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♦ See referenced article, J. Biol. Chem. 2010, 285, 14210–14216
Peroxynitrite (ONOO−) is a potent, unstable intermediate formed from the reaction between nitric oxide (•NO) and superoxide (O2). Due to its extremely short life-span, ONOO− is very difficult to monitor in cells, leading to some controversy regarding this reaction. For example, indirect measurements have suggested that ONOO− yield follows a bell-shaped curve with a maximum at an NO:O2 molar ratio of 1:1; however, these results might be based off of secondary radical reactions and thus may be erroneous. In this Paper of the Week, Jacek Zielonka and colleagues used a novel fluorogenic probe, coumarin-7-boronic acid (CBA), to assay ONOO− formation; CBA reacts rapidly and stoichiometrically with ONOO− to produce the fluorescent molecule 7-hydroxycoumarin. They found that in conditions using constant O2, ONOO− formation increased linearly with increased NO amounts until the ratio reached 1:1, at which point yields plateaued, but did not decrease as in previous work. Zielonka and colleagues performed these assays further in the presence of superoxide dismutase and catalase, and the lack of inhibition confirmed that ONOO−, and not other nitrogen species, was reacting with the CBA. This elegantly simple study lends crisp new insight into redox-related cellular reactions and also highlights the utility of boronate derivatives for detecting such molecules.