| Abstract: | The levels of both free and conjugated ecdysteroids, maternally labeled from 14C]cholesterol, of six different age groups of Manduca sexta eggs were quantitatively determined. Eggs 0–1-h old contain about 2.5 and 35 μ/g of the 2- and 26-phosphates of 26-hydroxyecdysone, respectively, and 1 μg/g of 26-hydroxyecdysone. During embryogenesis of 26-hydroxyedcdysone 26-phosphate is hydrolyzed to 26-hydroxyecdysone, which reaches a peak titer in 1–18-h-old eggs; the level of 26-hydroxyecdysone 2-phosphate remains rather constant. Additionally, other metabolic modifications such as hydroxylation, conjugation, epimerization, and oxidation are occurring; and as the level of the 26-hydroxyecdysone 26-phosphate decreases there is a progression of other ecdysteroids. C-20 hydroxylation first appears in 24–40-h-old eggs and reaches peak activity in 48–64-h-old eggs, where 20-hydroxyecdysone and 20, 26-dihydroxyecdysone are both present at peak titer but the latter is the major free ecdysteroid. Ecdysone is observed at measurable levels only in the three age groups of eggs between 1 and 64 h-old. C-3 epimerase activity also appears at 24–40 h and continually increases throughout embryogenesis to the point that 3-epi-26-hydroxyecdysone and 3-epi-20, 26-dihydroxyecdysone are the major free ecdysteroids in 96-h-old eggs. A new ecdysteroid conjugate, 26-hydroxyecdysone 22-glucoside, first appears at 24–40h and becomes the major conjugate in 72–80-h-old eggs; it represents an apparent end-product as its peak titer is reached and maintained throughout the final embryonic stages. 20-Hydroxyecdysonoic acid occurs in 48–64-h-old eggs, and along with 3-epi-20-hydroxyecdysonoic and ecdysonoic acids in 72–88-h-old eggs. 20-Hydroxyecdysonoic acid peaks during the latter time interval, and as its titer subsequently falls, there is a concurrent increase in the level of 3-epi-20-hydroxyecdysonoic which was identified as the second major component of the ecdysteroid conjugate fraction of 0–1-h-old larvae. Our results indicate that there is little or no biosynthesis of ecdysteroids during embryogenesis; that the materal ecdysteroid conjugate 26-hydroxyecdysone 26-phosphate serves as source for 26-hydroxyecdysone and the numerous metabolites; that 26-hydroxyecdysone and 20,26-dihydroxyecdysone may be the active hormones during embryonic development; and that glucosylation, epimerization, and formation of acids cosntitute inactivation processes. A scheme of the proposed pathways involved in the metabolism of 26–hydroxyecdysone 26-phosphate in the developing eggs of m. sexta is presented. |
