Rethinking Dithiothreitol-Based Particulate Matter Oxidative Potential: Measuring Dithiothreitol Consumption versus Reactive Oxygen Species Generation

Abstract

We measured the rate of generation of reactive oxygen species (ROS) [hydroxyl radicals (OH) and hydrogen peroxide (HO)] catalyzed by ambient particulate matter (PM) in the dithiothreitol (DTT) assay. To understand the mechanism of ROS generation, we tested several redox-active substances, such as 9,10-phenanthrenequinone (PQ), 5-hydroxy-1,4-naphthoquinone (5H-1,4NQ), 1,2-naphthoquinone (1,2-NQ), 1,4-naphthoquinone (1,4-NQ), copper(II), manganese(II), and iron (II and III). Both pure compounds and their mixtures show different patterns in DTT oxidation versus ROS generation. The quinones, known to oxidize DTT in the efficiency order of PQ > 5H-1,4NQ > 1,2-NQ > 1,4-NQ, show a different efficiency order (5H-1,4NQ > 1,2-NQ ≈ PQ > 1,4-NQ) in the ROS generation. Cu(II), a dominant metal in DTT oxidation, contributes almost negligibly to the ROS generation. Fe is mostly inactive in DTT oxidation, but shows synergistic effect in OH formation in the presence of other quinones (mixture/sum> 1.5). Ten ambient PM samples collected from an urban site were analyzed, and although DTT oxidation was significantly correlated with HO generation (Pearson's r = 0.91), no correlation was observed between DTT oxidation and OH formation. Our results show that measuring both DTT consumption and ROS generation in the DTT assay is important to incorporate the synergistic contribution from different aerosol components and to provide a more inclusive picture of the ROS activity of ambient PM.

Relationship between chemical composition and oxidative potential of secondary organic aerosol from polycyclic aromatic hydrocarbons

“…In the future, a more detailed understanding of SOA binding with metal components and the effects on the oxidative health outcomes will be essential. It should also be noted that the DTT assay alone may not be entirely representative of physiological ROS variations (Tuet et al, 2017a;Xiong et al, 2017). More in vitro and in vivo work should be performed to establish the relationship between chemical composition and the OP of aerosol.…”

“…S3). Here, we used the total DTT decay rate, DTT t (µM DTT min −1 ), to report the total oxidative capacity, as well as the mass normalized DTT decay rate, DTT m (pmol DTT min −1 µg −1 organics), to report the OP (Charrier et al, 2016;Jiang et al, 2017;Xiong et al, 2017). Detailed information about the chemicals used in this assay is shown in Sect.…”

“…To date, both organics (quinone) and transition metals (Cu, Fe, Mn, etc. ) have been shown to be redox-active (Charrier and Anastasio, 2012;Xiong et al, 2017). Metals in ambient particles can range from insoluble substances to soluble cations, leading to var- ious health outcomes after deposition onto the human respiratory tract (Gojova et al, 2007;Oberdörster et al, 2005).…”

“…ROS include a variety of oxidants such as superoxide (O q − 2 ), hydroxyl radical ( q OH), and hydrogen peroxide (HOOH), which could either be introduced into human body directly from inhaled PM or generated by targeted cells (Nel et al, 1998;Pöschl and Shiraiwa, 2015;Rhee, 2006;Verma et al, 2015b). The generation of ROS during multiphase in-teractions between air pollutants and the human respiratory tract is closely related to the chemical composition, since the combination of various pollutants may influence chemical reactivity as well as bioavailability of PM while having synergistic or nonlinear influences on its OP (Antiñolo et al, 2015;Charrier et al, 2015;Fang et al, 2015;Shiraiwa et al, 2012;Xiong et al, 2017).…”

“…There may also be correlations between the average carbon oxidation state (OSc) and OP of SOA at different stages of oxidation (Tuet et al, 2017a). Moreover, mixing between organics and metals was also found to change the OP of specific components in SOA (Xiong et al, 2017), but the mechanisms remain the tip of iceberg.…”