Isolation of 5 alpha- and 5 beta-dihydrorubrosterone from Silene otites L. (Wib)

Small-scale analysis of phytoecdysteroids in seeds by HPLC-DAD-MS for the identification and quantification of specific analogues, dereplication and chemotaxonomy

INTRODUCTION

Phytoecdysteroids are analogues of arthropod steroids occurring in plants. They contribute to invertebrate deterrence. A wide diversity of ecdysteroids occurs in phytoecdysteroid-containing plant species, sometimes in high amounts. Ecdysteroids demonstrate potentially useful pharmaceutical actions in mammals.

OBJECTIVES

Establish reversed-phase high-performance liquid chromatography with tandem mass spectrometry (RP-HPLC-MS/MS) and RP-HPLC-DAD-MS (diode array detector mass spectrometry) methods for the separation, identification and quantification of ecdysteroids to screen for species containing significant amounts of 20-hydroxyecdysone (20E) and other useful ecdysteroids.

MATERIALS AND METHODS

Micro-extracts of seed samples (ca. 30 mg) in 50% ethanol were subjected to RP-SPE (solid-phase extraction) purification prior to analysis by RP-HPLC-MS/MS and RP-HPLC-DAD-MS. The method was initially applied to genera (Amaranthus, Centaurea, Lychnis, Ourisia, Serratula, Silene and Trollius) where high-accumulating species had been previously encountered. Seeds of 160 randomly selected species, many of which have not previously been assessed, were then analysed. HPLC-MS/MS with a short analysis time initially identifies ecdysteroid-positive extracts and quantifies 20E. The positive extracts (20 ng 20E) are then analysed by HPLC-MS/MS with a longer analysis time to identify and quantify 17 common phytoecdysteroids and, finally, HPLC-DAD-MS (0.1-0.25 μg 20E) is used to obtain UV- and MS-spectra to confirm identifications or as a basis for characterisation of partially identified or novel analogues.

RESULTS

Lychnis coronaria, Silene fimbriata and Silene hookeri ecdysteroids are characterised for the first time and those of Cucubalus baccifer and Ipheion uniflorum are more extensively characterised.

CONCLUSIONS

The procedure provides a rapid/sensitive method for screening small plant samples for the presence, quantification and identification of ecdysteroids. It permits ready dereplication of samples, identifying extracts containing large amounts or novel analogues.

Diversity of Secondary Metabolites in the Genus Silene L. (Caryophyllaceae)—Structures, Distribution, and Biological Properties

The genus Silene (family Caryophyllaceae) comprises more than 700 species, which are widely distributed in temperate zones of the Northern Hemisphere, but are also present in Africa and have been introduced in other continents. Silene produces a high diversity of secondary metabolites and many of them show interesting biological and pharmacological activities. More than 450 compounds have been isolated; important classes include phytoecdysteroids (which mimic insect molting hormones), triterpene saponins (with detergent properties), volatiles, other terpenoids and phenolics. This review focusses on the phytochemical diversity, distribution of Silene secondary metabolites and their biological activities.

Distribution of phytoecdysteroids in the Caryophyllaceae

Certain genera within the Caryophyllaceae (especially Silene and Lychnis) have received a significant amount of attention with regard to the isolation and identification of ecdysteroids. However, the taxonomy of this family is difficult. Hence, the occurrence of phytoecdysteroids in members of the Caryophyllaceae is presented, and combined with new data on ecdysteroid agonist (phytoecdysteroid) and antagonist activities, in order to survey the distribution of phytoecdysteroid-containing species within this large family, and to assess the utility of phytoecdysteroids as chemotaxonomic markers. The new data presented (representing ca. 110 species) have been obtained by the application of sensitive biological/biochemical methods for the detection of ecdysteroid agonists and antagonists, using Drosophila melanogaster B(II) bioassay and ecdysteroid-specific immunoassays. In the antagonist version of the B(II) bioassay, only weak ecdysteroid antagonist activities were detected in a few of the extracts. From both new and previously available data, it was found that phytoecdysteroids were present predominantly in the Genera Lychnis, Petrocoptis, Sagina and Silene. Comparison of ecdysteroid occurrence with a molecular phylogeny for the tribe Sileneae [Taxon 44 (1995) 525] revealed close association of ecdysteroid occurrence with certain groups of this tribe. In 14 species of Silene examined, there is a reasonable, but not absolute, relationship between the presence of ecdysteroids in the seeds and in other plant parts. Where ecdysteroids are present in the plant, highest concentrations are generally present in the roots.

