Specific somatostatin-binding to cytosol of bovine gallbladder mucosa

The binding of somatostatin was studied in the cytosolic fraction of bovine gallbladder mucosa. The binding reaction depended on time, temperature and pH, and was reversible, saturable and specific. Stoichiometric data suggested the presence of two classes of binding sites: a class with high affinity (K _d=23.6 nM) and low capacity (3.7 pmol somatostatin/mg protein) and a class with low affinity (K _d=284.6 nM) and high capacity (85.0 pmol somatostatin/mg protein) at 37°C and pH 7.4. The binding sites were highly specific for somatostatin since peptides such as [Leu]enkephalin, neurotensin, vasoactive intestinal peptide and substance P showed practically no effect upon somatostatin binding. The presence of somatostatin-binding sites in the cytosolic fraction of gallbladder mucosa, together with the known occurrence of somatostatin nerve endings in the gallbladder strongly suggests that this peptide may be involved in the physiology and physiopathology of gallbladder mucosa.     

Neurotensin effect on Na+, K+-ATPase is CNS area- and membrane-dependent and involves high affinity NT1 receptor

We have previously shown that peptide neurotensin inhibits cerebral cortex synaptosomal membrane Na⁺, K⁺-ATPase, an effect fully prevented by blockade of neurotensin NT₁ receptor by antagonist SR 48692. The work was extended to analyze neurotensin effect on Na⁺, K⁺-ATPase activity present in other synaptosomal membranes and in CNS myelin and mitochondrial fractions. Results indicated that, besides inhibiting cerebral cortex synaptosomal membrane Na⁺, K⁺-ATPase, neurotensin likewise decreased enzyme activity in homologous striatal membranes as well as in a commercial preparation obtained from porcine cerebral cortex. However, the peptide failed to alter either Na⁺, K⁺-ATPase activity in cerebellar synaptosomal and myelin membranes or ATPase activity in mitochondrial preparations. Whenever an effect was recorded with the peptide, it was blocked by antagonist SR 48692, indicating the involvement of the high affinity neurotensin receptor (NT₁), as well as supporting the contention that, through inhibition of ion transport at synaptic membrane level, neurotensin plays a regulatory role in neurotransmission.     

Lack of Rapid Desensitization of Ca2+ Responses in Transfected CHO Cells Expressing the Rat Neurotensin Receptor Despite Agonist-Induced Internalization

Abstract: Transfected Chinese hamster ovary cells were used as a model for the study of the desensitization of the neurotensin receptor at the second messenger level. Stimulation with nanomolar concentrations of neurotensin elicited rapid rises in the cytosolic calcium concentration ([Ca²⁺]_i), which remained elevated throughout the peptide application. A significant response was already detected with neurotensin concentrations as low as 0.01 nM. This high efficiency of neurotensin in mediating this calcium response contrasts with the nanomolar affinity of the peptide for its receptor measured in binding experiments. Evidence indicated that the initial elevation of the [Ca²⁺]_i resulted from release of Ca²⁺ from intracellular stores, whereas the sustained response involved an influx of extracellular origin. Return to the basal level was only reached after extensive washing of the peptide or its displacement with the neurotensin receptor antagonist SR48692. After washing, further stimulations were still able to mediate an increase in the [Ca²⁺]_i, indicating an apparent absence of rapid desensitization of the intracellular signaling pathway that mediates calcium mobilization. In contrast with this absence of response desensitization, the neurotensin receptors were found to internalize after stimulation with the peptide. This internalization was maximal after 30 min and accounted for ～70% of the number of neurotensin binding sites located at the cell surface. These results indicate that despite the functional properties of the rat neurotensin receptor present in Chinese hamster ovary cells after transfection, the intracellular signaling pathway triggered by stimulation with neurotensin seems to be resistant to desensitization. This might be related to the high efficiency of the intracellular signaling pathway coupled to the neurotensin receptor observed in these cells. A possible absence of desensitization of the neurotensin receptor itself is also discussed.     

