Comparison of the Binding Characteristics of Bovine Thyrotropin and Human Chorionic Gonadotropin to Thyroid Plasma Membranes

Abstract

Bovine thyrotropin (bTSH) and human chorionic gonadotropin (hCG) interactions with bovine and human thyroid plasma membranes were evaluated quantitatively in order to understand the basis for hyperthyroidism in molar pregnancy and choriocarcinoma. Bovine [125I]iodoTSH 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 .mu.Ci/.mu.g. Scatchard analysis of labeled bTSH binding to bovine thyroid plasma membranes at pH 7.5 revealed 2 affinity constants, 2.0 .times. 1012 M-1, and 7.2 .times. 108 M-1, which were consistent with negative cooperativity between binding sites according to a Hill plot analysis. Fifty percent displacement of bovine [125I]iodoTSH was observed with 3.6 .times. 10-9 M unlabeled bTSH or 1.4 .times. 10-7 M hCG. Both bTSH and hCG displaced bovine [125I]iodoTSH from human thyroid plasma membrane preparations: 50% inhibition occurred with 8.2 .times. 10-9 M bTSH or 8.6 .times. 10-8 M hCG. Affinity constants of 2.4 .times. 109 M-1 and 9.3 .times. 107 M-1 were derived from a Scatchard plot of the displacement of bovine [125]iodoTSH from the human thyroid membrane particles by bTSH. The corresponding Hill plot analysis was consistent with negative cooperativity among a single class of binding sites. Addition of a small amount of unlabeled hormone enhanced the binding of bovine [125I]iodoTSH and suggested positive cooperativity at low hormone concentrations. Human [125I]iodoCG with high testicular membrane binding activity was derived from a commercial preparation of hCG following iodination by the chloramine-T method. The high resolution chromatographic fractions of this material with retention times corresponding to highly purified hCG displayed 17-20% maximal binding to the rat testis membrane preparation. The maximum binding of these human [125I]iodoCG fractions to human thyroid membrane preparations, on the other hand, was only 3%. Scatchard analysis of the displacement of this material by unlabeled hCG provided binding constants of 5.5 .times. 109 M-1 and 2.1 .times. 107 M-1. The high affinity binding of hCG to human thyroid plasma membranes provides a basis for the thyrotropic activity of hCG.