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Cytochrome P450 metabolism of furanocoumarins by parsnip webworms

Parsnip webworms ingest large quantities of furanocoumarins on a daily basis.  The webworms appear to lack any behavioral mechanism for direct assessment of furanocoumarin concentrations in their food.  A small amount of ingested furanocoumarins are incorporated into silk, but the bulk of it is metabolized and excreted in the frass as highly florescent metabolites.  As seen below, guts dissected taken from larvae that fed on a diet containing the furanocoumarin bergapten are, under visible light, indistinguishable from guts of larvae fed furanocoumarin-free diet.  Under UV illumination, however, the bright blue florescence of guts from bergapten-fed larvae stands out.
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                  white light illumination                                         near UV illumination

 

 

  Interactions between furanocoumarins

Furanocoumarin metabolism of webworms is increased when furanocoumarins are incorporated into its diet.  Metabolism rates of specific furanocoumarins vary.   In general the linear furanocoumarins (e.g., bergapten and xanthotoxin) are metabolized rapidly, while the angular furanocoumarins (e.g., sphondin) are metabolized more slowly.  When furanocoumarins are present in mixtures, as they would be in the host plant,  metabolism rates are dramatically reduced, suggesting competitive inhibition among these substrates (see graph at right).

 

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Is the furanocoumarin metabolism profile of webworms genetically fixed or can it be adjusted according to what the insect consumes?

last updated 3/11/1999