Caspase-3 is a cysteine protease located in both the cytoplasm and mitochondrial 
intermembrane space that is a central effector of many apoptotic pathways. In 
resting cells, a subset of caspase-3 zymogens is S-nitrosylated at the active 
site cysteine, inhibiting enzyme activity. During Fas-induced apoptosis, 
caspases are denitrosylated, allowing the catalytic site to function. In the 
current studies, we sought to identify the subpopulation of caspases that is 
regulated by S-nitrosylation. We report that the majority of mitochondrial, but 
not cytoplasmic, caspase-3 zymogens contain this inhibitory modification. In 
addition, the majority of mitochondrial caspase-9 is S-nitrosylated. These 
studies suggest that S-nitrosylation plays an important role in regulating 
mitochondrial caspase function and that the S-nitrosylation state of a given 
protein depends on its subcellular localization.