A highly selective ligand for brain delta opiate receptors, a cyclopropyl(E)Phe(4)-enkephalin analog, suppresses mu receptor-mediated thermal analgesia by morphine.

[D-Ala(2)(2R,3S)-delta(E)Phe(4)Leu(5)]enkephalin (CP-OH) [delta denoting cyclopropyl; superscript E indicating the E-configuration about the cyclopropane ring], a highly selective opioid ligand for delta receptors in rat brain, but not for those in the mouse vas deferens, was examined for in vivo biological activities by intracerebroventricular administration. CP-OH (5-20 micrograms) showed no analgesic activity in the hot plate (51 degrees C) test using rats. However, it suppressed completely the analgesic effects of intraperitoneally administered morphine (3 mg/kg rat) in a dose-dependent manner. CP-OH showed no binding affinity for brain kappa receptors to which dynorphin, an opioid peptide that inhibits morphine analgesia, binds predominantly. These results suggest that, besides the conventional delta receptors which mediate analgesia, the rat brain contains another delta-like receptor which has a modulatory role to attenuate morphine-induced analgesia mediated through the mu receptors, and that this modulatory receptor does not exist in the mouse vas deferens.

Enzyme inhibition by dipeptides containing 2,3-methanophenylalanine, a sterically constrained amino acid.

Both isomers of (E)-2,3-methanophenylalanine (delta EPhe), a sterically restricted amino acid, were incorporated into peptides in order to examine their possible enzyme inhibitory activity. Both (2R,3S)- and (2S,3R)- delta EPhe-Phe(or Leu)-OMe were found to inhibit effectively the hydrolysis of Ac-Tyr-OEt by chymotrypsin in a competitive manner. The ester groups of these dipeptides were quite resistant to chymotrypsin hydrolysis, and the delta EPhe-Phe peptide bond was also entirely stable. The inhibition constant (Ki) of the most potent dipeptide of H-(2R,3S)-delta EPhe-Phe-OMe was 0.16 mM at 25 degrees C. The inhibitory action of delta Phe-containing peptides was found to depend on the configuration of the delta Phe residue. The electrophilic nature of the cyclopropane ring which is conjugated with both the phenyl ring and the ester carbonyl group appears to be relevant to the inhibitory activity. Fully irreversible inactivation of chymotrypsin was achieved by its incubation with H-(2R,3S)- delta EPhe-Leu-OMe. An enzyme carboxylate group is thought to be responsible for nucleophilic attack on the cyclopropane ring leading to irreversible inactivation.

delta EPhe4-enkephalin analogs. Delta receptors in rat brain are different from those in mouse vas deferens.

Conformationally restricted enkephalin analogs containing E-cyclopropylphenylalanine (delta EPhe), [D-Ala2, (2R,3S)-delta EPhe4,Leu5]enkephalin and its (2S,3R) isomer, were evaluated in receptor-binding assays using rat brain and in assays using muscle preparations. The (2S,3R) isomer was almost completely inactive in all assays. In contrast, the (2R,3S) isomer showed a very high affinity for the delta and a very weak affinity for the mu receptors in rat brain. The extent of delta affinity and the selectivity of this isomer were almost equal to those of [D-Pen2,D-Pen5]enkephalin. However, the (2R,3S) isomer was inactive in both the mouse vas deferens and guinea pig ileum assays, and showed no antagonistic activity in these tissues. These results indicate that the (2R,3S) isomer interacts with the delta receptors in rat brain, but not with those in the mouse vas deferens, and they suggest that the delta receptors in the central and peripheral nervous systems are different from each other.

Synthesis of N-hydroxythiourea.

The synthesis of the title compound (1) was accomplished by the conversion of 2,4-dimethoxybenzylamine (2) into an isothiocyanate (3) using thiocarbonyl diimidazole. Treatment of 3 with hydroxylamine and removal of the DMB group with trifluoroacetic acid gave 1. N-Hydroxythiourea (1) showed no activity in the L1210 mouse tumor.

3-hydroxyisoxazole-5-hydroxamic acid.

