The amphibians skin produces in quantity a large range of hormones, neuropeptides and defence peptides similar or next from those produce in the nervous central system, endoctrine gland and gastro intestinal tractus of mammalian. Some batracian heptapeptides have a hight affinity and bond-selectivity for the opioid receptors (membrane proteins). These affinity and selectivity are greater than those observed for endogenous opioid ligands.These heptapeptides extract from batracian skin present a peptidic sequence very different from these of endogenous opioid peptides (enkephalins, dynorphins and β endorphins isolated from the brain, the pituitary gland and the adrenal medulla).

Phyllomedusa sauvagei frog (skin source of dermorphin)

Specificity α  Dermenkephalin :  Tyr-D-Met-Phe-His-Leu-Met-Asp-NH2 Deltorphin I :  Tyr-D-Ala-Phe-Asp-Val-Val-Gly-NH2 Specificity μ Hybrid :  Tyr-D-Met-Phe-His-Tyr-Pro-Ser-NH2 Dermorphin :  Tyr-D-Ala-Phe-Gly-Tyr-Pro-Ser-NH2 Studied heptapeptides, extract from the amphibien skin (except the synthetized hybride), with their opioid receptors specificity.

In spite of nearby primary structures, these heptapeptides present a great μ or δ selectivity difference, whereas the morphin with a distinct molecular structure has no discriminant affinity to the three μ, δ and χ receptors in the nervous central system that induces numerous pharmacologic effects.
The heptapeptide opioids of amphibians all have the same N-terminal tripeptide, Tyr¹-D Ala² or D Met²-Phe³, which acts as a consensus binding motif for the μ and δ-opioid receptors. Their selectivity for the μ- (Dermorphin) or the δ-receptors (Deltorphin, Dermenkephalin) is the result of their C-terminal part.

• Study perspectives

  We have tested the exitence, in solution, of conformational populations which could induce the μ or δ specificity.

• NMR and molecular modeling

  From NOE (Nuclear Overhauser Effect) in NMR (Nuclear Magnetic Resonance) and with simulated annealing calculations performed on a Silicon Graphics we have shown that two opioids of δ-specificity (Dermenkephalin and Deltorphin-I) have bend structures with a great similar conformation for the N-terminal tetrapeptide segment (Naim et al 1998). So the specific factor of δ-receptors fixation is this segment-conformation. This conformation results of the head-to-tail interactions which allow δ-opioid peptides to bind selectively to the δ-opioid receptor of peptide
  The C-terminal part is also important for the N-terminal part conformation as the hybrid heptapeptide study has pointed out (cf  Figure N°1  on hybrid heptapeptide structure). This hybrid possess also a common N-terminal but no similar conformation with dermenkephalin (Riand et al, 1999). (cf  Figure N°2 on heptapeptide structure comparison).

• Dermorphin

  Comparison of dermorphin (powerfull analgesic with μ-selectivity) and dermenkephalin (δ-selectivity) shows this selectivity difference could be due to the C-terminal part with a different composition/conformation (Riand et al, 2002).
  The heptapeptide dermorphin was the first opioid peptide isolated from amphibian skin in 1981. It is also the first natural peptide containing a D-amino acid which has been isolated in a vertebrate animal.

• Cis-trans isomerism in dermorphin

  Most of the studies of dermorphin-like molecule are complicated by cis-trans isomerism equilibrium from the peptide bond Xaa-Pro (Tyr⁵-Pro⁶ in dermorphin), requiring the specific assignment of the whole observed interactions. The μ opioid heptapeptide dermorphin is under 70% of trans forms for the Tyr⁵-Pro⁶ peptide bond in solution (CDCl₃/DMSO-d₆ 1/1 vol/vol).(cf  Figure N°3).

• Structural and conformationnal requirements

  - The tripeptide Tyr-D-X-Phe (X=Ala, Met) of N-terminal part is essential to form the bond with the μ or δ receptors.
  Lastly the nature and the conformation of heptapeptides C-terminal part are closely connect to their μ or δ selectivity.
  - Two preponderant conformational families were obtained for all the studied peptides. (cf  Figure N°4: dermorphin example).
  - Both families in δ peptides corresponded only to folded structures, while the minor family in hybrid already showed some extended trends. Finally, about 40 % of extended forms are needed to explain the experimental data in this dermorphin study.
  - The μ opioids conformation in solution is very different from those of δ opioids, almost without interactions between head and tail. This is a consequence either of a strong bending in the N-terminal moiety or an extension of the C-terminal part of the molecule.

Trans isomer structure of hybride: DREK[1-4]-DRM[5-7].

Hybride and dermenkephalin structure comparison.

Tyr-Pro cis-trans isomerism in dermorphin.

Two preponderant conformational families of dermorphine in solution.

Bibliography