New development in the tritium labelling of peptides and proteins using solid catalytic isotopic exchange with spillover-tritium

Tritium Labeling of Neuromedin S by Conjugation with [3H]N-Succinimidyl Propionate

The human neuropeptide neuromedin S (NMS) consists of 33 amino acids. The introduction of tritium atoms into NMS has not been described so far. This represents a gap for using [³H]NMS in radioreceptor binding assays or in tracking and monitoring their metabolic pathway. Two approaches for the incorporation of tritium into NMS were explored in this study: (1) halogenation at the His-18 residue followed by catalyzed iodine-127/tritium exchange and (2) conjugation of tritiated N-succinimidyl-[2,3-³H₃]propionate ([³H]NSP) to at least one of the three available primary amines of amino acids Ile-1, Lys-15, and Lys-16 in the peptide sequence. Although iodination of histidine was achieved, subsequent iodine-127/deuterium exchange was unsuccessful. Derivatization at the three possible amino positions in the peptide using nonradioactive NSP resulted in a mixture of unconjugated NSM and 1- to 3-conjugations at different amino acids in the peptide sequence. Each labeling position in the mixture was assigned following detailed LC-MS/MS analysis. After separating the mixture, it was shown in an in vitro fluorometric imaging plate reader (FLIPR) and in a competitive binding assay that the propionyl-modified NMS derivatives were comparable to the unlabeled NMS, regardless of the degree of labeling and the labeling position(s). A molecular simulation with NMS in the binding pocket of the protein neuromedin U receptor 2 (NMUR₂) confirmed that the possible labeling positions are located outside the binding region of NMUR₂. Tritium labeling was achieved at the N-terminal Ile-1 using [³H]NSP in 7% yield with a radiochemical purity of >95% and a molar activity of 90 Ci/mmol. This approach provides access to tritiated NMS and enables new investigations to characterize NMS or corresponding NMS ligands.

In silico Screening and Behavioral Validation of a Novel Peptide, LCGA-17, With Anxiolytic-Like Properties

The aim of the study was to develop better anxiolytics and antidepressants. We focused on GABA_A receptors and the α2δ auxiliary subunit of V-gated Ca²⁺ channels as putative targets because they are established as mediators of efficacious anxiolytics, antidepressants, and anticonvulsants. We further focused on short peptides as candidate ligands because of their high safety and tolerability profiles. We employed a structural bioinformatics approach to develop novel tetrapeptides with predicted affinity to GABA_A receptors and α2δ. In silico docking studies of one of these peptides, LCGA-17, showed a high binding score for both GABA_A receptors and α2δ, combined with anxiolytic-like properties in a Danio rerio behavioral screen. LCGA-17 showed anxiolytic-like effects in the novel tank test, the light–dark box, and the social preference test, with efficacy comparable to fluvoxamine and diazepam. In binding assays using rat brain membranes, [³H]-LCGA-17 was competed more effectively by gabapentinoid ligands of α2δ than ligands of GABA_A receptors, suggesting that α2δ represents a likely target for LCGA-17. [³H]-LCGA-17 binding to brain lysates was unaffected by competition with ligands for GABA_B, glutamate, dopamine, serotonin, and other receptors, suggesting specific interaction with α2δ. Dose-finding studies in mice using acute administration of LCGA-17 (i.p.) demonstrated anxiolytic-like effects in the open field test, elevated plus maze, and marble burying tests, as well as antidepressant-like properties in the forced swim test. The anxiolytic effects were effectively blocked by bicuculline. Therefore, LCGA-17 is a novel candidate anxiolytic and antidepressant that may act through α2δ, with possible synergism by GABA_A receptors.

Binding of Synthetic LKEKK Peptide to Human T-Lymphocytes

The synthetic peptide LKEKK corresponding to sequence 16-20 of human thymosin-α1 and 131-135 of human interferon-α2 was labeled with tritium to specific activity 28 Ci/mol. The [3H]LKEKK bound with high affinity (Kd = 3.7 ± 0.3 nM) to donor blood T-lymphocytes. Treatment of cells with trypsin or proteinase K did not abolish [3H]LKEKK binding, suggesting the non-protein nature of the peptide receptor. The binding was inhibited by thymosin-α1, interferon-α2, and cholera toxin B subunit (Ki = 2.0 ± 0.3, 2.2 ± 0.2, and 3.6 ± 0.3 nM, respectively). Using [3H]LKEKK, we demonstrated the existence of a non-protein receptor common for thymosin-α1, interferon-α2, and cholera toxin B-subunit on donor blood T-lymphocytes.

