Peroxynitrite (Cay-81565) is a highly reactive nitrogen species formed from the reaction of nitric oxide (NO) and superoxide. FeTPPS is a ferric porphyrin complex that causes the decomposition of peroxynitrite by catalytic isomerization to produce nitrate both in vitro and in vivo. The conversion of this reactive nitrogen species to nitrate results in cytoprotection (EC50 = 5 µM). FeTPPS does not complex with NO and does not alter superoxide directly. It is commonly used to elucidate the roles of peroxynitrite in oxidative stress, cell damage, and intracellular signaling.