Thioredoxins (Trxs), glutaredoxins (Grxs), and peroxiredoxins (Prxs) have been characterized as electron donors, guards of the intracellular redox state, and “antioxidants”. Today, these redox catalysts are increasingly recognized for their specific role in redox signaling. The number of publications published on the functions of these proteins continues to increase exponentially. The field is experiencing an exciting transformation, from looking at a general redox homeostasis and the pathological oxidative stress model to realizing redox changes as a part of localized, rapid, specific, and reversible redox-regulated signaling events. This review summarizes the almost 50 years of research on these proteins, focusing primarily on data from vertebrates and mammals. The role of Trx fold proteins in redox signaling is discussed by looking at reaction mechanisms, reversible oxidative post-translational modifications of proteins, and characterized interaction partners. On the basis of this analysis, the specific regulatory functions are exemplified for the cellular processes of apoptosis, proliferation, and iron metabolism. The importance of Trxs, Grxs, and Prxs for human health is addressed in the second part of this review, that is, their potential impact and functions in different cell types, tissues, and various pathological conditions. Antioxid. Redox Signal. 19, 1539–1605.

Thioredoxin and glutaredoxin systems in mammalian cells utilize thiol and selenol groups to maintain a reducing intracellular redox state acting as antioxidants and reducing agents in redox signaling with oxidizing reactive oxygen species. During the last decade, the functional roles of thioredoxin in particular have continued to expand, also including novel functions such as a secreted growth factor or a chemokine for immune cells. The role of thioredoxin and glutaredoxin in antioxidant defense and the role of thioredoxin in controlling recruitment of inflammatory cells offer potential use in clinical therapy. The fundamental differences between bacterial and mammalian thioredoxin reductases offer new principles for treatment of infections. Clinical drugs already in use target the active site selenol in thioredoxin reductases, inducing cell death in tumor cells. Thioredoxin and binding proteins (ASK1 and TBP2) appear to control apoptosis or metabolic states such as carbohydrate and lipid metabolism related to diseases such as diabetes and atherosclerosis.

Significance: Inflammation often complicates diseases associated with oxidative stress. This study shows that inflammatory macrophages release proteins with specific forms of cysteine oxidation to disulfides, particularly glutathionylation. Redox proteomics identified peroxiredoxin 2 (PRDX2) as a protein released in glutathionylated form by inflammation both in vivo and in vitro. Extracellular PRDX2 then triggers the production of TNF-α. These data indicate that redox-dependent mechanisms, in an oxidative cascade, can induce inflammation.

Reactive oxygen species (ROS) and the cellular thiol redox state are crucial mediators of multiple cell processes like growth, differentiation, and apoptosis. Excessive ROS production or oxidative stress is associated with several diseases, including cardiovascular disorders like ischemia-reperfusion. To prevent ROS-induced disorders, the heart is equipped with effective antioxidant systems. Key players in defense against oxidative stress are members of the thioredoxin-fold family of proteins. Of these, thioredoxins and glutaredoxins maintain a reduced intracellular redox state in mammalian cells by the reduction of protein thiols. The reversible oxidation of Cys-Gly-Pro-Cys or Cys-Pro(Ser)-Tyr-Cys active site cysteine residues is used in reversible electron transport. Thioredoxins and glutaredoxins belong to corresponding systems consisting of NADPH, thioredoxin reductase, and thioredoxin or NADPH, glutathione reductase, glutathione, and glutaredoxin, respectively. Thioredoxin as well as glutaredoxin activities appear to be very important for the progression and severity of several cardiovascular disorders. These proteins function not only as antioxidants, they inhibit or activate apoptotic signaling molecules like apoptosis signal-regulating kinase 1 and Ras or transcription factors like NF-κB. Thioredoxin activity is regulated by the endogenous inhibitor thioredoxin-binding protein 2 (TBP-2), indicating an important role of the balance between thioredoxin and TBP-2 levels in cardiovascular diseases. In this review, we will summarize cardioprotective effects of endogenous thioredoxin and glutaredoxin systems as well as the high potential in clinical applications of exogenously applied thioredoxin or glutaredoxin or the induction of endogenous thioredoxin and glutaredoxin systems.

Glutaredoxins catalyze glutathione-dependent thiol disulfide oxidoreductions via a GSH-binding site and active cysteines. Recently a second human glutaredoxin (Grx2), which is targeted to either mitochondria or the nucleus, was cloned. Grx2 contains the active site sequence CSYC, which is different from the conserved CPYC motif present in the cytosolic Grx1. Here we have compared the activity of Grx2 and Grx1 using glutathionylated substrates and active site mutants. The kinetic studies showed that Grx2 catalyzes the reduction of glutathionylated substrates with a lower rate but higher affinity compared with Grx1, resulting in almost identical catalytic efficiencies (k_cat/K_m). Permutation of the active site motifs of Grx1 and Grx2 revealed that the CSYC sequence of Grx2 is a prerequisite for its high affinity toward glutathionylated proteins, which comes at the price of lower k_cat. Furthermore Grx2 was a substrate for NADPH and thioredoxin reductase, which efficiently reduced both the active site disulfide and the GSH-glutaredoxin intermediate formed in the reduction of glutathionylated substrates. Using this novel electron donor pathway, Grx2 reduced low molecular weight disulfides such as CoA but with particular high efficiency glutathionylated substrates including GSSG. These results suggest an important role for Grx2 in protection and recovery from oxidative stress.