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Biomedical subjects

T M Nguyen

Publications and source records attributed to T M Nguyen.

At least 91 records · Page 5Linked to original sources
PubMed1989

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.

Amides↗
PubMed1989

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.

Animals↗
PubMed1988

Mechanism of target cytolysis by peptide defensins. Target cell metabolic activities, possibly involving endocytosis, are crucial for expression of cytotoxicity.

In a previous study, potent tumor cytolysis mediated by human neutrophil peptide defensins occurred slowly over 3 to 15 h. Because these kinetics suggested a requirement for target cell metabolic processes before tumor killing could be realized, the mechanism of lysis by these purified peptides was further investigated. 125I-labeled defensin bound extensively to peptide-sensitive K562 targets with biphasic kinetics. Binding was inhibited in parallel with cytotoxicity when both assays were performed at low temperature or in the presence of FCS. The albumin content of serum could account for the inhibitory effects of FCS. Cytotoxicity was also antagonized by agents that interfered with target cell energy metabolism (azide and 2-deoxyglucose), the cytoskeletal apparatus (cytochalasin B and dihydrocytochalasin B), lysosomal function (NH4Cl and chloraquin), or calmodulin-mediated activities (trifluoperazine). FCS also completely removed membrane-bound defensin when it was added after 5 min of binding at 37 degrees C. However, significantly less defensin was removed when FCS was added at later time points after binding was initiated. Cytochalasin B and azide/2-deoxyglucose did not prevent binding of defensin to targets but it significantly inhibited the development of FCS resistance in membrane-bound peptide. However, these two classes of inhibitors acted during distinct time windows: cytochalasin-sensitive events were complete by 1 h, whereas azide/2-deoxyglucose continued to be inhibitory when added as late as 2 h after defensins. These latter data indicated that critical energy-dependent events continue after the cytochalasin-sensitive phase has been completed. The results suggest that defensin-mediated cytotoxicity requires initial binding of defensin molecules to targets and subsequent cytoskeletal- and energy-dependent translocation or internalization. Although the defensins are low m.w. peptides, the initial processes required for their cytotoxic activity resemble those of more complex bacterial, plant and mammalian cytotoxins.

Animals↗
PubMed1988

Thyroid and parathyroid-independent increase in plasma 1,25-dihydroxyvitamin D during late pregnancy in the rat.

The effect of thyroparathyroidectomy (TPTX) on the plasma concentrations of the vitamin D metabolites (25-(OH)D, 24,25-(OH)2D and 1,25-(OH)2D) has been studied in pregnant rats and their fetuses during the last quarter of gestation. Maternal and fetal vitamin D metabolites were not significantly affected by TPTX. A significant increase in plasma 1,25-(OH)2D concentrations was observed in both TPTX and control mothers and fetuses from days 19 to 21. Fetal and maternal plasma 25-(OH)D were positively correlated in both control and TPTX groups. Such a correlation was also found for 24,25-(OH)2D in the two groups. In contrast, a positive correlation between maternal and fetal plasma concentrations of 1,25-(OH)2D was found in TPTX but not in control rats. These data suggest that major alterations in calcium metabolism, such as that produced by maternal TPTX, are insufficient to affect the changes in maternal and fetal plasma 1,25-(OH)2D during late pregnancy significantly. They also suggest that parathyroid hormone, thyroxine, and/or calcitonin may control a possible placental transfer of 1,25-(OH)2D in the rat.

24,25-Dihydroxyvitamin D 3↗
PubMed1987

Superactive analogs of the atrial natriuretic peptide (ANP).

