Endothelial cells, macrophages, neutrophils, and neuronal cells generate superoxide (O2-) and nitric oxide (.NO) which can combine to form peroxynitrite anion (ONOO-). Peroxynitrite, known to oxidize sulfhydryls and to yield products indicative of hydroxyl radical (.OH) reaction with deoxyribose and dimethyl sulfoxide, is shown herein to induce membrane lipid peroxidation. Peroxynitrite addition to soybean phosphatidylcholine liposomes resulted in malondialdehyde and conjugated diene formation, as well as oxygen consumption. Lipid peroxidation was greater at acidic and neutral pH, with no significant lipid peroxidation occurring above pH 9.5. Addition of ferrous (Fe+2) or ferric (Fe+3) iron did not enhance lipid peroxide formation over that attributable to peroxynitrite alone. Diethylenetetraminepentacetic acid (DTPA) or iron removal from solutions by ion-exchange chromatography decreased conjugated diene formation by 25-50%. Iron did not play an essential role in initiating lipid peroxidation, since DTPA and iron depletion of reaction systems were only partially inhibitory. In contrast, desferrioxamine had an even greater concentration-dependent inhibitory effect, completely abolishing lipid peroxidation at 200 microM. The strong inhibitory effect of desferrioxamine on lipid peroxidation was due to direct reaction with peroxynitrous acid in addition to iron chelation. We conclude that the conjugate acid of peroxynitrite, peroxynitrous acid (ONOOH), and/or its decomposition products, i.e., .OH and nitrogen dioxide (.NO2), initiate lipid peroxidation without the requirement of iron. These observations demonstrate a potential mechanism contributing to O2-(-)and .NO-mediated cytotoxicity.