Direct demonstration of electron transfer between tryptophan and tyrosine in proteins

Influence of Ionizing Radiation on Spontaneously Formed Aggregates in Proteins or Enzymes Solutions

We have shown that many proteins and enzymes (ovalbumin, β-lactoglobulin, lysozyme, insulin, histone, papain) undergo concentration-dependent reversible aggregation as a result of the interaction of the studied biomolecules. Moreover, irradiation of those protein or enzyme solutions under oxidative stress conditions results in the formation of stable soluble protein aggregates. We assume that protein dimers are mainly formed. A pulse radiolysis study has been made to investigate the early stages of protein oxidation by N3• or ^•OH radicals. Reactions of the N3• radical with the studied proteins lead to the generation of aggregates stabilized by covalent bonds between tyrosine residues. The high reactivity of the ^•OH with amino acids contained within proteins is responsible for the formation of various covalent bonds (including C-C or C-O-C) between adjacent protein molecules. In the analysis of the formation of protein aggregates, intramolecular electron transfer from the tyrosine moiety to Trp^• radical should be taken into account. Steady-state spectroscopic measurements with a detection of emission and absorbance, together with measurements of the dynamic scattering of laser light, made it possible to characterize the obtained aggregates. The identification of protein nanostructures generated by ionizing radiation using spectroscopic methods is difficult due to the spontaneous formation of protein aggregates before irradiation. The commonly used fluorescence detection of dityrosyl cross-linking (DT) as a marker of protein modification under the influence of ionizing radiation requires modification in the case of the tested objects. A precise photochemical lifetime measurement of the excited states of radiation-generated aggregates is useful in characterizing their structure. Resonance light scattering (RLS) has proven to be an extremely sensitive and useful technique to detect protein aggregates.

Radiolysis Studies of Oxidation and Nitration of Tyrosine and Some Other Biological Targets by Peroxynitrite-Derived Radicals

The widespread interest in free radicals in biology extends far beyond the effects of ionizing radiation, with recent attention largely focusing on reactions of free radicals derived from peroxynitrite (i.e., hydroxyl, nitrogen dioxide, and carbonate radicals). These radicals can easily be generated individually by reactions of radiolytically-produced radicals in aqueous solutions and their reactions can be monitored either in real time or by analysis of products. This review first describes the general principles of selective radical generation by radiolysis, the yields of individual species, the advantages and limitations of either pulsed or continuous radiolysis, and the quantitation of oxidizing power of radicals by electrode potentials. Some key reactions of peroxynitrite-derived radicals with potential biological targets are then discussed, including the characterization of reactions of tyrosine with a model alkoxyl radical, reactions of tyrosyl radicals with nitric oxide, and routes to nitrotyrosine formation. This is followed by a brief outline of studies involving the reactions of peroxynitrite-derived radicals with lipoic acid/dihydrolipoic acid, hydrogen sulphide, and the metal chelator desferrioxamine. For biological diagnostic probes such as ‘spin traps’ to be used with confidence, their reactivities with radical species have to be characterized, and the application of radiolysis methods in this context is also illustrated.

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.

Initiation and Prevention of Biological Damage by Radiation-Generated Protein Radicals

Ionizing radiations cause chemical damage to proteins. In aerobic aqueous solutions, the damage is commonly mediated by the hydroxyl free radicals generated from water, resulting in formation of protein radicals. Protein damage is especially significant in biological systems, because proteins are the most abundant targets of the radiation-generated radicals, the hydroxyl radical-protein reaction is fast, and the damage usually results in loss of their biological function. Under physiological conditions, proteins are initially oxidized to carbon-centered radicals, which can propagate the damage to other molecules. The most effective endogenous antioxidants, ascorbate, GSH, and urate, are unable to prevent all of the damage under the common condition of oxidative stress. In a promising development, recent work demonstrates the potential of polyphenols, their metabolites, and other aromatic compounds to repair protein radicals by the fast formation of less damaging radical adducts, thus potentially preventing the formation of a cascade of new reactive species.

Spectral Probe for Electron Transfer and Addition Reactions of Azide Radicals with Substituted Quinoxalin-2-Ones in Aqueous Solutions

The azide radical (N₃^●) is one of the most important one-electron oxidants used extensively in radiation chemistry studies involving molecules of biological significance. Generally, it was assumed that N₃^● reacts in aqueous solutions only by electron transfer. However, there were several reports indicating the possibility of N₃^● addition in aqueous solutions to organic compounds containing double bonds. The main purpose of this study was to find an experimental approach that allows a clear assignment of the nature of obtained products either to its one-electron oxidation or its addition products. Radiolysis of water provides a convenient source of one-electron oxidizing radicals characterized by a very broad range of reduction potentials. Two inorganic radicals (SO₄^●-, CO₃^●-) and Tl²⁺ ions with the reduction potentials higher, and one radical (SCN)₂^●- with the reduction potential slightly lower than the reduction potential of N₃^● were selected as dominant electron-acceptors. Transient absorption spectra formed in their reactions with a series of quinoxalin-2-one derivatives were confronted with absorption spectra formed from reactions of N₃^● with the same series of compounds. Cases, in which the absorption spectra formed in reactions involving N₃^● differ from the absorption spectra formed in the reactions involving other one-electron oxidants, strongly indicate that N₃^● is involved in the other reaction channel such as addition to double bonds. Moreover, it was shown that high-rate constants of reactions of N₃^● with quinoxalin-2-ones do not ultimately prove that they are electron transfer reactions. The optimized structures of the radical cations (7-R-3-MeQ)^●+, radicals (7-R-3-MeQ)^● and N₃^● adducts at the C2 carbon atom in pyrazine moiety and their absorption spectra are reasonably well reproduced by density functional theory quantum mechanics calculations employing the ωB97XD functional combined with the Dunning's aug-cc-pVTZ correlation-consistent polarized basis sets augmented with diffuse functions.

