Thiosulfate-utilizing enzymes

HCN is a systemic poison toxicity is due to inhibition of cytochrome oxidase, which prevents cellular utilization of oxygen. Inhibition of the terminal step of electron transport in cells of the brain results in loss of consciousness, respiratory arrest, and ultimately, death. Stimulation of the chemoreceptors of the carotid and aortic bodies produces a brief period of hyperpnea cardiac irregularities may also occur. The biochemical mechanisms of cyanide action are the same for all mammalian species. HCN is metabolized by the enzyme rhodanese which catalyzes the transfer of sulfur from thiosulfate to cyanide to yield the relatively nontoxic thiocyanate. 

While sulfide is toxic to cytochrome (c) oxidase the most important enzyme of the respiratoiy chain, at concentrations in the tens of pM, thiosulfate is not Thus, the animal has produced a soluble and excretable detoxification product and protected animal respiration. This thiosulfate still contains large amounts of energy and can serve as a substrate for bacterial chemoautotrophic metabolism. The bacterial endosymbionts can utilize the thiosulfate along with sulfide to fix CO2 (5Q) and supply the host s nutritional needs. 

Successful antidotes for acute cyanide poisoning are available. Sodium thiosulfate, a sulfur donor, can detoxify HCN by promoting its conversion to thiocyanate when catalyzed by the enzyme rhodanase (Baumeister et al., 1975 Egekeze and Oehme, 1980) however, the detoxification effect is slow. Another procedure utilizes cobalt compounds (hydroxoco-balamine) to bind directly with cyanide to form physiologically inactive cyanocobalamine (Offterdinger and Weger, 1969). 

---