Ah receptor for 2,3,7,8-tetrachlorodibenzo-p-dioxin in rat liver: Lack of sensitivity to alkaline phosphatase when compared with that of glucocorticoid receptor

Rat hepatic cytosol was treated with alkaline phosphatase in order to determine if dephosphorylation altered the ability of Ah receptor to bind 2,3,7,8-[³H]tetrachlorodibenzo-p-dioxin (TCDD). Glucocorticoid receptor was studied for comparison. As previously had been shown in other laboratories, treatment of cytosol with purified alkaline phosphatase dramatically reduced the subsequent ability of glucocorticoid receptor to bind hormone. However, alkaline phosphatase had no effect on the ability of Ah receptor to bind [³H]TCDD. If either glucocorticoid receptor or Ah receptor was occupied by its ligand prior to exposure to alkaline phosphatase there was no loss in ligand binding capacity. Crude alkaline phosphatase (containing some protease activity) substantially reduced the ability of glucocorticoid receptor to bind hormone and shifted the sedimentation position of the glucocorticoid receptor from ≈8 S to ≈2 S. Crude alkaline phosphatase did not reduce the ability of Ah receptor to bind [³H]TCDD and did not alter sedimentation of the 9 S [³H]TCDD. Ah receptor complex. Although the Ah receptor appears to be a member of the steroid receptor superfamily, the lack of effect of alkaline phosphatase on Ah receptor (compared to the sensitivity of glucocorticoid receptor) highlights another significant difference in molecular characteristics between the Ah receptor and the receptors for steroid hormones.