Interaction of cyanide and nitric oxide with cytochrome c oxidase: implications for acute cyanide toxicity

Regulation of mitochondrial processes by protein S-nitrosylation

BACKGROUND

Nitric oxide (NO) exerts powerful physiological effects through guanylate cyclase (GC), a non-mitochondrial enzyme, and through the generation of protein cysteinyl-NO (SNO) adducts-a post-translational modification relevant to mitochondrial biology. A small number of SNO proteins, generated by various mechanisms, are characteristically found in mammalian mitochondria and influence the regulation of oxidative phosphorylation and other aspects of mitochondrial function.

SCOPE OF REVIEW

The principles by which mitochondrial SNO proteins are formed and their actions, independently or collectively with NO binding to heme, iron-sulfur centers, or to glutathione (GSH) are reviewed on a molecular background of SNO-based signal transduction.

MAJOR CONCLUSIONS

Mitochondrial SNO-proteins have been demonstrated to inhibit Complex I of the electron transport chain, to modulate mitochondrial reactive oxygen species (ROS) production, influence calcium-dependent opening of the mitochondrial permeability transition pore (MPTP), promote selective importation of mitochondrial protein, and stimulate mitochondrial fission. The ease of reversibility and the affirmation of regulated S-nitros(yl)ating and denitros(yl)ating enzymatic reactions support hypotheses that SNO regulates the mitochondrion through redox mechanisms. SNO modification of mitochondrial proteins, whether homeostatic or adaptive (physiological), or pathogenic, is an area of active investigation.

GENERAL SIGNIFICANCE

Mitochondrial SNO proteins are associated with mainly protective, bur some pathological effects; the former mainly in inflammatory and ischemia/reperfusion syndromes and the latter in neurodegenerative diseases. Experimentally, mitochondrial SNO delivery is also emerging as a potential new area of therapeutics. This article is part of a Special Issue entitled: Regulation of cellular processes by S-nitrosylation.

Putative mechanisms of antitumor activity of cyano-substituted heteroaryles in HeLa cells

Six recently synthesized cyano-substituted heteroaryles, which do not bind to DNA but are highly cytotoxic against the human tumor cell line HeLa, were analyzed for their antitumor mechanisms of action (MOA). They did not interfere with the expression of human papillomavirus oncogenes integrated in the HeLa cell genome, but they did induce strong G1 arrest and result in the activation of caspase-3 and apoptosis. A computational analysis was performed that compared the antiproliferative activities of our compounds in 13 different tumor cell lines with those of compounds listed in the National Cancer Institute database. The results indicate that interference with cytoskeletal function and inhibition of mitosis are the likely antitumor MOA. Furthermore, a second in silico investigation revealed that the tumor cells that are sensitive to the cyano-substituted compounds show differences in their expression of locomotion genes compared with that of insensitive cell lines, thus corroborating the involvement of the cytoskeleton. This MOA was also confirmed experimentally: the cyano-substituted heteroaryles disrupted the actin and the tubulin networks in HeLa cells and inhibited cellular migration. However, further analysis indicated that multiple MOA may exist that depend on the position of the cyano-group; while cyano-substituted naphthiophene reduced the expression of cytoskeletal proteins, cyano-substituted thieno-thiophene-carboxanilide inhibited the formation of cellular reactive oxygen species.

Intracellular conversion of environmental nitrate and nitrite to nitric oxide with resulting developmental toxicity to the crustacean Daphnia magna

BACKGROUND

Nitrate and nitrite (jointly referred to herein as NO(x)) are ubiquitous environmental contaminants to which aquatic organisms are at particularly high risk of exposure. We tested the hypothesis that NO(x) undergo intracellular conversion to the potent signaling molecule nitric oxide resulting in the disruption of endocrine-regulated processes.

