Effects of the pyrimidine-containing cytochrome P-450 inhibitor, fenarimol, on the formation of 20-OH ecdysone in flies

Effects of anti-ecdysteroid quaternary derivatives of azole analogues of metyrapone on the post-embryonic development of the red cotton bug (Dysdercus cingulatus F)

In order to improve the larvicidal activity of the azole analogues of metyrapone, previously found to have a strong inhibitory activity on ecdysone 20-monooxygenase (E-20-M) from the fleshfly Neobellieria bullata Parker, soft-alkylated compounds (3-(1,1-dimethyl-2-oxo-2-phenylethyl)-1-dodecanoyloxymethyl-1H-imidazolium chloride, sPIM) and (1-(1,1-dimethyl-2-oxo-2-phenylethyl)-4-dodecanoyloxymethyl-1H-1,2,4-triazolium chloride, sPTM), derivatives of phenyl-imidazolyl-metyrapone (PIM) and phenyl-1,2,4-triazolyl-metyrapone (PTM), respectively, were synthesized. Both sPIM and sPTM, designed as propesticides, inhibited E-20-M in vitro at 10(-4) M concentration, which was unexpected since they had been expected to be inactive in vitro and to gain activity only within the organism. sPTM significantly delayed the pupariation of N. bullata larvae and this effect could be reversed by the simultaneous application of 20-hydroxyecdysone (20E), supporting the hypothesis that sPTM can act by interfering with the moulting hormone system. Due to this in vitro activity, sPTM and sPIM cannot be considered to be simple drug precursors, and their structure should contain structural elements (pharmacophores) responsible for the observed biological effects. In order to examine this hypothesis, derivatives of sPTM and sPIM were synthesised in which the hydrolytically labile N(+)-CH2O(CO)- moiety was changed to the more stable N(+)-CH2CH2(CO)-group. In three new stable derivatives, a dodecylamino or a phenyl group, respectively, is attached to the carbonyl group to obtain PTM and PIM derivatives quaternised with a 2-dodecylcarbamoylethyl or a 3-oxo-3-phenylpropyl group. In one derivative, the 2-oxo-2-phenylethyl quaternising group has one fewer carbon atom. In addition to their moderate activity (LC50 = 10(-6)-10(-5) M) against the red cotton bug Dysdercus cingulatus F, they delayed development and caused developmental abnormalities, including mortality in the pharate phase, mortality during moulting and wing deformations. These symptoms and the delay in development are characteristic of known compounds inhibiting the synthesis of 20E or interfering in the moulting processes. The facts that the frequent appearance of insects with developmental abnormalities and the delay in development could be reversed by co-application of 20E indicate that the moulting system might be the site of action. We presume that the quaternary azole derivatives of PIM and PTM can themselves also interact with the moulting system.

Characterization of ecdysteroid 26-hydroxylase: an enzyme involved in molting hormone inactivation

Insect molting hormone (ecdysteroid) inactivation occurs by several routes, including 26-hydroxylation and further oxidation to the 26-oic acids. Thus, the ecdysteroid 26-hydroxylase is a critical enzyme involved in precise regulation of ecdysteroid titers during insect development. Administration of the ecdysteroid agonist, RH-5849 (1,2-dibenzoyl, 1-tert-butyl hydrazone), or 20-hydroxyecdysone to the tobacco hornworm, Manduca sexta, results in induction of ecdysteroid 26-hydroxylase activity in midgut mitochondria and microsomes. The biochemical and kinetic properties of the ecdysteroid 26-hydroxylase were investigated. The mitochondrial enzyme was found to have optimal activity at a pH of 7. 5 in a Hepes or sodium phosphate buffer at 30-37 degrees C. The apparent K(m) of the microsomal 26-hydroxylase for 20-hydroxyecdysone substrate was lower than that of the mitochondrial enzyme for either 20-hydroxyecdysone or ecdysone substrate. The V(max) of the 26-hydroxylase in both subcellular fractions was slightly higher using 20-hydroxyecdysone as substrate compared to ecdysone. Demonstration that activity of the mitochondrial 26-hydroxylase was inhibited by incubation in a CO (or N(2)) atmosphere, taken together with the requirement for reducing cofactor and the efficacy of the P450 inhibitors, ketoconazole and fenarimol, provided strong evidence that the hydroxylase is cytochrome P450-dependent. Indirect evidence suggested that the mitochondrial and microsomal ecdysteroid 26-hydroxylase(s) could exist in a less active dephosphorylated state or more active phosphorylated state. Using Escherichia coli alkaline phosphatase to remove covalently bound phosphate groups, the activity of the 26-hydroxylase was decreased and, conversely, activity was enhanced using a cAMP-dependent protein kinase with appropriate cofactors. In addition, the protein kinase was shown to reactivate the 26-hydroxylase activity in alkaline phosphatase-treated fractions.

Early steps in ecdysteroid biosynthesis: evidence for the involvement of cytochrome P-450 enzymes

The first step in the biosynthesis of ecdysteroids by Manduca sexta prothoracic glands, the conversion of cholesterol to 7-dehydrocholesterol, is mediated by an enzyme with characteristics of a microsomal cytochrome P-450, i.e. sensitivity to CO and fenarimol, and a requirement for NADPH. The enzyme responsible for hydroxylation at C-25 of the putative 3-dehydroecdysone precursor, 14-hydroxy-5 beta-cholest-7-en-3,6-dione, is also microsomal, while those mediating hydroxylations at C-22 and C-2 of 3,14,25-trihydroxy-5 beta-cholest-7-en-6-one are mitochondrial. Indirect evidence revealed that the steps between 7-dehydrocholesterol and the trideoxyecdysteroids occur in the mitochondria, suggesting that extensive shuttling of intermediates between the endoplasmic reticulum and mitochondria takes place in the prothoracic gland cell during ecdysteroid biosynthesis. During the fifth larval instar, cholesterol 7,8-dehydrogenase activity is evident from days 2 to 9, while the conversion to [3H]ecdysteroids is not significant prior to the ecdysteroid commitment peak on day 4. Terminal hydroxylase activity shows little change throughout the instar. These data support the hypothesis that regulation of the biosynthetic pathway by PTTH occurs at the step immediately following the formation of 7-dehydrocholesterol. The steroid biosynthesis inhibitor, fenarimol, has been shown to inhibit each of these P-450 enzymes, as well as fat body ecdysone 20-monooxygenase, with an I50 of 10(-4) M in disrupted glands, suggesting that it is a general P-450 inhibitor. The secretion of ecdysteroids by the glands in vitro is very sensitive to fenarimol, i.e. I50 of 10(-6) M. RH5849, 1,2-dibenzoyl-1-tert-butylhydrazine, fails to inhibit any of these prothoracic gland reactions, yet strongly inhibits fat body ecdysone 20-monooxygenase activity. This suggests that RH5849 is a specific ecdysteroid substrate/product mimic in this reaction.