Molecular dynamics simulations of opioid peptide analogs containing multiple conformational restrictions

Molecular dynamics simulations were performed on the potent and slightly mu-receptor selective cyclic dermorphin analog H-Tyr-D-Orn-Phe-Glu-NH2 as well as on analogs containing a conformationally restricted phenylalanine derivative in place of Phe in the 3 position of the peptide sequence. Peptides studied included the potent and highly mu-selective analogs H-Tyr-D-Orn-Aic-Glu-NH2 (Aic = 2-aminoindan-2-carboxylic acid), H-Tyr-D-Orn-Atc-Glu-NH2 (Atc = 2-aminotetralin-2-carboxylic acid) and H-Tyr-D-Orn-D-Atc-Glu-NH2, and the weakly active analog H-Tyr-D-Orn-Tic-Glu-NH2 (Tic = tetrahydroisoquinoline-3-carboxylic acid). Four different starting conformations were chosen for each peptide, and after equilibration each simulation was allowed to proceed for 100 picoseconds at 600 degrees K. The 14-membered ring structures in the Phe-, Aic-, L- and D-Atc-containing analogs showed moderate structural flexibility, while the peptide ring in the Tic-containing analog was more rigid. As theoretically predicted, the phi 3 and psi 3 angles of the Aic-, L- and D-Atc-containing analogs were limited to values of either about +50 degrees or -50 degrees during almost the entire period of the simulations. In the Tic-containing analog the phi 3 and psi 3 angles were 0 degrees and 90 degrees, respectively, and did not change for the entire duration of the simulation. The side chains of the constrained amino acids showed limited movement, but transitions between the allowed conformations did occur on the time scale of the simulations.(ABSTRACT TRUNCATED AT 250 WORDS)

Highly efficient photoaffinity labeling of the hormone binding domain of atrial natriuretic factor receptor

A high-efficiency photoaffinity derivative of atrial natriuretic factor (ANF) was developed for studying the peptide binding domain of the receptor protein and for better characterization of this receptor in tissues with a low density of binding sites. The position of the photosensitive residue was chosen on the basis of a molecular conformational model and on structure-activity relationship studies which both indicate that the carboxy-terminal end of the peptide is part of a hydrophobic pole likely to interact deeply within the ANF binding pocket of the receptor. Selection of the photoreactive residue p-benzoylphenylalanine (BPA) as a substitute for arginine in position 125 of the peptide sequence led to a photoaffinity derivative with a high (63%) efficiency of covalent incorporation to the receptor protein. This derivative (BPA-ANF) has a 10-fold lower affinity when compared with ANF, but it is a full agonist in stimulating cGMP production and inhibiting aldosterone secretion in bovine adrenal zona glomerulosa. Photoaffinity labeling with BPA-ANF specifically identifies ANF-R1 and ANF-R2 receptor proteins with a 10-fold higher efficiency than with azido derivatives of ANF or with cross-linking agents. This new ANF derivative therefore appears to be useful for studying ANF receptors in tissues with low levels of expression, for locating receptor following cellular internalization, and for tagging proteolytic fragments of the receptor amenable to amino acid microsequencing.

14-membered cyclic opioids related to dermorphin and their partially retro-inverso modified analogues. I. Synthesis and biological activity

As a continuation of our program to study structure-activity relationships of opiate peptides, we report the syntheses and biological activities of a series of 14-membered cyclic dermorphin analogues closely related to enkephalin analogue Tyr-c[D-A2bu-Gly-Phe-Leu] incorporating a phenylalanine at the third position in place of glycine. In addition to two parent dermorphin analogues Tyr-c[D-A2bu-Phe-Phe-(L and D)-Leu], four stereoisomeric retro-inverso modified analogues Tyr-c[D-A2bu-Phe-gPhe-(S and R)-mLeu] with a reversed amide bond between residues four and five, and Tyr-c[D-Glu-Phe-gPhe-(L and D)-rLeu] with two reversed amide bonds between residues four and five, and between residue five and the side chain of residue two have been synthesized. The results from the guinea pig ileum (GPI) and mouse vas deferens (MVD) assays show that all analogues are superactive at either one or both opiate receptors and in general display higher activities as compared to the corresponding enkephalin analogues with a glycine at the third position. Results from the in vitro biological assays and conformational analysis using 1H-NMR spectroscopy (adjoining paper) will provide useful information to understand the role of the Phe3 aromatic side chain in dermorphin, and that of the Phe4 aromatic side chain in enkephalin, on opiate activity since these cyclic dermorphin analogues contain two Phe residues at both the third and fourth positions.

