Phytophagous insects, steroid metabolism

Variability in Steroid Metabolism Among Phytophagous Insects... 

Although this discussion has dealt only with some unusual variations in sterol utilization and metabolism among phytophagous insects, similar reviews of this area of insect biochemistry in zoophagous and omnivorous species could include equally interesting information. Considering the small fraction of the more than one million Identified species of the class Insecta that have been investigated, there will undoubtedly be many more discoveries of unique aspects of steroid biochemistry in insects. 

SvoBODA, J. A. and M. J. Thompson, Variability in steroid metabolism among phytophagous insects, in Ecology and Metabolism of Plant Lipids (G. Fuller and W. D. Nes, eds.), ACS Symposium Series 325,176-186, American Chemical Society, Washington, DC, 1987. 

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. 

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. 

Dietary sterols of sunflower seeds were incorporated essentially unchanged into the tissues and, when injected, neither radiolabeled campesterol nor sitosterol was metabolized to cholesterol. Apparently, there was some selective uptake of dietary cholesterol, indicated by an enrichment of cholesterol in the insect sterols compared to the cholesterol concentration in the seed sterols. A C28 ecdysteroid, makisterone A (Figure 1), was identified as the major ecdysteroid of milkweed bug eggs (18) and, subsequently, makisterone A was identified as the major ec steroid in hemolymph of last stage milkweed bug nymphs and two other phytophagous species of the Pentatomomorpha group of Hemiptera (19). Makisterone A was... 

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