Endomorphin-1 and endomorphin-2 are partial agonists at the human mu-opioid receptor

Recently two tetrapeptide ligands that bind preferentially to the mu-opioid receptor were identified and named endomorphin-1 and endomorphin-2. We examined the ability of these peptides to stimulate G protein activation in human mu-opioid receptor transfected B82 fibroblasts as measured by [35S]GTPgammaS binding to cell membranes. Both endomorphin-1 and -2 act as partial agonists in this assay system compared with the mu-selective agonist [D-Ala2,N-Me-Phe4, Gly-ol5]enkephalin (DAMGO). In addition, endomorphins demonstrate efficacy similar to morphine. These findings demonstrate that endomorphin peptides have similar activity at the mu-opioid receptor as morphine and suggest that these peptides have the potential to modulate neuronal activity in vivo.

Prevention of reflex natriuresis after acute unilateral nephrectomy by melanocortin receptor antagonists

gamma-Melanocyte-stimulating hormone (gamma-MSH), atrial natriuretic peptide (ANP), and oxytocin have been identified as candidate hormonal mediators of the reflex natriuresis that follows acute unilateral nephrectomy (AUN). Pharmacological characterization of the third melanocortin receptor (MC3-R) indicates that it uniquely responds to physiological concentrations of gamma-MSH. We tested the roles of gamma-MSH, ANP, and oxytocin in the postnephrectomy natriuresis by carrying out AUN during continuous intrarenal infusion of specific antagonists for their cognate receptors. In anesthetized Sprague-Dawley rats, urinary sodium excretion (UNaV) increased from 0.34 +/- 0.04 to 1.12 +/- 0.11 mu eq/min 90 min after AUN (P < 0.001). No change in UNaV occurred in rats undergoing a sham AUN procedure. Plasma immunoreactive gamma-MSH concentration was 53 +/- 8 fmol/ml after sham AUN but 112 +/- 17 fmol/ml after AUN (P < 0.01). SHU-9119 and SHU-9005 are substituted derivatives of alpha-MSH with potent antagonism at the MC3-R in vitro. Infusion of these compounds at 5 pmol/min completely blocked the natriuretic response to AUN despite a similar elevation in plasma gamma-MSH (111 +/- 12 vs. 49 +/- 8 fmol/ml in sham rats, P < 0.01). Intrarenal infusion of the ANP receptor antagonist A-71915 (5 pmol/min) or the oxytocin receptor antagonist [d(CH2)(5)1, Tyr(Me)2,Orn8] vasotocin (10 pmol/min) effectively inhibited the natriuresis induced by intravenous infusion of ANP or oxytocin (each at 1 pmol/min), respectively, but did not block the natriuresis after AUN. Plasma immunoreactivity of these peptides was not increased after AUN. These results indicate that reflex natriuresis after AUN is accompanied by an increase in plasma gamma-MSH but not ANP or oxytocin concentration and is prevented by intrarenal infusion of receptor antagonists with selectivity for MC3-R. The data indicate that gamma-MSH or a closely related peptide mediates postnephrectomy natriuresis and provide further support for the possibility that gamma-MSH may play a wider role in sodium homeostasis.

Modifications of the 4,4'-residues and SAR studies of Biphalin, a highly potent opioid receptor active peptide

Modifications of 4,4' residues of Biphalin have resulted in greater binding selectivity and biological potency for the mu opioid receptor. A higher partition coefficient across the phospholipid bilayer membrane has been achieved by using a beta-branched unusual amino acids.

Gradient- and sensitivity-enhanced TOCSY experiments

A pulsed field gradient version of the sensitivity-enhanced 2D TOCSY experiment is proposed which yields high-quality spectra with improved sensitivity and a minimum of two scans per t1 increment. For rapid acquisition of 1D TOCSY spectra, the 1D DPFGSE-TOCSY experiment was modified to include phase-encoded multiple-selective excitation followed by a simple spectral editing. Combination of these two building blocks is used in a sensitivity-enhanced 2D analog of the 3D TOCSY-TOCSY experiment which provides an efficient tool for resolving severely overlapped signals of oligomers in short experimental time.

