STUDIES ON THE REACTION MECHANISM FOR REDUCTIVE NITROSYLATION OF FERRIHEMOPROTEINS IN BUFFER SOLUTIONS

Ferrihemoproteins in buffer solutions bind nitric oxide to yield their nitric oxide adducts. Reversible binding of NO was found for ferricytochrome c (CytIII) and metmyoglobin (MbIII) at pH values lower than ca. 7.0. The equilibrium constants were obtained as (1.6 ± 0.1) × 104 M-1 for CytIII and (1.3 ± 0.1) × 104 M-1 for MbIII. At higher pH, the reversible formation of the NO adducts is no longer observed; the NO adduct of CytIII (CytIII−NO) undergoes reduction to ferrocytochrome c, CytII, and that of MbIII (MbIII−NO) to the nitrosyl adduct of MbII (MbII−NO). Methemoglobin (HbIII) reacts readily with NO even at pH < 6 to give the nitrosyl adduct of hemoglobin (HbII−NO). The rates for the formation of CytII, MbII−NO, and HbII−NO were measured as functions of NO and OH- concentrations. Kinetic analysis indicates that CytIII−NO and MbIII−NO undergo nucleophilic attack by OH- at higher pH to yield CytII and MbII, respectively. MbII thus produced further reacts with NO to give MbII−NO. For HbIII, the nitrosyl ...