Radiopharmacological characterization of ⁶⁴Cu-labeled α-MSH analogs for potential use in imaging of malignant melanoma

The melanocortin-1 receptor (MC1R) plays an important role in melanoma growth, angiogenesis and metastasis, and is overexpressed in melanoma cells. α-Melanocyte stimulating hormone (α-MSH) and derivatives are known to bind with high affinity at this receptor that provides the potential for selective targeting of melanoma. In this study, one linear α-MSH-derived peptide Nle-Asp-His-D-Phe-Arg-Trp-Gly-NH2 (NAP-NS1) without linker and with εAhx-β-Ala linker, and a cyclic α-MSH derivative, [Lys-Glu-His-D-Phe-Arg-Trp-Glu]-Arg-Pro-Val-NH2 (NAP-NS2) with εAhx-β-Ala linker were conjugated with p-SCN-Bn-NOTA and labeled with (64)Cu. Radiochemical and radiopharmacological investigations were performed with regard to transchelation, stability, lipophilicity and in vitro binding assays as well as biodistribution in healthy rats. No transchelation reactions, but high metabolic stability and water solubility were demonstrated. The linear derivatives showed higher affinity than the cyclic one. [(64)Cu]Cu-NOTA-εAhx-β-Ala-NAP-NS1 ([(64)Cu]Cu-2) displayed rapid cellular association and dissociation in murine B16F10 cell homogenate. All [(64)Cu]Cu-labeled conjugates exhibited affinities in the low nanomolar range in B16F10. [(64)Cu]Cu-2 showed also high affinity in human MeWo and TXM13 cell homogenate. In vivo studies suggested that [(64)Cu]Cu-2 was stable, with about 85 % of intact peptide in rat plasma at 2 h p.i. Biodistribution confirmed the renal pathway as the major elimination route. The uptake of [(64)Cu]Cu-2 in the kidney was 5.9 % ID/g at 5 min p.i. and decreased to 2.0 % ID/g at 60 min p.i. Due to the prospective radiochemical and radiopharmacological properties of the linear α-MSH derivative [(64)Cu]Cu-2, this conjugate is a promising candidate for tracer development in human melanoma imaging.

Effects of Trehalose on Thermodynamic Properties of Alpha-synuclein Revealed through Synchrotron Radiation Circular Dichroism

Many neurodegenerative diseases, including Huntington’s, Alzheimer’s and Parkinson’s diseases, are characterized by protein misfolding and aggregation. The capability of trehalose to interfere with protein misfolding and aggregation has been recently evaluated by several research groups. In the present work, we studied, by means of synchrotron radiation circular dichroism (SRCD) spectroscopy, the dose-effect of trehalose on α-synuclein conformation and/or stability to probe the capability of this osmolyte to interfere with α-synuclein’s aggregation. Our study indicated that a low trehalose concentration stabilized α-synuclein folding much better than at high concentration by blocking in vitro α-synuclein’s polymerisation. These results suggested that trehalose could be associated with other drugs leading to a new approach for treating Parkinson’s and other brain-related diseases.

Glutathione Transferase (GST)-Activated Prodrugs

Glutathione transferase (formerly GST) catalyzes the inactivation of various electrophile-producing anticancer agents via conjugation to the tripeptide glutathione. Moreover, several data link the overexpression of some GSTs, in particular GSTP1-1, to both natural and acquired resistance to various structurally unrelated anticancer drugs. Tumor overexpression of these proteins has provided a rationale for the search of GST inhibitors and GST activated cytotoxic prodrugs. In the present review we discuss the current structural and pharmacological knowledge of GST-activated cytotoxic compounds.

Peptide-receptor ligands and multivalent approach

The overexpression of peptide receptors in human tumours makes peptide-ligands attractive agents for the development of specific diagnostic imaging and/or therapy of cancers. Solid-phase peptide synthesis, modern phage display technology and combinatorial peptide chemistry have profoundly affected the pool of available targeting peptides for efficient and specific delivery of imaging or therapeutic label molecules. Additionally, the availability of a wide range of bifunctional chelating agents for the radiolabelling of bioactive peptides with radionuclides has produced a wide variety of useful radiopharmaceutical molecules. This review article examines the principal receptors-binding peptides and their overexpression on tumour cells. We discuss the advantage and the challenges in developing multivalent peptide-based ligands summarising their design strategies and applications.

Antamanide, a derivative of Amanita phalloides, is a novel inhibitor of the mitochondrial permeability transition pore

Antamanide is a cyclic decapeptide derived from the fungus Amanita phalloides. Here we show that antamanide inhibits the mitochondrial permeability transition pore, a central effector of cell death induction, by targeting the pore regulator cyclophilin D. Indeed, (i) permeability transition pore inhibition by antamanide is not additive with the cyclophilin D-binding drug cyclosporin A, (ii) the inhibitory action of antamanide on the pore requires phosphate, as previously shown for cyclosporin A; (iii) antamanide is ineffective in mitochondria or cells derived from cyclophilin D null animals, and (iv) abolishes CyP-D peptidyl-prolyl cis-trans isomerase activity. Permeability transition pore inhibition by antamanide needs two critical residues in the peptide ring, Phe6 and Phe9, and is additive with ubiquinone 0, which acts on the pore in a cyclophilin D-independent fashion. Antamanide also abrogates mitochondrial depolarization and the ensuing cell death caused by two well-characterized pore inducers, clotrimazole and a hexokinase II N-terminal peptide. Our findings have implications for the comprehension of cyclophilin D activity on the permeability transition pore and for the development of novel pore-targeting drugs exploitable as cell death inhibitors.

