IMMUNOCHEMICAL APPROACH TO THE MAPPING OF AN ASSEMBLED EPITOPE OF HUMAN CHORIONIC-GONADOTROPIN - PROXIMITY OF CTP-ALPHA TO THE RECEPTOR-BINDING REGION OF THE BETA-SUBUNIT

A single-step solid-phase RIA (SS-SPRIA) developed in our laboratory u sing hybridoma culture supernatants has been utilised for the quantita tion of epitope-paratope interactions. Using SS-SPRIA as a quantitativ e tool for the assessment of epitope stability, it was found that seve ral assembled epitopes of human chorionic gonadotropin (hCG) are diffe rentially stable to proteolysis and chemical modification. Based on th ese observations an approach has been developed for identifying the am ino acid residues constituting an epitopic region. This approach has n ow been used to map an assembled epitope at/near the receptor binding region of the hormone. The mapped site forms a part of the seat belt r egion and the cystine knot region (C34-C38-C88-C90-H106). The carboxy terminal region of the alpha-subunit forms a part of the epitope indic ating its proximity to the receptor binding region. These results are in agreement with the reported receptor binding region identified thro ugh other approaches and the X-ray crystal structure of hCG.