Sensitivity- and gradient-enhanced hetero (omega1) half-filtered TOCSY experiment for measuring long-range heteronuclear coupling constants

An enhanced version of the X (omega1) half-filtered TOCSY experiment for measurement of long-range heteronuclear coupling constants is proposed which yields high-quality spectra with substantially increased sensitivity and resolution. The modified method features gradient-enhanced X filtering sequences, broadband homonuclear decoupling during t1, optional 1JXH scaling in the F1 domain, and gradient coherence selection in combination with the sensitivity-enhanced protocol for the TOCSY transfer. These modifications extend the applicability of the method--coupling constants can be measured accurately for natural abundance samples at low concentrations and for compounds yielding complex spectra. Computer-aided analysis of E.COSY-type multiplets is applied for the determination of heteronuclear long-range coupling constants.

Sensitivity- and gradient-enhanced heteronuclear coupled/decoupled HSQC-TOCSY experiments for measuring long-range heteronuclear coupling constants

A pulsed field gradient version of the sensitivity-enhanced 2D HSQC-TOCSY experiment is proposed for measurement of long-range heteronuclear coupling constants. The coupling constants are obtained by computer-aided analysis of mixed-phase multiplets with and without the heteronuclear splitting. Generation of pure phase data is not required. Since large 1JXH and JHH couplings are used for coherence transfer, small nJXH can be measured accurately, which could be difficult to obtain from purely heteronuclear polarization transfer experiments.

Cyclic enkephalin analogues with exceptional potency and selectivity for delta-opioid receptors

Superpotent and highly delta-opioid receptor selective cyclic peptides of the general formula H-Tyr-c[D-Pen-Gly-Phe(p-X)-Pen]-Phe-OH (where X = hydrogen or halogen) have been synthesized. In the binding assays the most selective and most potent compound is the p-bromophenyl-alanine-4 analogue (IC50 value = 0.19 nM, selectivity ratio = 21,000 for delta vs mu). In the GPI and MVD bioassays the most selective and most potent analogue is the p-fluoro-substituted analogue Tyr-[D-Pen-Gly-Phe(p-F)-Pen]-Phe-OH. In the MVD assay it has an exceptionally low IC50 value of 0.016 nM and a delta vs mu selectivity ratio of 45,000.

Interaction of a highly potent dimeric enkephalin analog, biphalin, with model membranes

Biphalin, (Tyr-D-Ala-Gly-Phe-NH)2, is a highly potent dimeric analog of enkephalin. Its analgesic efficacy is due in part to its ability to permeate the blood-brain barrier. To aid in understanding the mechanism of the transmembrane movement we determined and analyzed the permeability and partition coefficients of biphalin and a series of analogues where F, Cl, I, NO2, or NH2 were placed in the para position of the aromatic rings of Phe4,4'. Liposomes composed of neutral phospholipids and cholesterol were used as the model membrane. The overall good correlation between permeability and water-membrane partition coefficients suggests that the movement of biphalins across the model membrane is controlled by diffusion and depends on the water-membrane partition coefficient. To explain the observed correlation between permeability and the electron withdrawing/donating character of the substituents in the phenylalanine ring, we examined various folding patterns of Leu-enkephalin, an endogenous pentapeptide that exhibits affinities toward the same classes of opioid receptors (delta and mu). The observed permeabilities and partition coefficients of biphalin and analogues, as well as the tyrosine side chain accessibility, are consistent with the presence of the type of folding where the tyrosine and phenylalanine side chains are in a close contact. We propose that the aromatic ring interaction can promote the peptide permeability by stabilizing a more compact structure of biphalin that would minimize the number of hydrogen bonds with water and therefore enhances partitioning into the model membrane.

