Triterpenoids in Echinoderms: Fundamental Differences in Diversity and Biosynthetic Pathways

Abstract

Echinoderms form a remarkable phylum of marine invertebrates that present specific chemical signatures unique in the animal kingdom. It is particularly the case for essential triterpenoids that evolved separately in each of the five echinoderm classes. Indeed, while most animals have Δ5-sterols, sea cucumbers (Holothuroidea) and sea stars (Asteroidea) also possess Δ7 and Δ9(11)-sterols, a characteristic not shared with brittle stars (Ophiuroidea), sea urchins (Echinoidea), and crinoids (Crinoidea). These particular Δ7 and Δ9(11) sterols emerged as a self-protection against membranolytic saponins that only sea cucumbers and sea stars produce as a defense mechanism. The diversity of saponins is large; several hundred molecules have been described in the two classes of these saponins (i.e., triterpenoid or steroid saponins). This review aims to highlight the diversity of triterpenoids in echinoderms by focusing on sterols and triterpenoid glycosides, but more importantly to provide an updated view of the biosynthesis of these molecules in echinoderms.

Structures and Bioactivities of Quadrangularisosides A, A1, B, B1, B2, C, C1, D, D1–D4, and E from the Sea Cucumber Colochirus quadrangularis: The First Discovery of the Glycosides, Sulfated by C-4 of the Terminal 3-O-Methylglucose Residue. Synergetic Effect on Colony Formation of Tumor HT-29 Cells of these Glycosides with Radioactive Irradiation

“…In the 1 H and 13 C NMR spectra of the aglycone part of 4 , the signals of holostane-type aglycone having 7(8)- and 25(26)-double bonds as well as a 16 β -acetoxy-group were observed ( Table 6 ). This aglycone is identical to that of pentactasides C [ 14 ] and II [ 15 ] isolated earlier from the same species and is frequently occurred in the glycosides of the sea cucumbers of the order Dendrochirotida [ 2 , 4 , 5 , 21 , 30 ].…”

“…The signals in the 13 C NMR spectrum of the aglycone part of 3 were very close to those in the spectrum of cucumarioside A 1 from E. fraudatrix [ 28 ] and pentactaside B [ 14 ], indicating the identity of their aglycones ( Table 5 ). This holostane-type aglycone with 7(8)- and 24(25)-double bonds and a 16 β -acetoxy-group is quite common for the glycosides of sea cucumbers of the order Dendrochirotida [ 2 , 4 , 5 ].…”

“…The signals in the 1 H and 13 C NMR spectra of the aglycone part of 5 were characteristic of holostane-type aglycone having 9(11)- and 25(26)-double bonds and a 16-keto-group ( Table 7 ). This aglycone is known as holotoxinogenin, and it was found first in the glycosides named as holotoxins A 1 and B 1 , which were isolated from the sea cucumber Apostichopus japonicus (Stichopodidae, Synallactida) [ 31 ] and are rather common for the glycosides of the sea cucumbers of different orders [ 2 , 4 , 5 ].…”

“…Actually, the 13 C NMR spectrum of the aglycone part of 7 was coincident with that of lefevreoside A 2 , isolated first from A. lefevrei [ 21 ] and identified in the glycosidic sum of C. quadrangularis . The aglycone identical to that of quadrangularisoside C 1 ( 7 ) is frequently occurred in the glycosides of representatives of the order Dendrochirotida [ 2 , 4 , 5 ].…”

“…Triterpene glycosides, biosynthesized by the sea cucumbers, are well-known secondary metabolites that are characterized by tremendous structural diversity [ 1 , 2 , 3 , 4 ], different kinds of biological activities [ 5 , 6 ], and taxonomic specificity [ 7 , 8 , 9 , 10 ]. Hence, the studies on this class of metabolites are still relevant.…”