15N chemically induced dynamic nuclear polarization during reaction of N-acetyl-L-tyrosine with the nitrating systems nitrite/hydrogen peroxide/horseradish peroxidase and nitrite/hypochloric acid

Solvent Cage Concept for the Homolytic Fragmentation of the Peroxynitrite-CO2 Adduct, ONOOCO2. Chem Res Toxicol 2022;35:1135-1145. [PMID: 35763359 DOI: 10.1021/acs.chemrestox.1c00355] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The toxicity of peroxynitrite, ONOO^-, is directed by carbon dioxide via the formation of the corresponding adduct, ONOOCO₂^-. Entity ONOOCO₂^- is believed to be a highly unstable compound that primarily decomposes to nitrate and carbon dioxide, but it also undergoes fractional homolysis to generate carbonate radical anion, CO₃^•-, and nitrogen dioxide, NO₂^•, in a so-called solvent (radical) cage reaction. Recently, Koppenol et al. reviewed their proposal that ONOOCO₂^- is a relatively long-lived intermediate, arguing that "the solvent cage as proposed is physically not realistic". To further address whether ONOOCO₂^- could be a long-lived species, bond dissociation enthalpies (BDE) were calculated by the composite reference method (SMD)W1BD. Anion ONOOCO₂^- can exist in two conformers, s-cis-gauche and s-trans-gauche with predicted gas-phase O-O BDEs of about 10.8 and 9.5 kcal mol^-1, respectively. Therefore, both conformers should have very short lifetimes. The (SMD)W1BD method was also used to evaluate the thermodynamic parameters of interest, revealing that the homolytic decomposition of ONOOCO₂^- is the most reasonable pathway. Moreover, previously reported experimental chemically induced dynamic nuclear polarization data also support the intermediacy of the radical cage and the formation of products CO₂ and NO₃^- at a total yield of about 70%. Because the solvent radical cage concept for the decay of ONOO^- in the presence of CO₂ is supported by a variety of spectrometric methods as well as by quantum chemical calculations at high levels of theory, it provides strong evidence against the "out-of-cage" construct. For clarification of the nature of the transient UV/vis absorption(s) between 600 and 700 nm, as observed by Koppenol et al., several experimental approaches are suggested.

15N-CIDNP investigations during tryptophan, N-acetyl-L-tryptophan, and melatonin nitration with reactive nitrogen species

Tryptophan and melatonin are nitrated by peroxynitrite; tryptophan residues in proteins are susceptible to attack by reactive nitrogen species. Nitrated tryptophan might therefore be used as a biomarker for the involvement of reactive species derived from nitrogen oxide in a variety of pathophysiological conditions. The radical character of the tryptophan (Trp) and N-acetyl-L-tryptophan (N-AcTrp) nitration with peroxynitrite is shown using (15)N-CIDNP. During the decay of peroxynitrite-(15)N in the presence of Trp at pH 5 in the probe of a (15)N-NMR spectrometer, the (15)N-NMR signals of various nitrated tryptophans ((15)NO(2)-Trp) show emission (E). The effects are built up in radical pairs [Trp( radical), 15NO2 ](F) formed by diffusive encounters of radicals 15NO2 and Trp( radical) generated during decay of peroxynitrite-(15)N in the presence of Trp. Similar (15)N-CIDNP effects are observed during reaction of Trp and/or N-AcTrp using the nitrating systems H(15)NO(3), H(15)NO(4) and H(2)O(2)/15NO2 /HRP, which are also built up in radical pairs [Trp, 15NO2 ](F). During nitration of melatonin (Mel) with peroxynitrite-(15)N and H(15)NO(4), the (15)N-NMR signal of 4-nitromelatonin (4-(15)NO(2)-Mel) shows emission arising from radical pairs [Mel, 15NO2 ](F) which are formed in an analogous manner.

