Conformational properties of deltorphin: new features of the delta-opioid receptor

Tailoring the nuclear Overhauser effect for the study of small and medium-sized molecules by solvent viscosity manipulation

The nuclear Overhauser effect (NOE) is a consequence of cross-relaxation between nuclear spins mediated by dipolar coupling. Its sensitivity to internuclear distances has made it an increasingly important tool for the determination of through-space atom proximity relationships within molecules of sizes ranging from the smallest systems to large biopolymers. With the support of sophisticated FT-NMR techniques, the NOE plays an essential role in structure elucidation, conformational and dynamic investigations in liquid-state NMR. The efficiency of magnetization transfer by the NOE depends on the molecular rotational correlation time, whose value depends on solution viscosity. The magnitude of the NOE between ¹H nuclei varies from +50% when molecular tumbling is fast to -100% when it is slow, the latter case corresponding to the spin diffusion limit. In an intermediate tumbling regime, the NOE may be vanishingly small. Increasing the viscosity of the solution increases the motional correlation time, and as a result, otherwise unobservable NOEs may be revealed and brought close to the spin diffusion limit. The goal of this review is to report the resolution of structural problems that benefited from the manipulation of the negative NOE by means of viscous solvents, including examples of molecular structure determination, conformation elucidation and mixture analysis (the ViscY method).

Conformational search of peptides and proteins: Monte Carlo minimization with an adaptive bias method applied to the heptapeptide deltorphin

The energy function of a protein consists of a tremendous number of minima. Locating the global energy minimum (GEM) structure, which corresponds approximately to the native structure, is a severe problem in global optimization. Recently we have proposed a conformational search technique based on the Monte Carlo minimization (MCM) method of Li and Scheraga, where trial dihedral angles are not selected at random within the range [-180 degrees,180 degrees ] (as with MCM) but with biased probabilities depending on the increased structure-energy correlations as the GEM is approached during the search. This method, called the Monte Carlo minimization with an adaptive bias (MCMAB), was applied initially to the pentapeptide Leu-enkephalin. Here we study its properties further by applying it to the larger peptide with bulky side chains, deltorphin (H-Tyr-D-Met-Phe-His-Leu-Met-Asp-NH(2)). We find that on average the number of energy minimizations required by MCMAB to locate the GEM for the first time is smaller by a factor of approximately three than the number required by MCM-in accord with results obtained for Leu-enkephalin.

Efficiency of the multicanonical simulation method as applied to peptides of increasing size: the heptapeptide deltorphin

The advantage of the multicanonical (MUCA) simulation method of Berg and coworkers over the conventional Metropolis method is in its ability to move a system effectively across energy barriers thereby providing results for a wide range of temperatures. However, a MUCA simulation is based on weights (related to the density of states) that should be determined prior to a production run and their calculation is not straightforward. To overcome this difficulty a procedure has been developed by Berg that calculates the MUCA weights automatically. In a previous article (Yaşar et al. J Comput Chem 2000, 14, 1251-1261) we extended this procedure to continuous systems and applied it successfully to the small pentapeptide Leu-enkephalin. To investigate the performance of the automated MUCA procedure for larger peptides, we apply it here to deltorphin, a linear heptapeptide with bulky side chains (H-Tyr(1)-D-Met(2)-Phe(3)-His(4)-Leu(5)-Met(6)-Asp(7)-NH(2)). As for Leu-enkephalin, deltorphin is modeled in vacuum by the potential energy function ECEPP. MUCA is found to perform well. A weak second peak is seen for the specific heat, which is given a special attention. By minimizing the energy of structures along the trajectory it is found that MUCA provides a good conformational coverage of the low energy region of the molecule. These latter results are compared with conformational coverage obtained by the Monte Carlo minimization method of Li and Scheraga.

Opioid peptides from frog skin

The skin of the South American frogs Phyllomedusa secretes, in addition to numerous mammalian-like hormones and neuropeptides, several gene-encoded opioid peptides that contain a D-amino acid in position 2 of their sequence. Dermorphin, Tyr-D-Ala-Phe-Gly-Tyr-Pro-Ser-NH2, dermenkephalin/deltorphin A, Tyr-D-Met-Phe-His-Leu-Met-Asp-NH2 and the deltorphins, Tyr-D-Ala-Phe-Xaa-Val-Val-Gly-NH2 (where Xaa is either Asp or Glu) are highly potent at, and exquisitely selective, for the mu- and delta-opioid receptors. D-Ala and D-Met present in dermorphin and related peptides are coded for by the usual codons in the corresponding messenger RNAs. Prepro-dermorphin/dermenkephalin and prepro-deltorphins have considerable sequence identities to precursors encoding 10-46-residue-long antimicrobial peptides--dermaseptins, brevinins, temporins, esculentins and gaegurins--originating from various amphibian species. The similarity between the prepro-regions of precursors encoding end products with strikingly different structures and biological activities supports the suggestion that the genes encoding these peptides are all members of the same family.

