Phytosterol metabolism insects

Resemblance of the tricyclic diterpene skeleton to the ABC-ring system of common steroids prompted the suggestion that the diterpenes interfered with the pink bollworm s steroid metabolism. Insect hormones are produced from dietary phytosterols which are absorbed, transported, and biochemically altered. Interference by the diterpene resin acids at any stage of this process could significantly reduce larval viability. To test this hypothesis, feeding studies were conducted using a levopimaric acid test diet with added cholesterol (addition of... 

Insect steroid metabolism has two biochemically distinctive components dealkylation of phytosterols to cholesterol and polyhydroxylation of cholesterol to ecdysone. We will focus on the first of these. Lacking the ability to synthesize sterols de novo, insects instead have evolved a dealkylation pathway to convert plant sterols to cholesterol(7-10). The dealkylation pathways are apparently absent in most other higher and lower organisms, which can convert mevalonate to squalene and thence into sterols( ). Specific insecticides are possible based on these biochemical differences. 

Arthropoda.—Steroid biosynthesis seems to be absent from all of this phylum. Examples of the class Arachnida, Diplopoda, Crustacea, and Insecta have been examined. Steroid metabolism in insects has been reviewed. " It should be borne in mind that insects can synthesise some terpenoids [e.g. (32) and (46)], but there is an absolute dietary requirement for steroids. Phytosterols such as -sitosterol are converted back into cholesterol derivatives apparently by the reverse of side chain alkylation (86 R = Et) (85 R = CHMe)—> (85 R = CHj)— (84)— (74). In addition a A -double bond is introduced. Parasites, and other organisms naturally present, may contribute to some of these reactions. ... 

A variety of steroidal natural products have been isolated from insects, even though, as mentioned previously, insects are not able to carry out de novo steroid biosynthesis. The steroidal nucleus, as it occurs in insect primary and secondary metabolites thus must ultimately come from dietary or symbiotic microbial sources. For many phytophagus insects, C28 and C29 phytosterols are converted into cholesterol (C27) through a series of dealkylation pathways, with cholesterol subsequently serving as the starting point for further metabolic transformations, and resulting in a wide variety of steroid-based natural products. In other cases, dietary phytosterols are sequestered and deployed unmodified, and as with other compound classes, the relative importance of dietary sequestration versus modification is not always clear. 

As discussed above, phytophagous insects generally are capable of converting dietary C2g- and C29-phytosterols to cholesterol. However, several kinds of insects which show unusual variations in the utilization and metabolism of dietary phytosterols were found recently by Svoboda et al. [165-172]. 

The effect of 22-dehydrocholesterols (97) on the growth of Drosophila was investigated by Kirchner et al. [180]. cis- and /ran5-22-dehydrocholesterols were added to media of diet for 10 species of Drosophila. The cis isomer prevented normal maturation of 4 species and the trans isomer was toxic to 9. These findings were corroborated by tests with 4 representative species on a sterol-deficient medium under axenic conditions. Addition of cholesterol to the latter overcame the toxicity of the trans isomer. trans-22-Dehydrocholesterol may be acting as a competitive inhibitor in the metabolism of phytosterols to cholesterol or ecdysone by the insects. 

It has become increasingly evident that considerable variability in steroid utilization and metabolism exists among phytophagous species of insects. In recent years, we have discovered several phytophagous species that are unable to convert C28 or C29 phytosterols to cholesterol. This Includes one species that dealkylates the C-24 substituent of the side chain but produces mostly saturated sterols and several species that totally lack the ability to dealkylate the sterol side chain. Certain members of this latter group are of particular interest because they have adapted to utilizing a Cos sterol as an ecdysteroid precursor and makisterone A (C28) has been identified as the major ecdysteroid of certain developmental stages of these species. 

Coleoptera. The confused flour beetle, Tribollum confusum, was the first phytophagous insect we found that produces an appreciable amount of a sterol other than cholesterol from radiolabeled dietary C28 and C29 phytosterols. We found this insect produced large quantities of 7-dehydrocholesterol, equivalent to as much as 70% of the total tissue sterols isolated (12). It was further determined that cholesterol and 7-dehydrocTfolesterol were in equilibrium in this flour beetle. Another new intermediate, 5,7,24-cholestatrien-3B-ol was identified as an intermediate between desmosterol and 7-dehydrocholesterol (Figure 3). We found very similar pathways of sterol metabolism to exist in the closely related flour beetle, Tribolium castaneum (13). However, another flour beetle, Tenebrio moHtor, nad only about one-third or less of the levels of 7-dehydrocholesterol as the two Tribolium species, but still much higher levels of this sterol than has been found in most species. Fucosterol 24,28-epoxide was also implicated as an intermediate in the synthesis of cholesterol from sitosterol in T. mol i tor (14). 

