Abstract
The reaction of hemoglobin (Hb) or myoglobin (Mb) with the monoanion (HN203", 1) of sodium trioxodinitrate was studied in anaerobic 50 mM potassium phosphate buffer at pH 7.0, 25 °C. The reaction showed two kinetic phases, and the initial more rapid reaction is the reaction of interest. That reaction was first-order in 1 and zero-order in [Hb]. The rate of disappearance of Hb was nearly twice that of 1, and in titrations, 1 mol of 1 consumed about 2 mol of Hb. The unimolecular rate-determining step in the disappearance of 1 in the presence of Hb is apparently identical with that in its absence. At 50-100 μ Hb, the products were methemoglobin (Hb+) and nitrosylhemoglobin (HbNO) in approximately equimolar amounts. At 1 mM Hb, HbNO was the chief product. Similar results to above were obtained with Mb. Formation of Hb+ and HbNO could suppress formation of both nitrite and N20, which are the normal decomposition products of 1 in the absence of Hb. The results are explained for the most part by rate-determining conversion of 1 to a reactive form (or to primary decomposition products), 1*, followed by reduction of 1* by Hb or Mb. The composition of products would appear to depend on the efficiency with which newly formed Hb+ traps nitrosyl hydride (nitroxyl, HNO), or related species, to form HbNO, relative to dimerization of HNO to form N20. The stoichiometry reported recently (Doyle, . P.; Mahapatro, S. N. J. Am. Chem. Soc. 1984, 106, 3678-3679) for the reaction of Hb with 1 is apparently incorrect and the mechanism inferred therefrom in question.