Glycoconjugates of opioid peptides. Synthesis and biological activity of [Leu5]enkephalin related glycoconjugates with amide type of linkage

Three N-glycoconjugates of the general formula H-Tyr-Gly-Gly-Phe-Leu-NH-R (R = carbohydrate residue) were synthesized in order to determine the influence of some carbohydrate molecules (6-amino-6-deoxy-D-glucopyranose, 2-amino-2-deoxy-D-glucopyranose, beta-D-glucopyranosylamine) on the biological activity, conformation, and stability of the opioid pentapeptide [Leu5]enkephalin. For the preparation of this compound different methods of peptide synthesis (active ester and mixed anhydride) were investigated. In comparison with [Leu5]enkephalin, all three N-glycoconjugates showed higher potency in the guinea pig ileum assay and lower potency in the mouse vas deferens assay, indicating a decrease in delta opioid receptor selectivity.

Structure-activity relationships of cyclic beta-casomorphin-5 analogues

Cyclic analogues of the beta-casein-derived opioid peptide beta-casomorphin-5 (H-Tyr-Pro-Phe-Pro-Gly-OH) were prepared through substitution of the Pro2 residue with various alpha,omega-diamino acid residues (lysine, ornithine, 2,4-diaminobutyric acid) and cyclization of the omega-amino group to the C-terminal carboxyl function. Compounds of this type, with D-configuration at the 2-position residue, showed high opioid receptor affinity with some preference for mu receptors over delta receptors, high potency in the guinea pig ileum assay and considerable activity in the mouse vas deferens assay. Configurational inversion at the 4-position in these cyclic analogues resulted in enhanced affinity for both mu and delta receptors, whereas N-methylation of the Phe3 residue produced a potency decrease.

Conformational restriction of the phenylalanine residue in a cyclic opioid peptide analogue: effects on receptor selectivity and stereospecificity

In an effort to determine the effect of side chain conformational restriction on opioid receptor selectivity, the cyclic phenylalanine analogues 2-aminoindan-2-carboxylic acid (Aic), 2-aminotetralin-2-carboxylic acid (Atc), and tetrahydroisoquinoline-3-carboxylic acid (Tic) were substituted for Phe in the potent cyclic opioid peptide analogue H-Tyr-D-Orn-Phe-Glu-NH2, which lacks significant opioid receptor selectivity. Compounds were tested in mu- and delta-opioid receptor representative binding assays and bioassays in vitro. The analogue H-Tyr-D-Orn-Aic-Glu-NH2 was found to be a potent agonist with high preference of mu receptors over delta receptors. Opening of the five-membered ring of Aic in the latter peptide, as achieved through substitution of C alpha-methylphenylalanine or o-methylphenylalanine, resulted in only slightly selective compounds, indicating that the high mu selectivity of the Aic analogue is exclusively the consequence of the imposed side chain conformational restriction. Both diastereoisomers of H-Tyr-D-Orn-(D,L)-Atc-Glu-NH2 were highly mu-selective and, in contrast to the weak affinity observed with the D-Phe3 analogue as compared to the L-Phe3 analogue, both had similar potency. Thus, stereospecificity was lost as a consequence of side chain conformational restriction. Further structure-activity data obtained with analogues containing L- or D-homophenylalanine (Hfe) or 3-(1'-naphthyl)alanine (Nap) in place of Phe3 and consideration of geometric interrelationships between Nap and the L and D isomers of Atc, Hfe, and Phe led to the proposal that the D-Phe3 and the D-Atc3 analogue may have different modes of binding to the receptor. The very low potency observed with H-Tyr-D-Orn-N alpha MePhe-Glu-NH2 (N alpha MePhe = N alpha-methylphenylalanine) and H-Tyr-D-Orn-Tic-Glu-NH2 indicated that N alpha-alkylation at the 3-position is detrimental to activity.

