BLACK SWALLOWTAIL (PAPILIO POLYXENES) ALLELES ENCODE CYTOCHROME P450STHAT SELECTIVELY METABOLIZE LINEAR FURANOCOUMARINS

Cytochrome P450 monooxygenases are involved in metabolism of hostplant allelochemicals by larval Lepidoptera. Biochemical purification of th e P450 polypeptide induced in Papilio polyxenes (black swallowtail) la rvae in response to xanthotoxin, a linear furanocoumarin, has allowed us to clone cDNAs encoding two allelic variants of the CYP6B1 locus. E xpression of these alleles in lepidopteran cell lines using baculoviru s expression vectors has demonstrated that both P450 isoforms metaboli ze substantial amounts of linear furanocoumarins, such as xanthotoxin and bergapten, but not angular furanocoumarins, such as angelicin and sphondin. These linear furanocoumarins are ubiquitous constituents of the hostplants of P. polyxenes. The efficiency of linear furanocoumari n metabolism is strongly affected by the nature of the substituents on the benzene ring; methoxy-derivatives are metabolized more efficientl y than are derivatives with smaller (hydroxy-) or larger (8-O isopente nyl) groups. Metabolism of either bergapten or xanthotoxin is inhibite d in the presence of the other. In addition, metabolism of linear fura nocoumarins is inhibited by the presence of nonmetabolizable angular f uranocoumarins, indicating that the active site of CYP6B1 binds angula r furanocoumarins. The reactivities described here indicate that P. po lyxenes larvae express at least two selective furanocoumarin-metabolic P450s: CYP6B1, which metabolizes a discrete set of linear furanocouma rins, and another P450, as yet unidentified, which metabolizes angular furanocoumarins more efficiently than does CYP6B1. (C) 1994 Academic Press, Inc.