Showing papers on "Isovitexin published in 1974"

Genetic control and biosynthesis of two new flavone-glycosides in the petals of Melandrium album.

Abstract: In a chemicogenetic analysis of the geographical distribution of flavone-glycosides in the petals of Melandrium album, we found two unknown flavone-glycosides in ten Hungarian and four German populations. By means of classical techniques for the identification and structure determination of flavonoids, the structure of these flavones turned out to be 6-C-glucosylglucosylapigenin and 7-O-glucosyl-6-C-glucosylglucosylapigenin, respectively. Genetic analysis showed that the coupling of glucose to the carbon-carbon bound glucose of isovitexin (6-C-glucosylapigenin) was controlled by a single dominant gene, Fg. Fg controls a UDP-glucose: isovitexin 6-C-glucosylglucosyltransferase. By means of ammonium sulfate fractionation and Sephadex chromatography, the enzyme was purified sixfold. The partly purified enzyme had a pH optimum between 8.0 and 8.5. The apparent K m value for UDP-glucose in the presence of 1.0 mm isovitexin was 2.2 mm. The apparent K m value for isovitexin in the presence of 1.8 mm UDP-glucose was 0.08 mm. The glucosyltransferase activity was stimulated by the divalent cations Mn, Mg, Co, and Ca. Neither 7-O-glucosyl nor 7-O-xylosyl isovitexin could serve as an enzyme substrate. Therefore, the biosynthesis of 7-O-glucosyl-6-C-glucosylglucosylapigenin found in the petals of M. album proceeds in a sequential manner: first the formation of 6-C-glucosylglucosylapigenin, followed by 7-O-glucosylation. Isovitexin 6-C-glucosylglucosyltransferase activity controlled by gene Fg could also be demonstrated in leaves of Fg plants. The enzyme probably uses another substrate in these green parts of the plant, because both isovitexin and isovitexin-glycosides are absent.

Dominance Relationships between Two Allelic Genes Controlling Glycosyltransferases with Different Substrate Specificity in Melandrium

Abstract: Genetical analysis showed that the genes gG and gX, which control, respectively, the glucosylation and xylosylation of the 7-hydroxyl group of isovitexin in the petals of Melandrium, are alleles. In petal extracts of plants possessing the gene gX an enzyme was present which catalyzed the transfer of the xylose moiety of UDP-xylose to the 7-hydroxyl group of isovitexin. The xylosyl-transferase controlled by the gene gX had a "true Km value" of 0.77 mM for UDP-xylose. The "true Km value" for isovitexin was << 0.04 mM. The transfer of glucose from UDP-glucose to the 7-hydroxyl group of isovitexin is catalyzed by the enzyme controlled by gene gG. In plants possessing both the alleles gG and gX, only the gene product of gG, i.e., isovitexin 7-O-glucoside, was found. In this respect gG is dominant over allele gX. In petal extracts of these gGgX plants, however, besides UDP-glucose: isovitexin 7-O-glucosyltransferase, also UDP-xylose: isovitexin 7-O-xylosyltransferase could be detected. This means that the dominance is not a consequence of transcriptional and/or translational control. Enzyme kinetic experiments showed that inhibition of the xylosyltransferase by the endproduct of the glucosyltransferase did not occur. Comparison of the enzyme kinetic parameters revealed that the dominance is probably caused by differences in Vmax between the two enzymes, both working at saturating isovitexin concentrations. A competition model is suggested which explains why the amount of isovitexin 7-O-glucoside in gGgG plants and the amount of isovitexin 7-O-xyloside in gXgX plants are about the same, whereas in gGgX plants isovitexin 7-O-xyloside escapes detection. The differences in distribution of the isovitexin glycosylation genes in the two species M. album and M. dioicum are discussed.

Dominance relations between isovitexin: 7-0-glycosyltransferase alleles in Melandrium

Abstract: In a hybrid swarm of Melandrium dioicum and Melandrium album, a plant was found in which the genes gG and gX, controlling an isovitexin 7-O-glucosyltransferase, respectively an isovitexin 7-O-xylosyltransferase, were co-dominant. In earlier experiments, gene gG was always dominant over its allele gX. Genetical and biochemical analysis revealed that this co-dominance was caused by an allele of gX, further designated gX′. Comparison of the enzyme-kinetic properties of the gX′ controlled xylosyltransferase with the gX controlled xylosyltransferase revealed no difference in substrate affinity for UDP-xylose nor for isovitexin. However, the maximal activity of the gX′ controlled transferase was 8+9 times higher than the gX controlled activity. These results support the competition model previously proposed to explain the dominance relationship between gG and gX. This model was based on the assumptions that 1) both the xylosyl- and the glucosyltransferase work at saturating isovitexin concentration and that 2) the lower Vmax of the xylosyltransferase, makes this enzyme a poorer competitor for isovitexin than the glucosyltransferase.