The kinetics of the reaction of carbon monoxide with fully oxygenated haemoglobin in solution and erythrocytes.

1. Spectrophotometric measurements, using a rapid mixing and stopped flow technique, have been made of the rate at which CO displaces O(2) from its combination with haemoglobin.2. In haemoglobin solutions, buffered at pH 7.2 and 9.1, the reaction proceeds by a unimolecular dissociation as proposed by Gibson & Roughton (1955). In a Ringer-Locke solution, equilibrated with a P(CO(2) ) of 3 cmHg and at pH 7.4, the reaction of HbO(2) with CO is a two-stage process, with a transition from one form of Hb(4)O(6) to another.3. An investigation of the reaction between CO and HbO(2) in erythrocytes, suspended in Ringer-Locke solution, indicates that the rate is determined by the chemical reaction and this also is a two-stage process.4. The transition is probably associated with the reaction of CO(2) with Hb(4)O(6), following the dissociation from fully saturated oxyhaemoglobin of an oxygen molecule. It alters the relative velocity constants of the reactions of O(2) and CO with Hb(4)O(6) by 100:1.5. The implications of these proposals of the equilibria of haemoglobin with CO and O(2) are discussed. The difference between the sigmoid equilibria curves at high HbO(2) and HbCO values can be explained as due to the different reaction pathways.