Hepatic Ah receptor for 2,3,7,8-tetrachlorodibenzo-p-dioxin. Partial stabilization by molybdate

In structure and general mode of action, the Ah receptor is very similar to the receptors for steroid hormones. Molybdate previously has been shown to be highly effective at preserving ligand-binding function in steroid receptors during their exposure to elevated temperature or high ionic strength and at stabilizing steroid receptors as high molecular weight oligomeric complexes. Since such stabilization by molybdate can be very useful during characterization and purification of receptors, we tested the effects of molybdate on the Ah receptor to determine if the Ah receptor, like the receptors for steroid hormones, might be stabilized. In hepatic cytosols from C57BL/6N mice and Sprague-Dawley rats, molybdate concentrations up to 30 mM in homogenizing and analysis buffers did not alter the concentration of specific Ah receptor sites detected by binding of [3H]2,3,7,8-tetrachlorodibenzo-p-dioxin. However, inclusion of 20 mM molybdate in the homogenizing buffer did significantly protect unliganded Ah receptor from thermal inactivation at 20 degrees C and from KCl-induced loss of ligand-binding ability. In accord with previous reports, 20 mM molybdate in homogenizing and analysis buffers greatly increased the concentration of detectable glucocorticoid receptor in rat hepatic cytosol and estrogen receptor in rat uterine cytosol. Exposure to 0.4 M KC1 caused the glucocorticoid receptor from rat liver to shift sedimentation from approximately equal to 8 S to approximately equal to 4 S and caused a severe loss of specific glucocorticoid binding. Presence of 20 mM molybdate stabilized the glucocorticoid receptor as a single discrete peak sedimenting at approximately equal to 8 S. In contrast, the Ah receptor from rat liver exposed to 0.4 M KC1 in the presence of molybdate sedimented as biphasic peaks; one peak (approximately equal to 9.5 S) corresponded to the form of Ah receptor observed at low ionic strength, while the other peak (approximately equal to 5.5 S) corresponded to the form of Ah receptor seen in cytosol treated with 0.4 M KC1 in the absence of molybdate. Addition of heparin to hepatic cytosols from mice or rats shifted sedimentation of Ah receptor from approximately equal to 9.5 S to approximately equal to 5.5 S. Molybdate, again, provided stabilization in the approximately equal to 9.5 S form, but only for about one-half the total Ah receptor content in both rat and mouse hepatic cytosols. In sum, molybdate is far less effective at stabilizing rodent Ah receptors than it is at stabilizing steroid receptors in the same species.