Interference of S-alkyl derivatives of glutathione with brain ionotropic glutamate receptors

N-Terminal Decarboxylation as a Probe for Intramolecular Contact Formation in γ-Glu-(Pro)n-Met Peptides

The kinetics of intramolecular-contact formation between remote functional groups in peptides with restricted conformational flexibility were examined using designed peptides with variable-length proline bridges. As probes for this motion, free radicals were produced using the ^•OH-induced oxidation at the C-terminal methionine residue of γ-Glu-(Pro)_n-Met peptides (n = 0-3). The progress of the radicals' motion along the proline bridges was monitored as the radicals underwent reactions along the peptides' backbones. Of particular interest was the reaction between the sulfur atom located in the side chain of the oxidized Met residue and the unprotonated amino group of the glutamic acid moiety. Interactions between them were probed by the radiation-chemical yields (expressed as G values) of the formation of C-centered, α-aminoalkyl radicals (αN) on the Glu residue. These radicals were monitored directly or via their reaction with p-nitroacetophenone (PNAP) to generate the optically detected PNAP^•- radical anions. The yields of these αN radicals were found to be linearly dependent on the number of Pro residues. A constant decrease by 0.09 μM J^-1 per spacing Pro residue of the radiation-chemical yields of G(αN) was observed. Previous reports support the conclusion that the αN radicals in these cases would have to result from (S∴N)⁺-bonded cyclic radical cations that arose as a result from direct contact between the ends of the peptides. Furthermore, by analogy with the rate constants for the formation of intramolecularly (S∴S)⁺-bonded radical cations in Met-(Pro)_n-Met peptides ( J. Phys. Chem. B 2016, 120, 9732), the rate constants for the formation of intramolecularly (S∴N)⁺-bonded radical cations are activated to the same extent for all of the γ-Glu-(Pro)_n-Met peptides. Thus, the continuous decrease of G(αN) with the number of Pro residues (from 0 to 3) suggests that the formation of a contact between the S-atom in the C-terminal Met residue and the N-atom of a deprotonated N-terminal amino group of Glu is controlled in peptides with 0 to 3 Pro residues by the relative diffusion of the S^•+ and unoxidized N-atom. The overall rate constants of cyclization to form the (S∴N)-bonded radical cations were estimated to be 3.8 × 10⁶, 1.8 × 10⁶, and 8.1 × 10⁵ s^-1 for peptides with n = 0, 1, and 2 Pro residues, respectively. If activation is the same for all of the peptides, then these rate constants are a direct indication for the end-to-end dynamics along the chain.

Unexpected Reaction Pathway of the Alpha-Aminoalkyl Radical Derived from One-Electron Oxidation of S-Alkylglutathiones

Laser flash photolysis and high-resolution mass spectrometry were used to investigate the mechanism of one-electron oxidation of two S-alkylglutathiones using 3-carboxybenzophenone (3CB) as a photosensitizer. This report indicates an unexpected reaction pathway of the α-aminoalkyl radical cation (αN⁺) derived from the oxidation of S-alkylglutathiones. Instead of a common hydrolysis reaction of αN⁺ reported earlier for methionine and other sulfur-containing aminoacids and peptides, an intramolecular ring-closure reaction was found for S-alkylglutathiones.

Sensitized photooxidation of s-methylglutathione in aqueous solution: intramolecular (S∴O) and (S∴N) bonded species

Nanosecond laser flash photolysis was used to generate sulfur radical cations of the thioether, S-methylglutathione (S-Me-Glu), via the one-electron oxidation of this thioether by triplet 4-carboxybenzophenone. The purpose of this investigation was to follow the neighboring group effects resulting from the interactions between the sulfur radical cationic sites and nearby lone-pair electrons on heteroatoms within the radical cation, especially the electron lone-pairs on heteroatoms in the peptide bonds. The tripeptide, S-Me-Glu, offers several possible competing neighboring group effects that are characterized in this work. Quantum yields of the various radicals and three-electron bonded (both intramolecular and intermolecular) species were determined. The pH dependence of photoinduced decarboxylation yields was used as evidence for the identification of a nine-membered ring, sulfur-nitrogen, three-electron bonded species. The mechanisms of the secondary reactions of the radicals and radical cations were characterized by resolving their overlapping transient-absorption spectra and following their kinetic behavior. In particular, sulfur-oxygen and sulfur-nitrogen three-electron bonded species were identified where the oxygen and nitrogen atoms were in the peptide bonds.

