Binding studies of novel, non-mammalian enkephalins, structures predicted from frog and lungfish brain cDNA sequences

Leu- and Met-enkephalin were the first endogenous opioid peptides identified in different mammalian species including the human. Comparative biochemical and bioinformatic evidence indicates that enkephalins are not limited to mammals. Various prodynorphin (PDYN) sequences in lower vertebrates revealed the presence of other enkephalin fingerprints in these precursor polypeptides. Among the novel enkephalins Ile-enkephalin (Tyr-Gly-Gly-Phe-Ile) was primarily observed in the African clawed frog (Xenopus laevis) PDYNs, while the structure of Phe-enkephalin (Tyr-Gly-Gly-Phe-Phe) was predicted by analyzing brain cDNA sequences encoding a PDYN of the African lungfish (Protopterus annectens). Ile-enkephalin can also be found in the PDYNs of four other fish species including the eel, bichir, zebrafish and tilapia, but no further occurrence for the Phe-enkephalin motif is available as yet. Based on sequencing data, the biological relevance of Phe- and Ile-enkephalin is suggested, because both of them can arise by regular posttranslational enzymatic processing of the respective neuropeptide precursors. In various receptor binding assays performed on rat brain membrane preparations both of the new peptides turned out to be moderate affinity opioids with a weak preference for the delta-opioid receptor (DOP) sites. Phe-enkephalin of the lungfish displayed rather unexpectedly low affinities toward the mu-opioid receptor (MOP) and DOP, while exhibiting moderate affinity toward the kappa-opioid receptor (KOP). In receptor-mediated G-protein activation assays measured by the stimulation of [(35)S]GTPgammaS binding, Met-enkephalin produced the highest stimulation followed by Leu-enkephalin, Ile-enkephalin and Phe-enkephalin, whereas the least efficacious among these endogenous peptides was still more effective than the prototype opiate agonist morphine in these functional tests.

[Research on synthetic peptides of biological interest]

Research on synthetic peptides at the Institute for Drug Research (IDR) is exemplified by an overview of the projects that resulted in significant results. The first synthesis of oxytocin, a pituitary hormone, in 1953 launched the research on synthetic peptides all over the world. This synthesis was reproduced by Bodanszky at the IDR in 1954, then, after some improvements, the process was presented to Richter to produce synthetic oxytocin for therapeutic purposes. Significant result was the first synthesis of the 39-member whole molecule of human ACTH, another pituitary hormone. A short SAR study on luteinizing hormone-releasing hormone (LHRH) led to an interesting analog, Cit-8-LHRH, and somewhat later, to the D-Cit-6-LHRH analogues, of which SB-75 become marketed under the name Cetrorelix. Studies on the brain peptides, enkephalins, resulted in GYKI-14,238, the first analog that showed analgesic activity upon systemic administration and whose human efficacy could also be proven during clinical examination. Significant results were also achieved in the research on anticoagulant peptides. The first highly potent peptide aldehyde inhibitor of thrombin, GYKI-14,166, was identified at the IDR as well as its stable analog, GYKI-14,766. This compound was selected for detailed preclinical study, licensed to Eli Lilly Company, got the generic name efegatran, and entered clinical trials. The first non-covalent peptide inhibitor of thrombin, GYKI-14,525, was also identified at the IDR. Thus IDR really provided the prototype of original thrombin inhibitors in the mid 70's, and analogues were prepared in many laboratories through two decades. IDR's current research program's objective includes a quest for peptide originals that can inhibit both thrombin and factor Xa in solution and also within plasma clots in which these enzymes are entrapped. Structures with such inhibitory profile were identified among the efegatran-related alpha-hydroxy acid and ethoxycarbonyl-amino acid derivatives. The follow-up molecules are even more promising as antithrombotics, and may also be useful for treatment of disseminated intravascular coagulation, an often fatal syndrome, so we continue working on this project.

Research on synthetic peptides of pharmaceutical interest at IDR

Research on synthetic peptides at the Institute for Drug Research (IDR) is exemplified by an overview of the projects that resulted in significant results.

Peptidyl beta-homo-aspartals (3-amino-4-carboxybutyraldehydes): new specific inhibitors of caspases

Interleukin-1 beta (IL-1 beta)-converting enzyme (ICE, caspase-1) processes the IL-1 beta precursor to mature inflammatory cytokine IL-1 beta. ICE has been identified as a unique cysteine protease, which cleaves Asp-X bonds, shows resistance to E-64 (an inhibitor of most cysteine proteases) and has a primary structure that is homologous to CED-3, a protein required for apoptosis (programmed cell death) in the nematode Caenorhabditis elegans, and to mammalian cysteine proteases that initiate and execute apoptosis, e.g., apopain/CPP32/caspase-3. The inhibitors of the ICE/CED-3 family or caspases, as they are called recently, may constitute therapeutic agents for amelioration of inflammatory and apoptosis-associated diseases. The most efficient ICE inhibitors are peptide aldehydes and peptidyl chloro or (acyloxy)methanes. A recent study revealed that both D- and L-Asp are accepted by ICE at the P1 of such inhibitors, and the peptidyl (acyloxy)methane analogues having the beta-homo-aspartyl residue [-NH-CH(CH2COOH)-CH2CO-] are inactive. These findings we reexamined in terms of two issues. (a) ICE's resistance to E-64. Since it was thought to be caused by the enzyme's unique substrate specificity, we prepared substrate-based analogues, which were not inhibitory suggesting significant structural difference between the active centers of ICE and papain-like enzymes. (b) Tolerance for D-stereochemistry at the P1 of these inhibitors. In view of the mechanism of cysteine protease inhibition by peptidyl X-methanes, we thought that this phenomenon should be a general characteristic of cysteine proteases and the hAsp-containing analogues should behave as reversible inhibitors. Here, we analyzed the inhibition of ICE and apopain in comparison with that of papain, thrombin, and trypsin by peptide L/D-alpha-aldehydes and their L-beta-homo-aldehyde [-NH-CH(R)-CH2-CHO] analogues. The following results were found. (1) The peptidyl L-beta-homo-aspartals are potent inhibitors for caspases. (2) The L-beta-homo analogues of peptide aldehyde inhibitors designed for other proteases are not inhibitory. (3) Unlike trypsin and thrombin (serine proteases), papain (cysteine protease) shows tolerance for D-stereochemistry at the P1 site of peptide aldehydes in proportion to the lability of the alpha-hydrogen of the P1-D-residue. The complete tolerance of ICE for P1-D-Asp may arise from this residue's high tendency to epimerization. (4) Reaction of cysteine proteases with peptide aldehyde or peptidyl X-methane inhibitors containing P1-D-residues may include alpha-proton abstraction followed by asymmetric induction leading to P1-L-residue-containing products.

Peptidyl beta-homo-aspartals: specific inhibitors of interleukin-1 beta converting enzyme and its homologues (caspases)

Inhibition of interleukin-1 beta converting enzyme (ICE), apopain, papain, thrombin and trypsin with substrate like peptidyl L- and D-alpha-aldehydes and their L-beta-homo-aldehyde analogues was investigated. The L-beta-homo-aspartals appear to be specific inhibitors for ICE and its homologues; the other enzymes were not inhibited with such L-beta-homo aldehydes. Papain shows tolerance for D-residues at P1 depending on their chiral stability.

