Hyoscyamine 6 beta-hydroxylase, an enzyme involved in tropane alkaloid biosynthesis, is localized at the pericycle of the root

Hyoscyamine 6 beta-hydroxylase (H6H; EC 1.14.11.11) catalyzes the first reaction in the biosynthetic pathway from hyoscyamine to scopolamine in several solanaceous plants. Four monoclonal antibodies were raised against H6H purified from cultured roots of Hyoscyamus niger. The IgG1 antibody mAb5 inhibited H6H activities present in cell-free extracts of H. niger roots and specifically recognized 38-40-kDa proteins from six different scopolamine-producing plant species in Western blot analysis after sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. The other three monoclonal antibodies all recognized SDS-denatured H6H protein from Hyoscyamus species, but did not bind to native H6H. Western blot analysis of protein extracts from various tissues of H. niger using these antibodies showed that H6H is abundant in cultured roots, present in plant roots, but absent in leaf, stem, calyx, cultured cells, and cultured shoots. Immunohistochemical studies using monoclonal antibody and immunogold-silver enhancement detected H6H only in the pericycle cells of the young root in several scopolamine-producing plants. Mature roots that underwent secondary growth and lacked the pericycle did not react with the antibody. This pericycle-specific localization of scopolamine biosynthesis provides an anatomical explanation for the tissue-specific biosynthesis of tropane alkaloids and may be important for translocation of tropane alkaloids from the root to the aerial parts.     

Tropinone reductases, enzymes at the branch point of tropane alkaloid metabolism

Two stereospecific oxidoreductases constitute a branch point in tropane alkaloid metabolism. Products of tropane metabolism are the alkaloids hyoscyamine, scopolamine, cocaine, and polyhydroxylated nortropane alkaloids, the calystegines. Both tropinone reductases reduce the precursor tropinone to yield either tropine or pseudotropine. In Solanaceae, tropine is incorporated into hyoscyamine and scopolamine; pseudotropine is the first specific metabolite on the way to the calystegines. Isolation, cloning and heterologous expression of both tropinone reductases enabled kinetic characterisation, protein crystallisation, and structure elucidation. Stereospecificity of reduction is achieved by binding tropinone in the respective enzyme active centre in opposite orientation. Immunolocalisation of both enzyme proteins in cultured roots revealed a tissue-specific protein accumulation. Metabolite flux through both arms of the tropane alkaloid pathway appears to be regulated by the activity of both enzymes and by their access to the precursor tropinone. Both tropinone reductases are NADPH-dependent short-chain dehydrogenases with amino acid sequence similarity of more than 50% suggesting their descent from a common ancestor. Putative tropinone reductase sequences annotated in plant genomes other that Solanaceae await functional characterisation.     

Molecular cloning and mRNA expression profiling of the first specific jasmonate biosynthetic pathway gene allene oxide synthase from <Emphasis Type="Italic">Hyoscyamus niger</Emphasis>

In the endeavor to enhance the production of pharmaceutically valuable tropane alkaloids including hyoscyamine and scopolamine in Hyoscyamus niger, methyl jasmonate (MeJA) showed significant stimulation both in tropane biosynthetic pathway enzymes activities and tropane alkaloids yields. Therefore it was speculated that genetic engineering of jasmonate biosynthetic pathway might enhance the endogenous jasmonates concentration, followed by stimulating the production of tropane alkaloids. Herein a full-length cDNA encoding allene oxide synthase (AOS, EC 4.2.1.92), the first committed step enzyme in jasmonate biosynthetic pathway was reported (named HnAOS, GenBank accession: EF532599). HnAOS was a novel member of the cytochrome P450 (CYP74A) subfamily. Real-time quantitative PCR analysis showed that HnAOS mRNA accumulated mainly in stems, and responded significantly to wounding or methyl jasmonate. The article is published in the original.     