Three analogs of the atrial natriuretic peptide ANP(105-126), lacking the N-terminal exocyclic peptide segment and containing 2-mercaptoacetic acid, 3-mercaptopropionic acid or 4-mercaptobutyric acid in place of the cysteine residue in position 105 of the peptide sequence, were synthesized by the solid-phase method. The resulting des-amino analogs showed 2 to 4 times higher diuretic/natriuretic activity than the most active natural ANP and displayed a potent hypotensive effect as well. All three analogs were relatively less potent in various in vitro bioassays and in a binding assay, indicating that their high activities in vivo may be due to resistance to enzymatic degradation and to reduced non-specific tissue adsorption. These compounds not only will serve as useful pharmacologic tools but also represent prototypes for the development of further reduced-size ANP analogs.

Adrenal Cortex↗
PubMed1987

Hybrid bivalent ligands with opiate and enkephalin pharmacophores.

Bivalent ligands consisting of oxymorphamine and [D-Glu2]enkephalin pharmacophores linked through a spacer attached to the 6-amino group of the former and D-Glu of the latter were synthesized in an effort to investigate the possible coexistence of mu and delta recognition sites in the same opioid receptor complex. Of the two bivalent ligands (1,2) synthesized, only 1 had substantially greater antinociceptive potency in mice than its monovalent analogues (1a, 1b). Testing of 1, 1a, and 1b in the guinea pig ileum preparation (GPI) revealed a potency profile similar to that found in vivo, whereas no correlation was observed in the mouse vas deferens (MVD). Binding data indicated the same rank-order affinities at delta receptors as the opioid activities in the GPI and in mice. However, mu binding exhibited no relationship with activity. These results are consistent with the simultaneous occupation of mu and delta by a single bivalent ligand 1, but they are also in harmony with the interaction of 1 with an opioid receptor and an accessory binding site.

Analgesics↗
PubMed1987

Structure-activity relationships of cyclic opioid peptide analogues containing a phenylalanine residue in the 3-position.

Ten analogues of the highly mu-receptor selective cyclic opioid peptide H-Tyr-D-Orn-Phe-Asp-NH2 (1) were synthesized by the solid phase method and were characterized in vitro in mu- and delta-receptor representative binding assays and bioassays. These cyclic analogues are structurally related to the linear opioid peptides dermorphin and beta-casomorphin (morphiceptin), which also contain a phenylalanine residue in the 3-position of the peptide sequence. The obtained results indicate that analogous structural modifications (configurational inversion at positions 2, 3, and 4 or N alpha-methylation of Phe3) in cyclic peptide 1 and in dermorphin-related peptides had qualitatively the same effect on opioid activity, whereas the corresponding modifications in beta-casomorphins had the opposite effect. These findings can be interpreted to indicate that the mode of receptor binding of H-Tyr-D-Orn-Phe-Asp-NH2 is identical with that of dermorphin, but differs from that of beta-casomorphins. The side-chain length of the aromatic residue in position 3 of cyclic analogue 1 was shown to be critical for receptor affinity and selectivity, suggesting that mu- and delta-receptors differ from one another in the relative topographical disposition of the binding sites for the Tyr1 tyramine moiety and the Phe3 aromatic ring. Cyclic lactam analogue H-Tyr-D-Asp-Phe-A2bu-NH2, containing a reduced-size (12-membered) ring structure, showed increased mu-receptor selectivity, whereas the more flexible, cystine-containing analogue H-Tyr-D-Cys-Phe-Cys-NH2 (11-membered ring) was less selective. The latter results indicate that both ring size and ring flexibility affect receptor affinity and selectivity.

Animals↗
PubMed1986

The biosynthesis of phosphatidylcholine by methylation of phosphatidylethanolamine derived from ethanolamine is not required for lipoprotein secretion by cultured rat hepatocytes.

