Design of peptides with improved affinities for anti-human chorionic gonadotropin monoclonal antibodies

Three monoclonal antibodies, LF-hCG-6, -26 and -28, raised against holo-human chorionic gonadotropin and directed against its beta-subunit, have been used to design synthetic peptides with improved affinity. By PEPSCAN analyses with pinbound, overlapping nonapeptides, it was shown that the peptide consisting of amino acids (125-133) of the beta-subunit (i.e., PPSLPSPSR) reacted with monoclonal antibody (LF-hCG-6; the sequence (135-143) (i.e., PGPSDTPIL), with LF-hCG-28; and the C-terminal nonapeptide (137-145) (i.e., PSDTPILPQ), with LF-hCG-26. To determine amino acids essential for binding and those comprising the necessary amino acid core, and to establish the optimal length for binding reactivity, sets of replacement nonapeptides and overlapping octa- to dodecapeptides derived from the beta-subunit sequences (122-134) (i.e., KAPPPSLPSPSRL) and (132-145) (i.e., SRLPGPSDTPILPQ) were synthesized. Amino acid cores were as follows: beta-hCG (125-131) (i.e., PPSLPSP) for monoclonal antibody LF-hCG-6, beta-hCG (135-142) for LF-hCG-28 and beta-hCG (139-145) for LF-hCG-26. Based on the optimal length, parent peptide sequences [beta-hCG (125-133), (137-145) and (135-145)] were synthesized by conventional solid-phase procedures. In addition, based on the results with the replacement peptides, peptide derivatives were produced in which specific single improvements had been combined. The affinities of the monoclonal antibodies for the peptide derivatives, compared to the parent peptide sequences, were at least 700 times better for LF-hCG-6 reactive peptides (6.9 x 10(6) M-1 and < 10(4) M-1, respectively) and 130 times better for LF-hCG-26 reactive peptides (1.3 x 10(6) M-1 and < 10(4) M-1, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)