Delta opioid receptor selectivity induced by conformational constraints in linear enkephalin-related peptides: 1H 400-MHz NMR study and theoretical calculations

Introduction into the structure of the linear hexapeptide DSLET (Tyr-D-Ser-Gly-Phe-Leu-Thr) or DTLET (Tyr-D-Thr-Gly-Phe-Leu-Thr) of tert-butyl groups as constraints different from cyclization leads to a large increase in the selectivity for delta opioid binding site in the case of DSTBULET [Tyr-D-Ser-(OtBu)-Gly-Phe-Leu-Thr] (Ki delta = 6.14 nM; Ki mu = 374 nM) and BUBU [Tyr-D-Ser(OtBu)-Gly-Phe-Leu-Thr(OtBu)] (Ki delta = 4.68 nM; Ki mu = 475 nM) or a loss of affinity for DTTBULET [Tyr-D-Thr(OtBu)-Gly-Phe-Leu-Thr] (Ki delta = 866 nM; Ki mu = 4500 nM). This puzzling behavior is studied here by 400-MHz 1H NMR spectroscopy in DMSO-d6 solution and by theoretical calculations. When DSLET and DTLET are compared, the reduction in energetically accessible phi and psi angles induced by the tert-butyl group in the D-Ser2 residue decreases the degree of freedom in the N-terminal part of the peptides. For DSTBULET and BUBU, the rigidification of the backbone evidenced by the appearance of the large NOE's of Phe4 NH-Gly3 alpha and Gly3 NH-alpha and by the loss of the C7 folding around the D-Ser2 residue found in DSLET could explain the drastic loss of affinity for mu opioid receptors. In DTTBULET, a large change in the spatial orientation around the D-Thr2 (OtBu) residue forces the aromatic rings far from each other.(ABSTRACT TRUNCATED AT 250 WORDS)     

The regulation of steroidogenesis by opioid peptides in porcine theca cells

The present study was designed to investigate basal and LH-induced steroidogenesis in porcine theca cells from large follicles in response to various concentrations (1-1000 nM) of mu opioid receptor agonists (beta-endorphin, DAMGO, FK 33-824), delta receptor agonists (met-enkephalin, leu-enkephalin, DPLPE) and kappa receptor agonists (dynorphin A, dynorphin B, U 50488). Agonists of mu opioid receptors suppressed basal androstenedione (A4), testosterone (T) and oestradiol-17beta (E2) secretion and enhanced LH-induced A4 and T release by theca cells. The inhibitory effect of the agonists on E2 secretion was abolished in the presence of LH. All delta receptor agonists depressed basal progesterone (P4) output. However, the influence of these agents on LH-treated cells was negligible. Among delta receptor agonist used only leu-enkephalin and DPLPE at the lowest concentrations inhibited basal A4 release. The presence of LH in culture media changed the influence of these opioids from inhibitory to stimulatory. Similarly, DPLPE reduced T secretion by non-stimulated theca cells and enhanced T secretion of stimulated cells. All of delta agonists inhibited basal E2 secretion and unaffected its release from LH-treated theca cells. Agonists of kappa receptors inhibited basal, non-stimulated, P4 secretion and two of them (dynorphin B, U 50488) potentiated LH-induced P4 output. Basal A4 and T release remained unaffected by kappa agonist treatment, but the cells cultured in the presence of LH generally increased both androgen production in response to these opioids. Basal secretion of E2 was also suppressed by kappa agonists. This inhibitory effect was not observed when the cells were additionally treated with LH. In view of these findings we suggest that opioid peptides derived from three major opioid precursors may directly participate in the regulation of porcine theca cell steroidogenesis.     