Cell-Penetrating Peptides: A Comparative Study on Lipid Affinity and Cargo Delivery Properties

A growing number of natural and/or synthetic peptides with cell membrane penetrating capability have been identified and described in the past years. These molecules have been considered promising tools for delivering bioactive compounds into various cell types. Although the mechanism of uptake is still unclear, it is reasonable to assume that the relative contribute of each proposed mechanism could differ for the same peptide, depending on experimental protocol and cargo molecule composition. In this work we try to connect the capability to interact with model lipid membrane and structural and chemical characteristics of CPPs in order to obtain a biophysical classification that predicts the behavior of CPP-cargo molecules in cell systems. Indeed, the binding with cell membrane is one of the primary step in the interaction of CPPs with cells, and consequently the studies on model membrane could become important for understanding peptide-membrane interaction on a molecular level, explaining how CPPs may translocate a membrane without destroying it and how this interactions come into play in shuttling CPPs via different routes with different efficiency. We analyzed by CD and fluorescence spectroscopies the binding properties of six different CPPs (kFGF, Nle54-Antp and Tat derived peptides, and oligoarginine peptides containing 6, 8 or 10 residues) in absence or presence of the same cargo peptide (the 392-401pTyr396 fragment of HS1 protein). The phospholipid binding properties were correlated to the conformational and chemical characteristics of peptides, as well as to the cell penetrating properties of the CPP-cargo conjugates. Results show that even if certain physico-chemical properties (conformation, positive charge) govern CPP capability to interact with the model membrane, these cannot fully explain cell-permeability properties.

Therapeutic prospect of Syk inhibitors

BACKGROUND

The non-receptor spleen tyrosine kinase (Syk; EC 2.7.10.2) is involved in signal transduction in a variety of cell types. In particular, it is a key mediator of immune receptors signaling in host inflammatory cells (B cells, mast cells, macrophages and neutrophils), important for both allergic and antibody-mediated autoimmune diseases. Deregulated Syk kinase activity also allows growth factor-independent proliferation and transforms bone marrow-derived pre-B cells that are able to induce leukemia. Consequently, the development of Syk kinase inhibitors could conceivably treat these disorders and so they have became a major focus in the pharmaceutical and biotech industry.

OBJECTIVE

In this review, we analyze the structure and role of Syk kinase, the use of small molecules, interacting with ATP-binding site, as inhibitors of kinase activity and finally the potential of using inhibitors of Syk kinase expression to attenuate pathological conditions.

CONCLUSION

Syk kinase inhibition is suggested as a powerful tool for the therapy of different pathologies.

Mechanistic studies of amide bond scission during acidolytic deprotection of Pip containing peptide

Unusual TFA catalyzed cleavage reaction is reported for peptide containing pipecolic acid residues. Although the use of TFA under standard cleavage conditions is sufficiently mild to prevent degradation of the desired products, the amide bond between consecutive pipecolic acid residues is unexpectedly hydrolyzed by standard TFA treatment. The hydrolysis is proposed to proceed via an oxazolinium ion intermediate. This mechanism is supported by H/D exchange as observed by ESI-MS and NMR experiments.

Malondialdehyde scavenging and aldose-derived Schiff bases' transglycation properties of synthetic histidyl-hydrazide carnosine analogs

Second-generation carnosine analogs bearing the histidyl-hydrazide moiety have been synthesized and tested for their efficiency in scavenging malondialdehyde (MDA) derived from lipid peroxidation and for their ability to reverse the glycation process in the glucose-ethylamine Schiff base model. The data obtained indicate that this class of compounds maintains the activity profile of carnosine and is a suitable candidate for the treatment of disorders caused by oxidative stress.

Neuroprotective actions of a histidine analogue in models of ischemic stroke

Histidine is a naturally occurring amino acid with antioxidant properties, which is present in low amounts in tissues throughout the body. We recently synthesized and characterized histidine analogues related to the natural dipeptide carnosine, which selectively scavenge the toxic lipid peroxidation product 4-hydroxynonenal (HNE). We now report that the histidine analogue histidyl hydrazide is effective in reducing brain damage and improving functional outcome in a mouse model of focal ischemic stroke when administered intravenously at a dose of 20 mg/kg, either 30 min before or 60 min and 3 h after the onset of middle cerebral artery occlusion. The histidine analogue also protected cultured rat primary neurons against death induced by HNE, chemical hypoxia, glucose deprivation, and combined oxygen and glucose deprivation. The histidine analogue prevented neuronal apoptosis as indicated by decreased production of cleaved caspase-3 protein. These findings suggest a therapeutic potential for HNE-scavenging histidine analogues in the treatment of stroke and related neurodegenerative conditions.

Spatial Conformation and Topography of the Tyrosine Aromatic Ring in Substrate Recognition by Protein Tyrosine Kinases

The side chain orientation of the tyrosine residue included in a peptide, which is an excellent substrate of Syk tyrosine kinase, was fixed in different conformations by either incorporating the tyrosine in cyclic structures (6-OH-Tic, 5-OH-Aic, and Hat derivatives) or adding a sterically bulky substituent in the tyrosine side chain moiety (beta-MeTyr). Synthetic peptides containing tyrosine analogues displaying different side chain orientations were analyzed by NMR techniques and tested as potential substrates of the nonreceptor tyrosine kinases Syk, Csk, Lyn, and Fyn. The "rotamer scan" of the phosphorylatable residue generated optimal substrates in terms of both phosphorylation efficiency and selectivity for Syk tyrosine kinase, while the peptidomimetics were not recognized by the other tyrosine kinases. In particular, l-beta-MeTyr and d-Hat containing peptides resulted to be both suitable substrates for the specific monitoring of Syk and consensus sequence scaffolds for the design of potential inhibitors highly selective for this tyrosine kinase.

Fluorescent, internally quenched, peptides for exploring the pH-dependent substrate specificity of cathepsin B

Cathepsin B is a cysteine protease that in tumor tissues is localized in both acidic lysosomes and extracellular spaces. It can catalyze the cleavage of peptide bonds by two mechanisms: endoproteolytic attack with a pH optimum around 7.4, and attack from the C-terminus with a pH optimum at 4.5-5.5. In this work, seven fluorescent, internally quenched, decapeptides have been synthesized using the prototypical cathepsin B selective substrate Z-Phe-Arg-AMC as a lead, and used to identify the structural factors determining the susceptibility of peptides to hydrolysis at acidic and neutral pH values. Each peptide differs from the others in one amino acid (residue 6) and contains a highly fluorescent Nma group linked to the alpha-amino function of the N-terminal Orn residue and a Dnp group linked to the side chain of the Lys(8) residue acting as a quencher. Proteolytic cleavage was monitored by measuring the increase of fluorescence at 440 nm upon excitation at 340 nm, and the cleavage sites were determined by HPLC followed by ESI-MS analysis. Peptides containing Ala or Phe at position 6 are good substrates for the enzyme at both pH 5.0 and 7.4. By contrast, those containing Glu, Asp, Lys or Val are not cleaved at all by cathepsin B at pH 7.4, and are poorly hydrolyzed at pH 5.0. These findings provide new information for the rational design of cathepsin B-activated peptide-containing anticancer drugs.