Oxidation of ethidium-based probes by biological radicals: mechanism, kinetics and implications for the detection of superoxide

Hydroethidine (HE) and hydropropidine (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {HPr}^{+}$$\end{document}HPr+) are fluorogenic probes used for the detection of the intra- and extracellular superoxide radical anion (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {O}_{ {2}}^{\bullet -}$$\end{document}O2∙-). In this study, we provide evidence that HE and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {HPr}^{+}$$\end{document}HPr+ react rapidly with the biologically relevant radicals, including the hydroxyl radical, peroxyl radicals, the trioxidocarbonate radical anion, nitrogen dioxide, and the glutathionyl radical, via one-electron oxidation, forming the corresponding radical cations. At physiological pH, the radical cations of the probes react rapidly with \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {O}_{ {2}}^{\bullet -}$$\end{document}O2∙-, leading to the specific 2-hydroxylated cationic products. We determined the rate constants of the reaction between \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {O}_{ {2}}^{\bullet -}$$\end{document}O2∙- and the radical cations of the probes. We also synthesized N-methylated analogs of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {HPr}^{+}$$\end{document}HPr+ and HE which were used in mechanistic studies. Methylation of the amine groups was not found to prevent the reaction between the radical cation of the probe and the superoxide, but it significantly increased the lifetime of the radical cation and had a substantial effect on the profiles of the oxidation products by inhibiting the formation of dimeric products. We conclude that the N-methylated analogs of HE and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {HPr}^{+}$$\end{document}HPr+ may be used as a scaffold for the design of a new generation of probes for intra- and extracellular superoxide.

Superoxide radicals react with peptide-derived tryptophan radicals with very high rate constants to give hydroperoxides as major products

Oxidative damage is a common process in many biological systems and proteins are major targets for damage due to their high abundance and very high rate constants for reaction with many oxidants (both radicals and two-electron species). Tryptophan (Trp) residues on peptides and proteins are a major sink for a large range of biological oxidants as these side-chains have low radical reduction potentials. The resulting Trp-derived indolyl radicals (Trp^•) have long lifetimes in some circumstances due to their delocalized structures, and undergo only slow reaction with molecular oxygen, unlike most other biological radicals. In contrast, we have shown previously that Trp^• undergo rapid dimerization. In the current study, we show that Trp^• also undergo very fast reaction with superoxide radicals, O₂^•-, with k 1-2 × 10⁹ M^-1 s^-1. These values do not alter dramatically with peptide structure, but the values of k correlate with overall peptide positive charge, consistent with positive electrostatic interactions. These reactions compete favourably with Trp^• dimerization and O₂ addition, indicating that this may be a major fate in some circumstances. The Trp^• + O₂^•- reactions occur primarily by addition, rather than electron transfer, with this resulting in high yields of Trp-derived hydroperoxides. Subsequent degradation of these species, both stimulated and native decay, gives rise to N-formylkynurenine, kynurenine, alcohols and diols. These data indicate that reaction of O₂^•- with Trp^• should be considered as a major pathway to Trp degradation on peptides and proteins subjected to oxidative damage.

Physiological Concentrations of Ascorbate Cannot Prevent the Potentially Damaging Reactions of Protein Radicals in Humans

The principal initial biological targets of free radicals formed under conditions of oxidative stress are the proteins. The most common products of the interaction are carbon-centered alkyl radicals which react rapidly with oxygen to form peroxyl radicals and hydroperoxides. All these species are reactive, capable of propagating the free radical damage to enzymes, nucleic acids, lipids, and endogenous antioxidants, leading finally to the pathologies associated with oxidative stress. The best chance of preventing this chain of damage is in early repair of the protein radicals by antioxidants. Estimate of the effectiveness of the physiologically significant antioxidants requires knowledge of the antioxidant tissue concentrations and rate constants of their reaction with protein radicals. Previous studies by pulse radiolysis have shown that only ascorbate can repair the Trp and Tyr protein radicals before they form peroxyl radicals under physiological concentrations of oxygen. We have now extended this work to other protein C-centered radicals generated by hydroxyl radicals because these and many other free radicals formed under oxidative stress can produce secondary radicals on virtually any amino acid residue. Pulse radiolysis identified two classes of rate constants for reactions of protein radicals with ascorbate, a faster one in the range (9-60) × 10⁷ M^-1 s^-1 and a slow one with a range of (0.5-2) × 10⁷ M^-1 s^-1. These results show that ascorbate can prevent further reactions of protein radicals only in the few human tissues where its concentration exceeds about 2.5 mM.