What peptides these deltorphins be

The deltorphins are a class of highly selective delta-opioid heptapeptides from the skin of the Amazonian frogs Phyllomedusa sauvagei and P. bicolor. The first of these fascinating peptides came to light in 1987 by cloning of the cDNA of from frog skins, while the other members of this family were identified either by cDNA or isolation of the peptides. The distinctive feature of deltorphins is the presence of a naturally occurring D-enantiomer at the second position in their common N-terminal sequence, Tyr-D-Xaa-Phe, comparable to dermorphin, which is the prototype of a group of mu-selective opioids from the same source. The D-amino acid and the anionic residues, either Glu or Asp, as well as their unique amino acid compositions are responsible for the remarkable biostability, high delta-receptor affinity, bioactivity and peptide conformation. This review summarizes a decade of research from many laboratories that defined which residues and substituents in the deltorphins interact with the delta-receptor and characterized pharmacological and physiological activities in vitro and in vivo. It begins with a historical description of the topic and presents general schema for the synthesis of peptide analogues of deltorphins A, B and C as a means to document the methods employed in producing a myriad of analogues. Structure activity studies of the peptides and their pharmacological activities in vitro are detailed in abundantly tabulated data. A brief compendium of the current level of knowledge of the delta-receptor assists the reader to appreciate the rationale for the design of these analogues. Discussion of the conformation of these peptides addresses how structure leads to further hypotheses regarding ligand receptor interaction. The review ends with a broad discussion of the potential applications of these peptides in clinical and therapeutic settings.

Solution conformations of deltorphin-I obtained from combined use of quantitative 2D-NMR and energy calculations: a comparison with dermenkephalin

Deltorphin-I, Tyr-D-Ala-Phe-Asp-Val-Val-Gly-NH2 and dermenkephalin, Tyr-D-Met-Phe-His-Leu-Met-Asp-NH2, two highly related opioid peptides from frog skin, display very similar N-termini but strikingly different C-terminal tails. Nevertheless, both peptides are highly potent at, and exquisitely selective for the delta-opioid receptor. To identify common determinants concuring to the remarkably efficient targeting of deltorphin-I and dermenkephalin, combined use of quantitative two-dimensional nuclear magnetic resonance (53 dipolar interactions studied at four temperatures) and energy calculations using simulated annealing generated five groups of deltorphin-I conformers. These groups were pooled into two families whose overall conformation could be described either by a left-handed helix (Family I) or by a big loop (Family II), both stabilized by H-bonds. Proximity of D-Ala2-Phe3-Asp4 and Val5-Val6-Gly7 triads is an obvious structural similarity between almost all groups in both families of structures. Whereas differences between the two families originated mostly from a transition at psi Asp4 backbone dihedral angle, the backbone structures at segment 1-4 are similar and spatial arrangements of Tyr1 (t) and Phe3 (g-) are identical in one group of each family. Moreover, these two groups have a N-terminal tetrapeptide whose conformation most closely resembles that of a well-defined group of structures for dermenkephalin. Altogether, these results suggest that conformational attributes that are common to dermenkephalin and deltorphin-I, i.e., the backbone conformation of the N-terminal tetrapeptide and preferential orientations in the side-chain of Tyr1 (t) and Phe3 (g-) underlie their ability to bind with high selectivity to the delta-opioid receptor.

