Kinetic and conformational studies on some partially synthetic ribonuclease S' analogues modified in position 8

Syntheses are described of two S-peptide analogues where the arginyl residue in position 10 has been replaced by ornithine and the phenylalanine in position 8 has been substituted by the unnatural amino acids cyclohexylalanine or p-fluorophenylalanine. In order to regenerate the arginyl residue, which is present in position 10 in the natural sequence, the S-peptide analogues beloning to the [Orn10]-series are transformed into the corresponding guanidinated derivatives by treatment with O-methylisourea. 1epsilon, 7epsilon, 10delta triguan-[Cha8, Orn10]-, 1epsilon, 7epsilon, 10delta-triguan-[pF-Phe8, Orn10]- and 1epsilon, 7epsilon, 10delta-triguan-[Tyr8, Orn10]-S-peptides were prepared. The ability to bind to and activate the S-protein of the synthetic S-peptide analogues, before and after guanidination, was tested by exploring their capacity to generate ribonuclease activity using RNA and C greater than p as substrates. The affinity of the different peptides for the S-protein in the absence of substrate was evaluated by difference spectroscopy.

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.

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.

Further evidences for neuroprotective effects of melatonin

The physiological roles of the pineal hormone melatonin are still not completely clarified. Recently it has been shown that melatonin is a potent, endogenous scavenger of reactive oxygen species suggesting that it might interfere with neurodegenerative processing involving free-radical formation and excitatory aminoacid release. These neuroprotective effects of melatonin may result, at least in part, from a sparing of glutathione reductase, which is decreased following administration of the neurotoxic agent kainate (KA) in rats. Moreover, KA causes a rapid decrease in glutathione (GSH) content of cultured cerebellar granule neurons but not in astrocytes. These cell types both express functional KA receptors, but only the former is sensitive to reactive oxygen species-dependent KA injury. Melatonin counteracts the changes in GSH, induced by KA, in cultured cerebellar granule neurons.

Intracellular glutathione levels determine cerebellar granule neuron sensitivity to excitotoxic injury by kainic acid

Glutathione (GSH) is a key component of the cellular defence cascade against injury caused by reactive oxygen species. Kainic acid (KA) is a potent central nervous system excitotoxin. KA-elicited neuronal death may result from the generation of ROS. The present study was undertaken to characterize the role of GSH in KA-induced neurotoxicity. Cultures of cerebellar granule neurons were prepared from 8-day-old rats, and used at 8, 14 and 20 days in vitro (DIV). Granule neurons displayed a developmental increase in their sensitivity to KA injury, as quantified by an ELISA-based assay with the tetrazolium salt MTT. At DIV 14 and 20, a 30-min challenge with KA (500 microM) reduced cell viability by 45% after 24 h, significantly greater (P<0.01) than the 22% cell loss with DIV 8 cultures. Moreover acute (30 min) KA exposure concentration-dependently reduced intracellular GSH and enhanced reactive oxygen species generation (evaluated by 2', 7'-dichlorofluorescein diacetate). In comparison to control, KA (500 microM) lowered GSH levels in DIV 8 granule neurons by 16% (P=0. 0388), and by 36% (P=0.0001) in both DIV 14 and DIV 20 neurons, after 30 min. Preincubation of granule neurons with the membrane permeant GSH delivery agent, GSH ethyl ester (5 mM), for 30 min significantly increased intracellular GSH content. Importantly, GSH ethyl ester reduced the toxic effects of KA, becoming significant at 1 mM (P=0.007 vs. KA-treated group), and was maximal at >/=2.5 mM (P<0.0001). GSH ethyl ester displayed a similar dose-dependence in its ability to counteract KA-induced depletion of cellular GSH. The data strengthen the notion that cellular GSH levels have a fundamental role in KA-induced neurotoxicity.