METHODOLOGY/PRINCIPAL FINDINGS

These experiments were performed with insect cells (Drosophila S2) and whole organisms Daphnia magna. We first evaluated the ability of cells to convert nitrate (NO(3)(-)) and nitrite (NO(2)(-)) to nitric oxide using amperometric real-time nitric oxide detection. Both NO(3)(-) and NO(2)(-) were converted to nitric oxide in a substrate concentration-dependent manner. Further, nitric oxide trapping and fluorescent visualization studies revealed that perinatal daphnids readily convert NO(2)(-) to nitric oxide. Next, daphnids were continuously exposed to concentrations of the nitric oxide-donor sodium nitroprusside (positive control) and to concentrations of NO(3)(-) and NO(2)(-). All three compounds interfered with normal embryo development and reduced daphnid fecundity. Developmental abnormalities were characteristic of those elicited by compounds that interfere with ecdysteroid signaling. However, no compelling evidence was generated to indicate that nitric oxide reduced ecdysteroid titers.

CONCLUSIONS/SIGNIFICANCE

Results demonstrate that nitrite elicits developmental and reproductive toxicity at environmentally relevant concentrations due likely to its intracellular conversion to nitric oxide.

Qo site of mitochondrial complex III is the source of increased superoxide after transient exposure to hydrogen peroxide

Transient exposure of cardiac myocytes to hydrogen peroxide (H(2)O(2)) results in further production of superoxide by the mitochondria as a result of increased influx of calcium through the L-type Ca(2+) channel and increased calcium uptake by the mitochondria. The response persists as a result of positive feedback on the channel and induces alterations in protein synthesis and cell size consistent with the development of myocyte hypertrophy. The aim of this study was to investigate the site of increased superoxide production within the mitochondria. Exposure of myocytes to 30 μM H(2)O(2) (5 min) then 10 U/mL catalase (5 min) increased dihydroethidium (DHE) signal by 58.7 ± 12.0% (n=4) compared to myocytes exposed to 0 μM H(2)O(2) for 5 min followed by 10 U/mL catalase (n=9). Complex I inhibitors DPI (n=5) and rotenone (n=7) attenuated the increase in DHE signal due to H(2)O(2). Complex III inhibitors myxothiazol (n=16) and stigmatellin (n=5) also attenuated the increase in DHE signal due to H(2)O(2). However, antimycin A (inhibitor of Q(i) site of complex III) had no effect. We "isolated" complex III in the intact cell by applying succinate in the patch pipette and exposing the cell to rotenone and antimycin A. Myxothiazol and TCA cycle inhibitors α-keto-β-methyl-n-valeric acid (KMV) and 4-hydroxynonenal (4-HNE) completely attenuated the increase in DHE signal. Direct activation of the L-type Ca(2+) channel by voltage-step mimicked the increase in DHE signal after transient exposure to H(2)O(2) (47.6 ± 17.8%, n=6) while intracellular application of catalase attenuated the increase in DHE signal due to H(2)O(2) (n=6). We propose that elevated superoxide production after transient exposure to H(2)O(2) occurs at the Q(o) superoxide generation site of complex III in cardiac myocytes and that an increase in TCA cycle activity plays a significant role in mediating the response.

Noninvasive in vivo monitoring of cyanide toxicity and treatment using diffuse optical spectroscopy in a rabbit model

Currently, no reliable noninvasive methods exist for monitoring the severity of in vivo cyanide (CN) toxicity, treatment, and resulting physiological changes. We developed a broadband diffuse optical spectroscopy (DOS) system to measure bulk tissue absorption and scattering. DOS was used to optically monitor CN toxicity and treatment with sodium nitrite (NaNO2). To perform experiments, the DOS probe was placed on the hind leg of rabbits. A sodium CN solution was infused intravenously. DOS and concurrent physiologic measurements were obtained. After completion of CN infusion, NaNO2 was infused to induce methemoglobinemia (MetHb). During infusion of CN, blood gas measurements showed an increase in venous partial pressure of oxygen (pO2), and following reversal, venous pO2 values decreased. DOS measurements demonstrated corresponding changes in hemoglobin oxygenation states and redox states of cytochrome-c oxidase (CcO) during CN infusion and NaNO2 treatment. Therefore, DOS enables detection and monitoring of CN toxicity and treatment with NaNO2.