Active site-directed thrombin inhibitors: alpha-hydroxyacyl-prolyl-arginals, new orally active stable analogues of D-Phe-Pro-Arg-H

D-alpha-Hydroxyacyl-prolyl-arginals, a new type of analogues of D-Phe-Pro-Arg-H (R1), have been prepared and evaluated. Unlike R1, whose terminal group is NH2, the new analogues with a terminal OH group are stable, as are the N-substituted derivatives of R1, that is, D-MePhe-Pro-Arg-H (R2), the highly potent and selective thrombin inhibitor, and Boc-D-Phe-Pro-Arg-H (R3), the much less favorable analogue. The most notable of the new analogues corresponds to the general formula D-Xaa-Pro-Arg-H, wherein Xaa means the acyl residue of mandelic acid (Man, 1), diphenyllactic acid (Dpl, 2), hexahydrophenyllactic acid (Hpl, 3), or hexahydromandelic acid (Hma, 4). In plasma clotting assays, 1 to 4 appeared to inhibit thrombin as well as some other clotting enzymes involved in thrombin generation, whereas R1 and R2 seemed to produce anticoagulation through inhibition of thrombin only. In the fibrin plate assay, 1 to 4 possessed even more moderate antifibrinolytic activities than R2. In in vivo evaluation in rats and rabbits, 2 to 4 proved to be potent anticoagulants/antithrombotics even on oral administration in a dose of 5 mg/kg. In view of these findings with the alpha-hydroxyacyl-prolyl-arginals, it is very likely that the less favorable biologic properties of Boc-D-Phe-Pro-Arg-H are due to the hydrophobicity and bulkiness of the terminal Boc-NH rather than its neutrality.

Effect of peptide aldehydes with IL-1 beta converting enzyme inhibitory properties on IL-1 alpha and IL-1 beta production in vitro

Tripeptide and pentapeptide aldehydes as substrate-base inhibitors of cysteine proteases were designed in our laboratory for the inhibition of interleukin-1 beta converting enzyme (ICE), a recently described cysteine protease responsible for the processing of IL-1 beta. The biological effectivity of the peptide aldehydes was studied in THP-1 cells and human whole blood. The released and cell-associated IL-1 alpha and IL-1 beta levels were determined by ELISA from the supernatants and cell lysates, respectively. The total IL-1 like bioactivity was assayed by the D10 G4.1 cell proliferation method. The tripeptide aldehyde (Z-Val-His-Asp-H) and pentapeptide aldehyde (Eoc-Ala-Tyr-Val-Ala-Asp-H) significantly reduced IL-1 beta levels in the supernatants in relatively high concentrations (10-100 microM), but the IL-1 alpha release was unaffected by these peptides. However, a considerable decrease in the cell-associated IL-1 beta and IL-1 alpha levels was observed. N-terminal extension of the tripeptide aldehyde yielded even more potent inhibitors. Amino acid substitution at the P2 position did not cause considerable changes in the inhibitory activity. The peptide aldehydes suppressed the IL-1 beta production in a reversible manner, whereas dexamethasone, a glucocorticoid, had a prolonged inhibitory effect. The inhibitory effect of these peptides and that of dexamethasone appeared to be additive. These findings indicate that these peptide aldehydes might be used as IL-beta inhibitory agents in experimental models in which IL-1 beta is a key mediator or ICE is implicated.

Active site-directed thrombin inhibitors: alpha-hydroxyacyl-prolyl-arginals. New orally active stable analogs of D-Phe-Pro-Arg-H

D-alpha-Hydroxyacyl-prolyl-arginals have been designed and synthesized as orally active stable analogs of D-Phe-Pro-Arg-H, the active site-directed peptidyl thrombin inhibitor prototype. Many of the new analogs possess high in vitro anticoagulant activity while having little effect on fibrinolysis. Compounds GYKI-66104 (2), -66131 (3) and -66132 (5) effectively delay the clotting time in rabbits ex vivo and prevent thrombus formation in various thrombosis models in rabbits and rats when applied in a single oral dose of 5 mg kg-1.

N-terminally protected penta- and tetrapeptide opioid antagonists based on a pentapeptide sequence found in the venom of Philippine cobra

Opioid receptor antagonist properties of the mu opioid receptor selective peptide, BOC-Tyr-Lys-Lys-Trp-Trp-NH2 and its systematically modified analogues were determined in guinea pig ileum, mouse vas deferens and rabbit vas deferens bioassays to locate the necessary structural features to develop kappa receptor selective antagonist(s) of substantial affinity. Replacing the tyrosine residue by phenylalanine as well as increasing the lipophilicity of the C-terminal by isoamylamide substitution yielded enhanced kappa receptor affinity. The presence of the C-terminal lipophilic Trp-Trp-NH2 region is necessary as revealed from the equilibrium dissociation constant values. Recognizing that only one lysine residue is required for the antagonist activity led to the synthesis of the tetrapeptide BOC-Tyr-Lys-Trp-Trp-NH2 having a kappa/mu selectivity of 22 and a Ke of 5.4 microM.

The antithrombotic and anticoagulant effects of a synthetic tripeptide and recombinant hirudin in various animal models

The pharmacologic activities of two thrombin inhibitors (D-MePhe-Pro-Arg-H, recombinant-hirudin) were compared in two animal models. The antithrombotic effect was investigated in vivo in rabbits using a modified Wessler stasis thrombosis model. During these experiments, blood was drawn for ex vivo testing to determine the coagulation profile and to determine plasma concentrations using pre-constructed calibration curves. A dose-dependent antithrombotic effect was observed for both agents. On an equigravimetric basis (100 micrograms/kg i.v.), r-hirudin showed a stronger antithrombotic effect than the tripeptide, which correlated well with the ex vivo anticoagulant effect. No adverse reactions were observed during this study. In a rabbit ear bleeding model, a dose and time dependent hemorrhagic effect was observed for both agents. Only slight bleeding effects were observed at 1.0 mg/kg dosages. These studies show that the tripeptide D-MePhe-Pro-Arg-H and r-hirudin are specific thrombin inhibitors with potent antithrombotic effects and a high therapeutic (antithrombotic/hemorrhagic) index. Furthermore, the results of these two animal models and ex vivo analyses can be used to determine the therapeutic index of thrombin inhibitors.

