Interactions of rutaecarpine alkaloids with specific binding sites for 2,3,7,8-tetrachlorodibenzo-p-dioxin in rat liver

Rutaecarpine alkaloids have the capacity to inhibit specific2,3,7,8-[1,6-³H]tetrachlorodibenzo-p-dioxin (TCDD) binding inrat liver cytosol, as analysed by electrofocusing in polyacrylamidegel. The IC₅₀ value for binding of 7,8-dehydrorutaecarpine wasestimated to 7 nM indicating a high-affinity interaction, whereasrutaecarpine appeared less active (IC₅₀ 110 nM). These findingsare of interest in view of the fact that analogues to thesecompounds may be formed following UV-irradiation of tryptophanand that such photo-products have been suggested to constitute(the) endogenous ligand(s) for the TCDD receptor. As furthersupport of this notion, the rutaecarpine alkaloids investigatedcould be fitted into a rectangle of 6.8x13.7 A, a characteristiccommon for most high affinity ligands of the TCDD receptor hithertostudied. In view of their structural similarity to dehydrorutaecarpineand the agreement of their mol. wt with that of the photoproductwith the highest affinity for the TCDD receptor, we suggestdeaza-analogues of dehydrorutaecarpine to represent possiblecandidates for the endogenous TCDD receptor ligand.