Diverse Effects of Glutathione and UPF Peptides on Antioxidant Defense System in Human Erythroleukemia Cells K562

The main goal of the present paper was to examine the influence of the replacement of γ-Glu moiety to α-Glu in glutathione and in its antioxidative tetrapeptidic analogue UPF1 (Tyr(Me)-γ-Glu-Cys-Gly), resulting in α-GSH and UPF17 (Tyr(Me)-Glu-Cys-Gly), on the antioxidative defense system in K562 cells. UPF1 and GSH increased while UPF17 and α-GSH decreased the activity of CuZnSOD in K562 cells, at peptide concentration of 10 μM by 42% and 38% or 35% and 24%, respectively. After three-hour incubation, UPF1 increased and UPF17 decreased the intracellular level of total GSH. Additionally, it was shown that UPF1 is not degraded by γ-glutamyltranspeptidase, which performs glutathione breakdown. These results indicate that effective antioxidative character of peptides does not depend only on the reactivity of the thiol group, but also of the other functional groups, and on the spatial structure of peptides.

Allosteric modulation of the calcium-sensing receptor by gamma-glutamyl peptides: inhibition of PTH secretion, suppression of intracellular cAMP levels, and a common mechanism of action with L-amino acids

γ-Glutamyl peptides were identified previously as novel positive allosteric modulators of Ca(2+)(o)-dependent intracellular Ca(2+) mobilization in HEK-293 cells that bind in the calcium-sensing receptor VFT domain. In the current study, we investigated whether γ-glutamyl-tripeptides including γ-Glu-Cys-Gly (glutathione) and its analogs S-methylglutathione and S-propylglutathione, or dipeptides including γ-Glu-Ala and γ-Glu-Cys are positive allosteric modulators of Ca(2+)(o)-dependent Ca(2+)(i) mobilization and PTH secretion from normal human parathyroid cells as well as Ca(2+)(o)-dependent suppression of intracellular cAMP levels in calcium-sensing receptor (CaR)-expressing HEK-293 cells. In addition, we compared the effects of the potent γ-glutamyl peptide S-methylglutathione, and the amino acid L-Phe on HEK-293 cells that stably expressed either the wild-type CaR or the double mutant T145A/S170T, which exhibits selectively impaired responses to L-amino acids. We find that γ-glutamyl peptides are potent positive allosteric modulators of the CaR that promote Ca(2+)(o)-dependent Ca(2+)(i) mobilization, suppress intracellular cAMP levels and inhibit PTH secretion from normal human parathyroid cells. Furthermore, we find that the double mutant T145A/S170T exhibits markedly impaired Ca(2+)(i) mobilization and cAMP suppression responses to S-methylglutathione as well as L-Phe indicating that γ-glutamyl peptides and L-amino acids activate the CaR via a common mechanism.

Sulfur radical cation-peptide bond complex in the one-electron oxidation of S-methylglutathione

