Identification of an UDP-glucose: Flavonol 3-O-glucosyl-transferase from cell suspension cultures of soybean (Glycine max L.).

A glucosyltransferase, which catalyses the glucosylation of flavonols, using uridine diphosphate-D-glucose as glucose donor, has been isolated and purified about 5-10 fold from cell suspension cultures of soybean (Glycine max L., var. Mandarin). The pH optimum for this reaction was ca. 8.5 in glycine-NaOH buffer, and no additional cofactors were required. The enzyme glucosylated the following flavonols predominantly at the 3-position: quercetin (Km 126 μM), kaempferol (Km 172 μM), isorhamnetin (Km 200 μM) and fisetin (Km 270 μM). With quercetin as substrate, the apparent Km value for uridine diphosphate-D-glucose was 0.3 M. Glucosylation of flavonols and flavones by this preparation occurred weakly also at the 7-position. No activity was found with dihydroquercetin, naringenin, 4,2',4'-trihydroxychalcone, daidzein or texasin. The enzyme was specific for flavonoid compounds, since no activity was observed towards cinnamic acids or simple phenols. However, the preparation was contaminated by a vanillic acid glucosyltransferase, from which it could be partially separated by ionexchange chromatography. The specific activity of the flavonol 3-O-glucosyltransferase increased with age of the culture, reaching a maximum late in the growth cycle of the culture.