Detection of surface free radical activity of respirable industrial fibres using supercoiled phi X174 RF1 plasmid DNA

The ability of a number of respirable industrial fibres, amosite and crocidolite asbestos, refractory ceramic fibres (RCFs) and man-made vitreous fibres (MMVFs) to cause free radical injury to plasmid phi X174 RFI DNA was assessed. The oxidative DNA damage was observed as depletion of supercoiled DNA after fibre treatment was quantified by scanning laser densitometry. The mechanism of fibre-mediated damage was determined by the use of the specific hydroxyl radical scavenger mannitol and the iron chelator desferrioxamine-B. The amosite and crocidolite asbestos caused substantial damage to DNA that was dose-related. The free radicals responsible for the asbestos-mediated DNA damage were hydroxyl radicals, as determined by inhibition with mannitol. Asbestos fibre-mediated damage to DNA was completely ameliorated by the chelation of fibre-associated iron with desferrioxamine-B. The amount of Fe(II) and Fe(III) released by equal numbers of the different fibre types at equal fibre number was determined. The fibres released very small amounts of Fe(II) and there were no significant differences between the fibre types. The fibres released substantial amounts of Fe(III); MMVF 21 released significantly more Fe(III) than any of the other fibres and short fibre amosite also released more Fe(III) than three of the MMVFs and two of the RCFs. When ability to release Fe(II) and Fe(III) was compared with ability to cause DNA damage there was not a good correlation, because only the long amosite and crocidolite caused substantial free radical injury to DNA; this contrasts with MMVF 21 and short amosite being the two fibres that released the greatest amounts of iron. The loss of ability to damage DNA in DSF-B-treated asbestos fibres shows that iron at the surface of asbestos fibres definitely has a role in generating hydroxyl radicals. However, it is clear that some fibres, such as short amosite and MMVF 21, release large quantities of iron without causing free radical damage, whilst neither long amosite nor crocidolite released more iron than the other fibres. The exact role of iron in fibre reactivity therefore remains unresolved, but fibre-bound iron not released from the surface of asbestos could be important. Further research is under way to investigate this possibility.