M. Hoshino et al., STUDIES ON THE REACTION-MECHANISM FOR REDUCTIVE NITROSYLATION OF FERRIHEMOPROTEINS IN BUFFER SOLUTIONS, Journal of the American Chemical Society, 118(24), 1996, pp. 5702-5707
Ferrihemoproteins in buffer solutions bind nitric oxide to yield their
nitric oxide adducts. Reversible binding of NO was found for ferricyt
ochrome c (Cyt(III)) and metmyoglobin (Mb(III)) at pH values lower tha
n ca. 7.0. The equilibrium constants were obtained as (1.6 +/- 0.1) x
10(4) M(-1) for Cyt(III) and (1.3 +/- 0.1) x 10(4) M(-1) for Mb(III).
At higher pH, the reversible formation of the NO adducts is no longer
observed; the NO adduct of Cyt(III) (Cyt(III)-NO) undergoes reduction
to ferrocytochrome c, Cyt(II), and that of Mb(III) (Mb(III)-NO) to the
nitrosyl adduct of Mb(II) (Mb(II)-NO). Methemoglobin (Hb(III)) reacts
readily with NO even at pH < 6 to give the nitrosyl adduct of hemoglo
bin (Hb(II)-NO). The rates for the formation of Cyt(II), Mb(II)-NO, an
d Hb(II)-NO were measured as functions of NO and OH- concentrations. K
inetic analysis indicates that Cyt(III)-NO and Mb(III)-NO undergo nucl
eophilic attack by OH- at higher pH to yield Cyt(II) and Mb(II), respe
ctively. Mb(II) thus produced further reacts with NO to give Mb(II)-NO
. For Hb(III), the nitrosyl adduct (Hb(III)-NO) was found to react wit
h both OH- and H2O to give Hb(II)-NO in the presence of excess NO. The
rate constants for the reaction between the nitrosyl ferrihemoprotein
s and OH- were determined as (1.5 +/- 0.1) x 10(3) M(-1) s(-1) for Cyt
(III)-NO, (3.2 +/- 0.2) x 10(2) M(-1) s(-1) for Mb(III)-NO, and (3.2 /- 0.2) x 10(3) M(-1) s(-1) for Hb(III)-NO. The reductive nitrosylatio
n of Hb(III) observed at pH < 6.0 is explained by reaction of H2O with
Hb(III)-NO: the rate constant is (1.1 +/- 0.1) x 10(-3) s(-1).