Sterol metabolism 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). 

Since the first rigorous demonstration of the dealkylative conversion of ergosterol into 22-dehydrocholesterol in the German cockroach Blattela germanica, several reports have appeared on the conversion of phytosterols into cholesterol in a variety of insects. The biochemical mechanism of the conversion has been investigated in 

Subsequently data from other insects support the biogenetic pathway show in Fig. 17. Goodwin s group found that the dealkylation of the synthetic [25- H, 26- C]clionasterol (C-24 epimer of sitosterol) to desmosterol by the insect, Tenebrio molitor, also involves migration of the C-25 hydrogen to the C-24 position [150-152]. The same mechanism was also proposed for the dealkylation of sitosterol in the locust Locusta migratoria L. [153]. 

For a detailed investigation of the mechanism of this dealkylation, it became necessary to determine the configuration of the epoxide (92). We developed a cell-free enzyme system prepared from the midguts of the silkworm B. mori. When samples of the supernatant obtained from a homogenate of silkworm guts were incubated separately with the [ H](24i ,28/ )-epoxide (92a) and [ H](24S,28S)-epoxide (92b), only the former was effectively converted to desmosterol (91) [154], However, subsequent in vivo and in vitro studies demonstrated no absolute stereo- 

In the dealkylation of campesterol (88), 24-methylenecholesterol has been identified as an intermediate. Recently, an intermediary role of the 24,28-epoxide of 24-methylenecholesterol has been demonstrated in the silkworm using [24- H]-, [25- H]- and [23,23,25- H3]campesterols as well as [23,23,25- H3]24-methylenecho-lesterol. The C-25 deuterium atom was shown to migrate to the 24 position of desmosterol during the dealkylation [159]. Similar results were also obtained in T. molitor using [23,23,25- H3]24-methylenecholesterol [160,161,166]. 

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

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