Abstract
Bovine thyrotropin (bTSH) and human chorionic gonadotropin (hCG) interactions with bovine and human thyroid plasma membranes have been evaluated quantitatively in order to understand the basis for hyperthyroidism in molar pregnancy and choriocarcinoma. Bovine [125I]iodo-TSH and [131I]iodoTSH were prepared by the lactoperoxidase method and the biological potency evaluated in the mouse TSH bioassay. Fifty-nine to 100% of biological activity was retained by bovine [125I]iodoTSH with specific activities of 8-236 μCi/μg. Scatchard analysis of labeled bTSH binding to bovine thyroid plasma membranes at pH 7.5 revealed two affinity constants, 2.0 × 1012M−1and 7.2 × 108M−1, which were consistent with negative cooperativity between binding sites according to a Hill plot analysis. Fifty per cent displacement of bovine [l25I]iodoTSH was observed with 3.6 × 10−9M unlabeled bTSH or 1.4 × 10−7M hCG. Both bTSH and hCG displaced bovine [125I]iodo TSH from human thyroid plasma membrane preparations: 50% inhibition occurred...
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