Possible binding site of thyrotropin binding inhibitor immunoglobulin (TBII) on the thyrotropin (TSH) receptor, which is different from TSH binding site

Abstract A synthetic decapeptide, P-194, which has the sequence No. 103 to 111 of hTSH receptor structure with an additional N-terminal tyrosine, did not bind TSH nor affected its receptor binding and thyroid stimulating activity. Preincubation of P-194 with sera from thyroid patients caused a significant decrease in TBII activity in almost all 12 TBII positive sera and an increase of thyroid stimulating activity in 3 of 7 Graves' IgG studied. In addition, [ 125 I] P-194 bound to serum IgG fraction from thyroid patients with a positive correlation with TBII (N=35, r=0.509, p