Screening for fibrinolysis inhibitory effect of synthetic thrombin inhibitors

Fibrin plate assay (FPA) and thrombelastography (TEG) were used to assess the antifibrinolytic effects of D-Phe-Pro-Arg-H (1), the prototype of peptide aldehyde inhibitors of thrombin, and two of its more stable derivatives, D-MePhe-Pro-Arg-H (2) and Boc-D-Phe-Pro-Arg-H (3). Inhibition of plasmin generation by tissue plasminogen activator, urokinase and streptokinase were studied by both FPA and TEG while that of plasmin could only be examined by FPA. TEG was more sensitive than FPA in general and for the detection of streptokinase inhibition in particular. Derivative (3) was 2-50 times more inhibitory than (1) or (2) depending on the enzyme studied and the assay system used. The thrombin selectivities of (1)-(3) were defined as the thrombin to fibrinolytic enzyme potency ratios. Data obtained by the FPA and thrombin time assay indicated (1) and (2) to be 2-80 times more selective for thrombin than (3). On the other hand, the values determined by TEG and recalcification assay showed the thrombin selectivity of (2) to be two to three times higher than that of (1), and (3) to have no such selectivity. According to TEG studies, (1) and (2) assisted rather than inhibited fibrinolysis by reducing the elasticity of human plasma clots.

Inhibition by D-MePhe-Pro-Arg-H (GYKI-14766) of thrombus growth in experimental models of thrombosis

The antithrombotic action of the highly effective synthetic thrombin inhibitor D-MePhe-Pro-Arg-H (GYKI-14766) was studied in various models of experimental thrombosis. The compound administered to rats and rabbits by i.v. bolus injections, continuous i.v. infusions, subcutaneously and orally, respectively, induced significant decrease in thrombus weight (i) in a quantitative venous thrombosis model with stasis based on vascular lesion in rats, (ii) in an extracorporeal arterio-venous shunt model in rabbits, and (iii) prevented the occlusion of the vessel in arterial thrombosis induced by mechanical damage in rats. By using the arterio-venous shunt model in rabbits the inhibitory effect on thrombus growth could be demonstrated as a function of dose and time in self-controlled experiments. Blood level of the inhibitor determined by a bioassay varied between 0.09-0.67 microgram/ml whole blood when doses of 15 and 20 mg/kg were administered orally. A correlation was found between thrombin time, platelet aggregation induced by thrombin ex vivo and the weight of thrombi formed.

Comparative studies in vitro and ex vivo on the anticoagulant effect of a reversible and an irreversible tripeptide inhibitor of thrombin

Comparative studies on the anticoagulant effect of D-Phe-Pro-Arg-H (ALD) and D-Phe-Pro-Arg-CH2Cl (CMK) were carried out in order to estimate whether the reversible or the irreversible tripeptide inhibitor of thrombin would be more suitable to develop as a novel anticoagulant. Conventional screening assay methods in vitro were focused on the functional stability of the compounds in whole blood and blood components while ex vivo the changes in whole blood clotting time under parenteral application of the inhibitors were investigated. The efficacy of ALD relative to that of CMK was found to depend on the complexity of the test systems. Thus CMK was the more inhibitory in citrated plasma, but ALD showed the higher potency in whole blood. When incubated in various systems such as human whole blood, serum, solutions of isolated plasma proteins, digestive juices and tissue homogenates, respectively, the inhibitory activity of ALD showed only slight decreases for several hours while marked or substantial loss of activity was observed with CMK under identical conditions. ALD administered parenterally to rabbits proved to be powerful anticoagulant; CMK exhibited only a weak and transient anticoagulant effect presumably due to its ability to bind irreversibly to various plasma and tissue proteins. Accordingly, the reversible inhibitor ALD should be more suitable to develop as an anti-coagulant than CMK, its irreversibly acting analogue.

Potent agonists of growth hormone-releasing hormone. II

Analogs of the 29-amino acid sequence of growth hormone-releasing hormone (GH-RH) with agmatine (Agm) or Lys-NH2 in position 29 have been synthesized by the solid-phase method, purified, and tested in vitro. Except for one peptide, all analogs contained desaminotyrosine (Dat) in position 1. All contained Nle27 in order to avoid oxidation of Met27. Some peptides contained one or more additional L- or D-amino acid substitutions in positions 2, 12, 15, 21, 27 and/or 28. Analogs [Dat1, Ala15, Nle27, Asn28]GH-RH(1-28)Agm (II, [Asn28]-Mz-2-51); [Dat1, Ala15, D-Lys21, Nle27, Asn28]GH-RH(1-28)Agm (III, MZ-3-125); and [Dat1, D-Asn8, Ala15, D-Lys21, Nl27, Asn28]GH-RH(1-28)Agm(IV, MZ-3-129) were 5.7, 2.8, and 3.9 times more potent in vitro, respectively, than GH-RH(1-29)NH2. However, if we compare the potencies of peptides II and III (analogs of the bovine sequence) with those of the analogs of human GH-RH (XII and XIII) [Dat1, Ala15, Nle27]GH-RH(1-28)Agm; [Dat1, Ala15, D-Lys21, Nle27]GH-RH(1-28)Agm, respectively, the GH-releasing potency was decreased by 50% and 33%, respectively, by the incorporation of Asn28. Our studies indicate that Lys-NH2 at the C-terminus of GH-RH(1-29) and/or beta-Ala, GABA (gamma-aminobutyric acid), and Phe in position 15 are disadvantageous, but potent GH-RH analogs can result from the combination of agmatine in position 29 with other substitutions.

In vitro inhibition of blood coagulation by tripeptide aldehydes--a retrospective screening study focused on the stable D-MePhe-Pro-Arg-H.H2SO4

A series of peptide aldehydes synthetized in our institute during the last 15 years were screened to detect their inhibitory effect on blood coagulation. Simple conventional clotting assays, platelet function tests and fibrinolytic methods were used to evaluate the inhibitory potency of the compounds in complex clotting systems as well as their supposed antifibrinolytic effect in vitro. Special attention was paid to the possible interactions with blood cells and plasma proteins, and to the functional stability of the inhibitors in several tissue homogenates. D-Phe-Pro-Arg-H (GYKI-14166, RGH-2958), Boc-D-Phe-Pro-Arg-H (GYKI-14451) and D-MePhe-Pro-Arg-H (GYKI-14766) were found to be the most potent inhibitors. The peptide aldehydes via formation of reversible complexes with thrombin impede the enzyme to react with the coagulation factors, platelet membrane and vessel wall. The compounds inhibit platelet aggregation induced by thrombin specifically without changing the sensitivity of platelets to other inducers. D-Phe-Pro-Arg-H and D-MePhe-Pro-Arg-H showed no antifibrinolytic effect. D-MePhe-Pro-Arg-H and Boc-D-Phe-Pro-Arg-H proved to be stable in dry state for years and in solution at room temperature for several days. The anticoagulant activity of the compounds was declared in NIH antithrombin units.