The role that phosphatidylcholine biosynthesis plays in the assembly and secretion of lipoproteins has been investigated in rat hepatocytes, since phosphatidylcholine is the major phospholipid in all serum lipoproteins. Phosphatidylcholine in rat hepatocytes can be made via the CDPcholine pathway or by the methylation of phosphatidylethanolamine. A specific inhibitor of cellular transmethylation, 3-deazaadenosine (10 microM), has been incubated with rat hepatocytes, and we have shown that the biosynthesis of phosphatidylcholine via the methylation of phosphatidylethanolamine derived from ethanolamine was inhibited by greater than 95%. However, incubation of 3-deazaadenosine with cultured rat hepatocytes for up to 18 h did not affect the secretion of any of the apoproteins into VLDL, LDL, HDL fractions or a fraction with density greater than 1.18 g/ml (albumin was the major protein). Nor was there any effect by 3-deazaadenosine on the amount of phosphatidylcholine secreted into the culture medium or into VLDL or HDL. After 18 h the amount of phosphatidylethanolamine that accumulated in the cells was doubled by treatment with 3-deazaadenosine, and the amount of phosphatidylethanolamine secreted into the medium was increased by approximately 70%. It is thus apparent that the synthesis of phosphatidylcholine from ethanolamine is not required for lipoprotein secretion by rat hepatocytes.

Animals↗
PubMed1986

Side chain to side chain cyclization of an enkephalin analog results in loss of opioid receptor selectivity.

The cyclic enkephalin analog H-Tyr-D-Lys-Gly-Phe-Glu-NH2 (I) and the structurally related open chain analogs H-Tyr-D-Nle-Gly-Phe-Gln-NH2 (II) and H-Tyr-D-Lys(For)-Gly-Phe-Abu-NH2 (III) were tested in mu and delta opioid receptor-representative binding assays and bioassays. Whereas both linear analogs showed a pronounced preference for mu receptors over delta receptors, the conformationally restricted cyclic peptide I was found to be unselective. This finding represents the first reported example of a peptide cyclization resulting in a loss of receptor selectivity. From this and earlier studies, it was concluded that the receptor selectivity of cyclized peptide analogs relative to that of their linear correlates may depend on the size and relative rigidity of their ring structures.

Animals↗
PubMed1985

A novel side-chain-linked antiparallel cyclic dimer of enkephalin.

The dimeric cyclic enkephalin analog, (H-Tyr-D-Lys-Gly-Phe-Glu-NH2)2, was isolated as a second major component from the crude product obtained in a solid-phase synthesis of the corresponding cyclic monomer, H-Tyr-D-Lys-Gly-Phe-Glu-NH2. In comparison with [Leu5]enkephalin the cyclic dimer is about equipotent in assays representative for mu-opioid receptor interactions and 1/10 as potent at the delta-receptor. The fact that the enkephalin dimer shows a receptor selectivity pattern distinct from that of the cyclic monomer and of the corresponding linear analog suggests that cyclodimerization via side-chain linkages might be generally useful as a means to produce shifts in the activity profiles of peptide hormones and neurotransmitters.

Animals↗
PubMed1985

Synthesis and biological activity of a linear fragment of the atrial natriuretic factor (ANF).

A linear fragment of the atrial natriuretic factor, ANF(106-125), unable to form an intramolecular cystine bridge, was synthesized by the solid-phase method. The fragment showed smooth muscle relaxant activity in the rabbit aorta and chick rectum assays, an inhibitory effect on aldosterone secretion from bovine adrenal zona glomerulosa cells, and had affinity for specific ANF receptors located in zona glomerulosa cell membranes. The potency of ANF(106-125) in these four assay systems was about two to three orders of magnitude lower than that of ANF(103-125) which contains the intact cyclic structure. The obtained results indicate that the disulfide linkage stabilizes the bioactive conformation of ANF peptides but is not an absolute requirement for biological activity.

Adrenal Cortex↗
PubMed1985

A novel cyclic opioid peptide analog showing high preference for mu-receptors.

