Generation of thiyl radical by nitric oxide: a spin trapping study

The free radical intermediates formed during the nitrosation of thiols including glutathione (GSH), L-cysteine (CystSH), captopril (CapSH) and N-acetylpenicillamine (NAPenSH) by NO. at physiological pH have been investigated using EPR spectroscopy combined with the spin trap DMPO. We have found that NO. in the presence of oxygen reacted with GSH, CystSH and CapSH to generate the corresponding thiyl radicals, which are spin trapped by DMPO to give the spin trapped adducts DMPO-SG, DMPOS-Cyst and DMPO-SCap respectively. In the case of NAPenSH no spin trapped adduct is detected. Desferrioxamine, an inhibitor of peroxynitrite (ONOO-), has no effect on the formation of either DMPO-SG or DMPO-SCyst, suggesting that ONOO- is not involved in the formation of Gs(.) and Cyst(.). In contrast, desferrioxamine inhibits the detection of DMPO-SCap, pointing to a role for ONOO- in the formation of Caps'. Our results demonstrate that (NO2)-N-., is the most likely reactive species in the nitrosation of GSH and CystSH by oxygenated NO., thereby generating the corresponding thiyl radicals. Furthermore, our data indicate that the scavenging of NO. by thiols may lead to the production of superoxide (O-2(.-)) via the formation of thiyl radicals.