Oxidative stress, free radicals and protein peroxides

Primary free radicals generated under oxidative stress in cells and tissues produce a cascade of reactive secondary radicals, which attack biomolecules with efficiency determined by the reaction rate constants and target concentration. Proteins are prominent targets because they constitute the bulk of the organic content of cells and tissues and react readily with many of the secondary radicals. The reactions commonly lead to the formation of carbon-centered radicals, which generally convert in vivo to peroxyl radicals and finally to semistable hydroperoxides. All of these intermediates can initiate biological damage. This article outlines the advantages of the application of ionizing radiations to studies of radicals, with particular reference to the generation of desired radicals, studies of the kinetics of their reactions and correlating the results with events in biological systems. In one such application, formation of protein hydroperoxides in irradiated cells was inhibited by the intracellular ascorbate and glutathione.

Production of lysozyme and lysozyme-superoxide dismutase dimers bound by a ditryptophan cross-link in carbonate radical-treated lysozyme

Despite extensive investigation of the irreversible oxidations undergone by proteins in vitro and in vivo, the products formed from the oxidation of Trp residues remain incompletely understood. Recently, we characterized a ditryptophan cross-link produced by the recombination of hSOD1-tryptophanyl radicals generated from attack of the carbonate radical produced during the bicarbonate-dependent peroxidase activity of the enzyme. Here, we examine whether the ditryptophan cross-link is produced by the attack of the carbonate radical on proteins other than hSOD1. To this end, we treated hen egg white lysozyme with photolytically and enzymatically generated carbonate radical. The radical yields were estimated and the lysozyme modifications were analyzed by SDS-PAGE, western blot, enzymatic activity and MS/MS analysis. Lysozyme oxidation by both systems resulted in its inactivation and dimerization. Lysozyme treated with the photolytic system presented monomers oxidized to hydroxy-tryptophan at Trp(28) and Trp(123) and N-formylkynurenine at Trp(28), Trp(62) and Trp(123). Lysozyme treated with the enzymatic system rendered monomers oxidized to N-formylkynurenine at Trp(28). The dimers were characterized as lysozyme-Trp(28)-Trp(28)-lysozyme and lysozyme-Trp(28)-Trp(32)-hSOD1. The results further demonstrate that the carbonate radical is prone to causing biomolecule cross-linking and hence, may be a relevant player in pathological mechanisms. The possibility of exploring the formation of ditryptophan cross-links as a carbonate radical biomarker is discussed.

Rapid reaction of superoxide with insulin-tyrosyl radicals to generate a hydroperoxide with subsequent glutathione addition

Tyrosine (Tyr) residues are major sites of radical generation during protein oxidation. We used insulin as a model to study the kinetics, mechanisms, and products of the reactions of radiation-induced or enzyme-generated protein-tyrosyl radicals with superoxide to demonstrate the feasibility of these reactions under oxidative stress conditions. We found that insulin-tyrosyl radicals combined to form dimers, mostly via the tyrosine at position 14 on the α chain (Tyr14). However, in the presence of superoxide, dimerization was largely outcompeted by the reaction of superoxide with insulin-tyrosyl radicals. Using pulse radiolysis, we measured a second-order rate constant for the latter reaction of (6±1) × 10(8) M(-1) s(-1) at pH 7.3, representing the first measured rate constant for a protein-tyrosyl radical with superoxide. Mass-spectrometry-based product analyses revealed the addition of superoxide to the insulin-Tyr14 radical to form the hydroperoxide. Glutathione efficiently reduced the hydroperoxide to the corresponding monoxide and also subsequently underwent Michael addition to the monoxide to give a diglutathionylated protein adduct. Although much slower, conjugation of the backbone amide group can form a bicyclic Tyr-monoxide derivative, allowing the addition of only one glutathione molecule. These findings suggest that Tyr-hydroperoxides should readily form on proteins under oxidative stress conditions where protein radicals and superoxide are both generated and that these should form addition products with thiol compounds such as glutathione.

Fast repair of protein radicals by urate

The repair of tryptophan and tyrosine radicals in proteins by urate was studied by pulse radiolysis. In chymotrypsin, urate repairs tryptophan radicals efficiently with a rate constant of 2.7 × 10(8)M(-1)s(-1), ca. 14 times higher than the rate constant derived for N-acetyltryptophan amide, 1.9 × 10(7)M(-1)s(-1). In contrast, no repair of tryptophan radicals was observed in pepsin, which indicates a rate constant smaller than 6 × 10(7)M(-1)s(-1). Urate repairs tyrosine radicals in pepsin with a rate constant of 3 × 10(8)M(-1)s(-1)-ca. 12 times smaller than the rate constant reported for free tyrosine-but not in chymotrypsin, which implies an upper limit of 1 × 10(6)M(-1)s(-1) for the corresponding rate constant. Intra- and intermolecular electron transfer from tyrosine residues to tryptophan radicals is observed in both proteins, however, to different extents and with different rate constants. Urate inhibits electron transfer in chymotrypsin but not in pepsin. Our results suggest that urate repairs the first step on the long path to protein modification and prevents damage in vivo. It may prove to be a very important repair agent in tissue compartments where its concentration is higher than that of ascorbate. The product of such repair, the urate radical, can be reduced by ascorbate. Loss of ascorbate is then expected to be the net result, whereas urate is conserved.

