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

The nitric oxide synthase inhibitor N G ‐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 α 1 ‐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.