Free radical generation in the toxicity of inhaled mineral particles: the role of iron speciation at the surface of asbestos and silica

Abstract

Free radical generation at the particle/biological fluid interface is one of the chemical processes that contributes to pathogenicity. In order to investigate the role played by iron, fibres of crocidolite asbestos have been modified by thermal treatments to alter their surface iron content. Two radical mechanisms, HO* from H2O2 and cleavage of a C-H bond, which are both active on the original fibres, have been tested on the modified fibres. C-H cleavage is dependent on Fe(II) abundance and location and is suppressed by surface oxidation while HO* release appears independent of the oxidation state of iron. Quartz specimens with different levels of iron impurities have been tested in a similar manner. A commercially available quartz (Min-U-Sil 5) containing trace levels of iron is also active in both tests, but reactivity is not fully suppressed by treatment with desferrioxamine, which should remove/inactivate iron. The radical yield attained is close to the level produced by a pure quartz dust, suggesting the presence of active sites other than iron. Ascorbic acid reacts with both crocidolite and quartz, with subsequent depletion of the level of antioxidant defences when particle deposition occurs in the lung lining layer. Following treatment with ascorbic acid the radical yield increases with quartz, but decreases with asbestos. Selective removal of iron and silicon from the surface may account for the differences in behaviour of the two particulates.

Inorganic Materials and Living Organisms: Surface Modifications and Fungal Responses to Various Asbestos Forms

“…[8,9,19] Iron chelators reduce free radical release from asbestos, [5,14] although no direct correlation may be established between the amount of iron removal and the reactivity at the fiber surface, which depends not only on the amount of surface iron but also on its coordination and oxidation state and on the fiber micromorphology. [15,27,32,33] The incubation with F. oxysporum, similarly to the treatment with chelators, causes a decrement in HOC release, with some differences among the different asbestos types examined. In the case of crocidolite and chrysotile, iron removal by the fungus fully blunted HOC radical activity.…”

The Iron‐Related Molecular Toxicity Mechanism of Synthetic Asbestos Nanofibres: A Model Study for High‐Aspect‐Ratio Nanoparticles

“…[11b] In the first mechanism, generally described as a Fenton-like reaction [Equation (1 a)] that occurs at the fibre surface, [19] H 2 O 2 reacts with ferrous iron ions:…”

“…Following a well-established technique described in previous studies, [19] the nature and quantity of the stabilised radical was measured by means of electron paramagnetic resonance (EPR) spectroscopy.…”

Quartz Inhibits Glucose 6-Phosphate Dehydrogenase in Murine Alveolar Macrophages

“…Thus iron is not likely to be the main responsible for the oxidative stress exerted by quartz in our experiments. Experiments with pure quartz and iron-deprived quartz dusts have already shown that hydroxyl radicals can be generated in the absence of trace iron at the crystal surface (29). We think that our data suggest a new mechanism of silica-evoked oxidative stress, wherein particles induce oxidative stress also by inhibiting one of the main antioxidant pathways of the cell.…”