Charge transport in vertically aligned, self-assembled peptide nanotube junctions

The self-assembly propensity of peptides has been extensively utilized in recent years for the formation of supramolecular nanostructures. In particular, the self-assembly of peptides into fibrils and nanotubes makes them promising building blocks for electronic and electro-optic applications. However, the mechanisms of charge transfer in these wire-like structures, especially in ambient conditions, are not yet fully understood. We describe here a layer-by-layer deposition methodology of short self-assembled cyclic peptide nanotubes, which results in vertically oriented nanotubes on gold substrates. Using this novel deposition methodology, we have fabricated molecular junctions with a conductive atomic force microscopy tip as a second electrode. Studies of the junctions' current-voltage characteristics as a function of the nanotube length revealed an efficient charge transfer in these supramolecular structures, with a low current attenuation constant of 0.1 Å(-1), which indicate that electron transfer is dominated by hopping. Moreover, the threshold voltage to field-emission dominated transport was found to increase with peptide length in a manner that depends on the nature of the contact with the electrodes. The flexibility in the design of the peptide monomers and the ability to control their sequential order over the nanotube by means of the layer-by-layer assembly process, which is demonstrated in this work, can be used to engineer the electronic properties of self-assembled peptide nanotubes toward device applications.

Formation of N-N cross-links in DNA by reaction of radiation-produced DNA base pair diradicals: a DFT study

This study employs DFT (density functional theory) to investigate the formation of hydrazine-like (N-N) cross-linked structures between DNA base pair diradicals that are likely to result from the interaction of high linear energy transfer (LET) radiation, such as ion-beam radiation, with DNA. In our calculations, we generated the guanine (G), cytosine (C), adenine (A), and thymine (T) radicals by removing one hydrogen atom from an N-H bond involved in the normal base pairing. The radical species formed are those that naturally result from one-electron oxidation of the bases followed by deprotonation. N-N cross-links between G and C or A and T diradicals were studied using the BHandHLYP, B3LYP, M06, and M06-2X density functionals and 6-31G* basis set. From a comparison to several test cases performed with the G3B3 method, which gives thermodynamically reliable values, we found that calculations employing the BHandHLYP/6-31G* method predict the best estimates of bonding energies for hydrazine-like structures. Our study shows that the N-N cross-link formed between guanine radical and a neutral cytosine is endothermic in nature but can form metastable structures. However, the reactions between two DNA base radicals (diradical) to form several N-N cross-linked structures are found to be highly exothermic in nature. The N-N cross-links formed between various G-C, G-G, and C-C diradicals have binding energies in the range of ca. -54 to -68, -41 to -47, and -67 to -75 kcal/mol, respectively, whereas A-T, A-A, and T-T have binding energies of -80, -60, and -98 kcal/mol, respectively. In all purine-pyrimidine N-N cross-linked structures, the highest occupied molecular orbital (HOMO) is found to be localized on the purine moiety and the lowest unoccupied molecular orbital (LUMO) is on the pyrimidine moiety.

Efficient repair of protein radicals by ascorbate

Protein radicals were selectively generated by reaction with azide radicals on Trp and Tyr residues in insulin, beta-lactoglobulin, pepsin, chymotrypsin, and bovine serum albumin at rate constants in the range (2.9-19)x10(8) M(-1) s(-1). Monohydrogen ascorbate reduced tryptophanyl radicals in chymotrypsin and pepsin with rate constants in the narrow range of (1.6-1.8)x10(8) M(-1) s(-1), whereas beta-lactoglobulin tryptophanyl radicals reacted almost 10 times slower. The corresponding values for the protein tyrosyl radicals were about an order of magnitude smaller. Comparison of the rate constants of reactions of free and protein-bound tryptophanyl and tyrosyl radicals showed that, in most cases, the location of the radicals in the protein chain did not constitute a major barrier to the reaction with monohydrogen ascorbate. The results suggest that, under physiological concentrations of dioxygen, monohydrogen ascorbate is likely to be a significant target of protein radicals. It seems likely, therefore, that reaction with protein radicals may be responsible for much of the well-documented loss of ascorbate in living organisms subjected to oxidative stress.

Influence of intramolecular electron transfer mechanism in biological nitration, nitrosation, and oxidation of redox-sensitive amino acids

Using both high-performance liquid chromatography (HPLC) and electron spin resonance (ESR) spin-trappng techniques, we developed an analytical methodology for investigating intramolecular electron transfer-mediated tyrosyl nitration and cysteine nitrosation in model peptides. Peptides N-acetyl-TyrCys-amide (YC), N-acetyl-TyrAlaCys-amide, N-acetyl-TyrAlaAlaCys-amide, and N-acetyl-TyrAlaAlaAlaAlaCys-amide were used as models. Product analysis showed that nitration and oxidation products derived from YC and related peptides in the presence of myeloperoxidase (MPO)/H(2)O(2)/NO(2)(-) were not detectable. The major product was determined to be the corresponding disulfide (e.g., YCysCysY), suggestive of a rapid electron transfer from the tyrosyl radical to the cysteinyl residue. ESR spin-trapping experiments with 5,5'-dimethyl-1-pyrroline N-oxide (DMPO) demonstrated that thiyl radical intermediates were formed from peptides (e.g., YC) treated with MPO/H(2)O(2) and MPO/H(2)O(2)/NO(2)(-). Blocking the thiol group in YC totally abrogated thiyl radical formation. Under similar conditions, we were, however, able to trap the tyrosyl radical using the spin trap dibromonitrosobenzene sulfonic acid (DBNBS). Competition spin-trapping experiments revealed that intramolecular electron transfer is the dominant mechanism for thiyl radical formation in YC peptides. We conclude that a rapid intramolecular electron transfer mechanism between redox-sensitive amino acids could influence both protein nitration and nitrosation reactions. This mechanism brings together nitrative, nitrosative, and oxidative mechanisms in free radical biology.