Neighboring group participation was investigated in the *OH-induced oxidation of S-methylglutathione in aqueous solutions. Nanosecond pulse radiolysis was used to obtain the spectra of the reaction intermediates and their kinetics. Depending on the pH, and the concentration of S-methylglutathione, pulse irradiation leads to different transients. The transients observed were an intramolecularly bonded [>S thereforeNH2]+ intermediate, intermolecularly S thereforeS-bonded radical cation, alpha-(alkylthio)alkyl radicals, alpha-amino-alkyl-type radical, and an intramolecularly (S thereforeO)+-bonded intermediate. The latter radical is of particular note in that it supports recent observations of sulfur radical cations complexed with the oxygen atoms of peptide bonds and thus has biological and medical implications. This (S thereforeO)+-bonded intermediate had an absorption maximum at 390 nm, and we estimated its formation rate to be >or=6x10(7) s(-1). It is in equilibrium with the intermolecularly S thereforeS-bonded radical cation, and they decay together on the time scale of a few hundred microseconds. The S thereforeS-bonded radical cation is formed from the monomeric sulfur radical cation (>S*+) and an unoxidized S-methylglutathione molecule with the rate constant of 1.0x10(9) M(-1) s(-1). The short-lived [>S thereforeNH2]+ intermediate is a precursor of decarboxylation, absorbs at approximately 390 nm, and decays on the time scale of hundreds of nanoseconds. Additional insight into the details of the association of sulfur radical cations with the oxygen atoms of the peptide bonds was gained by comparing the behavior of the S-methylglutathione (S thereforeO+-bonded five-membered ring) with the peptide gamma-Glu-Met-Gly (S thereforeO+-bonded six-membered ring). Conclusions from experimental observations were supported by molecular modeling calculations.

Intracerebroventricular injection of glutathione and its derivative induces sedative and hypnotic effects under an acute stress in neonatal chicks

Glutathione-related enzymes glyoxalase 1 and glutathione reductase 1 regulates anxiety in mice. To clarify the central function of glutathione as a neurotransmitter in the stress reaction, the effect of intracerebroventricular (i.c.v.) injection of reduced (GSH) (0.5, 1, 2 micromol) and oxidized (GSSG) glutathione (0.25, 0.5, 1 micromol) were investigated under an isolation-induced stress in the neonatal chick. Both GSH and GSSG dose-dependently decreased distress vocalizations and induced sleep-like behavior in chicks under acute stressful conditions. However, which glutathione is actually responsible for inducing sleep was unclear since glutathione cycles between GSH and GSSG in which two tripeptides are linked by a disulfide bond. Therefore, the behavior of chicks was monitored following the i.c.v. injection of S-methylglutathione (SMG) (0.0625, 0.25, 1 micromol). SMG does not form a disulfide bond due to the methylation of the SH group of the cysteine moiety. SMG had similar effects as observed in GSH and GSSG. In conclusion, glutathione and its derivative have sedative and hypnotic effects, and might be effective in improving psychic stress such as anxiety.

Interference of S-nitrosoglutathione with the binding of ligands to ionotropic glutamate receptors in pig cerebral cortical synaptic membranes

The interactions of S-nitrosoglutathione (GSNO) with the ionotropic glutamate receptors were studied on synaptic membranes isolated from the pig cerebral cortex. GSNO displaced the binding of [3H]glutamate, 3-[(R)-2-carboxypiperazin-4-yl] [3H]propyl-1-phosphonate ([3H]CPP), a competitive N-methyl-D-aspartate (NMDA) antagonist, and [3H]kainate, with IC50 values in the low micromolar range. It failed to displace (S)-5-fluoro-[3H]willardiine, a selective agonist of 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors. Reduced and oxidized glutathione were almost as effective as GSNO in glutamate and CPP binding. Of the three, GSNO was the most potent in kainate binding. They all stimulated [3H]dizocilpine binding in a concentration-dependent manner. This effect was additive to that of glycine and not mimicked by NO donors such as S-nitroso-N-acetylpenicillamine, 5-amino-3-morpholinyl-1,2,3-oxadiazolium chloride (SIN-1) and nitroglycerin. We assume that GSNO may act as an endogenous ligand at the NMDA and non-NMDA classes of glutamate receptors. In this manner it may facilitate NO transfer and target its delivery to specific sites in these receptors.