5-Alpha- and 5-beta-2-deoxyintegristerone A, a 5-alpha and 5-beta isomer pair of ecdysteroids isolated from the Silene genus

5-Alpha-2-deoxyintegristerone A and 5-beta-2-deoxyintegristerone A were isolated from the aerial parts of Silene italica ssp. nemoralis (Waldst. and Kit.) Nyman using a specific combination of absorption column chromatography, preparative thin-layer chromatography and preparative HPLC. Both normal-phase and reversed-phase modes of HPLC were employed for isolation. Structural elucidation of 5-alpha-2-deoxyintegristerone A was completed by X-ray diffraction. Both 5-alpha-2-deoxyintegristerone A and 5-beta-2-deoxyintegristerone A were firstly isolated from this plant. We propose that 5-alpha-2-deoxyintegristerone A is not an artifact but an integral part of the ecdysteroid spectrum of Silene italica ssp. nemoralis (Waldst. and Kit.) Nyman.

Minor Ecdysteroid Components of Leuzea carthamoides

Fourteen minor ecdysteroid components were isolated and identified from the roots of Leuzea carthamoides (Willd.) DC. Two of them are new phytoecdysteroids: leuzeasterone (1) a six-member side-chain lactone, and (24Z)-29-hydroxy-24(28)-dehydromakisterone C (4) a structurally related sitostane type analogue and assumed biogenetic precursor of 1. The next one, 5α-20-hydroxyecdysone (6), is a rare A/B-ring trans-annelated epimer of the most common phytoecdysteroid 20-hydroxyecdysone. Further compounds: makisterone C (3), 3-epi-20-hydroxyecdysone (5), integristerone A (7), integristerone B (8), 22-oxo-20-hydroxyecdysone (10), taxisterone (11), rubrosterone (12), dihydrorubrosterone (13) and poststerone (14), are new constituents of L. carthamoides, though already reported as compounds isolated from other natural sources. Two earlier reported minor Leuzea ecdysteroids: the five-membered side-chain lactone carthamosterone (2) and the 11-hydroxy-substituted analogue isovitexirone (9), are also included because they are now better characterised. Certain previously described Leuzea ecdysteroids were not found in our material, which may indicate geographic, seasonal or cultivar variations.

Identification and quantitative analysis of the phytoecdysteroids in Silene species (Caryophyllaceae) by high-performance liquid chromatography. Novel ecdysteroids from S. pseudotites

Many species in the genus Silene (Caryophyllaceae) have previously been shown to contain ecdysteroids and this genus is recognised as a good source of novel ecdysteroid analogues. We have used ecdysteroid-specific radioimmunoassays and the microplate-based Drosophila melanogaster B(II) cell bioassay for ecdysteroid agonist and antagonist activities to identify further phytoecdysteroid-containing species in this genus. The main ecdysteroid components from 10 Silene species (S. antirrhina, S. chlorifolia, S. cretica, S. disticha, S. echinata, S. italica, S. portensis, S. pseudotites, S. radicosa, S. regia) were isolated and identified, mainly by normal-phase and reversed-phase high-performance liquid chromatography. The amount of each ecdysteroid was determined by comparing chromatogram peak areas with those for reference 20-hydroxyecdysone (20E) on reversed-phase HPLC. 20E is the most abundant ecdysteroid in each of the Silene extracts. Polypodine B, 2-deoxy-20-hydroxyecdysone and ecdysone are also common ecdysteroids in these Silene species, but the proportions of these ecdysteroids vary between the Silene species. HPLC proved to be a quick and effective way to screen Silene species, determine ecdysteroid profiles and, hence, identify extracts containing novel analogues. An extract of the aerial parts of S. pseudotites was found to contain several new ecdysteroids. These have been isolated and identified spectroscopically (by NMR and mass spectrometry) as 2-deoxyecdysone 22beta-D-glucoside, 2-deoxy-20,26-dihydroxyecdysone and 2-deoxypolypodine B 3beta-D-glucoside. Additionally, (5alpha-H)-2-deoxyintegristerone A (5alpha-2H 91%, 5alpha-1H 9%) was isolated as an artefact. This study contributes to the understanding of ecdysteroid distribution in Silene species and provides further information on the chemotaxonomic significance of ecdysteroids in Silene species.