In vitro analysis of stable, receptor-selective neurotensin[8-13] analogues

A set of neurotensin[8-13] (NT[8-13]) analogues featuring substitution of non-natural cationic amino acids in the Arg(8) position have been synthesized and tested for binding potencies against the three cloned human NT receptors (hNTR-1, hNTR-2, hNTR-3), functional agonism of the hNTR1 and for rat serum stability. Three distinct classes of peptides have been identified: Class 1 features alkyl-Arg analogues at Arg(8), Class 2 features alpha-azido-cationic amino acids at Arg(8), and Class 3 feature modified Arg(8) and Tyr(11) residues. Most of the peptides maintain or exceed the binding potency of NT[8-13] to hNTR-1. Class 2 analogues exceed the binding potency of NT[8-13] to hNTR-2 with KK19 binding with higher affinity to hNTR-2 than hNTR-1. Peptides with enhanced binding potencies for hNTR-3 were not found. All analogues are functional agonists of the hNTR1 receptor as indicated by phosphoinositide (PI) determination. Serum stability increased with peptide classification where the half-life of Class 1 < Class 2 < Class 3 which are stable to rat serum for > 24 h.

Monitoring neurotensin[8-13] degradation in human and rat serum utilizing matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

A method was developed to quantify neurotensin (NT) fragment [8-13] and a novel NT[8-13] derivative, KK1, in human and rat serum utilizing matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS). The method allows for simultaneous quantification of the major NT[8-13] metabolite, NT[9-13] (according to molecular mass), and detection of the major KK1 metabolite, KK1M (according to molecular mass). The degradation rates of NT[8-13] and KK1 were calculated to be 24.1+/-1.0 and 193+/-8min in human serum and 5.90+/-0.22 and 153+/-4min in rat serum, respectively. The method utilizes a novel sample drying technique and spectrum acquisition protocol. In addition, an internal standard dissimilar in structure to the analytes was used. This method may be broadly applicable to the quantification of NT[8-13] and other peptide analogues of varying structure.

Synthesis and analysis of potent, more lipophilic derivatives of the bradykinin B2 receptor antagonist peptide Hoe 140

Bradykinin (BK) is an endogenous peptide that has been implicated in several pathological conditions, hence antagonists of its activity have therapeutic potential. The decapeptide Hoe 140 is currently one of the best BK antagonists, but interest remains in finding even more potent compounds. A library of Hoe 140 derivatives was synthesized that incorporated non-natural analogs of the cationic, naturally occurring amino acids arginine (Arg) and lysine (Lys). The modified amino acids were designed to form enhanced ionic interactions due to an increase in local hydrophobicity, which promotes desolvation of the cation in water. The potencies of the resulting peptides were determined by competitive binding assays in human A431 cells expressing the BK B2 receptor. Two of the peptides synthesized were equipotent to Hoe 140 (IC(50s) 2.99 and 3.36 nM) and the most potent was demonstrated as a functional antagonist in vitro by blocking BK-mediated phosphorylation of mitogen-activated protein (MAP) kinases. The new derivatives are more hydrophobic than Hoe 140 and thus may exhibit changes in pharmacokinetic properties when evaluated in vivo.

Topography of the neurotensin (NT)(8-9) binding site of human NT receptor-1 probed with NT(8-13) analogs

