IFN-gamma-activated macrophages: detection by electron paramagnetic resonance of complexes between L-arginine-derived nitric oxide and non-heme iron proteins.

Interferon-gamma induces the L-Arginine-dependent pathway that leads to the formation of nitrogen oxides in murine macrophages with subsequent inhibition of mitochondrial non-heme iron-dependent enzymes. To evaluate a possible role of nitric oxide through binding to enzymes containing iron-prosthetic groups, we used Electron Paramagnetic Resonance spectroscopy. In IFN-gamma-activated macrophages, we observed the appearance of a signal in the g = 2.04 region which is consistent with that given by nitrosyl-iron-sulfur complexes. Appearance of this signal was dependent on the presence of L-Arginine in the culture medium. Furthermore, we detected a virtually identical signal in macrophages non stimulated by IFN-gamma, following exposure to nitric oxide (after addition of an excess of nitrite in the presence of ascorbate). These data suggest that L-Arginine-derived nitric oxide may alter the configuration of the catalytic site of certain mitochondrial enzymes by coordinating to iron at their iron-sulfur cluster(s).