In vivo anticoagulant and antiplatelet effect of D-Phe-Pro-Arg-H and D-MePhe-Pro-Arg-H

D-Phe-Pro-Arg-H and D-MePhe-Pro-Arg-H synthetized in our institute were administered to mice, rats, rabbits and beagle dogs. The kinetics of the anticoagulant and antiplatelet effect was recorded by measuring various clotting parameters, platelet count and aggregation, and evaluated as proposed by Verstraete and Verwilghen. The minimum effective doses were found to be 0.25-0.5 mg kg-1h-1 by intravenous continuous infusions and 0.5-1.0 mg/kg by single injections. The dose-dependent prolongation of clotting times appeared after application within minutes and returned to baseline values as a function of dose. Blood level of the inhibitors was determined by a bioassay. Unlike heparin, no higher starting dose was required to reach the anticoagulant threshold level, i.e. 0.03-0.1 microgram/ml whole blood. The peptides did not cause significant changes in platelet count and function or in hemodynamic parameters (blood pressure, heart rate and ECG) and in respiration. They blocked platelet aggregation induced by thrombin ex vivo specifically. No rebound effect or bleeding could be demonstrated even after subtoxic doses of the compounds. The onset of the anticoagulant and antithrombotic effect appeared within 60 min after single oral doses and lasted for 3-6 h. In close correlation with the anticoagulant effect a complete or significant inhibition of platelet aggregation induced by thrombin ex vivo could also be recorded by using 5-10 mg/kg doses.

Potent agonists of growth hormone-releasing hormone. Part I

Analogs of the 29 amino acid sequence of growth hormone-releasing hormone (GH-RH) with agmatine (Agm) in position 29 have been synthesized by the solid phase method, purified, and tested in vitro and in vivo. The majority of the analogs contained desaminotyrosine (Dat) in position 1, but a few of them had Tyr1, or N-MeTyr1. Some peptides contained one or more additional L- or D-amino acid substitutions in positions 2, 12, 15, 21, 27, and/or 28. Compared to the natural sequence of GH-RH(1-29)NH2, [Dat1,Ala15]GH-RH(1-28)Agm (MZ-3-191) and [D-Ala2,Ala15]GH-RH(1-28)Agm (MZ-3-201) were 8.2 and 7.1 times more potent in vitro, respectively. These two peptides contained Met27. Their Nle27 analogs, [Dat1,Ala15,Nle27]GH-RH(1-28)Agm(MZ-2-51), prepared previously (9), and [D-Ala2,Ala15,Nle28]GH-RH(1-28)Agm(MZ-3-195) showed relative in vitro potencies of 10.5 and 2.4, respectively. These data indicate that replacement of Met27 by Nle27 enhanced the GH-releasing activity of the analog when the molecule contained Dat1-Ala2 residues at the N-terminus, but peptides containing Tyr1-D-Ala2 in addition to Nle27 showed decreased potencies. Replacement of Ser28 with Asp in multi-substituted analogs of GH-RH(1-28)Agm resulted in a decrease in in vitro potencies compared to the parent compound. Thus, the Ser28-containing MZ-2-51, and [Dat1,Ala15,D-Lys21,Nle27]GH-RH(1-28)Agm, its Asp28 homolog (MZ-3-149), possessed relative activities of 10.5 and 5.6, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Analogues of luteinizing hormone-releasing hormone containing cytotoxic groups

In an attempt to produce better cytotoxic analogues, chemotherapeutic antineoplastic radicals including an alkylating nitrogen mustard derivative of D-phenylalanine (D-melphalan), reactive cyclopropane, anthraquinone derivatives [2-(hydroxymethyl)anthraquinone and the anticancer antibiotic doxorubicin], and an antimetabolite (methotrexate) were coupled to suitably modified agonists and antagonists of luteinizing hormone-releasing hormone (LH-RH). Analogues with D-lysine6 and D-ornithine6 or N epsilon-(2,3-diaminopropionyl)-D-lysine and N delta-(2,3-diaminopropionyl)-D-ornithine were used as carriers for one or two cytotoxic moieties. The enhanced biological activities produced by the incorporation of D amino acids into position 6 of the agonistic analogues were further increased by the attachment of hydrophobic cytotoxic groups, resulting in compounds with 10-50 times higher activity than LH-RH. Most of the monosubstituted agonistic analogues showed high affinities for the membrane receptors of human breast cancer cells, while the receptor binding affinities of peptides containing two cytotoxic side chains were lower. Antagonistic carriers [Ac-D-Nal(2)1,D-Phe(4Cl)2,D-Trp3,Arg5,D-Lys6,D-Ala10] LH-RH [where Nal(2) is 3-(2-naphthyl)alanine], [Ac-D-Nal(2)1,D-Phe(4Cl)2,D-Trp3,Arg5,N epsilon-(2,3-diaminopropionyl)-D-Lys6,D-Ala10]LH-RH, and their D-Pal(3)3 homologs [Pal(3) is 3-(3-pyridyl)alanine] as well as [Ac-D-Nal(2)1,D-Phe(4Cl)2,D-Pal(3)3,Tyr5,N epsilon-(2,3-diamino-propionyl)-D-Lys6,D-Ala10]LH-RH were linked to cytotoxic compounds. The hybrid molecules inhibited ovulation in rats at doses of 10 micrograms and suppressed LH release in vitro. The receptor binding of cytotoxic analogues was decreased compared to the precursor peptides, although analogues with 2-(hydroxymethyl)anthraquinone hemiglutarate had high affinities. All of the cytotoxic analogues tested inhibited [3H]thymidine incorporation into DNA in cultures of human breast and prostate cancer cell lines. Some cytotoxic analogues also significantly suppressed the growth of mammary and prostate cancers in vivo in animal models.

Synthesis and in vitro and in vivo activity of analogs of growth hormone-releasing hormone (GH-RH) with C-terminal agmatine

In the search for more active analogs of human growth hormone-releasing hormone (GH-RH), 37 new compounds were synthesized by solid phase methodology, purified, and tested biologically. Most of the analogs contained a sequence of 27 amino acids and N-terminal desaminotyrosine (Dat) and C-terminal agmatine (Agm), which are not amino acids. In addition to Dat in position 1 and Agm in position 29, the majority of the analogs had Ala15 and Nle27 substitutions and one or more additional L- or D-amino acid modifications. [Dat1, Ala15, Nle27]GH-RH(1-28)Agm (MZ-2-51) was the most active analog. Its in vitro GH-releasing potency was 10.5 times higher than that of GH-RH(1-29)NH2 and in the i.v. in vivo assay, MZ-2-51 was 4-5 times more active than the standard. After s.c. administration to rats. MZ-2-51 showed an activity 34 times higher at 15 min and 179 times greater at 30 min than GH-RH(1-29)NH2 and also displayed a prolonged activity. D-Tyr10, D-Lys12, and D-Lys21 homologs of MZ-2-51 also showed enhanced activities. Thus, [Dat1, D-Tyr10, Ala15, Nle27]GH-RH(1-28)Agm (MZ-2-159), [Dat1, D-Lys12, Ala15, Nle27]GH-RH(1-28)AGM (MZ-2-57), and [Dat1, Ala15, D-Lys21, Nle27]GH-RH(1-28)Agm (MZ-2-75) were 4-6 times more active in vitro than GH-RH(1-29)NH2. In vivo, after i.v. administration, analog MZ-2-75 was equipotent and analogs MZ-2-159 and MZ-2-57 about twice as potent as the standard.(ABSTRACT TRUNCATED AT 250 WORDS)

Prolonged inhibition of luteinizing hormone and testosterone levels in male rats with the luteinizing hormone-releasing hormone antagonist SB-75

