Several tritiated chlorinated dibenzo-p-dioxins and dibenzofurans have been prepared with specific activities in the range of 30-50 Ci/mmol in order to investigate the effects of structure on the kinetics of their association with the rat cytosolic Ah receptor. The compounds were 2,3,7-trichlorodibenzo-p-dioxin (TrCDD), 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PeCDD), 2,3,7,8-tetrachlorodibenzofuran (TCDF), 1,2,7,8-TCDF, and 1,2,3,7,8-pentachlorodibenzofuran (PeCDF). Although these congeners differed by up to 2 orders of magnitude in their biochemical and toxic potencies, their affinities for the Ah receptor as determined by conventional Scatchard analysis varied by less than 2-fold (range of KD values 5.0-9.3 nM). The rate of association of these ligands with the Ah receptor was studied at several temperatures, taking into account the competing thermal inactivation of the unbound receptor. The equilibrium constants (KD) were also obtained as the ratio of the rate constants for dissociation and formation, respectively, of the receptor-ligand complexes. The following conclusions were derived from the kinetic studies: (1) 2,3,7-TrCDD and 1,2,7,8-TCDF bound significantly more slowly to the Ah receptor than the other radioligands at all temperatures (13.5-37 °C), and this paralleled the lower biochemical potencies of the congeners; (2) the KD values obtained kinetically were in the subnanomolar range, with the smallest KD values observed for those ligands which bound most rapidly to the receptor; and (3) the temperature dependence of the KD values indicated that receptor-ligand association was favorable both enthalpically and entropically.
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