Effect of Ligand Structure on Formation and DNA Binding Properties of the Transformed Rat Cytosolic Aryl Hydrocarbon Receptor

The saturation binding of transformed rat hepatic aryl hydrocarbon (Ah) receptor with [³²P]- dioxin responsive element was determined using gel mobility shift assays. The assay was carried out with ligands which exhibit both high and low Ah receptor agonist activity, namely: (high) 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 2,3,7,8-tetrachlorodibenzofuran (TCDF), 1,2,3,7,8- pentachlorodibenzo-p-dioxin, and 1,2,3,7,8-pentachlorodibenzofuran; and (low) 1,2,7,8-TCDF, 2,3,7-trichlorodibenzo-p-dioxin and 6-methyl-1,3,8-trichlorodibenzofuran (MCDF). Woolf plot analysis of the saturation binding data revealed that for the concentrations of ligands used in this study the B_max values for the high-affinity ligands were significantly higher (36.9–39.4 fmol/mg) than observed for the low-affinity ligands (6.15–13.8 fmol/mg). In contrast, there was not a structure-dependent trend in the equilibrium constant for dissociation K_D values or in the half-lives (t_1/2) for decomposition of the transformed receptor complexes. MCDF, a partial Ah receptor agonist/antagonist, also transformed the cytosolic Ah receptor and inhibited TCDDinduced transformation as determined by gel mobility shift assays. However, the diagnostic value of this assay to detect partial Ah receptor antagonists is questionable since similar results were obtained with 1,2,7,8-TCDF, a ligand which does not exhibit partial antagonist activity.