Differences in Binding Properties of μ and δ Opioid Receptor Subtypes from Rat Brain: Kinetic Analysis and Effects of Ions and Nucleotides

Differences in binding properties of mu and delta opioid receptors were investigated using DAGO (Tyr-D-Ala-Gly-MePhe-Gly-ol) and DTLET (Tyr-D-Thr-Gly-Phe-Leu-Thr), which occur, respectively, as the most selective mu and delta radioligands available. At high concentration, each agonist is able to interact with its nonspecific sites. Competition experiments indicated that a two-site competitive model was adequate to explain the interactions of DAGO and DTLET with [3H]DTLET and [3H]DAGO binding sites, respectively. The weak cross-reactivity (congruent to 10%) of DTLET for mu sites was taken into account in these experiments. On the other hand, DAGO and DTLET exhibit differential binding kinetics. Thus, at 35 degrees C, the lifetime of DTLET within its receptor site is about 14 times longer than that of the mu agonist. Sodium and manganese ions decrease the maximal number of high affinity mu and delta sites, but the sensitivity of mu receptors is three times higher towards Na+ and 20-fold higher towards Mn2+ than that of delta receptors. GTP reduces similarly the mu and delta binding whereas only the DAGO binding was modified by the nonhydrolyzable analogue guanylylimidodiphosphate [GMP-P(NH)P]. However, in the presence of Na+ ions, GMP-P(NH)P inhibits the DTLET binding in a concentration-dependent manner. The effects of Na+ and GMP-P(NH)P could be explained by a sequential transformation of delta receptors to low-affinity states. This model predicts that Na+, by lowering the affinity of a fraction of sites, produces a decrease in the maximal number of high-affinity delta receptors and that GMP-P(NH)P enhances the Na+ effect. Moreover, the binding kinetic to this high-affinity state was also modified by Na+ and nucleotides. All of these data support the existence of two independent mu and delta binding sites, the properties of which are differentially regulated by these endogenous effectors.     

Synthesis and evaluation of derivatives of leucine enkephalin as potential affinity labels for delta opioid receptors

As part of an effort to develop peptide-based affinity labels for opioid receptors, [Leu(5)]enkephalin (LeuEnk) and DTLET (Tyr-D-Thr-Gly-Phe-Leu-Thr), potent agonists for delta receptors, were selected as the parent peptides for further modification. The affinity label derivatives were prepared using standard Fmoc solid-phase peptide synthesis in conjunction with Fmoc-Phe(p-NHAlloc) (Fmoc: 9-flourenylmethoxycarbonyl;) and selective modification of the p-amino group on this residue. The electrophilic isothiocyanate and bromoacetamide groups were introduced into the para position of Phe(4); the corresponding free amine-containing peptides were also prepared for comparison. The pure peptides were evaluated in radioligand binding assays using Chinese hamster ovary (CHO) cells expressing delta and micro opioid receptors. Modification of Phe(4) in LeuEnk and DTLET significantly decreased delta-receptor binding affinity (40 to >2,000-fold). Among the synthesized analogues, [Phe(p-NH(2))(4)]DTLET showed the highest delta-receptor binding affinity (IC(50) = 39 nM) and enhanced selectivity for delta receptors compared to DTLET while other derivatives exhibited much lower delta receptor affinity. The differences in affinities between the two series of analogues and between the derivatives of LeuEnk and N,N-dibenzyl[Leu(5)]Enk reported previously suggest subtle differences in interactions of Phe(4) with delta receptors depending on other modifications in the sequences.     

Synthesis and binding properties of specific photoaffinity ligands for mu and delta opioid receptor subtypes

We report the synthesis and binding properties of specific photoaffinity ligands for mu and delta opioid receptor subtypes. These ligands are derived from DAGO: Tyr-D-Ala-Gly-NMePhe-Gly-ol, a mu selective probe and DTLET: Tyr-D-Thr-Gly-Phe-Leu-Thr, a delta selective probe by modifying the Phe 4 residue. These modifications are: i) a nitro group on the para position of Phe ring as Phe(4 NO2) or Nip, ii) an azido group as Phe(4 N3) or AZ. Pharmacological responses on mouse vas deferens (delta sites) and guinea pig ileum (mu sites), as well as competition experiments with [3H] DAGO and [3H] DTLET on crude rat brain membranes have been performed. The nitro group on the phenyl ring of the Phe residue preserves the affinity and selectivity of each probe: NipDAGO for the mu sites, NipDTLET for the delta ones. However the nitro probes do not appear to be photoactivable by u.v. irradiation. Likewise, azidation of the phenyl ring of the Phe residue does not change the receptor selectivity of each probe, but AZDAGO has less affinity than its parent molecule DAGO, while AZDTLET has more affinity than DTLET. These compounds are photoactivable and provide an efficient tool to characterize and isolate the different receptor subtypes, especially the delta site.     

