Crustaceans sterols

In crustaceans cholesterol is a predominant sterol, but considerable amounts of desmosterol and small amounts of C28- and C29-sterols are also present. It is now established that [ C]acetate and [ C]mevalonate are not incorporated into sterols, indicating incapability for the de novo synthesis of sterols. A nutritional requirement for sterol has been demonstrated for the prawn Penaeus Japonicus [109]. After feeding prawns phytosterol, the sterol isolated was predominantly cholesterol [110]. Dealkylation of 24-methyl and 24-ethyl sterols to yield cholesterol has been demon-... 

Dealkylation of phytosterols to cholesterol is now well known in insects and crustaceans. The transformation of [ H]fucosterol into cholesterol and desmosterol was also observed in two species of molluscs [134]. Desmosterol is a widely distributed sterol in molluscs, and in some species the content is very high (30-35%) [127,135]. This sterol can be considered as the dealkylation product of 24-al-kylsterols, and it is likely that desmosterol accumulates because of low activity of the A -sterol reductase. The capacity for both alkylation and dealkylation at the C-24 position in molluscs remains to be clarified. 

The ability of crustaceans to biosynthesize sterols has been investigated by several groups and it is now established that they are incapable of de novo sterol synthesis. Dealkylation of 24-methyl and 24-ethyl sterols, giving cholesterol, was demonstrated in. several crustaceans by Teshima and Kanazawa [HI]. 

The sterol requirements of invertebrates are frequently satisfied by modification of dietary steroids. Thus, cholesterol is formed from 24-alkylated steroids, such as ergosterol and /5-sitosterol, by Crustaceans and insects. The mechanism of this process seems to be the reverse of their mode of formation. The 24-ethyl group of -sitosterol is converted into a 24-ethylidene group with fucosterol, and cholesta-5,24-dienol is formed on loss of the alkyl group. Cholesterol is required in insects for metabolism to the hormone ecdysone (84). However, plants also produce ecdysone and both organisms metabolize cholesterol to ecdysone. which is then further metabolized to ecdysterone (85)." ... 

Phytosterols are of particular importance to insects, nematodes, and certain crustaceans, because they cannot synthesize cholesterol de novo. These organisms degrade dietary C28 and C29-phytosterols to C2 -sterols (usually cholesterol) or obtain C27-sterols directly from other organisms (Ikek-awa, 1983). These steroidal products are then used in the synthesis of biologically active sterols that the organisms require. There is evidence that dealkylation of phytosterols proceeds as indicated in (Fig. 23.14) (Harrison, 1985 Ikek-awa, 1983). 

Ambia KBM, Goad LJ, Hkycko S, Garneau F-X, Belanger J, ApSimon JW (1987) The sterols of the sea cucumber Psolus phantapus. Comp Biochem Physiol 86B 191-192 Anderson JW, Neff JM, Cox BA, Tatem HE, Hightower GM (1974) Characteristics of dispersions and water-soluble extracts of crude and refined oils and their toxicity to estuarine crustaceans and fish. Mar Biol 27 75-88... 

Sterols are involved in the stabilization of cell membranes (E 2.2). Ergosterol is used in the technical preparation of vitamin Dg (D 6.4.9). Ecdysone and ecdy-sterone are molting hormones in insects and crustaceans (E 3.1). Similar compounds (ecdysteroids), e.g., ponasterone and cyasterone, which are widespread in higher plants, deter feeding insects (E 5.5.3). The hormone activity of certain ecdyste-roids in insects is up to 20 times that of ecdysone. Certain Drosophila species are... 

Teshima, S.-I. (1991) Sterols of Crustaceans, molluscs and fish, in Physiok and Biochemistry of Sterols (eds G.N. Patterson and W.D. Nees), American Oil Chemists Society, Boulder, CO., Urbana, IL, pp. 229-256. The part devoted to molluscs can be found on pp. 230-236. 

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