Purification and characterization of norcoclaurine synthase. The first committed enzyme in benzylisoquinoline alkaloid biosynthesis in plants.
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Norcoclaurine synthase (NCS; EC ) catalyzes the condensation of dopamine and 4-hydroxyphenylacetaldehyde (4-HPAA) as the first committed step in benzylisoquinoline alkaloid biosynthesis in plants. NCS was purified 1590-fold to homogeneity from cell suspension cultures of meadow rue (Thalictrum flavum ssp. glaucum). The purification procedure, which resulted in a 4.2% yield, involved hydrophobic interaction, anion exchange, hydroxyapatite, and gel filtration chromatography. Purified NCS displayed native and denatured molecular masses of approximately 28 and 15 kDa, respectively, suggesting that the enzyme is composed of two subunits. Two-dimensional polyacrylamide gel electrophoresis revealed two major and two minor isoforms with pI values between 5.5 and 6.2. NCS activity was maximal at pH 6.5 to 7.0 and temperatures between 42 and 55 degrees C and was not affected by divalent cations. The enzyme showed hyperbolic saturation kinetics for 4-HPAA (K(m) = 335 microm) but sigmoidal saturation kinetics for dopamine (Hill coefficient = 1.8) suggesting cooperativity between the dopamine binding sites on each subunit; thus, NCS might play a regulatory, or rate-limiting, role in controlling the rate of pathway flux in benzylisoquinoline alkaloid biosynthesis. Product inhibition kinetics performed at saturating levels of one substrate and with norlaudanosoline as the inhibitor showed that NCS follows an iso-ordered bi-uni mechanism with 4-HPAA binding before dopamine. NCS activity was highest in soluble protein extracts from roots followed by stems, leaves, and flower buds. (+info)
Cell type-specific localization of transcripts encoding nine consecutive enzymes involved in protoberberine alkaloid biosynthesis.
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Molecular clones encoding nine consecutive biosynthetic enzymes that catalyze the conversion of l-dopa to the protoberberine alkaloid (S)-canadine were isolated from meadow rue (Thalictrum flavum ssp glaucum). The predicted proteins showed extensive sequence identity with corresponding enzymes involved in the biosynthesis of related benzylisoquinoline alkaloids in other species, such as opium poppy (Papaver somniferum). RNA gel blot hybridization analysis showed that gene transcripts for each enzyme were most abundant in rhizomes but were also detected at lower levels in roots and other organs. In situ RNA hybridization analysis revealed the cell type-specific expression of protoberberine alkaloid biosynthetic genes in roots and rhizomes. In roots, gene transcripts for all nine enzymes were localized to immature endodermis, pericycle, and, in some cases, adjacent cortical cells. In rhizomes, gene transcripts encoding all nine enzymes were restricted to the protoderm of leaf primordia. The localization of biosynthetic gene transcripts was in contrast with the tissue-specific accumulation of protoberberine alkaloids. In roots, protoberberine alkaloids were restricted to mature endodermal cells upon the initiation of secondary growth and were distributed throughout the pith and cortex in rhizomes. Thus, the cell type-specific localization of protoberberine alkaloid biosynthesis and accumulation are temporally and spatially separated in T. flavum roots and rhizomes, respectively. Despite the close phylogeny between corresponding biosynthetic enzymes, distinct and different cell types are involved in the biosynthesis and accumulation of benzylisoquinoline alkaloids in T. flavum and P. somniferum. Our results suggest that the evolution of alkaloid metabolism involves not only the recruitment of new biosynthetic enzymes, but also the migration of established pathways between cell types. (+info)
Four new cycloartane glycosides from Thalictrum fortunei.
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Four new cycloartane glycosides were isolated from the aerial parts of Thalictrum fortunei (Ranunculaceae). The chemical structures of these new glycosides were elucidated as 3-O-beta-D-glucopyranosyl-(1-->4)-beta-D-fucopyranosyl (22S,24Z)-cycloart-24-en-3beta,22,26-triol 26-O-beta-D-glucopyranoside, 3-O-beta-D-glucopyranosyl-(1-->4)-beta-D-fucopyranosyl (22S,24Z)-cycloart-24-en-3beta,22,26-triol 26-O-beta-D-quinovopyranosyl-(1-->6)-beta-D-glucopyranoside, 3-O-beta-D-glucopyranosyl-(1-->4)-beta-D-fucopyranosyl (22S,24Z)-cycloart-24-en-3beta,22,26-triol 26-O-beta-D-xylopyranosyl-(1-->6)-beta-D-glucopyranoside, and 3-O-beta-D-glucopyranosyl-(1-->4)-beta-D-fucopyranosyl (22S,24Z)-cycloart-24-en-3beta,22,26-triol 26-O-alpha-L-arabinopyranosyl-(1-->6)-beta-D-glucopyranoside by extensive NMR methods, HR-ESI-MS, and hydrolysis. This is the first report of (22S,24Z)-3beta,22,26-trihydroxycycloartan-24-ene (thelictogenin A, 5) being glycosylated at C-26. (+info)
Cloning, expression, crystallization and preliminary X-ray data analysis of norcoclaurine synthase from Thalictrum flavum.
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