Comparative conformational analysis of [D-Pen2,D-Pen5]enkephalin (DPDPE): a molecular mechanics study

A theoretical conformational analysis (molecular mechanics study) of the delta opioid receptor-selective enkephalin analog H-Tyr-D-Pen-Gly-Phe-D-Pen-OH (DPDPE) was performed, based on the use of the SYBYL software. The study led to the identification of several conformers that were significantly lower in energy than previously reported candidate conformers of DPDPE which, for comparative purposes, were also minimized by using the standard SYBYL force field. The results revealed a considerable degree of conformational flexibility of the DPDPE molecule, and suggested that incorporation of further conformational constraints into this enkephalin analog will be necessary in order to elucidate its receptor-bound conformation.

Conformational studies of stereoisomeric 14-membered cyclic enkephalin analogues containing 1-naphthylalanine at the fourth position: chirality effect of leucine at the fifth position on biological activity and receptor selectivity

In order to study structure-activity relationships of enkephalin-related analogues, we report the biological activity and conformational analysis of four 14-membered cyclic enkephalin analogues with beta-(1-naphthyl) alanine in place of phenylalanine at the fourth position, Tyr-c[D-A2bu-Gly-(L and D)-beta Nal(1)-(L and D)-Leu]. The L-beta Nal(1)-containing analogues display higher activity at both the mu and delta receptors than the corresponding analogues with the L-Phe residue. In contrast to the linear enkephalins, the cyclic analogues with the D-beta Nal(1) residue are also active at the mu receptor since the relative spatial arrangement of functional groups required for biological activity is achieved by the constrained nature of the cyclic molecules. A comparison of the findings from the conformational analysis and biological assays establishes that relatively extended structures, in which the two aromatic side chains are oriented in opposite directions with a approximately 14 A separation, is required for activity at the mu receptor. On the other hand, folded conformations with nearly parallel orientations and a close proximity (less than 10A) of the aromatic rings of the Tyr and beta Nal(1) residues are required for activity at the delta receptor. It should be noted that the overall structures and thus the biological profiles of the 14-membered cyclic enkephalin analogues are strongly dependent on the conformation of the second residue. The folded conformations with parallel orientation of the two aromatic side chains of Tyr-c[D-A2bu-Gly-L-beta Nal(1)-D-Leu] is stabilized by an interaction between the Tyr phenolic OH proton and beta Nal(1) C*O groups. This analogue, which shows the highest activity at both the mu and delta receptors among the four stereoisomers studied, displays an increase of the fraction of the side-chain chi 1 = t conformer for the beta Nal(1) residue. It is concluded that the incorporation of the D-Leu residue at the fifth position increases the relative fraction of the folded conformations with parallel orientation of the aromatic side chains, and hence enhances activity at the delta receptor as compared to the corresponding L-Leu containing analogue.

Direct electrophilic fluorination of tyrosine in dermorphin analogues and its effect on biological activity, receptor affinity and selectivity

In a preliminary communication we reported [(Tetrahedron Lett. 31, 619 (1990)] that acetyl hypofluorite can be used efficiently to introduce fluorine regiospecifically (ortho to OH) into the phenolic ring of tyrosine-containing peptides. This procedure has been applied to the fluorination of a number of mu-selective opioid peptides derived from dermorphin. While the procedure can be used even when the side chains of Arg, Lys, and Tyr are left unprotected, the sulfoxide of a Met(O)-containing analogue was oxidized to sulfone faster than fluorination of the phenolic ring. This method can also be used when the peptide is attached to Merrifield resin. Thus, Tyr(3-F)-D-Ala-Phe-Gly-NH2 and Tyr(3-F)-D-Arg-Phe-Lys-NH2 (F-DALDA) have been prepared, purified, and characterized. Affinities of these fluorinated peptides for both mu- and delta- opioid receptors are reduced (two- to nine-fold) relative to their nonfluorinated analogues, but their selectivity for mu-opioid receptors is not significantly altered. Similarly, the in vitro biological potencies (GPI and MVD assays) of the fluorinated analogues are reduced (two- to seven-fold) relative to their nonfluorinated parent peptides. Thus, F-DALDA, which has high affinity (Ki mu = 15.2 nM) and selectivity (Ki delta/Ki mu = 5390) for mu-opioid receptors, has potential use in biochemical studies which utilize 19F or 18F- labeled compounds.

