UDP-D-Xylose: Flavonol 3-O-Xylosyltransferase from Young Leaves of Euonymus alatus

From the young leaves of Euonymus alatus f. ciliato-dentatus, a novel enzyme, UDPD-xylose: flavonol 3-O-xylosyltransferase (F3XT), catalyzing the transfer of D-xylose from UDP-D-xylose to the 3 position of 3,5,7,4'-tetrahydroxyflavone (kaempferol), was detected and purified about 16-fold by precipitation with ammonium sulfate and DEAE-cellulose CC, by which F3XT was separated from two coexisting flavonol O-glucosyltransferases (FGT). Thus, F 3 XT was isolated as a soluble enzyme with a pH optimum of 7.0 in Tris-HCl buffer. The molecular weight of F3XT, which had an isoelectric point at pH 6.1, was estimated by elution from a column of Sephadex G-100 to be about 48 kDa. The activity of F3XT was stimulated by 14 mM 2-ME and strongly inhibited by 1 mM Cu2+, 1 mM Zn2+, and various re­ agents that react with sulfhydryl groups. Among the substrates tested for F3XT , kaempferol was the best. The Km values for kaempferol and UDP-xylose were determined to be 0.83 jim and 25 |iM, respectively. F3XT mediated the transfer of xylose exclusively to the 3-hydroxyl group of kaempferol. Isorhamnetin, quercetin and fisetin also can function as xylosyl acceptor though less efficiently, but neither the 7-O-glucosides nor the 3-O-glucosides of kaempferol and quercetin were able to accept D-xylose. Dihydroflavonols were not xylosylated.