Microwave-facilitated extraction of furanocoumarins onto paper substrates: an imaging technique to analyse spatial distribution and abundance in leaves.

An impediment to studying the implications of fine-scale variation in the distribution of secondary compounds within plants has been an inability to map their distribution. Conventional approaches require fine-scale dissections, followed by extraction and analysis, a strategy which is often limited by the large number of extractions required and by the difficulty in quantifying compounds in very small samples. A method has been developed which allows the microwave-facilitated transfer of furanocoumarins from fresh leaves to two-dimensional substrates (phase separation paper). The fluorescence of these compounds under UV light is enhanced by treatment with base, which probably opens the lactone ring of the compounds. The developed prints are then photographed under UV light and the images analysed with commonly available software programs. Images so obtained for furanocoumarins in wild parsnip foliage revealed that the total fluorescence from extracted furanocoumarins is correlated with the amount of furanocoumarins remaining in the leaf. Thus, the images provide information not only on spatial distribution but also quantitative data.

Characterization of furanocoumarin metabolites in parsnip webworm, Depressaria pastinacella.

Although metabolites of furanocoumarins have been characterized in a wide range of organisms, to date they have been identified in only a single insect species, Papilio polyxenes. Depressaria pastinacella, the parsnip webworm, like P. polyxenes a specialist on Apiaceae, routinely consumes plant tissues higher in furanocoumarin content than does P. polyxenes and is capable of faster cytochrome P-450-mediated detoxification of these compounds. In this study, we characterized metabolites of xanthotoxin, a linear furanocoumarin, and sphondin, an angular furanocoumarin, in midguts and frass of parsnip webworms. Two metabolites were isolated and identified from webworms fed artificial diet containing xanthotoxin. LC-ESI-MS analysis resulted in the determination of a MW of 266 for the compound in the frass and one of the compounds in the midgut; 1H NMR confirmed its structure as 6-(7-hydroxy-8-methoxycoumaryl)-hydroxyacetic acid (HCHA). The second compound from the midgut had a MW of 252 and was identified by 1H NMR and 13C NMR analysis as 6-(7-hydroxy-8-methoxycoumaryl)-hydroxyethanol) (HMCH). Whereas HCHA has been found in frass of Papilio polyxenes fed xanthotoxin, HMCH has not been reported previously in insects. Although the first step of metabolism of xanthotoxin in webworms as well as P. polyxenes is likely the formation of an epoxide on the furan ring, angular furanocoumarin metabolism in webworms appears to differ. The principal metabolite of sphondin was identified as demethylated sphondin (6-hydroxy-2H-furo[2,3-h]-1-benzopyran-2-one) by LC-ESI-MS and confirmed by 1H NMR and 13C NMR analyses. That webworms produce metabolites of xanthotoxin in common not only with other Lepidoptera (e.g., HCHA) but with other vertebrates (e.g., HMCH) suggests a remarkable conservatism in the metabolic capabilities of cytochrome P-450s and raises the possibility that insects may share other detoxification reactions with vertebrates with respect to toxins in foodplants.