The effect of Hg(II) ions on the free radicals of humic substances and their model compounds

Humic acids (HAs) and humin (HU) complexes with Hg(II) ions were studied using EPR, FTIR and CP MAS (13)C NMR spectroscopies. The analysis of the EPR spectra showed, especially for HA, a significant decrease in the g-factor value for the resulting Hg(II) radical species, as compared with that observed for the parent radicals. The concentration of the formed radical complexes was very low, precluding their detailed characterization. For this reason hydroxybenzenes and their benzoic acids were used as models of the humic substances' radical centres able to form radical ligands towards Hg(II) ions. The resulting radicals, characterized by a g-factor lower than that of the free electron (2.0006-2.00017), were efficiently produced upon Hg(II) complexation only for the hydroxybenzenes with their OH groups in the ortho position and, what is more important, the presence of carboxylic substituents was not necessary. It is most likely that formation of the radicals characterised by low g-factor proceeded in two steps. The first is oxidation process of the hydroxybenzenes and their derivatives resulting in a typical phenolic/semiquinone type radical while in the second step the radical is complexed by the excess of Hg(II), as the result spin density of the radical is shifted towards aromatic ring leading to the observed lowering of the g parameter value. The redox character of the process starting from Hg(II) was confirmed by the formation of elemental mercury. The g-factor of the semiquinone radical obtained from 2,4,6-trihydroxybenzoic acid was not changed by Hg(II) ions coordination, as indicated by g =2.0034.

Peroxidative metabolism of beta2-agonists salbutamol and fenoterol and their analogues

Phenolic beta(2)-adrenoreceptor agonists salbutamol, fenoterol, and terbutaline relax smooth muscle cells that relieve acute airway bronchospasm associated with asthma. Why their use sometimes fails to relieve bronchospasm and why the drugs appear to be less effective in patients with severe asthma exacerbations remains unclear. We show that in the presence of hydrogen peroxide, both myeloperoxidase, secreted by activated neutrophils present in inflamed airways, and lactoperoxidase, which is naturally present in the respiratory system, catalyze oxidation of these beta(2)-agonists. Azide, cyanide, thiocyanate, ascorbate, glutathione, and methimazole inhibited this process, while methionine was without effect. Inhibition by ascorbate and glutathione was associated with their oxidation to corresponding radical species by the agonists' derived phenoxyl radicals. Using electron paramagnetic resonance (EPR), we detected free radical metabolites from beta(2)-agonists by spin trapping with 2-methyl-2-nitrosopropane (MNP). Formation of these radicals was inhibited by pharmacologically relevant concentrations of methimazole and dapsone. In alkaline buffers, radicals from fenoterol and its structural analogue, metaproteronol, were detected by direct EPR. Analysis of these spectra suggests that oxidation of fenoterol and metaproterenol, but not terbutaline, causes their transformation through intramolecular cyclization by addition of their amino nitrogen to the aromatic ring. Together, these results indicate that phenolic beta(2)-agonists function as substrates for airway peroxidases and that the resulting products differ in their structural and functional properties from their parent compounds. They also suggest that these transformations can be modulated by pharmacological approaches using appropriate peroxidase inhibitors or alternative substrates. These processes may affect therapeutic efficacy and also play a role in adverse reactions of the beta(2)-agonists.

Study of Photoinduced N-Hydroxy-arylnitroxide Radicals (ArNO•OH) by Time-Resolved EPR

The photoreduction of aromatic nitro compounds by alcohols is a well-known reaction; however, the first stages of its mechanism remain controversial. This study aims at characterizing the "primary" radicalar transients involved in this reaction by EPR spectroscopy. Laser flash photolysis (lambda = 266 nm) of nitrobenzene, 5-nitrouracil, p-nitroacetophenone, o-propylnitrobenzene, and 2-nitroresorcinol in ethylene glycol was followed by time-resolved EPR spectroscopy. In all reported TR-EPR spectra, except those obtained from the photolysis of 2-nitroresorcinol, the key intermediate N-hydroxy-arylnitroxide radicals (ArNO*OH, 1-4) could be identified unambiguously. In 2-nitroresorcinol, the radical anion (ArNO*O(-), 5) and a sigma iminoxy radical (6) were observed, and a third radical (7) remains unidentified. These observations indicate that two radicalar mechanisms (by H* transfer and by electron transfer) are competing in the photoreduction mechanism. The attribution of the EPR spectra was helped by DFT calculations of the hyperfine coupling constants (hcc's).

Are free radicals and not quinones the haptenic species derived from urushiols and other contact allergenic mono- and dihydric alkylbenzenes? The significance of NADH, glutathione, and redox cycling in the skin

The induction of allergic contact dermatitis to urushiols from poison ivy and related plants is generally believed to involve an initial oxidation event by which a protein-reactive quinone is formed. However, this does not readily account for the contact allergenicity of closely related mono- and dihydric alkylbenzenes such as the alkylphenols and alkylresorcinols which are not so easily oxidised to quinones in vitro. When the redox processes known to occur in living tissues are taken into consideration, a more plausible unifying mechanism involving the formation of protein-reactive radical species becomes apparent. Experiments described here examine the autoxidation of p-benzoquinone and various mono- and dihydric benzenes and alkylbenzenes, and their reactions with the diphenylpicrylhydrazyl radical, cysteine, glutathione, and NADH. We have also demonstrated that administration to mice of 2-oxo-4-thiazolidine carboxylate, a compound known to elevate intracellular glutathione levels, inhibits the irritancy and sensitising activity of 3-pentadecylphenol. This work suggests that redox cycling in the skin following penetration of allergenic mono- and dihydric alkylbenzenes initially depletes local levels of endogenous reducing equivalents such as glutathione and NADH; once depleted, further cycling results in the uncontrolled generation of radical species which may reasonably be expected to exhibit protein reactivity.