Marked difference in cytochrome c oxidation mediated by HO(*) and/or O(2)(*-) free radicals in vitro

Cytochrome c (cyt c) is an electron carrier involved in the mitochondrial respiratory chain and a critical protein in apoptosis. The oxidation of cytochrome c can therefore be relevant biologically. We studied whether cytochrome c underwent the attack of reactive oxygen species (ROS) during ionizing irradiation-induced oxidative stress. ROS were generated via water radiolysis under ionizing radiation (IR) in vitro. Characterization of oxidation was performed by mass spectrometry, after tryptic digestion, and UV-visible spectrophotometry. When both hydroxyl and superoxide free radicals were generated during water radiolysis, only five tryptic peptides of cyt c were reproducibly identified as oxidized according to a relation that was dependent of the dose of ionizing radiation. The same behavior was observed when hydroxyl free radicals were specifically generated (N(2)O-saturated solutions). Specific oxidation of cyt c by superoxide free radicals was performed and has shown that only one oxidized peptide (MIFAGIK+16), corresponding to the oxidation of Met80 into methionine sulfoxide, exhibited a radiation dose-dependent formation. In addition, the enzymatic site of cytochrome c was sensitive to the attack of both superoxide and hydroxyl radicals as observed through the reduction of Fe(3+), the degradation of the protoporphyrin IX and the oxidative disruption of the Met80-Fe(3+) bond. Noteworthy, the latter has been involved in the conversion of cyt c to a peroxidase. Finally, Met80 appears as the most sensitive residue towards hydroxyl but also superoxide free radicals mediated oxidation.

Oxidation of myosin by haem proteins generates myosin radicals and protein cross-links

Previous studies have reported that myosin can be modified by oxidative stress and particularly by activated haem proteins. These reactions have been implicated in changes in the properties of this protein in food samples (changes in meat tenderness and palatability), in human physiology (alteration of myocyte function and force generation) and in disease (e.g. cardiomyopathy, chronic heart failure). The oxidant species, mechanisms of reaction and consequences of these reactions are incompletely characterized. In the present study, the nature of the transient species generated on myosin as a result of the reaction with activated haem proteins (horseradish peroxidase/H2O2 and met-myoglobin/H2O2) has been investigated by EPR spectroscopy and amino-acid consumption, product formation has been characterized by HPLC, and changes in protein integrity have been determined by SDS/PAGE. Multiple radical species have been detected by EPR in both the presence and the absence of spin traps. Evidence has been obtained for the presence of thiyl, tyrosyl and other unidentified radical species on myosin as a result of damage-transfer from oxidized myoglobin or horseradish peroxidase. The generation of thiyl and tyrosyl radicals is consistent with the observed consumption of cysteine and tyrosine residues, the detection of di-tyrosine by HPLC and the detection of both reducible (disulfide bond) and non-reducible cross-links between myosin molecules by SDS/PAGE. The time course of radical formation on myosin, product generation and cross-link induction are consistent with these processes being interlinked. These changes are consistent with the altered function and properties of myosin in muscle tissue exposed to oxidative stress arising from disease or from food processing.

Intramolecular Electron Transfer in Lysozyme Studied by Time-Resolved Chemically Induced Dynamic Nuclear Polarization

The kinetics of the chemically induced dynamic nuclear polarization (CIDNP) produced in reactions of hen lysozyme with photosensitizers have been studied for the native state of the protein at pH 3.8 and for two denatured states. The latter were generated by raising the temperature to 80 degrees C or by combining a temperature rise (to 50 degrees C) with the addition of chemical denaturant (10 M urea). Detailed analysis of the CIDNP time dependence on a microsecond time scale revealed that, in both denatured states, intramolecular electron transfer (IET) from a tyrosine residue to the cation radical of a tryptophan residue (rate constant k(f)) is highly efficient and plays a decisive role in the evolution of the nuclear polarization. To describe the observed CIDNP kinetics with a self-consistent set of parameters, IET in the reverse direction, from a tryptophan residue to a tyrosine residue radical (rate constant k(r)), has also to be taken into account. The IET rate constants determined by analysis of the CIDNP kinetics are, at 80 degrees C: k(f) = 1 x 10(5) s(-1) and k(r) = 1 x 10(4) s(-1); at 50 degrees C in the presence of 10 M urea: k(f) = 7 x 10(4) s(-1), k(r) = 1 x 10(4) s(-1). IET does not appear to influence the CIDNP kinetics of the native state.

Roles of Trp144 and Tyr203 in copper-containing nitrite reductase from Achromobacter cycloclastes IAM1013

The roles of the Trp144 and Tyr203 residues near the type 1 Cu site of Achromobacter cycloclastes nitrite reductase (AcNIR) have been examined with mutants of AcNIR. Tyr203 is located on the protein surface near the type 1 Cu site of AcNIR, and Trp144 is between the Tyr203 and the type 1 Cu center in AcNIR. Single mutation of Trp144 or Tyr203 in AcNIR to Leu resulted in decreased rate constants of intermolecular electron transfer from its cognate pseudoazurin (AcPAZ) (k(ET)=1.9x10(5), 2.2x10(5), and 7.3x10(5)M(-1)s(-1) for W144L, Y203L, and wild-type AcNIR, respectively). The intermolecular electron transfer rate constant of double mutant AcNIR (W144L/Y203L) was the same as those of single mutants (k(ET)=1.9x10(5)M(-1)s(-1) for W144L/Y203L). The redox potentials, coordination structures of the type 1 Cu, and the enzyme activities of AcNIR were affected little by the mutation.