Binding of synthetic peptide TPLVTLFK to nonopioid beta‐endorphin receptor on rat brain membranes

The synthetic peptide TPLVTLFK corresponding to the sequence 12–19 of β‐endorphin (referred to as octarphin) was found to bind to high‐affinity naloxone‐insensitive binding sites on membranes isolated from the rat brain cortex (K_d = 2.6 ± 0.2 nM ). The binding specificity study revealed that these binding sites were insensitive not only to naloxone but also to α‐endorphin, γ‐endorphin, [Met⁵]enkephalin, and [Leu⁵]enkephalin, as well. The [³H]octarphin specific binding with brain membranes was inhibited by unlabeled β‐endorphin (K_i = 2.4 ± 0.2 nM ) and a selective agonist of nonopioid β‐endorphin receptor decapeptide immunorphin SLTCLVKGFY (K_i = 2.9 ± 0.2 nM ). At the same time, unlabeled octarphin completely (by 100%) inhibited the specific binding of [³H]immunorphin with membranes (K_i = 2.8 ± 0.2 nM ). Thus, octarphin binds with a high affinity and specificity to nonopioid receptor of β‐endorphin on rat brain cortex membranes. Copyright © 2010 European Peptide Society and John Wiley & Sons, Ltd.     

NTS2-selective neurotensin mimetics with tetrahydrofuran amino acids

Stimulation of the NTS2 neurotensin receptor causes antipsychotic effects and leads to a promotion of the μ-opioid-independent antinociception, which is important in the modulation of tonic pain sensitivity. We report the synthesis and properties of a small library of peptidic agonists based on the active neurotensin fragment NT(8–13). Two tetrahydrofuran amino acid derivatives were synthesized to replace Tyr¹¹ in NT(8–13). Additionally, Arg⁸, Arg⁹, and Ile¹² of the lead peptide were exchanged by Lys, Lys, and Gly, respectively. The new compounds showed substantial NTS2 binding affinity and up to 1000-fold selectivity over NTS1. The highest selectivity (K_i(NTS2): 29 nM, K_i(NTS1): 35,000 nM) was observed for the peptide analog 17R^trans.     

Aggregate formation in the intercalation of long-chain fatty acid esters into liposomes

Various hydrophobic benzenediacetic esters, the corresponding benzenedipropionic esters, and branched alkyl esters were intercalated into DMPC liposomes, where the molar ratio (n/n) of ester:DMPC was 1:5. In the case of the very long-chain derivatives, double carbonyl peaks were observed in the ¹³C NMR spectrum. This doubling phenomenon was observed only for the carbonyl peaks, whose chemical shift is most sensitive to solvent polarity, and disappeared when the ester:DMPC molar ratio drops below 1:15. This doubling reflects the presence of two populations in these samples: one group includes those molecules which are intercalated within the liposome and feel the polarity corresponding to the liposomal microenvironment; the other consists of aggregates of these long-chain derivatives located in the extra-liposomal aqueous phase.     

Synthetic peptide TPLVTLFK (octarphin) reduces the corticosterone production by rat adrenal cortex through nonopioid β‐endorphin receptor

The synthetic peptide octarphin (TPLVTLFK) corresponding to the sequence 12–19 of β‐endorphin, a selective agonist of nonopioid β‐endorphin receptor, was labeled with tritium to a specific activity of 29 Ci/mmol. [³H]Octarphin was found to bind to high‐affinity naloxone‐insensitive binding sites on membranes isolated from rat adrenal cortex (K_d = 35.7 ± 2.3 nM, B_max = 41.0 ± 3.6 pmol/mg protein). The binding specificity study revealed that these binding sites were insensitive not only to naloxone but to α‐endorphin, γ‐endorphin, [Met⁵]enkephalin, and [Leu⁵]enkephalin as well. At the same time, the [³H]octarphin‐specific binding with adrenal cortex membranes was inhibited by unlabeled β‐endorphin (K_i = 32.9 ± 3.8 nM). Octarphin at concentrations of 10^?9–10^?6 M was found to inhibit the adenylate cyclase activity in adrenocortical membranes, whereas intranasal injection of octarphin at doses of 5 and 20 µg/rat was found to reduce the secretion of corticosterone from the adrenals to the bloodstream. Thus, octarphin decreases the adrenal cortex functional activity through the high affinity binding to nonopioid receptor of β‐endorphin. Copyright © 2012 European Peptide Society and John Wiley & Sons, Ltd.     