Differences of binding characteristics of non-selective opiates towards mu and delta receptor types

[3H]ET (etorphine), which is considered either as an "universal" ligand or a mu agonist, interacts with identical affinities KD = 0.33-0.38 nM to hybrid cells and rabbit cerebellum, pure delta and mu-enriched opioid receptor preparations, respectively. In rat brain tissue, [3H]ET binding is inhibited by DAGO (Tyr-D-Ala-Gly-(Me)-Phe-Gly-ol), a mu selective agonist, in a competitive manner without apparent modification of the maximal number of sites. Furthermore, even at a DAGO concentration (300 nM) which should be sufficient to block [3H]ET interaction with mu sites, no variation in the total capacity of the tritiated ligand is observed. In contrast, DTLET (Tyr-D-Thr-Gly-Phe-Leu-Thr), a delta-preferential agonist, blocks [3H]ET binding in rat brain at a concentration able to saturate delta-sites. At higher concentrations, where DTLET cross reacts with mu-sites, this ligand exhibits similar properties to those of DAGO. These data are very different from those obtained with [3H]EKC (ethylketocyclazocine), another "universal" ligand, the binding properties of which are easily explained by the occurrence in rat brain tissue of independent sites exhibiting pharmacological profiles of mu, delta and kappa sites. Our results underline the possible misinterpretation of binding data obtained by using [3H] etorphine as a non selective ligand.     

Synthesis, biochemical and pharmacological properties of BUBUC, a highly selective and systemically active agonist for in vivo studies of delta-opioid receptors

Based on the results of conformational studies of linear and cyclic delta-opioid peptides such as BUBU [Tyr-D-Ser(OtBu)-Gly-Phe-Leu-Thr(OtBu)] and DPLPE c[Tyr-D-Pen-Gly-Phe-Pen], a new enkephalin-related peptide, Tyr-D-Cys(StBu)-Gly-Phe-Leu-Thr(OtBu) (BUBUC) was synthesized and tested for its opioid activity and selectivity at both the peripheral and central levels. Amongst all the synthetic compounds described so far, BUBUC appears to be the most highly delta-selective probe [KI (mu) = to 2980 nM, KI (delta): 2.9 nM, KI (mu)/KI (delta) approximately 1000]. This selectivity was confirmed by the results of pharmacological studies, including measurements of supraspinal analgesia and behavioral changes in mice. In the later test, BUBUC was shown to increase the rearing activity after IV administration at very low concentrations (0.1 mg/kg) and this effect was reversed by the delta-selective antagonist naltrindole. No antinociceptive response was observed at a 10-fold higher concentration. Thanks to its enzymatic stability and its hydrophobicity. BUBUC is the first systemically active, highly selective delta agonist and should therefore be useful to characterize the physiological role of delta-opioid receptors.     

Modifications of the cyclic mu receptor selective tetrapeptide Tyr-c[D-Cys-Phe-D-Pen]NH2 (Et): effects on opioid receptor binding and activation.

