Free radical generation at the solid/liquid interface in iron containing minerals

Our plain-English summary of Free radical generation at the solid/liquid interface in iron containing minerals, by B. Fubini, L. Mollo, E. Giamello (Free Radical Research, 1995).

The study

• Authors: B. Fubini, L. Mollo, E. Giamello
• Journal: Free Radical Research
• Published: 1995;23(6):593-614
• Cited by: 123 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

The potential for free radical release has been measured by means of the spin trapping technique on three kinds of iron containing particulate: two asbestos fibers (chrysotile and crocidolite); an iron-exchanged zeolite and two iron oxides (magnetite and haematite). DMPO (5,5'-dimethyl-1-pirroline-N-oxide), used as spin trap in aqueous suspensions of the solids, reveals the presence of the hydroxyl and carboxylate radicals giving rise respectively to the two adducts [DMPO-OH] and [DMPO-CO2], each characterized by a well-defined EPR spectrum. Two target molecules have been considered: the formate ion to evidence potential for hydrogen abstraction in any biological compartment and hydrogen peroxide, always present in the phagosome during phagocytosis. The kinetics of decomposition of hydrogen peroxide has also been measured on all solids. Ferrozine and desferrioxamine, specific chelators of Fe(II) and Fe(III) respectively, have been used to remove selectively iron ions. Iron is implicated in free radical release but the amount of iron at the surface is unrelated to the amount of radicals formed. Only few surface ions in a particular redox and coordination state are active. Three different kinds of sites have been evidenced: one acting as H abstracter, the other as a heterogeneous catalyst for hydroxyl radical release, the third one related to catalysis of hydrogen peroxide disproportionation. In both mechanisms of free radical release, the Fe-exchanged zeolite mimics the behaviour of asbestos whereas the two oxides are mostly inert. Conversely magnetite turns out to be an excellent catalyst for hydrogen peroxide disproportionation while haematite is inactive also in this reaction. The results agree with the implication of a radicalic mechanism in the in vitro DNA damage and in the in vivo toxicity of asbestos.

In plain English

This cell-free laboratory study measured whether iron-containing mineral particles produce free radicals (reactive molecules) in water. The authors tested two asbestos fibers, Canadian chrysotile and crocidolite, plus an iron-exchanged zeolite and two iron oxides, magnetite and haematite. They measured radicals formed from formate and from hydrogen peroxide, including on ground samples and on samples soaked in iron-removing chemicals (chelators). The two asbestos types behaved similarly. Grinding created new sites that reacted with formate. These sites were used up after one test, while hydroxyl radical release from hydrogen peroxide continued on repeated tests. On the asbestos, desferrioxamine removed more iron than ferrozine (50 versus 2.5 micromoles per gram for chrysotile, 108 versus 10 for crocidolite) and abolished reactivity with hydrogen peroxide; ferrozine did not. The zeolite produced more than twice the formate-derived radicals of freshly ground fibers, and both oxides showed very low activity. Magnetite broke down hydrogen peroxide most efficiently, and haematite was inactive. The authors concluded that radical release was related not to how much iron was present but to the redox and coordination state of surface iron.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: cell-free laboratory chemistry experiment (spin trapping with EPR spectroscopy on aqueous suspensions of mineral particles)
• Who was studied: Mineral samples, not people: UICC Canadian chrysotile (B) and crocidolite asbestos, an iron-exchanged NaY zeolite, haematite (from Merck) and magnetite (from BDH)
• Exposure: 45 mg of each solid was suspended with the spin trap and either formate or hydrogen peroxide at 37°C and pH 7.4. Samples were also tested after mild grinding and after three days of incubation in desferrioxamine, ferrozine or EDTA.
• Main results:
  • Chrysotile and crocidolite behaved similarly in both tests. Grinding created new sites for the reaction with formate, with no significant change in the hydrogen peroxide test.
  • On both asbestos types, the sites reacting with formate were consumed in the first test. Activity towards hydrogen peroxide did not vary significantly on repeat tests, which the authors read as a catalytic process.
  • Table 1 iron removal with desferrioxamine versus ferrozine was 50 versus 2.5 micromoles per gram for chrysotile and 108 versus 10 micromoles per gram for crocidolite. For EDTA the figures were 38 for chrysotile and 42 for crocidolite. The text describes desferrioxamine removal as nearly tenfold that of ferrozine.
  • On both asbestos types, all chelators destroyed or reduced reactivity with formate. Desferrioxamine also destroyed reactivity with hydrogen peroxide, whereas ferrozine did not.
  • Both iron oxides showed very low radical release despite their high iron content. Haematite was always at the level of the blank, and magnetite showed a small activity only when ground.
  • The iron-exchanged zeolite produced more than twice the carboxylate radicals (from formate) found on freshly ground asbestos fibers. Its reactivity towards hydrogen peroxide was of the same order of magnitude.
  • In breaking down hydrogen peroxide, magnetite was the most efficient catalyst and haematite was fully inactive. This catalytic activity was not related to the iron content of the solids.
• The authors' conclusion: The authors concluded that radical release is related not to iron abundance but to the redox and coordination state of iron ions at the surface. They considered iron-triggered radical reactions in some minerals one of the possible causes of the harmful response to inhaled dusts and fibers. They added that this does not mean all iron-containing minerals are toxic.
• Limitations the authors state:
  • Spin trapping measurements vary intrinsically, so each test was repeated at least three times and results were averaged.
  • Results from the formate test and the hydrogen peroxide test cannot be compared quantitatively with each other, because the two radicals differ in stability.
  • The authors described their alternative explanation (iron in a higher oxidation state at the surface) as only speculative, since no direct evidence for it has been given.

Topics

Medical terms in this study, explained in our Lexicon: Chrysotile, Crocidolite, Zeolite, Phagocytosis, Hydroxyl Radical.

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