Influence of nano‐zinc oxide on tropane alkaloid production,h6h gene transcription and antioxidant enzyme activity in Hyoscyamus reticulatus L. hairy roots

The use of nanotechnology and biotechnology to improve the production of plant bioactive compounds is growing. Hyoscyamus reticulatus L. is a major source of tropane alkaloids with a wide therapeutic use, including treatment of Parkinson's disease and to calm schizoid patients. In the present study, hairy roots were obtained from two‐week‐old cotyledon explants of H. reticulatus L. using the A7 strain of Agrobacterium rhizogenes. The effects of different concentrations of the signaling molecule nano‐zinc oxide (ZnO) (0, 50, 100 and 200 mg/L), with three exposure times (24, 48 and 72 h), on the growth rate, antioxidant enzyme activity, total phenol contents (TPC), tropane alkaloid contents and hyoscyamine‐6‐beta‐hydroxylase (h6h) gene expression levels were investigated. Growth curve analysis revealed a decrease in fresh and dry weight of ZnO‐treated hairy roots compared to the control. ANOVA results showed that the antioxidant activity of the enzymes catalase, guaiacol peroxidase and ascorbate peroxidase was significantly higher in the ZnO‐treated hairy roots than in the control, as was the TPC. The highest levels of hyoscyamine (37%) and scopolamine (37.63%) were obtained in hairy roots treated with 100 mg/L of ZnO after 48 and 72 h, respectively. Semi‐quantitative RT‐PCR analysis revealed the highest h6h gene expression was in hairy roots treated with 100 mg/L of ZnO after 24 h. It can be concluded that ZnO is as an effective elicitor of tropane alkaloids such as hyoscyamine and scopolamine due to its enhancing effect on expression levels of the biosynthetic h6h gene.     

Tropane alkaloids production in transgenic Hyoscyamus niger hairy root cultures over-expressing Putrescine N-methyltransferase is methyl jasmonate-dependent

The cDNA from Nicotiana tabacum encoding Putrescine N-methyltransferase (PMT), which catalyzes the first committed step in the biosynthesis of tropane alkaloids, has been introduced into the genome of a scopolamine-producing Hyoscyamus niger mediated by the disarmed Agrobacterium tumefaciens strain C58C1, which also carries Agrobacterium rhizogenes Ri plasmid pRiA4, and expressed under the control of the CaMV 35S promoter. Hairy root lines transformed with pmt presented fivefold higher PMT activity than the control, and the methylputrescine (MPUT) levels of the resulting engineered hairy roots increased four to fivefold compared to the control and wild-type roots, but there was no significant increase in tropane alkaloids. However, after methyl jasmonate (MeJA) treatment, a considerable increase of PMTase and endogenous H6Hase as well as an increase in scopolamine content was found either in the transgenic hairy roots or the control. The results indicate that hairy root lines over-expressing pmt have a high capacity to synthesize MPUT, whereas their ability to convert hyoscyamine into scopolamine is very limited. Exposure to MeJA strongly stimulated both polyamine and tropane biosynthesis pathways and elicitation led to more or less enhanced production simultaneously.     

Alkaloid production in cultured roots of three species of Duboisia

Cultured roots were obtained from calluses of Duboisia leichhardtii, D. myoporoides and D. hopwoodii. Cultured roots of all these species produced both tropane and pyridine-type alkaloids. The selected cultured roots of D. leichhardtii showed high contents of tropane alkaloids (hyoscyamine 0.53%, scopolamine 1.16%, on a dry weight basis).     

Incorporation of arginine, ornithine and phenylalanine into tropane alkaloids in suspension-cultured cells and aseptic roots of intact plants oi Atropa belladonna

