Rapid and convenient detection of ascorbic acid using a fluorescent nitroxide switch

Ascorbic acid is a small-molecule reductant with multiple functions in vivo. Reducing ascorbic acid intake leads to a lack of hydroxylation of prolines and lysines, causing a looser triple helix and resulting in scurvy. Ascorbic acid also acts as an antioxidant to prevent oxidative stress. Because ascorbic acid is related to disease states, rapid and convenient detection of ascorbic acid should be useful in diagnosis. Nitroxide is reduced to the corresponding hydroxylamine by ascorbic acid and a sensitive and novel approach to its detection employs covalent coupling of nitroxide with a fluorophore, leading to intramolecular quenching of fluorescence emission by electron-exchange interactions. Here, we developed a new fluorophore-nitroxide probe, Naph-DiPy nitroxide, for ascorbic acid. Naph-DiPy nitroxide rapidly reacted with ascorbic acid and showed fluorescence enhancement, but not in response to other reductants or reactive oxygen species. To confirm the practical usefulness of the fluorophore-nitroxide probe, we demonstrated the use of Naph-DiPy nitroxide for the measurement of ascorbic acid in the plasma of osteogenic disorder Shionogi rats when fed an ascorbic acid-deficient diet. The results suggest that this novel fluorophore-nitroxide probe could sensitively and easily detect ascorbic acid and be useful as a tool for the diagnosis of disease states.

Nitroxides prevent exacerbation of indomethacin-induced gastric damage in adjuvant arthritis rats

Nonsteroidal anti-inflammatory drugs are the drugs of choice in the treatment of rheumatoid arthritis (RA) because of their rapid analgesic effect. However, they induce severe gastric damage in RA patients and animals by a process mediated by reactive oxygen species (ROS). Nitroxides (nitroxyl radicals) are widely used as imaging agents and antioxidants to explore the role of ROS generation in the pathogenesis of disease. In this study, the effectiveness of the newly synthesized nitroxides 8-aza-7,7,9,9-tetramethyl-1,4-dioxaspiro[4.5]undecan-8-oxyl (compound 1) and 4-oxo-2,2,6,6-tetraethylpiperidine-1-oxyl (compound 2) in the prevention of gastric ulcers in adjuvant arthritis rats treated with indomethacin was evaluated by monitoring the reaction of reactive oxygen species in gastric tissue with Overhauser-enhanced magnetic resonance imaging (OMRI). Pretreatment with all tested nitroxides suppressed the ulcers induced by indomethacin treatment in arthritic rats. OMRI using compounds 1 and 2 as well as 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL) demonstrated a redox imbalance in the stomach of these rats. Lipid peroxide and interleukin (IL)-1β levels in the gastric mucosa were significantly suppressed by compound 1 and TEMPOL, whereas CINC/gro, a member of the IL-8 family, was significantly suppressed by compound 1 only. These results suggest that the preventive effects of nitroxides on gastric ulcers may operate by different mechanisms.

In vivo evaluation of novel nitroxyl radicals with reduction stability

Nitroxyl radicals (nitroxide) have great potential advantages as spin probes, antioxidants, contrast agents, and radiation-protecting agents. However, they are readily reduced by reductants in cells and lose their paramagnetic nature. Recently, tetraethyl-substituted nitroxyl radicals have been reported to have high stability toward reduction by ascorbic acid (AsA). We report the general considerations of tetraethyl nitroxyl radicals for in vivo application. The reason for the low reactivity to AsA reduction was the positive value of Gibbs energy between the tetraethyl nitroxyl radical and AsA. Further, these compounds had an inhibitory effect on lipid peroxidation despite having AsA resistance. They had low antiproliferative effects in HepG2 cells and HUVECs and did not have a lowering effect on blood pressure in animals. Further, after intravenous injection, the ESR signal intensities of tetraethyl-substituted piperidine nitroxyl radicals were very stable in mice over 20 min. These results suggest that tetraethyl-substituted nitroxyl radicals have stability against bioreduction with reductants such as AsA and confer onto them features as antioxidants and paramagnetic tracers/contrast agents. Hence, they will be useful in identifying the foci of oxidative stress in vivo using redox-based imaging approaches.