The toxicity of a given arsenical is related to the rate of its clearance from the body and therefore to its degree of accumulation in tissues. In general, toxicity increases in the sequence of organic arsenicals <As5+ <As3+ < arsine (AsH3). The organic arsenicals contain arsenic covalently linked to a carbon atom, where arsenic exists in the trivalent or pentavalent state. Arsphenamine contains trivalent arsenic; sodium arsanilate contains arsenic in the pentavalent form. The organic arsenicals usually are excreted more rapidly than are the inorganic forms.
Arsenate (pentavalent) uncouples mitochondrial oxidative phosphorylation by a mechanism whereby arsenate substitutes for inorganic phosphate in the formation of ATP, with subsequent formation of an unstable arsenate ester that is hydrolyzed rapidly. This process is termed arsenolysis.
Trivalent arsenicals, including inorganic arsenite, are regarded primarily as -SH reagents. As such, trivalent arsenicals inhibit many enzymes by reacting with biological ligands containing available -SH groups. The pyruvate dehydrogenase system is especially sensitive to trivalent arsenicals because of their interaction with two -SH groups of lipoic acid to form a stable 6-membered ring.
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