Inhibitory effects of the potent antagonist of luteinizing hormone-releasing hormone N-Ac-[3-(2-naphthyl)-D-alanine1,4-chloro-D-phenylalanine2,3- (3-pyridyl)-D- alanine3,D-citrulline6,D-alanine10]luteinizing hormone-releasing hormone (SB-75) free of edematogenic effects were investigated in male rats. In a study to determine the effect on luteinizing hormone levels in castrated male rats, SB-75 was injected s.c. in doses of 0.625, 1.25, 2.5, 5.0, and 10 micrograms. Blood samples were taken at different intervals for 48 hr. All doses of SB-75 significantly decreased luteinizing hormone levels for greater than 6 hr (P less than 0.01); this inhibition lasted for greater than 24 hr (P less than 0.01) with a dose of 5.0 micrograms and greater than 48 hr with 10 micrograms (P less than 0.05). Serum testosterone levels were also measured in intact male rats injected with SB-75 in doses of 25, 50, and 100 micrograms. All doses produced a dramatic fall in testosterone to castration levels 6 hr after injection (P less than 0.01); this inhibition of serum testosterone was maintained for greater than 72 hr, but only the 100-micrograms dose could keep testosterone in the castration range for greater than 24 hr (P less than 0.01). In another study using a specific RIA, we obtained the pharmacokinetic release pattern of SB-75 from two sustained delivery formulations of SB-75 pamoate microgranules and examined their effect on serum testosterone. After a single i.m. injection of 20 mg of one batch of microgranules, a large peak corresponding to SB-75 at 45.8 ng/ml was observed, corresponding to the "burst" effect. Levels of the analog decreased to 19.6 ng/ml on day 2, gradually reached a concentration of 4.7 ng/ml on day 7, and kept declining thereafter. Testosterone levels were reduced on day 1 (P less than 0.01) and were maintained at low values for greater than 7 days (P less than 0.05). In rats injected with 10 mg of SB-75 pamoate microgranules of the second batch, SB-75 serum levels rose to 33 ng/ml 3 hr after administration and then fell gradually to approximately 3.4 ng/ml on day 16, but a second small peak was seen on day 28. Subsequently, the analog levels decreased slowly to 2.9 ng/ml on day 42. At this time, testosterone serum levels were still significantly lower than in controls. These overall results demonstrate the efficacy of SB-75 in the suppression of the pituitary-gonadal axis. This modern luteinizing hormone-releasing hormone antagonist can possibly be used for treating sex hormone-sensitive cancers and other disorders.

Oligopeptides interfering with calcium channels inhibit prolactin and growth hormone release by cultured anterior pituitary cells of the rat

A number of oligopeptides, protected at their N termini and possessing an aldehyde residue at their C terminal amino acids, are able to inhibit 45Ca2+ influx into anterior pituitary cells grown in monolayer culture and depolarized with high extracellular potassium concentration. In addition, the same oligopeptides interfere with hormone release, especially with that produced by lactotrophs. Our findings imply that oligopeptides may represent a new class of calcium channel ligands, and the pituitary cells are sensitive targets for them.

Highly active and selective anticoagulants: D-Phe-Pro-Arg-H, a free tripeptide aldehyde prone to spontaneous inactivation, and its stable N-methyl derivative, D-MePhe-Pro-Arg-H

D-Phe-Pro-Arg-H sulfate (GYKI-14166) is a highly active and selective inhibitor of thrombin both in vitro and in vivo. Recent studies on the stability of D-Phe-Pro-Arg-H in neutral aqueous solution at higher temperature have revealed that it is transformed into inactive 5,6,8,9,10,10a-hexahydro-2-(3'- guanidinopropyl)-5-benzyl-6-oxo- imidazo[1,2-a]pyrrolo[2,1-c]pyrazine. No such inactivation could be observed with Boc-D-Phe-Pro-Arg-H (GYKI-14451), but this compound was far less specific than the free peptide as it inhibited thrombin and, for instance, plasmin equally well. Assuming that the transformation of free tripeptide aldehyde, mentioned above, can only be initiated by a primary amino terminus, the N-alkyl derivatives of D-Phe-Pro-Arg-H were prepared. Of the new analogues, D-MePhe-Pro-Arg-H (GYKI-14766) proved to be as highly active and selective anticoagulant as its parent compound and was not inactivated by transformation into a heterocyclic compound.

Highly potent analogues of luteinizing hormone-releasing hormone containing D-phenylalanine nitrogen mustard in position 6

The nitrogen mustard derivatives of 4-phenylbutyric acid and L-phenylalanine, called chlorambucil (Chl) and melphalan (Mel), respectively, have been incorporated into several peptide hormones, including luteinizing hormone-releasing hormone (LH-RH). The alkylating analogues of LH-RH were prepared by linking Chl, as an N-acyl moiety, to the complete amino acid sequence of agonistic and antagonistic analogues. These compounds, in particular the antagonistic analogues, showed much lower potency than their congeners carrying other acyl groups. To obtain highly potent alkylating analogues of LH-RH, the D enantiomer of Mel was incorporated into position 6 of the native hormone and some of its antagonistic analogues. Of the peptides prepared, [D-Mel6]LH-RH (SB-05) and [Ac-D-Nal(2)1,D-Phe(pCl)2,D-Pal(3)3,Arg5,D-Mel6,D-Ala10++ +]LH-RH [SB-86, where Nal(2) is 3-(2-naphthyl)alanine and Pal(3) is 3-(3-pyridyl)alanine] possessed the expected high agonistic and antagonistic activities, respectively, and also showed high affinities for the membrane receptors of rat pituitary cells, human breast cancer cells, human prostate cancer cells, and rat Dunning R-3327 prostate tumor cells. These two analogues exerted cytotoxic effects on human and rat mammary cancer cells in vitro. Thus these two D-Mel6 analogues seem to be particularly suitable for the study of how alkylating analogues of LH-RH could interfere with intracellular events in certain cancer cells.

Highly potent metallopeptide analogues of luteinizing hormone-releasing hormone

Metal complexes related to the cytotoxic complexes cisplatin [cis-diamminedichloroplatinum(II)] and transbis(salicylaldoximato)copper(II) were incorporated into suitably modified luteinizing hormone-releasing hormone (LH-RH) analogues containing D-lysine at position 6. Some of the metallopeptides thus obtained proved to be highly active LH-RH agonists or antagonists. For instance, SB-40, a PtCl2-containing metallopeptide in which platinum is coordinated to an N epsilon-(DL-2,3-diaminopropionyl)-D-lysine residue [D-Lys(DL-A2pr] at position 6, showed 50 times higher LH-releasing potency than the native hormone. SB-95, [Ac-D-Nal(2)1,D-Phe(pCl)2, D-Pal(3)2, Arg5,D-Lys[DL-A2pr(Sal2Cu)]6,D-Ala10]LH-RH, where Nal(2) is 3-(2-naphthyl)alanine, Pal(3) is 3-(3-pyridyl)alanine, and copper(II) is coordinated to the salicylideneimino moieties resulting from condensation of salicylaldehyde with D-Lys(DL-A2pr)6, caused 100% inhibition of ovulation at a dose of 3 micrograms in rats. Most metallopeptide analogues of LH-RH showed high affinities for the membrane receptors of rat pituitary and human breast cancer cells. Some of these metallopeptides had cytotoxic activity against human breast cancer and prostate cancer cell lines in vitro (this will be the subject of a separate paper on cytotoxicity evaluation). Such cytostatic metallopeptides could be envisioned as targeted chemotherapeutic agents in cancers that contain receptors for LH-RH-like peptides.

