Competitive binding to the cytosolic 2,3,7,8-tetrachlorodibenzo-p-dioxin receptor

The proposed mechanism of action of the toxic halogenated aromatics, typified by 2,3,7,8-tetrachlorodibenzo- p -dioxin (TCDD), involves the initial binding to a high-affinity, low-capacity, cytosolic receptor protein. Previous studies have shown that several 4'-halo-2,3,4,5-tetrachlorobiphenyls bind to the TCDD receptor and that a lateral substituent on both phenyl rings is required for activity. Using an extended series of eighteen 4'-substituted-2,3,4,5-tetrachlorobiphenyls as probes, the effects of a variable lateral substituent on receptor binding affinity and the induction of aryl hydrocarbon hydroxylase (AHH) in vivo and in rat hepatoma H-4-II E cells have been determined. For most substituents, there was an excellent correlation between the rank-order potency for receptor binding and the rank-order potency for AHH induction. Based on in vitro binding affinities ( ec 50 values) of the 4'-substituted tetrachlorobiphenyls, a multiparameter regression equation was formulated correlating the binding constants to physicochemical substituent parameters. For thirteen compounds out of the present series, multiple regression analysis of the binding data led to the following equation: log(1/ ec 50 ) = 1.530σ + 1.47π + 1.09HB + 4.08, r = 0.978. The results suggest that halogen substitution on both phenyl rings is not a requirement for binding and that hydrophobic (π) and electronic (σ) substituent constants and a variable for hydrogen bond (HB) formation are significant parameters describing relative binding avidities of this series of substituted biphenyls for the TCDD receptor.