Biological and conformational studies of [Val4]morphiceptin and [D-Val4]morphiceptin analogs incorporating cis-2-aminocyclopentane carboxylic acid as a peptidomimetic for proline

We report the synthesis, biological activity, and conformational analysis of tetrapeptide analogs related to [Val4]morphiceptin and [D-Val4]morphiceptin in which the proline at the second position has been replaced with cis-2-aminocyclopentane carboxylic acid (cis-2-Ac5c). Since the cis-2-Ac5c residue contains a normal amide, only the trans form has been observed about the amide bond between the first and second residues. The cis-2-Ac5c is a beta amino acid with two chiral centers resulting in two possible configurational isomers, namely (1S, 2R) and 1R, 2S) forms. The analogs containing the (1S, 2R)-Ac5c residue show activity at the mu-receptor but are inactive at the delta-receptor, resulting in a high selectivity for the mu-receptor. The (1R,2S)-Ac5c containing analogs are completely inactive at both the mu- and delta-receptors. The conformational analysis indicates that the separation of the aromatic rings of the tyrosine and phenylalanine residues, as expressed by the center-to-center distance, is 10.1-12.7 A for the preferred conformations of the bioactive analogs containing the (1S,2R)-Ac5c residue while a range of 4.8-7.0 A is observed for the preferred conformations of the inactive analogs with the (IR,2S)-Ac5c residue. A comparison of the findings from the conformational analysis and biological assays establishes the fact that a relatively large separation of the two aromatic side chains is required for the mu-opioid receptor activity of these molecules. Since the tetrapeptide amides studied in this investigation show similar biological profiles to those of the morphiceptin-related analogs, we have compared the preferred conformations estimated for the cis-2-Ac5c containing analogs with those of morphiceptin. One of the low energy conformations calculated for morphiceptin with the cis form about the tyrosine and proline residues has considerable topological similarity with the bioactive analogs containing the (1S,2R)-Ac5c residue, indicating that the cis from about these two residues is required for the biological activity of the morphiceptin-related analogs containing the proline at the second position.

Spectroscopic analysis of [Trp3]-beta-casomorphin analogs. Comparative structure conformation-activity studies