Comparison of different agonists and antagonists of luteinizing hormone-releasing hormone for receptor-binding ability to rat pituitary and human breast cancer membranes

A sensitive multipoint assay capable of measuring receptors for LHRH and its analogs using 5-10 micrograms membrane protein/incubation tube was used to determine binding characteristics of different agonists and antagonists of LHRH in membranes of male rat pituitary and human breast cancer specimens. This method also permitted Scatchard analysis of the receptor binding in pellet fractions of human breast cancer biopsies remaining from estrogen and progesterone receptor assays. The potent agonist [D-Trp6]LHRH bound to at least two classes of receptor sites, one with high affinity and one with low affinity in both rat pituitary and human breast cancer samples. The analysis of displacement curves of LHRH by agonists and antagonists showed that LHRH also bound to two classes of receptor sites in pituitary and one receptor site with lower affinity in human breast cancer membranes. Among the antagonists synthesized in our laboratory, SB-030, SB-077, SB-088, and SB-090 appeared to be the most potent in displacing labeled [D-Trp6]LHRH and showed the highest binding affinity to the pituitary and breast cancer membranes. Labeled antagonists showed somewhat less affinity to membranes of pituitaries and human cancers than the agonists and bound to only a single class of receptor population. Both agonists and antagonists were able to bind to membranes of human breast cancer samples, and some antagonists were very potent in this respect. Certain LHRH agonists or antagonists could be capable of exerting direct inhibitory effects on breast cancers depending upon the presence and characteristics of LHRH receptors.

Calcium ion uptake, somatotropin release, and fine structure of somatotrophs in cultures of the rat anterior pituitary upon the action of an oligopeptide (Boc-Gln-Leu-Lysinal)

Cultured cells from the anterior pituitary glands of adult rats were treated with the tripeptide aldehyde proteinase inhibitor, Boc-Gln-Leu-Lys-H. The oligopeptide had a profound releasing effect on growth hormone, whereas the prolactin release remained unchanged at 10(-3) mol/l drug concentration after an incubation for 2 h. In the presence of the oligopeptide a time- and dose-dependent calcium influx into cultured cells has been shown which was proved to be almost completely antagonized with magnesium ions but not with Nifedipine. In addition, radioactive calcium ions could be detected in a number of cells by light microscopic autoradiography when cultures were treated with Boc-Gln-Leu-Lys-H for short periods. The selective Gel action of the oligopeptide on growth hormone producing cells has been demonstrated also in fine structural investigations: multigranular and single exocytotic profiles have been observed. Accordingly, we have postulated that Boc-Gln-Leu-Lysinal mimics the effects of the known ionophores. Its mode of action needs, however, further studies especially on isolated somatotrophs.

New antagonists of LHRH. II. Inhibition and potentiation of LHRH by closely related analogues

Modifications of the previously described LHRH antagonists, [Ac-D-Nal(2)1, D-Phe(4Cl)2, D-Trp3, D-Cit6, D-Ala10]LHRH and the corresponding D-Hci6 analogue, have been made to alter the hydrophobicity of the N-terminal acetyl-tripeptide portion. Substitution of D-Trp3 with the less hydrophobic D-Pal(3) had only marginal effects on the antagonistic activities and receptor binding potencies of the D-Cit/D-Hci6 analogues, but it appeared to further improve the toxicity lowering effect of D-Cit/D-Hci6 substitution. Antagonists containing D-Pal(3)3 and D-Cit/D-Hci6 residues, i.e. [Ac-D-Nal(2)1, D-Phe(4Cl)2, D-Pal(3)3, D-Cit6, D-Ala10]LHRH (SB-75) and [Ac-D-Nal(2)1, D-Phe(4Cl)2, D-Pal(3)3, D-Hci6, D-Ala10]LHRH (SB-88), were completely free of the toxic effects, such as cyanosis and respiratory depression leading to death, which have been observed in rats with the D-Trp3, D-Arg6 antagonist and related antagonists. Replacement of the N-acetyl group with the hydrophilic carbamoyl group caused a slight decrease in antagonistic activities, particularly in vitro. Introduction of urethane type acyl group such as methoxycarbonyl (Moc) or t-butoxycarbonyl (Boc) led to analogues that showed LHRH-potentiating effect. The increase in potency induced by these analogues, e.g. [Moc-D-Nal(2)1, D-Phe(4Cl)2, D-Trp3, D-Cit6, D-Ala10]LHRH and [Boc-D-Phe1, D-Phe(4Cl)2, D-Pal(3)3, D-Cit6, D-Ala10]LHRH, was 170-260% and persisted for more than 2 h when studied in a superfused rat pituitary system.

The tripeptide aldehyde, Boc-DPhe-Phe-Lysinal, is a novel Ca2+ channel inhibitor in pituitary cells

The effect of Boc-DPhe-Phe-Lysinal (Boc-DPPL) on the 45Ca2+ uptake of rat anterior pituitary monolayer cultures was investigated. The compound decreased the basal Ca2+ uptake at 3 x 10(-4) mol/l. The 45Ca2+ uptake stimulated by potassium-induced depolarization was more sensitive to Boc-DPPL inhibition, a slight decrease was seen with 3 x 10(-6) mol/l and there was a half maximal inhibition at 3 x 10(-5) mol/l. Boc-DPPL is known to inhibit pituitary hormone release in similar concentrations, an effect might also be due to its calcium antagonist property.

Highly potent antagonists of luteinizing hormone-releasing hormone free of edematogenic effects

To eliminate the undesirable edematogenic effect of the luteinizing hormone-releasing hormone (LH-RH) antagonists containing basic D amino acids at position 6, exemplified by [Ac-D-Phe(pCl)1,2,D-Trp3,D-Arg6,D-Ala10]LH-RH [Phe(pCl) indicates 4-chlorophenylalanine], analogs with D-ureidoalkyl amino acids such as D-citrulline (D-Cit) or D-homocitrulline (D-Hci) at position 6 were synthesized and tested in several systems in vitro and in vivo. HPLC analysis revealed that the overall hydrophobicity of the D-Cit/D-Hci6 analogs was similar to that of the basic D-Arg6 antagonists. In vitro, most of the analogs completely inhibited LH-RH-mediated luteinizing hormone release in perfused rat pituitary cell systems at an antagonist to LH-RH molar ratio of 5:1. In vivo, the most active peptides, [Ac-D-Nal(2)1,D-Phe(pCl)2,D-Trp3,D-Cit6,D-Ala10]LH-RH [Nal(2) indicates 3-(2-naphthyl)alanine] and its D-Hci6 analog, caused 100% inhibition of ovulation in cycling rats in doses of 3 micrograms and suppressed the luteinizing hormone level in ovariectomized female rats for 47 hr when administered at doses of 25 micrograms. Characteristically, these peptides did not exert any edematogenic effects even at 1.5 mg/kg. These properties of the D-Cit/D-Hci6 antagonists may make them useful clinically.

