Bosworth, CA; Toledo, J; Zmijewski, JW; Li, Q; Lancaster, JR
Nitrosothiols (RSNO), formed from thiols and metabolites of nitric oxide (•NO), have been implicated in a diverse set of physiological and pathophysiological processes, although the exact mechanism(s) by which they are formed biologically is unknown. Several candidate nitrosative pathways involve the reaction of •NO with O2, reactive oxygen species (ROS), and transition metals. We developed a strategy utilizing extracellular ferrocyanide to determine that under our conditions intracellular protein RSNO formation occurs from reaction of •NO inside the cell, as opposed to cellular entry of nitrosative reactants from the extracellular compartment. Using this method we found that in RAW 264.7 cells RSNO formation occurs only at very low (< 8μM) O2 concentrations, and exhibits zero-order dependence on •NO concentration. Indeed, RSNO formation is not inhibited even at oxygen levels < 1 µM. Additionally, chelation of intracellular labile iron reduces RSNO formation by more than 50%. One possible metal-dependent, oxygen independent nitrosative pathway is the reaction of thiols with dinitrosyliron complexes (DNIC), which are formed in cells from the reaction of •NO with intracellular labile iron (Toledo et al., J. Biol Chem., in press). Under our conditions, DNIC formation, like RSNO formation, is inhibited by ~50% following chelation of labile iron. Both DNIC and RSNO are also increased during overproduction of ROS by the redox cycler 5,8-dimethoxy-1,4-naphthoquinone. Taken together, these data strongly suggest that cellular RSNO are formed from free •NO via transnitrosation from DNIC derived from the cellular labile iron pool. to our knowledge, this study is the first to examine in detail the kinetics and mechanism of RSNO formation inside cells.
