The relationship between L-arginine-dependent nitric oxide synthesis, nitrite release and dinitrosyl-iron complex formation by activated macrophages

We identified the source of the nitrogen included into nitric oxide (NO) and studied the relationship between formation of NO, intracellular dinitrosyl ferrous iron complex (DNIC) and release of nitrite by murine bone-marrow-derived macrophages stimulated with E. coli lipopolysaccharide (LPS). NO was trapped in the cell membrane by iron-diethyldithiocarbamate complex (FeDETC) and was detected as a paramagnetic NOFe(DETC)2 complex by electron paramagnetic resonance (EPR) spectroscopy. Macrophages stimulated for 7 h up to 48 h with LPS and then incubated for 2 h with DETC exhibited an anisotropic EPR signal of axial symmetry with g- factor values g ⊥ = 2.035 , g ∥ = 2.02 and a triplet hyperfine structure (hfs) at g ⊥ characteristic of NOFe(DETC)2. In cells incubated with [15NG] l -arginine instead of [14NG] l -arginine the EPR signal of [15N]OFe(DETC)2 was detected with a doublet hfs at g ⊥ , indicating that NO was generated exclusively from the terminal guanidino-nitrogen of extracellular l -arginine. The ratio of NO formation and of nitrite release changed with time of exposure to LPS, nitrite exceeding NO at early stages of macrophage activation, and NO exceeding nitrite at later stages. DNIC with thiolate ligands (0.5 nmol/107 cells) was observed in stimulated macrophages not loaded with DETC. Furthermore, DNIC released from macrophages was trapped in the extracellular medium by bovine serum albumin (BSA) (1 nmol/107 cells per 2 h) by formation of a paramagnetic DNIC with BSA. DNIC release not only provides a route for iron loss from activated macrophages, but may also play a role in the cytotoxic and microbiostatic activity of macrophages.