Biochemical strategy of sequestration of pyrrolizidine alkaloids by adults and larvae of chrysomelid leaf beetles

Tracer feeding experiments with ¹⁴C-labeled senecionine and senecionine N-oxide were carried out to identify the biochemical mechanisms of pyrrolizidine alkaloid sequestration in the alkaloid-adapted leaf beetle <i>Oreina cacaliae</i> (Chrysomelidae). The taxonomically closely related mint beetle (<i>Chrysolina coerulans</i>) which in its life history never faces pyrrolizidine alkaloids was chosen as a ‘biochemically naive’ control. In <i>C. coerulans</i> ingestion of the two tracers resulted in a transient occurrence of low levels of radioactivity in the hemolymph (1–5% of radioactivity fed). With both tracers, up to 90% of the radioactivity recovered from the hemolymph was senecionine. This indicates reduction of the alkaloid N-oxide in the gut. Adults and larvae of <i>O. cacaliae</i> sequester ingested senecionine N-oxide almost unchanged in their bodies (up to 95% of sequestered total radioactivity), whereas the tertiary alkaloid is converted into a polar metabolite (up to 90% of total sequestered radioactivity). This polar metabolite, which accumulates in the hemolymph and body, was identified by LC/MS analysis as an alkaloid glycoside, most likely senecionine <i>O</i>-glucoside. The following mechanism of alkaloid sequestration in <i>O. cacaliae</i> is suggested to have developed during the evolutionary adaptation of <i>O. cacaliae</i> to its alkaloid containing host plant: (i) suppression of the gut specific reduction of the alkaloid <i>N</i>-oxides, (ii) efficient uptake of the alkaloid <i>N</i>-oxides, and (iii) detoxification of the tertiary alkaloids by <i>O</i>-glucosylation. The biochemical mechanisms of sequestration of pyrrolizidine alkaloid <i>N</i>-oxides in Chysomelidae leaf beetles and Lepidoptera are compared with respect to toxicity, safe storage and defensive role of the alkaloids.