Echinoderms metabolites

The phylum Echinodermata comprises about 7000 living species [177]. Echinoderm means spiny-skinned and these organisms are characterised by the tube feet, which they use to move about. These have suction discs on the ends, which operate by an internal bulb pumping water in and out of the foot, causing expansion and contraction. The phylum is sub-divided into five classes the asteroids (sea stars), the holothurians (sea cucumbers), the crinoids (sea lilies), the echinoids (sea urchins) and the ophiuroids (brittle stars) [178]. As stated in the introduction to this review, sulfated sterols and saponins, which comprise the majority of echinoderm metabolites containing sulfur, are not included here. 

Steroids with 0-heterocyclic fragments (pyrans and 0-macroheterocycles) as bioactive metabolites from echinoderms andporifera 97G771. 

It is apparent that the aquatic echinoderm and terrestrial mammal deal with a chemical probe by very different metabolic pathways. The exclusive formation of oxidized ((T-demethylated) product in the mouse may partly reflect the animal s highly oxidizing environment, while the relatively anoxic marine environment is represented in the observed reduced metabolites of the sea urchin. 

Echinoderms have yielded a smaller range of secondary metabolites than other marine invertebrates such as sponges or ascidians, perhaps due to their ability to deter predators by other means. The ophiuroids are able to burrow into the substratum and reduce exposure to predators. Many echinoderms are nocturnal, moving and feeding when predator activity is at a minimum. The outer layer of most echinoderms is either calcified or composed of insoluble proteins, making the organism unfavorable as prey.120... 

The compounds most characteristic of the phylum are the saponins, glycosolated sterols, most of which are sulfated. The suite of secondary metabolites produced by the classes Asteroidea, Ophiuroidea, and Holothuroidea are dominated by these sterols. It is believed that echinoderms do not generally undertake de novo synthesis of these sterols, but create them by modifying precursors obtained through feeding.90... 

The Hemichordata are a phylum of lower deuterostomes and, therefore, are more closely related to echinoderms and chordates than to annelids. They are discussed here because they are separate lineages that have evolved distinctive metabolites and also because of their feeding mechanisms. Very little work has been done on their natural products, however. 

FIGURE 3.12 Selected metabolites from echinoderms. 3.12.1 Comasteride A from Comasterias lurida.224 3.12.2 A polyhydroxylated sterol from Stylaster caroli 225 3.12.3 Fuscusine from Perknaster fuscus.226... 

Stonik, V. A. and Elyakov, G. B., Secondary metabolites from echinoderms as chemotaxonomic markers, in Bioorganic Marine Chemistry, Scheuer, P. J., Ed., Springer-Verlag, Berlin, 2, 43, 1988. 

Another class of compound that can be difficult to purify is the saponins found in Echinoderms. Almost all echinoderms examined to date contain either polyhydroxylated sterols or terpene glycosides, many of which contain sulfate ester functionality. The purification of compounds (Schemes 6-9) (40) from the starfish Nardoa tuberculata exemplifies the procedure most often used to purify these metabolites (Fig. 2). 

In the past fewyears a large number of metabolites have been isolated from echinoderms, mainly... 

The present paper is devoted to the more recent developments in the field of natural products from echinoderms, with emphasis given to those metabolites recently (1989-93) isolated in the laboratory of the Authors. 

Two more factors contributing to the persistence of xenobiotics in polychaetes, molluscs, crustaceans and echinoderms, and presumably other marine invertebrates, are the formation of macromolecular adducts and the slow release of free metabolites. The incorporation of PAH into more stable compartments , possibly as a result of cytochrome P-450-mediated adduct formation, is particularly evident in molluscs (see Fig. 3). The metabolites of many xenobiotics are lost more slowly than the parent compound, resulting in a build-up in the tissues during both exposure and subsequent depuration periods. The primary function of metabolism, therefore, appears to be detoxication, by preventing the formation of specific reactive metabolites and, probably more importantly, the non-specific toxic action of lipophilic xenobiotics caused by their penetration of membrane systems. Loss of metabolites down a concentration gradient occurs, reducing body burden of the xenobiotic, but is obviously limited by the absence or ineffectiveness of specific transport and excretory systems for the polar metabolites. 

The marine flora and animal world are a rich source of biologically active compounds. A large number of sometimes highly toxic metabolites has been isolated inter alia from protozoans, sponges, coelenter-ates, echinoderms, molluscs, nemertines, sea snakes, and fishes (1-4) and the great interest in these chemically varied compounds has resulted in extensive publications (5-13), Among the best known of the toxic substances are tetrodotoxin, saxitoxin, and the polypeptides from sea anemones, but they will not be the main subject of this report. 

Figure 3.2 illustrates the same distribution for the invertebrates, where it can be seen that there is no correlation between the percentages of nitrogenous secondary metabolites and the position of each phylum on Luden Cuenot s Tree of Life. It is therefore dearly apparent that to find new nitrogenous metabolites, it is better to seek them among tunicates, bryozoans or sponges than among cni-darians, echinoderms, or mollusks. 

Among the characteristics of the baderial metabolites, it is notable that there are significant proportions of aromatic derivatives, quinones and pyrones, and espedaUy the presence of macrocycles (macrolactones, macrodepsipep-tides), which probably have chelating properties for the metal cations, and in particular for iron (siderophores). Compounds presented previously - for instance, the esters assodating a phenazine to L-quinovose - also deserve to be mentioned because this sugar is rare in marine environments, except in echinoderms. 

The chemotaxonomic relations observed in the phylum Echinodermata are particularly clear each class is characterized by a particular set of secondary metabolites that is probably specific to the class. For the class Crinoidea, these are anthraquinonic pigments, often sulfated, which have been found in all species studied. The other four classes of echinoderms also contain quinonic pigments but these are naphthoquinones, which are widespread among Echinoidea (spinochromes), rare in Asteroidea and Ophiuroidea, and exceptional among Holothuroidea (Scheuer, 1973). 

Fusetani, N. (1987) Marine metabolites which inhibit development of echinoderm embryos, in Biootganic Marine Chemistry, vol. 1 (ed. P.J. Scheuer), Springer-Verlag, Berlin, Heidelberg, pp. 61-92. 

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