The sigmoidal “apparent zero order” behavior of the nitrite reaction with deoxyhemoglobin. Deoxyhemoglobin (50 μM) reaction with nitrite (10 mM) at pH 7.4 and 37°C. (A) Time-resolved absorption spectra were deconvoluted to determine the percentage of each species as a function of time. Deoxyhemoglobin is observed to form equal amounts of methemoglobin and iron-nitrosyl-hemoglobin. Deviation from first-order behavior is evident in the curve for decay of deoxyhemoglobin, having a sigmoidal shape. In experiments with excess nitrite, the spectrum of nitrite bound to methemoglobin (nitrite-methemoglobin) should be included. (B) The instantaneous rate of the reaction shown in panel A where the negative of the slope of the decay curve for deoxyhemoglobin is plotted as a function of time. The figure is reproduced from Grubina et al15 with permission.