Label from U-¹⁴C-arginine (Arg), -ornithine (Orn) and -phenylalanine(Phe) was incorporated into hyoscyam-ine and scopolamine byboth dissected roots of intact plants and homogeneous or aggregatingsuspension cultures of Atropa belladonna L. The early biosyntheticcompounds (hygrine, tropinone, tropanol) were found only inthe roots, and alkaloid synthesis proceeded as far as to scopolaminethere. In the synthesis of the tropane skeleton, Orn was usedmore efficiently than Arg. Phe was quickly metabolized bothin roots and suspension cultures, and the label was incorporatedinto both ethanol-insoluble compounds, particularly proteinsand different ethanol-soluble compounds, especially phenolics. When the callus, used to initiate suspension cultures, was repeatedlysubcultured, the degree of organization of the suspension-culturedcells (second suspension passage) grown in the presence of 2.5µM 2-naphthaleneacetic acid (NAA), changed first fromhomogeneous to aggregating and later to heavily rooty. Simultaneouslywith an increasing degree of organization, alkaloid synthesisdecreased. At first, the rate of incorporation of Arg and Phe,and later Orn into alkaloids decreased. In heavily rooty suspensionsneither hyoscyamine nor scopolamine were found, but traces oftropanol were detected. Hence, the synthesis of tropane alkaloidsseems to be regulated at the later biosynthetic steps. Key words: Arginine, Atropa, hyoscyamine, omithine, phenylalanine, roots, scopolamine, suspension culture, tropane alkaloids     

Tropane alkaloid production by Agrobacterium rhizogenes transformed hairy root cultures of Atropa baetica Willk. (Solanaceae)

Atropa baetica hairy root cultures were induced after infecting stem segments with Agrobacterium rhizogenes strain ATCC 15834. Accumulation of the tropane alkaloids atropine and scopolamine by hairy roots cultured in half- and full-strength Murashige and Skoog (MS) medium was high, although this was not growth associated. These alkaloids were also released into both liquid media. Higher tropane alkaloids present both in hairy roots and liquid medium occurred in half MS medium, showing a clear relationship between slow growth of cultures and higher product accumulation. The pH of both nutrient media varied as culture progressed, and seemed to be associated with the release of scopolamine. GC-MS analyses showed the presence of a new compound, namely tigloylpseudotropine; moreover, 3α-isobutyryloxytropane, formerly found only in plant leaf tissue, was also identified in the hairy roots. Received: 18 August 1997 / Revision received: 30 November 1997 / Accepted: 20 January 1998     

In vitro organogenesis and alkaloid accumulation in Datura innoxia

The kinetics of tropane alkaloids accumulation in different organs such as roots, leaves, stems, flowers and seeds of Datura innoxia was investigated by GC-MS. Twenty-six tropane alkaloids were detected. The ester derivatives of tropine (3alpha-tigloyloxytropine and 3-tigloyloxy-6-hydroxytropine) are the major compounds. Undifferentiated callus were established from the stem explants of Datura innoxia using Murashige and Skoog (MS) medium supplied with 6-benzylaminopurine (BA, 1 mg l(-10) and indole-3-acetic acid (IAA, 0.5 mg l(-1)) in combination for 6 weeks. Callus differentiation was initiated by subculture onto solid MS medium, free from hormones, for more than 10 months. Initially, shoots were formed after four weeks from subculture. Further subculturing in basal MS medium without growth regulators initiated the rooting of a shooty callus after 6 weeks. Investigation of the alkaloid content of the unorganized and organized callus revealed that callus (either green or brown) yielded only trace amounts of alkaloids. On the other hand, re-differentiated shoots contained mainly scopolamine while re-differentiated roots biosynthesized hyoscyamine as the main alkaloid.     

Overexpression of <Emphasis Type="Italic">odc</Emphasis> (ornithine decarboxylase) in <Emphasis Type="Italic">Datura innoxia</Emphasis> enhances the yield of scopolamine

Scopolamine is widely used for its anticholinergic properties. Because of higher physiological activity and less side effects the world demand of scopolamine is estimated to be ten times greater than other anticholinergic agents, hyoscyamine and atropine. Since natural production is limited, alternatives are required to boost the production. We report the introduction of mouse odc gene of polyamine biosynthesis pathway which is also the primary pathway of tropane alkaloids in Datura innoxia. Polyamines, mainly putrescine, serve as the common metabolite for tropane alkaloids and nicotine. We have overexpressed odc gene to modulate the metabolic flux downstream and eventually achieved higher accumulation of scopolamine in transgenic plants. Among six independent transformed lines one line (O10) produced scopolamine (0.258 μg/g dry weight) almost six times higher than that produced by control plants (0.042 μg/g DW). To our knowledge, this is the first report of odc overexpression in D. innoxia leading to higher scopolamine yield.     

