A tracer analysis study on the redistribution and oxidization of endogenous carbon monoxide in the human body

Past studies have suggested that some carbon monoxide (CO) moves from blood haemoglobin to tissue cells and that mitochondrial cytochrome c oxidase oxidizes CO to carbon dioxide (CO(2)). However, no study has demonstrated this redistribution and oxidization of CO under physiological conditions. The objective of this study was to trace the redistribution and oxidization of CO in the human body by detecting (13)CO(2) production after the inhalation of (13)CO. In Experiment 1, we asked a healthy subject to inhale 50 ppm (13)CO gas. In Experiment 2, we circulated heparinized human blood in a cardio-pulmonary bypass circuit and supplied 50 ppm (13)CO gas to the oxygenator. We sequentially sampled exhaled and output gases and measured the (13)CO(2)/(12)CO(2) ratios. In Experiment 1, the exhaled (13)CO(2)/(12)CO(2) ratio increased significantly between 4 to 31 h of (13)CO inhalation. In Experiment 2, the output (13)CO(2)/(12)CO(2) ratio showed no significant increase within 36 h of (13)CO input. Experiment 1 demonstrated the oxidization of CO in the human body under physiological conditions. Experiment 2 confirmed that oxidization does not occur in the circulating blood and indicated the redistribution of CO from blood carboxyhaemoglobin to tissue cells.