Anxiolytic activity of the neuroprotective peptide HLDF-6 and its effects on brain neurotransmitter systems in BALB/c and C57BL/6 mice

This study is focused on a new amide derivative of the peptide HLDF-6 (Thr-Gly-Glu-Asn-His-Arg). This hexapeptide is a fragment of Human Leukaemia Differentiation Factor (HLDF). It displays a broad range of nootropic and neuroprotective activities. We showed, for the first time, that the peptide HLDF-6-amide has high anxiolytic activity. We used 'open field' and 'elevated plus maze' tests to demonstrate anxiolytic effects of HLDF-6-amide (0.1 and 0.3 mg/kg intranasally), which were comparable to those of the reference drug diazepam (0.5 mg/kg). Five daily equipotent doses of HLDF-6-amide selectively mitigated anxiety and increased the density of NMDA receptors in the hippocampus of stress-susceptible BALB/c mice, and had no effect on stress-resilient C57BL/6 mice. The subchronic administration of HLDF-6-amide showed no effect on the density of GABAA and nicotine receptors but was accompanied by a nonselective decrease of the 5-HT2A serotonin receptor density in frontal cortex of both strains. The mechanism of the specific anxiolytic activity of HLDF-6-amide may include its action on the NMDA-glutamatergic receptor system of the hippocampus and on serotonin 5-HT2A-receptors in the prefrontal cortex. The psychotropic activity of HLDF-6-amide is promising for its introduction to medical practice as a highly effective anxiolytic medicine for mental and neurological diseases.

Use of deuterium labeling by high-temperature solid-state hydrogen-exchange reaction for mass spectrometric analysis of bradykinin biotransformation

RATIONALE

Studies of molecular biodegradation by mass spectrometry often require synthetic compounds labeled with stable isotopes as internal standards. However, labeling is very expensive especially when a large number of compounds are needed for analysis of biotransformation. Here we describe an approach for qualitative and quantitative analysis using bradykinin (BK) and its in vitro degradation metabolites as an example. Its novelty lies in the use of deuterated peptides which are obtained by a high-temperature solid-state exchange (HSCIE) reaction.

METHODS

Deuterated and native BK were analyzed by positive electrospray ionization high-resolution mass spectrometry (ESI-HRMS) using an Orbitrap Fusion mass spectrometer. High-energy collision-induced dissociation (HCD) experiments were performed on [M+H](+) and [M+2H](2+) ions in targeted-MS(2) mode with adjusted normalized HCD value.

RESULTS

After the HSCIE reaction, each amino acid residue of the deuterated peptide contained deuterium atoms and the average degree of substitution was 5.5 atoms per the peptide molecule. The deuterated peptide demonstrated the same chromatographic mobility as the unlabeled counterpart, and lack of racemization during substitution with deuterium. Deuterium-labeled and unlabeled BKs were incubated with human plasma and their corresponding fragments BK(1-5) and BK(1-7), well known as the major metabolites, were detected.

CONCLUSIONS

Quantitative assays demonstrated applicability of the heavy peptide for both sequencing and quantification of generated fragments. Applicability of the HSCIE deuterated peptide for analysis of routes of its degradation has been shown in in vitro experiments. Copyright © 2016 John Wiley & Sons, Ltd.

