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14. CHEMICAL AND CLINICAL ASPECTS OF METAL-CONTAINING ANTIDOTES FOR POISONING BY CYANIDE

Sigridur G. Suman and Johanna M. Gretarsdottir

Abstract

Physiological metabolism of cyanide takes place by a single major pathway that forms non-toxic thiocyanate that is subsequently excreted. Rhodanese is the primary enzyme to execute metabolism of cyanide with minor pathways from other sulfurtransferases in vivo. The rhodanese enzyme depends on sulfur donor availability to metabolize cyanide and poisoning occurs at elevated cyanide concentrations in vivo. Cyanide interacts with over 40 metalloenzymes, but its lethal action is non-competitive inhibition of cytochrome c oxidase, halting cellular respiration and causing hypoxic anoxia. Only a handful of antidotes for treatment of cyanide poisoning are known; they are primarily inorganic compounds and metal complexes which are intended to intercept cyanide before it inhibits cellular respiration. The inorganic compounds manipulate hemoglobin, forming methemoglobin, or supply sulfur for the rhodanese enzyme. The metal complexes intercept the cyanide and bind it before reaching its target. Cobalt complexes of corrins and vitamin B12 derivatives are the state-of-the-art agents, while the longest employed complex, Co2EDTA, is designed to deliver “free” cobalt for binding of cyanide. Compounds that are in development are discussed from the point of how they are designed to intercept cyanide. The challenge of reversing the cyanide inhibition of cytochrome c oxidase is based on the catalytic active site structure and reactivity. General information about history and occurrence of poisoning and clinical symptoms is discussed and the challenges related to analytical methods available to analyze blood cyanide levels and to confirm the presence of cyanide poisoning.

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