Fractionation of oxygen isotopes due to equilibration of oxygen with hemoglobin

The aim of the study was to answer the question as to whether the chemical reaction of oxygen with hemoglobin exhibits a source of isotopic fractionation, which could be of significance in forming the overall fractionation effect of respiration recently determined in man. Investigations were performed on bovine hemoglobin solutions adjusted to normal values of Hb concentration and pH. After degassing in vacuo, the hemoglobin solution was equilibrated in a closed system with pure oxygen of suitable pressure so that oxygen saturation levels of 30, 50 and 100% were achieved. After complete equilibration, isotope analysis of oxygen by mass spectrometry resulted in 16O18O/16O2 ratios which were 0.35 +/- 0.02% lower in the oxygen bound to hemoglobin than that of the gas phase during all levels of oxygenation. Model calculations suggest the following: (1) the fractionation during oxygen uptake in the lung at rest is primarily determined by the reaction with hemoglobin, (2) the overall fractionation effect of respiration can be explained as due to single effects of the constituent processes when assuming the oxygen transport to be limited by utilization.