Picrotoxane sesquiterpenoids: chemistry, chemo- and bio-syntheses and biological activities

Covering: up to December 2021Picrotoxane sesquiterpenoids are a special category of natural products known to have a picrotoxane skeleton and are characterised by a highly oxidised cis-hydrindene core, lactone rings, and epoxide functionalities. Ever since the first picrotoxane was isolated from Menispermum cocculus in the early 19^th century, these compounds have long attracted the attention of natural product chemists, synthetic chemists, and pharmacologists for their particular structures and powerful biological activities. This review extensively summarizes a total of 132 naturally occurring picrotoxane sesquiterpenoids, taking into account their distributions, structural classifications, chemical and bio-synthetic researches, and bioactivities. It provides a comprehensive and in-depth perspective for further investigation on picrotoxane sesquiterpenoids.

Natural plant toxins in honey: An ignored threat to human health

Consumers often believe that "natural food" is harmless, however naturally occurring toxins in food represent a health risk to humans. Honey as a natural, nutritious sweetener, is one of the most commonly consumed foods throughout the world. However, food safety concerns for honey arise when honeybees collect nectar from poisonous plants such as Rhododendron sp., Coriaria arborea, and Tripterygium wilfordii Hook F. Such honey contains natural plant toxins. Humans may develop intoxication symptoms after consuming toxic honey; in some cases, it can be fatal. As a result, toxic honey poses an often-ignored threat to public health. Typical plant toxins such as grayanotoxins, triptolides, tutin and pyrrolizidine alkaloids, have been identified in toxic honey. Although different toxic honeys elicit similar symptoms, such as vomiting, nausea, and dizziness, the mechanism of toxicity may be different. Thus, it is necessary to determine the exact toxicity mechanism of different toxins to further develop effective antidotes and cures. Another important challenge is preventing toxic honey from entering the food chain. Liquid chromatography-mass spectrometry has a wide range of applications in the detection of different toxins due to its accuracy and simplicity. More methods, however, are urgently needed to detect multiple plant-derived toxins in honey and its derivatives. Developing uniform international standards for toxin detection during quarantine using advanced techniques is critical for preventing human consumption of toxic honey.

LC-ESI/LTQ-Orbitrap-MS Based Metabolomics in Evaluation of Bitter Taste of Arbutus unedo Honey

Strawberry tree honey is a high-value honey from the Mediterranean area and it is characterised by a typical bitter taste. To possibly identify the secondary metabolites responsible for the bitter taste, the honey was fractionated on a C18 column and the individual fractions were subjected to sensory analysis and then analysed by liquid chromatography coupled with high-resolution tandem mass spectrometry in negative ion mode, using a mass spectrometer with an electrospray source coupled to a hybrid high resolution mass analyser (LC-ESI/LTQ-Orbitrap-MS). A chemometric model obtained by preliminary principal component analysis (PCA) of LC-ESI/LTQ-Orbitrap-MS data allowed the identification of the fractions that caused the perception of bitterness. Subsequently, a partial least squares (PLS) regression model was built. The studies carried out with multivariate analysis showed that unedone (2-(1,2-dihydroxypropyl)-4,4,8-trimethyl-1-oxaspiro [2.5] oct-7-en-6-one) can be considered responsible for the bitter taste of strawberry tree honey. Confirmation of the bitter taste of unedone was obtained by sensory evaluation of a pure standard, allowing it to be added to the list of natural compounds responsible for giving the sensation of bitterness to humans.

Two new picrotoxane-type sesquiterpenoid lactones from Dendrobium williamsonii

Two new picrotoxane-type sesquiterpenoid lactones, dendrowillins A (1) and B (2), together with five known sesquiterpenoids, (-)-picrotin (3), α-dihydropicrotoxinin (4), dendronobilin B (5), amoenin (6), and (-)-10β,13,14-trihydroxyalloaromadendrane (7), were isolated from the whole plants of Dendrobium williamsonii. Their structures were elucidated by means of extensive spectroscopic analysis.

Glycosides of the Neurotoxin Tutin in Toxic Honeys Are from Coriaria arborea Phloem Sap, Not Insect Metabolism

Some honeys contain the neurotoxin tutin (1) plus hyenanchin (2), 2-(β-d-glucopyranosyl)tutin (3), and 2-[6'-(α-d-glucopyranosyl)-β-d-glucopyranosyl]tutin (4). These honeys are made by bees collecting honeydew from passionvine hoppers feeding on the sap of tutu plants ( Coriaria spp.). We report a LC-MS study showing that all these picrotoxanes are of plant, not insect, origin. Hyenanchin was barely detectable and the diglucoside was not detectable in C. arborea leaves, but tutu phloem sap contained all four compounds at concentrations up to the highest found in honeydew. It is proposed that the diglucoside may function as a transport form of tutin, analogous to sucrose transport in phloem.

