Circular dichroism study of the carbohydrate-modified opioid peptides

Novel glycosylated [Lys7]-dermorphin analogues: synthesis, biological activity and conformational investigations

Syntheses of the [Lys(7)]- and [Hyp(6),Lys(7)]-dermorphin analogues in which either Tyr(5) or Hyp(6) are O-glucosylated are described. For comparison, the carbohydrate-free peptides have also been prepared. Structural investigations by FT-IR and CD measurements were carried out on the synthetic analogues and some preliminary pharmacological experiments were also performed. The biological potency of the glucosylated analogues was compared with that of the micro-opioid receptor agonist dermorphin in GPI preparations. Glucosylation of either Tyr(5) or Hyp(6) reduces the potency of both [Lys(7)]-dermorphin and [Hyp(6),Lys(7)]-dermorphin. The effect induced by the Tyr(5) glucosylation is quite strong and the potency of both peptides is reduced by about 150 times. A similar but less dramatic effect is induced by the glucosylation of the Hyp(6) residue, and the potency of the parent peptide is reduced by about 15 times. The presence of acetyl groups on the sugar hydroxyl functions further reduces the agonistic potency of the glucosylated analogues. The analgesic potency of [Hyp(6),Lys(7)]-, [Hyp(betaGlc)(6),Lys(7)]- and [Tyr(betaGlc)(5),Lys(7)]-dermorphin were also tested in vivo by the tail-flick test. The glucosylated hydroxyproline-containing analogue is 8-10 times less active than the parent peptide, but its analgesic effect lasts significantly longer.

Analogs of Insulin-like Peptide 3 (INSL3) B-chain Are LGR8 Antagonists in Vitro and in Vivo

Insulin-like peptide 3 (INSL3) is a member of the insulin superfamily that plays an important role in mediating testes descent during fetal development. More recently, it has also been demonstrated to initiate oocyte maturation and suppress male germ cell apoptosis. These actions are mediated via a specific G-protein-coupled receptor, LGR8. Little is known regarding the structure and function relationship of INSL3, although it is believed that the principal receptor binding site resides within its B-chain. We subsequently observed that the linear B-chain alone (INSL3B-(1-31)) bound to LGR8 and was able to antagonise INSL3 stimulated cAMP accumulation in HEK-293T cells expressing LGR8. Sequentially N- and C-terminally shortened linear analogs were prepared by solid phase synthesis and subsequent assay showed that the minimum length required for binding was residues 11-27. It was also observed that increased binding affinity correlated with a corresponding increase in alpha-helical content as measured by circular dichroism spectroscopy. Molecular modeling studies suggested that judicious placement of a conformational constraint within this peptide would increase its alpha-helix content and result in increased structural similarity to the B-chain within native INSL3. Consequently, intramolecularly disulfide-linked analogs of the B-chain showed a potentiation of INSL3 antagonistic activity, as well as exhibiting increased proteolytic stability, as assessed in rat serum in vitro. Administration of one of these peptides into the testes of rats resulted in a substantial decrease in testis weight probably due to the inhibition of germ cell survival, suggesting that INSL3 antagonists may have potential as novel contraceptive agents.

Conformationally constrained single-chain peptide mimics of relaxin B-chain secondary structure

Relaxin is a member of the insulin superfamily and has many biological actions including angiogenesis and collagen degradation. It is a 6 kDa peptide hormone consisting of two peptide chains (A and B) tethered by two disulphide bonds. Past structure-function relationship studies have shown the key receptor binding site of relaxin to be principally situated within the B-chain alpha-helix. Molecular dynamic simulations were performed to aid the design of conformationally constrained relaxin B-chain analogues that possess alpha-helical structure and relaxin-like activity. Restraints included disulphide bonds, both single and double, and lactam bonds. Each peptide was prepared by solid phase synthesis and, following purification, subjected to detailed conformational analysis by circular dichroism spectroscopy. Of 15 prepared relaxin B-chain mimetics, one was able to mimic the secondary structure of the native ligand as indicated by biomolecular recognition/interaction analysis using surface enhanced laser desorption ionization mass spectroscopy together with a relaxin antibody. However, none of the mimetics possess characteristic relaxin-like biological activity which strongly indicates that the pharmacophore comprises additional structural elements other than the relaxin B-chain alpha-helix. These findings will assist in the design and preparation of novel relaxin agonists and antagonists.

O-Linked glycopeptides retain helicity in water

A 17-residue O-linked glycopeptide model incorporating a central alpha-mannosyl serine residue, and its unglycosylated analog both demonstrate substantial helicity in water. The peptide sequence was derived from previous studies in which differences in overall helicity as a function of single amino acid substitutions were measured by circular dichroism (CD). The helical content was predicted by molecular modeling, and confirmed by CD and NMR. Moreover, the glycopeptide retained its helicity in the presence of SDS micelles, whereas the native peptide lost secondary structure in the presence of micelles. The inference is that the peptide sequence is a more important helix determinant than glycosylation per se.