Radiation chemical studies of Gly-Met-Gly in aqueous solution

Influence of plasma halide, pseudohalide and nitrite ions on myeloperoxidase-mediated protein and extracellular matrix damage

Myeloperoxidase (MPO) mediates pathogen destruction by generating the bactericidal oxidant hypochlorous acid (HOCl). Formation of this oxidant is however associated with host tissue damage and disease. MPO also utilizes H₂O₂ to oxidize other substrates, and we hypothesized that mixtures of other plasma anions, including bromide (Br^-), iodide (I^-), thiocyanate (SCN^-) and nitrite (NO₂^-), at normal or supplemented concentrations, might modulate MPO-mediated HOCl damage. For the (pseudo)halide anions, only SCN^- significantly modulated HOCl formation (IC₅₀ ∼33 μM), which is within the normal physiological range, as judged by damage to human plasma fibronectin or extracellular matrix preparations detected by ELISA and LC-MS. NO₂^- modulated HOCl-mediated damage, in a dose-dependent manner, at physiologically-attainable anion concentrations. However, this was accompanied by increased tyrosine and tryptophan nitration (detected by ELISA and LC-MS), and the overall extent of damage remained approximately constant. Increasing NO₂^- concentrations (0.5-20 μM) diminished HOCl-mediated modification of tyrosine and methionine, whereas tryptophan loss was enhanced. At higher NO₂^- concentrations, enhanced tyrosine and methionine loss was detected. These analytical data were confirmed in studies of cell adhesion and metabolic activity. Together, these data indicate that endogenous plasma levels of SCN^- (but not Br^- or I^-) can modulate protein modification induced by MPO, including the extent of chlorination. In contrast, NO₂^- alters the type of modification, but does not markedly decrease its extent, with chlorination replaced by nitration. These data also indicate that MPO could be a major source of nitration in vivo, and particularly at inflammatory sites where NO₂^- levels are often elevated.

Evaluation of Hydroxyl Radical Reactivity by Thioether Group Proximity in Model Peptide Backbone: Methionine versus S-Methyl-Cysteine

Hydroxyl radicals (HO^•) have long been regarded as a major source of cellular damage. The reaction of HO^• with methionine residues (Met) in peptides and proteins is a complex multistep process. Although the reaction mechanism has been intensively studied, some essential parts remain unsolved. In the present study we examined the reaction of HO^• generated by ionizing radiation in aqueous solutions under anoxic conditions with two compounds representing the simplest model peptide backbone CH₃C(O)NHCHXC(O)NHCH₃, where X = CH₂CH₂SCH₃ or CH₂SCH₃, i.e., the Met derivative in comparison with the cysteine-methylated derivative. We performed the identification and quantification of transient species by pulse radiolysis and final products by LC-MS and high-resolution MS/MS after γ-radiolysis. The results allowed us to draw for each compound a mechanistic scheme. The fate of the initial one-electron oxidation at the sulfur atom depends on its distance from the peptide backbone and involves transient species of five-membered and/or six-membered ring formations with different heteroatoms present in the backbone as well as quite different rates of deprotonation in forming α-(alkylthio)alkyl radicals.

Photo- and Radiation-Induced One-Electron Oxidation of Methionine in Various Structural Environments Studied by Time-Resolved Techniques

Oxidation of methionine (Met) is an important reaction that plays a key role in protein modifications during oxidative stress and aging. The first steps of Met oxidation involve the creation of very reactive and short-lived transients. Application of complementary time-resolved radiation and photochemical techniques (pulse radiolysis and laser flash photolysis together with time-resolved CIDNP and ESR techniques) allowed comparing in detail the one-electron oxidation mechanisms initiated either by ^●OH radicals and other one-electron oxidants or the excited triplet state of the sensitizers e.g., 4-,3-carboxybenzophenones. The main purpose of this review is to present various factors that influence the character of the forming intermediates. They are divided into two parts: those inextricably related to the structures of molecules containing Met and those related to external factors. The former include (i) the protection of terminal amine and carboxyl groups, (ii) the location of Met in the peptide molecule, (iii) the character of neighboring amino acid other than Met, (iv) the character of the peptide chain (open vs cyclic), (v) the number of Met residues in peptide and protein, and (vi) the optical isomerism of Met residues. External factors include the type of the oxidant, pH, and concentration of Met-containing compounds in the reaction environment. Particular attention is given to the neighboring group participation, which is an essential parameter controlling one-electron oxidation of Met. Mechanistic aspects of oxidation processes by various one-electron oxidants in various structural and pH environments are summarized and discussed. The importance of these studies for understanding oxidation of Met in real biological systems is also addressed.