Sweet Poisons: Synthetic Strategies towards Tutin Glycosides

The polycyclic, polyoxygenated picrotoxane tutin was subjected to various glycosylation reaction conditions in an effort to synthesise β-linked tutin glycosides, recently found in toxic honeys. Cationic palladium-mediated glycosylation of tutin was successful; however, the α-linked tutin tetrabenzyl glucoside was obtained as the major product (5 : 1, α : β). Hydrogenolysis of the benzyl ether protecting groups resulted in concomitant tutin double-bond migration. Epoxide opening and rearrangement were observed upon acetylation of the tutin glucoside.

Sweet Poisons: Honeys Contaminated with Glycosides of the Neurotoxin Tutin

Poisonings due to consumption of honeys containing plant toxins have been reported widely. One cause is the neurotoxin tutin, an oxygenated sesquiterpene picrotoxane, traced back to honeybees (Apis mellifera) collecting honeydew produced by passionvine hoppers (Scolypopa australis) feeding on sap of the poisonous shrub tutu (Coriaria spp.). However, a pharmacokinetic study suggested that unidentified conjugates of tutin were also present in such honeys. We now report the discovery, using ion trap LC-MS, of two tutin glycosides and their purification and structure determination as 2-(β-d-glucopyranosyl)tutin (4) and 2-[6'-(α-d-glucopyranosyl)-β-d-glucopyranosyl]tutin (5). These compounds were used to develop a quantitative triple quadrupole LC-MS method for honey analysis, which showed the presence of tutin (3.6 ± 0.1 μg/g honey), hyenanchin (19.3 ± 0.5), tutin glycoside (4) (4.9 ± 0.4), and tutin diglycoside (5) (4.9 ± 0.1) in one toxic honey. The ratios of 4 and 5 to tutin varied widely in other tutin-containing honeys. The glycosidation of tutin may represent detoxification by one or both of the insects involved in the food chain from plant to honey.

Human pharmacokinetic study of tutin in honey; a plant-derived neurotoxin

Over the last 150 years a number of people in New Zealand have been incapacitated, hospitalised, or died from eating honey contaminated with tutin, a plant-derived neurotoxin. A feature of the most recent poisoning incident in 2008 was the large variability in the onset time of clinical signs and symptoms of toxicity (0.5-17 h). To investigate the basis of this variability a pharmacokinetic study was undertaken in which 6 healthy males received a single oral dose of tutin-containing honey giving a tutin dose of 1.8 μg/kg body weight. The serum concentration-time curve for all volunteers exhibited two discrete peaks with the second and higher level occurring at approximately 15 h post-dose. Two subjects reported mild, transient headache at a time post-dose corresponding to maximum tutin concentrations. There were no other signs or symptoms typical of tutin intoxication such as nausea, vomiting, dizziness or seizures. Pharmacokinetic analysis using a two-site absorption model resulted in a good fit to the observed concentration data. A novel analytical method subsequently revealed the presence of glycoside conjugates of tutin in addition to unconjugated tutin in honey. These pharmacokinetic data will be important to better define a safe maximum tutin concentration in honey.

Nuclear magnetic resonance and chemometrics to assess geographical origin and quality of traditional food products

In this globalization era, the opening of the markets has put at almost everybody's disposal a wide variety of foods, allowing everybody to taste food flavors and aromas from different nations. Notwithstanding this opportunity, countries try to preserve their markets by developing protection policies. A few countries have adopted different denominations to label their "typical food" products in order to give them additional value. Besides, the term "typical food" is widely thought of as something anchored to the local traditions, with geographical meaning and made with typical raw materials. Then a "typical food" starts to be considered "traditional" when it is made following specific and old recipes. As a matter of fact, these products acquire particular organoleptic characteristics that are not reproducible when produced in different places. In this review, NMR studies coupled to multivariate statistical analysis are presented with the aim of determining geographical origin and key quality characteristics.

Geographical characterization of polyfloral and acacia honeys by nuclear magnetic resonance and chemometrics

The importance of geographical origin determination is an increasing and pressing requirement for all foods. Honey is one of the largest studied foods due to its nutritional and medicinal properties in a correct diet. In this paper, a total of 41 honey samples (polyfloral and acacia) from different countries have been analyzed in terms of (1)H NMR spectroscopy coupled with multivariate statistical methods. Unsupervised principal component analysis resulted as an efficient tool in distinguishing (1)H NMR spectra of polyfloral and acacia honey samples and for geographical characterization of the latter ones. Hierarchical projection to latent structures discriminant analysis was successfully applied for the discrimination among polyfloral honey samples of different geographical origins. (13)C NMR spectroscopy was applied to honey samples with the aim to investigate possible sugar isoforms differentiation. Our preliminary data indicated a different isoforms ratio between betaFP and betaFF only for polyfloral Argentinean samples, while Hungarian samples showed resonance shifts for some carbons of alphaFF, betaFP, betaFF, and alphaGP isoforms for both varieties. These data confirmed the potentiality of (13)C spectroscopy in food characterization, especially in sugar-based foods.

NOESY on neurotoxins: NMR and conformational assignments of picrotoxins

Nuclear Overhauser effect spectroscopy (NOESY) gave full assignments of the 1H-NMR spectra of the picrotoxane neurotoxins tutin, hyenanchin, picrotoxinin and picrotin, as well as the solution conformations of these compounds, consistent with molecular modelling. Fully assigned 13C-NMR data are reported.