Coleoptera. Sterol metabolism studies with another important stored products pest, the khapra beetle, Trogoderma granarium, revealed another phytophagous Insect that is unable to dealkylate and convert C28 and C29 phytosterols to cholesterol (23). Similar results were obtained whether a diet consisting of cracked wheat and brewer s yeast or an artificial diet coated with radiolabeled sterols was used (24). There was some selective uptake of cholesterol from tFe dietary sterols, as indicated by an enrichment of cholesterol in the pupal sterols 1.2% of total), compared to the dietary sterols (0.5% of total). Unlike the previously discussed stored product coleopteran pests, T. confusum and T. castaneum, both of which had high levels of 7-dehycTrochoiesterol, Tfo 7-dehydrocholesterol could be identified in the sterols from the khapra beetle. 

Since insects are unable to biosynthesize the steroid nucleus, they require dietary sterols for structural and physiological (hormonal) purposes. Cholesterol will satisfy this dietary need in most cases, but since phytophagous insects ingest little or no cholesterol from dietary materials, they must convert dietary C28 and Q9 phytosterols to cholesterol or other sterols. Through evolutionary development, certain insects have acquired the ability to metabolize dietary sterols in unique ways and to produce and utilize a variety of ecdysteroids (molting hormones) for hormonal control of development and reproduction. Thus, insects are able to flourish in virtually every conceivable ecological niche. Certain comparative studies that illustrate these evolutionary processes will be discussed in this chapter. 

The firebrat, Thermobia domestica of the order Thysanura, is the most primitive insect found to be capable of dealkylating phytosterols to cholesterol (6). In this study, dietary sitosterol was shown to be converted to cholesterol, and with the inclusion in the diet of an azasteroid that inhibits A -reductase and causes an accumulation of desmosterol, it was also determined that desmosterol was an intermediate in the conversion of sitosterol to cholesterol. To date, this is the most primitive insect that has been examined with respect to sterol metabolism. 

As mentioned earlier, much of the information on dealkylation and conversion of C28 and C29 phytosterols to cholesterol in insects (Figure 2) has been acquired through research with two lepidopteran species, the tobacco homworm, Af. sexta (3, and references therein) and the silkworm, B. mori (5, and references therein). Studies with Af. sexta established that desmosterol is the terminal intermediate in the conversion of all phytosterols to cholesterol, and that fucosterol and 24-methylenecholesterol were the first intermediates in the metabolism of sitosterol and campesterol, respectively, to cholesterol (5). In-depth metabolic studies with B. mori first demonstrated the involvement of an epoxidation of the A - -bond of fucosterol or 24-methylenecholesterol in the dealkylation of sitosterol and campesterol (5,45). More recently, the metabolism of stigmasterol was elucidated in detail in another lepidopteran, Spodoptera littoralis, and the side chain was shown to be dealkylated via a A " -bond and a 24,28-epoxide as were sitosterol and campesterol (46). The only significant differences in the metabolism of stigmasterol are the involvement of the additional 5,22,24-triene intermediate preceding desmosterol in the pathway and reduction of the A -bond prior to reduction of the A -bond (Figure 2). 

It is notable that of all Lepidoptera examined to date, none have been found to differ significantly from what is considered to be the normal pathways of dealkylation and production of cholesterol. This relatively highly evolved order of insects appears to have considerable uniformity with respect to the utilization and metabolism of dietary phytosterols. 

Some of this variability in sterol metabolism capabilities could well be exploited in developing new selective control methodology. As an example, it was found that certain inhibitors that block conversion of C28 and C29 phytosterols to cholesterol and adversely affect growth and development in Lepidoptera and species of other orders that dealkylate, had no effect on the honey bee, which is unable to dealkylate 58). Any technology that would limit the availability of utilizable sterol in a pest insect could provide a useful addition to our arsenal of weapons to use against insect pests. 

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