Broad details of NO signaling involving NO synthesis by nitric oxide synthase and NO activation of soluble guanylate cyclase (sGC) are reasonably well-understood, though critical molecular aspects of function remain unanswered.  On the contrary, relatively little is known about sGC-independent NO signaling.  Most sGC- independent hypotheses involve S-nitrosation of low molecular weight thiols or protein thiols.  Some speculate that metal-catalyzed oxidation of a thiol and subsequent reaction with NO, thus forming a nitrosothiol, could be playing a major role in this type of signaling.  More interesting is the possibility that the thiol could be the nucleophilic amino acid of an important enzyme - for instance, a cysteine protease.

Many important questions must be answered to establish S-nitrosation as a deliberate cellular modification and not a stochastic result of nitrosative stress.  Are there enzymes, such as kinases and phosphatases, to nitrosate and denitrosate specific protein targets?  The tools needed to answer these questions do not exist. In an effort to begin to understand cellular signaling by protein S-nitrosation, we have recently studied the interaction of thioredoxin (Trx) with caspase 3 (Casp). Casp plays an important role in vivo as the executioner of cellular apoptosis. We showed that Casp may be S-nitrosated at Cys163 by transfer of NO from Cys73 of Trx in vitro and that this nitrosation event inhibits the activity of Casp towards celullar apoptosis in vivo.