Influence of hairy root ecotypes on production of tropane alkaloids in Brugmansia candida

Hyoscyamine and scopolamine are tropane alkaloids widely applied in medicine. Differences in alkaloid production and growth kinetics have been observed in Argentinian and Colombian ecotypes of Brugmansia candida hairy roots. The aim of this work was to analyze the production of key intermediates in tropane alkaloid synthesis in both ecotypes to determine differences in the biosynthetic pathway. Additionally, rolC gene expression was analyzed to determine its correlation with hairy root growth. The results showed a higher accumulation of polyamines in Colombian hairy roots, suggesting that there may be a rate-limiting enzyme in the last steps of hyoscyamine biosynthesis. Additionally, rolC gene expression was correlated with an improvement in hairy root growth, which supports the function of rol genes as growth modulators and suggests that metabolic engineering approaches involving rolC manipulation may be useful for the development of more efficient B. candida hairy root cultures for biotechnological applications.     

Formation of acetyltropine in Datura callus cultures

The production of tropoylesters in suspension cultures of Datura innoxia stem callus was significantly stimulated by dl-tropic acid, phenylpyruvate or tropine, but was little affected by l-phenylalanine or l-ornithine. Analyses have shown that acetyltropine is synthesized in large quantities by cultured cells when tropine has been supplied to various cultures of D. innoxia and D. tatula. Acetyltropine has been isolated from either the culture medium or cells supplied with tropine. These results indicate that tropine absorbed by the cultured cells of Datura is esterified predominantly by acetic acid to form acetyltropine, instead of other tropane alkaloids.     

Molecular cloning and catalytic characterization of a recombinant tropine biosynthetic tropinone reductase from Withania coagulans leaf

Tropinone reductases (TRs) are small proteins belonging to the SDR (short chain dehydrogenase/reductase) family of enzymes. TR-I and TR-II catalyze the conversion of tropinone into tropane alcohols (tropine and pseudotropine, respectively). The steps are intermediary enroute to biosynthesis of tropane esters of medicinal importance, hyoscyamine/scopolamine, and calystegins, respectively. Biosynthesis of tropane alkaloids has been proposed to occur in roots. However, in the present report, a tropine forming tropinone reductase (TR-I) cDNA was isolated from the aerial tissue (leaf) of a medicinal plant, Withania coagulans. The ORF was deduced to encode a polypeptide of 29.34 kDa. The complete cDNA (WcTRI) was expressed in E. coli and the recombinant His-tagged protein was purified for functional characterization. The enzyme had a narrow pH range of substantial activity with maxima at 6.6. Relatively superior thermostability of the enzyme (30% retention of activity at 60 °C) was catalytic novelty in consonance with the desert area restricted habitat of the plant. The in vitro reaction kinetics predominantly favoured the forward reaction. The enzyme had wide substrate specificity but did not cover the substrates of other well-known plant SDR related to menthol metabolism. To our knowledge, this pertains to be the first report on any gene and enzyme of secondary metabolism from the commercially and medicinally important vegetable rennet species.     

Enhancement of scopolamine production in Hyoscyamus muticus L. hairy root cultures by genetic engineering

 In order to test the possibility of enhancing the production of pharmaceutically valuable scopolamine in transgenic cultures, the 35S-h6h transgene that codes for the enzyme hyoscyamine-6β-hydroxylase (EC 1.14.11.11) was introduced into Hyoscyamus muticus L. strain Cairo (Egyptian henbane). This plant was chosen for its capability to produce very high amounts of tropane alkaloids (up to 6% of the dry weight in the leaves of mature plant). To our knowledge, this is the first time such a large population of transgenic cultures has been studied at the morphological, chemical and genetic levels. A great variation was observed in the tropane alkaloid production among the 43 positive transformants. The best clone (KB7) produced 17 mg/l scopolamine, which is over 100 times more than the control clones. However, conversion of hyoscyamine to scopolamine was still incomplete. The expression of h6h was found to be proportional to the scopolamine production, and was the main reason behind the variation in the scopolamine/hyoscyamine ratio in the hairy-root clones. These results indicate that H. muticus strain Cairo has a potential for even more enhanced scopolamine production with more efficient gene-expression systems. Received: 24 December 1998 / Accepted: 13 January 1999     

