W. E. Robbins

Ecdysones and Analogs: Effects on Development and Reproduction of Insects

Ingestion of certain synthetic ecdysone analogs inhibited larval growth and development in several species of insects, whereas 20-hydroxyecdysone was inactive or considerably less active. Natural 20-hydroxyecdysone and ponasterone A, and a synthetic ecdysone analog inhibited ovarian maturation and egg production in the adult housefly. These effects appeared to be related to hormonal activity.

Insect steroid metabolism

Insects are unable to biosynthesize the steroid nucleus and generally require an exogenous source of sterols. Two salient areas of insect steroid metabolism are the dealkylation and conversion of dietary C28 and C29 plant sterols to cholesterol and other C27 sterols, and the biosynthesis and metabolism of the steroidal insect molting hormones. Certain azasteroids and nonsteroidal amines block this conversion of 24-alkyl sterols to cholesterol and/or disrupt molting and development in insects. These inhibitors have served in charting metabolic pathways for steroids in insects and are serving as models in developing selective pesticidal chemicals and chemotherapeutic agents for use against insects and other invertebrate pests and parasites. The mode of action of some of these inhibitors on molting and development has been investigated in vivo and in vitro. Certain of these inhibitors represent a new class of insect hormonal compounds with a novel mode of action—the disruption of molting hormone metabolism. Research on sterol metabolism in insects provides important information on the comparative biochemistry and physiological functions of steroids in living systems.

Assay for ecdysone (Molting hormone) activity using the house fly, Musca domestica L.

Abstract Larvae of the house fly, Musca domestica L., were used as the test insect in a modified assay for ecdysone. The house fly is easily reared by using standard procedures and provides a high yield of organisms suitable for bioassay. The quantity of α-ecdysone needed for about 60% pupation in the abdomens of ligated house fly larvae was 0.005–0.006 μg, one-third to one-fourth the amount needed to effect the same Response in the Calliphora test.

Insect Hormones: Alpha Ecdysone and 20-Hydroxyecdysone in Bracken Fern

The two major molting hormones of insects, alpha ecdysone and 20-hydroxyecdysone, were isolated in crystalline form from dry pinnae of the bracken fern, Pteridium aquilinum (L.) Kuhn. Three unidentified substances with molting hormone activity were also detected. Bracken is the first plant found to contain both of the major insect ecdysones, and it is the first known plant source of alpha ecdysone.

26-Hydroxyecdysone: New Insect Molting Hormone from the Egg of the Tobacco Hornworm

Five kilograms of tobacco hornworm eggs (48 to 64 hours old) afforded 26.5 milligrams of a new crystalline insect molting hormone identified as 26-hydroxyecdysone. The three known insect ecdysones—α-ecdysone, 20-hydroxyecdysone, and 20,26-dihydroxyecdysone—were also present but in much smaller quantities. The new hormone is the predominant molting hormone in the hornworm during this stage of embryonic development. These results takent in context with the current knowledge of the chemistry and biochemistry of the molting hormones during postembryonic development in the hornworm indicate quantitative and qualitative differences in the biosynthetic-mnetabolic pathways as well as in the ecdysones in different developmental stages of this insect.

Ecdysones and synthetic analogs: Molting hormone activity and inhibitive effects on insect growth, metamorphosis and reproduction

Abstract Five ecdysones and 19 ecdysone analogs were assessed for molting hormone activity in the house fly assay. When these 5β-steroids were further tested in the house fly, the confused flour beetle, the yellow fever mosquito, and the German cockroach, many of the compounds inhibited growth, development and/or reproduction. The relationship of structure to both the molting hormone and inhibitive activity of these compounds is discussed.

Makisterone A:a 28-carbon hexahydroxy molting hormone from the embryo of the milkweed bug

Makisterone A is the predominant ecdysone in the 96 +/- 4-hour-old embryo of the large milkweed bug and it is the first molting hormone with a C-24 alkyl substituent of the side chain to be isolated and identified from an insect. In addition, unknown compounds that may represent other C28 ecdysones were detected in very low concentrations. The milkweed bug could well possess a biosynthetic-metabolic pathway for C28 molting hormones instead of or in addition to known pathways for the C27 ecdysones.

Metabolism of Steroids in Insects

Publisher Summary This chapter discusses metabolism of steroids in insects. Insects require a dietary or exogenous source of sterol for normal growth, metamorphosis, and reproduction, and the only known exceptions to this are those species in which a sterol source can be attributed to associated symbiotes. The hornworm utilizes the major sterols of its primary host plant, tobacco—campesterol, sitosterol, and stigmasterol—through their dealkylation and conversion to cholesterol. These conversions proceed through a series of oxidation-reductive steps, and a number of the metabolites and intermediates involved can be characterized. Cholesterol serves both as a structural component of cells and tissues and as a precursor for the ecdysones or molting hormones. While the sulfate and glucoside conjugates of sterols and ecdysteroids function, primarily, in the inactivation and excretion of steroids in insects, these conjugates can have other equally important roles related to the biosynthesis, metabolism, and transport of steroids, or the regulation of hormone titer.

Desmosterol, an intermediate in dealkylation of β-sitosterol in the tobacco hornworm

The mechanism of dealkylation of phytosterols such as β-sitosterol to cholesterol in insects is an important and little understood biochemical transformation. This alteration of the sterol side chain provides a means for omnivorous and particularly phytophagous insects to obtain their essential cholesterol from foodstuffs that are either very low or completely lacking in cholesterol but which contain substantial quantities of plant sterols. Dealkylation has been reported or inferred for a number of insects and has definitely been demonstrated to occur in certain species through the use of semidefined diets, radiotracer labeled sterols, and/or gas-liquid chromatographic analysis (1,2,3). In these insects, only the final metabolite of this process, cholesterol or a cholestane derivative, has been identified, and nothing is known of the intermediate reactions. Recent research has greatly enhanced our understanding of the alkylation process that occurs during the biosynthesis of plant sterols (4,5,6). It would be of extreme interest from a comparative biochemical standpoint to determine whether the alkylation and dealkylation processes share certain common intermediates or proceed through common pathways. During a study in our laboratory on the rate of conversion of H3-β-sitosterol to cholesterol in the tobacco hornworm, Manducasexta (Johannson), an unknown H3-labeled sterol component was detected. The isolation and identification of this sterol as desmosterol (Δ5,24-cholestadien-3β-o1) and proof of its occurrence as an intermediate in the dealkylation process is the topic of this report.

Metabolic pathways of steroids in insects

This paper discusses sterol metabolism and utilization in insects with particular emphasis on the two known major metabolic pathways for steroids in insects: (1) The conversion of C28 and C29 plant sterols to cholesterol, including a comprehensive survey of the intermediates involved in the conversion of phytosterols to cholesterol, certain inhibitors of phytosterol metabolism, and the end products of sterol metabolism in insects. (2) The biosynthesis and metabolism of the steroid moulting hormones of insects (ecdysones), including the present state of our knowledge of the sterol precursors of the ecdysones, the ecdysteroid intermediates in their biosynthesis, and the inactivation of the ecdysteroids in insects. The biological and physiological significance of these metabolic pathways in insects is discussed, as well as their relationship to analogous pathways in plants and vertebrates.