Otto Heinrich Warburg quotes:

The toxic effect of carbon monoxide on man has nothing to do with inhibition of cellular respiration by carbon monoxide but is based on the reaction of carbon monoxide with blood iron.

The concentration of the ferment iron in living substance is very small, being in the region of 1 g to 10 million g of cellular substance.

Catalytic oxidation in living substances rests upon change of valency in an iron compound which is the respiratory oxygen-transferring ferment.