Sterols in crustaceans

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- 

Generally the sterols in Mollusca are a complex mixture of Cje-, C27-, C28- and C29-sterols [124]. In the classes of Gastropoda, Bivalvia and Cephalopoda, A -sterols including cholesterol are the most abundant sterols. In the class Amphineura, A7-sterols are predominant in all species, and 5 -cholest-7-en-3)8-ol is the major sterol. Cephalopoda contain cholesterol as about 90% of the total sterols. Gastropoda and Bivalvia also contain cholesterol as the major sterol (50-60%). In some species of Bivalvia, the sterol fraction is a mixture of C27-, C28- and Cjg-sterols, and in some species dihydrobrassicasterol is the major sterol (34-65%). The most 

In 1970, a new C26-sterol was first isolated from Placopecten magellanicus by Idler [126], and its structure was determined as (22 )-24-norcholesta-5,22-dien- -ol (83). Subsequently Cj -sterols were found in many marine sources. These sterols may be of dietary origin from algae, but the mode of biosynthesis is still unknown. (22Z)-Cholesta-5,22-dien-3j8-ol was reported from several molluscs [127]. 

A new C3o-sterol, (24Z)-24-propylidenecholest-5-en-3-j8-ol (84) was isolated from the scallop, P. magellanicus [128]. Occelasterol (85), (22 ,24S)-27-nor-24-methyl-cholesta-5,22-dien-3)8-ol, and its 5,6-dihydro analogue, patinosterol, were isolated from the scallop, Patinopecten yessoensis [129] (Fig. 11). 

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. 

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