Sterols in Insects

Insects, unlike most vertebrates and plants, lack the capacity for de novo sterol synthesis and require dietary sterol for their normal growth, development and reproduction. This sterol requirement is in most cases satisfied by cholesterol (86) which is one of the principal sterols in insects, serving as component of the cell membranes and as a precursor of ecdysone (107). The zoophagous species such as the house fly Mucosa domestica are unable to convert phytosterol to cholesterol. For this reason, cholesterol is an essential nutrient for these species. In phytophagous and omnivorous insects, sterols such as sitosterol (87), campesterol (88), and stigmasterol (89) are dealkylated to cholesterol. Thus, 24-dealkylation is one of the essential metabohc processes in phytophagous insects (Fig. 15). 

It may be concluded that probably all insects require dietary sterol and, unlike the vertebrate, cannot use vitamin D. Nothing is yet known of the function of sterols in insects. 

Figure 7.8 The de-alkylation of plant sterols in insect gut to cholesterol...

Phytosterol dealkylation can be harnessed in insects to release a fluoroacetate equivalent from a 29-fluorinated sterol. Moreover, the fluorocitrate which then results from the "lethal synthesis" can be isolated and chemically characterized. hope that the range of insects susceptible to the 29-fluorophytosterols and more commercially viable analogs will be further explored. Furthermore, we urge wider scrutiny of insect biochemical pathways in search of possible targets for suicide substrates or latent toxin release. 

Balance between the active and inactive forms of ecdysone and/or other ecdysteroids may be accomplished by the formation of conjugates such as sulfate esters or glucosides (20,27). The sulfation of phenols and a variety of sterols has been 3emonstrated in insect tissues and this, in close association with an appropriate sulfatase, would constitute a readily reversible mechanism whereby the required balance between active and inactive forms of ecdysteroids could be regulated (27). 

To finish this duscussion on lipophorin biosynthesis we will mention studies on the origins of PLs, hydrocarbons, sterols, and carotenoids. It has been reported that in adult M. sexta and Rhodnius prolixm PL can be transferred from fat body to lipophorin (Van Heusden et al., 1991 Correa et al., 1992). This transfer of PL is independent of de novo synthesis of lipophorin however, the mechanism by which it occurs is unknown. Hydrocarbon transport by lipophorin has been studied only in P. ameri-cana. Katase and Chino (1982) have shown, in in vitro incubations, that a fat body rich in oenocytes, one type of cell in the hemolymph, which is the major site of hydrocarbon biosynthesis (Diehl, 1975), can release labeled hydrocarbon to lipophorin. It was also shown, using in vitro incubations, that the labeled hydrocarbon in lipophorin was delivered to the epidermis, the normal site of hydrocarbon deposition in insects. The sterols and carotenoids that are present in lipophorin must arise from the diet, because insects cannot biosynthesize either sterols or carotenoids de novo. Chino and Gilbert (1971) have shown that sterol can be transferred from the midgut to lipophorin, and the same is most likely true for carotenoids. The mechanism by which hydrocarbons, sterols, and carotenoids are transferred from either oenocytes or midgut epithelial cells to lipophorin is unknown. 

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. 

In the case of stigmasterol (89), the proposed mechanism in Fig. 18 was evaluated as follows. [23- H]-, [24- H]- and [25- H]stigmasterols were synthesized and the fate of the deuterium atoms during the dealkylation was followed by mass spectrometry. The transfer of the deuterium atom from C-25 to C-24 was established in silkworm larvae [162]. Also, the chemically synthesized (24F)- and (24Z)-A -dienes (93 and 94) were found to satisfy the sterol requirement of the silkworm. The A -diene (96) and desmosterol (91) were identified in significant amounts from the insects in accord with the previous observation by the Beltsville group. However, mass fragmentographic analysis of the sterols of insects fed on stigmasterol (89), the... 

Another series of inhibitors of sterol metaboUsm in insects were synthesized by our group. These are 24,28-iminofucosterol (104) [176], stigmasta-5,24(28),28-trien-3)8-ol (105) [177], and cholesta-5,23,24-trien-3(j8-ol (106) [177] (Fig. 19). When the imine (104) or the allene (105) was administered in the silkworm diet in combination with sitosterol or cholesterol, the growth and development of B. mori were markedly retarded. The imine was expected to inhibit the conversion of fucosterol epoxide to desmosterol, and this was verified by in vitro experiments where the imine, at the same level as the substrate [ H]fucosterol epoxide (92), completely blocked the transformation into desmosterol. However, the imine may not exert its effect solely by limitation of desmosterol or cholesterol formation because cholesterol as the sole dietary sterol was unable to prevent the inhibitory effect. In contrast, the allene (106) seemed to exert little effect on sitosterol dealkylation because the sterols in silkworms fed on the allene (106) in combination with sitosterol were essentially the same as in controls. 

Various aspects of steroid biosynthesis were included in a Royal Society Symposium. The published proceedings and other reviews have dealt with cyclase enzymes,water-soluble steroids and triterpenoids, the involvement of a 14(15)- or 8(14)-double bond and its reductionin cholesterol biosynthesis, biosynthesis of sterols, steroid metabolism in insects, pregnane steroids, cardenolides, and bufadienolides. ... 

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)." ... 

We will discuss some of our comparative sterol metabolism studies and provide specific examples to illustrate some of these unusual variations in steroid utilization and metabolism in insects, and to show how this information is useful in predicting differences in ecdysteroid biosynthesis in certain species. We will also point out instances in which these variations in neutral sterol metabolism can be related to phylogenetic relationships between species. 

Although this discussion has dealt only with some unusual variations in sterol utilization and metabolism among phytophagous insects, similar reviews of this area of insect biochemistry in zoophagous and omnivorous species could include equally interesting information. Considering the small fraction of the more than one million Identified species of the class Insecta that have been investigated, there will undoubtedly be many more discoveries of unique aspects of steroid biochemistry in insects. 

Work on sterol ester metabolism in insects has been reviewed by Thompson et al. (1973). 

Ikekawa, N., Sterol metabolism in insects and biosynthesis of ecdysone in the silkworm, Experientia, 39, 466-472 (1983). 

Kircher, W. C., Sterols in the leaves of Cheirodendron gaudichau-dii tree and their relation to Hawaiian Drospohila ecology, J. Insect Physiol., 15, 1167-1173 (1969). 

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... 

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