A Comparison of Hydroxyl Radical and Hydrogen Peroxide Generation in Ambient Particle Extracts and Laboratory Metal Solutions.

Generation of reactive oxygen species (ROS) - including superoxide ((•)O(2) (-)), hydrogen peroxide (HOOH), and hydroxyl radical ((•)OH) - has been suggested as one mechanism underlying the adverse health effects caused by ambient particulate matter (PM). In this study we compare HOOH and (•)OH production from fine and coarse PM collected at an urban (Fresno) and rural (Westside) site in the San Joaquin Valley (SJV) of California, as well as from laboratory solutions containing dissolved copper or iron. Samples were extracted in a cell-free, phosphate-buffered saline (PBS) solution containing 50 μM ascorbate (Asc). In our laboratory solutions we find that Cu is a potent source of both HOOH and (•)OH, with approximately 90% of the electrons that can be donated from Asc ending up in HOOH and (•)OH after 4 h. In contrast, in Fe solutions there is no measurable HOOH and only a modest production of (•)OH. Soluble Cu in the SJV PM samples is also a dominant source of HOOH and (•)OH. In both laboratory copper solutions and extracts of ambient particles we find much more production of HOOH compared to (•)OH: e.g., HOOH generation is approximately 30 - 60 times faster than (•)OH generation. The formation of HOOH and (•)OH are positively correlated, with roughly 3 % and 8 % of HOOH converted to (•)OH after 4 and 24 hr of extraction, respectively. Although the SJV PM produce much more HOOH than (•)OH, since (•)OH is a much stronger oxidant it is unclear which species might be more important for oxidant-mediated toxicity from PM inhalation.