Mechanistic studies of the oxygen-mediated oxidation of nitrosylhemoglobin

Nitrosylhemoglobin (HbFe(II)NO) has been shown to be generated in vivo from the reaction of deoxyHb with NO(*) as well as with nitrite. Despite the physiological importance attributed to this form of Hb, its reactivity has not been investigated in detail. In this study, we showed that the rate of oxidation of HbFe(II)NO by O(2) does not depend on the O(2) concentration. The reaction time courses had to be fitted to a two-exponential expression, and the obtained rates were approximately 2 x 10(-)(4) and 1 x 10(-)(4) s(-)(1), respectively. In the presence of the allosteric effector inositol hexaphosphate (IHP), the value for the fast component of the rate was significantly larger (44 x 10(-)(4) s(-)(1)) whereas that for the slow step was only slightly higher (2.5 x 10(-)(4) s(-)(1)). Moreover, we found that both in the absence and in the presence of IHP the rate of the O(2)-mediated oxidation of HbFe(II)NO is essentially identical to that of NO(*) dissociation from HbFe(II)NO, determined under analogous conditions by replacement of NO(*) with CO in the presence of an excess of dithionite. Taken together, our data show that the reaction between O(2) and HbFe(II)NO proceeds in three steps via dissociation of NO(*) (rate-determining step), binding of O(2) to deoxyHb, and NO(*)-mediated oxidation of oxyHb to metHb and nitrate.