[The Qualitative Analysis of the Amide Derivative of HLDF-6 Peptide and Its Metabolites with the Use of Tritium- and Deuterium-Labeled Derivatives]

The goal of the study was to elaborate the pharmacokinetics methods of the amide derivative of peptide HLDF-6 (TGENHR-NH2) and its range of nootropic and neuroprotective activity is wide. The hexapeptide 41TGENHR46 is a fragment of the HDLF differentiation factor. It forms the basis for the development of preventive and therapeutic preparations for treating cerebrovascular and neurodegenerative conditions. Pharmacokinetic and molecular mechanisms of the action of the HLDF-6 peptide were studied using tritium- and deuterium-labeled derivatives of this peptide, produced with the use of the high-temperature solid-state catalytic isotope exchange reaction (HSCIE). This reaction was employed to produce the tritium-labeled peptide [3H]TGENHR-NH2 with a molar radioactivity of 230 Ci/mmol and the deuterium-labeled peptide [2H]TGENHR-NH2 with an average deuterium incorporation equal to 10.5 atoms. It was shown by the NMR spectroscopy that the isotope label distribution over the labeled peptide's molecule was uniform, which allowed qualitative analysis ofboth the peptide itself and its fragments in the organism's tissues to be conducted. The newly developed pharmacokinetics method makes it possible to avoid almost completely losses of the peptides under study due to biodegradation during the analysis of tissues. These labeled peptides were used in mice, rats and rabbits to study the pharmacokinetics of the peptide and to calculate the values of its principal pharmacokinetic parameters. Characteristics of its pharmacokinetic profile in the blood were obtained, the hypothesis of pharmacokinetics linearity tested, its metabolism analyzed and its bioavailability value, 34%, calculated. It has been shown that the studied TGENHR-NH2 peptide shows high resistance to hydrolysis in the blood plasma, with dipeptidyl aminopeptidases making the largest contribution to its hydrolysis.

Comparative study of the neuroprotective and nootropic activities of the carboxylate and amide forms of the HLDF-6 peptide in animal models of Alzheimer's disease

A comparative study of the neuroprotective and nootropic activities of two pharmaceutical substances, the HLDF-6 peptide (HLDF-6-OH) and its amide form (HLDF-6-NH2), was conducted. The study was performed in male rats using two models of a neurodegenerative disorder. Cognitive deficit in rats was induced by injection of the beta-amyloid fragment 25-35 (βA 25-35) into the giant-cell nucleus basalis of Meynert or by coinjection of βA 25-35 and ibotenic acid into the hippocampus. To evaluate cognitive functions in animals, three tests were used: the novel object recognition test, the conditioned passive avoidance task and the Morris maze. Comparative analysis of the data demonstrated that the neuroprotective activity of HLDF-6-NH2, evaluated by improvement of cognitive functions in animals, surpassed that of the native HLDF-6-OH peptide. The greater cognitive/ behavioral effects can be attributed to improved kinetic properties of the amide form of the peptide, such as the character of biodegradation and the half-life time. The effects of HLDF-6-NH2 are comparable to, or exceed, those of the reference compounds. Importantly, HLDF-6-NH2 exerts its effects at much lower doses than the reference compounds.

Increase in the specific radioactivity of tritium-labeled compounds obtained by tritium thermal activation method

A method of tritium introduction into different types of organic molecules that is based on the interaction of atomic tritium with solid organic target is described. Tritium atoms are formed on the hot W-wire, which is heated by the electric current. Such an approach is called “tritium thermal activation method”. Here we summarize the results of labeling globular proteins (lysozyme, human and bovine serum albumins); derivatives of pantothenic acid and amino acids; ionic surfactants (sodium dodecylsulfate and alkyltrimethylammonium bromides) and nonionic high-molecular weight surfactants – pluronics. For the first time it is observed that if the target-compound is fixed and its radicals are stable the specific radioactivity of the labeled product can be drastically increased (up to 400 times) when the target temperature is ca. 295 K compared with the results obtained at 77 K. The influence of labeling parameters as tritium gas pressure, exposure time and W-wire temperature was tested for each target temperature that results in the optimum labeling conditions with high specific radioactivity and chemical yield of the resulting compound.