The previously described cyclic mu opioid receptor-selective tetrapeptide Tyr-c[D-Cys-Phe-D-Pen]NH2 (Et) (JOM-6) was modified at residues 1 and 3 by substitution with various natural and synthetic amino acids, and/or by alteration of the cyclic system. Effects on mu and delta opioid receptor binding affinities, and on potencies and efficacies as measured by the [35S]-GTPgammaS assay, were evaluated. Affinities at mu and delta receptors were not influenced dramatically by substitution of Tyr1 with conformationally restricted phenolic amino acids. In the [35S]-GTPgammaS assay, all of the peptides tested exhibited a maximal response comparable with that of fentanyl at the mu opioid receptor, and all showed high potency, in the range 0.4-9nM. However, potency changes did not always correlate with affinity, suggesting that the conformation required for binding and the conformation required for activation of the opioid receptors are different. At the delta opioid receptor, none of the peptides were able to produce a response equivalent to that of the full delta agonist BW 373,U86 and only one had an EC50 value of less than 100nM. Lastly, we have identified a peptide, D-Hat-c[D-Cys-Phe-D-Pen]NH2 (Et), with high potency and > 1,000-fold functional selectivity for the mu over delta opioid receptor as measured by the [35S]-GTPgammaS assay.     

Stereospecific synthesis of (2S)-2-methyl-3-(2',6'-dimethyl-4'-hydroxyphenyl)-propionic acid (Mdp) and its incorporation into an opioid peptide

To examine the effect of replacing the N-terminal amino group in opioid peptides with a methyl group on biological activity, a stereospecific synthesis of the tyrosine analogue (2S)-2-methyl-3-(2',6'-dimethyl-4'-hydroxyphenyl)-propionic acid (Mdp) was performed. The enkephalin analogue (2S)-Mdp-D-Ala-Gly-Phe-Leu-NH2 turned out to be a quite potent delta opioid antagonist and a somewhat less potent mu antagonist, indicating that a positively charged N-terminal amino group is not a conditio sine qua non for the binding of opioid peptides to delta and mu receptors but may be required for signal transduction.     

Characterization of Opioid Receptor Subtypes in Solution

Stable opioid receptor binding activity that retains distinct subtype specificities (mu, delta, and kappa) has been obtained in high yields in digitonin extracts of rat brain membranes that had been preincubated with Mg2+ prior to solubilization. The dependence on Mg2+ ions for receptor activity is also expressed in the soluble state, where the presence of Mg2+ leads to high-affinity and high-capacity opioid peptide binding to the delta, mu, and kappa sites (the latter subtype measured by the binding of [3H]dynorphin1-8). Binding of opiate alkaloids to soluble receptor sites is less dependent on Mg2+ than is opioid peptide binding. Soluble opioid binding activity shows the same sensitivity to Na+ ions and guanine nucleotides as the membrane-bound receptor. The ligand-receptor interactions give evidence of strong positive cooperativity, which is interpreted in terms of association-dissociation of receptor subunits on ligand binding in solution. Binding of enkephalin peptides is associated with the large macromolecules present (apparent Stokes radii greater than 60 A), whereas both those and several small species present (less than 60 A) bind opiate alkaloids and dynorphin1-8.     

1H-n.m.r. investigation of conformational features of cyclic, penicillamine-containing enkephalin analogs

Conformational features of a series of cyclic, penicillamine-containing enkephalin analogs, all of which display selectivity for the delta opioid receptor, were studied by 1H n.m.r. in aqueous solution. Comparison of chemical shifts, coupling constants, and temperature dependence of amide proton chemical shifts suggests different conformational features among the analogs, some of which can be related to the different primary sequences of these peptides. The observation that some of the analogs display disparate individual conformational features while exhibiting similar opioid potency and receptor selectivity suggests that such analogs may share a similar overall topography or at the least maintain the same relative orientations of key portions of the molecule.     

Conformation-activity relationships of opioid peptides with selective activities at opioid receptors

