Metabolic fate of the allelochemical nicotine in the tobacco hornworm Manduca sexta
1994
Snyder, M.J. | Walding, J.K. | Feyereisen, R.
The uptake, distribution and metabolism of nicotine vas studied in fifth instar Manduca sexta larvae. Significant levels of nicotine and its metabolites were found in hemolymph, midgut, hindgut, gut contents, and the remaining carcass after feeding on a 0.75% (fresh weight) nicotine diet. Most of the ingested nicotine was rapidly excreted, not as unmetabolized nicotine as previously thought, but almost exclusively as nicotine metabolites. The pattern of nicotine metabolites changed with time after the addition of nicotine to the diet and this change vas concomitant with the induction of midgut microsomal cytochrome P450 activities. The major metabolite found after induction was identified as cotinine-N-oxide by mass spectral and [3H]NMR analyses. The induction of nicotine metabolism was also reflected in a rapid clearance of unmetabolized nicotine from the hemolymph. 15 min after injection of nicotine one third of the injected dose was still circulating in nicotine-naive larvae whereas 97% of the dose had been cleared or metabolized in larvae previously, exposed to dietary nicotine. The extent of nicotine metabolism decreased and cotinine-N-oxide was no longer found in the feces within 24h after returning the larvae to a nicotine-free diet. These results show that rapid and reversible induction of nicotine metabolism is a major adaptation of the tobacco hornworm to this host plant alkaloid.
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