Nitrite-mediated antagonism of cyanide inhibition of cytochrome c oxidase in dopamine neurons

Cyanide inhibits aerobic metabolism by binding to the binuclear heme center of cytochrome c oxidase (CcOX). Amyl nitrite and sodium nitrite (NaNO(2)) antagonize cyanide toxicity in part by oxidizing hemoglobin to methemoglobin (mHb), which then scavenges cyanide. mHb generation is thought to be a primary mechanism by which the NO(2)(-) ion antagonizes cyanide. On the other hand, NO(2)(-) can undergo biotransformation to generate nitric oxide (NO), which may then directly antagonize cyanide inhibition of CcOX. In this study, nitrite-mediated antagonism of cyanide inhibition of oxidative phosphorylation was examined in rat dopaminergic N27 cells. NaNO(2) produced a time- and concentration-dependent increase in whole-cell and mitochondrial levels of NO. The NO scavenger 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxy 3-oxide (PTIO) reversed this increase in cellular and mitochondrial NO. NO generated from NaNO(2) decreased cellular oxygen consumption and inhibited CcOX activity. PTIO reversed the NO-mediated inhibition, thus providing strong evidence that NO mediates the action of NaNO(2). Under similar conditions, KCN (20muM) inhibited cellular state-3 oxygen consumption and CcOX activity. Pretreatment with NaNO(2) reversed KCN-mediated inhibition of both oxygen consumption and CcOX activity. The NaNO(2) antagonism of cyanide was blocked by pretreatment with the NO scavenger PTIO. It was concluded that NaNO(2) antagonizes cyanide inhibition of CcOX by generating of NO, which then interacts directly with the binding of KCN x CcOX to reverse the toxicity. In vivo antagonism of cyanide by NO(2)(-) appears to be due to both generation of mHb and direct displacement of cyanide from CcOX by NO.

Intramuscular cobinamide sulfite in a rabbit model of sublethal cyanide toxicity

STUDY OBJECTIVE

Exposure to cyanide in fires and industrial exposures and intentional cyanide poisoning by terrorists leading to mass casualties is an ongoing threat. Current treatments for cyanide poisoning must be administered intravenously, and no rapid treatment methods are available for mass casualty cyanide exposures. Cobinamide is a cobalamin (vitamin B(12)) analog with an extraordinarily high affinity for cyanide that is more water-soluble than cobalamin. We investigate the use of intramuscular cobinamide sulfite to reverse cyanide toxicity-induced physiologic changes in a sublethal cyanide exposure animal model and determine the ability of an intramuscular cobinamide sulfite injection to rapidly reverse the physiologic effects of cyanide toxicity.

METHODS

New Zealand white rabbits were given 10 mg sodium cyanide intravenously over 60 minutes. Quantitative diffuse optical spectroscopy and continuous-wave near-infrared spectroscopy monitoring of tissue oxyhemoglobin and deoxyhemoglobin concentrations were performed concurrently with blood cyanide level measurements and cobinamide levels. Immediately after completion of the cyanide infusion, the rabbits were injected intramuscularly with cobinamide sulfite (n=6) or inactive vehicle (controls, n=5).

RESULTS

Intramuscular administration led to rapid mobilization of cobinamide and was extremely effective at reversing the physiologic effects of cyanide on oxyhemoglobin and within deoxyhemoglobin extraction. Recovery time to 63% of their baseline values in the central nervous system occurred within a mean of 1,032 minutes in the control group and 9 minutes in the cobinamide group, with a difference of 1,023 minutes (95% confidence interval 116 to 1,874 minutes). In muscle tissue, recovery times were 76 and 24 minutes, with a difference of 52 minutes (95% confidence interval 7 to 98 minutes). RBC cyanide levels returned toward normal significantly faster in cobinamide sulfite-treated animals than in control animals.

CONCLUSION

Intramuscular cobinamide sulfite rapidly and effectively reverses the physiologic effects of cyanide poisoning, suggesting that a compact cyanide antidote kit can be developed for mass casualty cyanide exposures.

