Abstract Nitric oxide ([formula]NO) is a signal transducing free radical which can modify oxidant stress by limiting superoxide[formula]-mediated injury. However, the product of[formula]NO reaction with[formula], peroxynitrite (ONOO −), is a potent oxidizing and nitrating agent. Exposure of a mixture containing phosphatidylcholine liposomes and surfactant apoprotein A (SP-A; 10% by weight) to increasing concentrations of[formula]NO, generated by spermine NONOate, and constant[formula]levels, produced by the action of xanthine oxidase on lumazine, suppressed[formula]-induced lipid peroxidation in the presence of Fe 3+-EDTA. On the other hand, an increase in the[formula]NO/[formula]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 μ m; nonspecific scavenger of both ONOO −and hydroxyl radical), but not mannitol (50 m m; 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,[formula]NO inhibit[formula]-induced lipid peroxidation. However, ONOO −, formed by the reaction of[formula]NO and[formula], nitrates SP-A leading to decreased ability to aggregate lipids and bind mannose.