The kinetics of the reaction between NO and O-2 as studied by a novel approach

The kinetics of the reaction between NO and O-2 was determined by measuring the time course of the decrease in tion of NO with a quench-flow technique . NO and O-2 were mixed rapidly and reacted for periods of time varying fro m 10 to 50 s. A second rapid mixing with a solution containing an excess of deoxyhemoglobin and sodium disulfite trapped free NO as nitrosylhemoglobin and reduced O-2. The spectrum of the mixture of deoxy and nitrosylhemoglob in was recorded within 30 s from the second mixing, before any appreciable dissociation of NO from the protein, by means of a flow-cell mounted on-lin e with the quench-flow apparatus. The amount of NO not consumed in the auto -oxidation reaction was calculated from the proportion of nitrosylhemoglobi n in the mixture. As NO and O-2 bind deoxyhemoglobin at comparable rates an d NO is oxidized to nitrate by oxyhemoglobin, the ratio of hemoglobin/(NO O-2) had to be optimized to avoid the interference of this oxidation react ion. The kinetics was first and second order with respect to O-2 and NO, re spectively and third order overall with a rate constant k = 4 X k(aq) = 4 X 2.23 (+/-0.26) X 10(6) M-2 s(-1) at 20 degrees C, invariant in the pH rang e 7-9, in agreement with published values obtained by different methodologi es. (C) 1999 Elsevier Science B.V. All rights reserved.