The modification of protein and non-protein thiols by oxidants including hydrogen peroxide (H(2)O(2)), peroxynitrite anion (ONOO(-)) and hypochlorous acid (HOCl) is well documented. Using an aromatic thiol, 5-thio-2-nitrobenzoic acid, and biologically relevant oxidants, we have identified higher oxidation states of sulfur including the sulfonic acid derivative and the disulfide S-oxide, a thiosulfinate, by HPLC and mass spectrometry. The initial reaction of ONOO(-) with 5-thio-2-nitrobenzoic acid yielded a transient red intermediate, the sulfenate anion. The red intermediate was observed when ONOO(-) and H(2)O(2) were used to oxidize 5-thio-2-nitrobenzoic acid and it persisted for several seconds at pH 7. HOCl oxidized the disulfide, 5,5'dithiobis(2-nitrobenzoic acid) to the corresponding sulfonic acid and no additional products were detected. Using this system, we can directly compare the thiol-oxidizing abilities of several oxidants. Because 5-thio-2-nitrobenzoic acid is the product of the reaction of Ellman's reagent with protein thiols, a detailed study of its stability in biological matrices where oxidants may be generated is warranted.
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