A series of [3-tryptophan]-beta-casomorphin-5([Trp3]-beta-CM-5) analogs were investigated by circular dichroism (CD) and fluorescence spectroscopy to explore their structure-conformation properties in solution. In addition, the comparative opioid activities of these compounds were evaluated using the in vitro guinea pig ileum (GPI) and mouse vas deferens (MVD) assays. Specifically, the pentapeptide sequence of [Trp3]-beta-CM-5, H-Tyr-Pro-Trp-Pro-Gly-OH (I) was modified at Pro-2 and Pro-4 by D-Pro substitutions to provide two diastereometric analogs, [Trp3-D-Pro-4]-beta-CM-5 (II) and [D-Pro2,4,Trp3]-beta-CM-5 (III). In the GPI and MVD assays, beta-CM-5 effected IC50 values of 1.3 microM and 8.9 microM, respectively, which confirmed its known mu/delta-selectivity on these two peripheral opioid receptor subtypes. The potencies of compounds I, II, and III were 0.2, 2.0, and less than 0.005 relative to beta-CM-5 on the GPI assay. Compounds I and II exhibited pronounced mu/delta-selectivities (greater than 18.9- and 12.4-fold respectively), whereas compound III was essentially inactive in both the GPI and MVD assays. CD studies of beta-CM-5 and its [Trp3]-beta-CM-5 analogs showed striking differences in their near-UV and far-UV spectra in aqueous or organic solvents. In the far UV CD spectra, weak (20%) alpha-helicity (maximum at 193 nm and minima at 208 and 222 nm) for beta-CM-5 was obtained in trifluoroethanol (TFE); however, none of the [Trp3]-beta-CM-5 analogs showed such CD bands. Of potential relevance to gamma-turn or C7 secondary structure was the observation of a strong negative band at 245 nm for compounds II and III which was not solvent-dependent in H2O or TFE, whereas compound I showed this CD band exclusively in TFE. In the near-UV CD at 275 nm (Trp electronic transition), the relative order of intensities of this band were determined for the [Trp3]-beta-CM-5 compounds to be II greater than I greater than III, which was identical to their relative biological potencies in both the GPI and MVD assays. Fluorescence energy transfer (FET) experiments of compounds I-III provided the intramolecular distances (r) between their Tyr (donor) to Trp (acceptor) side-chains, by the Förster method, and were as follows: [Trp3]-beta-CM-5, r = 10.6 A; [Trp3, D-Pro4]-beta-CM-5, r = 9.6 A; and [D-Pro2,4,Trp3]-beta-CM-5, r = 11.0 A.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of morphine, H-Tyr-D-Arg-Phe-Lys-NH2 (DALDA) and B-HT920 on non-cholinergic nerve-mediated bronchoconstriction in pithed guinea-pigs

1. Electrical stimulation (1 ms, 5 Hz, 80 V) for 5 or 15 s at the level of C4-T1 in the spinal canal of artificially respired pithed guinea-pigs (which had received intravenously (i.v.) (+)-tubocurarine chloride 2 mg kg-1, atropine sulphate 2 mg kg-1 and pentolinium tartrate 5 mg kg-1) caused constriction of airways, indicated by increased insufflation pressure. 2. This non-cholinergic constriction was inhibited by morphine (1-3 mg kg-1, i.v.), the peripherally acting mu-receptor agonist, H-Tyr-D-Arg-Phe-Lys-NH2 (DALDA, 0.1-1 mg kg-1, i.v.) or the alpha 2-adrenoceptor agonist B-HT920 (1-3 mg kg-1, i.v.). 3. The effects of either morphine (3 mg kg-1, i.v.) or DALDA (1 mg kg-1, i.v.) were inhibited by naloxone (3 mg kg-1, i.v.). Idazoxan (3 mg kg-1, i.v.) inhibited the anti-constrictor effect of B-HT920 (3 mg kg-1, i.v.), but not that of DALDA (0.1 mg kg-1, i.v.). 4. Thus activation of peripheral mu-opioid receptors or alpha 2-adrenoceptors inhibits airways constriction induced by non-cholinergic nerve stimulation in the pithed guinea-pig. This preparation therefore provides a further method for the in vivo examination of the effects of drugs on non-cholinergic tracheobronchial constrictor nerve function.

Synthesis and biological activity profiles of atrial natriuretic factor (ANF) analogs

Several analogs of the atrial natriuretic factor (ANF) were synthesized by the solid-phase method using the acetamidomethyl (Acm) group for sulfhydryl protection. The compounds were tested in a receptor binding assay using bovine adrenal zona glomerulosa cell membranes and in the rat diuresis/natriuresis assay. Substitution of tyrosine in position 116 of ANF(101-126) and of the analog [3-Mpr105]ANF(105-126)(3-Mpr = 3-mercaptopropionic acid) did not alter the biological activity profiles and, therefore, these two analogs in radioiodinated form will be useful for enzymatic degradation and clearance studies. Replacement of 3-mercaptopropionic acid with 2-mercaptopropionic acid in [3-Mpr105]ANF(105-126) resulted in an analog with very low potency in both assay systems, presumably as a consequence of the steric bulk and/or local conformational restriction produced by the methyl group attached to the alpha-carbon in position 105. The analog [3-Mpr105,Nva109]ANF(105-126)(Nva = norvaline) showed very low affinity in the receptor binding assay but displayed considerable diuretic/natriuretic activity. The obtained biological activity profiles suggest that in comparison with other ANF peptides the des-amino ANF(105-126) analogs may have a somewhat longer half-life in vivo, or alternatively, may indicate a more complex situation of ANF receptor or binding site heterogeneity.