An evaluation of intravenous, subcutaneous, and in vitro activity of new agmatine analogs of growth hormone-releasing hormone hGH-RH (1-29)NH2

The effects of new Agmatine (Agm) analogs of human growth hormone-releasing hormone (GH-RH) were compared to GH-RH (1-29)NH2 and to (D-Ala2)GH-RH(1-29)NH2 after intravenous (IV) and subcutaneous (SC) administration to pentobarbital-anesthetised male rats and in vitro using superfused rat pituitary cell system. After IV administration, the analogs: (D-MeAla2,Nle27)GH-RH(1-28)Agm(JG-75), (desamino-Tyr1,D-Ala2,Nle27)GH-RH(1-28)Agm(JG-77), (D-Ala2,Nle27)GH-RH(1-28)Agm(JG-73) and (D-Ala2)GH-RH(1-29)NH2 showed a potency 2.6-3.9 times greater than GH-RH(1-29)NH2 at 5 min and 1.6-2.7 times higher at 15 min. After SC administration these analogs were 30-74 times more potent than GH-RH(1-29)NH2. The ratio between the IV and SC GH-releasing activity of the analogs ranged from 2 to 5, while GH-RH(1-29)NH2 was about 50 times more active IV than SC. This indicates that 20-50% of the analogs can be absorbed from SC tissues, but only 2% of GH-RH(1-29)NH2. The in vitro activity of the agmatine analogs on GH release closely paralleled their IV potency and was 2.8-3.9 times greater than that of GH-RH(1-29)NH2. No significant difference in potency was found between (D-Ala2)GH-RH(1-29)NH2 and JG-75 after IV administration and in vitro, although JG-75 contained only 28 amino acids. We conclude that the reason for the large discrepancies between the previously reported activities of (D-Ala2)GH-RH(1-29)NH2 was simply due to the different ways of administration of this analog, SC vs IV, and not to species specificity. The replacement of Arg29 by Agmatine in (D-Ala2,Nle27)GH-RH(1-29)NH2 causes a 3 fold increase in SC potency, but the replacement of D-Ala2 with D-MeAla2 reduces the SC, but not the IV and in vitro activity in half.

Inhibition of angiotensin-converting enzyme by angiotensin I analogue peptide inhibitors. A kinetic study

Angiotensin I analogues with a phosphonic acid group replacing the C-terminal carboxyl group were shown to be competitive inhibitors of angiotensin-converting enzyme. This new class of inhibitors was used to study the binding requirements of the angiotensin I-like ligands to the enzyme's active site. These studies indicate that angiotensin-converting enzyme recognizes at least five amino acid residues at the C-terminus of the peptide. The effect of pH on the binding of the most potent inhibitor peptide was compared to Captopril. The two inhibitors showed similar Ki-pH profiles despite their structural differences. Chloride enhanced the binding of the peptide inhibitor at both pH 9.0 and pH 6.5. At pH 9.0 the inhibitor peptide and the anion bind randomly to the enzyme, while at pH 6.5 the mechanism is ordered. In the latter case, the anion binds first to the enzyme.

Overlapping imprinting of oligopeptides in Chang liver cells. Data on the mechanism of hormone evolution

Imprinting was induced with synthetic oligopeptides in Chang liver cell cultures to test these molecules for signal molecule value. Investigations into imprinting overlaps (cross-imprinting) have shown that all oligopeptides (di-, tetra- and pentapeptides) carrying a terminal proline group were able to imprint the cells for the pentapeptide Tyr-D-Met-Gly-Phe-Pro-NH2, which displayed an outstanding imprinting potential for itself and an extraordinary opioid activity as well. The fact that exclusively the proline-deficient oligopeptide (a tetrapeptide) failed to imprint for the pentapeptide in question, indicates a decisive role of proline in the transformation of molecules to signal carriers (hormones). The pentapeptide in question did imprint for the related molecules (except the dipeptide) but to a much lesser degree than for itself. The marked inferiority of the pentapeptide's cross-imprinting potential to its self-imprinting potential supports the hypothetical implication that a considerable difference between the specific and non-specific binding capacities of a molecule, if not the loss of non-specific binding was an essential prerequisite of transformation to a signal molecule, i.e. of hormone evolution.

Suitability of oligopeptides for induction of hormonal imprinting--implications on receptor and hormone evolution

Hormonal imprinting induced in Tetrahymena and in Chang liver cells with di-, tri-, tetra- and pentapeptides (synthetic opioids and their fragments) has shown that both cell types are able to differentiate the related molecules from one another. The dipeptide phenylalanine + proline induced a measurable imprinting in the liver cells, and chain length increase, especially terminal coupling with tyrosine enhanced the imprinting potential enormously. Intra-chain changes in the amino acid sequence had a measurable effect on the intensity of imprinting. The molecules showing the relatively strongest physiological action accounted for the most intensive imprinting in both cell types; this indicates that, in all probability, induction of binding site formation plays a key role in the development of signal molecules, and thereby in hormone evolution.

Tripeptide aldehyde protease inhibitors may depress in vitro prolactin and growth hormone release

The effects of novel nontoxic tripeptide aldehyde inhibitors of proteolytic enzymes were examined in order to investigate the possibility that serine-thiol protease(s) may be involved in PRL and GH secretion. Rat anterior pituitary cells maintained in culture for 7-8 days or freshly taken pituitary quarters were treated with BOC-DPhe-Pro-Arg-H (BOC-dPPA), DPhe-Pro-Arg-H (dPPA), BOC-DPhe-Leu-Lys-H (BOC-dPLL), or BOC-DPhe-Phe-Lys (BOC-dPPL). Newly synthetized [3H]PRL and [3H]GH as well as immunoreactive (i) hormones (iPRL, iGH) were measured in the incubation media and cell homogenates. Four hours of incubation in the presence of 0.1 mM dPPA resulted in a 30% decrease of [3H]PRL and iPRL release by cell cultures; the inhibition by BOC-dPPA was 60% and 48%, respectively. [3H]PRL biosynthesis was unchanged or slightly decreased. The effect of these tripeptide aldehydes on [3H]GH and iGH release was less pronounced but statistically significant. Pituitary quarters treated with 1.0 or 3.0 mM BOC-dPPA release 20% and 57% less [3H]PRL than the controls. In the same system BOC-dPPA in a 1.0 mM concentration did not effect GH secretion, and 3.0 mM BOC-dPPA inhibited [3H]GH output by 27%. Forty micromolars of BOC-dPPL decreased by 47%, 0.2 mM by 79%, and 1.0 mM by 94% [3H]PRL release from pituitary quarters. GH secretion was not influenced. A similar selectivity was observed when BOC-dPLL was used. It is clear that by serine-thiol protease inhibitors whose effects are sequence and dose dependent, PRL and GH release are decreased. The relative inhibiting potency on PRL release was BOC-dPPL greater than BOC-dPLL greater than BOC-dPPA greater than dPPA. The biosynthesis of [3H]PRL was reduced only in the presence of the highest tripeptide aldehyde concentrations or long (8 h) exposure, and only 1.0 mM Boc-dPPL reduced [3H]GH biosynthesis by 30%. The data suggest that proteolysis may be involved in the process of PRL and GH release and the enzyme(s) in question may be serine-thiol protease(s).