Helix-inducing alpha-aminoisobutyric acid in opioid mimetic deltorphin C analogues

The achiral symmetric alpha-aminoisobutyric acid (Aib) replaced the critical N-terminal residues of the amphibian skin opioid deltorphin C (H-Tyr-D-Ala-Phe-Asp-Val-Val-Gly-NH2) without detriment to the physicochemical requirements for delta opioid receptor recognition. Substitutions by the alpha, alpha-dialkyl amino acid in place of D-Ala2 or Phe3, or both, exhibited high delta receptor affinity (Ki delta = 0.12-3.6 nM) and 5-9-fold greater selectivity (Ki mu/Ki delta = 5000-8500) than the parent compound. This is the first definitive demonstration that the D-chirality of alanine and the aromaticity of phenylalanine are replaceable by an achiral alpha, alpha-dialkylated residue without detrimental effects on ligand binding. Incorporation of the mono-alpha-alkyl amino acid L- or D-Ala at the third position also produced highly selective delta ligands (Ki mu/Ki delta = 2000-3500), albeit with reduced delta affinities (Ki delta = 6-15 nM). Replacement of the anionic residue Asp4 by Aib yielded an opioid peptide that fit two-site binding models for the delta receptor (eta = 0.763; P < 0.0001) and displayed dual high affinity for both delta and mu receptors, emphasizing the repulsive effect by a negative charge at mu receptor sites and the insignificance of Asp for delta affinity. Molecular dynamics conformation analyses suggested that Aib residues caused distinct changes in deltorphin C secondary structure when substituted for D-Ala2, Asp4, and simultaneously D-Ala2 and Phe3 but not when substituted for Phe3. These conformational changes might be critical factors for the proper orientation of reactive constituents of residues in the N-terminal region of deltorphin C. Disparities between binding data and functional bioassays of [Aib3] indicated that Phe3 was required for bioactivity in mouse vas deferens but not for interaction with delta opioid receptors in rat brain membranes.

Structural modifications of the N-terminal tetrapeptide segment of [D-Ala2]deltorphin I: effects on opioid receptor affinities and activities in vitro and on antinociceptive potency

A series of deltorphin I analogs containing D- or L-N-methylalanine (MeAla), D- or L-proline (Pro), alpha-aminoisobutyric acid (Aib), sarcosine (Sar) or D-tert-leucine (Tle) in place of D-Ala2, or phenylalanine in place of Tyr1, was synthesized. The opioid activity profiles of these peptides were determined in mu and delta opioid receptor-representative binding assays and bioassays in vitro as well as in the rat tail flick test in vivo. In comparison with the deltorphin I parent, both the L- and the D-MeAla2-analog were slightly more potent delta agonists in the mouse vas deferens (MDV) assay, and the D-MeAla2-analog showed two-fold higher antinociceptive potency in the analgesic test. In view of the fact that deltorphin analogs with an unsubstituted L-amino acid residue in the 2-position generally lack opioid activity, the observed high delta opioid potency of [L-MeAla2]deltorphin I is postulated to be due to the demonstrated presence of a conformer with a cis Tyr1-MeAla2 peptide bond, since the cis conformer allows for a spatial arrangement of the pharmacophoric moieties in the N-terminal tripeptide segment similar to that in active deltorphin analogs containing a D-amino acid residue in the 2-position. Substitution of Aib in the 2-position led to a compound, H-Tyr-Aib-Phe-Asp-Val-Val-Gly-NH2, which displayed lower delta receptor affinity than the parent peptide but higher delta selectivity and, surprisingly, three times higher antinociceptive potency. The D- and L-Pro2-, Sar2- and D-Tle2-analogs showed much reduced delta receptor affinities and were inactive in the tail flick test. Replacement of Tyr1 in deltorphin I with Phe produced a 32-fold decrease in delta receptor affinity but only a 7-fold drop in antinociceptive potency.

Folded conformations of the delta-selective opioid dermenkephalin with head-to-tail interactions. A simulated annealing study through NMR restraints

Despite similar tripeptide N-termini, dermorphin (Tyr-D-Ala-Phe-Gly-Tyr-Pro-Ser-NH2) and dermenkephalin (Tyr-D-Met-Phe-His-Leu-Met-Asp-NH2), naturally occuring opioid peptides from frog skin, exhibit high affinity but contrasting selectivity for the mu- and delta-opioid receptors, respectively. Structure-activity relationship studies have shown that the N-terminal tripeptide, Tyr-D-Xaa-Phe (where Xaa is either Ala or Met), is necessary for binding with both the mu- and delta-receptors while the nature and/or the conformation of the C-terminus His-Leu-Met-Asp-NH2 of dermenkephalin are responsible for addressing the peptide to the delta-receptor. In order to examine the conformational characteristics that are related to the selectivity of dermenkephalin towards the delta-receptor, 50 NOE restraints (10 between non-adjacent residues), and 7 dihedral angles, derived from a two-dimensional 1H-NMR study of dermenkephalin in dimethyl sulfoxide, were used in simulated annealing and energy minimization procedures. Twenty-four resulting conformers (60% of the generated structures) with no severe distance restraint violation were pooled into seven groups and three related families. These 24 conformers show close proximity between the two methionine residues, S-shaped structures, mean planes of N-terminal and C-terminal moieties almost at right angles to each other, a C-terminus region above the plane of the N-terminal region and g- as preferential orientation in the side chain of Phe. Aside these similarities, families of conformers differ by the preferential orientation in the side chain of Tyr (t or g-) and proximity between Tyr and Asp, or Tyr and the C-terminus. In contrast to previous models, practically no beta-turn structures exist for dermenkephalin, most of the NH hydrogen bonds participating to gamma-turns. The possible relationship between the conformational characteristics of dermenkephalin and the delta-opioid receptor selectivity is discussed.