Para-substituted phenylalanine-4 analogues of [L-Ala3]DPDPE: highly selective delta opioid receptor ligands

Several para-substituted Phe4 analogues of the delta 1-selective antagonist [L-Ala3]DPDPE (DPADPE) were prepared and evaluated for their brain-binding and in vitro pharmacological effects. Unlike the p-haloPhe4 analogues of DPDPE and the deltorphins, similar analogues of DPADPE with electron-withdrawing groups substituted at the para-position of the Phe4 aromatic ring did not all have increased potency and selectivity for delta opioid receptors, but all retained high potency and selectivity for delta opioid receptors greater than DPDPE.

Brain and spinal cord distribution of biphalin: correlation with opioid receptor density and mechanism of CNS entry

Biphalin [(Tyr-D-Ala-Gly-Phe-NH)2] is a bivalent, opioid peptide containing two pharmacophores linked by a hydrazine bridge. When administered intracerebroventricularly, it has been shown to be more potent than morphine and etorphine at eliciting antinociception. Biphalin has also been shown to cross both the blood-brain and blood-cerebrospinal fluid barriers. To understand the basis of biphalin's potency, regional brain and spinal cord distribution studies with [125I-Tyr1]biphalin were performed 5, 20, and 40 min after intravenous bolus injections. A statistically greater amount of [125I-Tyr1]biphalin was detected in the nucleus accumbens compared with other brain regions (p < 0.05). This correlates with the high density of delta- and mu-opioid receptor mRNA and binding sites shown to be expressed in the nucleus accumbens. Also, a statistically greater amount of [125I-Tyr1] biphalin was detected in two other circumventricular organs, the choroid plexus and pituitary, when compared with other brain regions. These studies provide evidence that biphalin can reach not only brain sites, but also spinal sites to elicit antinociception. The overall CNS distribution of [125I-Tyr1]biphalin was decreased with naloxone, D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2, or naltrindole pretreatment, showing that biphalin detected in the brain and spinal cord is binding to delta- and mu-opioid receptors. Additional in situ brain perfusion experiments identified a saturable component contributing to CNS entry of [125I-Tyr1]biphalin, which could be described by Michaelis-Menten kinetics with a Km of 2.6 +/- 4.8 microM, Vmax of 14.6 +/- 2.89 pmol(-1) x min(-1) x g(-1), and Kd of 0.568 +/- 0.157 microl x min(-1) x g(-1). Brain entry of [125I-Tyr1]biphalin was sensitive to 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid and L-phenylalanine, suggesting use of the large neutral amino acid carrier. This work provides evidence that biphalin is a promising, potent analgesic that has a unique mechanism for reaching both spinal and supraspinal opioid receptor sites.

beta-Methylation of the Phe7 and Trp9 melanotropin side chain pharmacophores affects ligand-receptor interactions and prolonged biological activity

Topographically modified melanotropin side chain pharmacophore residues Phe7 and Trp9 in a cyclic peptide template (Ac-Nle4-c[Asp-His-Xaa7-Arg-Yaa9-Lys]-NH2) and Phe7 in a linear peptide template (Ac-Ser-Tyr-Ser-Nle4-Glu-His-Xaa7-Arg-Trp-Gly-Lys-Pro-Val-NH2) result in differences in potency and prolonged biological activity in the frog and lizard skin bioassays. These topographic modifications included the four isomers of beta-methylphenylalanine (beta-MePhe)7 and beta-methyltryptophan (beta-MeTrp)9 and the two isomers of 1,2,3,4-tetrahydro-beta-carboline (Tca)9 Modifications in the cyclic template resulted in up to a 1000-fold difference in potency for the beta-MePhe7 stereoisomeric peptides; up to a 476-fold difference in potency resulted for the beta-MeTrp9 peptides, and about a 50-fold difference between the Tca9-containing peptides. Up to a 40-fold difference in potency resulted for the beta-MePhe7 stereoisomeric peptides using the linear template in these assays. The relative potency ranking for modifications in the cyclic template of beta-MePhe7 were 2R,3S > 2S,3S = 2S,3R > 2R,3R in the frog assay and 2S,3R > 2R,3S > 2S,3S > 2R,3R in the lizard assay. The relative potencies for modifications in the cyclic template of beta-MeTrp9 were 2R,3S > 2R,3R > 2S,3S > > 2S,3R in the frog assay and 2S,3S = 2R,3R > 2R,3S > 2S,3R in the lizard assay. The relative potencies for modifications in the cyclic template of Tca9 were DTca > LTca in both assays. Significant differences in prolonged (residual) activities were also observed for these modified peptides and were dependent upon stereochemistry of the beta-methyl amino acid, peptide template, and bioassay system. Furthermore, comparisons of beta-MeTrp9 stereoisomeric peptides on the frog, lizard, and human MC1 receptors suggest that structure-activity relationships on both the classical frog and lizard skin bioassays do not necessarily predict corresponding SAR profiles for the human melanocortin receptors, indicating a remarkable species specificity of the MC1 receptor requirements.

