Detection and identification of oxidants formed during •NO/O2•⁻ reaction: a multi-well plate CW-EPR spectroscopy combined with HPLC analyses.

New techniques and probes are routinely emerging for detecting short-lived free radicals such as superoxide radical anion (O₂(•-)), nitric oxide ((•)NO), and transient oxidants derived from peroxynitrite (ONOO(-)/ONOOH). Recently, we reported the profiles of oxidation products (2-hydroxyethidium, ethidium, and various dimeric products) of the fluorogenic probe hydroethidine (HE) in the (•)NO/O₂(•-) system (Zielonka et al. 2012). In this study, we used HPLC analyses of HE oxidation products in combination with continuous wave electron paramagnetic resonance (CW-EPR) spin trapping with 5-tert-butoxycarbonyl-5-methyl-1-pyrroline N-oxide (BMPO) to define the identity of the oxidizing species formed in the (•)NO/O₂(•-) system. EPR spin-trapping technique is still considered as the gold standard for characterization of free radicals and their intermediates. We monitored formation of BMPO-superoxide (BMPO-(•)OOH) and BMPO-hydroxyl (BMPO-(•)OH) radical adducts. Simultaneous analyses of results from EPR spin-trapping and HPLC measurements are helpful in the interpretation of the mechanism of formation of products of HE oxidation.