The in vitro bioactivity of the human TSH subunit was investigated utilizing eleven overlapping synthetic peptides representing the entire 112 residue sequence. The peptides were tested for both stimulatory and inhibitory activity in two sensitive bioassay systems: the first based on cAMP production in FRTL-5 rat thyroid cells, and the second based on stimulation of iodine trapping by the same continuous cell line. Peptides from three distinct regions of the -subunit showed concentration dependent inhibition of TSH bio-activity, including l-15, ll-25, 31-45, 81-95, and 91-105 with IC50 values ranging from 150 to 304 pM. An additional peptide representing the entire sequence of the “intercysteine loop” region of TSH, 31-52, also inhibited TSH activity with somewhat higher potency than its fragment peptide 31-45 (IC50 of 87.5 14.7 uM for 31-52 versus 207 92.4 pM for 31-45). Three of these, l-15, 31-45, and 31-52, also inhibited binding of TSH to the receptor in a radio-receptor assay, as previously reported (1), supporting their importance in receptor interaction. None of the synthetic peptides stimulated either cAMP production or iodine trapping. Two other overlapping peptides, 81-95 and 91-105, possessed bio-inhibitory activity but did not inhibit binding of labeled TSH. Computer analysis of this sequence predicted an extended turn structure for this region. This region has been referred to as the “determinant loop” as it is bounded by cysteine residues at positions 88 and 95 that many believe form a disulfide bond in the native subunit. The current data suggests the 88-95 region may play a role in receptor activation after initial binding of hormone to receptor.
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