Alpha-lipoic acid protects against potassium cyanide-induced seizures and mortality

This study was proposed to investigate the potential protective effect of alpha-lipoic acid (α-LA) against potassium cyanide (KCN)-induced seizures and lethality in mice. The intraperitoneal ED(50) value of KCN, as measured by induction of clonic and tonic seizures was increased by pretreatment of mice with α-LA (25, 50 and 100 mg/kg) intraperitoneally in a dose-dependent manner. Similarly, the intraperitoneal LD(50) value of KCN, based on 24h mortality, was increased by pretreatment with α-LA in a dose-dependent manner. Intraperitoneal injection of the estimated ED(50) of KCN (4.8 mg/kg) into mice increased, 1h later, nitric oxide (NO) production and brain glutamate and malondialdehyde (MDA) levels. The estimated ED(50) of KCN also decreased brain intracellular reduced glutathione (GSH) level and glutathione peroxidase (GSH-Px) activity in these animals. Administration of the estimated LD(50) of KCN (6 mg/kg) produced, 24h later, similar marked biochemical alterations in surviving animals. Pretreatment of mice with α-LA inhibited; dose-dependently KCN (ED(50) and LD(50))-induced an increase in NO production and brain MDA level as well as a decrease in brain intracellular GSH level and GSH-Px activity. The elevation induced by KCN in brain glutamate level was not inhibited by α-LA. It can be concluded that the protective effect of α-LA against KCN-induced seizures and lethality may be due to inhibition of NO overproduction and maintenance of intracellular antioxidant defense mechanisms.

Cyanide-induced apoptosis of dopaminergic cells is promoted by BNIP3 and Bax modulation of endoplasmic reticulum-mitochondrial Ca2+ levels

Cyanide is a potent neurotoxicant that can produce dopaminergic neuronal death in the substantia nigra and is associated with a Parkinson-like syndrome. In this study involvement of Bcl-2/adenovirus E1B 19-kDa interacting protein 3 (BNIP3), a BH3-only Bcl-2 protein, in cyanide-induced death of dopaminergic cells was determined in mice and Mes 23.5 cells. Treatment of mice with cyanide up-regulated BNIP3 and Bax expression in tyrosine hydroxylase (TH)-positive cells of the substantia nigra, and progressive loss of TH-positive neurons was observed over a 9-day period. In Mes 23.5 dopaminergic cells, cyanide stimulated translocalization of BNIP3 to both endoplasmic reticulum (ER) and mitochondria. In ER, BNIP3 stimulated release of Ca(2+) into the cytosol, followed by accumulation of mitochondrial Ca(2+), resulting in reduction of mitochondrial membrane potential (Deltapsi(m)) and eventually cell death. Cyanide also activated Bax to colocalize with BNIP3 in ER and mitochondria. Forced overexpression of BNIP3 activated Bax, whereas gene silencing reduced Bax activity. Knockdown of Bax expression by small interfering RNA blocked the BNIP3-mediated changes in ER and mitochondrial Ca(2+) to block cyanide-induced mitochondrial dysfunction and cell death. These findings show that BNIP3-mediates cyanide-induced dopaminergic cell death through a Bax downstream signal that mobilizes ER Ca(2+) stores, followed by mitochondrial Ca(2+) overload.

Cyanide-induced death of dopaminergic cells is mediated by uncoupling protein-2 up-regulation and reduced Bcl-2 expression

Cyanide is a potent inhibitor of mitochondrial oxidative metabolism and produces mitochondria-mediated death of dopaminergic neurons and sublethal intoxications that are associated with a Parkinson-like syndrome. Cyanide toxicity is enhanced when mitochondrial uncoupling is stimulated following up-regulation of uncoupling protein-2 (UCP-2). In this study, the role of a pro-survival protein, Bcl-2, in cyanide-mediated cell death was determined in a rat dopaminergic immortalized mesencephalic cell line (N27 cells). Following pharmacological up-regulation of UCP-2 by treatment with Wy14,643, cyanide reduced cellular Bcl-2 expression by increasing proteasomal degradation of the protein. The increased turnover of Bcl-2 was mediated by an increase of oxidative stress following UCP-2 up-regulation. The oxidative stress involved depletion of mitochondrial glutathione (mtGSH) and increased H2O2 generation. Repletion of mtGSH by loading cells with glutathione ethyl ester reduced H2O2 generation and in turn blocked the cyanide-induced decrease of Bcl-2. To determine if UCP-2 mediated the response, RNAi knock down was conducted. The RNAi decreased cyanide-induced depletion of mtGSH, reduced H2O2 accumulation, and inhibited down-regulation of Bcl-2, thus blocking cell death. To confirm the role of Bcl-2 down-regulation in the cell death, it was shown that over-expression of Bcl-2 by cDNA transfection attenuated the enhancement of cyanide toxicity after UCP-2 up-regulation. It was concluded that UCP-2 up-regulation sensitizes cells to cyanide by increasing cellular oxidative stress, leading to an increase of Bcl-2 degradation. Then the reduced Bcl-2 levels sensitize the cells to cyanide-mediated cell death.

