Novel glutathione analogues containing the dithiol and disulfide form of the Cys-Cys dyad

Anti-Oxidant and Tyrosinase Inhibitory In Vitro Activity of Amino Acids and Small Peptides: New Hints for the Multifaceted Treatment of Neurologic and Metabolic Disfunctions

Oxidative damage is among the factors associated with the onset of chronic pathologies, such as neurodegenerative and metabolic diseases. Several classes of anti-oxidant compounds have been suggested as having a protective role against cellular stressors, but, in this perspective, peptides’ world represents a poorly explored source. In the present study, the free radical scavenging properties, the metal ion reducing power, and the metal chelating activity of a series of sulfurated amino acids and tripeptides were determined in vitro through canonical assays (DPPH, ABTS, CUPRAC, FRAP, PM, and EECC) and estimated in comparison with the corresponding activities of synthetic peptide semicarbazones, incorporating the peculiar non-proteinogenic amino acid, tert-leucine (tLeu). The compounds exhibited remarkable anti-oxidant properties. As expected, sulfurated compounds 1–5 were found to be the most efficient radical scavengers and strongest reductants. Nevertheless, tLeu-containing peptides 7 and 8 disclosed notable metal reducing and chelating activities. These unprecedented results indicate that tLeu-featuring di- and tripeptide backbones, bearing the semicarbazone chelating moiety, are compatible with the emergence of an anti-oxidant potential. Additionally, when tested against a panel of enzymes usually targeted for therapeutic purposes in neurodegenerative and metabolic disorders, all samples were found to be good inhibitors of tyrosinase.

Nitrobenzoxadiazole-based GSTP1-1 inhibitors containing the full peptidyl moiety of (pseudo)glutathione

CONTEXT

The inhibition of glutathione S-transferase P1-1 (GSTP1-1) is a sound strategy to overcome drug resistance in oncology practice.

OBJECTIVE

The nitrobenzoxadiazolyl (NBD) S-conjugate of glutathione and the corresponding γ-oxa-glutamyl isostere (compounds 1 and 5, respectively) have been disclosed as GST inhibitors. The rationale of their design is discussed in juxtaposition to non-peptide NBD thioethers.

MATERIALS AND METHODS

Synthesis of derivatives 1 and 5 and in vitro evaluation on human GSTP1-1 and M2-2 are reported.

RESULTS

Conjugates 1 and 5 were found to be low micromolar inhibitors of both isoforms. Furthermore, they display a threefold reduction in selectivity for GSTM2-2 over the P1-1 isozyme in comparison with the potent non-peptide inhibitor nitrobenzoxadiazolyl-thiohexanol (NBDHEX).

DISCUSSION AND CONCLUSIONS

Spectroscopic data are congruent with the formation of a stable sigma-complex between GSH and the inhibitors in the protein active site. Conjugate 5 is suitable for in vivo modulation of GST activity in cancer treatment.

Synthesis and neuromodulatory effects of TRH-related peptides: inhibitory activity on catecholamine release in vitro

BACKGROUND

A detailed comprehension of central mechanisms underlying feeding behavior holds considerable promise for the treatment of alimentary disorders.

METHODS

In order to elucidate the tight interrelationships occurring at the hypothalamic neuronal endings between aminergic neurotransmitters and co-localized appetite modulators, we initially studied the effects of two anorexigenic peptides structurally related to thyrotropin-releasing hormone (TRH, 1), namely cyclo(His-Pro) (CHP, 2) and pGlu-His-Gly-OH (3), on [(3)H]-norepinephrine and [(3)H]-dopamine release from perfused rat hypothalamic synaptosomes. Furthermore, a number of TRH and CHP analogues were synthesized and tested for their ability to influence neurotransmitter release in the selected neuronal model.

RESULTS

Peptide 3 showed only a slight inhibitory activity on norepinephrine release, whereas no effect was observed for compound 2. TRH analogue 8, metabolically stabilized by the replacement of pyroglutamate with the pyrohomocysteic acid (pHcs), was found to be inactive. Conversely, a significant inhibitory effect on dopamine and norepinephrine release was observed for the CHP-related diketopiperazines cyclo(Leu-Pro) (11) and cyclo(His-Gly) (14).

CONCLUSIONS

These results suggest a potential role for cyclo-dipeptides 11 and 14 in the hypothalamic modulation of appetite suppressant circuitry.

No selenium required: reactions catalyzed by mammalian thioredoxin reductase that are independent of a selenocysteine residue

