Synthesis and conformation of constrained peptides with hypoglycaemic activity derived from human growth hormone.

The alpha-aminosuccinimide (Asu11) octapeptide analogue of human growth hormone hGH[6-13] (Leu6-Ser-Arg-Leu-Phe-Asu-Asn-Ala13) has been reported [Robson et al. (1990) Biol. Chem. Hoppe Seyler 371, 423-431] to have hypoglycaemic activity whilst the corresponding peptide with Asp at position 11 is inactive. In order to determine whether this change in activity is caused by conformational and/or stereo-electronic effects, the incorporation of two different isomeric gamma-lactam structures at position 11 has been investigated. One lactam structure (i) is of the type developed by Freidinger and coworkers [Freidinger et al. (1982) J. Org. Chem. 47, 102-107], whilst the isomeric gamma-lactam structure (ii) represents a new type of constrained synthon for use in peptide synthesis. The chiral type-ii gamma-lactam was synthesized via a suitably protected desoxodipeptide prepared in several ways from L-aspartic acid. The solution conformations of the [Asu11]- and the [gamma-lactam11]-containing hGH[6-13] peptide analogues were investigated with the aid of two-dimensional NMR (COSY and NOESY) spectroscopy. Conformational similarities were found for these hGH[6-13] peptide analogues. For example, for all peptide analogues studied, weak NOEs were evident between the Phe10 ring protons and protons of the amino acid residues at the C-terminus. Overall, however, the NOESY NMR spectra of the [Asu11]- and the [gamma-lactam11]-containing peptides related to hGH[6-13] suggest the presence of an extended structure in solution with a possible weak type II' beta-turn at position 11. The extent of conformational constraint introduced into these hGH[6-13] peptide analogues by substitution of the Asu11 residue with either isomeric gamma-lactam structure was reflected as differences in their hypoglycaemic activity. In particular, the hGH[6-13] peptide analogue derived from the new chiral type-ii gamma-lactam exhibits both lower activity in intravenous insulin tolerance tests in vivo and weaker NOEs than the isomeric hGH[6-13] peptide analogue derived from the type (i) gamma-lactam structure. The relative change in blood glucose levels from 20 to 90 min for the racemic (R,S)-form of the type-ii gamma-lactam compared to the control values was approximately half that of the (S)-stereoisomer.