The velocity constant, k4, of the reaction Hb4O8 + CO, in sheep erythrocytes and dilute haemoglobin solutions

The rate of dissociation of HbO₂ in erythrocytes and solution has been measured, using a split-beam spectrophotometric detector, after rapid mixing and $\text{1:20}$ dilution in a medium equilibrated with a high pCO. The observations were made on sheep HbA and HbB over the temperature range 5–38°. In solution, the value of $\text{k}{}_4$ was 44.5 sec⁻¹ at 20° and pH 7.4 and the activation energy 19.5 kcal for HbA; the corresponding values for HbB were 54.8 sec⁻¹ and 19 kcal. In the erythrocyte, similar measurements gave the values 38.8 sec⁻¹ and 17 kcal for HbA and 51.2 sec⁻¹ and 16 kcal for HbB. The rate constant $\text{k}{}_4$ for HbA was reduced by 10% in alkaline solutions and that of HbB by 25%. No effect of increasing the ionic strength in the cell on $\text{k}{}_4$ was observed. An analysis of the results suggests that there are two forms of HbB and the haem-haem interaction is reduced when the haemoglobin is in the cell.