IMMUNOCHEMICAL MAPPING OF HUMAN LUTROPIN .1. DELINEATION OF A CONFORMATIONAL ANTIGENIC DETERMINANT

Lutropin (LH), follitropin (FSH), thyrotropin (TSH) and choriogonadotropin (CG) are assembled of two non-covalently alpha (alpha) and beta (beta) subunits. We studied the discontinuous antigenic regions recognized by a monoclonal anti-hLH antibody designated as LH05 which binds to hLH, hCG and hTSH and does not cross-react with either the free subunits or hFSH. LH05 and an antibody designated HT13, recognizing an epitope partly comprizing the alpha 64-76 region, did not bind simultaneously to hCG. Furthermore, LH05 was unable to combine with an anti-peptide antit;ody (LHP03) directed to residues 43-52 of hLH beta. Thus, LH05 recognizes an epitope partly overlapping with those recognized by HT13 and LHP03. Using various hybrid molecules, we showed that the human alpha-subunit plays a critical role in the assembly of the epitope that, in contrast, contains amino acid residues conserved in the various beta-subunit of several species. Together, our results suggest that the amino acids Leu(49)-Pro(50), Tyr(59)-Arg(60) and Leu(86)-Ser(87) in the hLH beta and the alpha 64-76 region are probably included in the epitope recognized by LH05 which appeared to be not accessible on the CG/LH receptor.