Reactive oxygen and nitrogen species (ROS/RNS) such as superoxide (O), hydrogen peroxide, lipid hydroperoxides, peroxynitrite, and hypochlorous and hypobromous acids play a key role in many pathophysiological processes. Recent studies have focused on mitochondrial ROS as redox signaling species responsible for promoting cell division, modulating and regulating kinases and phosphatases, and activating transcription factors. Many ROS also stimulate cell death and senescence. The extent to which these processes occur is attributed to ROS levels (low or high) in cells. However, the exact nature of ROS remains unknown. Investigators have used redox-active probes that, upon oxidation by ROS, yield products exhibiting fluorescence, chemiluminescence, or bioluminescence. Mitochondria-targeted probes can be used to detect ROS generated in mitochondria. However, because most of these redox-active probes (untargeted and mitochondria-targeted) are oxidized by several ROS species, attributing redox probe oxidation to specific ROS species is difficult. It is conceivable that redox-active probes are oxidized in common one-electron oxidation pathways, resulting in a radical intermediate that either reacts with another oxidant (including oxygen to produce O) and forms a stable fluorescent product or reacts with O to form a fluorescent marker product. Here, we propose the use of multiple probes and complementary techniques (HPLC, LC-MS, redox blotting, and EPR) and the measurement of intracellular probe uptake and specific marker products to identify specific ROS generated in cells. The low-temperature EPR technique developed to investigate cellular/mitochondrial oxidants can easily be extended to animal and human tissues.

Hydroethidine (HE) and hydropropidine ($$\hbox {HPr}^{+}$$ HPr + ) are fluorogenic probes used for the detection of the intra- and extracellular superoxide radical anion ($$\hbox {O}_{ {2}}^{\bullet -}$$ O 2 ∙ - ). In this study, we provide evidence that HE and $$\hbox {HPr}^{+}$$ HPr + react rapidly with the biologically relevant radicals, including the hydroxyl radical, peroxyl radicals, the trioxidocarbonate radical anion, nitrogen dioxide, and the glutathionyl radical, via one-electron oxidation, forming the corresponding radical cations. At physiological pH, the radical cations of the probes react rapidly with $$\hbox {O}_{ {2}}^{\bullet -}$$ O 2 ∙ - , leading to the specific 2-hydroxylated cationic products. We determined the rate constants of the reaction between $$\hbox {O}_{ {2}}^{\bullet -}$$ O 2 ∙ - and the radical cations of the probes. We also synthesized N-methylated analogs of $$\hbox {HPr}^{+}$$ HPr + and HE which were used in mechanistic studies. Methylation of the amine groups was not found to prevent the reaction between the radical cation of the probe and the superoxide, but it significantly increased the lifetime of the radical cation and had a substantial effect on the profiles of the oxidation products by inhibiting the formation of dimeric products. We conclude that the N-methylated analogs of HE and $$\hbox {HPr}^{+}$$ HPr + may be used as a scaffold for the design of a new generation of probes for intra- and extracellular superoxide.