4-Fluoroproline derivative peptides: effect on PPII conformation and SH3 affinity

Eukaryotic signal transduction involves the assembly of transient protein-protein complexes mediated by modular interaction domains. Specific Pro-rich sequences with the consensus core motif PxxP adopt the PPII helix conformation upon binding to SH3 domains. For short Pro-rich peptides, little or no ordered secondary structure is usually observed before binding interactions. The association of a Pro-rich peptide with the SH3 domain involves unfavorable binding entropy due to the loss of rotational freedom on forming the PPII helix. With the aim of stabilizing the PPII helix conformation in the Pro-rich HPK1 decapeptide PPPLPPKPKF (P2), a series of P2 analogues was prepared, in which specific Pro positions were alternatively occupied by 4(S)- or 4(R)-4-fluoro-L-proline. The interactions of these peptides with the SH3 domain of the HPK1-binding partner HS1 were quantitatively analyzed by the NILIA-CD approach. A CD thermal analysis of the P2 analogues was performed to assess their propensity to adopt the PPII helix conformation. Contrary to our expectations, the K(d) values of the analogues were lower than that of the parent peptide P2. These results clearly show that the induction of a stable PPII helix conformation in short Pro-rich peptides is not sufficient to increase their affinity toward the SH3 domain and that the effect of 4-fluoroproline strongly depends on the position of this residue in the sequence and the chirality of the substituent in the pyrrolidine ring.

Carnosine and carnosine-related antioxidants: a review

First isolated and characterized in 1900 by Gulewitsch, carnosine (beta-alanyl-L-hystidine) is a dipeptide commonly present in mammalian tissue, and in particular in skeletal muscle cells; it is responsible for a variety of activities related to the detoxification of the body from free radical species and the by-products of membrane lipids peroxidation, but recent studies have shown that this small molecule also has membrane-protecting activity, proton buffering capacity, formation of complexes with transition metals, and regulation of macrophage function. It has been proposed that carnosine could act as a natural scavenger of dangerous reactive aldehydes from the degradative oxidative pathway of endogenous molecules such as sugars, polyunsaturated fatty acids (PUFAs) and proteins. In particular, it has been recently demonstrated that carnosine is a potent and selective scavenger of alpha,beta-unsaturated aldehydes, typical by-products of membrane lipids peroxidation and considered second messengers of the oxidative stress, and inhibits aldehyde-induced protein-protein and DNA-protein cross-linking in neurodegenerative disorders such as Alzheimer's disease, in cardiovascular ischemic damage, in inflammatory diseases. The research for new and more potent scavengers for HNE and other alpha,beta-unsaturated aldehydes has produced a consistent variety of carnosine analogs, and the present review will resume, through the scientific literature and the international patents, the most recent developments in this field.

Synthesis and Evaluation of Neuroprotective α,β-Unsaturated Aldehyde Scavenger Histidyl-Containing Analogues of Carnosine

The synthesis, scavenging activity, and cytoprotective profiles of histidyl-containing carnosine analogues bearing hydrazide or 1,2-diol moieties is reported. Some compounds have demonstrated higher aldehyde-sequestering efficiency than carnosine and were also efficient in protecting SH-SY5Y neuroblastoma cells and rat hippocampal neurons from 4-hydroxy-trans-2,3-nonenal (HNE)-mediated death. The cytoprotective efficacy of these compounds suggests their potential use as therapeutic agents for disorders that involve excessive membrane lipids peroxidation and HNE-mediated neuronal toxicity.

Tat cell-penetrating peptide has the characteristics of a poly(proline) II helix in aqueous solution and in SDS micelles

Tat cell-penetrating peptide (GRKKRRQRRRPPQG) is able to translocate and carry molecules across cell membranes. Using CD spectroscopy the conformation of this synthetic peptide was studied in aqueous and membrane-mimicking, micellar SDS solutions at different temperatures. The CD spectrum of the Tat cell-penetrating peptide in SDS micellar solution was virtually unchanged from that in aqueous solution, and at low temperature it was close to that of a poly(proline) II helix.

Fine analysis of the short arm of chromosome 1 in sporadic and familial pheochromocytoma

OBJECTIVE

Despite the very recent discovery that about 25% of apparently sporadic forms of pheochromocytoma are actually due to germline mutations of RET, VHL, SDHB or SDHD genes, the genetic bases of the tumourigenesis of this type of cancer are still incompletely understood. Recent studies provided evidence that a new tumour suppressor gene, mapping on the short arm of chromosome 1, could be involved in early tumourigenesis of pheochromocytoma.

DESIGN

We have performed a fine analysis of loss of heterozygosity (LOH) of this region. In particular, we have analysed 31 highly polymorphic microsatellites distributed at 3.8 Mege base (Mb) mean intervals along the short arm of the chromosome 1 in paired samples of DNA extracted from peripheral blood lymphocytes and tumour tissues.

PATIENTS

The study was carried out on 38 patients with pheochromocytoma that had been grouped, by careful clinical and molecular investigation, in the following classes: 21 sporadic, five multiple endocrine neoplasia type 2 (MEN2), two type 1 neurofibromatosis (NF1), five von Hippel-Lindau (VHL), one somatic VHL mutated and four nonsyndromic familial cases.

