Kinetic study of littorine rearrangement in Datura innoxia hairy roots by (13)C NMR spectroscopy

Tailoring enzyme strategies and functional groups in biosynthetic pathways

Covering: 2000 to 2022Secondary metabolites are assembled by drawing off and committing some of the flux of primary metabolic building blocks to sets of enzymes that tailor the maturing scaffold to increase architectural and framework complexity, often balancing hydrophilic and hydrophobic surfaces. In this review we examine the small number of chemical strategies that tailoring enzymes employ in maturation of scaffolds. These strategies depend both on the organic functional groups present at each metabolic stage and one of two tailoring enzyme strategies. Nonoxidative tailoring enzymes typically transfer electrophilic fragments, acyl, alkyl and glycosyl groups, from a small set of thermodynamically activated but kinetically stable core metabolites. Oxidative tailoring enzymes can be oxygen-independent or oxygen-dependent. The oxygen independent oxidoreductases are often reversible nicotinamide-utilizing redox catalysts, flipping the nucleophilicity and electrophilicity of functional groups in pathway intermediates. O₂-dependent oxygenases, both mono- and dioxygenases, act by orthogonal, one electron strategies, generating carbon radical species. At sp³ substrate carbons, product alcohols may then behave as nucleophiles for subsequent waves of enzymatic tailoring. At sp² carbons in olefins, electrophilic epoxides have opposite reactivity and often function as "disappearing groups", opened by intramolecular nucleophiles during metabolite maturation. "Thwarted" oxygenases generate radical intermediates that rearrange internally and are not captured oxygenatively.

Alkaloids of the Genus Datura: Review of a Rich Resource for Natural Product Discovery

The genus Datura (Solanaceae) contains nine species of medicinal plants that have held both curative utility and cultural significance throughout history. This genus’ particular bioactivity results from the enormous diversity of alkaloids it contains, making it a valuable study organism for many disciplines. Although Datura contains mostly tropane alkaloids (such as hyoscyamine and scopolamine), indole, beta-carboline, and pyrrolidine alkaloids have also been identified. The tools available to explore specialized metabolism in plants have undergone remarkable advances over the past couple of decades and provide renewed opportunities for discoveries of new compounds and the genetic basis for their biosynthesis. This review provides a comprehensive overview of studies on the alkaloids of Datura that focuses on three questions: How do we find and identify alkaloids? Where do alkaloids come from? What factors affect their presence and abundance? We also address pitfalls and relevant questions applicable to natural products and metabolomics researchers. With both careful perspectives and new advances in instrumentation, the pace of alkaloid discovery—from not just Datura—has the potential to accelerate dramatically in the near future.

Analysis of plant secondary metabolism using stable isotope-labelled precursors

INTRODUCTION

Analysis of biochemical pathways typically involves feeding a labelled precursor to an organism, and then monitoring the metabolic fate of the label. Initial studies used radioisotopes as a label and then monitored radioactivity in the metabolic products. As analytical equipment improved and became more widely available, preference shifted the use stable 'heavy' isotopes like deuterium (² H)-, carbon-13 (¹³ C)- and nitrogen-15 (¹⁵ N)-atoms as labels. Incorporation of the labels could be monitored by mass spectrometry (MS), as part of a hyphenated tool kits, e.g. Liquid chromatography (LC)-MS, gas chromatography (GC)-MS, LC-MS/MS. MS offers great sensitivity but the exact location of an isotope label in a given metabolite cannot always be unambiguously established. Nuclear magnetic resonance (NMR) can also be used to pick up signals of stable isotopes, and can give information on the precise location of incorporated label in the metabolites. However, the detection limit for NMR is quite a bit higher than that for MS.

OBJECTIVES

A number of experiments involving feeding stable isotope-labelled precursors followed by NMR analysis of the metabolites is presented. The aim is to highlight the use of NMR analysis in identifying the precise fate of isotope labels after precursor feeding experiments. As more powerful NMR equipment becomes available, applications as described in this review may become more commonplace in pathway analysis.

CONCLUSION AND PROSPECTS

NMR is a widely accepted tool for chemical structure elucidation and is now increasingly used in metabolomic studies. In addition, NMR, combined with stable isotope feeding, should be considered as a tool for metabolic flux analyses.

