Nitric oxide acts substantially in cellular signal transduction through stimulus-coupled S-nitrosylation of cysteine residues. The mechanisms that might subserve protein denitrosylation in cellular signaling remain uncharacterized. Our search for denitrosylase activities focused on caspase-3, an exemplar of stimulus-dependent denitrosylation, and identified thioredoxin and thioredoxin reductase in a biochemical screen. In resting human lymphocytes, thioredoxin-1 actively denitrosylated cytosolic caspase-3 and thereby maintained a low steady-state amount of S-nitrosylation. Upon stimulation of Fas, thioredoxin-2 mediated denitrosylation of mitochondria-associated caspase-3, a process required for caspase-3 activation, and promoted apoptosis. Inhibition of thioredoxin-thioredoxin reductases enabled identification of additional substrates subject to endogenous S-nitrosylation. Thus, specific enzymatic mechanisms may regulate basal and stimulus-induced denitrosylation in mammalian cells.

S-Nitrosylation, the redox-based modification of Cys thiol side chains by nitric oxide, is a common mechanism in signal transduction. Dysregulated S-nitrosylation contributes to a range of human pathologies. New roles for protein denitrosylation in regulating S-nitrosylation are being revealed. Recently, several denitrosylases - the enzymes that mediate Cys denitrosylation - have been discovered, of which two enzyme systems in particular, the S-nitrosoglutathione reductase and thioredoxin systems, have been shown to be physiologically relevant. These highly conserved enzymes regulate signalling through multiple classes of receptors and influence diverse cellular responses. In addition, they protect from nitrosative stress in microorganisms, mammals and plants, thereby exerting profound effects on host-microbe interactions and innate immunity.

Key Words: angiogenesis Ⅲ apoptosis Ⅲ atherosclerosis Ⅲ cardiac electrophysiology Ⅲ excitation-contraction coupling Ⅲ adrenergic contractility M olecular oxygen (O 2 ) and carbon dioxide (CO 2 ) are critical components of cardiovascular physiology (and pathophysiology). These gaseous molecules are central to tissue physiology and cellular respiration, and it has long been understood that disturbances in O 2 or CO 2 processing are both causative and indicative of pathophysiology. 1 Over time, however, it has become increasingly clear that nitric oxide (NO) is also an endogenous regulator in cardiovascular physiology and cellular respiration, operating at considerably lower concentrations than O 2 or CO 2 . These observations have led to the proposal that NO is the "third gas" of the respiratory cycle. [2][3][4] The major sources of NO in vivo are the NO synthase (NOS) isoforms. These include predominantly the neuronal Original