Synergistic and Antagonistic Interactions among the Particulate Matter Components in Generating Reactive Oxygen Species Based on the Dithiothreitol Assay

We assessed the interactions among the particulate matter (PM) components in generating the reactive oxygen species (ROS) based on a dithiothreitol (DTT) assay. We started with the standard solutions of known redox-active substances, i.e., quinones (9,10-phenanthraquinone, 1,2-naphthoquinone, 1,4-naphthoquinone, and 5-hydroxy-1,4-naphthoquinone) and metals [Fe (II), Mn (II), and Cu (II)]. Both DTT consumption and hydroxyl radical (center dot OH) generation were measured in the DTT assay. The interactions of Fe were additive with quinones in DTT consumption but strongly synergistic in center dot OH generation. Cu showed antagonistic interactions with quinones in both DTT consumption and center dot OH generation. Mn interacted synergistically with quinones in DTT oxidation but antagonistically in center dot OH generation. The nature of the interactions of these metals (Fe, Mn, and Cu) with ambient humic-like substances (HULLS) resembled that with quinones, although the intensity of interactions were weaker in DTT consumption than center dot OH generation. Finally, we demonstrated that the DTT consumption capability of ambient PM can be well explained by HULIS, three transition metals (Fe, Mn, and Cu), and their interactions, but center dot OH generation involves a contribution (similar to 50%) from additional compounds (aliphatic species or metals other than Fe, Mn, and Cu) present in the hydrophilic PM fraction. The study highlights the need to account for the interactions between organic compounds and metals, while apportioning the relative contributions of chemical components in the PM oxidative potential.