The physiological role and pharmacological potential of nitric oxide in neutrophil activation.

There is contention over whether human neutrophils produce physiologically significant levels of nitric oxide (NO) during inflammatory reactions. Nevertheless, regardless of its cell source, NO does exert regulatory effects on neutrophil function. Depending on experimental conditions, NO can either inhibit or enhance neutrophil activation, in both cases probably acting through cyclic GMP. The explanation for these apparently contradictory findings may be that the effect depends upon the concentration of NO: low concentrations of NO being stimulatory and high concentrations inhibitory. Nitrite, produced at high concentrations from NO during inflammation, can react with neutrophil myeloperoxidase-derived hypochlorous acid (HOCl) to form the active oxidant nitryl chloride, a species capable of nitrating tyrosine and tyrosyl residues on proteins. Whether nitryl chloride acts to limit or amplify the oxidant effects of myeloperoxidase is not yet clear, although formation of nitrotyrosine has been linked with nitration of phagocytosed bacteria. Clearly, a better understanding of the inflammatory effects of NO on neutrophils is needed before the therapeutic potential of NO donors or inhibitors in inflammation can be realised.