Oxygen-dissociation kinetics in the blood of smokers and non-smokers: interaction between oxygen and carbon monoxide at the hemoglobin molecule

Summary

The importance of smoking as a possible factor in coronary heart disease (CHD) may be related to the effect of carbon monoxide (CO) on oxygen exchange at the hemoglobin molecule (Hb). We examined the kinetics of this process in vitro and ex vivo using a fast-reaction technique (stopped-flow) whereby the dissociation-rate constant of oxygen was determined in the blood of smokers and non-smokers and at fixed HbCO-concentrations in non-smokers blood. In non-smokers, carbon monoxide saturated blood was obtained by gassing with carbon monoxide and mixing the samples with appropriate carbon monoxide free blood to achieve HbCO-concentration in the range of 10–60%. The reaction time course for the oxygen-dissociation was divided into a non-linear-initial phase (loss of the first oxygen molecule) and a subsequent linear phase. The oxygen-dissociation velocity decreased from 96.5 × 10³ ms⁻¹ to 42.7 × 103 ms⁻¹ in the linear phase at pH 7.4 and decreased from 29.2 × 103 ms⁻¹ to 20.9 × 103 ms⁻¹ at pH 9.2 when the HbCO-concentration was increased to 63%. For the initial phase at pH 7.4, the dissociation velocity decreased depending on the HbCO-concentration. In non-smokers 50% of the bound oxygen was released in 17.5±2.3 ms (n = 13) whereas in smokers 19.4±1.8 ms (n = 14) (p<0.05) was required. For non-smokers the initial reaction proceeded with a dissociation-rate constant of k = 50.4±11.9 × 10³ ms⁻¹ (n = 13), in smokers this process was slower (k = 42.7±5.9 × 10³ ms⁻¹ (n = 14) (p<0.05). In the latter phase the reaction (occuring over 14–30 ms) the rate of oxygen-release in smokerswas also reduced (k = 77.1±7.3 × 10³ ms⁻¹ versus k = 85.1±13.7 × 10³ ms⁻¹; p = 0.05). These differences were presumably due to the presence of 8.9±2.3% (n = 14) HbCO in smokers. In non-smokers theHbCO-concentration was 0.81±0.11% (n = 13). These results support the view that increased HbCO-levels are responsible for a decreased oxygen capacity and a slower oxygen dissociation-rate from hemoglobin in smokers. This phenomenon can impair oxygen availability in CHD patients who smoke.