Echinodermata sterols

Sea cucumbers (Holothuroidea, Echinodermata) appear to be unique in their mode of squalene oxide (37) cyclization. Tritium-labeled lanosterol (33), cycloartenol (32) and parkeol (38) were individually administered to the sea cucumber Holothuria arenicola. While the former two triterpenes were not metabolized [22], parkeol was efficiently transformed into 14x-methyl-5a-cho-lest-9(l l)-en-3/ -ol (39) (Scheme 3). Other A1 sterols present in H. arenicola were not found to be radioactive and were thus assumed to be of dietary origin. The intermediacy of parkeol was confirmed by the feeding of labeled mevalonate (23) and squalene (26) to the sea cucumber Stichopus californicus [15]. Both precursors were transformed into parkeol, but not lanosterol nor cycloartenol, aqd to 4,14a-dimethyl-5a-cholest-9(ll)-en-3/J-ol (40) and 14a-methyl-5a-cholest-9(ll)-en-3/ -ol. Thus, while all other eukaryotes produce either cycloartenol or lanosterol, parkeol is the intermediate between triterpenes and the 14-methyl sterols in sea cucumbers. 

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. 

Further work in the phylum Echinodermata shows a variable ability to biosynthesize steroids. In the class Holothuroidea and Echinoidea, the representatives examined could synthesize squalene but not triterpenoids or sterols from acetate. However, several examples from the class Asteroidea were able to synthesize squalene, lanosterol, and other steroids. In the later stages of steroid metabolism it was shown that cholesterol was converted into cholest-7-enol via cholestanol. 

The major sterols of Echinodermata are C27-sterols. Cholesterol is the principal sterol in echinoids (70-80%), ophiuroids, and crinoids. 5a-Cholest-7-en-3i8-ol is the major sterol in asteroids and sea cucumbers. A C26-sterol, 24-norcholesta-5,22-dien-3 8-01 (83) is also a common sterol in Echinodermata the content is especially high (6-10%) in ophiuroids and crinoids [79]. Asterosterol (70), a A -C26-sterol, is common in asteroids and sea cucumbers [80], and amuresterol (71) was isolated from Asterias amurensis [81]. 

Acanthasterol [82] (acansterol) [83,84] (72), the isomer of gorgosterol, was isolated from Acanthaster planci by Scheuer. Gorgostanol [85] and demethyl-acanthasterol (73) [86] were also found in A. planci [86] (Fig. 8). Transformation from A - to A -sterol may be a common bioconversion in Echinodermata, because gorgonians are components of the diet of A. planci in which gorgosterol cannot be found. 

In addition to cholesterol and 5a-cholest-7-en-3y8-ol, many C2g and C29 conventional sterols are present in Echinodermata. These sterols are probably derived from the diet. However, echinoderms are able to synthesize sterols. Thus, [ C]mevalonic acid was incorporated into squalene, lanosterol and desmosterol by the sea urchin. Echinus esculentus [87]. The ability of ophiurpids to synthesize sterol from [ C]acetate has also been demonstrated [88]. Sterol biosynthesis by a holothuroid was first investigated by Numura [89], and sterol biosynthesis from [ C]acetate in sea cucumbers has also been reported [90]. 

Goad, LJ. (1978) The sterols of marine invertebrates Composition, biosynthesis, and metabolites, in Marine Natural Products Chemical and Biolcgical Perspectives, vol. II (ed. P.J. Scheuer), Academic Press, New York, London, pp. 75-172. The parts devoted to Echinodermata can he found on pp. 129-135 and 139-156. 

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