Effect of pmt gene overexpression on tropane alkaloid production in transformed root cultures of Datura metel and Hyoscyamus muticus

In order to increase the production of the pharmaceuticals hyoscyamine and scopolamine in hairy root cultures, a binary vector system was developed to introduce the T-DNA of the Ri plasmid together with the tobacco pmt gene under the control of CaMV 35S promoter, into the genome of Datura metel and Hyoscyamus muticus. This gene codes for putrescine:SAM N-methyltransferase (PMT; EC. 2.1.1.53), which catalyses the first committed step in the tropane alkaloid pathway. Hairy root cultures overexpressing the pmt gene aged faster and accumulated higher amounts of tropane alkaloids than control hairy roots. Both hyoscyamine and scopolamine production were improved in hairy root cultures of D. metel, whereas in H. muticus only hyoscyamine contents were increased by pmt gene overexpression. These roots have a high capacity to synthesize hyoscyamine, but their ability to convert it into scopolamine is very limited. The results indicate that the same biosynthetic pathway in two related plant species can be differently regulated, and overexpression of a given gene does not necessarily lead to a similar accumulation pattern of secondary metabolites.     

Enhanced production of hyoscyamine and scopolamine from genetically transformed root culture of <Emphasis Type="Italic">Hyoscyamus reticulatus</Emphasis> L. elicited by iron oxide nanoparticles

The medicinal plant Hyoscyamus reticulatus L. is a rich source of hyoscyamine and scopolamine, the tropane alkaloids. The use of hairy root cultures has focused significant attention on production of important metabolites such as stable tropane alkaloid production. Elicitation is an effective approach to induce secondary metabolite biosynthetic pathways. Hairy roots were derived from cotyledon explants inoculated with Agrobacterium rhizogenes and elicited by iron oxide nanoparticles (FeNPs) at different concentrations (0, 450, 900, 1800, and 3600 mg L^?1) for different exposure times (24, 48, and 72 h). The highest hairy root fresh and dry weights were found in the medium supplemented with 900 mg L^?1 FeNPs. Antioxidant enzyme activity was significantly increased in induced hairy roots compared to non-transgenic roots. The highest hyoscyamine and scopolamine production (about fivefold increase over the control) was achieved with 900 and 450 mg L^?1 FeNPs at 24 and 48 h of exposure time, respectively. This is the first report of the effect of FeNP elicitor on hairy root cultures of a medicinal plant. We suggest that FeNPs could be an effective elicitor in hairy root cultures in order to increase tropane alkaloid production.     

Alkaloids in plants and root cultures of Atropa belladonna overexpressing putrescine N-methyltransferase

Putrescine N-methyltransferase (PMT) is the first alkaloid-specific enzyme for nicotine and tropane alkaloid formation. The pmt gene from Nicotiana tabacum was fused to the CaMV 35S promoter and integrated into the Atropa belladonna genome. Transgenic plants and derived root cultures were analysed for gene expression and for levels of alkaloids and their precursors. Scopolamine, hyoscyamine, tropine, pseudotropine, tropinone, and calystegines were found unaltered or somewhat decreased in pmt-overexpressing lines compared to controls. When root cultures were treated with 5% sucrose, calystegine levels were elevated in control roots, but were not affected in pmt-overexpressing roots. 1 microM auxin reduced calystegine levels in control roots, while in pmt-overexpressing roots all alkaloids remained unaltered. Expression level of pmt alone is apparently not limiting for tropane alkaloid formation in A. belladonna.