W.H. Koppenol

A practical method for preparing peroxynitrite solutions of low ionic strength and free of hydrogen peroxide.

The reaction of ozone (approximately 5% in oxygen) with sodium azide (0.02-0.2 M in water) at pH 12 and 0-4 degrees C is shown to yield concentrated, stable peroxynitrite solutions of up to 80 mM. The product of this reaction is identified based on a broad absorption spectrum with a maximum around 302 nm and by its first-order rate of decomposition (k = 0.40 +/- 0.01 s-1 at pH 7.05 and 25 degrees C). These peroxynitrite solutions can be obtained essentially free of hydrogen peroxide (detection limit 1 microM) and only traces of azide (detection limit 0.1 mM). They are low in ionic strength and have a pH of about 12 but without buffering capacity; therefore, they can be adjusted to any pH by addition of buffer. These preparations of peroxynitrite frozen at -20 degrees C show negligible decomposition for about 3 weeks of storage and follow a first-order decomposition with a halflife of about 7 days at refrigerator temperatures (approximately 5 degrees C). These preparations give reactions that are characteristic of peroxynitrite. For example, at pH 7.0, they react with L-tyrosine to give a 7.3 mol % yield of nitrotyrosine(s), and with dimethyl sulfoxide to give a 8.2 mol % yield of formaldehyde, based on starting peroxynitrite concentration.

The centennial of the Fenton reaction

Abstract A short account is given of Fentons life and research, with special emphasis on the Fenton reactions.

THE KINETICS OF THE OXIDATION OF L-ASCORBIC ACID BY PEROXYNITRITE

Peroxynitrite [O = NOO-, oxoperoxonitrate(1-)bd is a strong oxidant that may be formed in vivo by the reaction of O2.- and NO(.). Oxoperoxonitrate(1-) reacts with molecules in aqueous acidic solutions via pathways that involve the highly reactive hydrogen oxoperonitrate either as an intermediate in a first-order reaction or as a reactive agent in a simple second-order reaction. ESR experiments show that hydrogen oxoperoxonitrate oxidizes monohydrogen L-ascorbate by one electron: when mixed at pH ca. 5 and passed through a flow cell within 0.1 s, the two-line ESR signal of the ascorbyl radical anion (aH = 0.18 T, g = 2.005) is observed. The overall stoichiometry of the reaction was 1 mol of ascorbate oxidized per mol of oxoperoxonitrate(1-) added. The kinetics of the reaction were studied over the pH range 4.0-7.5 by stopped-flow spectrometry. Hydrogen oxoperoxonitrate, observed between 300 and 350 nm, and the oxoperoxonitrate(1-) anion, at 302 nm, disappear faster than predicted for the first-order isomerization to NO3-. The rate increases from pH 4 to 5.8, and then decreases with increasing pH. The rate variation suggests a bimolecular reaction either between the oxoperoxonitrate(1-) anion and ascorbic acid or between hydrogen oxoperoxonitrate and the monohydrogen ascorbate anion. Although the two pathways are kinetically indistinguishable, the pKa values of ascorbic acid and hydrogen oxoperoxonitrate strongly suggest that the reacting species are hydrogen oxoperoxonitrate and monohydrogen ascorbate. The second-order rate constant for this reaction is 235 +/- 4 M-1s-1 at 25 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

The hydroxylation of tryptophan

Products of the chemical hydroxylation of tryptophan by Fenton and Udenfriend reactions are similar to those obtained by ionizing radiation. When tryptophan is exposed to either of these systems, a mixture of four hydroxytryptophans, oxindole-3-alanine, and N-formylkynurenine is formed. This observation indicates that the hydroxyl radical attacks the aromatic nucleus as well as the 2 and 3 positions of the pyrrole ring. During gamma-radiolysis of nitrous oxide-saturated tryptophan solution and in the absence of oxygen or ferric edta, the hydroxyl radical adduct (or hydroxycyclohexadienyl radical) of tryptophan undergoes dimerization and polymerization, which results in a yellow product with maximal absorbance at 425 nm. In the presence of ferric edta, or in a Fenton system, the hydroxyl radical adduct disproportionates, and hydroxylated derivatives are formed. The yields of the hydroxytryptophans are proportional to the concentration of ferric edta to a limiting yield of 54% of the theoretical yield, which is taken to be one hydroxylated product per two hydroxyl radicals. Under these conditions, 4-, 5-, 6-, and 7-hydroxy-derivatives of tryptophan are found in the proportion 4:2:2:3, respectively. The presence of dioxygen during gamma-radiolysis increases the yield of N-formylkynurenine, but does not affect the total yield of hydroxytryptophans. Similarly, tryptophan subjected to the Udenfriend reaction yields 4-, 5-, 6-, and 7-hydroxytryptophan and N-formylkynurenine in approximately equal amounts.

