The overall fractionation effect of isotopic oxygen molecules during oxygen transport and utilization in humans

The aim of the study was to clarify (1) whether fractionation effects of isotopic oxygen molecules due to respiratory processes can be neglected in tracer studies, (2) whether additional information about respiratory processes can be obtained from measuring isotope fractionations. Experiments were performed on 7 healthy humans at rest. Samples were taken from inspiratory and expiratory gas. After having removed the carbon dioxide from the samples, the oxygen was completely burnt with graphite to CO2, and the 16O18O/16O2 ratio of the CO2 generated was analysed by mass spectrometry. The 16O18O/16O2 ratio of the expiratory gas was 0.21 +/- 0.07% greater than that of the inspiratory air. The overall rate constant for uptake, subsequent transport and utilization was higher for 16O2 by 0.91 +/- 0.24% than for 16O18O. These results together with model calculations including data from literature suggest: (1) the fractionation effects between 16O18O and 16O2 as well as between 18O2 and 16O2 are small enough for both isotopic species to be considered appropriate labels in the investigation of respiratory processes, (2) the effect measured could be due to limitations of oxygen transport by utilization, (3) additional processes such as the reaction of oxygen with hemoglobin could be involved in forming the overall fractionation effect.