Beetles, sterol metabolism

Bacteria, synthesis, 65 Bacteriohopane skeleton formation, pentacyclic C35, See Pentacyclic C35 bacteriohopane skeleton formation Bees, sterol metabolism, 131,132/133 Beetles, sterol metabolism, 133-134... 

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. 

The khapra beetle, T. granarlum, inhabits an environment more similar to that of the confused flour beetle than to the usual environment of other members of the family Dermestidae to which the khapra beetle belongs. However, with respect to utilization of dietary sterols, the khapra beetle Is more similar to other dermestids such as the hide beetle, Dermestes maculatus, (31) which usually feeds on animals or animal products rather than plant derived material. Apparently, the khapra beetle has not had to alter Its sterol metabolism to adapt to living In Its stored product environment. It will be of Interest to examine the ecdysterolds of the khapra beetle to see If the synthesis of molting hormones In this species has been modified to utilize other than a C27 sterol as an ecdysterold precursor. 

In another example, metamorphosis of the beetle Xyleb-orus ferrugineus will occur only if the larva receives the metabolic assistance of the fungus Fusarium solani. The latter organism has the ability to introduce a double bond into the nucleus of dietary sterols such functionality is an essential feature of all ecdysteroids. Larvae without the symbiotic fungus will metamorphose only if provided with er-gosterol (39) (Fig. 23.17), a sterol that has this double bond (Kircher, 1982). 

The lampyrid beetles (fireflies. Chapter 9) produce lucibufagjns (Figure 10.18) to make themselves unpalatable to predators. These substances come from dietary sterols, and therefore represent a group of more highly metabolized compounds. They are related in structure to the cardiac glycosides and toad poisons. 

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