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

Abstract A series of overlapping peptides were synthesized representing the entire amino acid sequence of the β-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 β-hCG subunit. The antisera did not react with pituitary glycoprotein hormones with similar sequences.