Immunochemical mapping of human lutropin: II. Characterization of two monoclonal antipeptide antibodies reacting with the native beta-subunit

To investigate the epitopes present on the beta-subunit of the human lutropin (hLHbeta) and their topographical relationship at the surface of the molecule, we produced two monoclonal antipeptide antibodies, designated LHP03 and LHP04, capable of binding to the radiolabeled 125I-hLHbeta and directed to the 43-52 and 110-117 regions of the hLHbeta, respectively. Analysis of the accessibility of the epitopes on hLH and on the beta-subunit of human chorionic gonadotropin (hCGbeta), equine LH (eLHbeta) and ovine LH (oLHbeta) indicated that: (i) LHP03 binds to both the free hLHbeta subunit and dimeric hLH whereas LHP04 binds preferentially to the free hLHbeta, (ii) LHP03 recognizes weakly the hCGbeta and oLHbeta in comparison to hLHbeta and (iii) LHP04 binds oLHbeta as well as hLHbeta but does not bind to hCGbeta and eLHbeta. The topographical relationship of epitopes recognized by LHP03 and monoclonal antibodies recognizing dimer specific epitopes on hLH allowed us to localize discontinuous antigenic sites that overlaps or are located outside the hHLbeta(43-52) region. Together, our results demonstrated that the hHLbeta(43-52) portion is accessible on both the free hLHbeta subunit and hLH whereas the COOH-terminal portion, hHLbeta(110-117), is probably buried at the alpha/beta interface of the hormone.