Metals associated with both the water-soluble and insoluble fractions of an ambient air pollution particle catalyze an oxidative stress

One potential mechanism of injury mediated by air pollution particles is through metal-catalyzed oxidant generation. In one emission source particle, soluble metals have been associated with biological effect and toxicity. However, a majority of metals in ambient air pollution particles can be associated with insoluble components. We tested the hypothesis that concentrations of catalytically active metal in ambient air pollution particles are not equivalent to the concentrations of water-soluble metal. Twelve filters collected from the North Prove, UT, monitoring station were agitated in deionized water. Both the aqueous extract and pellet were isolated, lyophilized, and defined as the water-soluble and insoluble fractions, respectively. The fractions were chemically characterized and ionizable concentrations of metals were measured using inductively coupled plasma emission spectroscopy. While the water-soluble fraction had significantly greater concentrations of ionizable metals per unit mass, the insoluble fraction also had measurable quantities. In vitro oxidant generation by the two fractions, measured as thiobarbituric acid-reactive-products of deoxyribose, corresponded to the concentrations of ionizable rather than total metals. The release of interleukin-8 by cultured respiratory epithelial cells after incubation with the two fractions also coincided with the ionizable metal concentrations. Finally, neutrophil influx and lavage protein levels 24 h after instillation of the two fractions in rats reflected the ionizable metal concentrations, in vitro oxidative stress, and mediator release. We conclude that catalytically active metals can be measured in both the soluble and insoluble fractions of an ambient air pollution particle. These metals corresponded to the biological activity of the two fractions. While in greater concentration in the water-soluble fraction, larger total quantities of catalytically and biologically active metals are likely to be associated with the insoluble fraction as a result of the abundance of the latter.