Conversion of (+)-dihydroquercetin to (+)-2,3-trans-3,4-cis-leucocyanidin and (+)-catechin with an enzyme extract from maturing grains of barley

Abstract

A soluble, NADPH-dependent reductase, catalyzing the reduction of (+)-dihydroquercetin to (+)-2,3-trans-3,4-cis-leucocyanidin ((2R, 3S, 4S)-3,4,5,7,3′,4′-hexahydroxyflavan), was demonstrated in an enzyme prepartation from maturing grains of wild type barley (Hordeum vulgare L., cv. Nordal). This reductase activity had a pH-optimum around 7.0 and was strongly inhibited by the product of the reaction. Furthermore, a second, less active NADPH-dependent reductase, catalyzing the reduction of (+)-2,3-trans-3,4-cis-leucocyanidin to (+)-catechin, was demonstrated by a double step reduction of (+)-dihydroquercetin to (+)-cartechin. The reaction product of (+)-dihydroquercetin reductase was identified by co-chromatography with an authentic standard of (+)-2,3-trans-3,4-cis-leucocyaidin, which was prepared chemically by acid epimerization of (+)-2,3-trans-3,4-trans-leucocyanidin (2R, 3S, 4R)-3,4,5,7,3′,4′-hexahydroxyflavan) and characterized byIH NMR spectroscopy in the free phenolic form. © 1986 Carlsberg Laboratory.