Research topics in the medicinal chemistry and molecular pharmacology of opioid peptides--present and future

Neuropeptidases

Neuropeptides are neurotransmitters and modulators distributed in the central nervous system (CNS) and peripheral nervous system. Their abnormalities cause neurological and mental diseases. Neuropeptidases are enzymes crucial for the biosynthesis and biodegradation of neuropeptides. We here focus on the peptidases involved in the metabolism of the well-studied opioid peptides. Bioactive enkephalins are formed from propeptides by processing enzymes—prohormone thiol protease, prohormone convertase 1 and 2 (PC 1 and 2), carboxypeptidase H/E, and Arg/Lys aminopeptidase. After they exert their biological effects, enkephalins are likely to be inactivated by degrading enzymes—angiotensin-converting enzyme (ACE), aminopeptidase N (APN), puromycin-sensitive aminopeptidase (PSA), and endopeptidase 24.11. Recently, a neuron-specific aminopeptidase (NAP), which was a putative enkephalin-inactivating enzyme at the synapses, was found. Neuropeptidases are useful drug targets and their inhibitors can be therapeutic. Synthetic anti-enkephalinases and anti-aminopeptidases are being developed. They are potent analgesics but have fewer side effects than the opiates.

Synthesis of novel morphiceptin analogues modified in position 3 and their binding to mu-opioid receptors in experimental mammary adenocarcinoma

Binding of the (125)I-labeled mu-opioid receptor selective ligands, morphiceptin (Tyr-Pro-Phe-Pro-NH(2)) and [D-Phe(3)]morphiceptin, to membranes isolated from experimental mouse mammary adenocarcinoma was examined in vitro using a cross-linking assay followed by a Western blot technique. The radioactive complex had a molecular weight of about 65 kDa and was detectable by anti-mu-opioid receptor antibody, indicating the presence of mu-opioid receptors in tumor membranes. A series of new morphiceptin analogues, modified at the pharmacophoric position 3, was synthesized in order to find the correlation between the lipophilicity, electronic and steric properties of the amino acid in this position and the in vitro affinity of new analogues for mu-opioid receptors on mouse brain and tumor membranes. In in vivo studies the uptake of (131)I-labeled analogues by experimental mammary adenocarcinoma was estimated. The highest affinity for mu-opioid receptors in both, in vitro and in vivo experiments was observed for [D-Phe(3)]morphiceptin and [D-ClPhe(3)]-morphiceptin.

Dalargin and [Cys-(O2NH2)]2 analogues of enkephalins and their selectivity for mu opioid receptors

1. Effects of the enkephalins Met-enk (M) and Leu-enk (L), of two newly synthesized analogues--[Cys-(O2NH2)]2-Met-enk (CM) and [Cys-(O2NH2)]2-Leu-enk (CL)--and of a hexapeptide--D-Ala2-Leu5-Arg6 (Dalargin; DL) on the spontaneous and electrically stimulated activity were examined with respect to their selectivity for the mu opioid receptors in the longitudinal layer of guinea pig ileum. 2. M and CM exerted relaxing and contractile effects on the spontaneous contractile activity while L, CL and DL produced only relaxation. The order of potency towards the relaxatory phase was DL > M > CM > L > CL and towards the contractile phase CM > M. 3. The effects of enkephalins on the spontaneous activity were naloxone and TTX sensitive except for the contractile phase of M and CM which persisted in the presence of TTX. NO was not involved in the neurotransmission of the relaxatory responses, while the blockade of alpha and beta adrenoceptors showed the participation of adrenergic mechanisms. Relaxation and contraction induced by enkephalins could not be directly attributed to cholinergic neurotransmission. 4. The naloxone-sensitive and concentration-dependent inhibitory effects of enkephalins and their analogues on the electrically stimulated cholinergic contractions were established. The order of the relative potency of opioids was: DL-3.8; M-1.0; L-0.4; CM-0.01; CL-0.005. 5. These data indicated that the D-Ala2 substitution and lengthening of the peptide chain by Arg6 in the molecule of L increased the potency at the mu opiate receptors, while the substitution in position 2 with Cys-(O2NH2) in the molecule of M and L yielded a less potent and selective mu agonists.

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.

Development of delta opioid peptides as nonaddicting analgesics

Although much effort has been devoted to opioid research since the identification of enkephalins, understanding of the physiological importance and mechanisms of action of endogenous opioids lags behind understanding of opiate alkaloids such as morphine. In recent years, several novel approaches have been refined with promise for the successful development of the long-awaited nonaddicting analgesics that act at the opioid delta receptor. The present communication reviews these efforts.