The discovery of endogenous opioid peptides 25 years ago opened up a new chapter in efforts to understand the origins and control of pain, its relationships to other biological functions, including inflammatory and other immune responses, and the relationships of opioid peptides and their receptors to a variety of undesirable or toxic side effects often associated with the nonpeptide opiates such as morphine including addiction, constipation, a variety of neural toxicities, tolerance, and respiratory depression. For these investigations the need for potent and highly receptor selective agonists and antagonists has been crucial since they in principle allow one to distinguish unequivocally the roles of the different opioid receptors (mu, delta, and kappa) in the various biological and pathological roles of the opioid peptides and their receptors. Conformational and topographical constraint of the linear natural endogenous opioid peptides has played a major role in developing peptide ligands with high selectivity for mu, delta, and kappa receptors, and in understanding the conformational, topographical, and stereoelectronic structural requirements of the opioid peptides for their interactions with opioid receptors. In turn, this had led to insights into the three-dimensional pharmacophore for opioid receptors. In this article we review and discuss some of the developments that have led to potent, selective, and stable peptide and peptidomimetic ligands that are highly potent and selective, and that have delta agonist, mu antagonist, and kappa agonist biological activities (other authors in this issue will discuss the development of other types of activities and selectivities). These have led to ligands that provide unique insight into opioid pharmacophores and the critical roles opioid ligands and receptor scan play in pain, addiction, and other human maladies.     

Use of hydrophilic diamines for bridging of two opioid peptide pharmacophores. Synthesis and receptor binding of two new analogues.

The bivalent ligand approach, which assumes that two pharmacophores are connected by a spacer, was used to design receptor type-selective ligands for opioid receptors. The first two opioid peptide bivalent ligands with different spacer lengths containing different numbers of hydroxyl groups, (Tyr-D-Ala-Gly-Phe-NH-CH2-CHOH-)2 (Tyr-D-Ala-Gly-Phe-NH-CH2-CHOH-CHOH-)2, were synthesized and their binding to mu, delta, and kappa opioid receptors was characterized. Both analogues were found to possess high opioid in vitro activities. The length of the hydrophilic spacer does not affect the affinity for delta receptors, whereas shorter spacer length increases affinity for mu and even more so for kappa receptors. Thus receptor type-selective peptides for opioid receptors can be designed using the bivalent approach.     

Cyclic, disulfide- and dithioether-containing opioid tetrapeptides: development of a ligand with high delta opioid receptor selectivity and affinity

Tetrapeptides of primary sequence Tyr-X-Phe-YNH2, where X is D-Cys or D-Pen (penicillamine) and where Y is D-Pen or L-Pen, were prepared and were cyclized via the side chain sulfurs of residues 2 and 4 to disulfide or dithioether-containing analogs. These peptides are related to previously reported penicillamine-containing pentapeptide enkephalin analogs but lack the central glycine residue of the latter and were designed to assess the effect of decreased ring size on opioid activity. Binding affinities of the tetrapeptides were determined to both mu and delta opioid receptors. Binding affinity and selectivity in the tetrapeptide series were observed to be highly dependent on primary sequence. For example, L-Pen4 analogs displayed low affinity and were nonselective, while the corresponding D-Pen4 diastereomers were of variable affinity and higher selectivity. Among the latter compounds were examples of potent analogs in which selectivity shifted from delta selective to mu selective as the ring size was increased. The relatively high binding affinity and delta receptor selectivity observed with one of the carboxamide terminal disulfide analogs led to the synthesis of the corresponding carboxylic acid terminal, Tyr-D-Cys-Phe-D-PenOH. This analog displayed delta receptor binding selectivity similar to that of the standard delta ligand, [D-Pen2,D-Pen5]enkephalin (DPDPE), and was found to have a 3.5-fold higher binding affinity than DPDPE. All the tetrapeptides were further evaluated in the isolated mouse vas deferens (mvd) assay and all displayed opioid agonist activity. In general, tetrapeptide potencies in the mouse vas deferens correlated well with binding affinities but were somewhat lower. Receptor selectivity in the mvd, assessed by examining the effect of opioid antagonists on the tetrapeptide concentration-effect curves, was similar to that determined in the binding studies.     

(4-Carboxamido)phenylalanine is a surrogate for tyrosine in opioid receptor peptide ligands

(S)-4-(Carboxamido)phenylalanine (Cpa) is examined as a bioisosteric replacement for the terminal tyrosine (Tyr) residue in a variety of known peptide ligands for the mu, delta and kappa opioid receptors. The Cpa-containing peptides, assayed against cloned human opioid receptors, display comparable binding affinity (Ki), and agonist potency (EC50) to the parent ligands at the three receptors. Cpa analogs of delta selective peptides show an increase in delta selectivity relative to the mu receptor. Cpa is the first example of an amino acid that acts as a surrogate for Tyr in opioid peptide ligands, challenging the long-standing belief that a phenolic residue is required for high affinity binding.     

