Measurement artifacts in the dithiothreitol (DTT) oxidative potential assay caused by interactions between aqueous metals and phosphate buffer.

Metals in particulate matter (PM) are hypothesized to have enhanced toxicity based on their ability to catalyze reactive oxygen species (ROS) formation. Acellular assays are used to measure the oxidative potential (OP) of PM and its individual components. Many OP assays, including the dithiothreitol (DTT) assay, use a phosphate buffer matrix to simulate biological conditions (pH 7.4 and 37 °C). Prior work from our group observed transition metal precipitation in the DTT assay, consistent with thermodynamic equilibria. In this study, we characterized the effects of metal precipitation on OP measured by the DTT assay. Metal precipitation was affected by aqueous metal concentrations, ionic strength, and phosphate concentrations in ambient PM sampled in Baltimore, MD and a standard PM sample (NIST SRM-1648a, Urban Particulate Matter). Critically, differences in metal precipitation induced differing OP responses of the DTT assay as a function of phosphate concentration in all PM samples analyzed. These results indicate that comparison of DTT assay results obtained at differing phosphate buffer concentrations is highly problematic. Further, these results have implications for other chemical and biological assays that use phosphate buffer for pH control and their use to infer PM toxicity.