Carboxy-PTIO reacts rapidly with NO to yield NO2 and has been used as a scavenger to test the importance of nitric oxide (NO) in various physiological conditions. This study investigated the effects of carboxy-PTIO on several NO- and peroxynitrite-mediated reactions. The scavenger potently inhibited NO-induced accumulation of cGMP in endothelial cells but potentiated the effect of the putative peroxynitrite donor SIN-1, Carboxy-PTIO completely inhibited peroxynitrite-induced formation of 3-nitrotyrosine from free tyrosine (EC50 = 36 +/- 5 microM) as well as nitration of bovine serum albumin. Peroxynitrite-mediated nitrosation of GSH was stimulated by the drug with an EC50 of 0.12 +/- 0.03 mM, whereas S-nitrosation induced by the NO donor DEA/NO (0.1 mM) was inhibited by the scavenger with an IC50 of 0.11 +/- 0.03 mM. Oxidation of NO with carboxy-PTIO resulted in formation of nitrite without concomitant production of nitrate. Our results demonstrate that the effects of carboxy-PTIO are diverse and question its claimed specificity as NO scavenger.