Apolipophorin III interaction with model membranes composed of phosphatidylcholine and sphingomyelin using differential scanning calorimetry

Apolipophorin III (apoLp-III) from Locusta migratoria was employed as a model apolipoprotein to gain insight into binding interactions with lipid vesicles. Differential scanning calorimetry (DSC) was used to measure the binding interaction of apoLp-III with liposomes composed of mixtures of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and sphingomyelin (SM). Association of apoLp-III with multilamellar liposomes occurred over a temperature range around the liquid crystalline phase transition (L_α). Qualitative and quantitative data were obtained from changes in the lipid phase transition upon addition of apoLp-III. Eleven ratios of DMPC and SM were tested from pure DMPC to pure SM. Broadness of the phase transition (T_1/2), melting temperature of the phase transition (T_m) and enthalpy were used to determine the relative binding affinity to the liposomes. Multilamellar vesicles composed of 40% DMPC and 60% SM showed the greatest interaction with apoLp-III, indicated by large T_1/2 values. Pure DMPC showed the weakest interaction and liposomes with lower percentage of DMPC retained domains of pure DMPC, even upon apoLp-III binding indicating demixing of liposome lipids. Addition of apoLp-III to rehydrated liposomes was compared to codissolved trials, in which lipids were rehydrated in the presence of protein, forcing the protein to interact with the lipid system. Similar trends between the codissolved and non-codissolved trials were observed, indicating a similar binding affinity except for pure DMPC. These results suggested that surface defects due to non-ideal packing that occur at the phase transition temperature of the lipid mixtures are responsible for apolipoprotein-lipid interaction in DMPC/SM liposomes.     

The actions of naloxazone on the binding and analgesic properties of morphiceptin (NH2Tyr-Pro-Phe-Pro-CONH2), a selective mu-receptor ligand

Active in both binding and biological assays, morphiceptin (NH₂ Tyr-Pro-Phe-Pro-CONH₂), a potent opioid peptide derivative of β-casamorphine, binds specifically and selectively to mu or morphine-type receptors with little affinity for delta sites. Displacement studies of a variety of ³H-labeled opiates and enkephalins show biphasic curves. Naloxazone, which blocks irreversibly and selectively high affinity opiate and enkephalin binding, abolishes morphiceptin's inhibition of binding at low concentrations, suggesting that the high affinity binding of enkephalins and opiates represents a mu or morphine-type receptor. Unlike the reversible antagonist naloxone, naloxazone treatment $\text{in}$$\text{vivo}$ inhibits for over 24 hours the analgesic activity of morphiceptin like it inhibits morphine, β-endorphin and enkephalin analgesia. Together, these studies imply that opiates and enkephalins bind with highest affinity to a mu receptor which mediates their analgesic activity. The ³H-D-ala²-D-leu⁵-enkephalin binding remaining after naloxazone treatment, representing a lower affinity site (K_D 4 nM), is quite insensitive to morphiceptin inhibition and has the characteristics of a delta receptor. However, the ³H-dihydromorphine binding present after naloxazone treatment, which also represents a lower affinity site (K_D 6 nM), is far more sensitive to both morphine and morphiceptin and may represent a second morphine-like, or mu, receptor.     

