Abstract
The thyrotropin-releasing hormone receptor from rat brain was solubilized in a stable unbound form. The natural glycoside digitonin was the only detergent of a variety tested capable of solubilizing the active receptor. The digitonin-solubilized receptor exhibited binding kinetics for 3-[3H]methylhistidine thyrotropin-releasing hormone virtually identical to membrane preparations and responded with a similar order of potency to a series of thyrotropin-releasing hormone analogues that inhibit binding of the labeled ligand. Gel filtration analysis and sucrose density gradient centrifugation indicated a Stokes radius of 5.2 nm, a sedimentation coefficient of 11.2 s, and a corresponding calculated molecular weight of 244,000 for the detergent-receptor complex. The soluble receptor shows a gradual loss of binding activity over 48 to 72 hr when kept at 4 degrees C. However, the preparation may be frozen at -20 degrees C with no significant loss of activity.