The role of phenylalanine at position 6 in glucagon's mechanism of biological action: multiple replacement analogues of glucagon

Extensive evidence gathered from structure-activity relationship analysis has identified and confirmed specific positions in the glucagon sequence that are important either for binding to its receptor or for signal transduction. Fifteen glucagon analogues have been designed and synthesized by incorporating structural changes in the N-terminal region of glucagon, in particular histidine-1, phenylalanine-6, and aspartic acid-9. This investigation was conducted to study the role of phenylalanine at position 6 on the glucagon mechanism of action. These glucagon analogues have been made by either deleting or substituting hydrophobic groups, hydrophilic groups, aromatic amino acids, or a D-phenylalanine residue at this position. The structures of the new analogues are as follows: [des-His1, des-Phe6, Glu9]glucagon-NH2 (1); [des-His1,Ala6,Glu9]glucagon-NH2 (2); [des-His1,Tyr6,Glu9]glucagon-NH2 (3); [des-His1,Trp6,Glu9]-glucagon-NH2 (4); [des-His1,D-Phe6,Glu9]glucagon-NH2 (5); [des-His1,Nle6,Glu9]glucagon-NH2 (6); [des-His1,Asp6,Glu9]glucagon-NH2 (7); [des-His1,des-Gly4,Glu9]glucagon-NH2 (8); [desPhe6,-Glu9]glucagon-NH2 (9); [des-Phe6]glucagon-NH2 (10); [des-His1, des-Phe6]glucagon-NH2 (11); [des-His1, des-Phe6,Glu9]glucagon (12); [des-Phe6,Glu9]glucagon (13); [des-Phe6]glucagon (14); and [des-His1, des-Phe6]glucagon (15). The receptor binding potencies IC50 values are 48 (1), 126 (2), 40 (3), 19 (4), 100 (5), 48 (6), 2000 (7), 52 (8), 113 (9), 512 (10), 128 (11), 1000 (12), 2000 (13), 500 (14), and 200 nM (15). All analogues were found to be antagonists unable to activate the adenylate cyclase system even at concentrations as high as 10(-5) M except for analogues 6 and 8, which were found to be weak partial agonists/partial antagonists with maximum stimulation between 6-12%. In competitive inhibition experiments, all the analogues caused a right shift of the glucagon-stimulated adenylate cyclase dose-response curve. The pA2 values were 8.20 (1), 6.40 (2), 6.20 (3), 6.25 (4), 6.30 (5), 6.30 (7), 6.05 (8), 6.20 (9), 6.30 (10), 6.25 (11), 6.10 (12), 6.20 (13), 6.20 (14), and 6.35 (15).

Structural model of a cyclic dynorphin A analog bound to dodecylphosphocholine micelles by NMR and restrained molecular dynamics