A series of neurotensin (NT)(8-13) analogs featuring substitution of the Arg8 and/or Arg9 residues with non-natural cationic amino acids was synthesized and evaluated for binding to the human NT receptor-1 (hNTR-1). The modifications were designed to probe specific steric and electrostatic requirements in the N-terminal cationic region of NT(8-13) for receptor binding as a general evaluation of the feasibility of incorporating minor structural changes into a peptide at a crucial polar receptor binding site. Many of the non-natural amino acids are more or less isosteric to Arg but more lipophilic as a result of addition of alkyl groups or through removal or replacement of NH character with methylene or methyl substituents, whereas others vary the distance between the cation and the alpha-amino acid carbon. Substitution of Arg8 with N(G)-alkylated Arg derivatives or homolysine (Hlys) maintained the subnanomolar affinity of NT(8-13) to the hNTR-1. Position 8 incorporation of Hlys produced the most favorable primary amine side-chain substitution to date. Moderate losses in affinity observed with position 9 substitutions were attributed to adverse steric effects. Doubly substituted [Hlys8, DAB9]NT(8-13), in which DAB is 2,4-diaminobutyric acid, was also prepared and tested as the shorter side-chain of DAB is known to be favored in position 9 of NT(8-13). This analog maintained 60% of NT(8-13) binding affinity making it the most favored des-guanidinium-containing analog known. These results demonstrate that adequate receptor binding affinity can be maintained over a structural range of Arg analogs, thus providing a range of peptides expected to exhibit altered pharmacokinetic properties. From the standpoint of the hNTR-1 cationic binding sites, these results help to map out the structural stringency inherent in the formation of a tight binding complex with NT(8-13) and related analogs.

Selectivity enhancement induced by substitution of non-natural analogues of arginine and lysine in arginine-based thrombin inhibitors

Seven non-natural analogues of arginine and lysine have been substituted in an established arginine-based thrombin inhibitor. Four of the new compounds exhibited significant thrombin inhibition (K(i)'s 0.53-3.95 microM) and were subsequently tested for selectivity against trypsin. The two best compounds gave selectivity ratios of 962 and 525 (trypsin/thrombin), improving upon the parent compound.

Design rationale, synthesis, and characterization of non-natural analogs of the cationic amino acids arginine and lysine

A series of non-natural isosteric analogs of the cationic, ion-pairing, natural amino acids arginine and lysine have been synthesized, characterized with regard to relevant physical parameters, and protected for routine inclusion in Merrifield solid-phase synthesis. The design of these molecules is based on the concept of steric inhibition of solvation, in that judicious placement of alkyl groups can destabilize aqueous ion solvation and favor ion-pairing [see Beeson & Dix (1993) J. Am. Chem. Soc. 115, 10275]. When the residues are substituted for the natural amino acids in biologically active peptides, enhanced ion-pairing of the peptides to their receptors to increase the peptides' biological activities can result. The increased lipophilicity of the non-natural residues can also improve pharmacokinetic parameters and agonist/antagonist behaviors of peptides. While the synthesis of the L-series is described, the D-isomers were also prepared using identical chemistry.

Synthesis of neurotensin(9-13) analogues exhibiting enhanced human neurotensin receptor binding affinities

Recent evidence is consistent with neurotensin (NT)(8-13) adopting a Type I beta-turn conformation while binding the NT receptor, which would place the cationic side-chains of Arg(8) and Arg(9) in close proximity. This was the basis for the design, synthesis and analysis of truncated NT(9-13) analogues 1-5 with dicationic position 9 side-chains to emulate the functions of the 8 and 9 side-chains of NT(8-13).

Synthesis and human neurotensin receptor binding activities of neurotensin(8-13) analogues containing position 8 alpha-azido-N-alkylated derivatives of ornithine, lysine, and homolysine

A series of neurotensin(8-13) (NT) analogues were synthesized through intermediates in which the N-terminal Arg(8) was replaced by various omega-bromo-2(S)-azido residues spanning 3-5 methylene units in side-chain length. Subsequent nucleophilic substitution of the omega-bromo groups with ammonia, methylamine, dimethylamine, or trimethylamine provided peptides containing N-terminal 2(S)-azido residues containing primary through quaternary N-alkylated side chains corresponding in length to ornithine, Lys, and homolysine. The synthetic procedure appears applicable for incorporation of a wide variety of amine-containing nonnatural amino acids into peptides. The particular modifications were intended to enhance physiochemical properties of NT(8-13) responsible for human NT 1 receptor (hNTR) binding, overall lipophilicity, and stability that may influence the potency of the peptides in vivo. When the peptides were tested for hNTR binding, the affinities in each series followed the order primary > secondary > tertiary > quaternary amine with the homolysine side-chain length being favored. All analogues possess binding affinities between acetyl-NT(8-13) and NT(8-13) indicating that the sterically less bulky alpha-azido may be inherently preferable to the acetyl group for N-terminal protection. This study extends the strategy of modifying amine-containing drugs through alkylations to peptide modification. The set of alkylated side chains also offers a new method of steric selection between receptor subtypes and could be used to modify the properties and biological effects of any peptide that contains cationic residues.

