Thyrotropin Binding and Long Acting Thyroid Stimulator Absorbing Activities in Subcellular Fractions from Isolated Thyroid Cells and Thyroid Homogenates

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Abstract
The ability of various [human, porcine] thyroid subcellular fractions to bind [125I]iodo TSH [thyrotropin] and to absorb long-acting thyroid stimulator (LATS) and thyroid stimulating immunoglobulins (TSI) activities was examined. Membranes purified from thyroid homogenates or isolated thyroid cells adsorbed LATS/TSI activities and specifically bound [125I]iodo TSH. Purified thyroglobulin, nuclei, mitochondria and ribosomes did not bind [125I]iodo TSH nor did they absorb LATS/TSI activities. Cell sap obtained by gentle lysis of isolated thyroid cells failed to absorb LATS/TSI activities and to bind labeled hormone. Freeze-thawing of the cells fragmented the membranes, releasing [125I]iodo TSH binding and LATS/TSI absorbing activities into the soluble (cell sap) fraction. The LATS/TSI antigen may be of cell surface origin, and includes the TSH receptor or larger membrane fragments containing the receptor, and its release into the soluble fraction is due to the fragmentation of the thyroid membrane during homogenization and preparative procedures.