Discovery of new muscarinic acetylcholine receptor antagonists from Scopolia tangutica

Scopolia tangutica (S. tangutica) is a traditional Chinese medicinal plant used for antispasmodics, anesthesia, analgesia and sedation. Its pharmacological activities are mostly associated with the antagonistic activity at muscarinic acetylcholine receptors (mAchRs) of several known alkaloids such as atropine and scopolamine. With our recent identification of four hydroxycinnamic acid amides from S. tangutica, we hypothesized that this plant may contain previously unidentified alkaloids that may also contribute to its in vivo effect. Herein, we used a bioassay-guided multi-dimension separation strategy to discover novel mAchR antagonists from S. tangutica. The core of this approach is to use label-free cell phenotypic assay to first identify active fractions, and then to guide purification of active ligands. Besides four tropanes and six cinnamic acid amides that have been previously isolated from S. tangutica, we recently identified two new tropanes, one new cinnamic acid amide, and nine other compounds. Six tropane compounds purified from S. tangutica for the first time were confirmed to be competitive antagonists of muscarinic receptor 3 (M3), including the two new ones 8 and 12 with IC₅₀ values of 1.97 μM and 4.47 μM, respectively. Furthermore, the cinnamic acid amide 17 displayed 15-fold selectivity for M1 over M3 receptors. These findings will be useful in designing lead compounds for mAchRs and elucidating mechanisms of action of S. tangutica.

Kinetics of glucosylated and non-glucosylated aryltetralin lignans in Linum hairy root cultures

Due to their pronounced cytotoxic activity, a number of aryltetralin lignans (ATLs), such as podophyllotoxin (PTOX), are used as antitumor compounds. The production of such molecules from entire plants or plant cell-tissue-organ cultures is thus of interest to the pharmaceutical industry. Hairy root cultures constitute a good tool not only for phytochemical production but also for investigating plant secondary metabolism. This work reports on the growth and ATL biosynthesis in two hairy root cultures of Linum album Kotschy ex Boiss. and Linum flavum. The kinetics of accumulation of the intermediates of MPTOX biosynthesis and of their glucosylated forms are described over a 21-day period of growth. An accumulation of non-glucosylated forms of the ATLs during the exponential phase of the cultures is followed by an accumulation of the glucosylated forms during the stationary phase. Our results show a strong coordination of the biosynthetic paths derived from deoxypodophyllotoxin via deoxypodophyllotoxin 6-hydroxylase and deoxypodophyllotoxin 7-hydroxylase, and a coordinated glucosylation of podophyllotoxin, methoxypodophyllotoxin, and 5'-demethoxymethoxypodophyllotoxin. Furthermore, our results suggest an important role of β-peltatin-6-glucoside formation in the control of ATL accumulation in Linum hairy root cultures.

Norlittorine and norhyoscyamine identified as products of littorine and hyoscyamine metabolism by (13)C-labeling in Datura innoxia hairy roots

The presence of two compounds, norlittorine and norhyoscyamine, has been reported in leaves and roots of Datura innoxia; however their metabolic origin in the tropane alkaloid pathway has remained unknown. Precise knowledge of this pathway is a necessary pre-requisite to optimize the production of hyoscyamine and scopolamine in D. innoxia hairy root cultures. The exact structure of norlittorine and norhyoscyamine was confirmed by LC-MS/MS and NMR analyses. Isotopic labeling experiments, using [1-(13)C]-phenylalanine, [1'-(13)C]-littorine and [1'-(13)C]-hyoscyamine, combined with elicitor treatments, using methyl jasmonate, coronalon and 1-aminocyclopropane-1-carboxylic acid, were used to investigate the metabolic origin of the N-demethylated tropane alkaloids. The results suggest that norlittorine and norhyoscyamine are induced under stress conditions by conversion of littorine and hyoscyamine. We propose the N-demethylation of tropane alkaloids as a mechanism to detoxify cells in overproducing conditions.

Kinetic study of the rearrangement of deuterium-labeled 4'-O-methylnorbelladine in Leucojum aestivum shoot cultures by mass spectrometry. Influence of precursor feeding on amaryllidaceae alkaloid accumulation

Alkaloids from plants of the family Amaryllidaceae have important pharmacological properties and can be regarded as derivatives of the common precursor 4'-O-methylnorbelladine (6) via intramolecular oxidative phenol coupling. Their biosynthetic pathway, particularly in Leucojum aestivum, has not yet been totally elucidated. Therefore, shoot cultures of this plant were subcultured in medium containing the labeled precursor 4'-O-methyl-d(3)-norbelladine (3) at various concentrations (0.05, 0.10, and 0.20 g/L) and were incubated for various periods of time (15, 30, and 40 days). The aim of this work was to study the influence of this precursor on both labeled and native alkaloid accumulation. Biotransformation into galanthamine (1) and lycorine (2) in shoot cultures was demonstrated using HPLC coupled to mass spectrometry. A maximal amount of 0.16% of 1 referred to the dry weight was obtained at day 15 in shoots fed with 0.10 g/L of precursor. In addition, a 20.5% dry weight of 2 was reached after 40 days of feeding with 0.20 g/L of precursor.

