The TCDD receptor in rat intestinal mucosa and its possible dietary ligands

Induction of aryl hydrocarbon hydroxylase (AHH) by polycyclic aromatic hydrocarbons and other inducers such as 2,3,8-tetrachlorodibenzo-p-dioxin (TCDD) is known to occur following binding of the inducer to a soluble receptor protein similar to steroid hormone receptors. This receptor is usually called the TCDD receptor, since TCDD has the highest affinity of all known ligands for the receptor. In the present paper a receptor for TCDD in cytosol from rat intestinal mucosa has been studied, using isoelectric focusing in polyacrylamide gel. This receptor's biochemical properties were found to be similar to those of the TCDD-receptor in rat liver cytosol. The dissociation constant (Kd) of the 3H-TCDD-receptor complex in rat intestinal mucosa was 0.7-3.1 nM, and it was present at a concentration of 70-80 fmol/mg protein. Starvation did not significantly increase the receptor level. The affinities of some potential dietary ligands for the TCDD receptor in rat intestinal mucosa were also studied. Indole-3-carbinol had 1/2,600 of the affinity of TCDD for the receptor protein. Butylated hydroxyanisole (BHA), transstilbene oxide and quercetinpentamethylether competed even more weakly with 3H-TCDD for binding to the receptor. The biological significance of the occurrence of low-affinity ligands of dietary origin for the TCDD receptor is uncertain at the present time.