A new fluorogenic assay for tyrosine-containing peptides

A new tyrosine-specific LC assay with pre-column fluorogenic derivatization is described for Tyr-Gly as model peptide. o-Hydroxylation of the tyrosine residue with tyrosinase in the presence of ascorbic acid, followed by oxidation to the corresponding quinone by potassium ferricyanide at room temperature and condensation with 1,2-diamino-1,2-diphenylethane in the presence of acetonitrile gave a highly fluorescent species. The resulting fluorescence signal was stable over the investigated period of 5 h and exhibited a linear response curve on a reversed-phase LC system. Under optimized reaction conditions, the lower limit of detection for Tyr-Gly was 200 fmol per injection. Examination of a series of dipeptides (L-Tyr-L-X; X = Gly, Ala, Val, Leu, Phe) showed no significant influence of neighbouring amino acids on the enzymatic hydroxylation by tyrosinase. This and the formation of a highly fluorescent signal for Leu-enkephalin suggests the general feasibility of the approach for the determination of tyrosine-containing peptides.

Conformational analysis of enkephalin analogs containing a disulfide bond. Models for delta- and mu-receptor opioid agonists

Conformational analysis of the cyclic opioids H-Tyr-D-Pen-Gly-Phe-D-Pen-OH (DPDPE) and H-Tyr-D-Cys-Gly-Phe-D-Cys-OH (DCDCE) have been performed using the AMBER program. DPDPE is considerably more selective for delta-receptors than DCDCE. Using the RNGCFM program, a large number of ways were found to close the 14-membered disulfide-containing ring structure. However, intramolecular hydrogen bonds were only possible in gamma-turn and inverse gamma-turn conformations centered on the glycine residue which were associated with opposite chiralities of the disulfide bond. With the cyclic part of the molecules in either a gamma-turn or inverse gamma-turn, a systematic conformational analysis was performed on the tyrosine and phenylalanine sidechains. This showed that conformers with the tyrosine and phenylalanine phenyl rings in the vicinity of the disulfide bond were preferred due to attractive van der Waals forces. For DPDPE, however, this was only possible with a positive dihedral angle for the disulfide bond due to the presence of the beta-carbon methyls of Pen2. In contrast, these preferred conformers were possible with both chiralities of the disulfide bond in DCDCE. Conformational entropies and free energies were computed from the translational, rotational, and vibrational energy levels available to each conformer. The conformational entropies were found to vary significantly and to result in a re-ordering of the lowest energy minima. Based on these conformational differences in DPDPE and DCDCE and their differing pharmacological selectivities, tentative conformational preferences for delta- and mu-receptor opioid peptides are proposed.

Importance of folded monomer and extended antiparallel dimer structures as enkephalin active conformation. Molecular dynamics simulations of [Met5]enkephalin in water

Simulations of the molecular dynamics of the [Met5]enkephalin monomer and dimer structures in water have been carried out. The dynamic trajectories have been analyzed in terms of the distances between intra- or intermolecular polar atoms. The time-correlated conformational transitions of an extended monomer structure have been converged into a stationary state among the beta-bend folded forms. However, the dynamics simulation of an extended antiparallel dimer structure has shown no noticeable conformation change. These results imply that both the beta-bend monomer and the extended dimer structures exist together as the fundamental conformation of enkephalins.

Effects of the enkephalin analogue FK33-824 on rectal temperature and respiratory rate in male mice

Subcutaneous administration of the enkephalin analogue FK33-824 (FK) elicited a dose-related decrease in rectal temperature and respiratory rate in male ddY strain mice. Naloxone and 3 days' implantation of morphine pellet decreased the effects of FK, suggesting the involvement of opioid receptors and cross-tolerance with morphine to both effects of FK. A positive correlation was found between the FK-induced decrease in rectal temperature and that in respiratory rate among the 6 strains of inbred mice including BALB/c, C3H, A/J, CBA, C57BL/6 and DBA/2. The degree of hypothermia elicited by FK was different among strains, whereas marginal strain difference was seen in the respiratory depression induced by FK. The strain difference in the FK responses may be due to the difference in the opioid receptor subtypes in the brain.

Conformational characteristics of receptor-selective opioid peptides. 1H n.m.r. and c.d. spectroscopic studies of delta-kephalin and [Val4]morphiceptin

An investigation on the conformations of highly receptor-selective opioid peptides was carried out to gain further understanding of the structure-activity relationship of endogenous enkephalins. The preferred conformations of a highly mu-selective [Val4]morphiceptin and a highly delta-selective delta-kephalin have been probed by 1H n.m.r. solvent-, concentration- and temperature-dependences of amide protons to take the folded conformations stabilized by an intramolecular hydrogen bond and the anti-parallely extended dimeric structures respectively. Their possible stereo-conformations were proposed, based on the analyses of the vicinal coupling constants (JHNC alpha H). The conformational difference between the mu- and delta-selective opioid peptides was further ascertained by the c.d. measurements. The c.d. spectra of the mu-selective peptides show negative bands in the range of 210-230 nm, while those of the delta-selective ones show the opposite positive bands. A correlation between c.d. spectra and receptor-selectivity was possible.