CIRCULAR-DICHROISM (CD) STUDIES OF ANTAGONISTS DERIVED FROM PARATHYROID HORMONE-RELATED PROTEIN

We have undertaken a study of the structure of antagonist peptides der ived from the parathyroid hormone-related protein (PTHrP) in the prese nce of amphiphiles using circular dichroism (CD). The results were use d to gain knowledge about bioactive conformations of the peptide when bound to a membrane. The substitutions within the PTHrP-(7-34)amide se quence resulted in differences in biological activity. Structural dete rmination by CD showed the presence of an alpha-helical structure. The antagonist activity was increased in constrained peptides in which i to (i + 4) side-chain to side-chain cyclization was used to form a lac tam, [Lys13,Asp17]PTHrP-(7-34)NH2. This peptide showed increased helic ity in the presence of a surfactant. Hydrophobic substitutions Leu and D-Trp at positions 11 (Lys) and 12 (Gly), respectively, in PTHrP-(7-3 4)NH2 resulted in increased potency, but the derivatives were not sign ificantly more helical than the unsubstituted peptide in the presence of surfactants. The combination of the hydrophobic substitutions with the constraint of lactam formation were mutually exclusive in terms of their biological activity and their alpha-helical content. We conclud e that hydrophobic substitutions contribute to an increase in binding affinity by increasing hydrophobic interactions which stabilize recept or-ligand complexes. Structural rigidification, on the other hand, inc reases the alpha-helical content, which is important for attaining a c onformation recognized by the receptor. (C) Munksgaard 1993.