Kinetic feasibility of nitroxyl reduction by physiological reductants and biological implications

Abstract

Nitroxyl (HNO), the one-electron reduced and protonated congener of nitric oxide (NO), is a chemically unique species with potentially important biological activity. Although HNO-based pharmaceuticals are currently being considered for the treatment of chronic heart failure or stroke/transplant-derived ischemia, the chemical events leading to therapeutic responses are not established. The interaction of HNO with oxidants results in the well-documented conversion to NO, but HNO is expected to be readily reduced as well. Recent thermodynamic calculations predict that reduction of HNO is biologically accessible. Herein, kinetic analysis suggests that the reactions of HNO with several mechanistically distinct reductants are also biologically feasible. Product analysis verified that the reductants had in fact been oxidized and that in several instances HNO had been converted to hydroxylamine. Moreover, a theoretical analysis suggests that in the reaction of HNO with thiol reductants, the pathway producing sulfinamide is significantly more favorable than that leading to disulfide. Additionally, simultaneous production of HNO and NO yielded a biphasic oxidative capacity.

A comparison of the chemistry associated with the biological signaling and actions of nitroxyl (HNO) and nitric oxide (NO)

“…The rate constant for the reaction of HNO with glutathione has been reported to be 2-8 × 10 6 M −1 s −1 [33,58] and reaction of HNO with a protein with an "activated" thiol (papain) is significantly faster (k = 2 × 10 7 M − 1s −1 ) [59] indicating it can be a very efficient thiol-modifying agent.…”

The use of cyclic nitroxide radicals as HNO scavengers

“…(5). The numbers in parenthesis are G-values, which represent the radiation yields in neutral water (in number of molecules formed per 100 eV corresponding to 0.1036 μM Gy −1 ) [35], and are about 7% higher in the presence of high solute concentrations.…”

“…The rate constant of HNO reaction with the stable cyclic nitroxide 4-hydroxy-2,2,6,6-tetramethyl piperidine-1-oxyl (TEMPOL) has been determined to be 8×10 4 M −1 s −1 using the HNO donor Angeli's salt (AS) and competition kinetics against metmyoglobin (MbFe III ) assuming that TEMPOL oxidizes HNO [4]. Surprisingly, its structural analog 2,2,6,6-tetramethyl piperidine-1-oxyl (TEMPO) has been reported to reduce HNO, and there is neither experimental evidence nor any explanation on how the authors arrived at this conclusion [5]. We have previously studied using AS and EPR spectrometry the reactions of HNO with TEMPOL, the nitronyl nitroxides 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO) and 2-(4-carboxyphenyl)-4,4,5,5,-tetramethylimidazoline-1-oxyl-3-oxide (C-PTIO), and have shown unequivocally that HNO reduces these nitroxides to their respective hydroxylamines (reaction (1)) [6,7].…”

The Specificity of Nitroxyl Chemistry Is Unique Among Nitrogen Oxides in Biological Systems

“…Some caution is therefore required when using a high concentration of HNO donor to evaluate biological properties in closed systems or in containers with low surface areas. Autoxidation of HNO is relatively slow [*10 3 M -1 s -1 (76,92,112)] compared with reactions with metal complexes and thiols, suggesting limited importance in the cytoplasm. However, in cell membranes where the concentrations of neutral gases are higher (93) and scavengers such as thiols are less abundant, autoxidation of HNO may become relevant.…”