Sterols in Insects

It is perhaps surprising that insects use sterol derivatives both for a few pheromones and more widely for defensive compounds when they are dependant upon their diet for a supply of them. Cholestanone is used as a trail pheromone by tent caterpillars (Malacosoma species) (Figure 7.9), cholesteryl oleate and other cholesteryl esters are attractant pheromones for some ticks, and two sterol glucosides, blatellostanosides A and B, are aggregation pheromones of the cockroach Blatta germanica. Note that these not only retain the side-chain ethyl group of sitosterol but have been chlorinated as well. It is noteworthy that a modified sterol andros-tenone known as 'boar taint steroid' acts as a sex pheromone in pigs.

Figure 7.9 Some sterol pheromones. Cholestanone is a trail pheromone of tent caterpillars and the two blatellostanosides are aggregation pheromones of the German cockroach

Some water beetles store in their defensive glands the same sterols that are adrenal hormones in mammals (Figure 7.10). These compounds provide protection against fish, frogs and small mammal predators. Clearly the beetles possess enzymes with which they can degrade cholesterol further and remove most or all of the side chain (Figure 7.10).

cholesterol cholesterol

Figure 7.10 Corticosteroids from water beetles, used as defensive secretions. Other hydroxylated derivatives of these compounds have also been found in water beetles

Figure 7.10 Corticosteroids from water beetles, used as defensive secretions. Other hydroxylated derivatives of these compounds have also been found in water beetles

[4-14C]Cholesterol, when injected, served as a precursor of all the corticosterones. Insects injected with labelled mevalonolactone, as expected, produced no labelled sterols. The pathway to dienones and enones presumably diverges at an early stage since there was a greater incorporation of [4-14C]cholestadienone into the two dienones (Figure 7.10), while [4-14C]progesterone was better incorporated into the two enones. Specific incorporation of labelled cholesterol varied from 0.51% for cortexone to 1.20% for dihydrocybisterone. Labelling experiments with cholesterol showed that the 4(3 and 7(3 hydrogen atoms were eliminated in forming the diene.

There is about 1 mg of cortexone (also known as deoxycorticosterone or systematically as 4-pregnene-21-ol-3,20-dione) in the pro thoracic defensive glands of each beetle of Cybister limbatus. In Ilybius fenestrate there is testosterone, oestradiol, three compounds related to cortexone and the quinoline compound in Figure 7.11, which too is toxic to small mammals.

testosterone oestradiol

Figure 7.11 Defensive compounds from Ilybius fenestratus

N COOCH3 OH

methyl 8-hydroxy-quinoline-2-carboxylate testosterone oestradiol

N COOCH3 OH

methyl 8-hydroxy-quinoline-2-carboxylate

Figure 7.11 Defensive compounds from Ilybius fenestratus

The adults of another group of beetles, the Chrysolinina, have cardiac glycosides in their defensive glands. These are steroids (cardenolides) with sugar attached, first found in plants of the Asteraceae, Ranuncu-laceae and Apidaceae, in which the steroid side-chain is converted to an oc,(3-unsaturated lactone. The cardiac glycosides from plants, like

CH3C00H xylose ¥ OH

CH3C00H xylose ¥ OH

Cardenolides R, = OH, R2 = H sarmentogenin xyloside R1 = H, R2 = OH periplogenin xyloside R1 = OH, R2 = OH bipindogenin xyloside

Figure 7.12 Cardiac glycosides containing xylose made by the beetle Chrysolina coerulans from cholesterol The asterisk indicates a I4C-label which is lost in the synthesis digitoxin, originally used in arrow poisons, cause vomiting in mammals and affect their nervous systems. In plants it is known that the sterol side-chain is degraded to two carbon atoms and the lactone re-built with an acetate unit. The beetles that make these compounds do not feed on plants that contain them. It has been shown with [1,2-2H,23-14C]cholesterol that the beetles make the cardenolide from cholesterol (Figure 7.12). In the process the deuterium is retained but the 14C is lost. It is concluded that the beetles make the cardenolides in much the same way as plants do. Other chrysomelid beetles attach 2-deoxyhexoses and others make the steroids without the sugars.

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