Metabolism of Epoxidised Withanolides by a Generalist and a Specialist Insect Species.

Major secondary metabolites of Physalis plants, epoxidized withanolides, have a potent feeding deterrent and growth inhibitory effect on most herbivorous insects. Caterpillars of the specialist moth species Heliothis (Chloridea) subflexa consume only Physalis fruits, whereas the closely related generalist Heliothis (Chloridea) virescens feeds on 14 different plant families, but not Physalis. The two species have different physiological responses to dietary withanolides, so it is wondered whether they metabolize withanolides differently. The Physalis peruviana plants are grown in a [¹³C]CO₂-supplied atmosphere, 4β-hydroxywithanolide E is isolated and purified from the leaves, and the compound is fed to the caterpillars. Subsequent high-performance liquid chromatography with diode array UV-vis detection coupled to high-resolution electrospray ionization mass spectrometry (HPLC-DAD-HRESIMS) and nuclear magnetic resonance (NMR) analyses of the main metabolite isolated from the frass show that both species convert 4β-hydroxywithanolide E mainly to withanolide S, probably by the action of an epoxide hydrolase. Withanolide S is completely characterized regarding its NMR and electronic circular dichroism data. To date, this is the first study to analyze the fate of withanolides after ingestion by insects.