Phytophagous insects dealkylation

Although dealkylation is restricted to arthropods, not all insects possess this ability, nor do all insects employ the same steroid nucleus. In general, phytophagous insects are capable of dealkylation while zoophagous insects lack this ability. 

Many phytophagous insects dealkylate (95) to produce cholesterol, and a preparation from silkworm gut was able to catalyse the formation of desmosterol (101) from 24,28-epoxyfucosterol (102).170 Further evidence for the operation of the pathway shown in Scheme 10 is provided by the demonstration that the imine (103) was a... 

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). 

For many years, it was believed that phytophagous insects in general were capable of dealkylating and converting dietary C28 and C29 phytosterols to cholesterol to satisfy their need for cholesterol (4). Also, a number of omnivorous species of insects are known to Fe capable of this conversion Thus, cholesterol... 

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 do not synthesize sterols, they are dependent upon a dietary supply of these essential compounds. Phytophagous insects transform the dietary C g- and Cj -sterols into C2. -sterols (usually cholesterol) before they are used for various l unctions, including structural components of membranes and moulting-hormone precursors. This dealkylation process has been re-viewed, and it appears that fucosterol (135) is an intermediate in the conversion of sitosterol (134) into cholesterol. Scheme 10 has been proposed as a main dealkylation pathway in Bombyx mori, based on the efficient incorporation of [ H]fucosterol 24,28-epoxide (136) into cholesterol (138) and also trapping of (136) as a probable intermediate in the conversion of fucosterol... 

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. 

One of the earliest biochemical studies on phytosterol dealkylation was done with the Virginia pine sawfly. Neodiprion pratti (25). This research demonstrated that dietary [ H]sitosterol was converted to [ H]cholesterol by this insect and that no cholesterol was produced from labeled acetate or mevalonate, the typical precursors in organisms that can biosynthesize sterols. To date, this rather primitive hymenopteran is the only known phytophagous hymenopteran capable of dealkylating phytosterols. 

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