126: Cyanide and Hydrogen Sulfide

CYANIDE POISONING

History and Epidemiology

Cyanide exposure is associated with smoke inhalation, laboratory mishaps, industrial incidents, suicide attempts, and criminal activity.^39,113 Cyanide is a chemical group that consists of one atom of carbon bound to one atom of nitrogen by three molecular bonds (C≡N). Inorganic cyanides (also known as cyanide salts) contain cyanide in the anion form (CN^–) and are used in numerous industries, such as metallurgy, photographic developing, plastic manufacturing, fumigation, and mining. Common cyanide salts include sodium cyanide (NaCN) and potassium cyanide (KCN). Sodium salts react readily with water to form hydrogen cyanide. Organic compounds that have a cyano group bonded to an alkyl residue are called nitriles. For example, methyl cyanide is also known as acetonitrile (CH₃CN). Hydrogen cyanide (HCN) is a colorless gas at standard temperature and pressure with a reported bitter odor. Cyanogen gas, a dimer of cyanide, reacts with water and breaks down into the cyanide anion. Cyanogen chloride (CNCl) is a colorless gas that is easily condensed; it is a listed agent by the Chemical Weapons Convention.

Many plants, such as the Manihot spp, Linum spp, Lotus spp, Prunus spp, Sorghum spp, and Phaseolus spp contain cyanogenic glycosides.¹¹¹ The Prunus species consisting of apricots, bitter almond, cherry, and peaches have pitted fruits containing the glucoside amygdalin. When ingested, amygdalin is biotransformed by intestinal β-d-glucosidase to glucose, aldehyde, and cyanide (Fig. 126–1). Laetrile, which contains amygdalin, was inappropriately suggested to have antineoplastic properties despite a lack of evidence to support such claims.⁸³ When laetrile was administered by intravenous infusion, amygdalin bypassed the necessary enzymes in the gastrointestinal tract to liberate cyanide and did not cause toxicity. However, ingested laetrile can cause cyanide poisoning. Despite data demonstrating its lack of utility in the treatment of cancer, it still is available via the Internet.

Cassava (Manihot esculenta) root is a major source of food for millions of people in the tropics. It is a hardy plant that can remain in the ground for up to 2 years and needs relatively little water to survive. Because the shelf life of a cassava root is short once it is removed from the stem, cassava root must be processed and sent to market as soon as it is harvested. However, proper processing must occur to assure the food’s safety. Processed cassava is called Gari. Linamarin (2-hydroxyixo-buty-nitrite-β-d-glycoside) is the major cyanogenic glycoside in cassava roots. It is hydrolyzed to hydrogen cyanide and acetone in two steps during the processing of cassava roots.¹²⁴ Soaking peeled cassava in water for a single day releases approximately 45% of the cyanogens, whereas soaking for 5 days causes 90% loss. If processing ...