Abstract
Abstract. The rate of consumption of dithiothreitol (DTT) is increasingly used to measure the oxidative potential of particulate matter (PM), which has been linked to the adverse health effects of PM. While several quinones are known to be very reactive in the DTT assay, it is unclear what other chemical species might contribute to the loss of DTT in PM extracts. To address this question, we quantify the rate of DTT loss from individual redox-active species that are common in ambient particulate matter. While most past research has indicated that the DTT assay is not sensitive to metals, our results show that seven out of the ten transition metals tested do oxidize DTT, as do three out of the five quinones tested. While metals are less efficient at oxidizing DTT compared to the most reactive quinones, concentrations of soluble transition metals in fine particulate matter are generally much higher than those of quinones. The net result is that metals appear to dominate the DTT response for typical ambient PM2.5 samples. Based on particulate concentrations of quinones and soluble metals from the literature, and our measured DTT responses for these species, we estimate that for typical PM2.5 samples approximately 80% of DTT loss is from transition metals (especially copper and manganese), while quinones account for approximately 20%. We find a similar result for DTT loss measured in a small set of PM2.5 samples from the San Joaquin Valley of California. Because of the important contribution from metals, we also tested how the DTT assay is affected by EDTA, a chelator that is sometimes used in the assay. EDTA significantly suppresses the response from both metals and quinones; we therefore recommend that EDTA should not be included in the DTT assay.
Highlights
Dithiothreitol (DTT, HSCH2(CH(OH))2CH2SH) is commonly used as a cell-free measure of the oxidative potential of particles (e.g., Cho et al, 2005; Shima et al, 2006; Sauvain et al, 2008; Li et al, 2009a; McWhinney et al, 2011; Verma et al, 2011)
To identify species that might contribute to DTT loss in particulate matter (PM) samples we measured the rate of DTT consumption in the presence of 18 individual quinones, polycyclic aromatic hydrocarbons (PAHs), and dissolved transition metals, all of which are commonly present in ambient PM (Connell et al, 2006; Shinyashiki et al, 2009; Vidrio et al, 2009; Walgraeve et al, 2010)
We believe that the DTT response in the PHEN solution is from a small amount of PQN contaminant, since this is an oxidation product of PHEN that could form over time in the solid
Summary
Dithiothreitol (DTT, HSCH2(CH(OH))2CH2SH) is commonly used as a cell-free measure of the oxidative potential of particles (e.g., Cho et al, 2005; Shima et al, 2006; Sauvain et al, 2008; Li et al, 2009a; McWhinney et al, 2011; Verma et al, 2011). Some later studies found correlations between DTT loss and the metal content of ambient PM, but these relationships were attributed to covariance between metals and carbonaceous redox-active organic species, which were thought to be responsible for DTT oxidation (Ntziachristos et al, 2007; Hu et al, 2008). Lin and Yu (2011) found DTT loss in laboratory solutions of Cu(II) and Zn(II), but not from Fe. While most of the current literature indicates that DTT assay responds strongly to certain organic species, other measures of oxidative potential from ambient PM indicate that metals are most important for ROS production. We use these results to quantify the contributions of metals and quinones to the DTT response, both for a hypothetical, typical particle composition as well as for six ambient PM2.5 samples from the San Joaquin Valley of California
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