The side-chain to side-chain cyclized opioid peptide analogs H-Tyr-D-Orn-Phe-Asp-NH2 (I) and H-Tyr-D-Lys-Phe-Glu-NH2 (II) were synthesized and tested in the guinea pig ileum and mouse vas deferens assays and in binding assays based on displacement of mu- and delta-opioid receptor-selective radioligands from rat brain membranes. The more rigid cyclic analog I containing a 13-membered ring structure showed very high preference for mu-receptors over delta-receptors, whereas the more flexible cyclic peptide II (15-membered ring) was non-selective. These results indicate that variation in the degree of conformational restriction of opioid peptides can produce drastic shifts in their receptor selectivity profile. Because of its high mu-receptor selectivity and rigidity cyclic analog I will be useful for determining the conformational requirements of mu-opioid receptors.

Animals↗
PubMed1985

Synthesis and activity profiles of novel cyclic opioid peptide monomers and dimers.

A new family of cyclic opioid peptide analogues of the type H-Tyr-D-Xxx-Phe-Yyy-NH2 was obtained through amide bond formation between side chain amino and carboxyl groups of Orn (or Lys) and Asp (or Glu) residues substituted in positions 2 and 4 of the peptide sequence. Peptides were synthesized entirely by solid-phase techniques, and aside from the cyclic monomers, cyclization on the benzhydrylamine resin also produced side chain linked antiparallel cyclic dimers due to intersite reaction. In binding studies based on displacement of mu- and delta-opioid receptor-selective radiolabels from rat brain membranes the highly rigid cyclic monomer H-Tyr-D-Orn-Phe-Asp-NH2 (1) (containing a 13-membered ring) was shown to be one of the most selective mu-receptor ligands reported to date, whereas the corresponding cyclic dimer, (H-Tyr-D-Orn-Phe-Asp-NH2)2 (1a), was nonselective. The difference in receptor selectivity observed between 1 and 1a is a consequence of the different conformational constraints present in the cyclic monomer and dimer. In contrast to 1, the conformationally less restricted cyclic analogue H-Tyr-D-Lys-Phe-Glu-NH2 (3) (15-membered ring) showed no receptor preference. Qualitatively similar potency relationships were observed in the guinea pig ileum (GPI) and mouse vas deferens (MVD) bioassays. However, in the case of analogues 1 and 3 discrepancies observed between potencies determined in the mu-receptor-representative GPI bioassay and in the mu-receptor-selective binding assay seemed to indicate that the conformational constraint present in these compounds may produce an "efficacy" enhancement. Corresponding analogues containing an Asp (or Glu) residue in the 2-position and an Orn (or Lys) residue in the 4-position showed similar selectivity relationships, but better agreement between bio- and binding assay data. These results indicate that incorporation of various conformational constraints into opioid peptides permits manipulation of both receptor selectivity and efficacy.

Amino Acid Sequence↗
PubMed1985

Synthesis and biological activity of linear and cyclic enkephalins modified at the Gly3-Phe4 amide bond.

As part of our continuing effort to define structure-activity relationships for enkephalin and design enzymatically resistant analogs, we report the synthesis and biological activities of linear and cyclic enkephalin analogs modified at the Gly3-Phe4 amide bond. The partial retro-inverso enkephalin analog Tyr-D-Ala-gGly-(R,S)-mPhe-Leu-NH2 and its cyclic counterpart, Tyr-cyclo[D-A2 bu-gGly-(R,S)-mPhe-Leu-], were synthesized as diastereomeric mixtures using solution methodology. The racemic benzylmalonate allowed the linear analog to be synthesized by fragment coupling at the reversed bond. Cyclization of the second analog was carried out at high concentration, eliminating formation of polymer by the use of an insoluble base. All gem-diaminoalkyl residues were prepared by conversion of peptidyl amides with benzene iodonium bis(trifluoroacetate). Diastereomers of both compounds were separable by reverse phase HPLC but those of the linear compound racemized rapidly under conditions of testing and were therefore tested together. All analogs tested had activities ranging from 6 to 14% of the activity of Leu enkephalin, indicating that the Gly3-Phe4 amide bond is important, though not crucial, for receptor binding.

Animals↗
PubMed1984

Activity profiles of novel side-chain to side-chain cyclized opioid peptide analogs.