Dimeric Pentapeptide and Tetrapeptide Enkephalins: New Tools for the Study of Delta Opioid Receptors

Abstract

When a dimeric ligand can react bivalently, one would expect an increase in affinity, selectivity, and possibly biological activity. On this premise, we have synthesized and characterized two series of dimers, viz.: Dimeric Pentapeptide Enkephalin (DPE_n = (H-Tyr-D-Ala-Gly-Phe-Leu-NH-)₂ · (CH₂)_n, and Dimeric Tetrapeptide Enkephalin (DTE_n) = (H-Tyr-D-Ala-Gly-Phe-NH)₂ · (CH₂)_n, with n = 2, 4, …, 12. These dimers display affinity, activity, and δ/μ selectivity which vary systematically with chain length (n). DPE₂ shows a seven-fold increase in affinity for the δ receptor of whole brain and NG108-15 cells, relative to monomer, while its activity for the μ receptor is similar to enkephalin monomers. DTE₁₂ shows a dramatic increase in δ selectivity relative to its monomer. The association rate constant for ³H-DPE₂ is increased two-fold and its dissociation rate constant is significantly reduced, relative to monomer. DPE₂ shows a loss of affinity in the presence of Na⁺ or Mn⁺⁺, while GTP unexpectedly increases its affinity under some conditions. DPE₂ shows equal potency with the agonist [D-Ala², D-Leu⁵] enkephalin in assays measuring their inhibitory effect on prostaglandin E₁-stimulated cAMP production by NG108-15 cells. DPE₂ is very potent in the mouse vas deferens assay; DTE₁₂ shows substantially less activity. These results suggest that δ opiate receptors may be closely clustered in the cell membrane, and provide new approaches to the development of δ and μ receptor ligands.     

Thermodynamics of partitioning and efflux of phenothiazines from liposomes

Summary The partitioning of nine phenothiazines between dimyristoylphosphatidylcholine (DMPC) liposomes and 0.9% wt/vol saline at pH 6 has been studied both below and above the phase transition temperature (T _c ) of the phospholipid. Higher partitioning was observed aboveT _c . Both the entropy and enthalpy of partitioning were positive below and aboveT _c , and a linear relationship between the entropy and enthalpy has been derived. In general, the partitioning and transport of alkylaminophenothiazines in DMPC liposomes over the temperature range of 5 to 40°C is entropically controlled. The entropies and enthalpies of partitioning of various groups in the phenothiazine structure have been calculated.No relationship was found between particle size of the DMPC liposomes and the equilibrium partition coefficient at 25°C. However, the particle size of liposomes did increase with increasing acyl chain length of the phospholipid.Using differential scanning calorimetry, the enthalpy and entropy of transition of the DMPC liposomes in the absence and presence of phenothiazines has been calculated. The temperature dependence of the first-order rate constant of trimeprazine tartrate transport in DMPC liposomes was investigated and was found to be maximum at theT _c of the phospholipid.     

Interaction of iodinated enkephalin analogues with opiate receptors.

Radioiodinated derivatives of the metabolically stable enkephalin analogues, [DAla²,Leu⁵]- and [DAla²,DLeu⁵]-enkephalin, have been prepared. Such derivatives show sterospecific binding to receptors in brain homogenates and some neuroblastoma cell lines such as NG108-15 and N4TG1. The relative effects of levorphanol and dextrorphan and Na⁺ and Mn⁺⁺ ions on enkephalin binding in brain and cells indicate that the iodinated derivatives are interacting with opiate receptors. Levorphanol is considerably more potent in displacing specifically bound enkephalin than dextrorphan. Sodium ions at physiological concentrations decrease enkephalin binding whereas manganese ions enhance it. Unlabelled monoiodo derivatives retain high potency in the guinea-pig ileum, mouse vas deferens and receptor binding assays. Unlabelled diiodo derivatives show far lower potency in these assays. It is concluded that radio-iodinated derivatives containing one iodine per molecule retain high affinity for the opiate receptor but diiodo derivatives do not.     