A conformational comparison of two stereoisomeric cyclic dermorphin analogues employing NMR and computer simulations

In a continuation of our program to study the structure-activity relationship of peptide opiates, we report the conformational analysis of two cyclic tetrapeptides related to dermorphin--Tyr-c[D-Orn-Phe-Asp]-NH2 and Tyr-c[D-Asp-Phe-Orn]-NH2. These analogues have similar binding properties marked by a high selectivity for the mu-opioid receptors because of a drastic decrease in the affinity for the delta-opioid receptor. The conformational preferences of these analogues of dermorphin determined from proton nmr, molecular dynamics, and energy minimizations are quite similar. The constraint of the 13-membered ring formed from cyclization is quite evident from the conformational analysis. The constrained ring system acts as a template maintaining the relative orientation of the exocyclic tyrosine and side chain of phenylalanine. Two intramolecular hydrogen bonds measured for the D-Orn analogue in DMSO were disrupted upon the addition of water. For the D-Asp analogue, two intramolecular hydrogen bonds were found stable in DMSO and water. The global conformations of the two peptides determined from nuclear Overhauser effects did not change with the solvent titration. The difference in the hydrogen bonding within the 13-membered ring may account for the slight differences observed in the efficacy of the analogues at the mu-opioid receptors.

Conformation-activity relationships of cyclic dermorphin analogues

A theoretical conformational analysis (molecular mechanics study) of nine cyclic tetrapeptides, structurally related to the highly mu-receptor-selective dermorphin analogue H-Tyr-D-Orn-Phe-Asp-NH2, was performed. These compounds display considerable diversity in their mu-receptor affinity and selectivity. A systematic search and subsequent energy minimization in absence of the exocyclic Tyr1 residue and Phe3 side chain revealed the constrained nature of the 11-13-membered ring structures contained in these analogues. No more than four low-energy conformers (within 2 kcal/mol of the lowest energy conformation) were found in each case. After attachment of the Tyr1 moiety and Phe3 side chain to the "bare" low-energy ring structures, a systematic search and energy minimization of these exocyclic moieties resulted in a limited number of low-energy conformational families for all compounds. Five analogues with high mu-receptor affinity--H-Tyr-D-Orn-Phe-Asp-NH2, H-Tyr-D-Orn-Phe-D-Asp-NH2, H-Tyr-D-Asp-Phe-Orn-NH2, H-Tyr-D-Asp-Phe-A2bu-NH2 (A2 bu: alpha, gamma-diaminobutyric acid) and H-Tyr-D-Cys-Phe-Cys-NH2--all showed a tilted stacking interaction between the Tyr1 and Phe3 aromatic rings in the lowest or second lowest energy conformation found. The same kind of stacking was not possible in low-energy conformers of the four analogues with poor affinity for the mu-receptor [H-Tyr-L-Orn-Phe-Asp-NH2, H-Tyr-D-Orn-D-Phe-Asp-NH2, H-Tyr-D-Orn-Phe(NMe)-Asp-NH2 [Phe(NMe): N alpha-methylphenylalanine], and H-Tyr-D-Orn-Phg-Asp-NH2 (Phg: phenylglycine)].(ABSTRACT TRUNCATED AT 250 WORDS)

Morphiceptin analogs containing 2-aminocyclopentane carboxylic acid as a peptidomimetic for proline

As part of a program to study the structure-activity relationship of peptide opioids we report the synthesis, conformational characterization and biological activity of four analogs related to morphiceptin in which the proline at position two has been substituted with 2-aminocyclopentane carboxylic acid (beta Ae5c). The beta Ac5c residue is a beta amino acid with two chiral centers resulting in four possible configurations; two configurational cis (R,S and S,R) and two configurational trans (R,R and S,S) forms. Utilizing high resolution n.m.r. at 500 MHz and computer simulations with NOE restraints the chirality of the beta Ac5c residues are assigned. The analog containing the R,S-beta Ac5c is active at both the mu and delta-opioid receptors, with a slight preference for the mu-receptor. The (S,R), (S,S), and (R,R) analogs show minimal activity at the mu-receptor and are inactive at the delta-receptor. A comparison of the findings from the conformational analysis and biological assays lends insight into the structure-activity relationship of this important peptide opiate.