15N CIDNP investigations of the peroxynitric acid nitration of l-tyrosine and of related compounds

Peroxynitric acid (O2NOOH) nitrates L-tyrosine and related compounds at pH 2-5. During reaction with O2(15)NOOH in the probe of a 15N NMR spectrometer, the NMR signals of the nitration products of L-tyrosine, N-acetyl-L-tyrosine, 4-fluorophenol and 4-methoxyphenylacetic acid appear in emission indicating a nitration via free radicals. Nuclear polarizations are built up in radical pairs [15NO2* , PhO*]F or [15NO2* , ArH*+]F formed by diffusive encounters of 15NO2 with phenoxyl-type radicals PhO or with aromatic radical cations ArH*+. Quantitative 15N CIDNP investigations with N-acetyl-L-tyrosine and 4-fluorophenol show that the radical-dependent nitration is the only reaction pathway. During the nitration reaction, the 15N NMR signal of 15NO3- also appears in emission. This is explained by singlet-triplet transitions in radical pairs [15NO2* , 15NO3*]S generated by electron transfer between O2(15)NOOH and H15NO2 formed as a reaction intermediate. During reaction of peroxynitric acid with ascorbic acid, 15N CIDNP is again observed in the 15N NMR signal of 15NO3- showing that ascorbic acid is oxidized by free radicals. In contrast to this, O2(15)NOOH reacts with glutathione and cysteine without the appearance of 15N CIDNP, indicating a direct oxidation without participation of free radicals.

Generation of peroxynitrite from reaction of N-acetyl-N-nitrosotryptophan with hydrogen peroxide over a wide range of pH values

The novel reaction of N-acetyl-N-nitrosotryptophan (NANT) with hydrogen peroxide to yield peroxynitrite is demonstrated. Quantum chemical calculations performed at CBS-QB3 level of theory predicted that the reaction of N-nitrosoindole with both H(2)O(2) and its corresponding anion is thermodynamically feasible. At pH 13, the formation of peroxynitrite from the bimolecular reaction of NANT with H(2)O(2) is unequivocally demonstrated by (15)N NMR spectrometry. In order to prove the intermediacy of peroxynitrite from the NANT-H(2)O(2) system at neutral (7.4) and acidic pH (4.5), the characteristic pattern of CIDNP (chemically induced dynamic nuclear polarization) signals were recorded, i.e. enhanced absorption in the (15)N NMR signal of nitrate and emission in the (15)N NMR signal of nitrite. Most interestingly, the NANT-H(2)O(2) system nitrated N-acetyltyrosine at pH 4 via recombination of freely diffusing nitrogen dioxide and tyrosyl radicals, but nitration was negligible at pH 7.4. Since the combination between NANT and H(2)O(2) is slow, endogenous N-nitrosotryptophan residues cannot act as a "carrier" for peroxynitrite.

Mechanistic insight into the peroxidase catalyzed nitration of tyrosine derivatives by nitrite and hydrogen peroxide

Peroxidases perform the nitration of tyrosine and tyrosyl residues in proteins, in the presence of nitrite and hydrogen peroxide. The nitrating species is still unknown but it is usually assumed to be nitrogen dioxide. In the present investigation, the nitration of phenolic compounds derived from tyrosine by lactoperoxidase and horseradish peroxidase was studied, with the aim of elucidating the mechanism of the reaction. The results indicate that nitrogen dioxide cannot be the only nitrating species and suggest the presence of two simultaneously operative pathways, one proceeding through enzyme-generated nitrogen dioxide and another through a more reactive species, assumed to be complexed peroxynitrite, which is generated by reaction of hydrogen peroxide with the enzyme-nitrite complex. The importance of the two pathways depends on peroxide and nitrite concentrations. With lactoperoxidase, nitration through the highly reactive intermediate is preferred except at very low nitrite concentration, while with horseradish peroxidase, the nitrogen dioxide driven mechanism is preferred except at very high nitrite concentration. The preferred mechanism for the two enzymes is that operative in the physiological nitrite concentration range.

Enzymatic nitration of phytophenolics: evidence for peroxynitrite-independent nitration of plant secondary metabolites

Peroxynitrite (ONOO(-)), a reactive nitrogen species, is capable of nitrating tyrosine residue of proteins. Here we show in vitro evidence that plant phenolic compounds can also be nitrated by an ONOO(-)-independent mechanism. In the presence of NaNO(2), H(2)O(2), and horseradish peroxidase (HRP), monophenolic p-coumaric acid (p-CA, 4-hydroxycinnamic acid) was nitrated to form 4-hydroxy-3-nitrocinnamic acid. The reaction was completely inhibited by KCN, an inhibitor for HRP. The antioxidant ascorbate suppressed p-CA nitration and its suppression time depended strongly on ascorbate concentration. We conclude that nitrogen dioxide radical (NO(2)(radical)), but not ONOO(-), produced by a guaiacol peroxidase is the intermediate for phytophenolic nitration.

