CO production by feces and urine.

Virtually all CO production in humans and animals is thought to result from the catabolism of heme, and endogenous CO production (VCO) is used as a measure of heme turnover. Using a rebreathing technique, we found that previously fed mice had an apparent VCO that was 40% greater than that of mice that had been fasted before the study. This unexpected result was shown to reflect an excess CO production by the excreta (feces, urine, or both) of fed mice relative to the excreta of fasted mice. Incubation of feces and urine of humans, rats, and mice demonstrated CO release that was not inhibited by autoclaving or acidification to pH 1. CO release from excreta was markedly reduced, however, under anaerobic conditions, and fed and fasted animals excreted CO at the same rate when their excreta were maintained in an anaerobic state. We conclude that feces and urine can release CO via nonenzymatic, oxidative reactions and, under some circumstances, this source of CO can influence measurements of VCO.