Inhibition of human surfactant protein A function by oxidation intermediates of nitrite

Nitration of protein tyrosine residues by peroxynitrite (ONOO⁻) has been implicated in a variety of inflammatory diseases such as acute respiratory distress syndrome (ARDS). Pulmonary surfactant protein A (SP-A) has multiple functions including host defense. We report here that a mixture of hypochlorous acid (HOCl) and nitrite (NO₂⁻) induces nitration, oxidation, and chlorination of tyrosine residues in human SP-A and inhibits SP-A’s ability to aggregate lipids and bind mannose. Nitration and oxidation of SP-A was not altered by the presence of lipids, suggesting that proteins are preferred targets in lipid-rich mixtures such as pulmonary surfactant. Moreover, both horseradish peroxidase and myeloperoxidase (MPO) can utilize NO₂⁻ and hydrogen peroxide (H₂O₂) as substrates to catalyze tyrosine nitration in SP-A and inhibit its lipid aggregation function. SP-A nitration and oxidation by MPO is markedly enhanced in the presence of physiological concentrations of Cl⁻ and the lipid aggregation function of SP-A is completely abolished. Collectively, our results suggest that MPO released by activated neutrophils during inflammation utilizes physiological or pathological levels of NO₂⁻ to nitrate proteins, and may provide an additional mechanism in addition to ONOO⁻ formation, for tissue injury in ARDS and other inflammatory diseases associated with upregulated ^•NO and oxidant production.