A Quasi-Michaelis-Menten Mechanism Explains the Nonlinear Relationship between Transition Metals and PM2.5 Oxidative Potential Measured by the Dithiothreitol (DTT) Assay.

Oxidative potential (OP), defined as the capacity of particulate matter (PM) to consume antioxidants and/or generate reactive oxygen species, is increasingly used as a metric for assessing PM toxicity. However, the concentration-response relationship between PM components and OP measured by the dithiothreitol (DTT) assay (OPDTT) is often nonlinear, and the underlying mechanism remains poorly understood. Here, we measured OPDTT of laboratory-prepared solutions and ambient PM2.5 samples and propose a quasi-Michaelis-Menten mechanism involving DTT, transition metals, and oxygen. The derived equations for copper (Cu) and manganese (Mn) quantitatively reproduced the nonlinear OPDTT response observed in both single-metal standard solutions and ambient samples, as well as the antagonistic effects of Cu-Mn mixtures. For 62 ambient PM2.5 samples collected in Beijing over one year, a model based on water-soluble Cu, Mn, and water-soluble organic carbon (WSOC), assuming a linear WSOC contribution, accounted for 98.6% of the measured OPDTT. These results support the proposed quasi-Michaelis-Menten mechanism as an explanation for the nonlinear concentration-response relationship and provide a basis for predicting OPDTT from PM2.5 chemical composition.