylase activity could be demonstrated, catalyzing hydroxylation of naringenin and dihydrokaempferol in 3′-position. Furthermore, a strict correlation was found between 3′-hydroxylase activity and the gene r which is known to control the formation of 3′, 4′-hydroxylated flavonoid compounds.