Nitric oxide ((·)NO) is a signal transducing free radical which can modify oxidant stress by limiting superoxide (O2/(·-))-mediated injury. However, the product of (·)NO reaction with O2/(·-), peroxynitrite (ONOO- ), is a potent oxidizing and nitrating agent. Exposure of a mixture containing phosphatidylcholine liposomes and surfactant apeprotein A (SP-A; 10% by weight) to increasing concentrations of (·)NO, generated by spermine NONOate, and constant O2/(·-) levels, produced by the action of xanthine oxidase on lumazine, suppressed O2/(·-)-induced lipid peroxidation in the presence of Fe3+-EDTA. On the other hand, an increase in the (·)NO/O2/(·-) value resulted in nitration of SP-A tyrosine residues, located in the carbohydrate recognition domain (CRD), and decreased the ability of SP-A to aggregate lipids and bind mannose, two functions that require an intact CRD. SP-A was also nitrated to a large extent following exposure to 3-morpholinosydnonimine (SIN-1) or tetranitromethane at pH 8. In each case, increased nitrotyrosine content correlated in a monotonic fashion with inhibition of lipid aggregation and mannose binding, correlated with the extent of functional inhibition. Superoxide dismutase (2400 U/ml) and urate (100 μM; nonspecific scavenger of both ONOO- and hydroxyl radical), but not mannitol (50 mM; hydroxyl radical scavenger), prevented the SIN-1-induced injury to SP-A. In contrast, spermine NONOate or xanthine oxidase plus lumazine alone neither inhibited SP-A function nor nitrated the protein. These results indicate that at high concentrations, (·)NO inhibit O2/(·- )-induced lipid peroxidation. However, ONOO-, formed by the reaction of (·)NO and O2/(·-), nitrates SP-A leading to decreased ability to aggregate lipids and bind mannose.