Enkephalin analogues containing beta-naphthylalanine at the fourth position

To examine the importance of the aromatic side chains of enkephalin on opiate activity, we report the synthesis and conformational analysis of a series of analogues related to enkephalin with beta-naphthylalanine in place of phenylalanine at the fourth position. Three linear analogues (Tyr-D-Ala-Gly-(L and D)-beta Nal(1)-Leu-NH2 and Tyr-D-Ala-Gly-beta Nal(2)-Leu-NH2) were initially synthesized to examine the effect of the substitution on biological activity. The increased activity of these peptides at the mu-opiate receptor, compared to native Leu-enkephalin, prompted us to examine the more conformational constrained analogues, Tyr-c[D-A2bu-Gly-(L and D)-beta Nal(1)-Leu], incorporating a alpha, gamma-diaminobutyric acid at the second position and cyclization to the carboxylic end of the leucine. These two cyclic analogues provide insight into the necessity for the L chirality of the aromatic residue at position 4. The Tyr-c[D-A2bu-Gly-L-beta Nal(1)-Leu] analogue is highly potent and displays a slight preference for the mu receptor. The conformational analysis indicates that despite the high flexibility of the tyrosine side chain, the aromatic rings of the tyrosine and naphthylalanine are relatively distant from each other. The presence of two intramolecular hydrogen bonds help maintain the conformation of the 14-membered backbone ring that keeps the side chains directed away from each other. These findings are in agreement with our model of an extended structure required for mu selectivity and a folded form with close aromatic ring placement for delta selectivity.

Intramuscular and subcutaneous administration of atrial natriuretic factor in the rat

The effects of ANF (99-126) and its active analog, [3-mercaptopropionic 105]-ANF (105-126), on diuresis and natriuresis were assessed in the rat after intramuscular (i.m.) or subcutaneous (s.c.) injection. Water and sodium excretion increased significantly after i.m. or s.c. administration of either peptide in the first 20-min urine collection. ANF (99-126) was more active i.m. than its analog, while they both produced similar diuresis and natriuresis when they were administered s.c. The rapid and significant increase of plasma IR-ANF levels, detected by radioimmunoassay and analyzed by high performance liquid chromatography after i.m. or s.c. injection of these peptides, indicates very rapid passage into the blood circulation without degradation. These data demonstrate that either the i.m. or s.c. route can be used to administer ANF or its analog.

Biological activities of cyclic enkephalin pseudopeptides containing thioamides as amide bond replacements

Analogs of H-Tyr-cyclo(N epsilon-D-Lys-Gly-Phe-Leu) have been prepared which contain thioamides at the 3-4 position (monothio), 3-4 and 5-2 positions (dithio), and 2-3, 3-4, and 5-2 positions (trithio). These compounds have been tested for opioid activity in mu- and delta-receptor selective bio- and binding assays. As the number of sulfurs increased, the biological activities dropped on the guinea pig ileum and fluctuated modestly on the mouse vas deferens assay. Surprisingly, the compounds displayed increasing delta selectivity as the number of sulfurs increased. In the binding assay, the thioamide analogs tended to retain affinity toward the mu receptor. The mono- and dithio-analogs were more mu selective than the parent, while the trithio-analog was more delta selective. These results suggest that the subtle exchange of sulfur for oxygen can have a significant impact on receptor selectivity and affinity, and probably reflect the different conformation/structural requirements for binding vs. the biological transduction event.