Immunostimulating effect of the synthetic peptide octarphin corresponding to β-endorphin fragment 12-19

We have synthesized the peptide TPLVTLFK corresponding to β-endorphin fragment 12-19 (dubbed octarphin) and its analogs (LPLVTLFK, TLLVTLFK, TPLVLLFK, TPLVTLLK, TPLVTLFL). The octarphin peptide was labeled with tritium (specific activity 28 Ci/mol), and its binding to murine peritoneal macrophages was studied. [3H]Octarphin was found to bind to macrophages with high affinity (K(d) = 2.3 ± 0.2 nM) and specificity. The specific binding of [3H]octarphin was inhibited by unlabeled b-endorphin and the selective agonist of nonopioid b-endorphin receptor synthetic peptide immunorphin (SLTCLVKGFY) (K(i) = 2.7 ± 0.2 and 2.4 ± 0.2 nM, respectively) and was not inhibited by unlabeled naloxone, a-endorphin, γ-endorphin, or [Met(5)]enkephalin (K(i) > 10 mM). Inhibitory activity of unlabeled octarphin analogs was more than 100 times lower than that of unlabeled octarphin. Octarphin was shown to stimulate activity of murine immunocompetent cells in vitro and in vivo: at concentration of 1-10 nM it enhanced the adhesion and spreading of peritoneal macrophages as well as their ability to digest bacteria of Salmonella typhimurium virulent strain 415 in vitro; the peptide administered intraperitoneally at a dose of 20 µg/animal on day 7, 3, and 1 prior to isolation of cells increased activity of peritoneal macrophages as well as spleen T- and B-lymphocytes.

[Effects and mechanism of action of synthetic peptide octarphin]

We have synthesized the peptide TPLVTLFK corresponding to the β-endorphin fragment 12-19 (the name given by the authors - octarphin), and its analogs (LPLVTLFK, TLLVTLFK, TPLVLLFK, TPLVTLLK, TPLVTLFL). The peptide octarphin was labeled with tritium (the specific activity of 28 Ci/mmol) and its binding to the murine peritoneal macrophages has been studied. [(3)H]Octarphin was found to bind to macrophages with high affinity (K(d) = 2.3 ± 0.2 nM) and specificity. The specific binding of [(3)H]octarphin is inhibited by unlabeled β-endorphin and selective agonist of non-opioid β-endorphin receptor synthetic peptide immunorphin (SLTCLVKGFY) (K(i) = 2.7 ± 0.2 and 2.4 ± 0.2 nM respectively) and not inhibited by unlabeled naloxone, α-endorphin, γ-endorphin and [Met(5)]enkephalin (K(i) > 10 μM). Inhibiting activity of unlabeled analogs of octarphin is more then 100 times lower the unlabeled octarphin. Octarphin stimulates activity of murine immunocompetent cells in vitro and in vivo: at the concentration of 1-10 nM enhances the adhesion and spreading of peritoneal macrophages as well as their capacity to digest bacteria of Salmonella typhimurium virulent strain 415 in vitro. Intraperitoneal administration of peptide at dose 20 μg/animal on day 7,3 and 1 prior to the isolation of cells increases activity of peritoneal macrophages as well as T- and B-spleen lymphocytes.

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 beta-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 alpha-endorphin, gamma-endorphin, [Met(5)]enkephalin, and [Leu(5)]enkephalin, as well. The [(3)H]octarphin specific binding with brain membranes was inhibited by unlabeled beta-endorphin (K(i) = 2.4 +/- 0.2 nM) and a selective agonist of nonopioid beta-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 [(3)H]immunorphin with membranes (K(i) = 2.8 +/- 0.2 nM). Thus, octarphin binds with a high affinity and specificity to nonopioid receptor of beta-endorphin on rat brain cortex membranes.

[Interaction of the synthetic immunomodulatory dipeptide bestim with murine macrophages and thymocytes]

The tritium-labeled dipeptide bestim (gamma-D-Glu-L-Trp) with a specific activity of 45 Ci/mmol was obtained by high-temperature solid-state catalytic isotope exchange. It was found that [3H]bestim binds with a high affinity to murine peritoneal macrophages (Kd 2.1 +/- 0.1 nM) and thymocytes (Kd 3.1 +/- 0.2 nM), as well as with plasma membranes isolated from these cells (Kd 18.6 +/- 0.2 and 16.7 +/- 0.3 nM, respectively). The specific binding of [3H]bestim to macrophages and thymocytes was inhibited by the unlabeled dipeptide thymogen (L-Glu-L-Trp) (Ki 0.9 +/- 0.1 and 1.1 +/- 0.1 nM, respectively). After treatment with trypsin, macrophages and thymocytes lost the ability to bind [3H]bestim. Bestim in the concentration range of 10(-10) to 10(-6) M reduced the adenylate cyclase activity in the membranes of murine macrophages and thymocytes.