RESULTS

In 12/21 sporadic cases (57.1%), in 4/5 MEN2 (80%), 2/4 non-syndromic familial cases (50%), and in 2/2 NF1 (100%), the entire short arm was deleted, while in 6/21 sporadic (28.6%) and 1/5 MEN2 (20%) cases a partial deletion was detected. On the other hand, none of the five cases due to VHL mutation (either germline or somatic) had LOH at chromosome 1. In total, complete or partial deletion of 1p was detected in 27/38 (71%) of the cases. The most frequently deleted marker was D1S2890, which maps at 1p32.1. This region, which spans from 50 to 62 Mb from telomere, was therefore further investigated with markers located at a mean interval of 1.3 Mb in the subset of cases that showed a partial deletion of 1p. This analysis showed that a small region between 55.1 and 59.0 Mb was most frequently missing, which could therefore contain a novel pheochromocytoma locus.

CONCLUSIONS

The results presented here confirm that the short arm of chromosome 1 harbours one or more genes responsible for the development of pheochromocytoma and suggest that one of them could map in a 3.9-Mb fragment between 1p32.3 and 1p32.1.

Conformational constraints of tyrosine in protein tyrosine kinase substrates: Information about preferred bioactive side-chain orientation

The side-chain orientation of a tyrosine residue located in a peptide, which is an excellent substrate of Syk tyrosine kinase (A. M. Brunati, A. Donella-Deana, M. Ruzzene, O. Marin, L. A. Pinna, FEBS Letters, 1995, Vol. 367, pp. 149-152), was fixed in the gauche (+) or gauche (-) conformation by using the 7-hydroxy-1,2,3,4-tetrahydro isoquinoline-3-carboxylic (Htc) structure. The tyrosine trans conformation was blocked by using an aminobenzazepine-type (Hba) structure. The proposed side-chain orientations were confirmed by the analysis of the (1)H-NMR parameters: chemical shifts, coupling constants, and nuclear Overhauser effects to the tyrosine constraints in the different analogs. This "rotamer scan" of the phosphorylatable residue allowed us to generate optimal substrates in terms of both phosphorylation efficiency and selectivity for Syk tyrosine kinase. In contrast, these conformationally restricted tyrosine analogs were not tolerated by the Src-related tyrosine kinases Lyn and c-Fgr.

N-benzhydryl-glycolamide: the first protecting group in peptide synthesis with a strong conformational bias

We have synthesized and examined the preferred conformation of a set of N-benzhydryl-glycolamide esters from N(alpha)-protected (or N(alpha)-blocked) alpha-amino acids. Experiments were performed in CDCl(3) solution by Fourier transform infrared absorption and (1)H-NMR techniques, and in the crystalline state by x-ray diffraction. The results of our analysis strongly support the view that this type of N(alpha)-acylated alpha-aminoacyl esters has a marked tendency to fold into a beta-turn conformation, the nature of which is dictated by the structural propensity of the amino acid constituent at the i+1 position.

Specific monitoring of Syk protein kinase activity by peptide substrates including constrained analogs of tyrosine

The ability of Syk protein tyrosine kinase (PTK) to phosphorylate peptides, where tyrosine had been replaced by conformationally constrained analogs, has been exploited to develop highly selective substrates suitable for the specific monitoring of Syk activity. In particular we have synthesized a peptidomimetic, RRRAAEDDE(L-Htc)EEV (syktide), with the 3(S)-7-hydroxy-1,2,3,4-tetrahydroisoquinoline-3-carboxyl acid residue (L-Htc) replaced for tyrosine, which is phosphorylated by Syk with remarkable efficiency (K(cat)=73 min(-1), K(m)=11 microM), while it is not affected to any appreciable extent by a number of PTKs tested so far. These properties make syktide the first choice substrate for the specific monitoring of Syk.

Antennapedia/HS1 chimeric phosphotyrosyl peptide: conformational properties, binding capability to c-Fgr SH2 domain and cell permeability

With the aim of interfering with the signaling pathways mediated by the SH2 domains of Src-like tyrosine kinases, we synthesized a tyrosyl-phospho decapeptide, corresponding to the sequence 392-401 of HS1 protein, which inhibits the secondary phosphorylation of HS1 protein catalyzed by the Src-like kinases c-Fgr or Lyn. This phospho-peptide was modified to enter cells by coupling to the third helix of Antennapedia homeodomain, which is able to translocate across cell membranes. Here we present CD and fluorescence studies on the conformational behavior in membrane-mimicking environments and on lipid interactions of Antennapedia fragment and its chimeric phosphorylated and unphosphorylated derivatives. These studies evidenced that electrostatic rather than amphiphilic interactions determine the peptide adsorption on lipids. Experiments performed with recombinant protein containing the SH2 domain of c-Fgr fused with GST and with isolated erythrocyte membranes demonstrated that the presence of the N-terminal Antennapedia fragment only slightly affects the binding of the phospho-HS1 peptide to the SH2 domain. In fact, it has been shown that in isolated erythrocyte membranes, both phospho-HS1 peptide and its chimeric derivative greatly affect either the SH2-mediated recruitment of the c-Fgr to the transmembrane protein band 3 and the following phosphorylation of the protein catalyzed by the Src-like kinase c-Fgr. The ability of the chimeric phospho-peptide to enter cells has been demonstrated by confocal microscopy analysis.

Ion-binding and pharmacological properties of Tyr6 and Tyr9 antamanide analogs

In order to investigate the antiproliferative properties of antamanide, we have synthesized and studied two antamanide analogs where the phenylalanine residue in positions 6 or 9 is substituted by tyrosine, their corresponding linear forms and the cyclic and linear des Phe5,Phe6-Tyr9-analogs. Antamanide and its biologically active synthetic analogs are able to form highly stable complexes with metal ions, particularly Na+, K+ and Ca2+. We studied the ion-binding properties of the Tyr-antamanide analogs by CD and Tb3+ -mediated fluorescence in acetonitrile. In this medium the far-and near-UV CD spectra of the neat Tyr6-antamanide analog are very similar to that of the parent cyclic decapeptide. Substantial differences occur on the contrary in the CD spectra of the neat Tyr9-antamanide, particularly in the regions at 220 nm and 270-290 nm. In acetonitrile, as already found for antamanide, the interaction with the above-mentioned metal ions always produces evident changes in the far- and near-UV CD spectra of both analogs. On the contrary, the CD spectra of the linear deca- and octa- and of the cyclic octa-analogs are affected by the presence of metal ions only in the near-UV region. In the same solvent the Tb3+ -mediated fluorescence spectra of all the synthetic peptides are remarkably affected by the addition of ions. On the basis of the spectral total changes, by using either or both the spectroscopic techniques, it has been possible to determine the ion binding constants for all the linear and cyclic Tyr-antamanide analogs and to compare them with that of the parent peptide. The antitoxic and antiproliferative activities of these antamanide analogs have been tentatively correlated to their ion-binding properties. A preliminary account of this work was given in (1).