Characterization of the bioactive form of linear peptide antagonists at the δ-opioid receptor

The stereochemical requirements for δ-opioid receptor binding of a series of linear peptide antagonists with a novel conformationally restricted Phe analogue (Tic) as a second residue were examined by using a variety of computational chemistry methods. The δ-opioid receptor analogues with significant affinity, Tyr-Tic-NH₂ (TI-NH₂), Tyr-Tic-Phe-OH (TIP), Tyr-Tic-Phe-NH₂(TIP-NH₂), Tyr-Tic-Phe-Phe-OH (TIPP), Tyr-Tic-Phe-Phe-NH₂) (TIPP-NH₂), and the low affinity δ-opioid peptides Tyr-Pro-Phe-Pro-NH₂ (morphiceptin) and Tyr-Phe-Phe-Phe-NH₂ (TPPP-NH₂), were included in this study. The conformational profiles of these peptides were obtained by consecutive cycles of high and low temperature molecular dynamic simulations, coupled to molecular mechanical energy minimization carried out until no new conformational minima were obtained. Comparing the results for TPPP-NH₂ and TIPP-NH₂, the presence of the conformationally restricted Tic residue did not greatly reduce the number of unique low energy conformations, but did allow low energy conformers involving cis bonds between the first two residues. The conformational libraries of these peptides were examined for their ability to satisfy the three key ligand components for receptor recognition already identified by previous studies of high affinity cyclic (Tyr¹-D -Pen²-Gly³-Phe⁴-D -Pen⁵) enkephalin (DPDPE) type agonists: a protonated amine group, an aromatic ring, and a lipophilic moiety in a specific geometric arrangement. Two types of conformations common to the five high δ-opioid affinity L -Tic analogues were found that satisfied these requirements, one with a cis and the other with a trans peptide bond between the Tyr¹ and Tic² residues. Moreover, both the Tic² and Phe³ residues could mimic the hydrophobic interactions with the receptor of the Phe⁴ moiety in the cyclic DPDPE type agonists, consistent with the appreciable affinity of both di-and tripeptides. The low δ-opioid receptor affinity of morphiceptin can be understood as the result of conformational preferences that prevent the fulfillment of this pharmacophore for recognition. © 1996 John Wiley & Sons, Inc.     

Clozapine Administration Modifies Neurotensin Effect on Synaptosomal Membrane Na<Superscript>+</Superscript>, K<Superscript>+</Superscript> -ATPase Activity

Na⁺, K⁺-ATPase is inhibited by neurotensin, an effect which involves the peptide high affinity receptor (NTS1). Neurotensin effect on cerebral cortex synaptosomal membrane Na⁺, K⁺-ATPase activity of rats injected i.p. with antipsychotic clozapine was studied. Whereas 3.5 × 10⁻⁶ M neurotensin decreased 44% Na⁺, K⁺-ATPase activity in the controls, the peptide failed to modify enzyme activity 30 min after a single 3.0, 10.0 and 30.0 mg/kg clozapine dose. Neurotensin decreased Na⁺, K⁺-ATPase activity 40 or 20% 18 h after 3.0 or 5.6 mg/kg clozapine administration, respectively, and lacked inhibitory effect 18 h after 17.8 and 30.0 mg/kg clozapine doses. Results indicated that the clozapine treatment differentially modifies the further effect of neurotensin on synaptosomal membrane Na⁺, K⁺-ATPase activity according to time and dose conditions employed. Taken into account that clozapine blocks the dopaminergic D2 receptor, findings obtained favor the view of an interplay among neurotensinergic receptor, dopaminergic D2 receptor and Na⁺, K⁺-ATPase at synaptic membranes.     

Chronic ethanol alters the receptor binding characteristics of the delta opioid receptor ligand,DAla2DLeu5 enkephalin in mouse brain

The binding characteristics of the delta opioid receptor ligand, ³HDAla²DLeu⁵ enkephalin, were markedly altered in brains obtained from mice fed an ethanol-containing diet for five days. Control mice exhibited both a high and low affinity site for ³HDAla²DLeu⁵ enkephalin, whereas those consuming the ethanol diet were found to possess only one binding site. This singular site has an intermediate K_D value with an increase in receptor number when compared to the high and low affinity sites observed in control mice. The $\text{in}$$\text{vitro}$ addition of ethanol to a brain membrane preparation obtained from untreated mice, at a concentration equivalent to that found in the blood of the ethanol-treated mice, did not markedly affect DAla²DLeu⁵ enkephalin binding characteristics. No alteration in the binding characteristics of ³H-naloxone, a mu receptor ligand, was noted following five days of ethanol consumption. Mice maintained on the ethanol-containing diet were tolerant to the activity-stimulating effects of acute ethanol administration. These results suggest that mice consuming an ethanol diet in sufficient quantities to render them tolerant exhibit a specific loss of a ³HDAla²DLeu⁵ enkephalin binding site, while the binding of ³H-naloxone was unchanged.     

