Receptor binding characteristics of 2,3,7,8-substituted polychlorinated dibenzofuran radioligands

The condensation of 2,6-dichloroaniline and 2,5-dichlorophenol gave 2-hydroxy-2′,3,6,6′-tetrachlorobiphenyl which was readily cyclized to 1,4,9-trichlorodibenzofuran (TrCDF). 1,4,9-TrCDF was converted into [1,4,9-³H₃]-dibenzofuran by catalytic exchange with tritium gas. The tritiated dibenzofuran was dissolved in methylene chloride and chlorine gas was bubbled into the solution for 20 minutes to give the following PCDF congeners which were separated and purified by HPLC; [³H]-2,3,7,8-tetrachlorodibenzofuran (TCDF, 57 Ci/mmol), [³H]-1,2,3,7,8-pentachlorodibenzofuran (PeCDF, 34 Ci/mmol) and [³H]-1,2,3,6,7,8-/1,2,3,4,7,8-hexachlorodibenzofuran (HCDF, 32.5 Ci/mmol). The dose-dependent binding of the new radioligands with different concentrations of cytosolic receptor protein was saturable. Scatchard plot analysis of the binding isotherms gave K_d values of 1.3 ± 0.07 (2), 5.9 ± 1.2 (2), 2.3 ± 1.5 (2) nM respectively for radiolabeled 2,3,7,8-TCDF, 1,2,3,7,8-PeCDF and 1,2,3,6,7,8-/1,2,3,4,7,8-HCDF respectively (no. of determination shown in brackets). Although the Scatchard plots with 2,3,7,8-TCDD frequently were curvilinear at low concentrations of the radioligand, the plots obtained with the three PCDF radioligands were all linear. In addition, the Hill plot analysis of the data for all four radioligands gave slopes which were equal to one and the results are consistent with noncooperative ligand-receptor binding.