Mu and delta receptors: their role in analgesia in the differential effects of opioid peptides on analgesia

Utilizing the mouse tail-flick assay, the rank order of analgesic potency for various opioids (i.c.v.) is beta h-endorphin greater than D-Ala2-D-Leu5-enkephalin greater than morphine greater than D-Ala2-met-enkephalinamide much greater than met-enkephalin much greater than leu-enkephalin. Assuming mu receptor mediation of analgesia, there is an affinity and analgesic potency (ie: D-Ala2-Leu5-enkephalin has 1/7 the affinity of morphine for the mu receptor but is 18X more potent as an analgesic). Additionally, sub-analgesic doses of various opioid peptides have opposite effects on analgesic responses. Leu-enkephalin, D-Ala2-D-Leu5-enkephalin or beta h-endorphin potentiate morphine or D-Ala2-met-enkephalinamide analgesia whereas met-enkephalin or D-Ala2-met-enkephalinamide antagonize opioid-induced analgesia. Using the enkephalins as the prototypic delta ligands (100 fold selective) and based on their effects on analgesia, we suggest that Leu-enkephalin-like peptides interact with the delta receptor as an "agonist" to facilitate and met-enkephalin-like peptides as an "antagonist" to attenuate analgesia. Given the biochemical evidence of a coupling between mu and delta receptors, we suggest that the mechanism of facilitation or attenuation of analgesia by the enkephalins is a direct in vivo consequence of this coupling. Further, the analgesic potencies of various opioid ligands can be better correlated to the combination of their simultaneous occupancy of mu and delta receptors.     

Delta opioidmimetic antagonists: prototypes for designing a new generation of ultraselective opioid peptides.

BACKGROUND: Tyr-Tic (1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid) and Tyr-Tic-Ala were the first peptides with delta opioid antagonist activity lacking Phe, considered essential for opioid activity based on the N-terminal tripeptide sequence (Tyr-D-Xaa-Phe) of amphibian skin opioids. Analogs were then designed to restrain the rotational flexibility of Tyr by the substitution of 2,6-dimethyl-L-tyrosine (Dmt). MATERIALS AND METHODS: Tyr and Dmt peptides were synthesized by solid phase and solution methods using Fmoc technology or condensing Boc-Dmt-OH or Boc-Tyr(But)-OH with H-L-Tic-OBut or H-D-Tic-OBut, respectively. Peptides were purified (> 99%) by HPLC and characteristics determined by 1H-NMR, FAB-MS, melting point, TLC, and amino acid analyses. RESULTS: H-Dmt-Tic-OH had high affinity (Ki delta = 0.022 nM) and extraordinary selectivity (Ki mu/Ki delta = 150,000); H-Dmt-Tic-Ala-OH had a Ki delta = 0.29 nM and delta selectivity = 20,000. Affinity and selectivity increased 8700- and 1000-fold relative to H-Tyr-Tic-OH, respectively. H-Dmt-Tic-OH and H-Dmt-Tic-NH2 fitted one-site receptor binding models (eta = 0.939-0.987), while H-Dmt-Tic-ol, H-Dmt-Tic-Ala-OH and H-Dmt-Tic-Ala-NH2 best fitted two-site models (eta = 0.708-0.801, F 18.9-26.0, p < 0.0001). Amidation increased mu affinity by 10- to 100-fold and acted synergistically with D-Tic2 to reverse selectivity (delta-->mu). Dmt-Tic di- and tripeptides exhibited delta antagonist bioactivity (Ke = 4-66 nM) with mouse vas deferens and lacked agonist mu activity (> 10 microM) in guinea-pig ileum preparations. Dmt-Tic analogs weakly interacted with kappa receptors in the 1 to > 20 microM range. CONCLUSIONS: Dmt-Tic opioidmimetic peptides represent a highly potent class of opioid peptide antagonists with greater potency than the nonopioid delta antagonist naltrindole and have potential application as clinical and therapeutic compounds.     

