Isolation and Characterization of a UDPGlucose: Flavonol O-Glucosyltransferase from Illuminated Red Cabbage (Brassica oleracea cv Red Danish) Seedlings

A UDPGlc:flavonol O³-glucosyltransferase (EC 2.4.1.91) that catalyzes the formation of quercetin and kaempferol O³-glucosides has been purified about 1450-fold from illuminated red cabbage (Brassica oleracea cv Red Danish) seedlings with a 3.3% yield. Purification of the enzyme was achieved by (NH₄)₂SO₄-precipitation, gel-filtration, ion-exchange chromatography on DEAE-Bio-Gel and Q-Sepharose, chromatofocusing, and electrophoresis in nondenaturing polyacrylamide (10%) gels. The enzyme preparation had a pH optimum between 5.8 and 6.2, isoelectric point in the pH range 4.25 to 4.55, a M_r of 59,000, and it was composed of two similar subunits of M_r 29,500. The glucosyltransferase reached half substrate saturation at 180 micromolar (UDPGlc) and 7 micromolar (quercetin) concentrations. Kaempferol, which was glucosylated at a relative rate of 87%, had a lesser affinity for the enzyme (K_m~12 micromolar). Flavanones, flavanols, flavones, dihydroflavonols, and anthocyanidins were not readily utilized as substrates, suggesting that the enzyme is specific for flavonol glucoside biosynthesis.