Specific glutathione binding sites in pig cerebral cortical synaptic membranes

Glutathione (gamma-glutamylcysteinylglycine) is a neuromodulator at glutamate receptors, but may also act as a neurotransmitter at sites of its own. The Na+-independent binding of [3H]glutathione to pig cortical synaptic membranes was characterized here using glycine, cysteine analogs, dipeptides and glutathione derivatives, and ligands selective for known glutamate receptors. L-Glutamate, pyroglutamate, quinolinate, (S)-5-fluorowillardiine and 6-nitro-7-sulfamoylbenzo[f]quinoxaline-2,3-dione were weak inhibitors at concentrations of 0.5 or 1 mM. D-Glutamate, L- and D-aspartate, glutamine, quisqualate, kynurenate, other N-methyl-D-aspartate receptor ligands and non-N-methyl-D-aspartate receptor ligands failed to displace [3H]glutathione. Except for weak inhibition by D-serine (0.5 mM), glycine and other ligands of the glycine co-activatory site in the N-methyl-D-aspartate receptors had no displacing effect. Similarly, metabotropic glutamate group I, II and III receptor agonists and antagonists and compounds acting at the glutamate uptake sites were generally inactive. Glutathione, oxidized glutathione, S-nitrosoglutathione, gamma-L-glutamylcysteine, cysteinylglycine, cysteine, cysteamine and cystamine were the most potent displacers (IC50 values in the micromolar range), followed by dithiothreitol, glutathione sulfonate and the S-alkyl derivatives of glutathione (S-methyl-, -ethyl-, -propyl-, -butyl- and -pentylglutathione). L-Homocysteinate and aminomethanesulfonate exhibited a moderate efficacy. Thiokynurenate, a cysteine analog and an antagonist at the N-methyl-D-aspartate receptor glycine co-activatory site, was a potent activator of glutathione binding. At 1 mM, some dipeptides also slightly activated the binding, gamma-L-glutamylleucine and gamma-L-glutamyl-GABA being the most effective. The specific binding sites for glutathione in brain synaptic membranes are not identical to any known excitatory amino acid receptor. The cysteinyl moiety is crucial in the binding of glutathione. The oxidation or alkylation of the cysteine thiol group reduces the binding affinity. The strong activation by thiokynurenate may indicate that the glutathione receptor protein contains a modulatory site to which co-agonists may bind and allosterically activate glutathione binding. The novel population of specific binding sites of glutathione gives rise to the possibility that they may have profound effects on synaptic functions in the mammalian central nervous system. The glutathione binding sites may be an important, and for the most part unrecognized, component in signal transduction in the brain.

Glutathione and signal transduction in the mammalian CNS

The tripeptide glutathione (GSH) has been thoroughly investigated in relation to its role as antioxidant and free radical scavenger. In recent years, novel actions of GSH in the nervous system have also been described, suggesting that GSH may serve additionally both as a neuromodulator and as a neurotransmitter. In the present article, we describe our studies to explore further a potential role of GSH as neuromodulator/neurotransmitter. These studies have used a combination of methods, including radioligand binding, synaptic release and uptake assays, and electrophysiological recording. We report here the characteristics of GSH binding sites, the interrelationship of GSH with the NMDA receptor, and the effects of GSH on neural activity. Our results demonstrate that GSH binds via its gamma-glutamyl moiety to ionotropic glutamate receptors. At micromolar concentrations GSH displaces excitatory agonists, acting to halt their physiological actions on target neurons. At millimolar concentrations, GSH, acting through its free cysteinyl thiol group, modulates the redox site of NMDA receptors. As such modulation has been shown to increase NMDA receptor channel currents, this action may play a significant role in normal and abnormal synaptic activity. In addition, GSH in the nanomolar to micromolar range binds to at least two populations of binding sites that appear to be distinct from all known excitatory amino acid receptor subtypes. GSH bound to these sites is not displaceable by glutamatergic agonists or antagonists. These binding sites, which we believe to be distinct receptor populations, appear to recognize the cysteinyl moiety of the GSH molecule. Like NMDA receptors, the GSH binding sites possess a coagonist site(s) for allosteric modulation. Furthermore, they appear to be linked to sodium ionophores, an interpretation supported by field potential recordings in rat cerebral cortex that reveal a dose-dependent depolarization to applied GSH that is blocked by the absence of sodium but not by lowering calcium or by NMDA or (S)-2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate antagonists. The present data support a reevaluation of the role of GSH in the nervous system in which GSH may be involved both directly and indirectly in synaptic transmission. A full accounting of the actions of GSH may lead to more comprehensive understanding of synaptic function in normal and disease states.