Interaction of ACTH synthetic fragments with rat adrenal cortex membranes

Synthetic peptide, corresponding to the amino acid sequence 11-24 of human adrenocorticotropic hormone (ACTH), was labeled with tritium (specific activity of 22 Ci/mmol). [(3)H]ACTH (11-24) was found to bind to rat adrenal cortex membranes with high affinity and specificity (K(d) = 1.8 +/- 0.1 nM). Twenty nine fragments of ACTH (11-24) have been synthesized and their ability to inhibit the specific binding of [(3)H]ACTH (11-24) to adrenocortical membranes has been investigated. Unlabeled fragment ACTH 15-18 (KKRR) was found to replace in a concentration-dependent manner [(3)H]ACTH (11-24) in the receptor-ligand complex (K(i) = 2.3 +/- 0.2 nM). ACTH (15-18) was labeled with tritium (specific activity of 20 Ci/mmol). [(3)H]ACTH (15-18) was found to bind to rat adrenal cortex membranes with high affinity (K(d) = 2.1 +/- 0.1 nM). The specific binding of [(3)H]ACTH (15-18) was inhibited by unlabeled ACTH (11-24) (K(i) = 2.2 +/- 0.1 nM). ACTH (15-18) at the concentration range of 1-1000 nM did not affect the adenylate cyclase activity in adrenocortical membranes.

[Stress-protective effect of the synthetic ACTH-like peptide leucocorticotropin]

We found that the tritium-labeled synthetic ACTH-like octapeptide leucocorticotropin corresponding to the 81-88 sequence of the precursor of human interleukin-1alpha ([3H]GKVLKKRR) is bound by the ACTH receptor of rat adrenal cortex with a high affinity and specificity (Kd 2.2 +/- 0.1 nM). This peptide was shown to exert no effect on the adenylate cyclase activity of the membranes of rat adrenal cortex in the concentration range from 1 to 1000 nM. Leucocorticotropin administration three times at doses of 10-20 microg/animal did not change the level of hydroxycorticosteroids (11-HOCS) in the rat adrenal glands in the absence of temperature action. At the same time, the peptide abolishes (at a dose of 20 microg/animal, three times) or significantly decreases (at a dose of 10 microg/animal, three times) the dramatic increase in the 11-HOCS content in the adrenal glands occurring in the case of cold or heat shock. Thus, leucocorticotropin normalizes the 11-HOCS level in the rat adrenal cortex during stress. The stress-protective effect of the peptide is mediated through the ACTH receptor.

Degradation of the ACTH(4-10) analog Semax in the presence of rat basal forebrain cell cultures and plasma membranes

Here a new approach of the elucidation of paths of proteolytic biodegradation of physiologically active peptides, based on the use of a peptide with isotopic label at all amino acid residues and the enrichment of HPLC samples with unlabeled peptide fragments in UV-detectable concentration, has been proposed. The method has been applied for the investigation of degradation dynamics of the neuroactive heptapeptide MEHFPGP (Semax) in the presence of plasma membranes, and cultures of glial and neuronal cells obtained from the rat basal forebrain. The splitting away of ME and GP, and formation of pentapeptides are the predominant processes in the presence of all tested objects, whereas the difference in patterns of resulting peptide products for glial and neuronal cells has been detected. In conclusion, the approach applied allows analyzing physiologically active peptide concentrations in biological tissues and degradation pathways of peptides in the presence of targets of their action.