Reversibility of Electron Transfer in Tryptophan−Tyrosine Peptide in Acidic Aqueous Solution Studied by Time-Resolved CIDNP

Time-resolved chemically induced dynamic nuclear polarization (CIDNP) has been used to study electron transfer reactions in tryptophan-tyrosine peptide under strongly acidic conditions. It is demonstrated that a decrease in pH from 2.4 to 1.6 reduces the overall efficiency of intramolecular electron transfer from the tyrosine residue to the oxidized tryptophan residue. A detailed analysis of the CIDNP kinetics revealed that the rate constant of this reaction k(f) stays unchanged upon pH variation, whereas the rate constant of electron transfer in the opposite direction k(r) increases with decreasing pH. The values of the rate constants extracted from model simulations are as follows: k(f) = (5.5 +/- 0.5) x 10(5) s(-1); k(r) = (5.5 +/- 1.0) x 10(4) s(-1) at pH 2.4, (1.2 +/- 0.2) x 10(5) s(-1) at pH 2.0, and (3.2 +/- 0.4) x 10(5) s(-1) at pH 1.6. The pH dependence of log K = log(k(f)/k(r)) is linear and allows for the determination of the difference between the one-electron reduction potentials of the tryptophanyl and tyrosyl radicals in the peptide. The efficiency of IET in acidic aqueous solution containing 10 M urea-d(4) was estimated.

The oxidative environment and protein damage

Proteins are a major target for oxidants as a result of their abundance in biological systems, and their high rate constants for reaction. Kinetic data for a number of radicals and non-radical oxidants (e.g. singlet oxygen and hypochlorous acid) are consistent with proteins consuming the majority of these species generated within cells. Oxidation can occur at both the protein backbone and on the amino acid side-chains, with the ratio of attack dependent on a number of factors. With some oxidants, damage is limited and specific to certain residues, whereas other species, such as the hydroxyl radical, give rise to widespread, relatively non-specific damage. Some of the major oxidation pathways, and products formed, are reviewed. The latter include reactive species, such as peroxides, which can induce further oxidation and chain reactions (within proteins, and via damage transfer to other molecules) and stable products. Particular emphasis is given to the oxidation of methionine residues, as this species is readily oxidised by a wide range of oxidants. Some side-chain oxidation products, including methionine sulfoxide, can be employed as sensitive, specific, markers of oxidative damage. The product profile can, in some cases, provide valuable information on the species involved; selected examples of this approach are discussed. Most protein damage is non-repairable, and has deleterious consequences on protein structure and function; methionine sulfoxide formation can however be reversed in some circumstances. The major fate of oxidised proteins is catabolism by proteosomal and lysosomal pathways, but some materials appear to be poorly degraded and accumulate within cells. The accumulation of such damaged material may contribute to a range of human pathologies.

Hydroxyl radical oxidation of cytochrome c by aerobic radiolysis

The reaction of radiolytically generated *OH with cytochrome c was investigated by mass spectrometry. Tryptic digestion and characterization of the oxidized peptides by MALDI-TOF and ESI tandem mass spectrometry identified eight different amino acid residues with oxidized side chains with no cleavage of the protein detected. Solvent-accessible aromatic and methionine residues are the most susceptible to oxidation by *OH. These results support the careful use of *OH in characterizing protein surfaces. Dose-response studies identified the residues most prone to oxidation to be Phe-36, Phe-46, and Met-80. Hydroxylation of Phe-36 and Phe-46 should serve as indicators of the presence of *OH in the mitochondrial intermembrane space. Using solutions containing 50 at.% (18)O, our study also provides a novel method of determining the source of oxygen during *OH-mediated oxidation of proteins and contributes to identification of the modified residue type, with Phe>Tyr>Met in (18)O incorporation. During aerobic radiolysis, UV-vis spectroscopy indicates that ferrocytochrome c reaches a steady state concomitant with reduction of the heme.

Repair of oxidative DNA damage by amino acids

Guanyl radicals, the product of the removal of a single electron from guanine, are produced in DNA by the direct effect of ionizing radiation. We have produced guanyl radicals in DNA by using the single electron oxidizing agent (SCN)2-, itself derived from the indirect effect of ionizing radiation via thiocyanate scavenging of OH. We have examined the reactivity of guanyl radicals in plasmid DNA with the six most easily oxidized amino acids cysteine, cystine, histidine, methionine, tryptophan and tyrosine and also simple ester and amide derivatives of them. Cystine and histidine derivatives are unreactive. Cysteine, methionine, tyrosine and particularly tryptophan derivatives react to repair guanyl radicals in plasmid DNA with rate constants in the region of approximately 10(5), 10(5), 10(6) and 10(7) dm3 mol(-1) s(-1), respectively. The implication is that amino acid residues in DNA binding proteins such as histones might be able to repair by an electron transfer reaction the DNA damage produced by the direct effect of ionizing radiation or by other oxidative insults.

