Metmyoglobin and Methemoglobin as Efficient Traps for Nitrosyl Hydride (Nitroxyl) in Neutral Aqueous Solution

Abstract

The reactions of metmyoglobin (Mb+, ferric myoglobin) and methemoglobin (Hb+, ferric hemoglobin) with trioxodinitrate monoanion (HN2O3-) in neutral aqueous solution have been studied at 25 °C under anaerobic conditions. The sole heme product of the reaction is nitrosylmyoglobin (MbNO) or nitrosylhemoglobin (HbNO). The reactions are approximately first-order in [HN2O3-] and zero-order in ferric heme protein concentration, and the rate of formation of the nitrosyl (ferrous) heme protein product is always less than that of HN2O3-decomposition. The HN2O3-/ferric heme protein mole ratio required for quantitative conversion to the nitrosyl heme protein is about 1.4 by titration and, in the case of Mb+, 1.25 by kinetic analysis. Product analyses show that nitrosyl heme protein formation occurs at the expense of N2O production, but not of nitrite production. The results are consistent with the view that HN2O3-decomposes in the rate-determining step into nitrosyl hydride (nitroxyl, HNO) plus nitrite and that HNO then partitions in fast reactions between dimerization/dehydration to form N2O and reaction with ferric heme protein to form nitrosyl heme protein. Hb+ shows kinetic evidence for cooperativity in the latter reaction. In an alternative possibility, HN2O3-may decompose into NO and (HONO)-(Doyle, M. P.; Mahapatro, S. N. J. Am. Chem. Soc. 1984, 106, 3678–3679). Subsequently (HONO)-reduces Mb+or Hb+ to Mb or Hb, and the latter captures NO to form MbNO or HbNO. This pathway is held to be unlikely on chemical grounds. © 1985, American Chemical Society. All rights reserved.