The synthesis of the title compound, 3-hydroxyisoxazole-5-hydroxamic acid (4b), by two procedures is described. The first, involving the treatment of dimethyl acetylenedicarboxylate with hydroxylamine, had previously been reported to give the 3-hydroxyisoxazole-5-carboxylic acid (4a). In the second, treatment of chlorofumaroyl dichloride with hydroxylamine also gave the intermediate chlorofumarodihydroxamic acid (6). Compound 6 was found to have some activity against P388 lymphocytic leukemia.

5-Carboxamido-4-amino-3-isoxazolidone, and asparagine analogue.

trans-Aziridine-2,3-dicarboxylic ester was used to prepare the required beta-chlorohydroxamic acid used in the synthesis of the title compound. The trans configuration of the asparagine analogue was established by hydrogenolysis to erythro-beta-hydroxyasparagine amide. Neither the title compound nor the intermediate aziridinehydroxamic acid (8) showed significant activity against the L1210 and P-388 tumors. The title compound was inactive as an inhibitor of asparagine synthetase from Novikoff hepatoma and did not inhibit the growth of some 25 bacteria and fungi.

Inhibition of pig kidney L-aromatic amino acid decarboxylase by 2,3-methano-m-tyrosines.

Both racemic (E)- and (Z)-2,3-methano-m-tyrosines (9E and 9Z) have been synthesized from a common intermediate, monoester (Z)-1-(ethoxycarbonyl)-2-[3-[(2-methoxyethoxy)methoxy]phenyl] cyclopropanecarboxylic acid (5). Quinine and ephedrine, respectively, were used to resolve their N-tert-butoxycarbonyl (Boc) derivatives. Among the compounds prepared, the (+)-(E)-diastereomer of 9 is the most potent inhibitor of L-aromatic amino acid decarboxylase (Dopa decarboxylase), having a Ki of 22 microM, with the (-)-Z-diastereomer (9Z) second at Ki = 49 microM. (+)-9E is a 45-fold more potent inhibitor of DDC than its acyclic analogue, D-m-tyrosine.

The enzyme stability of dehydro-enkephalins.

Dehydro-enkephalins [delta Ala2]-, [delta Ala3]-, [delta Phe4]-, and [delta Leu5]enkephalins, were examined for their stability to enzymatic hydrolysis by carboxypeptidase Y [EC 3.4.16.1]. The successively liberated amino acids were determined quantitatively by amino acid analyses. The saturated leucine-enkephalin was rapidly hydrolyzed from the COOH-terminus. However, peptide linkages with alpha, beta-dehydroamino acid residues placed in the enkephalin molecule were strongly resistant to the enzyme at the carboxyl side and completely resistant at the amino side of the dehydro residue.

The synthesis of N-benzyloxycarbonl-L-prolyl-dehydrophenylalanyl-L-histidyl-L-leucine: a converting-enzyme inhibitor.

The direct oxidation of a dipeptide azlactone (1) by DDG affords an optically active unsaturated dipeptide azlactone (2). It was shown that the double bond had the Z-configuration and that no racemization of the proline moiety occurred during the oxidation. The dehydrotripeptide, Z-Pro-delta-Phe-Phe OH (9b) was prepared by direct coupling of azlactone (2) with phenylalanine tetramethyl-guanidinium salt and was shown to be stable to chymotrypsin. The tetrapeptide, Z-Pro-delta-Phe-His-Leu OH (11), was prepared directly from the azlactone (2) and by saponification of the tetrapeptide ester (10) prepared by mixed anhydride coupling of Z-Pro-deltaPhe OH (7) with the dipeptide ester. The dehydropeptide (11) inhibits angiotensin I converting enzyme (IC50, 1X10-4 M).

Dehydro-enkephalins. III. Synthesis and biological activity of [delta Ala2, Leu5]-enkephalin.

[delta Ala2, Leu5]-enkephalin has been prepared and shown to be more active than the parent saturated enkephalin in a binding assay using rat brain membranes and [3H]dihydromorphine as a tracer. In a comparison of potencies against [3H]dihydromorphine and [3H]-[D-Ala2, D-Leu5]-enkephalin as tracers, [delta Ala2, Leu5]-enkephalin showed preference for micro opiate receptors, possibly due to the hydrophobicity of the delta Ala2 residue. A synthetic tetrapeptide enkephalin [delta Ala2]-desLeu5-enkephalin had weak activity and high selectivity for the micro receptors. O-Acylation of a serine residue in the peptide was achieved by coupling between the peptide and a carboxylic acid using DCC and a catalytic amount of 4-dimethylaminopyridine.