Reductive Stress of Sulfur-Containing Amino Acids within Proteins and Implication of Tandem Protein-Lipid Damage

Reductive radical stress represents the other side of the redox spectrum, less studied but equally important compared to oxidative stress. The reactivity of hydrogen atoms (H^•) and hydrated electrons (e^-_aq) connected with peptides/proteins is summarized, focusing on the chemical transformations of methionine (Met) and cystine (CysS-SCys) residues into α-aminobutyric acid and alanine, respectively. Chemical and mechanistic aspects of desulfurization processes with formation of diffusible sulfur-centered radicals, such as methanethiyl (CH₃S^•) and sulfhydryl (HS^•) radicals, are discussed. These findings are further applied to biomimetic radical chemistry, modeling the occurrence of tandem protein-lipid damages in proteo-liposomes and demonstrating that generation of sulfur-centered radicals from a variety of proteins is coupled with the cis-trans isomerization of unsaturated lipids in membranes. Recent applications to pharmaceutical and pharmacological contexts are described, evidencing novel perspectives in the stability of formulations and mode of action of drugs, respectively.

Radiation- and Photo-Induced Oxidation Pathways of Methionine in Model Peptide Backbone under Anoxic Conditions

Within the reactive oxygen species (ROS) generated by cellular metabolisms, hydroxyl radicals (HO^•) play an important role, being the most aggressive towards biomolecules. The reactions of HO^• with methionine residues (Met) in peptides and proteins have been intensively studied, but some fundamental aspects remain unsolved. In the present study we examined the biomimetic model made of Ac-Met-OMe, as the simplest model peptide backbone, and of HO^• generated by ionizing radiation in aqueous solutions under anoxic conditions. We performed the identification and quantification of transient species by pulse radiolysis and of final products by LC-MS and high-resolution MS/MS after γ-radiolysis. By parallel photochemical experiments, using 3-carboxybenzophenone (CB) triplet with the model peptide, we compared the outcomes in terms of short-lived intermediates and stable product identification. The result is a detailed mechanistic scheme of Met oxidation by HO^•, and by CB triplets allowed for assigning transient species to the pathways of products formation.

Early Events of Photosensitized Oxidation of Sulfur-Containing Amino Acids Studied by Laser Flash Photolysis and Mass Spectrometry

The mechanism of photooxidation of methionine (N-Ac-Met-NH-CH₃, 1) and methyl-cysteine (N-Ac-MeCys-NH-CH₃, 2) analogues by 3-carboxybenzophenone triplet (3CB*) in neutral aqueous solution was studied using techniques of nanosecond laser flash photolysis and steady-state photolysis. The short-lived transients derived from 3CB and sulfur-containing amino acids were identified, and their quantum yields and kinetics of formation and decay were determined. The stable photoproducts were analyzed using liquid chromatography coupled with high-resolution mass spectrometry. Substantial differences in the mechanisms were found for methionine and S-methyl-cysteine analogues for both primary and secondary photoreactions. A new secondary reaction channel (back hydrogen atom transfer from the ketyl radical to the carbon-centered α-thioalkyl radical yielding reactants in the ground states) was suggested. The detailed mechanisms of 3CB* sensitized photooxidation of 1 and 2 are proposed and discussed.

Chlorination and oxidation of the extracellular matrix protein laminin and basement membrane extracts by hypochlorous acid and myeloperoxidase

