Nitric oxide (NO) can interact with a number of biomolecular carriers to form products that retain biological activity. It has been suggested that physiological effects of nitric oxide (e.g.. those associated with endothelium-derived relaxing factors) may be related to NO-adducts, such as a nitrosothiols (Creager MA. Roddy MA. Boles K. Stamler JS. N-acetylcysteine does not influence the activity of endothelium-derived relaxing factor in vivo. Hypertension. 29(2):668-72, 1997). Antioxidant effects of NO are believed to play an important role in its multifunctional physiological activity. The two major antioxidant mechanisms of NO are: (i) direct radical scavenging, and (ii) interaction with hemoproteins that prevent formation of potent oxidants, oxoferryl-associated radicals. In this study, we compared effects of an NO-donor, NOC-15. and two nitrosothiols. NO-GS and NO-Cys, on cytotoxicity and oxidative stress induced by tert-butyl hydroperoxide (tBuOOH)/oxy-hemoglobin (oxyHb) in rat mesenteric smooth muscle cells. We found that tBuOOH/oxyHb induced pronounced peroxidation of major membrane phospholipids-phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol, and phosphatidylserine in the cells as measured by a metabolically integrated oxidation-sensitive fluorescent probe, cis-parinaric acid (PnA). PnA-labeled phospholipids in cells were significantly protected against oxidation by NOC-15. Neither NO-Cys nor NO-GS were able to inhibit tBuOOH/ oxyHb-induced oxidation. Similarly, the tBuOOH/oxyHb-induced decrease in cell survival was not affected by either NO-GS or NO-Cys. A complete protection was, however. provided by NOC-15. Our ESR and spectrophoto- metric measurements demonstrated formation of heme nitrosylated Hb in the presence of NOC-15 and lack of heme nitrosylation by either NO-GS or NO-Cys. We conclude that nitrosothiols do not act as antioxidants against hemoglobin-catalyzed oxidative stress induced by peroxides.
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