Characterization of triton-solubilized TSH receptors from human thyroid plasma membranes

The TSH receptor from human thyroid plasma membranes has been solubilized in 10 mM Tris/HCl, 50 mM NaCl, pH 7.4 containing 0.5% triton X-100. Binding of [125I]TSH to the soluble receptor showed rapid and reversible kinetics and reached a maximum within 30 min at 37 degrees C, by 1 h at 25 degrees C and by 24 h 4 degrees C. Optimal pH was 7.4. The soluble receptor retained specificity with cross-reactivity only to crude hCG (0.03%). Scatchard plots were curvilinear indicating the presence of at least two binding sites. The high affinity site showed an affinity content of 1.1 X 10(9) M-1 with binding capacity of 1.3 X 10(-10) M/mg protein. TSH-binding inhibitor immunoglobulins from patients with Graves' disease inhibited [125I]TSH binding to the soluble receptor in a dose-dependent manner. NaCl inhibited the TSh binding and this was ascribed to the decrease in the receptor capacity. Trypsin, neuraminidase and phospholipase C treatment of the solubilized receptor had no effect on TSH binding. The apparent molecular weight of the receptor, determined by gel filtration of Sepharose 6B, was approximately 300 000.