Mercaptoethylguanidine and related compounds are scavengers of peroxynitrite

Part of the nitric oxide (NO) related eytotoxicity is mediated by peroxynitrite, a toxic oxidant species produced from NO and superoxide. Inhibitors of iNOS and scavengers of peroxynitrite may be of therapeutic potential. Certain guanidines are inhibitors of the inducible isoform of NO synthase (iNOS), and have antiinflammatory effects. Here we demonstrate that MEG, and related guanidines. in addition to being inhibitors of iNOS, are scavengers of peroxynitrite. Peroxynitrile-induced oxidations were measured by fluorometric detection of peroxynitrite-induced oxidation of dihydrorhodamine 123 to rhodamine 123 by the spectrophotometric detection of peroxynitrite-induced oxidation of cytochrome C²⁺, and by the fluorometric detection of peroxynitrite-induced hydroxylation of benzoate. In the dihydrorhodamine assay, MEG inhibited peroxynitrite-induced oxidations with a potency comparable to that of glutathione and cysteine. Derivatives of MEG, such as selenoethylguanidine, N-methyl-MEG, N,N₁-dimethyl-MEG, mercaptopropylguamdine, and guanidinoethyldisulfide (GED), as well as aminoguanidine inhibited the peroxy nitrite-induced oxidation of dihydrorhodamine 123, but with potencies lower than that of MEG. In the other assays used (benzoate and cytochrome C²⁺). MEG also exerted a potent inhibitory effect. Using stopped flow spectroscopy, we then determined that MEG reacts with peroxynitrite with a second order rate constant of 1900±64 M₁ s^-1, at 37 °C. In contrast to MEG, neither GED nor aminoguanidine accelerated the rate of peroxynitrite decomposition, indicating that they do not react in a second order process with peroxynitrite. We speculate that the antiinflammatory effect of MEG is due to a combined mode of action: iNOS inhibition and peroxynitrite scavenging.