Various proteinase inhibitors decrease prolactin and growth hormone release by anterior pituitary cells

Proteinase inhibitors were tested for their ability to inhibit prolactin (PRL) and growth hormone (GH) release by cultured anterior pituitary cells of the rat. Inhibitors of microbial origin (chymostatin, elastatinal, leupeptin) had either no or a moderate effect on hormone release while some tripeptide aldehydes, especially those with lysine at their C terminus, inhibited markedly PRL and to a lesser extent GH release. Boc-DPhe-Phe-lysinal was the most effective on lactotrophs inhibiting PRL release more than 50% at 10(-4) M. The site(s) of action of tripeptide aldehydes remain to be elucidated.

Met5-enkephalin-Arg6-Phe7 inhibition of noradrenaline and acetylcholine release from peripheral organs

The effect of the opioid heptapeptide Met5-enkephalin-Arg6-Phe7 was measured on the contractions of the longitudinal muscle of the guinea-pig ileum and mouse vas deferens; on release of endogenous and newly synthesized acetylcholine from the longitudinal muscle strip preparation of the guinea-pig ileum and of 3H-noradrenaline from mouse vas deferens. Met5-enkephelin-Arg6-Phe7 depressed the contractions of the longitudinal muscle strip of guinea-pig ileum elicited by low frequency stimulation (IC50 366.5 +/- 71.5 nM). The Ke value of naltrexone against the heptapeptide was found to be 0.50 +/- 0.08 nM. Bestatin prolonged and strongly potentiated the action of the opioid peptide in this tissue while captopril proved to be only slightly effective. Met5-enkephalin-Arg6-Phe7 also inhibited neuroeffector transmission in the mouse vas deferens with an IC50 value of 9.44 +/- 2.24 nM. The Ke value of naltrexone was 6.33 +/- 1.22 nM against the heptapeptide. Met5-enkephalin-Arg6-Phe7 (10(-5) M) failed to affect the low frequency stimulated release of endogenous acetylcholine from the guinea-pig ileum preparation. Addition of either bestatin or captopril with the heptapeptide resulted in a reduction of acetylcholine output. The opioid peptide by itself decreased the release of newly synthesized acetylcholine outflow from this organ. Met5-enkephalin-Arg6-Phe7 also reduced the 3H-noradrenaline outflow from mouse vas deferens and this effect was antagonized by naltrexone. From our data we conclude that Met5-enkephalin-Arg6-Phe7 affects the neural transmission of peripheral autonomically innervated organs by depressing the release of neurotransmitter. In vitro it has enkephalin-like character with a preference for the so called delta receptor interaction.

Species differences in the relative analgesic potencies of some classical opiates and opioid peptides

The analgesic ED50 values of some classical morphine congeners (morphine, methadone, fentanyl, azidomorphine) in the rat and mouse tail-flick tests were found to be similar. However, several synthetic derivatives of the natural enkephalins were more potent in mice than in rats. (These analogs contain D-amino acid in position 2 and D- or L-sulfonic (or phosphonic) acid residue in position 5). beta-Endorphin, D-Met2, Pro5-enkephalinamide and two partial agonists showed intermediate interspecies relative potencies. According to the data obtained, similar opiate receptors might mediate the analgesic action of classical opiates in rats and in mice. However, the opiate receptors responsible for the antinociceptive effects of the above mentioned enkephalin analogues must be dissimilar in the two species examined. The results are discussed in terms of the role of mu- and delta-receptors in mediation of the analgesic effect induced by different types of opioids.

Study of the specificity of thrombin with tripeptidyl-p-nitroanilide substrates

The kinetic behaviour of human thrombin has been studied with 26 tripeptidyl-p-nitroanilide substrates protected at the N terminus and with 9 unprotected ones. By the regression analysis of experimentally determined 1/Km, kcat and kcat/Km values the individual contribution of each side chain of the various substrates to the kinetic parameters was calculated. The contributions to the kinetic parameters of the best substrates provide information about the structure of the binding site. The interaction of subsites S1 and P1, which determines primary specificity, proved to be marginal on the basis of contribution values, though it depends upon this contact whether the substrate is hydrolyzed at all. At subsite S2 proline appeared to be favourable. Subsite S3 plays an important role in efficiency. The best parameters were obtained here with the D configurations of bulky amino acid residues. The aromatic protecting groups applied did not improve the properties of substrates. BZDPhe-Pro-Arg-Nan was predicted by calculation to be better than the protected substrates assayed. The compound was synthesized and tested. Its experimentally determined 1/Km, 55.1 mM-1, was in good agreement with 50.9 mM-1 found by calculation.

Investigation of the substrate-binding site of trypsin by the aid of tripeptidyl-p-nitroanilide substrates

The kinetic parameters of the tryptic hydrolysis of tripeptidyl-p-nitroanilide substrates were determined and the data were studied by regression analysis. The sequence of substrates optimal from the viewpoint of kinetic constants 1/Km, kcat and kcat/Km was established and the influence of amino acid side chains on the binding and reactivity of substrates was calculated. At subsite P3 [notation of Schechter and Berger (1967) Biochem. Biophys, Res. Commun. 27, 157] polar side chains (Asn, D-Arg) are favourable as regards 1/Km, whereas hydrophobic side chains are preferred definitely from the viewpoint of catalytic efficiency, just as at subsite P2. In the side chain contributions, calculated for the kinetic parameters, the P3-S3 interaction predominates, in spite of the fact that the properties of the residue at subsite P1 decide whether hydrolysis occurs at all. The ZAsn-Ile-Arg-Nan sequence was predicted as a better substrate than those tested experimentally. The compound was synthesized, and the calculated value of its 1/Km (116.4 mM-1) was in a good agreement with the measured value (100.2 mM-1). Comparing the data obtained with trypsin with those observed with thrombin, elastase and subtilisin, we can establish that the homology of these enzymes can be characterized at each binding subsite by the aid of tripeptidyl-p-nitroanilide substrates. The quantities derived allow one to envisage a novel type of comparison of the proteases.

Enkephalin-like character and analgesia

The opioid activities of enkephalin analogues bearing D- or L-aminopentane-sulfonic/phosphonic acid at position 5 were studied in vitro, in electrically stimulated longitudinal muscle strip of guinea-pig ileum and mouse vas deferens preparations and in vivo in the rat tail-flick test. Using their in vitro effects Met-enkephalin-like, beta-endorphin-like, (nor)morphine-like and derivatives of intermediate character could be differentiated. Correlating the in vitro activities with the analgesic activity in vivo it is concluded that the enkephalin-like character in a pentapetide may hinder the expression of analgesic activity, when the compounds are given into the cerebroventricular system.