The mechanism underlying nitroxyl and nitric oxide formation from hydroxamic acids

Background: The pharmacological effects of hydroxamic acids (RC(O)NHOH, HX) are partially attributed to their ability to serve as HNO and/or NO donors under oxidative stress. Given the development and use of HXs as therapeutic agents, elucidation of the oxidation mechanism is needed for more educated selection of HX-based drugs. Methods: Acetohydroxamic and glycine-hydroxamic acids were oxidized at pH 7.0 by a continuous flux of radiolytically generated ^·OH or by metmyoglobin and H₂O₂ reactions system. Gas chromatography and spectroscopic methods were used to monitor the accumulation of N₂O, N₂, nitrite and hydroxylamine. Results: Oxidation of HXs by ^·OH under anoxia yields N₂O, but not nitrite, N₂ or hydroxylamine. Upon the addition of H₂O₂ to solutions containing HX and metmyoglobin, which is instantaneously and continuously converted into compound II, nitrite and, to a lesser extent, N₂O are accumulated under both anoxia and normoxia. Conclusions: Oxidation of HXs under anoxia by a continuous flux of ^·OH, which solely oxidizes the hydroxamate moiety to RC(O)NHO^·, forms HNO. This observation implies that bimolecular decomposition of RC(O)NHO^· competes efficiently with unimolecular decomposition processes such as internal disproportionation, hydrolysis or homolysis. Oxidation by metmyoglobin/H₂O₂ involves relatively mild oxidants (compounds I and II). Compound I reacts with HX forming RC(O)NHO^· and compound II, which oxidizes HX, RC(O)NHO^·, HNO and NO. The latter reaction is the main source of nitrite. General significance: HXs under oxidative stress release HNO, but can be considered as NO-donors provided that HNO oxidation is more efficient than its reaction with other biological targets.