Comparative metabolism of the phototoxic allelochemical α-terthienyl in three species of lepidopterans

Abstract

The comparative metabolism of α-terthienyl (2,2′:5′,2″-terthienyl), a secondary metabolite found in several tribes of the plant family Asteraceae, was investigated in three species of economically important lepidopterous larvae. The in vivo study involved feeding α-[ 3H]T to fifth instar larvae and monitoring the ratio of α-T:metabolite in the body and feces at 24 and 72 hr following administration. Analysis of the body (minus gut contents) of the larvae of the three species showed slightly (but not significantly) higher levels of the α-T:metabolite ratio in Manduca sexta as compared to Heliothis virescens or Ostrinia nubilalis. On the other hand, significantly increased levels of α-T:metabolite in the frass were observed for M. sexta as opposed to H. virescens or O. nubilalis. A significant amount (>40%) of label excreted was parent α-T in all of the three species. Striking differences in the capacity of the midgut microsomes to generate metabolites were observed in an in vitro study. α-T was metabolized 16 and 30 times faster by H. virescens and O. nubilalis microsomal fractions, respectively, as compared to M. sexta microsomal fractions. Evidence gained with the use of piperonyl butoxide indicated that polysubstrate monooxygenases are major enzymes involved in metabolism. Examination of cytochrome P450 levels in the three species showed increasing levels in the order: M. sexta, O. nubilalis, and H. virescens with the latter two species having 3 and 4 times more cytochrome P450, respectively, than M. sexta. Administration of α-T at 10 and 30 μg/g in the diet had no significant effect on cytochrome P450 levels in M. sexta and O. nubilalis. However, for H. virescens, a slight but significant decrease of cytochrome P450 levels was observed at these two concentrations. The results suggest that increased metabolism, mediated by polysubstrate monooxygenases leading to rapid clearance of this phototoxic thiophene, is one method by which phytophagous insect species deal with this type of allelochemical in host plants.