Re-evaluation of intramolecular long-range electron transfer between tyrosine and tryptophan in lysozymes. Evidence for the participation of other residues

One-electron oxidation of six different c-type lysozymes from hen egg white, turkey egg white, human milk, horse milk, camel stomach and tortoise was studied by gamma- and pulse-radiolysis. In the first step, one tryptophan side chain is oxidized to indolyl free radical, which is produced quantitatively. As shown already, the indolyl radical subsequently oxidizes a tyrosine side chain to the phenoxy radical in an intramolecular reaction. However this reaction is not total and its stoichiometry depends on the protein. Rate constants also vary between proteins, from 120 x s(-1) to 1000 x s(-1) at pH 7.0 and room temperature [extremes are hen and turkey egg white (120 x s(-1)) and human milk (1000 x s(-1))]. In hen and turkey egg white lysozymes we show that another reactive site is the Asn103-Gly104 peptidic bond, which gets broken radiolytically. Tryptic digestion followed by HPLC separation and identification of the peptides was performed for nonirradiated and irradiated hen lysozyme. Fluorescence spectra of the peptides indicate that Trp108 and/or 111 remain oxidized and that Tyr20 and 53 give bityrosine. Tyr23 appears not to be involved in the process. Thus new features of long-range intramolecular electron transfer in proteins appear: it is only partial and other groups are involved which are silent in pulse radiolysis.

Secretory peptide hormones are biochemical antioxidants: structure-activity relationship

The secretory peptides luteinizing hormone-releasing hormone, enkephalin, angiotensin, and oxytocin are biochemical antioxidants in aqueous medium. These hormones scavenge free peroxyl radicals, prevent the oxidation of low-density lipoprotein, and inhibit lipid peroxidation in brain membranes. Their capacity to directly suppress free radical-mediated reactions is demonstrated by electron-spin resonance spectroscopy. Electrospray ionization-mass spectrometry analysis of the free radical-quenching reaction reveals distinct oxidation products, including peptide dimers. Moreover, secretory peptide hormones can scavenge reactive nitrogen species derived from nitric oxide and peroxynitrite. An analysis of the structure-activity relationship indicates that their antioxidant activity is derived from the occurrence of solvent-exposed tyrosine and tryptophan residues, which is consistent with the mass spectrometry results. Significant effects in vitro can be observed at nanomolar concentrations, which makes these peptides comparable in potency with classic antioxidants having low molecular mass. Secretory peptide hormones may constitute an important part of the antioxidant defense system, and the sequences of the described antioxidant peptides may be unique lead structures for the rational design of novel antioxidant drugs having an improved pharmacological profile.

Protection of radiation-induced protein damage by curcumin

Free radical reactions of lysozyme (Lz), tryptophan and disulfides were studied with curcumin, a lipid-soluble antioxidant from turmeric, in aqueous solution using a pulse radiolysis technique. The binding of curcumin with lysozyme was confirmed using absorption, fluorescence and stopped-flow techniques. The free radicals of curcumin generated after repairing radicals of disulfides, lysozyme and tryptophan absorb at 500-510 nm. Implication of this in evaluating the antioxidant behavior of curcumin in protecting proteins is discussed.

Oxidative dimerization of proteins: role of tyrosine accessibility

PURPOSE

To investigate the importance of two possible mechanisms of tyrosine oxidation on the yield of protein dimerization. The model chosen is hen and turkey egg-white lysozymes, which differ by seven amino acids, among which one tyrosine is in the 3 position.

MATERIALS AND METHODS

Aqueous solutions of proteins were oxidized by OH(*) or N(*)(3) free radicals produced by gamma or pulse irradiation in an atmosphere of N(2)O. Protein dimers were quantified by SDS-PAGE and reverse-phase HPLC. Dityrosines were identified by absorption and fluorescence.

RESULTS

Using N(*)(3) free radicals, the initial yields of dimerization are equal to (8.6 +/- 0.7) x 10(-9) mol J(-1) for both proteins. Using OH(*) free radicals, they become equal to (1.23 +/- 0.1) x 10(-8) and (4.42 +/- 0.1) x 10(-8) mol J(-1) for hen and turkey egg-white lysozymes, respectively (gamma radiolysis). DISCUSSION. N(*)(3) radicals react primarily with tryptophan residues only. Tyrosine gets oxidized by intramolecular long-range electron migration, whereas OH(*) may react directly with tyrosines. We propose a low participation of Tyr3 in turkey protein in the intramolecular process, because Tyr3 is far from all tryptophans. On the other hand, Tyr3 is very accessible to solvent and in a flexible area; thus collisions with OH(*) could easily be followed by intermolecular dimerization.