Simultaneous analysis of hyoscyamine, scopolamine, 6β-hydroxyhyoscyamine and apoatropine in Solanaceous hairy roots by reversed-phase high-performance liquid chromatography

A new high-performance liquid chromatographic method is described for tropane alkaloid analysis in genetically transformed root cultures of Datura innoxia Mill. and Atropa belladonna L. Sample preparation, tropane alkaloid extraction with chloroform-methanol-concentrated ammonia 15:5:1 (v/v/v), was followed by solid-phase extraction on Supelclean LC-18 cartridges. Optimized conditions and careful pH control resulted in high recovery and reproducibility. Simultaneous determination of apoatropine, 6beta-hydroxyhyoscyamine, hyoscyamine and scopolamine was performed by HPLC on C18 (2) reversed-phase column. The application of Luna new-generation silica-based stationary phase resulted in excellent peak shapes using an ion-pair reagent and triethanolamine free mobile phase and allowed to exploit the full power of pH-dependent selectivity. Simplicity and improved selectivity make this method a preferred alternative of published ion-pair chromatographic methods. Validation studies proved that the global method has good repeatability and satisfactory recovery. Absolute limits of detection were 0.6, 0.6, and 0.8 ng for hyoscyamine, 6beta-hydroxyhyoscyamine, and scopolamine respectively.

NMR analysis of plant nitrogen metabolism

The analysis of primary and secondary nitrogen metabolism in plants by nuclear magnetic resonance (NMR) spectroscopy is comprehensively reviewed. NMR is a versatile analytical tool, and the combined use of (1)H, (2)H, (13)C, (14)N and (15)N NMR allows detailed investigation of the acquisition, assimilation and metabolism of nitrogen. The analysis of tissue extracts can be complemented by the in vivo NMR analysis of functioning tissues and cell suspensions, and by the application of solid state NMR techniques. Moreover stable isotope labelling with (2)H-, (13)C- and (15)N-labelled precursors provides direct insight into specific pathways, with the option of both time-course and steady state analysis increasing the potential value of the approach. The scope of the NMR method, and its contribution to studies of plant nitrogen metabolism, are illustrated with a wide range of examples. These include studies of the GS/GOGAT pathway of ammonium assimilation, investigations of the metabolism of glutamate, glycine and other amino acids, and applications to tropane alkaloid metabolism. The continuing development of the NMR technique, together with potential applications in the emerging fields of metabolomics and metabolic flux analysis, leads to the conclusion that NMR will play an increasingly valuable role in the analysis of plant nitrogen metabolism.

Occurrence of circadian rhythms in hairy root cultures grown under controlled conditions

Hairy roots obtained by transformation via Agrobacterium rhizogenes provide an artificial plant material devoid of aerial parts with high growth on hormone-free media. Fundamental knowledge of hairy root physiology is essential to develop and control its culture. In contrast to shake-flask cultures, a bioreactor set-up combined with on-line data logging provides an efficient tool to study rapid physiological variations in hairy root cultures. Datura innoxia hairy roots were grown in a bioreactor equipped with on-line data analyses of pH, dissolved oxygen (pO2), conductivity, oxygen, and carbon dioxide. The experiments were done at a constant temperature and in the absence of light cues. The results obtained showed that the carbon dioxide evolution rate (CER) presented regular oscillations during the culture. Similar oscillations were also observed for the oxygen uptake rate (OUR). These signals were treated mathematically to look for the existence of a rhythm. An autocorrelation function was used to detect any periodic components. The results demonstrate that hairy root respiration exhibited peaks of 1 day. These oscillations, having a period of about 24 h, were also observed in pH and conductivity signals, although not for the pO2 signal. The data acquired in the absence of hairy roots showed that the observed periodic behavior was not an artifact. No effect on rhythms was observed by the imposition of an external "day/night" cycle. The fact that oscillations persisted in the absence of external stimuli, with a free-running period of 24 h, suggests that a circadian rhythm exists in hairy roots of D. innoxia.