Inhibition of superoxide generation from fMLP-stimulated leukocytes by high concentrations of nitric oxide or peroxynitrite: Characterization by electron spin resonance spectroscopy

This present study examined the effects of high concentrations of nitric oxide (NO*) and peroxynitrite (ONOO^-) on superoxide (O₂^•-) production from formyl-methionyl-leucyl-phenylalanine (fMLP)-stimulated polymorphonuclear leukocytes (PMNs) by using electron spin resonance (ESR) and spin trapping with 5-(diethoxyphosphoryl)-5-methyl-1-pyrroline-N-oxide (DEPMPO). We demonstrated that ONOO^- (100 μM) decreased the ESR signal of DEPMPO-OOH from fMLP-activated PMNs, indicating the inhibition of O₂^•- generation, while it enhanced the signal of DEPMPO-OH. Inhibition of the respiratory burst was also observed when PMNs were pre-exposed to high concentrations of NO* (100 μM), generated by the NO* donor NOR-1, 30 min prior to stimulation with fMLP. NOR-1 inhibited O₂^•- generation more effectively under conditions in which ONOO^- was formed concurrently. The ability of high concentrations of either ONOO^- or NO* to inhibit O₂^•- generation from fMLP-stimulated PMNs is relevant to pathophysiological conditions, such as severe inflammation, in which NO* or ONOO^- production can be significantly elevated.