Intraprotein electron transfer between tyrosine and tryptophan in DNA photolyase from Anacystis nidulans

Light-induced electron transfer reactions leading to the fully reduced, catalytically competent state of the flavin adenine dinucleotide (FAD) cofactor have been studied by flash absorption spectroscopy in DNA photolyase from Anacystis nidulans. The protein, overproduced in Escherichia coli, was devoid of the antenna cofactor, and the FAD chromophore was present in the semireduced form, FADH., which is inactive for DNA repair. We show that after selective excitation of FADH. by a 7-ns laser flash, fully reduced FAD (FADH-) is formed in less than 500 ns by electron abstraction from a tryptophan residue. Subsequently, a tyrosine residue is oxidized by the tryptophanyl radical with t(1)/(2) = 50 microseconds. The amino acid radicals were identified by their characteristic absorption spectra, with maxima at 520 nm for Trp. and 410 nm for TyrO. The newly discovered electron transfer between tyrosine and tryptophan occurred for approximately 40% of the tryptophanyl radicals, whereas 60% decayed by charge recombination with FADH- (t(1)/(2) = 1 ms). The tyrosyl radical can also recombine with FADH- but at a much slower rate (t(1)/(2) = 76 ms) than Trp. In the presence of an external electron donor, however, TyrO. is rereduced efficiently in a bimolecular reaction that leaves FAD in the fully reduced state FADH-. These results show that electron transfer from tyrosine to Trp. is an essential step in the process leading to the active form of photolyase. They provide direct evidence that electron transfer between tyrosine and tryptophan occurs in a native biological reaction.

Oxidation reactions of a bovine serum albumin-bilirubin complex. A pulse radiolysis study

Using the technique of pulse radiolysis, oxidation studies of the bovine serum albumin-bilirubin (BSA-BR) system with radicals like CCl3OO., N3., (SCN)2.-, Br2.- and OH. generated in neutral and alkaline medium are reported. In a neutral solution, BSA protects the bound BR very efficiently from the attack of these radicals. The experimental k/k' values for the reaction of CCl3OO., N3. and Br2.- radicals are 2.46, 1.78 and 2.55 respectively, where k and k' are the bimolecular rate constants for the formation of the semi-oxidized BSA and BR radicals respectively. The calculated ratios from our measurements of rate constants k and k' are 0.16, 0.28 and 1.38 for CCl3OO., N3. and Br2.- respectively. These values indicate protection of BR by BSA from free radical attack. For Br2.- radical-induced oxidation of the BSA-BR system, a radical transfer from protein to BR was observed. OH. shows very fast adduct formation with both BSA and BR. The bimolecular rate constant for the formation of BR-OH adducts at PH 8+/- 0.2 is 9.5 x 10(9) dm3 mol-1 s-1 (540 nm). OH. adds to BSA at neutral pH with a rate constant of 3.0 +/- 1.0 x 10(10) dm3 mol-1 s-1 (305 nm). In the BSA-BR complex, BSA fully protects BR from OH. attack and the (BSA-BR)-OH adduct further reacts with free BR molecule if present in solution.

Quenching of UV-induced fluorescence by ascorbic acid in the aqueous humour

Spectrophotometry and spectrofluorimetry of bovine aqueous humour have been performed. The question of whether mechanisms for fluorescence quenching exist in the eye media was evaluated. It turned out that the marked fluorescence of tryptophane and protein was considerably quenched by the ascorbic acid, an observation not shown before. Thus, the aqueous humour minimizes UV-radiation to the lens in diurnal animals through three different mechanisms: 1) Absorption. 2) Fluorescence-quenching. 3) Fluorescence-mediated ray transformation of energy-rich short wavelengths to less potent longer wave-lengths. The impact of each of them is significantly influenced by the ascorbate concentration in the aqueous humour. The phenomenon of fluorescence-mediated ray transformation is, of course, per se independent of ascorbate, but the transformational input is significantly reduced due to ascorbate.

Formation of protein-bound 3,4-dihydroxyphenylalanine in collagen types I and IV exposed to ultraviolet light

Collagen was exposed to an ultraviolet (UV) lamp that emitted predominantly in the UVB range. The cross-linking of collagen type I and type IV by UV irradiation was observed. Amino acid analyses revealed that Tyr residues in both collagen types I and IV were decreased by irradiation. In collagen type IV, losses of His and Met residues were also observed. These losses of collagen type IV may be due to the degradation of Trp, which exists in collagen type IV and decreased drastically during UV irradiation. To clarify the mechanism of Tyr modification in both types of collagen, the degradation products of Tyr were analyzed. Dityrosine, which is a dimer of the Tyr residue, could not be detected in the acid hydrolysates of UV-irradiated collagen. However, 3,4-dihydroxyphenylalanine, DOPA, was detected in the hydrolysates using HPLC with an electrochemical detector. The amounts of DOPA in the acid hydrolysates of collagen exposed to UV light for 24 h were approximately 350 pmol/mg protein (collagen type IV) and 80 pmol/mg protein (collagen type I). The DOPA formed may partially contribute to photoaging of the skin.

Aggregation of collagen exposed to UVA in the presence of riboflavin: a plausible role of tyrosine modification

Riboflavin-sensitized photodynamic modification of collagen led to significant formation of cross-linked molecules. Sodium azide or 1,4-diazabicyclo(2,2,2)octane, which are known to be singlet oxygen quenchers, and catalase could not inhibit the modification. Surprisingly, the collagen modification was accelerated in the presence of superoxide dismutase. The aggregation was accompanied by the loss of tyrosine and histidine residues in the collagen. An inhibitory effect of dissolved oxygen on the modification of collagen was observed. Similarly, the loss of tyrosine residues in the irradiated collagen was inhibited in the presence of dissolved oxygen. Dityrosine formation was also observed with the loss of tyrosine. These results indicate that photodynamic modification of tyrosine probably contributes to the riboflavin-sensitized cross-linking of collagen through the formation of dityrosine.