The hydroxylation of the salicylate anion by a fenton reaction and Γ-radiolysis: A consideration of the respective mechanisms

The yield of 2,3- and 2,5-dihydroxybenzoates (dHBs) from the reaction of .OH radicals with salicylate (SA) ions has been measured as a function of pH and in the presence of oxidants. Under steady-state radiolysis conditions, the production of these products occurs via the reactions .OH + SA----HO-SA. (radical adduct) HO-SA. H+.OH+----2-carboxyphenoxyl radical (SA.) + H2O HO-SA. + SA.----2,3-/2,5-dHB + SA The addition of the oxidants O2, Fe3+ edta, or Fe(CN)63- increases the relative yield of 2,5-dHB/2,3-dHB from about 0.2 to 1. A model to account for this effect is presented. Steady-state radiolyses of 3- and 4-hydroxybenzoate give dihydroxybenzoate products consistent with the phenol group being an ortho-para director in the electrophilic attack of the hydroxyl radical on the aromatic ring. A comparison of product distributions from the reaction of ferrous edta with hydrogen peroxide using salicylate as a scavenger strongly suggests that the same hydroxyl radical adducts are formed as in the radiation experiments.

Reactions of iron(II) nucleotide complexes with hydrogen peroxide

The rate constants for the reactions of hydrogen peroxide with ferrous complexes of ATP, ADP, UTP, citrate and pyrophosphate were measured at pH 7.2. These ligands are potential chelators of iron(II) in the low‐molecular weight iron pool that may catalyze oxidative degradation of tissues. The second‐order rate constants range from 5.5 × 103 M−1s−1 (UTP) to 1 × 105 M−1s−1 (pyrophosphate) at pH 7.2. The kinetic dependences of the ATP reaction are consistent with a mechanism involving a ‘ferryl’ (FeIV) transient which decays to the hydroxyl radical and ferric ATP.

Thermodynamic considerations on the formation of reactive species from hypochlorite, superoxide and nitrogen monoxide Could nitrosyl chloride be produced by neutrophils and macrophages?

Hypohalous acids are poor one‐electron oxidizing agents, such that reactions with hydrogen peroxide to yield radical species are not feasible. However, the oxidation of superoxide by hypohalous acids can be a source of hydroxyl or haline radicals. The oxidation of nitrogen monoxide by hypochlorous acid is favourable, but in all likelihood cannot compete with the diffusion‐controlled reaction with superoxide to yield peroxynitrite. The reaction of the latter with hypochlorous acid may lead to nitrosyl chloride, a strongly oxidizing agent [E°′(NOCl/NO•, Cl) = 1.0 V] that is capable of nitrosylating organic compounds and thereby generating mutagens or promutagens.

Spirohydantoin inhibitors of aldose reductase inhibit iron- and copper-catalysed ascorbate oxidation in vitro

Transition metal-catalysed oxidations have been implicated in the complications of diabetes. We report here that some experimental inhibitors of the enzyme aldose reductase (implicated in diabetes mellitus via its ability to catalyse glucose reduction to sorbitol) are also potent inhibitors of transition metal-catalysed ascorbate oxidation. The inhibition appears to be dependent upon the presence of a spirohydantoin group. It is conceivable that the copper- and iron-binding capacity of these compounds may contribute to some of their observed biological effects and may provide a starting point for a new generation of experimental drugs for the treatment of diabetes mellitus.

Oxyradicals and multivitamin tablets

Abstract Ingestion of a single multivitamin tablet leads to hydroxyl radical production equivalent to a radiation dose rate of 53 Gy/h.