Design and synthesis of 1-aminocycloalkane-1-carboxylic acid-substituted deltorphin analogues: unique delta and mu opioid activity in modified peptides

Deltorphin analogues were substituted by a series of achiral C alpha,alpha-dialkyl cyclic alpha-amino acids (1-aminocycloalkane-1-carboxylic acids, Ac chi c, where chi = a hexane, pentane, or propane cycloalkane ring) in position 2, 3, 4, or 2 and 3 in deltorphin C, and in position 2 in [Ac6c2,-des-Phe3]deltorphin C hexapeptide. Receptor assays indicated that even though Ac6c2 and Ac6c3 exhibited a diminished Ki delta by ca. 20-fold (2.5-3.3 nM) relative to deltorphin C (Ki delta = 0.15 nM), selectivity was marginally elevated (Ki mu/Ki delta = 1250) or enhanced by about 70%, and both peptides fitted stringent iterative calculations for a two-site binding model (eta = 0.625 and 0.766, respectively, P < 0.0001). The disubstituted [Ac6c2,3]- or [Ac6c2,des-Phe3]deltorphin analogues yielded peptides with decreased Ki delta, such that the latter peptide was essentially inactive. The presence of Ac5c or Ac3c in place of Phe3 further diminished Ki delta (15.4 to 19.0 nM), yet delta selectivity only fell about one-half (Ki mu/Ki delta = 440 and 535, respectively), and only the former peptide fitted a two-site binding model (eta = 0.799). The replacement of Asp4 by Ac6c, Ac5c, or Ac3c produced essentially nonselective analogues through the acquisition of high mu affinities (2.5, 0.58 and 0.27 nM, respectively) while maintaining high delta affinities (Ki delta = 0.045-0.054 nM) which were about 3-fold greater than that of deltorphin C. Using pharmacological assays in vitro (mouse vas deferens and guinea pig ileum), position 3-substituted analogues all indicated substantial losses in bioactivity, whereas substitution by 1-aminocycloalkanes at the fourth position retained high delta activity. In fact, the bioactivity of [Ac3c4]deltorphin C indicated a peptide with relatively weak delta selectivity, which was comparable to the observations with the receptor binding data. In summary, the data confirmed that (i) delta selectivity occurs in the absence of D-chirality at position 2, (ii) the aromaticity of Phe3 is replaceable by an achiral residue with a hydrophobic ring-saturated side chain, and (iii) the acquisition of dual high-affinity analogues occurs through the elimination of the anionic function at position 4 and replacement by an amino acid with a hydrophobic side chain.

Differential effects of deltorphin on arginine and galanin-induced growth hormone secretion in healthy man

Recently we demonstrated the inhibitory action on Growth Hormone (GH) secretion of an opioid heptapeptide, deltorphin (DT), that is highly selective in binding delta-opioid receptors. To investigate the possible mechanism leading to the decrease in GH secretion by specific activation of delta-opioidergic pathway in man, we compared, in normal subjects, the effect of DT on GH secretion responses to two different GH secretagogues, namely arginine (ARG) and galanin (GAL). DT completely blunted the GH response to ARG, whereas it attenuated the GH response to GAL, but not at a statistically significant level. We suggest that the specific activation of delta-opioid receptors in man may exert an inhibitory influence on GH secretion principally by modulating endogenous hypothalamic somatostatin (SRIH) release.