Dynamics and Cleavability at the alpha-cleavage site of APP(684-726) in different lipid environments

The occurrence of late-onset Alzheimer's disease has been related to the lipid homeostasis. We tested whether the membrane lipid environment affects the dynamics and cleavability of a model peptide corresponding to the amino acid sequence 684-726 of the amyloid precursor protein APP reconstituted in liposomes. Solid-state NMR with ²H-Ala⁷¹³, which is located within the putative transmembrane domain, suggested that the peptide observes less rotational motion in egg phosphatidylcholine (PhC) membranes than in dimyristoyl-phosphatidylcholine (DMPC) bilayers above the main phase transition temperature T_c. The residue ¹⁵N-Ala⁶⁹², which is in the vicinity of the α-cleavage site, i.e., Lys⁶⁸⁷, showed less motion after reconstitution in distearoyl-phosphatidylcholine liposomes <T_c than in PhC, DMPC, or sphingomyelin vesicles. In all tested liposomal systems the α-cleavage site was accessible for hydrolysis by trypsin. However, the catalytic rate constant was higher in the PhC and DMPC than in the sphingomyelin and distearoyl-phosphatidylcholine systems. In conclusion, the dynamics of APP(684-726) on the transmembrane level as well as the motion of the α-cleavage site and its hydrolysis by a model enzyme are dependent on the bilayer characteristics. This could be relevant for the processing of APP in vivo.     

Synthesis and receptor binding of opioid peptide analogues containing β3‐homo‐amino acids

β‐Amino acids containing hybrid peptides and β‐peptides show great potential as peptidomimetics. In this paper we describe the synthesis and affinity toward the µ‐ and δ‐opioid receptors of β‐peptides, analogues of Leu‐enkephalin, deltorphin I, dermorphin and α,β‐hybrides, analogues of deltorphin I. Substitution of α‐amino acid residues with β³‐homo‐amino acid residues, in general resulted in decrease of affinity to opioid receptors. However, the incorporation β³h‐D ‐Ala in position 2 or β³hPhe in position 3 of deltorphin I resulted in potent and selective ligand for δ‐opioid receptor. The NMR studies of β‐deltorphin I analogue suggest that conformational motions in the central part of the peptide backbone are partially restricted and some conformational preferences can be expected. Copyright © 2009 European Peptide Society and John Wiley & Sons, Ltd.     

Differential Calcitonin Gene-Related Peptide (CGRP) and Amylin Binding Sites in Nucleus Accumbens and Lung: Potential Models for Studying CGRP/Amylin Receptor Subtypes

