Leishmanicidal activity of peroxynitrite

Nitric oxide reacts with superoxide to produce peroxynitrite which has been reported to be highly microbicidal to Trypanosoma cruzi in phosphate buffer but ineffective against Leishmania major in culture medium. This contradiction and the potential importance of peroxynitrite as a cytotoxic effector molecule of both macrophages and neutrophils led us to re-examine its leishmanicidal effects. Our results demonstrate that peroxynitrite inhibits growth of Leishmania amazonensis promastigotes in a concentration-dependent manner both in phosphate buffer and culture medium (DMEM containing 20% fetal calf serum). In the latter, 43% growth inhibition was observed with 4 mM peroxynitrite whereas in buffer a 70% inhibition was already observed with 0.5 mM peroxynitrite. Treated parasites presented reduced motility and became round in shape further confirming the leishmanicidal activity of peroxynitrite. The latter was attenuated by reduced glutathione supporting the view that peroxynitrite-mediated oxidation of critical thiol groups is a major mechanism accounting for its trypanocidal activity.