SUPEROXIDE AND PEROXYNITRITE GENERATION FROM INDUCIBLE NITRIC-OXIDE SYNTHASE IN MACROPHAGES

Superoxide (O-2(radical anion)) and nitric oxide (NO) act to kill inva ding microbes in phagocytes. In macrophages NO is synthesized by induc ible nitric oxide synthase (iNOS, NOS 2) from L-arginine (L-Arg) and o xygen; however O-2(radical anion) was thought to be produced mainly by NADPH oxidase, Electron paramagnetic resonance. (EPR) spin trapping e xperiments performed in murine macrophages demonstrate a novel pathway of O-2(radical anion) generation. It was observed that depletion of c ytosolic L-Arg triggers O-2(radical anion) generation from iNOS, This MOS-mediated O-2(radical anion) generation was blocked by the NOS inhi bitor N-nitro-L-arginine methyl ester or by L-Arg, but not by the noni nhibitory enantiomer N-nitro-D-arginine methyl ester, In L-Arg-deplete d macrophages iNOS generates both O-2(radical anion) and NO that inter act to form the potent oxidant peroxynitrite (ONOO-), which was detect ed by luminol luminescence and whose formation was blocked by superoxi de dismutase, urate, or L-Arg, This iNOS-derived ONOO- resulted in nit rotyrosine formation, and this was inhibited by iNOS blockade, iNOS-me diated O-2(radical anion) and ONOO- increased the antibacterial activi ty of macrophages, Thus, with reduced L-Arg availability iNOS products O-2(radical anion) and ONOO- that modulate macrophage function. Due t o the existence of L-Arg depletion in inflammation, iNOS-mediated O-2( radical anion) and ONOO- may occur and contribute to cytostatic/cytoto xic actions of macrophages.