Abstract: Calcitonin gene-related peptide (CGRP), a 37-amino-acid peptide, is a member of a small family of peptides including amylin or islet amyloid polypeptide and salmon calcitonin. These related peptides have been shown to display similar effects on in vitro and in vivo carbohydrate metabolism. The present study was initiated to identify and characterize the binding sites for these peptides in lung and nucleus accumbens membranes prepared from pig and guinea pig. Both tissues in either species displayed high-affinity (2-[¹²⁵I]iodohistidyl¹⁰)humanCGRPα ([¹²⁵I]hCGRPα) binding (IC₅₀ = 0.4–7.7 nM), which was displaced by hCGRP_8–37α with equally high affinity (IC₅₀ = 0.4–7.3 nM). High-affinity binding for [¹²⁵I]Bolton-Hunter human amylin ([¹²⁵I]BH-h-amylin) was also observed in these tissues (IC₅₀ = 0.2–6.0 nM). In membranes from the nucleus accumbens of both species, salmon calcitonin competed for amylin binding sites with high affinity (IC₅₀ = 0.1 nM) but was poor in competing for amylin binding in lung membranes. Rat amylin_8–37 competed for [¹²⁵I]hCGRPα binding with higher affinity (IC₅₀ = 5.4 nM) compared with [¹²⁵I]BH-h-amylin binding (IC₅₀ = 200 nM) in porcine nucleus accumbens, whereas in guinea pig nucleus accumbens, the IC₅₀ values for rat amylin_8–37 were 117 and 12 nM against [¹²⁵I]hCGRPα and [¹²⁵I]BH-h-amylin, respectively. Also, functional studies evaluating the activation of adenylate cyclase and generation of cyclic AMP in response to these agonists indicated that hCGRPα (EC₅₀ = 0.3 nM), h-amylin (EC₅₀ = 150 nM), and salmon calcitonin (EC₅₀ = 1,000 nM) activated adenylate cyclase, resulting in increased cyclic AMP production in porcine lung membranes that was antagonized by hCGRP_8–37α. The affinity of hCGRP_8–37α was similar for all three peptides. The cyclic AMP responses to amylin and salmon calcitonin were significantly (p < 0.05) lower than that of hCGRPα and not additive, suggesting that they are acting as partial agonists at the same CGRP1-type receptor in porcine lung membranes. Similar observations were made for guinea pig lung membranes. However, human amylin and salmon calcitonin were weaker than hCGRPα in activating lung adenylate cyclase. None of these peptides activated adenylate cyclase in membranes prepared from the nucleus accumbens of both species. The data from these studies demonstrate both species and tissue differences in the existence of distinct CGRP and amylin binding sites and present a potential opportunity to study further CGRP and amylin receptor subtypes.     

Coimmobilization of Superoxide Dismutase,Catalase, and Peroxidase

Various orders of sequential coimmobilization of superoxide dismutase (SOD), catalase, and horseradish peroxidase (HRP) were tested in order to prepare a multienzyme antioxidant complex of these enzymes. Simultaneous coimmobilization of catalase with a preliminarily cross-linked complex between SOD and HRP was found to be the optimum procedure. The catalytic enzyme activity and working stability of catalase was tested kinetically in the multienzyme complexes prepared by different methods. The effects of ascorbic acid, glutathione, and ethanol on the kinetic parameters of catalase were studied. A possible scheme of H₂O₂ degradation catalyzed by coimmobilized SOD, catalase, and HRP in the presence of reducing substrates is suggested.     

Synthesis and evaluation of derivatives of leucine enkephalin as potential affinity labels for delta opioid receptors

As part of an effort to develop peptide-based affinity labels for opioid receptors, [Leu(5)]enkephalin (LeuEnk) and DTLET (Tyr-D-Thr-Gly-Phe-Leu-Thr), potent agonists for delta receptors, were selected as the parent peptides for further modification. The affinity label derivatives were prepared using standard Fmoc solid-phase peptide synthesis in conjunction with Fmoc-Phe(p-NHAlloc) (Fmoc: 9-flourenylmethoxycarbonyl;) and selective modification of the p-amino group on this residue. The electrophilic isothiocyanate and bromoacetamide groups were introduced into the para position of Phe(4); the corresponding free amine-containing peptides were also prepared for comparison. The pure peptides were evaluated in radioligand binding assays using Chinese hamster ovary (CHO) cells expressing delta and micro opioid receptors. Modification of Phe(4) in LeuEnk and DTLET significantly decreased delta-receptor binding affinity (40 to >2,000-fold). Among the synthesized analogues, [Phe(p-NH(2))(4)]DTLET showed the highest delta-receptor binding affinity (IC(50) = 39 nM) and enhanced selectivity for delta receptors compared to DTLET while other derivatives exhibited much lower delta receptor affinity. The differences in affinities between the two series of analogues and between the derivatives of LeuEnk and N,N-dibenzyl[Leu(5)]Enk reported previously suggest subtle differences in interactions of Phe(4) with delta receptors depending on other modifications in the sequences.