Biochemical strategy of sequestration of pyrrolizidine alkaloids by adultsand larvae of chrysomelid leaf beetles

Tracer feeding experiments with C-14-labeled senecionine and senecionine N- oxide were carried out to identify the biochemical mechanisms of pyrrolizid ine alkaloid sequestration in the alkaloid-adapted leaf beetle Oreina cacal iae (Chrysomelidae). The taxonomically closely related mint beetle (Chrysol ina coerulans) which in its life history never faces pyrrolizidine alkaloid s was chosen as a 'biochemically naive' control. In C. coerulans ingestion of the two tracers resulted in a transient occurrence of low levels of radi oactivity 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 l arvae of O, cacaliae sequester ingested senecionine N-oxide almost unchange d in their bodies (up to 95% of sequestered total radioactivity), whereas t he tertiary alkaloid is converted into a polar metabolite (up to 90% of tot al sequestered radioactivity). This polar metabolite, which accumulates in the hemolymph and body, was identified by LC/MS analysis as an alkaloid gly coside, most likely senecionine O-glucoside. The following mechanism of alk aloid sequestration in O. cacaliae is suggested to have developed during th e evolutionary adaptation of O. cacaliae to its alkaloid containing host pl ant: (i) suppression of the gut specific reduction of the alkaloid N-oxides , (ii) efficient uptake of the alkaloid N-oxides, and (iii) detoxification of the tertiary alkaloids by O-glucosylation. The biochemical mechanisms of sequestration of pyrrolizidine alkaloid N-oxides in Chysomelidae leaf beet les and Lepidoptera are compared with respect to toxicity, safe storage and defensive role of the alkaloids. (C) 1999 Elsevier Science Ltd. All rights reserved.