PRINCIPLES OF TOXICOLOGY

4.15 ANTIDOTES FOR HYDROGEN SULFIDE AND CYANIDE POISONING 107
4.15 ANTIDOTES FOR HYDROGEN SULFIDE AND CYANIDE POISONING
Unfortunately, there are no failproof antidotes for hydrogen sulfide poisoning, although
methods that induce methemoglobinemia have been suggested. In instances of cyanide
poisoning, and occasionally hydrogen sulfide exposure, the administration of nitrite in the
form of amyl nitrite or intravenous sodium nitrite is recommended to purposely convert the
patient’s blood to a safe-level of methemoglobin. Methemoglobin has a very strong binding
affinity for cyanide and hydrogen sulfide. The relative large amount of methemoglobin binds up
and acts as a sink to remove cyanide or hydrogen sulfide from cellular spaces and the mitochondria.
Once bound to methemoglobin, cyanide and hydrogen sulfide are no longer available to bind
to (and thus inhibit) cytochrome oxidase, an mitochondrial enzyme essential to the aerobic
metabolism of glucose. The chemicals are eventually released into the blood where they can
be metabolized to thiocyanate (in the case of cyanide) and sulfite/sulfate (in the case of
hydrogen sulfide). The ability of methemoglobin to trap cyanide and hydrogen sulfide is
illustrated in Figure 4.6.
Figure 4.6 Schematic depiction of the electron transport chain through which the oxidation of NADH derived
from sugar metabolism generates ATP. Both the cyanide (CN – ) and hydrogen sulfide (HS – ) anions bind to and
inhibit cytochrome oxidase. However, both anions also bind the Fe +++ ion methemoglobin (MetHb) formed by the
oxidation of hemoglobin with nitrate (NO2 − ). CN-MetHb denotes cyanmethemoglobin; HS-MetHb denotes
sulfmethemoglobin.