Nitric oxide exerts a plethora of biological effects via protein S-nitrosylation, a redox-based reaction that converts a protein Cys thiol to a S-nitrosothiol. However, although the regulation of protein S-nitrosylation has been the subject of extensive study, much less is known about the systems governing protein denitrosylation. Most recently, thioredoxin/thioredoxin reductases were shown to mediate both basal and stimulus-coupled protein denitrosylation. We now demonstrate that protein denitrosylation by thioredoxin is regulated dynamically by thioredoxin-interacting protein (Txnip), a thioredoxin inhibitor. Endogenously synthesized nitric oxide represses Txnip, thereby facilitating thioredoxin-mediated denitrosylation. Autoregulation of denitrosylation thus allows cells to survive nitrosative stress. Our findings reveal that denitrosylation of proteins is dynamically regulated, establish a physiological role for thioredoxin in protection from nitrosative stress, and suggest new approaches to manipulate cellular S-nitrosylation.It has become increasingly appreciated that protein S-nitrosylation, the covalent attachment of a nitroso group to a cysteine thiol side chain, is a principle mechanism by which nitric oxide modulates numerous cellular functions and phenotypes (1, 2). These include G-protein-coupled receptor signaling (3-5), death receptor-mediated apoptosis (6 -9), vesicular trafficking (10 -12), stimulation of prostaglandin synthesis (13-15), hypoxia-dependent control of blood flow (16 -18), and the unfolded protein response (19). In addition, aberrant S-nitrosylation is implicated in pathologies such as tumor initiation and growth (20, 21), neurodegeneration (19,22,23), and pulmonary hypertension (24).The three isoforms of nitric-oxide synthase (neuronal NOS/ NOS1, iNOS/NOS2, 2 and endothelial NOS/NOS3) are well established mediators of S-nitrosylation, and numerous studies have demonstrated that their localization is critical for S-nitrosylation of target proteins. For example, binding of iNOS to COX2 is required for S-nitrosylation and activation of prostaglandin synthesis (13), whereas the subcellular localization of endothelial NOS is a major determinant of S-nitrosylation-mediated protein trafficking (25). Moreover, although it is generally assumed that S-nitrosylation reactions are nonenzymatic, there is precedence for hemoglobin-dependent S-nitrosylation of the anion exchanger protein AE1 (26) and ceruloplasmin-dependent S-nitrosylation of glypican-1 (27) and GSH (28), consistent with a recent report that metalloproteins may play a general role in SNO synthesis (29).By contrast with progress in elucidating the enzymatic determinants of S-nitrosylation, the intracellular mediators of denitrosylation and their possible contributions to overall Snitrosylation status have only recently gained attention. By analogy to phosphorylation (where kinases and phosphatases together regulate phosphorylation), the steady-state level of S-nitrosylation is the net difference between nitrosylation and denitros...