Stereospecific, high affinity binding of 2,3,7,8-tetrachlorodibenzo-p-dioxin by hepatic cytosol. Evidence that the binding species is receptor for induction of aryl hydrocarbon hydroxylase

Abstract

We previously hypothesized that the genetic trait of aromatic hydrocarbon nonresponsiveness (the failure in certain inbred strains of mice of polycyclic hydrocarbons to induce aryl hydrocarbon hydroxylase activity, and the diminished sensitivity to the more potent inducer 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is due to mutation which results in an induction receptor with a diminished affinity for the inducing compound. Following the intraperitoneal administration of [14C]TCDD (6 nmol/kg), hepatic accumulation of the radiolabel was greatest in C57BL/6J mice, intermediate in the hybrid B6D2F1/J mice, and least in DBA/2J mice, a pattern which mirrors the strain sensitivity to hydroxylase induction by TCDD (C57BL/6J greater than B6D2F1/J greater than DBA/2J). These data are compatible with receptor mutation theory and suggested that the hepatic uptake of TCDD is determined by the affinity of the receptor. In vitro experiments on the binding of [3H]TCDD to hepatic cytosol from C57BL/6J mice revealed a small pool of high affinity sites which stereospecifically and reversibly bind TCDD. The specific binding of [3H]TCDD to hepatic cytosol had an equilibrium dissociation constant KD of 0.27 nM and a maximum binding capacity of 84 fmol/mg of cytosol protein. Much less high affinity specific binding of [3H]TCDD was observed in hepatic cytosol from DBA/2J mice, but the KD was not estimated because of the limited aqueous solubility of the ligand. The binding affinity of 23 halogenated dibenzo-p-dioxins and dibenzofurans for this hepatic cytosol-binding species closely correlated with the potencies of these compounds as inducers of hepatic aryl hydrocarbon hydroxylase activity. The polycyclic hydrocarbons that induce hepatic hydroxylase activity competed with [3H]TCDD for hepatic cytosol binding, but phenobarbital, pregnenolone-16alpha-carbonitrile, and the steroid hormones had no specific binding. The data suggest that the hepatic cytosol species which binds TCDD is the receptor for the induction of hepatic aryl hydrocarbon hydroxylase activity, and that the mutation in nonresponsive mice results in an altered receptor with a diminished affinity for inducing compounds.