Involvement of mu-opioid receptors in the modulation of pituitary-adrenal axis in normal and stressed rats

The availability of the most selective, high-affinity, natural opioid agonists for mu-receptors (dermorphin-DM) and delta-receptors (deltorphin-DT) has provided the possibility for in vivo studying of the role of acute and chronic activation of mu- and delta-opioid receptors on the functional activity of the hypothalamus-pituitary-adrenocortical (HPA) axis, both in basal conditions and in response to an acute stress in adult male rats. Plasma corticosterone (CS) and beta-endorphin-like-immunoreactivity (beta-EP-LI) levels were measured by specific radioimmunoassays before and after 5 and 30 minutes from the exposure to cold (3 +/- 0.5 C) water and forcing them to swim for 10 minutes (acute cold swimming stress). Acute administration of DM, the specific mu-receptor agonist, enhanced basal and stress induced plasma levels of CS and beta-EP-LI. These effects were antagonized by pretreatment with naloxone, specific mu-opioid receptor antagonist, but not by naltrindole, a delta-opioid receptor antagonist. Long-term administration of DM did not alter resting plasma levels of CS and beta-EP-LI, but significantly reduced stress-induced increase of these hormones. Both the acute and chronic administration of the DT, highly selective delta-opioid receptors agonist, failed to modify resting and stress induced hormone levels. Our present data show that DM throughout mu-opioid receptors, but not DT, modulates the response of HPA axis to acute stress in rats, increasing or decreasing the release of CS and beta-EP-LI when acutely or chronically administered, respectively.

Comparative conformational analyses of mu-selective dermorphin and delta-selective deltorphin-II in aqueous solution by 1H-NMR spectroscopy

Two-dimensional 1H-NMR methods have been used to obtain complete proton resonance assignments and possible solution conformations of dermorphin (H-Tyr-D-Ala-Phe-Gly-Tyr-Pro-Ser-NH2) and deltorphin-II (H-Tyr-D-Ala-Phe-Glu-Val-Val-Gly-NH2), naturally occurring mu- and delta-selective opioids, respectively, in order to examine the conformational characteristics that are closely related to the selectivities towards mu/delta-opioid receptors. With the use of the proton-proton distances derived from ROESY measurements in aqueous solution, 50 possible 3D structures are generated by means of distance geometry calculations. The conformers which satisfy the distance constraints and the torsion angles estimated from JNHC alpha H vicinal coupling constants within the allowable range are then subjected to molecular dynamics simulations for 10 ps after equilibration. Although dermorphin and deltorphin-II are both in equilibrium among many flexible conformers, some conformational differences are observed between these peptides: many conformers of dermorphin show a structure rounded at the N-terminal Tyr-D-Ala-Phe-Gly-Tyr and C-terminal Gly-Tyr-Pro-Ser-NH2 moieties, which are almost at right angles to each other, while those of deltorphin-II are characterized by a 'hook'-shaped backbone structure in which the nearly extended conformation of the Val-Val-Gly-NH2 sequence is located under the folded conformation of the N-terminal Tyr-D-Ala-Phe-Glu sequence. The possible relationship between these conformational characteristics and the mu/delta-opioid receptor selectivities is discussed.

Conversion of enkephalin and dermorphin into delta-selective opioid antagonists by single-residue substitution

The properties of di- and tri-peptides containing 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid (Tic) in second position suggest that the message domain of opioid peptides can be composed of only two residues [Temussi, P. A., Salvadori, S., Amodeo, P., Guerrini, R., Tomatis, R., Lazarus, L. H., Picone, D. & Tancredi, T. (1994) Biochem. Biophys. Res. Commun. 198, 933-939]. As a crucial test of the possibility that the Tyr-Tic segment be a message domain in longer peptide sequences, we have inserted it in the sequences of two typical opioid peptides: [Leu]enkephalin, a non-selective agonist, and dermorphin, a selective mu agonist. Here we report the synthesis and biological activity of [L-Tic2]enkephalin, [L-Tic2]dermorphin, [L-Tic2]dermorphin carboxylic acid and [D-Tic2]dermorphin: all [L-Tic2]peptides were converted from agonists to delta-selective antagonists. The NMR conformational study in a dimethylsulfoxide/water cryoprotective mixture at low temperature shows diagnostic side-chain--side-chain NOEs in the spectra of all [L-Tic2]peptides and hints that the 90 degrees arrangement of the the two aromatic rings found in the cis-Tyr-L-Tic moiety, typical of N-methyl naltrindole and other delta-selective opiate antagonists, is responsible for the antagonist activity of all these peptides.

