The effects of cyanide have a rapid onset and are extremely toxic because it is easily absorbed through the skin, lungs, eyes, and gastrointestinal tract. Cyanide produces histotoxic cellular hypoxia by initially binding to the protein portion of cytochrome oxidase and then to the ferric form of iron. This reaction forms a relatively stable but reversible complex. Over 40 enzyme systems in the human body are potentially inactivated by cyanide. The most important of these are the cytochrome oxidases. This intramitochondrial system consists of the cytochrome a-a3 complex of the electron transport chain. By binding to this enzyme complex, cyanide inhibits electron transfer to molecular oxygen, thus blocking ATP generation. This results in the reduced cellular utilization of oxygen and increased venous Po2. The impairment of cellular oxygen utilization reduces aerobic respiration with a decrease in pyruvate conversion in the Krebs citrate cycle; lactic acid formation then increases, producing metabolic acidosis (Sa!┬žO.O.e.y 1994).

Elimination of cyanide is primarily through the enzymes rhodanese and mercaptopyruvate (80 per cent), which catalyze the transfer of sulfur from the sulfur donor to cyanide. This forms thiocyanate which is subsequently excreted by the kidneys. The half-life of thiocyanate is 2.7 days. Renal insufficiency may lead to thiocyanate toxicity. The endogenous sulfur pool is small and the reaction is too slow to prevent acute cyanide toxicity. The reaction can be accelerated (as much as 30-fold) by adding sulfur in the form of sodium thiosulfate (Sylvester. ..a/: 1983). The remaining cyanide can react with hydroxocobalamin (vitamin B12a) to form cyanocobalamin

(vitamin B12) or oxidize to CO2 and formate with small amounts of HCN.

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