Solution conformational analysis of sodium complexed [Gly6]- and [Gly9]-antamanide analogs

To investigate the conformational flexibility of metal-complexed cyclodecapeptides, we synthesized and studied two antamanide analogs, in which the phenylalanine residue in position 6 or 9 of the sequence was substituted by Gly. Previous conformational studies on antamanide suggested that these backbone regions are affected by conformational variation. The NMR conformational study showed a high degree of flexibility for the two analogs. With sodium ions, on the other hand, [Gly9]-antamanide was able to form a fairly stable equimolar complex, whereas [Gly6]-antamanide showed a conformational heterogeneity, with one prevailing conformer. For the [Gly9]-antamanide analog, the whole NMR data, combined with extensive theoretical calculations, were consistent with the presence of 1) two beta-turns of type I, centered on Gly9-Phe10 and Ala4-Phe5, respectively; 2) a central cavity with a six-carbonyl oxygen cage, optimal for a Na+ hexacoordination; 3) strongly H-bonded amide protons for residues 1 and 6, both involved in the formation of the two type I beta-turns, which, however, exhibited some fluctuations during the molecular dynamics simulations. For the [Gly6]-antamanide-Na+ complex the prevailing conformer was consistent with a more open structure, with the partial solvent exposure of all the amide protons; that is, the Gly residue in position 6 increases the flexibility of this critical site more than does the Gly in position 9. These data in some way parallel the results of the cytotoxicity tests on B16-F10 transformed cells for the two analogs: [Gly9]-antamanide is cytotoxic after 48 h exposure, whereas [Gly6]-antamanide is almost inactive. On the contrary, both analogs are practically inactive in vivo against phalloidin.

Linear and cyclic peptides as substrates for Lyn tyrosine kinase

Two Tyr residues are supposed to play a crucial role in the regulation of protein tyrosine kinases of the Src family. Autophosphorylation of Src Tyr416 correlates with enzyme activation, while phosphorylation of C-terminal Tyr527 by Csk gives rise to inactive forms of Src kinases. It has previously been demonstrated that the Src-like tyrosine kinase expressed by the oncogene lyn displays a particularly high affinity (Km 20 microm) toward the dimeric linear and cyclic derivatives of the heptapeptide H-Glu-Asp-Asn-Glu-Tyr-Thr-Ala-OH which reproduces the main autophosphorylation site of most of the Src enzymes. Under the experimental conditions used only one Tyr residue of the dimeric sequence can be phosphorylated [P. Ruzza, A. Calderan, B.Filippi, B. Biondi, A. Donella Deana, L. Cesaro, L. A. Pinna & G. Borin (1995) Int. J. Peptide Protein Res. 45, 529-539]. The present study addresses the problem of the efficiency displayed by Lyn towards the two Tyr residues located at positions 5 and 12 of the dimeric peptide. To this purpose, two tetradecapeptides were synthesized by the classical solution method, each containing one of the two Tyr residues alternatively replaced by Phe, and the corresponding univocal cyclic form. A possible correlation between the different structural properties induced by the modifications of the native sequence and the ability of the peptides to act as Lyn substrates was noted. The kinetic data obtained indicate that Lyn phosphorylates the residues located at different positions in the two linear analogues differently. In particular, while the Tyr5, Phe12 derivative presents a Km value similar to those obtained for the dimeric linear and cyclic unmodified analogues, the Km value of the Phe5, Tyr12 derivative is two-fold higher than those found for the above-mentioned peptides. Moreover, as previously reported for the linear and cyclic dimeric forms of the native sequence, in the mono-tyrosine containing series of dimers the still conformationally flexible cyclic derivative shows a phosphorylation efficiency two-fold higher than those found for the linear derivatives.

An exploration of the effects of constraints on the phosphorylation of synthetic protein tyrosine kinase peptide substrates

We synthesized by classical solution methods three conformational constrained analogues of EDNEYTA, a heptapeptide sequence that represents the common major autophosphorylation site of the protein tyrosine kinases (PTKs) of the Src family. The correlation between the different structural properties induced by the modifications of the native sequence and the propensity of the peptides to act as PTK substrates was examined. The kinetic data obtained indicate that the introduction of the tyrosine-analogue constraints Tic(OH) and MeTyr, which block the ring flexibility, completely prevents the phosphorylation catalysed by the kinases Lyn and Fgr. On the other hand PTKIIB/p38syk can phosphorylate the two derivatives albeit with an efficiency lower than that found with the native sequence. A third derivative contained side chain to side chain cyclization. This analogue, in which the freedom of the phenolic moiety is not altered, can be phosphorylated by all the PTKs tested with kinetic constants comparable to the parent peptide.

Synthetic Tyr-phospho and non-hydrolyzable phosphonopeptides as PTKs and TC-PTP inhibitors

Tyrosine-specific protein kinases and phosphatases are important signal transducing enzymes in normal cellular growth and differentiation and have been implicated in the etiology of a number of human neoplastic processes. In order to develop agents which inhibits the function of these two classes of enzymes by interfering with the binding of their substrates, we synthesized analogs derived from the peptide EDNEYTA. This sequence reproduces the main autophosphorylation site of Src tyrosine kinases. In this work we report the synthesis, by classical solution methods, of the phosphotyrosyl peptide EDNEYpTA as well as of three analogs in which the phosphotyrosine is replaced by a phosphinotyrosine and by two unnatural, non-hydrolyzable amino acids 4-phosphonomethyl-L-phenylalanine and 4-phosphono-L-phenylalanine. The Src peptide and its derivatives were tested as inhibitors of three non-receptor tyrosine kinases (Lyn, belonging to the Src family, CSK and PTK-IIB) and a non-receptor protein tyrosine phosphatase obtained from human T-cell (TC-PTP). The biomimetic analogues, which do not significantly affect the activity of CSK, PTK-IIB and TC-PTP, act as efficient inhibitors on Lyn, influencing both the exogenous phosphorylation and, especially, its autophosphorylation. In particular, the Pphe derivative may provide a basis for the design of a class of inhibitors specific for Lyn and possibly Src tyrosine kinases, capable of being used in vivo and in vitro conditions.

