Hepatic Ah receptor from the Wistar rat: role of solvation in receptor structure and inactivation

Repeated freezing and thawing, the addition of salts, and elevated temperatures all promote the inactivation of the rat hepatic Ah receptor. The reduced availability of bulk water to solvate the protein is proposed to be the factor linking all these routes for inactivation. Prospective protocols for purification of unliganded Ah receptor should therefore minimize the number of freeze/thaw cycles; long-term freezing of cytosolic samples at -20 degrees C is inadequate to maintain long-term viability of the unliganded receptor. The stability of rat hepatic receptor is greatly increased upon binding the ligand, and the extent of ligand-induced stabilization is much greater than what is observed with steroid hormone receptors. Concentrations of NaCl and K2HPO4 up to 0.5 M inactivate the unbound Ah receptor irreversibly, with the loss of approximately 50% of the specific binding. At 2.0 M NaCl, a further reversible reduction in ligand binding activity is observed. The results at lower salt concentrations are interpreted in terms of the irreversible dissociation of a single binding unit from the trimeric cytosolic Ah receptor (which consists of two ligand-binding units and a 90-kDa heat shock protein), with the release of bound ligand from that subunit.