Delineation of tyrosine-containing epitopes within the β subunit of bovine thyrotropin

The epitopes recognised by two monoclonal antibodies (mAb 279 and mAb 299), specific for the β subunit of bovine thyroid-stimulating hormone (bTSH), have been localised using a technique in which the tyrosine residues in the bTSH β subunit were subjected to modification when the bTSH β subunit was complexed with either mAb or in the free, unbound state. The epitope recognised by mAb 279 was localised to the C-terminal region of bTSH β with the tyrosine residue Tyr104 protected from modification by the presence of this mAb. In addition, the experimental results indicate that the tyrosine residues Tyr18 and/or Tyr112 are also involved in the mAb 279 epitope. The epitope recognised by mAb 299 was localised to the region 59-74 of bTSH β as both Tyr59 and Tyr74 were protected from modification by the presence of this mAb. Since both mAbs have been previously found to inhibit receptor binding, the sequence regions/amino acid positions recognised by these mAbs are likely to represent determinants for receptor binding. Moreover, these data indicate that the identified amino acid residues are located on the surface of the molecule, consistent with predictions of the tertiary structure of the bTSH β subunit based on the recently elucidated X-ray crystal structure of human chorionic gonadotropin.