Quantitative two-dimensional NMR study of dermenkephalin, a highly potent and selective delta-opioid peptide

Dermenkephalin, H-Tyr-(D)Met-Phe-His-Leu-Met-Asp-NH2, a highly potent and selective delta-opioid peptide isolated from frog skin, was studied in DMSO-d6 solution by two-dimensional nmr spectroscopy, including the determination of NH temperature coefficients, the evaluation of 3J coupling constants from phase-sensitive correlated spectroscopy (COSY) and the volumes of nuclear Overhauser effect (NOE) correlations. The two-dimensional NOE spectroscopy (NOESY) spectrum of dermenkephalin revealed sequential, medium-, and long-range effects. To put this information on a quantitative basis, special attention was devoted to J cross-peak suppression, quantification of the NOE volumes and analysis of the overlaps, normalization of the NOEs against diagonal peaks and H beta beta' geminal interactions. Although most of the dihedral angles deduced from the 3JN alpha coupling constants together with several Ni alpha i and alpha iNi+1 NOEs pointed to a partially extended peptide backbone, several NiNi+1 NOEs and beta iNi+1 interactions argued in favor of a folded structure. Moreover, several long-range correlations of strong intensities were found that supported a close spatial proximity between the side chains of D-Met2 and Met6, Tyr1 and His4, Tyr1 and Asp7, and His4 and the C-terminal amide group. In Phe, the g- rotamer in the side chain is deduced from the 3J alpha beta coupling constants and alpha beta and N beta NOE correlations. Whereas the amide proton dependency was not indicative of stable hydrogen bonds, the nonuniform values of the temperature coefficient may reflect an equilibrium mixture of folded and extended conformers. The overall data should provide realistic starting models for energy minimization and modelization studies.

Investigation of the structural parameters involved in the delta-opioid selectivity of several families of opioid peptides

Three series of highly delta-opioid selective peptides are now available, and each family is used as template to investigate the structural parameters involved in delta-receptor recognition and in the modulation of the selectivity of the parent peptide. The first series includes cyclic derivatives such as Tyr-D-Pen-Gly-Phe-D-Pen(DPDPE) and Tyr-D-Pen-Gly-Phe-Pen(DPLPE); the second are the synthetic linear constrained peptides [Tyr-D-Ser(OtBu)-Gly-Phe-Leu-Thr(DSTBULET), Tyr-D-Ser(OtBu)-Gly-Phe-Leu-Thr (OtBu)(BUBU) and especially Tyr-D-Cys(StBu)-Gly-Phe-Leu-Thr(OtBu) (BUBUC)] and the last one the natural peptides [Tyr-D-Met-Phe-His-Leu-Met-Asp-NH2 (deltorphin or dermenkephalin) and Tyr-D-Ala-Phe-Asp-Val-Val-GlyNH2 ([D-Ala2] deltorphin I)]. In the present study, the possibly of transposing some of the decisive factors of delta-selectivity evidenced in the two other families, to the linear constrained peptides series was examined. With this aim in view, residues such as Phe3, pClPhe4 or Asp were introduced in the sequence of DSTBULET, BUBU or BUBUC. Direct comparison between the biochemical profiles of the [pClPhe4] analogs of the linear constrained peptides and their parent compounds shows that the addition of an electronegative atom on the Phe4 residue of enkephalin sequences is not an absolute parameter for delta-selectivity improvement. The hydrophobic delta-receptor subsite seems able to receive a range of molecular volumes and electronegativities. By contrast, this subsite cannot interact with a Phe3 aromatic ring introduced in this series of peptides. Moreover, the results obtained with linear peptides including additional negatively charged residues demonstrate that the proposed location of the delta-receptors in a cationic membrane environment is not adequate to explain the selectivity profile of a number of compounds.

Conformation of deltorphin-II in membrane environment studied by two-dimensional NMR spectroscopy and molecular dynamics calculations

Two-dimensional homonuclear Hartmann-Hahn spectroscopy and NOESY (nuclear Overhauser effect and exchange spectroscopy) 1H-NMR techniques have been used to obtain complete proton resonance assignments and to perform a conformational investigation of deltorphin-II (Tyr-D-Ala-Phe-Glu-Val-Val-Gly-NH2), a naturally occurring delta-selective opioid peptide, in the membrane-mimetic micelles of perdeuterated dodecylphosphocholine. This was done in order to examine conformational characteristics that would be closely related to the selectivity towards the delta-opioid receptor. With the use of the proton-proton distances derived from NOESY measurements, 50 possible three-dimensional structures were generated by means of distance-geometry calculations, and 25 of them were subjected to the molecular-dynamics simulations of 10 ps, which were energetically constrained for the NOE interproton distances. Most of the possible conformers simulated showed a common feature such that the conformation of deltorphin-II is characterized by the S-shaped back-bone structure in which the turn conformation of the Val-Val-Gly-NH2 sequence is located under the helically folded conformation of the N-terminal Tyr-D-Ala-Phe-Glu sequence. The possible relationship between this conformational characteristic and the delta-opioid-receptor selectivity has been discussed.

