Facilitated diffusion. The case of carbon monoxide.

An application of a singular perturbation method to the case of carbon monoxide similar to that used previously for the case of oxygen (MURRAY, J. D., Proc. Roy. SOC. London B Biol. Sci., 178, 95 (1971)) shows that the absence of any observed facilitation of diffusion of this ligand by either hemoglobin or myoglobin in all of the experiments so far performed results from the very high affinity of both proteins for the gas. At every point in the solution both proteins were essentially at equilibrium with the gas but it was evidently impossible to reduce the pressure of carbon monoxide on the low pressure face of the solution sufficiently to remove a significant amount of the carbon monoxide from combination with the macromolecular carrier. The principle that a macromolecule can only function as a carrier under conditions in which its saturation with the ligand is incomplete in some region of the solution is quite general.