Reduced peptide bond cyclic somatostatin based opioid octapeptides. Synthesis, conformational properties and pharmacological characterization.

The conformational and pharmacological properties that result from peptide bond reduction as well as the use of secondary amino acids in a series of cyclic peptides related to the mu opioid receptor selective antagonist D-Phe1-Cys2-Tyr3-D-Trp4-Orn5-Thr6-Pen7+ ++-Thr8-NH2 (IV), have been investigated. Peptide analogues that contain [CH2NH] and [CH2N] pseudo-peptide bonds (in primary and secondary amino acids, respectively) were synthesized on a solid support. Substitution of Tyr3 in IV by the cyclic, secondary amino acid 1,2,3,4-tetrahydroisoquinoline carboxylate (Tic) and of D-Trp4 with D-1,2,3,4-tetrahydro-beta-carboline(D-Tca4), gave peptides 4 and 1, respectively. Both analogues displayed reduced affinities for mu opioid receptors. Conformational analysis based on extensive NMR investigations demonstrated that the backbone conformations of 1 and 4 are similar to those of the potent and selective analogue D-Phe-Cys-Tyr-D-Trp-Lys-Thr-Pen-Thr-NH2 (I), while the conformational properties of the side chains of Tic3 (4) and D-Tca4 (1) resulted in topographical properties that were not well recognized by the mu opioid receptor. Peptide bond modifications were made including (Tyr3-psi[CH2NH]-D-Trp4), 3; (Tyr3-psi[CH2N]-D-Tca4), 2; and (Cys2-psi[CH2N]-Tic3), 6. These analogues showed decreases in their mu opioid receptor affinities relative to the parent compounds IV, 1, and 4, respectively. 1H NMR based conformational analysis in conjunction with receptor binding data led to the conclusion that the reduced peptide bonds in 2, 3, 5, and 6 do not contribute to the process of discrimination between mu and delta opioid receptors, and in spite of their different dynamic behaviors (relative to 1 and 4), they are still capable of attaining similar receptor bound conformations, possibly due to their increased flexibility.     

Opioid receptor three-dimensional structures from distance geometry calculations with hydrogen bonding constraints.

Three-dimensional structures of the transmembrane, seven alpha-helical domains and extracellular loops of delta, mu, and kappa opioid receptors, were calculated using the distance geometry algorithm, with hydrogen bonding constraints based on the previously developed general model of the transmembrane alpha-bundle for rhodopsin-like G-protein coupled receptors (Biophys. J. 1997. 70:1963). Each calculated opioid receptor structure has an extensive network of interhelical hydrogen bonds and a ligand-binding crevice that is partially covered by a beta-hairpin formed by the second extracellular loop. The binding cavities consist of an inner "conserved region" composed of 18 residues that are identical in delta, mu, and kappa opioid receptors, and a peripheral "variable region," composed of 19 residues that are different in delta, mu, and kappa subtypes and are responsible for the subtype specificity of various ligands. Sixteen delta-, mu-, or kappa-selective, conformationally constrained peptide and nonpeptide opioid agonists and antagonists and affinity labels were fit into the binding pockets of the opioid receptors. All ligands considered have a similar spatial arrangement in the receptors, with the tyramine moiety of alkaloids or Tyr1 of opioid peptides interacting with conserved residues in the bottom of the pocket and the tyramine N+ and OH groups forming ionic interactions or H-bonds with a conserved aspartate from helix III and a conserved histidine from helix VI, respectively. The central, conformationally constrained fragments of the opioids (the disulfide-bridged cycles of the peptides and various ring structures in the nonpeptide ligands) are oriented approximately perpendicular to the tyramine and directed toward the extracellular surface. The results obtained are qualitatively consistent with ligand affinities, cross-linking studies, and mutagenesis data.