Studies on Steroid Metabolism in the Echinoderm Asterias Rubens

Abstract

Since the pioneering work of Bergmann1 on the sterol composition of marine invertebrates, it has been apparent that the echinoderm classes Asteroidea (starfish) and Holothuroidea (sea cucumber) present unique problems in relation to steroid metabolism. It is now well established2 that the echinoderm classes Crinoidea (feather stars), Ophiuroidea (brittle stars) and Echinodea (sea urchins) contain sterol mixtures which, in common with the majority of animals, are predominently Δ5-compounds, and these animals have the capacity to synthesise cholesterol de novo from mevalonic acid via the intermediates squalene and lanosterol. The large quantities of C28 and C29 Δ5 sterols found in these animals are presumed to be derived from the diet, since a C-24 methylation reaction has not been demonstrated. By contrast, the free sterols found in starfish and sea cucumbers are complex mixtures of Δ7-sterols, with usually less than 5% of the mixture being comprised of stanols and Δ5-sterols2. Both classes of animals have been shown to have a limited ability for de novo sterol biosynthesis, but the sequence apparently stops at the Δ7-sterol stage2 3, thus leading to the accumulation of 5α-cholest-7-en-3β-ol as the principal free sterol component found in many species. In starfish, the fate of dietary sterols has been examined, and it has been established that the usual dietary Δ5-sterols can be metabolised to give Δ7-sterols. Thus, dietary cholesterol is converted into 5α-cholest-7-en-3s-ol, while the various C26 C28 and C29 Δ7-sterols found in starfish are derived by metabolism of the corresponding Δ5sterol taken in the diet2,4,5. The principal metabolic route from cholesterol proceeds with an oxidation catalysed by a 3β-hydroxysteroid dehydrogenase enzyme system to produce cholest-4-en-3-one, which is converted by two NADPH-requiring reductions into 5α-cholestan3β-ol. Introduction of a Δ7-bond into the latter compound then produces cholest-7-en-3α-ol4.