The identification of peptide sequences of human chorionic gonadotropin containing a conformational epitope

A series of overlapping peptides were synthesized representing the entire amino acid sequence of the beta-subunit of human chorionic gonadotropin (hCG) and these were reacted with a monoclonal antibody shown to be specific for hCG. One linear peptide (residues 40-52 of the sequence) reacted significantly with the monoclonal antibody but a conjugate of this peptide to diphtheria toxoid (DT) failed to elicit significant levels of antibodies reactive to hCG in rabbits. The subsequent preparation of an extended peptide (residues 38-57) in which the two cysteines were oxidized to form a loop peptide yielded a highly immunogenic antigen when conjugated to DT. Antibody levels reactive with hCG from loop peptide immunizations of rabbits exceeded those found after immunization with a 37 residue peptide representing the carboxyl terminus of the beta-hCG subunit. The antisera did not react with pituitary glycoprotein hormones with similar sequences.