Role of guanidine group at the N-terminal proctolin chain in cardioexcitatory effects in insects

Six proctolin analogues (I-VI) modified in position 1 of the peptide chain by the following amino acids: homo-Arg, Gac, Gav, Gap, Phe (p-guanidino) and Orn, were synthesized by conventional liquid phase method. The myotropic activity of the obtained peptides was investigated in cardioexcitatory test on two insect species, cockroach, Periplaneta americana L., and yellow mealworm, Tenebrio molitor.     

Insect Trypsin Modulating Oostatic Factor (Neb-TMOF) and Its Analogs: Preliminary Structure/Biological Function Relationship Studies

Neb-TMOF, the trypsin modulating oostatic factor of gray fleshfly Neobellieria bullata, is a hexapeptide with the following sequence: H-Asn-Pro-Thr-Asn-Leu-His-OH. It has been isolated from vitellogenic ovaries in 1994. TMOF, the newly discovered insect peptide, inhibits trypsin biosynthesis in the gut, lowers yolk polypeptide concentration in the hemolymph and strongly inhibits ecdysone biosynthesis by larval ring glands. It is interesting that this short non-protected peptide contains in its molecule two Asn residues at positions 1 and 4 and His at its C-terminus. To obtain information about the role of the His-6 and Asn-4 residues we synthesised two series of Neb-TMOF analogs, modified: (1) in position 6 by D-His (I), His(Bzl) (II) and Phe(p-X) derivatives, where X = NH₂ (III), NO₂ (IV), OEt (V) and OH (VI) and (2) in position 4 by such amino acid residues as Ser (VII), Thr (VIII), Gly (IX), Asp (X), Glu (XI) and D-Asn (XII). The influence of these peptides on trypsin biosynthesis in N. bullata was determined in vivo. In preliminary investigations, we found that Neb-TMOF, [Phe(NH₂)⁶], and [Phe(NO₂)⁶]-Neb-TMOF inhibited trypsin biosynthesis, whereas [D-His)⁶]- and [D-His(Bzl)⁶]-Neb-TMOF were inactive. In further biological studies performed in vitro on heart of Tenebrio molitor we found that Neb-TMOF and [Phe(p-NH₂)⁶-Neb-TMOF showed weak cardioexcitatory activity, about 30% of the cardioexcitatory activity of proctolin, an insect neuromodulating peptide.     

Novel nociceptin analogues

A series of new nociceptin analogues containing cysteine was tested for their nociceptive effects in tail-flick test on mice after icv injection. The cysteines were introduced in order to get irreversibly binding analogues based on the assumption that the cysteines in the ligand can interact with the cysteines from the receptor to form an S-S bridge. In vivo tests revealed that Cys1-nociceptin (1-13)-NH2 (Cys1-NC) is an antagonist, whereas Cys7-NC is an agonist. Gly1[Phe(p-NO2)]4-NC was less active indicating that the antagonist properties of Cys1-NC are associated with the presence of the sulfhydryl group of cysteine. The analogues D-Cys2 and Cys3 were also almost inactive.     

Further studies on proctolin analogues modified in position 2 of the peptide chain and their influence on heart-beat frequency of insects

Seven proctolin analogues (I-VII) modified in position 2 of the peptide chain by Phe (p-guanidino) (I), Phe (p-OEt) (II), Tyr (3'-NH2) (III), Tyr (3'-NO2) (IV), Afb (p-OH) (V) (Afb = 3-amino-4-phenyl-L-butyric acid), Afb (p-NH2) (VI), Afb (p-NO2) (VII), and the tetrapeptide Tyr (3'-NH2)-Leu-Pro-Thr (VIII) were synthesized by the classic liquid-phase method. The biological effects of the peptides were investigated in cardioexcitatory tests on two insect species, the cockroach Periplaneta americana L., and the yellow mealworm, Tenebrio molitor L. Within physiological concentrations (10(-9)-10(-7) M) peptides II, III, and IV stimulated the heart action of P. americana like proctolin itself. Under identical conditions, in the case of T. molitor, only peptide III showed cardiostimulatory properties, whereas other compounds (including II and IV) were inactive at concentrations up to 10(-7) M. Results reported here reflect, with reference to the analogues I-VII, selective recognition of receptors on myocardium of both insect species. The tetrapeptide VIII revealed a weak deacceleratory effect on P. americana and T. molitor heart action.     

Type II Thioesterase ScoT, Associated with Streptomyces coelicolor A3(2) Modular Polyketide Synthase Cpk, Hydrolyzes Acyl Residues and Has a Preference for Propionate

Type II thioesterases (TE IIs) were shown to maintain the efficiency of polyketide synthases (PKSs) by removing acyl residues blocking extension modules. However, the substrate specificity and kinetic parameters of these enzymes differ, which may have significant consequences when they are included in engineered hybrid systems for the production of novel compounds. Here we show that thioesterase ScoT associated with polyketide synthase Cpk from Streptomyces coelicolor A3(2) is able to hydrolyze acetyl, propionyl, and butyryl residues, which is consistent with its editing function. This enzyme clearly prefers propionate, in contrast to the TE IIs tested previously, and this indicates that it may have a role in control of the starter unit. We also determined activities of ScoT mutants and concluded that this enzyme is an α/β hydrolase with Ser90 and His224 in its active site.     

Solid phase synthesis of peptides containing novel amino acids, substituted 3-benzimidazolealanines

A direct solid-phase synthesis of a series of substituted benzimidazole-containing peptides is described. The method involves on-resin formation of new amino acids containing benzimidazole derivatives in the side chain. The heterocycle conjugates were obtained by reaction between aldehydes and peptides containing β-(3,4-diaminophenyl)alanine residue, immobilized on a polymeric solid support.     

Cyclic enkephalin analogs that are hybrids of DPDPE-related peptides and metenkephalin-Arg-Gly-Leu: prohormone analogs that retain good potency and selectivity for delta opioid receptors.

We report here on the binding affinity and bioassay results of cyclic enkephalin analogs comprising a cyclic moiety and C-terminal fragment of MERGL, where ME denotes methionine enkephalin. MERGL (YGGFMRGL) has been suggested to be cleaved enzymatically by membrane-bound enkephalinase 24.11 to leave ME and the tripeptide RGL. In our study we have synthesized hybrids of DPDPE or DPLCE and the C-terminal tripeptide RGL in order to mimic a prohormone able to cross the blood-brain barrier. The study has shown that of the homologs presented here, analogs of DPLCE often are more potent at delta opioid receptors both in binding affinity and in bioactivity at the MVD, than DPDPE. Our hypothesis that hybrids (consisting of the drug and the spacer for the carrier) could be designed which would either have no opioid activity or, alternatively, be by themselves very active, has been verified.