The role of protein nitration in the inhibition of platelet activation by peroxynitrite

Peroxynitrite at low concentrations (3–10 μM) inhibited agonist-induced platelet aggregation by a mechanism not dependent on the formation of cyclic guanosine monophosphate. Platelets recovered completely from peroxynitrite-induced inhibition within 30 min. Peroxynitrite induced nitration of cytosolic proteins, but this diminished to near basal levels within 60 min of exposure to the oxidant. During this period there was a reduction in tyrosine phosphorylation of specific proteins such as syk, but this was not due to direct nitration of these same proteins. The inhibition of phosphorylation was reversible with platelet proteins recovering the ability to be phosphorylated within 15 min of exposure to peroxynitrite. Conversely, peroxynitrite increased phosphorylation of other proteins, but again these events were not directly linked to nitration. Nitration may affect the phosphorylation of tyrosine residues in a number of proteins, but by an indirect route, possibly by acting on proteins upstream in the signalling cascades. We suggest that low concentrations of peroxynitrite reversibly inhibit platelet aggregation by preventing the phosphorylation of key signalling proteins.