The compound c[Cys5,11]dynorphin A-(1-11)-NH2, 1, is a cyclic dynorphin A analog that shows similar selectivity and potency at the kappa-opioid receptor when compared to the native form of the peptide in central nervous system assays. Previous molecular mechanics calculations have shown that the ring portion of the isoform that is trans about the Arg9-Pro10 omega bond contains either a beta-turn from residues Arg6 to Arg9 or an alpha-helical conformation. Our results from solution state NMR indicate that the compound exhibits cis-trans isomerism about the Arg9-Pro10 omega bond in both aqueous solution and when bound to dodecylphosphocholine micelles. Restrained molecular dynamics calculations show that the cis isoform of the peptide contains a type III beta-turn from residues Arg7 to Pro10. Similar calculations on the trans isoform show it to contain a beta-turn from residues Cys5 and Arg8. In this report we describe the generation of three-dimensional models from NMR data for the ring portions of both the cis and trans isoforms of 1 bound to dodecylphosphocholine micelles. Comparison with other dynorphin A structural information indicates that both the cis and trans isoforms of the peptide may be active as kappa-opioid agonists.

Discovery of prototype peptidomimetic agonists at the human melanocortin receptors MC1R and MC4R

[Nle4, DPhe7]-alpha-MSH (NDP-MSH), a highly potent analogue of alpha-melanocyte-stimulating hormone (alpha-MSH), possesses nanomolar efficacies at all the melanocortin receptor subtypes except the MC2R. Evaluation of the melanocortin "message" sequence of [Nle4, DPhe7]-alpha-MSH was performed on the human melanocortin receptor subtypes designated hMC1, hMC3R, hMC4R, and hMC5R. Tetrapeptides and tripeptides were stereochemically modified to explore topochemical preferences at these receptors and to identify lead peptides possessing agonist activity and subtype selectivity. Four peptides were discovered to only bind to the hMC1 and hMC4 receptor subtypes. The tetrapeptide Ac-His-DPhe-Arg-Trp-NH2 (1) possessed 0.6 microM binding affinity at the hMC1R, 1.2 microM binding affinity at the hMC4R, and agonist activity at both receptors. The tripeptides Ac-DPhe-Arg-Trp-NH2 (6) and Ac-DPhe-Arg-DTrp-NH2 (7) possessed 2.0 and 9.1 microM binding affinities, respectively, only at the hMC4R, and both compounds effected agonist activity. The tetrapeptide Ac-His-Phe-Arg-DTrp-NH2 (4) possessed 6.3 microM affinity and full agonist activity at the hMC1R, while only binding 7% at the hMC3R, 36% at the hMC4R, and 11% at the hMC5R at a maximal concentration of 10 microM. These data demonstrate that the His-Phe-Arg-Trp message sequence of the melanocortin peptides does not bind and stimulate each melanocortin receptor in a similar fashion, as previously hypothesized. Additionally, this study identified the simplest structural agonists for the hMC1R and hMC4R receptors reported to date.

Synthesis and biological properties of beta-MePhe3 analogues of deltorphin I and dermenkephalin: influence of biased chi 1 Phe3 residues on peptide recognition for delta-opioid receptors

Using the method of conformational constraint, we have designed and synthesized analogues of deltorphin I and dermenkephalin containing each of the four stereoisomers (2S,3S; 2S,3R; 2R,3S; 2R,3R) of the unusual amino acid beta-methylphenylalanine in position three. The potency and selectivity of these analogues were evaluated by radioreceptor binding assays in the rat brain using [3H]CTOP (mu-ligand) and[3H]p-C1Phe4]DPDPE (delta-ligand), and by bioassay using the mouse vas deferens (delta-receptor assay) and guinea pig ileum (mu-receptor assay) assays. The substitution of a beta-MePhe for Phe3 in deltorphin I and dermenkephalin has a large and variable effect on the bioactivities of the synthesized analogues. The synthesized analogues are somewhat less potent than the native peptides. Both [(2S,3R)-beta-MePhe3]deltorphin and [(2S,3R)-beta-MePhe3] dermenkephalin are more selective, however, and interact essentially specifically with the receptor in the binding assays and bioassays. The bioassay data in vitro of the synthesized analogues of deltorphin I and dermenkephalin follow the same general trends as the receptor binding data. These results demonstrate that topographical modifications of the side-chain conformation of critical structural moieties in a ligand can significantly modulate both the potency and receptor selectivity for ligands that have multiple sites of biological activity, and they illustrate that this approach has general application to peptide and peptidomimetic ligand design.