Common Identity of UDP-Glucose: Anthocyanidin 3-O-Glucosyltransferase and UDP-Glucose: Flavonol 3-O-Glucosyltransferase in Flowers of Petunia hybrida

In an attempt to distinguish between the UDP-glucose: flavonol 3-O-glucosyltransferase (3G T ) and the UDP-glucose:anthocyanidin 3-O-glucosyltransferase in flower buds o f Petunia hybrida, several properties of these activities were determined. The 3-glucosylation o f anthocyanidin had a pH-activity optimum of 7.2, that o f flavonol pH 9.2 to 9.5. Anthocyanidin 3G T activity was lowered in the presence o f EDTA or /?-mercaptoethanol, but this was due to an effect on the anthocyanidin substrate. The two 3-glucosylating activities were to a similar extent inhibited by an increasing ionic strength in the enzyme assay and showed an identical iso-electric point (5.2) as determined by chromatofocusing. Molecular weights were identical: 26000, 52000 or 78000 daltons as determined by gel-filtration. Antiserum raised against partially purified 3G T gave identical immunoprecipitation curves with flavonol 3G T and anthocyanidin 3G T. Special attention was given to the 3-O-glucosyltransferase in mutants with low levels o f 3G T activity. These mutants are unable to form significant amounts o f anthocyanins but contain wildtype amounts of flavonols. The enzyme o f such mutants had the same iso-electric point and identical titration-curves with antiserum as the enzyme from wildtype plants. 3G T from wildtype or mutant plants glucosylated flavonols at higher rates than anthocyanidins.