Preparation and receptor binding affinities of cyclic C-terminal neurotensin (8-13) and (9-13) analogues

Cyclic analogues of neurotensin (NT) C-terminal fragments NT(8-13) and NT(9-13) were produced via intramolecular nucleophilic substitution of the Tyr(11) phenoxide anion on a 6-bromohexanoyl side chain substituted at position 8 or 9 and tested for NT receptor binding affinity.

Oxidation of DNA bases, deoxyribonucleosides and homopolymers by peroxyl radicals

DNA base oxidation is considered to be a key event associated with disease initiation and progression in humans. Peroxyl radicals (ROO. ) are important oxidants found in cells whose ability to react with the DNA bases has not been characterized extensively. In this paper, the products resulting from ROO. oxidation of the DNA bases are determined by gas chromatography/MS in comparison with authentic standards. ROO. radicals oxidize adenine and guanine to their 8-hydroxy derivatives, which are considered biomarkers of hydroxyl radical (HO.) oxidations in cells. ROO. radicals also oxidize adenine to its hydroxylamine, a previously unidentified product. ROO. radicals oxidize cytosine and thymine to the monohydroxy and dihydroxy derivatives that are formed by oxidative damage in cells. Identical ROO. oxidation profiles are observed for each base when exposed as deoxyribonucleosides, monohomopolymers and base-paired dihomopolymers. These results have significance for the development, utilization and interpretation of DNA base-derived biomarkers of oxidative damage associated with disease initiation and propagation, and support the idea that the mutagenic potential of N-oxidized bases, when generated in cellular DNA, will require careful evaluation. Adenine hydroxylamine is proposed as a specific molecular probe for the activity of ROO. in cellular systems.

Redox-mediated activation of latent transforming growth factor-beta 1

Transforming growth factor beta 1 (TGF beta) is a multifunctional cytokine that orchestrates response to injury via ubiquitous cell surface receptors. The biological activity of TGF beta is restrained by its secretion as a latent complex (LTGF beta) such that activation determines the extent of TGF beta activity during physiological and pathological events. TGF beta action has been implicated in a variety of reactive oxygen-mediated tissue processes, particularly inflammation, and in pathologies such as reperfusion injury, rheumatoid arthritis, and atherosclerosis. It was recently shown to be rapidly activated after in vivo radiation exposure, which also generates reactive oxygen species (ROS). In the present studies, the potential for redox-mediated LTGF beta activation was investigated using a cell-free system in which ROS were generated in solution by ionizing radiation or metal ion-catalyzed ascorbate reaction. Irradiation (100 Gray) of recombinant human LTGF beta in solution induced 26% activation compared with that elicited by standard thermal activation. Metal-catalyzed ascorbate oxidation elicited extremely efficient recombinant LTGF beta activation that matched or exceeded thermal activation. The efficiency of ascorbate activation depended on ascorbate concentrations and the presence of transition metal ions. We postulate that oxidation of specific amino acids in the latency-conferring peptide leads to a conformation change in the latent complex that allows release of TGF beta. Oxidative activation offers a novel route for the involvement of TGF beta in tissue processes in which ROS are implicated and endows LTGF beta with the ability to act as a sensor of oxidative stress and, by releasing TGF beta, to function as a signal for orchestrating the response of multiple cell types. LTGF beta redox sensitivity is presumably directed toward recovery of homeostasis; however, oxidation may also be a mechanism of LTGF beta activation that can be deleterious during disease mechanisms involving chronic ROS production.