Interaction of deltorphin with opioid receptors: molecular determinants for affinity and selectivity

Opioid receptor analyses of deltorphin A (H-Tyr-D-Met-Phe-His-Leu-Met-Asp-NH2) analogues indicated the following: (a) increased negativity differentially affected affinities (Ki) and selectivity (Ki mu/Ki delta); (b) shifted sequence heptapeptides, [Asp5,Leu6,Met-NH2(7)] and [Asp4,His5,Leu6,Met-NH2(7)], reversed selectivity (delta-->mu); (c) substitutions at positions 4, 5, and 6 diminished selectivity, with changes in residue 5 being the most detrimental; (d) C-terminal deletions differentially effected Ki. These are the first data to demonstrate a reversal of delta selectivity in heptapeptides containing a negative charge and indicate that modifications in affinity occur through changes in both anionic and hydrophobic properties of residues at specific positions in the peptide. Deltorphin analogues might also be applied to differentiate between opioid receptor subsites.

Opioid activity of dermenkephalin analogues in the guinea-pig myenteric plexus and the hamster vas deferens

1. To elucidate the structural features required for selective and potent action of dermenkephalin at the delta-opioid receptor, a series of analogues of dermenkephalin and dermorphin were tested for their effectiveness in depressing electrically-evoked contractions of the vas deferens of the hamster (delta-opioid receptors) and the guinea-pig myenteric plexus-longitudinal muscle preparation (mu- and kappa-opioid receptors). 2. Dermenkephalin was more selective and more potent at delta-receptors than the delta-ligand [D-Pen2, D-Pen5]-enkephalin. The responses to dermenkephalin in the hamster vas deferens were increased by addition of peptidase inhibitors; the maximum effect was obtained with 3 microM thiorphan. 3. [L-Met2]-dermenkephalin had 0.2% and [L-Ala2]-dermorphin 0.01% of the agonist activity of the corresponding endogenous peptides which have D-amino acids in position 2. The pharmacological activity of these analogues was unaffected by inhibition of peptidases. This emphasizes the role that the D-configuration plays in determining the bioactive folding of these highly active peptides. 4. Dermenkephalin-(1-6)-NH2 was more potent at delta-receptors than at mu-receptors whereas, dermenkephalin-(1-4)-NH2 is a selective mu-agonist, having no activity at delta-receptors. 5. Substitution of the C-terminal tripeptide of dermorphin with the C-terminal tripeptide of dermenkephalin abolished the mu-receptor preference of dermorphin. The resulting hybrid peptide, Tyr-D-Ala-Phe-Gly-Leu-Met-Asp-NH2 was as potent as dermenkephalin at delta-receptors. A shift towards a preference for delta-receptors was obtained when the C-terminal tetrapeptide of dermorphin was replaced by the C-terminal tetrapeptide of dermenkephalin. 6. Substitution of Asp by Asn in position 7 of dermenkephalin caused an increase in mu-receptor potency and a decrease in delta-receptor potency, resulting in a 20 fold decrease in mu-receptor selectivity. Dermenkephalin-(1-6)-NH2 and [Asn7]-dermenkephalin have almost identical delta-receptor agonist potencies and ratios of IC50 in the myenteric plexus to IC50 in the hamster vas deferens. 7. The results obtained emphasise the importance of a negative charge at the C-terminus of dermenkephalin for selectivity at the delta-opioid receptor. Furthermore, the hydrophobic residues Leu5 and Met6 may be critical in ensuring tight binding to the receptor which results in high agonist potency.

