On the Relative Contribution of Iron and Organic Compounds, and Their Interaction in Cellular Oxidative Potential of Ambient PM2.5

Previous studies have indicated the roles of both organic compounds and metals in driving the cellular generation of reactive oxygen species (ROS); however, their contribution has not been adequately quantified using mechanistic approaches. We developed a novel fractionation scheme for the stepwise removal of various classes of organic compounds and metals using a combination of solid phase extraction columns. We applied this fractionation scheme to 10 PM2.5 samples collected from the midwestern United States. Because both water-soluble organic carbon (WSOC) and Fe have shown good correlations with cellular ROS, we separated them into different fractions and measured their ability to generate ROS in rat alveolar macrophages. Most of the PM2.5 cellular ROS was attributed to the metallic fraction. To further explore the reason for the correlation of WSOC with ROS, we investigated the water solubility of Fe by measuring the total Fe in PM2.5 samples. The water-soluble fraction of Fe was tightly correlated with WSOC (r ≥ 0.69), indicating WSOC may have an additional role in cellular oxidative potential, probably through complexation of Fe, in enhancing its water solubility and macrophage ROS activity. This work reveals the role of both Fe and organic compounds through different mechanisms in contributing to PM2.5-driven cellular ROS.