A new family of opioid peptide analogs cyclized through amide bond formation between side-chain amino and carboxyl groups was tested in mu- and delta-opioid receptor selective bio- and binding assays. Within the series of compounds investigated considerable variation in potency and receptor selectivity was observed as a consequence of the introduced conformational restrictions. In particular, the cyclic analogs H-Tyr-D-Glu-Gly-Phe-Lys-NH2 and H-Tyr-D-Glu-Phe-Lys-NH2 showed high preference for mu-receptors over delta-receptors. The development of highly selective receptor ligands constitutes one of the most important prerequisites in efforts aimed at correlating the various types of opioid receptors with specific opioid activities. Recently, the principle of conformational restriction has been successfully applied in the development of selective opioid peptide analogs. In particular, cyclic enkephalin analogs selective for either mu-receptors (1) or delta-receptors (2) have been reported. In the present paper we describe the activity profiles of a new family of side-chain to side-chain cyclized opioid peptide analogs which were obtained through substitution of an alpha, omega-diamino acid and a glutamic or aspartic acid residue in appropriate positions followed by amide bond formation between the side-chain amino and carboxyl groups (cf. Table I). The syntheses of the analogs have been reported elsewhere (3).

Animals↗
PubMed1983

Cyclic and acyclic partial retro-inverso enkephalinamides: Mu receptor selective enzyme resistant analogs.

The cyclic partial retro-inverso modified enkephalins, H-Tyr-cyclo[-D-Glu-Gly-gPhe-D-Leu-] (I), H-Tyr-cyclo[-D-A2bu-Gly-gPhe-R&S-mLeu-] (IIf, IIs), H-Tyr-cyclo[-D-Glu-Gly-Phe-gLeu-] (III), and their corresponding acyclic analogs, H-Tyr-D-Ala-Gly-gPhe-D-Leu-For (Ia), H-Tyr-D-Ala-Gly-gPhe-R,S-mLeu-NH2 (IIa), H-Tyr-D-Ala-Gly-Phe-gLeu-For (IIIa) have been evaluated in in vitro bioassays. Relative to Leu-enkephalin, the modified compounds are more potent in the guinea pig ileum assay and more effectively displace [3H]naloxone from rat brain receptors, while they are generally less potent in the mouse vas deferens assay and only weakly able to displace [3H] (D-Ala2, D-Leu5)enkephalin. The analogs are highly resistant to proteolytic degradation by rat brain membrane preparations that readily hydrolyze Leu-enkephalin.

Amino Acid Sequence↗
PubMed1983

Comparison of mu-, delta- and kappa-receptor binding sites through pharmacologic evaluation of p-nitrophenylalanine analogs of opioid peptides.

For the purpose of comparing the structural requirements of opioid receptor subsites interacting with phenylalanine residues of opioid peptides, analogs containing p-nitrophenylalanine (Phe(pNO2) ) were synthesized and tested in the guinea pig ileum (GPI) and mouse vas deferens (MVD) assay as well as in mu- and delta-receptor selective binding assays. Whereas substitution of Phe(pNO2) in position 4 of the mu-selective analog H-Tyr-c[-N epsilon-D-Lys-Gly-Phe-Leu-] and of the delta-selective analog H-Tyr-D-Ser-Gly-Phe-Leu-Thr-OH produced a significant potency increase in all assays, the corresponding substitution in the kappa-selective dynorphin-(1-13) fragment resulted in a decreased potency in the GPI-assay. These findings suggest that the electronic requirements of the Phe4 binding site at the mu- and delta-receptor are identical, but different from those of the corresponding site at the kappa-receptor. The observation that Phe(pNO2) substitution in position 3 of morphiceptin and dermorphin also produces a drastic potency drop indicates that the Phe3 side-chain of the latter peptides at the mu- and delta-receptor may bind to a site different from that interacting with the Phe4 side-chain of enkephalins.

Animals↗