Basement membranes are specialized extracellular matrices that underlie arterial wall endothelial cells, with laminin being a key structural and biologically-active component. Hypochlorous acid (HOCl), a potent oxidizing and chlorinating agent, is formed in vivo at sites of inflammation via the enzymatic action of myeloperoxidase (MPO), released by activated leukocytes. Considerable data supports a role for MPO-derived oxidants in cardiovascular disease and particularly atherosclerosis. These effects may be mediated via extracellular matrix damage to which MPO binds. Herein we detect and quantify sites of oxidation and chlorination on isolated laminin-111, and laminin in basement membrane extracts (BME), by use of mass spectrometry. Increased modification was detected with increasing oxidant exposure. Mass mapping indicated selectivity in the sites and extent of damage; Met residues were most heavily modified. Fewer modifications were detected with BME, possibly due to the shielding effects. HOCl oxidised 30 (of 56 total) Met and 7 (of 24) Trp residues, and chlorinated 33 (of 99) Tyr residues; 3 Tyr were dichlorinated. An additional 8 Met and 10 Trp oxidations, 14 chlorinations, and 18 dichlorinations were detected with the MPO/H₂O₂/Cl^- system when compared to reagent HOCl. Interestingly, chlorination was detected at Tyr₂₄₁₅ in the integrin-binding region; this may decrease cellular adhesion. Co-localization of MPO-damaged epitopes and laminin was detected in human atherosclerotic lesions. These data indicate that laminin is extensively modified by MPO-derived oxidants, with structural and functional changes. These modifications, and compromised cell-matrix interactions, may promote endothelial cell dysfunction, weaken the structure of atherosclerotic lesions, and enhance lesion rupture.

Structural and functional changes in RNAse A originating from tyrosine and histidine cross-linking and oxidation induced by singlet oxygen and peroxyl radicals

Oxidation can be induced by multiple processes in biological samples, with proteins being important targets due to their high abundance and reactivity. Oxidant reactions with proteins are not comprehensively understood, but it is known that structural and functional changes may be a cause, or a consequence, of disease. The mechanisms of oxidation of the model protein RNAse A by singlet oxygen (¹O₂) were examined and compared to peroxyl radical (ROO^•) oxidation, both common biological oxidants. This protein is a prototypic member of the RNAse family that exhibits antiviral activity by cleaving single-stranded RNA. RNAse A lacks tryptophan and cysteine residues which are major oxidant targets, but contains multiple histidine, tyrosine and methionine residues; these were therefore hypothesized to be the major sites of damage. ¹O₂ and ROO^• induce different patterns and extents of damage; both induce cross-links and side-chain oxidation, and ¹O₂ exposure modulates enzymatic activity. Multiple products have been characterized including methionine sulfoxide and sulfone, alcohols, DOPA, 2-oxohistidine, histidine-derived ring-opened species and inter- and intra-molecular cross-links (di-tyrosine, histidine-lysine, histidine-arginine, tyrosine-lysine). In addition to methionine modification, which appears not to be causative to activity loss, singlet oxygen also induces alteration to specific histidine, tyrosine and proline residues, including modification and cross-linking of the active site histidine, His12. The high homology among the RNAse family suggests that similar modifications may occur in humans, and be associated with the increased risk of viral infections in people with diabetes, as markers for ¹O₂ have been found in early stages of this pathology.

Peroxyl radical- and photo-oxidation of glucose 6-phosphate dehydrogenase generates cross-links and functional changes via oxidation of tyrosine and tryptophan residues

Protein oxidation is a frequent event as a result of the high abundance of proteins in biological samples and the multiple processes that generate oxidants. The reactions that occur are complex and poorly understood, but can generate major structural and functional changes on proteins. Current data indicate that pathophysiological processes and multiple human diseases are associated with the accumulation of damaged proteins. In this study we investigated the mechanisms and consequences of exposure of the key metabolic enzyme glucose-6-phosphate dehydrogenase (G6PDH) to peroxyl radicals (ROO^•) and singlet oxygen (¹O₂), with particular emphasis on the role of Trp and Tyr residues in protein cross-linking and fragmentation. Cross-links and high molecular mass aggregates were detected by SDS-PAGE and Western blotting using specific antibodies. Amino acid analysis has provided evidence for Trp and Tyr consumption and formation of oxygenated products (diols, peroxides, N-formylkynurenine, kynurenine) from Trp, and di-tyrosine (from Tyr). Mass spectrometric data obtained after trypsin-digestion in the presence of H₂¹⁶O and H₂¹⁸O, has allowed the mapping of specific cross-linked residues and their locations. These data indicate that specific Tyr-Trp and di-Tyr cross-links are formed from residues that are proximal and surface-accessible, and that the extent of Trp oxidation varies markedly between sites. Limited modification at other residues is also detected. These data indicate that Trp and Tyr residues are readily modified by ROO^• and ¹O₂ with this giving products that impact significantly on protein structure and function. The formation of such cross-links may help rationalize the accumulation of damaged proteins in vivo.