Solid phase isotope exchange with spillover hydrogen in amino acids, peptides, and proteins

We summarize here information on the theoretical and experimental study of high-temperature (150-200 degrees C) solid phase catalytic isotope exchange (HTSPCIE) carried out with amino acids, peptides, and proteins under the action of spillover hydrogen. Main specific features of the HTSPCIE reaction, its mechanism, and its use for studying spatial interactions in polypeptides are discussed. A virtually complete absence of racemization makes this reaction a valuable preparative method. The main regularities of the HTSPCIE reaction with the participation of spillover tritium have been revealed in the case of peptides and proteins, and the dependence of reactivity of peptide fragments on the spatial organization of their molecules has been studied. An important peculiarity of this reaction is that HTSPCIE proceeds at 150-200 degrees C with a high degree of chirality retention in amino acids and peptides. This is provided by its reaction mechanism, which consists in a synchronous one-center substitution at the saturated carbon atom characterized by the formation of pentacoordinated carbon and a three-center bond between the carbon and the incoming and outgoing hydrogen atoms.

Synthetic peptide immunocortin stimulates the production of 11-oxycorticosteroides by rat adrenal cortex through ACTH receptors

Adrenocorticotropic hormone (ACTH)-like peptide immunocortin (IMC) VKKPGSSVKV, corresponding to the amino acid sequence 11-20 of the variable part of human immunoglobulin G (IgG) 1 heavy chain, at concentrations of 10(-9)-10(-6) I was found to increase the adenylate cyclase activity in adrenal cortex membranes, while intramuscular injection of immunocortin at doses of 10-100 microg/kg was found to stimulate the secretion of 11-oxycorticosteroids (CS) from the adrenals to the bloodstream. Immunocortin was labeled with tritium to specific activity of 22 Ci/mmol. Receptor binding studies revealed that [(3)H]immunocortin ([(3)H]IMC) bound with high affinity and specificity to ACTH receptors on rat adrenal cortex membranes (K(d)=2.1+/-0.2 nM, B(max)=1.1+/-0.1 pmol/mg protein).

The effect of three-dimensional structure on the solid state isotope exchange of hydrogen in polypeptides with spillover hydrogen

The effect of the three-dimensional structure of polypeptides and proteins on their ability to undergo isotopic exchange under the action of spillover hydrogen (SH) in the high temperature solid state catalytic isotope exchange reaction (HSCIE) was theoretically and experimentally studied. The HSCIE reaction in the beta-galactosidase protein from Thermoanaerobacter ethanolicus (83kDa) was studied. The influence of the beta-galactosidase structure on isotopic exchange as peptide fragments with spillover tritium was studied. The most accessible peptide fragment, which does not contribute to alpha-helix and beta-strand formations (KEMQKE215-220), had the largest relative reactivity. The one located in the contact area between the subunits (YLRDSE417-422) showed the smallest relative reactivity. The relative reactivities of these peptides differ more than 150 times. Data collected during a study devoted to the HSCIE reaction of the beta-galactosidase protein indicate that the HSCIE reaction might be employed for acquiring information about their three-dimensional structure and protein-protein interactions. The results of ab initio calculations have shown that alpha-helix formation in polypeptides decreases the reactivity in HSCIE. Hydrogen exchange in the alpha-helical fragment Trp1-Leu8 of zervamycin IIB was also analyzed using theoretical methods. It was shown by ab initio quantum-chemical calculations that the high degree of substitution of C(alpha)H for tritium in Gln3 might be associated with the participation of electron donor O and N atoms in transition state stabilization in the HSCIE reaction.

Beta-endorphin-like peptide SLTCLVKGFY reduces the production of 11-oxycorticosteroids by rat adrenal cortex through nonopioid beta-endorphin receptors

Beta-endorphin-like peptide immunorphin (SLTCLVKGFY), a selective agonist of nonopioid beta-endorphin receptor, was labeled with tritium to specific activity of 24 Ci/mmol. It was used for the detection and characterization of nonopioid beta-endorphin receptors on rat adrenal cortex membranes (Kd = 31.6 +/- 0.2 nM, Bmax = 37.4 +/- 2.2 pmol/mg protein). Immunorphin at concentrations of 10(-9) to 10(-6) M was found to inhibit the adenylate cyclase activity in adrenal cortex membranes, while intramuscular injection of immunorphin at doses of 10-100 microg/kg was found to reduce the secretion of 11-oxycorticosteroids from the adrenals to the bloodstream.