SUPEROXIDE MODULATES THE OXIDATION AND NITROSATION OF THIOLS BY NITRIC OXIDE-DERIVED REACTIVE INTERMEDIATES

Thiol-containing proteins are key to numerous cellular processes, and their functions can be modified by thiol nitrosation or oxidation. Nit rosation reactions are quenched by O-2(radical anion), while the oxida tion chemistry mediated by peroxynitrite is quenched by excess flux of either NO or O-2(radical anion). A solution of glutathione (GSH), a m odel thiol containing tripeptide, exclusively yielded S-nitrosoglutath ione when exposed to the NO donor, Et2NN(O)NONa. However, when xanthin e oxidase was added to the same mixture, the yield of S-nitrosoglutath ione dramatically decreased as the activity of xanthine oxidase increa sed, such that there was a 95% reduction in nitrosation when the fluxe s of NO and O-2(radical anion) were nearly equivalent. The presence of superoxide dismutase reversed O-2(radical anion)-mediated inhibition, while catalase had no effect. Increasing the flux of O-2(radical anio n) yielded oxidized glutathione (GSSG), peaking when the flux of NO an d O-2(radical anion) were approximately equivalent. The results sugges t that oxidation and nitrosation of thiols by superoxide and NO are de termined by their relative fluxes and may have physiological significa nce.