Product formation and kinetic simulations in the pH range 1–14 account for a free-radical mechanism of peroxynitrite decomposition

The yields of nitrate and nitrite from decomposition of peroxynitrite in phosphate buffer at 37 degrees C were determined in the pH range 1-14. The NO(2)(-)/NO(3)(-) yields showed a stepwise variation with pH, with inflection points at approximately pH 3.1, 5.8, 6.8, 8.0, and 11.9. Nitrite formation increased strongly above pH 7 at the expense of nitrate, but above pH 12 nitrate again became the major product (80% at pH 14). At this pH, the Arrhenius parameters were E(a)=24.1+/-0.2kcal mol(-1) and A=(4.9+/-1.3)x10(12)s(-1). The yields of NO(2)(-), NO(3)(-), and O(2) measured at pH 5.8, 7.4, and 8.5 as a function of the initial peroxynitrite concentration (50-1000 microM) were linear only at pH 5.8. In the presence of carbon dioxide, oxygen production at pH 7.5 and pH 10 was found to be linear on the CO(2) concentration. The experimental observations were satisfactorily reproduced by kinetic simulations including principal component analyses. These data strongly suggest that the chemistry of peroxynitrite is exclusively mediated by z.rad;NO(2) and HO(z.rad;) radicals in the absence, and by z.rad;NO(2) and CO(3)(z.rad;-) radicals in the presence of CO(2).

15N CIDNP study of formation and decay of peroxynitric acid: evidence for formation of hydroxyl radicals

The reaction of nitrous acid with hydrogen peroxide leads to nitric acid as the only stable product. In the course of this reaction, peroxynitrous acid (ONOOH) and, in the presence of CO(2), a peroxynitrite-CO(2) adduct (ONOOCO(2)(-)) are intermediately formed. Both intermediates decompose to yield highly oxidizing radicals, which subsequently react with excess hydrogen peroxide to yield peroxynitric acid (O(2)NOOH) as a further intermediate. During these reactions, (15)N chemically induced dynamic nuclear polarization (CIDNP) effects are observed, the analysis of the pH dependency of which allows the elucidation of mechanistic details. The formation and decay of peroxynitric acid via free radicals NO(2)(*) and HOO(*) is demonstrated by the appearance of (15)N CIDNP leading to emission (E) in the (15)N NMR signal of O(2)NOOH during its formation and to enhanced absorption (A) during its decay reaction. Additionally, the (15)N NMR signal of the nitrate ion (NO(3)(-)) appears in emission at pH approximately 4.5. These observations are explained by proposing the intermediate formation of short-lived radical anions O(2)NOOH(*)(-) probably generated by electron transfer between peroxynitric acid and peroxynitrate anion, followed by decomposition of O(2)NOOH(*)(-) into NO(3)(-) and HO(*) and NO(2)(-) and HOO(*) radicals, respectively. The feasibility of such reactions is supported by quantum-chemical calculations at the CBS-Q level of theory including PCM solvation model corrections for aqueous solution. The release of free HO(*) radicals during decomposition of O(2)NOOH is supported by (13)C and (1)H NMR product studies of the reaction of preformed peroxynitric acid with [(13)C(2)]DMSO (to yield the typical "HO(*) products" methanesulfonic acid, methanol, and nitromethane) and by ESR spectroscopic detection of the HO(*) and CH(3)(*) radical adducts to the spin trap compound POBN in the absence and presence of isotopically labeled DMSO, respectively.

The pathobiochemistry of nitrogen dioxide

Nitrogen dioxide (*NO2) is an oxidizing free radical which can initiate a variety of destructive pathways in living systems, and several diseases are suspected to be connected with both exogenously and endogenously formed *NO2. Peroxynitrite (ONOO-/ONOOH) is believed to be an important endogenous source of *NO2 radicals, but other sources, among them enzymatically ones, have been identified recently. It also became clear during the last few years that in vivo formation of 3-nitrotyrosine strictly depends on the availability of *NO2 radicals. Since nitrogen dioxide is a very toxic compound an arsenal of antioxidants (e.g. vitamin C, glutathione, vitamin E, and beta-carotene) must eliminate this harmful radical in vivo. Here the recently identified superoxide (O2*-)-dependent formation of peroxynitrate (O2NOO-) and the central role of vitamin C are of special importance.