Linear and cyclic synthetic peptides related to the main autophosphorylation site of the Src tyrosine kinases as substrates and inhibitors of Lyn

Tyrosine protein kinases (TPKs) of the src family contain two major phosphoacceptor sites which are homologous to the Tyr 416 and Tyr 527 of pp60c-src. The former represents the main autophosphorylation sites of these enzymes, and its phosphorylation correlates with increased kinase activity. It has previously been demonstrated that the Src-like tyrosine kinase expressed by the oncogene lyn displays a high affinity toward the heptapeptide H-Glu-Asp-Asn-Glu-Tyr-Thr-Ala-OH, which reproduces the main autophosphorylation site of the Src family enzymes [Donella-Deana, A., Marin, O., Brunati, A.M. & Pinna, L.A. (1992) Eur. J. Biochem. 204, 1159-1163]. Our study was addressed to the synthesis of some derivatives of this sequence in order to obtain both peptide substrates suitable for the detection of the Src-like tyrosine kinase activity and active site-directed inhibitors specific for this class of enzymes. For this purpose we synthesized by classical solution methods the heptapeptide and its dimeric form. Moreover, in order to improve the proteolytic resistance of these peptides we also synthesized their cyclic derivatives and their N-terminal acetylated and C-terminal amidated analogs. The correlation between the different structural properties induced by the modifications of the native sequence and the propensity of the peptides to act as Lyn substrates was examined. The kinetic data obtained indicate that the extent of the peptide phosphorylation varies considerably depending on the flexibility and length of the analogs. While the cyclization and the C-terminal amidation of the heptapeptide are detrimental for the Lyn activity, dimeric derivatives display very favourable kinetic constants. In particular the cyclic dimer is an especially suitable substrate for the tyrosine kinase and a powerful inhibitor of both the phosphorylation activity of Lyn and the enzyme autophosphorylation.

Synthesis and conformational studies on peptides corresponding to a putative autophosphorylation site of abl TPK

The transforming gene of Abelson murine leukaemia virus (v-abl) codes for a membrane-associated tyrosine-specific protein kinase (abl TPK). Analysis of the v-abl gene has shown that both the fibroblast-transforming and tyrosine-protein kinase activities reside within a minimal region encoding a protein of 43 kDa (p43v-abl), which represents the most active, isolated form of this enzyme. Since the cellular substrates for p43v-abl are yet to be identified, we synthesized by classical solution methods the octapeptide H-Gly-Asp-Thr-Tyr-Thr-Ala-His-Ala-OH, corresponding to the structural sequence of the main putative autophosphorylation site (Tyr 515) of the abl TPK, as well as some of its analogs modified in positions -2, -1, +1 and +3. The synthetic peptides were tested as substrates for the p43v-abl. The kinetic data obtained indicate that the rates of their phosphorylation vary considerably depending on the sequence of the peptide, as expected. As a rule, no significant increment of the efficiency results from each substitution in the parent sequence. While the replacement of the two charged residues, namely Asp-2 and His-7, with neutral Ala is well tolerated, the substitution with amino acids bearing opposite charges is detrimental. The correlation between secondary structure of our synthetic octapeptides and their substrate recognition by p43v-abl was studied using CD and fluorescence spectroscopy in 5 mM Tris, in 98% TFE/Tris and in 30 mM SDS solutions. The comparison of the spectroscopic data with the kinetic parameters does not confirm a close relationship between the conformational properties of these peptides and their enzymatic role.

Specificity of T-cell protein tyrosine phosphatase toward phosphorylated synthetic peptides

The local specificity determinants for a T-cell protein tyrosine phosphatase (TC-PTP) have been inspected with the aid of a series of synthetic peptides, either enzymically or chemically phosphorylated, derived from the phosphoacceptor sites of phosphotyrosyl proteins. The truncated form of T-cell PTP, deprived of its C-terminal down-regulatory domain, readily dephosphorylates submicromolar concentrations of eptapeptides to eicosapeptides, reproducing the C-terminal down-regulatory site of pp60c-src (Tyr527), the phosphorylated loop IV of calmodulin and the main autophosphorylation site of two protein tyrosine kinases of the src family (Tyr416 of pp60c-src and Tyr412 of p51fgr). However, phosphopeptides of similar size, derived from phosphoacceptor tyrosyl sites of the abl and epidermal-growth-factor receptor protein tyrosine kinases, the phosphorylated loop III of calmodulin, and phosphoangiotensin II undergo either very slow or undetectable dephosphorylation, even if tested up to 1 microM concentration. The replacement of either Ser-P or O-methylated phosphotyrosine for phosphotyrosine within suitable peptide substrates gives rise to totally inert derivatives. Moreover, amino acid substitutions around phosphotyrosine in the peptides src(412-418), src(414-418) and abl-(390-397) deeply influence the dephosphorylation efficiency. From these data and from a comparative analysis of efficient versus poor phosphopeptide substrates, it can be concluded that acidic residues located on the N-terminal side of phosphotyrosine, with special reference to position -3, play a crucial role in substrate recognition, while basic residues in the same positions act as negative determinants. In any event, the presence of at least two aminoacyl residues upstream of phosphotyrosine represents a necessary, albeit not sufficient, condition for detectable dephosphorylation to occur. By replacing the truncated form of TC-PTP with the full length TC-PTP, the dephosphorylation efficiencies of all peptides tested are dramatically impaired. Such an effect is invariably accounted for by a substantial increase in Km values, accompanied by a more or less pronounced decrease in Vmax values. These data support the concept that the C-terminal regulatory domain of TC-PTP exerts its function primarily by altering the affinity of the enzyme toward its phosphotyrosyl targets.

