Scavenging of hydroxyl radicals but not of peroxynitrite by inhibitors and substrates of nitric oxide synthases

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Abstract
The nitric oxide synthase inhibitor NG‐nitro‐l‐arginine methyl ester (l‐NAME) is widely used to study the role of NO• in physiological and pathological processes, including its role in the generation of the cytotoxic species peroxynitrite (ONOO−) and of reactive oxygen radicals such as hydroxyl (OH•). Often l‐NAME is applied to tissues at mM concentrations. At such high concentrations, it might act as a free radical scavenger. A similar possibility might apply to the use of high levels of arginine to study the role of NO. in atherogenesis. We therefore examined the rate of scavenging of OH• by l‐NAME and found that l‐NAME reacts more quickly with OH. than the established ‘OH. scavenger’ mannitol and the widely used ‘OH• trap’ salicylate. However, d‐NAME can scavenge OH• at rates equal to l‐NAME. Both l‐ and d‐arginine were also good OH• scavengers, comparable in effectiveness to mannitol. Neither l‐NAME, d‐NAME, l‐arginine nor d‐arginine was able to inhibit ONOO−‐dependent nitration of tyrosine, suggesting that they are unlikely to be scavengers of ONOO−‐derived nitrating species. Neither l‐NAME, d‐NAME, l‐arginine nor d‐arginine was able to inhibit the inactivation of α1‐antiproteinase by ONOO−, suggesting that they cannot prevent direct oxidations by peroxynitrite. We conclude that l‐NAME has sufficient activity as an OH• scavenger to confound certain pharmacological experiments. However, this explanation of its biological effects can be ruled out if control experiments show that d‐NAME has no effect and that l‐arginine (also a free radical scavenger) antagonizes the action of l‐NAME.