Inhibition of thyrotropin binding to receptor by synthetic human thyrotropin β peptides

In order to study the structure and function relationships of the thyrotropin (TSH)-specific β-subunit, we produced 11 synthetic overlapping peptides containing the entire 112-amino acid sequence of human βTSH and tested them for activity in TSH radioreceptor assay using both human and porcine thyroid membranes. Synthetic peptides representing four regions of the β-subunit demonstrated the ability to inhibit binding of ¹²⁵I-bovine TSH to crude thyroid membranes. The peptide representing the -COOH terminus of the subunit (β101-112) possessed highest binding activity, inhibiting binding of labeled TSH with an EC₅₀ of 80 μM. The remaining active peptides were: β71-85 (104 μM), β31-45 (186 μM), β41-55 (242 μM), and β1-15 (331 μM). Specificity of the binding activity was shown by the inability of the peptides representing the remainder of the subunit to inhibit binding of label and by the inability of any of the peptides to inhibit binding of ¹²⁵I-epidermal growth factor to the same thyroid membranes. The low affinity of the peptides as compared with native hormone is in agreement with previous studies of synthetic α-subunit peptides and, further, suggests that the interaction of βTSH with receptor is multifaceted, requiring cooperative binding of these sites for the observed high affinity of the whole hormone. These studies are in agreement with previous predictions of active regions by chemical modification but add two regions to the list, showing the utility of the synthetic peptide strategy in the study of peptide hormone structure-activity relationships.