Modulation of peroxynitrite- and hypochlorous acid-induced inactivation of α₁-antiproteinase by mercaptoethylguanidine

1. Peroxynitrile is a cytotoxic species that can be formed, among other mechanisms, by the rapid reaction of superoxide with nitric oxide. Peroxynitrite formation has been implicated in a wide range of neurodegenerative and chronic inflammatory diseases, as has the formation of hypochlorous acid by myeloperoxidase. 2. There is considerable interest in the development of peroxynitrite scavengers as therapeutic agents. The thiol compound mercaptoethylguanidine has been suggested to fulfil this role since it has recently been shown to be not only a potent inhibitor of inducible nitric oxide synthase but also a scavenger of peroxynitrite. Indeed, it has been shown to be protective in some experimental models of circulatory shock and inflammation at plasma levels in the approximate range 100-300 μM. 3. One protein inactivated by peroxynitrite is the major inhibitor of serine proteinases in human body fluids, α₁-antiproteinase. At high (250-1000 μM) concentrations, mercaptoethylguanidine was found to be effective in preventing peroxynitrite-mediated tyrosine nitration and α₁-AP inactivation. 4. By contrast, lower concentrations of mercaptoethylguanidine (1-60 μM) enhanced the inactivation of α₁-antiproteinase by peroxynitrite. 5. At all concentrations tested (1-1000 μM), mercaptoethylguanidine decreased the inactivation of α₁-antiproteinase by hypochlorous acid. 6. We suggest that products of reaction of mercaptoethylguanidine with peroxynitrite or peroxynitrite-derived products could cause damage to α₁-antiproteinase, and possibly other proteins in vivo, whereas scavenging of hypochlorous acid by mercaptoethylguanidine could contribute to its anti-inflammatory action in vivo.