Purification of proteinase-like and Na+/K(+)-ATPase stimulating substance from plasma of insulin-dependent diabetics and its identification as alpha 1-antitrypsin

The purpose of this study was to purify and identify the proteinase-like substance previously recognized as responsible for the Na+/K(+)-ATPase stimulating property of plasma from insulin-dependent diabetic subjects. Anion-exchange chromatography followed by two-step heparin affinity chromatography resulted in a fraction highly enriched in both potent Na+/K(+)-ATPase stimulating activity and potent proteolytic activity. Approx. 400 micrograms of purified protein was isolated from 62 mg of starting plasma proteins. When analyzed on sodium dodecyl sulfate gels the active fraction consisted mainly of one polypeptide band with an apparent molecular mass of 66 kDa under either reducing or nonreducing conditions. The proteinase-like properties of the purified fraction were further revealed by its ability to clot plasma, split fibrinogen with production of fibrinopeptide A and induce shape change in human platelets and irreversible platelet aggregation in the presence of the stable analogue of endoperoxides U46619. Its additional capacity to affect platelet phosphoinositol metabolism was shown by the stimulation of protein kinase C-dependent phosphorylation of 47 kDa platelet membrane protein. In designing an identification protocol for the purified fraction, it was postulated that plasma proteinases are probably bound to their inhibitors, to form a stable covalently linked complex. The possibility that a proteinase-proteinase inhibitor complex was purified instead of single proteinase(s) was investigated. Neither trypsin nor neutrophil elastase were present in the active fraction whereas, among the possible plasma proteinase inhibitors tested, immunoreactivity was observed only in the presence of alpha 1-antitrypsin (alpha 1 AT) antiserum. Double immunodiffusion showed that control human alpha 1 AT and the plasma-purified fraction shared common antigens. Furthermore, both isoelectric focusing and amino acid composition analysis showed that the two substances were similar. The results obtained indicate that alpha 1 AT is apparently the only active component of the purified fraction from the plasma of insulin-dependent diabetics, thus suggesting that an altered form of the inhibitor is responsible for the broad range of proteinase-like effects elicited by the plasma-purified fraction.

Separation of acidic peptides by reversed-phase ion-pair chromatography. Analytical application to a series of acidic substrates of casein kinases

A series of small peptides including clusters of glutamyl residues, synthesized to study the site specificity of rat liver (L-CK2) and yeast (Y-CK2) casein kinase-2, are analytically characterized by ion-pair high-performance liquid chromatography using tetrabutylammonium as counter-ion and acetonitrile as modifier of the aqueous phase. Under these conditions peptides of slightly different acidity can be separated and the elution order parallels the hydrophobicity of the ion-pair-peptide complexes, which increases with the number of the acidic functions present in the sequence.

Conformational and binding studies on peptides related to domains I and III of calmodulin

The conformational and ion-binding properties of two peptide fragments of 25 amino acid residues corresponding to the helix-loop sequences of domains I and III of calmodulin (CaM) were investigated by CD and Tb(3+)-mediated fluorescence spectroscopy. Both peptides exhibit very similar ion binding properties either in water or trifluoroethanol (TFE), and do not allow the differentiation of the two domains in the native protein in terms of their binding capacity. An aggregation phenomenon was observed in TFE with increase of the alpha-helical content. We suggest that the aggregation involves an interaction between the hydrophilic surfaces of amphiphilic alpha-helices in a way similar to inverse micelle formation.

Synthetic peptides including acidic clusters as substrates of yeast casein kinase-2

The synthesis is reported of a series of glutamyl peptide analogs of the model substrate H-Ser-Glu-Glu-Glu-Glu-Glu-OH of casein kinase-2 (CK-2). A convenient HPLC method for the separation of slightly different acidic peptides is also reported. The site specificity of yeast casein kinase-2 (Y-CK2) is examined with the aid of synthesized peptide substrates.

Synthetic peptides reproducing the EGF-receptor segment homologous to the pp60v-src phosphoacceptor site. Phosphorylation by tyrosine protein kinases

The octapeptide E-E-K-E-Y-H-A-E, corresponding to the amino acid sequence 841-845 of EGF receptor, whose tyrosine-845 is homologous to the main phosphorylation site of pp60v-src, has been synthesized together with seven shorter peptides encompassing variable segments around the tyrosine residue. The peptides have been employed as model substrates for inspecting the local structural determinants of three tyrosine protein kinases (TPKs), namely; TPK-IIB and TPK-III, isolated from lymphoid cells (Eur. J. Biochem. 172, 451-457 (1988] and the TPK encoded by the oncogene of Abelson murine leukemia virus. The phosphorylation order with the different peptide substrates is variable depending on the TPK used: in particular, the lysine residue at position -2 relative to tyrosine proved especially harmful with TPK-IIB, the peptides K-E-Y-H and K-E-Y-H-A-E being very poor substrates compared with their shorter derivatives devoid of the N-terminal lysine (E-Y-H and E-Y-H-A-E, respectively). Conversely, such a basic residue is well tolerated by the other two TPKs. The negative effect of the N-terminal lysine on TPK-IIB-catalyzed phosphorylation is accounted for by an increase of Km and can be overcome by the presence of additional glutamic acid(s) on that side. On the other hand, the C-terminal acidic doublet Ala-Glu specifically impairs the phosphorylation efficiency of abl-TPK, by lowering the Vmax value, the heptapeptide E-K-E-Y-H-A-E being much less readily phosphorylated than E-K-E-Y-H. Collectively, these results would indicate that the site specificity of tyrosine protein kinases results from the balance of positive and negative determinants whose influence on the catalytic activity of the individual enzymes can differ greatly.

