Structural studies on chemical constituents of echinoderms

Abstract

This chapter focuses on the more recent developments in the field of natural products from echinoderms. The phylum Echinodermata, which comprises about 6000 living species, is divided into five classes: Crinoidea (sea lilies and feather stars), Holoturoidea (sea cucumbers or holothurians), Echinoidea (sea urchins), Asteroidea (sea stars or starfishes), and Ophiuroidea (brittle stars). Among the echinoderms, starfishes and sea cucumbers usually contain saponins, which are responsible of their general toxicity. Chemically, saponins derived from sea cucumbers are triterpenoid glycosides, whereas those from starfishes are steroidal glycosides. The presence of oligoglycosides in both Holothuroidea and Asteroidea classes gives support to the opinion that sea cucumbers and starfishes are phylogenetically related. In starfishes and sea cucumbers Δ 7 -sterols, which are probably a consequence of the presence of haemolytic saponins, are also predominant, whereas the other three classes contain the usual Δ 5 -sterols. Haemolysis is caused by the abstraction of membrane cholesterol by the saponins. Saponins show a much lower affinity for Δ 7 -sterols and this helps to explain the apparent immunity of starfishes and sea cucumbers to their own saponins.