Author: Yadollah Bahrami
Bahrami, Yadollah, 2015 Discovery of novel saponins as potential future drugs from sea cucumber viscera, Flinders University, School of Medicine
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Sea cucumbers are prolific producers of a wide range of bioactive compounds, which are potential sources of agrichemical, nutraceutical, pharmaceutical and cosmeceutical products. Sea cucumbers expel their internal organs as a defence mechanism called evisceration. We hypothesize that the reason for their ingenious form of defence is because their internal organs contain high levels of compounds that repel predators. To our knowledge, no study has investigated the contribution of saponins from the viscera of any sea cucumber species. Therefore, this project is aimed at the characterisation of the triterpene glycosides, saponins, from the viscera (and body wall) of selected Australian sea cucumber species using high-throughput technologies such as high performance centrifugal partition chromatography (HPCPC) and mass spectrometry. The longer term aim is to develop the novel compounds for pharmaceutical or nutraceutical or cosmeceutical application. We will describe the saponin distributions of Holothuria lessoni and Stichopus hermanni in detailed as representatives of two different families to reveal how their saponin profiles are different. The saponins were extracted from the viscera or body wall and enriched by a standard liquid-liquid partition process followed by adsorption column chromatography and partition of the eluate into isobutanol. The isobutanol saponin-enriched mixture was further purified by HPCPC to a high level of purity and recovery. The resultant purified polar samples were analysed using matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS)/MS and electrospray ionization mass spectrometry (ESI-MS)/MS to identify saponin congeners and characterise their molecular structures. Our results revealed over 100 saponin congeners in the viscera and body wall of H. lessoni with a high range of structural diversity, including 45 new sulphated, non-sulphated and acetylated triterpene glycosides. This study also identified the presence of more than 85 saponin congeners in the viscera of S. hermanni of which around half are new compounds. The majority of identified triterpene glycosides from the viscera of S. hermanni were acetylated, but non-sulphated compounds, contacting six monosaccharaide units, whereas the abundant saponin congeners from the viscera of H. lessoni were mainly sulphated compounds. All of these highlighted the chemical diversity of triterpene glycosides from sea cucumber species. Moreover, the identified saponin congeners have shown strong antifungal property in addition to antioxidant and antiviral activity. The conventional procedures to differentiate between isomeric saponins, including chemical derivatization and stereoscopic analysis, are tedious and time-consuming. Tandem mass spectrometry was conducted to obtain more structural information about the saccharide moiety and elucidate their structural features. Collision-Induced Dissociation (CID) preferentially cleaves glycosides at glycosidic linkages, which makes the assignment of the sugar residues and elucidation of the structure relatively straight forward. This study revealed the presence of the highest number of saponin congeners reported from any sea cucumber species in the viscera of examined species, H. lessoni and S. hermanni. These congeners contain a diverse range of molecular weights and structures. The mass of reported saponins for these species ranged from 759 Da to 1600 Da. So far we have identified more than 15 aglycone structures in these species. This research discovered over 100 new compounds from the viscera and body wall of different sea cucumber species with a high range of structural diversity, including sulphated, non-sulphated, and acetylated congeners. In conclusion, our findings showed that the viscera were found to be an excellent repository of numerous unique and novel saponins which have a broad range of potential applications in the health industry as nutraceutical, pharmaceutical, and cosmeceutical products.
Keywords: Saponins, sea cucumbers, triterpene glycosides, mass spectrometry, matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), electrospray ionization mass spectrometry (ESI-MS), high performance centrifugal partition chromatography (HPCPC), holothurian, bioactive compounds, marine natural products, marine invertebrate, structure elucidation, antifungal, anti cancer,
Subject: Medical Biotechnology thesis
Thesis type: Doctor of Philosophy
School: School of Medicine
Supervisor: Prof Chris Franco