Holothuria triterpene glycosides: a comprehensive guide for their structure elucidation and critical appraisal of reported compounds

Sea cucumbers or holothurians are marine invertebrates, belonging to the phylum Echinodermata (kingdom Animalia). In Asia, they are commonly used as food, while they are applied in traditional medicine as well. A wide range of secondary metabolites from these animals has been reported, but triterpene glycosides are the most predominant and most studied and are considered as valuable chemotaxonomic markers. With respect to this, the genus Holothuria is an important genus of sea cucumbers, given the high number and variety of triterpene glycosides previously reported. Holothuria triterpene glycosides are typically composed of a 3β-hydroxyholost-9(11)-ene aglycon and a glycosidic moiety comprised of one up to six sugar units, connected to position 3 of the sapogenin. In the past decade (2010–2020) as much as 63 saponins were identified for the first time in the Holothuria genus, of which 36 had not been reported before. Identification was mainly carried out by LC–MS (liquid chromatography—mass spectrometry) analysis, while also NMR (nuclear magnetic resonance) spectroscopy is often applied for structure elucidation. However, while carrying out a literature search on these compounds, various inconsistencies were observed. Therefore, this review intends to provide an overview of typical signals that can occur in NMR and MS data, accompanied with an extensive explanation of their interpretation, in order to facilitate any future research on identification/structure elucidation of Holothuria saponins. Moreover, the data published in the past decade were critically reviewed and various questionable results are discussed.