Ascorbate assay as a measure of oxidative potential for ambient particles: Evidence for the importance of cell-free surrogate lung fluid composition

In this study, we investigated the cell-free ascorbic acid assay (OPAA response) to quantify the oxidative potential (OP) of particle matter (PM), as a promising metric for studying the association between the chemical properties and toxicological effects of PM. With the purpose of designing an experimental set-up mostly representative of the intracellular oxidation, the assay was performed in different media, representing an artificial respiratory tract lining fluid (RTLF), i.e., simple ascorbate (Asc) or mixtures of reduced glutathione (GSH), urate (UA) and citrate (Cit). The study was performed on real PM2.5 samples collected at an urban and rural site in the Po Valley (northern Italy). For comparison, standard solutions of redox-active species were investigated, i.e., Cu2+, Fe2+, 1,2-naphthoquinone, 1,4-naphthoquinone and 9,10-phenantrenequinone, that are known to give positive response to the AA assay. The composition of the synthetic RTLF strongly effected the OPAA responses, as they decreased with increasing the mixture complexity, following the order: Asc > Asc + Cit > Asc + UA > Asc + GSH > Asc + Cit + GSH similar to Asc + Cit + GSH + UA. Based on comparison of the dependence of OPAA on RTLF composition, we could infer that Cu2+ and quinones were the redox active species most responsible of the OPAA response of the analysed PM2.5 samples.