Dermorphin analogues carrying an increased positive net charge in their "message" domain display extremely high mu opioid receptor selectivity

According to the membrane compartment concept the receptor specificity of ligands is based not only on ligand-receptor complementarity but also on specific ligand-membrane interactions. Elaboration of this concept for opioid peptide-receptor interactions had led to the assumption that mu- and delta-receptors are located in anionic and cationic membrane compartments, respectively, and to the prediction that positively charged opioid receptor ligands should display mu-receptor selectivity. To assess the validity of this model, we synthesized a series of dermorphin analogues carrying a net positive charge and tested them in mu- and delta-receptor representative binding assays and bioassays. Some but not all of the prepared compounds showed the receptor-selectivity profile expected on the basis of the membrane compartment concept. In particular, gradual augmentation of the positive charge from 1+ to 3+ in a series of dermorphin-(1-4) tetrapeptide analogues produced an enhancement of mu-receptor affinity and a progressive decrease in delta-receptor affinity, resulting in increasingly higher mu-receptor selectivity. The most selective compound was [D-Arg2,Lys4]dermorphin-(1-4)-amide (DALDA), showing a selectivity ratio (Ki delta/Ki mu = 11,400) more than 10 times higher than that of DAGO (Ki delta /Ki mu = 1050) and, thus, displaying unprecedented mu-receptor specificity. Because of its high positive charge (3+), DALDA may be particularly useful as a very specific agonist for studying peripheral mu-receptor interactions.

Cyclic enkephalin analogues containing alpha-amino-beta-mercapto-beta,beta-pentamethylenepropionic acid at positions 2 or 5

Analogues of the highly potent and delta-receptor-selective enkephalins 1-4 were prepared with alpha-amino-beta-mercapto-beta,beta-pentamethylenepropionic acid (Apmp) replacing the beta,beta-dimethylcysteine (Pen) at positions 2 or 5. The peptides 5-8 were prepared by employing D,L-Apmp and, following oxidative cyclization, the resulting diastereomeric peptides were separated and purified by preparative high performance liquid chromatography. Compounds 7 and 8, with D- or L-Apmp substituted at position 5 are approximately 5 orders of magnitude more potent in the MVD assay than analogues 5 or 6 with D- or L-Apmp at position 2. While displaying less delta-receptor selectivity than the corresponding Pen-containing compounds, 7 and 8 are an order of magnitude more potent. All the analogues showed diminished delta-receptor selectivity in the rat brain binding assay. Compounds 7 and 8 displayed delta-receptor affinity comparable to the corresponding Pen-containing analogues.

Role of the conformational element in peptide-receptor interactions. Studies with cyclic opioid peptide analogs

Biological activity profiles of three different families of cyclic opioid peptide analogs are presented. It is illustrated that conformational constraints introduced through peptide cyclizations can have drastic effects on receptor affinity, selectivity and 'efficacy' ('intrinsic activity'). Conformational studies of cyclic opioid peptides by various physico-chemical techniques have been initiated and have already produced insight into the conformational requirements of the various opioid receptor types. On the basis of the results obtained, conformational restriction of opioid peptides may represent a first promising step towards the goal of developing peptide mimetics.

Synthesis and biological activity of [Leu5]enkephalin derivatives containing D-glucose

The synthesis of some [Leu5]enkephalin derivatives is described in which D-glucose has been linked to the opioid pentapeptide through the ester bond involving the carboxyl function at the C-terminal with C-1 or C-6 of the D-glucopyranose moiety. Enkephalin derivatives were assayed for opioid activity and found to be full agonists in bioassays based on inhibition of electrically evoked contractions of the guinea pig ileum (GPI) and of the mouse vas deferens (MVD). The obtained results suggest that the opioid activity of the tested glucoconjugates depend upon the ester bond position in the molecule. Whereas 1-O conjugate 5 was somewhat more potent than [Leu5]enkephalin in the GPI assay, the 6-O conjugates, with the exception of 1-O-benzyl derivative 11, were considerably less potent. All enkephalin derivatives were delta-receptor selective; in particular, the acetylated analog 8 was three times more delta-receptor selective than [Leu5]enkephalin.