Mammalian thioredoxin reductase (TR) contains a rare selenocysteine (Sec) residue in a conserved redox-active tetrapeptide of sequence Gly-Cys(1)-Sec(2)-Gly. The high chemical reactivity of the Sec residue is thought to confer broad substrate specificity to the enzyme. In addition to utilizing thioredoxin (Trx) as a substrate, other substrates are protein disulfide isomerase, glutaredoxin, glutathione peroxidase, NK-lysin/granulysin, HIV Tat protein, H(2)O(2), lipid hydroperoxides, vitamin K, ubiquinone, juglone, ninhydrin, alloxan, dehydroascorbate, DTNB, lipoic acid/lipoamide, S-nitrosoglutathione, selenodiglutathione, selenite, methylseleninate, and selenocystine. Here we show that the Cys(2) mutant enzyme or the N-terminal reaction center alone can reduce Se-containing substrates selenocystine and selenite with only slightly less activity than the wild-type enzyme, in stark contrast to when Trx is used as the substrate when the enzyme suffers a 175-550-fold reduction in k(cat). Our data support the use of alternative mechanistic pathways for the Se-containing substrates that bypass a critical ring-forming step when Trx is the substrate. We also show that lipoic acid can be reduced through a Sec-independent mechanism that involves the N-terminal reaction center. These results show that the broad substrate specificity of the mammalian enzyme is not due to the presence of the rare Sec residue but is due to the catalytic power of the N-terminal reaction center. We hypothesize that the N-terminal reaction center can reduce substrates (i) with good leaving groups such as DTNB, (ii) that are highly electrophilic such as selenite, or (iii) that are activated by strain such as lipoic acid/lipoamide. We also show that the absence of Sec only changed the IC(50) for aurothioglucose by a factor of 1.7 in the full-length mammalian enzyme (83-142 nM), but surprisingly the truncated enzyme showed much stronger inhibition (25 nM). This contrasts with auranofin, where the absence of Sec more strongly perturbed inhibition.

Synthesis and activity of novel glutathione analogues containing an urethane backbone linkage

The new GSH analogues H-Glo(-Ser-Gly-OH)-OH (5), its O-benzyl derivative 4, and H-Glo(-Asp-Gly-OH)-OH (9), characterized by the replacement of central cysteine with either serine or aspartic acid, and containing an urethanic fragment as isosteric substitution of the scissile gamma-glutamylic junction, have been synthesized and characterized. Their ability to inhibit human GST P1-1 (hGST P1-1) in comparison with H-Glu(-Ser-Gly-OH)-OH and H-Glu(-Asp-Gly-OH)-OH, which are potent competitive inhibitors of rat GST 3-3 and 4-4, has been evaluated. In order to further investigate the effect of the isosteric substitution on the binding abilities of the new GSH analogues 4, 5 and 9, the previously reported cysteinyl-containing analogue H-Glo(-Cys-Gly-OH)-OH has been also evaluated as a co-substrate for hGSTP1-1.

Synthesis and biological evaluation of the disulfide form of the glutathione analogue gamma-(L-glutamyl)-L-cysteinyl-L-aspartyl-L-cysteine

By using the chain to chain mode of cyclization the title glutathione analogue (4), containing the 11-membered disulfide ring replacing the native -Cys-Gly fragment, has been synthesized and characterized together with its reduced dithiol form gamma-Glu-Cys-Asp-Cys (5). The activity of (4) with gamma-glutamyl-transferase and glutathione reductase has been evaluated and compared with those of the two conformationally restricted glutathione analogues (2) and (3) previously reported.

Synthesis and biological evaluation of a novel pyroglutamyl-modified TRH analogue

The TRH analogue 3, incorporating the (S)-isothiazolidine-1,1-dioxide-3-carboxylic acid (1) moiety in place of the native L-pyroglutamic acid (pGlu) residue, has been synthesized and fully characterized by 1H and 13C NMR. The effects of replacing pGlu with its sulphonamido counterpart on biological activity have been investigated. This peptide, which is significantly stabilized towards hydrolysis by pyroglutamyl peptidase type I (PP I, EC 3.4.19.3), has shown to maintain in vitro prolactin-releasing activity.

Exploring the interest of 1,2-dithiolane ring system in peptide chemistry. Synthesis of a chemotactic tripeptide and x-ray crystal structure of a 4-amino-1,2-dithiolane-4-carboxylic acid derivative

Due to their relevant biological functions and specific chemical reactivity 1,2-dithiolanes (five-membered cyclic disulfides) represent an emerging class of heterocyclic compounds. However, despite the extensive research centered on lipoic acid and its analogues, only very few data are at the present available on peptides containing this ring system. We report here synthesis, conformation and bioactivity of a fMLF-OMe analogue, namely For-Met-Adt-Phe-OMe (7), in which the residue of the 4-amino-1,2-dithiolane-4-carboxylic acid (Adt) (4) replaces the central L-leucine. The crystal conformation of the synthetic intermediate Boc-Adt-OMe (5) is also described and compared to that of lipoic acid (R-1,2-dithiolane-3-pentanoic acid) (3) and asparagusic acid (1,2-dithiolane-4-carboxylic acid) (2).

4-Amino-1,2-dithiolane-4-carboxylic acid (Adt) as cysteine conformationally restricted analogue. Synthetic protocol for Adt containing peptides

An efficient and versatile protocol to incorporate the achiral and C(alpha,alpha)-tetrasubstituted 4-amino-1,2-dithiolane-4-carboxylic acid Adt (1) residue into peptides is described. The 2,2-bis[(benzylthio)methyl]glycine N-carboxy anhydride (5) was found to be the key reactive intermediate from which both Boc-Adt-OMe (8) and the glutathione analogue H-Glu(-Adt-Gly-OH)-OH (12) can be obtained.