A spectrophotometric study of the competition of methemoglobin and cytochrome oxidase for cyanide in vitro.

Although the efficacy of induced methemoglobinemia in cyanide poisoning has been demonstrated both therapeutically and prophylactically (l),l the tissue reactions involved in the protection of animals from the lethal effects of the cyanide ion by methemoglobin are only imperfectly understood. Presumably, cyanide interferes with cellular respiration by forming a slightly dissociable complex with cytochrome oxidase (2-4). Kobert (5) first showed that methemoglobin combines with cyanide to form cyanmethemoglobin. The often reported antidotal action of methemoglobin-forming substances in cyanide poisoning may then be regarded as a competitive reaction between methemoglobin and cytochrome oxidase for cyanide ion. It is the purpose of the present work to determine whether this competition may be demonstrated in vitro and to obtain some idea of the comparative dissociabilities of cyanmethemoglobin and cytochrome oxidase-cyanide complex.