Opioid receptor selectivity reversal in deltorphin tetrapeptide analogues

Deltorphin N-terminal tetrapeptides [DEL A: H-Tyr-D-Met-Phe-His-R, where R = -NH2, -NH-NH2, -OCH3, -OH, -NH-NH-CO-R' (R' = -CH3 or adamantane); DEL C: H-Tyr-D-Ala-Asp-R (R = -OH, -NHCH3)], were used in a receptor binding assay with [3H]DADLE and [3H]DPDPE for delta sites, and [3H]DAGO for mu sites; tetrapeptide Ki delta values were similar with either [3H]-delta ligand. DEL A tetrapeptides C-terminally substituted with -NH2, -NH-NH2, -OCH3, and -OH had 10 to greater than 1,000-fold decreased Ki delta values, while Ki mu increased 5 to 100-fold to yield mu selectivity. C-Terminal substitution with -NH-NH2 and -OCH3 conferred highest mu selectivities; adamantyl and acetyl hydrazide derivatives were non-selective. DEL-(1-4)-OH peptides had decreased delta and mu affinities: DEL A-[Asp4]-(1-4)-OH and DEL C-(1-4)-OH had low affinities (greater than 1 microM), however, the Ki delta of the former was 5-fold greater than the latter, and the Ki mu was less by 15-fold. The data suggest that the "message" domain of DEL exhibits receptor selectivity different from that of the heptapeptide.

New insights on mu/delta selectivity of opioid peptides: conformational analysis of deltorphin analogues

The message domain of dermorphin (Tyr-D-Ala-Phe), a natural mu-opioid heptapeptide, has long been considered the main cause of the high mu selectivity of this peptide and of its analogues. The recent discovery, in the skin of Phyllomedusa sauvagei (i.e., the same natural source of dermorphin) and of Phyllomedusa bicolor of deltorphins, challenges this belief. Deltorphins, in fact, are three heptapeptides characterized by a message domain typical of mu-selective peptides, but endowed of an extremely high delta selectivity, the highest of all natural opioid peptides. A conformational analysis of dermorphin and deltorphins, based on nmr studies in DMSO and cryoprotective mixtures and internal energy calculations, showed that the enormous differences in receptor selectivity can be interpreted on the basis of receptor models for mu and delta opioids that recognize the same beta-turn in the N-terminal part, but discriminate for the conformation and polarity of the C-terminal part. Here we present the synthesis, biological activity, and conformational analysis in solution of three deltorphin analogues with very similar constitution, but with different net charge, different location of negative residues, or even without negative residues, which confirm these hypotheses and show that His4 can play a specific structural role.

Delta opioid receptor-selective ligands: [D-Pen2,D-Pen5]enkephalin-dermenkephalin chimeric peptides

A number of DPDPE-dermenkephalin chimeric peptides have been synthesized in which the putative C-terminal delta-address of dermenkephalin has been linked to the highly delta opioid selective cyclic peptide [D-Pen2,D-Pen5]enkephalin (DPDPE). Asp, Met-Asp and Leu-Met-Asp have been added to the C-terminus of DPDPE and both the carboxyl terminal and the carboxamide terminal series have been prepared. The bioassays using the mouse vas deferens and guinea pig ileum preparations have revealed a steady decrease in potency (compared to DPDPE) at delta and mu receptors as the dermenkephalin sequences were added. Some of the analogues, however, retained high delta selectivity. Similar results were obtained using radioligand binding assays. These findings suggest that the C-terminal amino acid sequence of dermenkephalin plays a role of delta-address which is specific to dermenkephalin itself, and is not additive with another delta selective ligand such as DPDPE.

Conformational preferences of [Leu5]enkephalin in biomimetic media. Investigation by 1H NMR

The conformation of [Leu5]enkephalin has been studied by 1H-NMR spectroscopy in media more like the actual environment in which the agonist-receptor interaction takes place than water, i.e. in three cryoprotective mixtures (dimethylformamide/water, methanol/water and ethylene glycol/water), in aqueous SDS and in two neat solvents, dimethylformamide and acetonitrile, whose dielectric constants (36.7 and 37.5) are intermediate between that of water and that of the lipid phase. In all cases examined, contrary to the studies in water or dimethylsulfoxide, we were able to detect numerous nuclear Overhauser effects, indicating that the media employed favour well-defined structures and/or reduce the internal motions of the peptide. Data from both organic solvents and cryoprotective mixtures suggest a 4----1 beta turn as the most probable structure of [Leu5]enkephalin in solution, whereas in SDS/H2O micelles the structural picture appears completely different, suggesting the presence of a 5----2 beta turn. The existence of two different preferred conformations of enkephalins may possibly be related to their ability to be effective towards both mu and delta opioid receptors.