Mechanism of site-selective DNA nicking by the hydrodioxyl (perhydroxyl) radical

In previous studies, the ability of the hydrodioxyl (perhydroxyl) radical [HOO., the conjugate acid of superoxide (O2.-] to "nick" DNA under biomimetic conditions was demonstrated, and a sequence selectivity was observed. A background level of nonspecific nicking also was noted. This paper provides support for 5'-hydrogen atom abstraction from the deoxyribose ring as the initial event in the sequence-selective nicking by 02.-/HOO.. Two experiments support the proposed mechanism. First, using a defined sequence 5'-32P-labeled restriction fragment as the DNA substrate, only free (unalkylated) 3'-phosphate is produced at the site of nicking. Second, using poly (dA).poly (T) as the substrate, furfural is formed in the reaction from deoxyribose ring breakdown. Both results are consistent with 5'-hydrogen atom abstraction for initiation of the site-selective nicking. Hydrogen atom abstraction at other sites of the deoxyribose ring and/or base oxidation and loss followed by strand scission likely are responsible for the nonspecific nicking. The 5'-abstraction mechanism contrasts to those elicited by other 02-derived and metal-associated oxidants, which may provide a biomarker for the reactivity of HOO. in vivo.

Hydrodioxyl (perhydroxyl), peroxyl, and hydroxyl radical-initiated lipid peroxidation of large unilamellar vesicles (liposomes): comparative and mechanistic studies

The mechanisms and relative efficiencies of lipid peroxidation initiation by biological O2-derived oxidants were studied using large unilammellar vesicle (LUV) liposomes, structural models for biological membranes, as targets for oxidation. LUVs, when prepared from dilinoleoylphosphatidylcholine (DLPC) containing either 0 or 5 mol% hydroperoxide (LOOH, added either as a linoleic acid or DLPC hydroperoxide), maintained structural integrity, which enabled evaluation of the relative ability of oxidants to initiate lipid peroxidation when generated outside of the bilayer. LUVs were more oxidazable than multilamellar vesicles or lipids dispersed in solution, supporting their appropriateness as biological membrane models. In parallel to previous results using lipid dispersions (J. Aikens and T. A. Dix, 1991, J. Biol. Chem. 266, 15091-15098), both perhydroxyl (HOO.) and peroxyl (ROO.) radicals initiated lipid peroxidation in LUVs. Oxidants that did not initiate included H2O2, organic hydroperoxides, and, most notably, superoxide (O2-). HOO. and ROO. initiated by different mechanisms: HOO. required the presence of the preexisting LOOHs for efficient initiation, indicating the direct reaction of HOO. with LOOH, whereas ROO. initiated by hydrogen atom abstraction at the bisallylic site of unsaturation on the fatty acid side chain of the PCs. Hydroxyl radicals (HO.s) were poor initiators in comparison to ROO.s (and, indirectly, HOO.s), which might be considered surprising as the latter species are chemically weaker oxidants. The decreased activity of HO. was not due to decreased access to the LUVs; rather, this oxidant appears to react to generate less viable lipid peroxidation propagating species. It was also demonstrated that the fluidity of the LUV membrane had little effect on the relative initiating activity of each oxidant. It is argued that HO. may initiate lipid peroxidation only indirectly in vivo (through the generation of carbon-based peroxyl radicals, ROO.s) and that greater effort should be made to understand the roles of HOO. and ROO. at lipid peroxidation initiation.

Cloning, nucleotide sequence, and expression of a p-hydroxybenzoate hydroxylase isozyme gene from Pseudomonas fluorescens

A gene encoding for a putative isozyme of p-hydroxy-benzoate hydroxylase (PHBH) has been isolated from Pseudomonas fluorescens (ATCC 13525). A comparison of the translated amino acid sequence with that of the known PHBH from P. fluorescens revealed that the new enzyme contains 3 additional amino acids and has 73% absolute homology to the previously known enzyme; conservation of secondary and active-site structures implied that the isozyme and known enzyme share the same general tertiary structure. Subsequent expression of the isozyme in Escherichia coli produced an enzyme with a specific activity about half that of the previously characterized PHBHs from P. fluorescens and Pseudomonas aeruginosa; in addition, somewhat weaker binding affinities for both NADPH and p-hydroxybenzoate were observed. Speculations are made on the reason for the existence of the isozyme, which does not appear to be expressed routinely in P. fluorescens.