Synthesis of the dodecapeptide corresponding to domain III of bovine brain calmodulin: alpha-beta shift side reactions during the synthesis by the classical method in solution

We report details of the chemical synthesis of the dodecapeptide corresponding to the calcium binding loop III of bovine brain calmodulin (sequence 93-104) and its fragments 96-04, 93-98, and 99-104. The preparation of the peptides employed classical solution methods and a fragment-condensation strategy. The major difficulties were encountered during the synthesis of the peptides containing the N-terminal sequences -Gly-Asn-Gly- and -Asp-Lys-Asp-Gly-Ans-Gly-, in which alpha-beta shift side reactions were observed.

Conformation and ion binding properties of peptides related to calcium binding domain III of bovine brain calmodulin

The conformational and ion binding properties of the sequences 93-104, 96-104, and 93-98 of domain III of bovine brain calmodulin (CaM) have been studied by CD and Tb3+-mediated fluorescence. In aqueous solution the interaction of all fragments with Ca2+ and Mg2+ ions is very weak and without any effect on the peptide conformation, which remains always random. In trifluoroethanol the interaction is very strong and the different fragments exhibit very distinct binding properties. In particular, the dodecapeptide fragment 93-104, and its N-terminal hexapeptide 98-104, bind calcium and magnesium with a very high binding constant (Kb greater than 10(5) M-1), undergoing a substantial conformational change. The structural rearrangement is particularly evident in the hexapeptide fragment, which tend to form a beta-bend. The C-terminal nonapeptide fragment 96-104 interacts with calcium and magnesium more weakly, and the binding process causes a decrease of ordered structure. These results suggest that, even in the entire dodecapeptide sequence corresponding to the loop of domain III of CaM, the calcium binding site is shifted toward the N-terminal hexapeptide segment. This interpretation is consistent with the results of crystallographic studies of CaM, which show that the calcium ions are located toward the amino terminal portion of the loop.

Substrate-specificity determinants for a membrane-bound casein kinase of lactating mammary gland. A study with synthetic peptides

A tissue-specific casein kinase, purified from the Golgi-enriched-membrane fraction of guinea-pig lactating mammary gland (GEF-CK), readily phosphorylates the synthetic peptide Ser-Glu5, a good substrate of casein kinase-2, and several derivatives varying for the number and position of acidic residues on the C-terminal side of serine, except those lacking an acidic side chain at position +2. The least acidic peptide, still significantly affected by GEF-CK, is Ser-Ala-Glu-Ala3 which is not a substrate for CK-2. Conversely, the peptides Ser-Ala2-Glu-Ala2, Ser-Ala2-Glu3, Ser-Ala2-Glu5 and Ser-Glu-Ala-Glu3, all of which are more or less readily phosphorylated by CK-2, are not appreciably affected by GEF-CK. On the other hand the presence of additional glutamyl residues, besides the one in the second position, improves the affinity of the peptide substrate for GEF-CK, as indicated by the Km values of Ser-Glu5, Ser-Glu2-Ala3 and Ser-Ala-Glu-Ala3 which are 80, 950 and 3950 microM respectively. It is concluded that although both CK-2 and GEF-CK require, for optimal activity, rather extended acidic clusters on the C-terminal side of the target serine, the most critical residue in the case of GEF-CK is not the one at position +3, which is required for CK-2 catalyzed phosphorylation [Marin, O. et al. (1986) Eur. J. Biochem. 160, 239-244], but the one lying at position +2. Additional differences, concerning the site specificities of these enzymes, have been outlined using the threonyl derivative of Ser-Glu5 and the peptide Arg-Ser-Glu3-Val-Glu. The former is still phosphorylated by CK-2 but not to any appreciable extent by GEF-CK, which apparently is strictly specific for seryl residues. On the contrary, the presence of an N-terminal basic residue, which greatly reduces phosphorylation by CK-2, is tolerated rather well by GEF-CK. On the other hand a C-terminal basic residue, interrupting the acidic cluster, compromises phosphorylation by GEF-CK, as indicated by the extremely high Km value of Ser-Glu3-Lys-Glu vs Ser-Glu3-Val-Glu (13,000 and 170 microM, respectively).

Synthetic peptide substrates for casein kinase 2. Assessment of minimum structural requirements for phosphorylation

Unlike the peptides SAEAAA and SEEAAA which are not substrates for casein kinase 2 (CK-2) their analogs SAAEAE and SAAEAA are still significantly phosphorylated. Their Km values, however, (13.3 and 18.9 mM, respectively) are almost two orders of magnitude higher than that of SEEEEE and their Vmax values are 3- and 14-fold lower than that of SAAEEE. The peptide ESEEEEE, but not ASEEEEE, is a slightly better substrate than SEEEEE, while both RSEEEEE and SEEEKE are very poor substrates compared to ASEEEEE and SEEEAE, respectively. SAAEAE is much more responsive to polylysine stimulation and polyphosphate inhibition than is SEEEEE. Taken together these data show that a single acidic residue at the third position from the C-terminal side of the phosphorylatable amino acid represents not only a necessary, but also a sufficient condition for site recognition by CK-2. Optimal phosphorylation efficiency, however, requires an extended C-terminal cluster of several acidic residues, and can be compromised by the presence of only a basic residue either inside the acidic cluster or adjacent to the N-terminal side of the phosphoacceptor amino acid. The structure of the phosphoacceptor site can greatly influence the efficacy of substrate-directed effectors of CK-2.

Synthesis of human [15-norleucine]little-gastrin-II and des-1-tryptophan-[12-norleucine]minigastrin-II

By the use of a newly developed procedure for the synthesis of tyrosine-O-sulfate peptides based on the direct incorporation of the suitably N alpha-protected tyrosine-O-sulfate residue along the synthetic route, the synthesis of two human gastrin-II analogues was successfully accomplished. Thereby acid labile side chain protection was applied in combination with the N alpha-benzyloxycarbonyl group in the intermediate chain elongation steps. Despite the pronounced acid-lability of the sulfate ester moiety, its hydrolysis during the final acidolytic deprotection step was significantly reduced under optimized conditions. Subsequent chromatographic purification led to the two gastrin analogues in satisfactory yields as highly pure compounds as judged by various indicative analytical assays.