Dehydro-enkephalins. VI. Dehydroalanine3-enkephalin: a potent enkephalin analog for the delta opiate receptor.

The Gly3 residue in Gly2- and D-Ala2-enkephalins has been replaced by delta Ala3 and Ser3 in order to examine the effect on binding to the delta and mu opiate receptors. [D-Ala2, delta Ala3, Leu5]-enkephalin maintains most of its receptor binding affinities for both sites. It is suggested that the proper conjunctions of positions 2 and 3 of the enkephalin sequence are important to receptor preference and that position 3 may have a very specific interaction with the delta receptor. The in vivo analgesic and CNS activities of the peptides are also discussed.

Synthesis of four diastereomeric enkephalins incorporating cyclopropyl phenylalanine.

Four diastereomeric D-Ala2, Leu5-enkephalins have been synthesized and their CD spectra and rat brain binding affinities determined. Only the peptides containing Z-cyclopropyl phenylalanine residues showed strong binding affinity. No correlation between CD spectra and bioactivity could be made.

Synthesis and conformational studies by 1H- and 13C-NMR spectroscopy of a novel, sterically constrained analogue of thyrotropin-releasing hormone.

A novel thyrotropin-releasing hormone (TRH) analogue, [2,4-MePro3]-TRH (2,4-MePro: 2-carboxy-2,4-methanopyrrolidine), has been synthesized using a rapid solid phase peptide synthesis method, and its conformational properties investigated by one- and two-dimensional (2D) nmr spectroscopy and by proton Overhauser measurements. Following a published approach, calibrated interproton Overhauser effects were used together with distance geometry analysis to deduce that the single conformation of the His-2,4-MePro tertiary amide bond is trans in aqueous solution. This conclusion was corroborated by 2D dipolar-correlated (NOESY) spectroscopy. A preferentially extended conformation is indicated by the nmr data, similar to that of TRH. The phi, psi conformational space of 2,4-MePro is, however, significantly different from that of trans proline and the structural consequences of these differences at the C-terminus are discussed. The distribution of histidine side-chain conformations in the TRH analogue was deduced from coupling constants and from the short-range interaction between the imidazole ring and one of the prochiral faces of the 2,4-MePro side chain.

Conformational preferences of cyclopropyl peptides. Crystal structure of (E)-DL-1-benzamido-1-methoxycarbonyl-2-chlorcyclopropane (BCP).

Crystal structure analysis of (E)-DL-1-benzamido-1-methoxycarbonyl-2-chlorocyclopropane (C12H12NO3Cl) is reported. The phi' (about N1-C1 bond) and psi' (about C1-C11 bond) torsional angles for this compound are -62.5 degrees and -33.0 degrees, respectively, and are close to the phi, psi values of the 3(10) helix and the alpha-helix. Semi-empirical potential energy calculations are performed on a cyclopropyl dipeptide which is a special case of alpha,alpha-disubstituted dipeptide where the alpha-carbon and the two substituent carbon atoms form a 3-membered ring. Our calculations show that different types of helics: alpha-, gamma-, pi-, omega-, 3(10-) and delta-helices, are energetically favorable. Another interesting possibility is the formation of a cyclic pentapeptide with five-fold symmetry. The effect of substitutions on C beta atom are also studied with the help of potential energy maps. Selective substitutions on C beta atom may be used effectively to restrict either phi or psi values into a very narrow range.

Synthesis and receptor binding affinity of both E- and Z-dehydrophenylalanine4 enkephalins.

The dehydrophenylalanine4-enkephalin having the E-configuration (delta EPhe; phenyl and C = 0, cis) was prepared by photoisomerization of the Z-isomer with 3100 A light, followed by reversed-phase HPLC separation of the resulting mixture of the Z- and E-isomers. In the radioligand receptor binding assays, the E-isomer of [D-Ala2, delta Phe4, Leu5]enkephalin exhibited an extremely diminished affinity as compared with the Z-isomer, namely 150-260-fold loss of affinity for the delta and mu opiate receptors. The results indicate that the interrelationship of the Tyr1 and Phe4 residues in the enkephalin molecule seems to be of great importance in receptor recognition.