Cytochrome cyanide

As a class of compounds, the two main toxicity concerns for nitriles are acute lethality and osteolathyrsm. A comprehensive review of the toxicity of nitriles, including detailed discussion of biochemical mechanisms of toxicity and stmcture-activity relationships, is available (12). Nitriles vary broadly in their abiUty to cause acute lethaUty and subde differences in stmcture can greatly affect toxic potency. The biochemical basis of their acute toxicity is related to their metaboHsm in the body. Following exposure and absorption, nitriles are metabolized by cytochrome p450 enzymes in the Hver. The metaboHsm involves initial hydrogen abstraction resulting in the formation of a carbon radical, followed by hydroxylation of the carbon radical. MetaboHsm at the carbon atom adjacent (alpha) to the cyano group would yield a cyanohydrin metaboHte, which decomposes readily in the body to produce cyanide. Hydroxylation at other carbon positions in the nitrile does not result in cyanide release. 

The poisoning effect of molecules such as CO and PF3 (p. 495) arises simply from their ability to bond reversibly to haem in the same manner as O2, but much more strongly, so that oxygen transport is prevented. The cyanide ion CN can also displace O2 from oxyhaemoglobin but its very much greater toxicity at small concentrations stems not from this but from its interference with the action of cytochrome a. 

Cyanides are strong Lewis bases that form a range of complexes with d-block metal ions. They are also famous as poisons. When they are ingested, they combine with certain protein molecules—the cytochromes—involved in the transfer of electrons and the supply of energy in cells, and the victim dies. 

Although only two protons are pumped out of the matrix, two others from the matrix are consumed in the formation of H2O. There is therefore a net translocation of four positive charges out of the matrix which is equivalent to the extrusion of four protons. If four protons are required by the chemiosmotic mechanism to convert cytosolic ADP + Pj to ATP, then 0.5 mol ATP is made for the oxidation of one mol of ubiquinol and one mol ATP for the oxidation of 2 mols of reduced cytochrome c. These stoichiometries were obtained experimentally when ubiquinol was oxidized when complexes I, II, and IV were inhibited by rotenone, malonate, and cyanide, respectively, and when reduced cytochrome c was oxidized with complex III inhibited by antimycin (Hinkle et al., 1991). (In these experiments, of course, no protons were liberated in the matrix by substrate oxidation.) However, in the scheme illustrated in Figure 6, with the flow of two electrons through the complete electron transport chain from substrate to oxygen, it also appears valid to say that four protons are extmded by complex I, four by complex III, and two by complex 1. 

Barbiturates such as amobarbital inhibit NAD-hnked dehydrogenases by blocking the transfer from FeS to Q. At sufficient dosage, they are fatal in vivo. Antin cin A and dimercaprol inhibit the respiratory chain between cytochrome b and cytochrome c. The classic poisons H2S, carbon monoxide, and cyanide inhibit cytochrome oxidase and can therefore totally arrest respiration. Malonate is a competitive inhibitor of succinate dehydrogenase. 

The purpose of sodium nitrite (or amyl nitrite in the absence of IV access) is to produce methemoglobin, which binds cyanide with greater affinity than mitochondrial cytochromes. In the presence of decreased oxygen carrying capacity, as in combined exposures to cyanide and carbon monoxide (e.g., some fires), sodium nitrite can be detrimental and should be avoided. 

The mechanism of hydrogen sulfide toxicity is in part similar to that of cyanide. Like cyanide, hydrogen sulfide can inhibit the enzyme cytochrome oxidase resulting in tissue hypoxia. Specific health effects are discussed in greater detail below. 

Smith L, Kruszyna H, Smith RP. 1977. The effect of methemoglobin on the inhibition of cytochrome c oxidase by cyanide, sulfide or azide. Biochem Pharmacol 26 2247-2250. 

The metabolism studies with cyanide present showed no dehydrogenation whatsoever of the substrate. It is thus considered likely that the resazurin and the resorufin interact with some metal bearing system, possibly the cytochromes participating in the hydrogen transfer. Although the resazurin (or resorufin) may interact in a system several steps removed from the dehydrogenation of the particular substrates, the relative rates of reduction of the indicator are still comparable with the relative oxidation rates of the substrates. 

Cyanide Cytochrome oxidase Blocks transfer of electrons to 02. Blocks at site III. 

In higher plants, elevated cyanide concentrations inhibited respiration (through iron complex-ation in cytochrome oxidase) and ATP production and other processes dependent on ATP, such as... 

Drinking water, 1000 mg KCN/L, exposure for 40 days Marked inhibition of cytochrome oxidase activity in liver, brain, and blood increased cyanide concentrations in all tissues inhibition of rhodanese activity diminished labile sulfur tissue levels 43... 

Usefulness of various biochemical indicators of cyanide poisoning, such as cytochrome oxidase inhibition (Gee 1987), estradiol and thyroxine levels in fish plasma (Ruby et al. 1986, 1993a), and pituitary gland histology (Ruby et al. 1993b)... 

Ballantyne, B., S.P. Boardman, J. Bright, DJ. Coffee, T.D. Weber, and P. Williams. 1972. Tissue cyanide concentrations and cytochrome oxidase activities in experimental cyanide poisoning. Brit. Jour. Pharmacol. 44(2) 382P-383P. 

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. 

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