Hemoglobin-Methemoglobin and KCN

Summary and Conclusions

These experiments show that KCN combines only with the trivalent form of hemoglobin. Since it does not combine with the bivalent form, the presence of CN^- in the blood system of animals poisoned by CN^- is not the factor which produces death; its lethal effect is rather due to its action on the oxidizing enzymes in the tissues.

Two substances have been used as antidotes for cyanide poisoning, methylene blue and NaNO₂ The action of these two substances is different and can now be clarified as follows: In the case of NaNO₂, methemoglobin is formed in the blood stream, cyanide combines readily with this trivalent form and is taken out of the system as the blood circulates forming cyanmethemoglobin. This of course inactivates a certain percentage of hemoglobin, but the enzymes of the tissues are freed and are again able to function.

In the case of methylene blue, no appreciable concentrations of methemoglobin are produced in the blood stream as the oxidation-reduction potentials of the combined system of methylene blue plus reductants in the blood are too low.¹ Therefore the dye produces its action by catalysis, activating oxygen and actual substitution in place of the poisoned enzymes transferring oxygen to the tissues until the enzymes are again able to function.

K ferri CN and NaNO₂ are strong oxidants compared with methylene blue. The ferro-ferricyanide system, according to Clark's scale 5 is far at the end of the scale near O₂ itself. NaNO₂ on addition to oxyhemoglobin still gives a weak band at 576 mμ, indicating that only a certain proportion of the oxyhemoglobin has been changed to methemoglobin. The methylene blue system is a weak oxidant located almost at neutrality in the pH-Eh oxidation-reduction scale.