Antagonism of nitric oxide toward the inhibition of cytochrome c oxidase by carbon monoxide and cyanide

The principle mitochondrial target where the respiratory inhibitors CO, CN(-), and NO act in the execution of their acute toxic effects is complex IV of the electron-transport chain, cytochrome c oxidase. However, there is a paucity of studies in the literature regarding the concerted effects of such poisons. Accordingly, the combined inhibitory effects of CO + CN(-), NO + CN(-), and NO + CO on the activity of cytochrome c oxidase preparations are reported. Only in the case of CO + CN(-) do the effects of the two inhibitors seem to be additive as expected. NO appears to be antagonistic toward the effects of the other two inhibitors; that is, the effects of both CO an CN(-) on enzyme activity are ameliorated by NO when present. To further clarify these observations, the ligand substitutions of heme-bound CN(-) by NO in cytochrome c oxidase and hemoglobin have also been briefly investigated. These results suggest that displacement of CN(-) from the ferric hemoproteins by NO is rate-limited by heme reduction-and in the case of the enzyme, the presence of nonligand-binding electron-transfer centers facilitates the reaction. The findings are discussed in relation to the idea that NO does not behave as a classic reversible (by dissociation) inhibitor.

Copper-Carbon Bonds in Mechanistic and Structural Probing of Proteins as well as in Situations where Copper is a Catalytic or Receptor Site

While copper-carbon bonds are well appreciated in organometallic synthetic chemistry, such occurrences are less known in biological settings. By far, the greatest incidence of copper-carbon moieties is in bioinorganic research aimed at probing copper protein active site structure and mechanism; for example, carbon monoxide (CO) binding as a surrogate for O2. Using infrared (IR) spectroscopy, CO coordination to cuprous sites has proven to be an extremely useful tool for determining active site copper ligation (e.g., donor atom number and type). The coupled (hemocyanin, tyrosinase, catechol oxidase) and non-coupled (peptidylglycine α-hydroxylating monooxygenase, dopamine β-monooxygenase) binuclear copper proteins as well as the heme-copper oxidases (HCOs) have been studied extensively via this method. In addition, environmental changes within the vicinity of the active site have been determined based on shifts in the CO stretching frequencies, such as for copper amine oxidases, nitrite reductases and again in the binuclear proteins and HCOs. In many situations, spectroscopic monitoring has provided kinetic and thermodynamic data on CuI-CO formation and CO dissociation from copper(I); recently, processes occurring on a femtosecond timescale have been reported. Copper-cyano moieties have also been useful for obtaining insights into the active site structure and mechanisms of copper-zinc superoxide dismutase, azurin, nitrous oxide reductase, and multi-copper oxidases. Cyanide is a good ligand for both copper(I) and copper(II), therefore multiple physical-spectroscopic techniques can be applied. A more obvious occurrence of a “Cu-C” moiety was recently described for a CO dehydrogenase which contains a novel molybdenum-copper catalytic site. A bacterial copper chaperone (CusF) was recently established to have a novel d-π interaction comprised of copper(I) with the arene containing side-chain of a tryptophan amino acid residue. Meanwhile, good evidence exists that a plant receptor site (ETR1) utilizes copper(I) to sense ethylene, a growth hormone. A copper olfactory receptor has also been suggested. All of the above mentioned occurrences or uses of carbon-containing substrates and/or probes are reviewed and discussed within the framework of copper proteins and other relevant systems.

The inhibition of mitochondrial cytochrome oxidase by the gases carbon monoxide, nitric oxide, hydrogen cyanide and hydrogen sulfide: chemical mechanism and physiological significance

The four gases, nitric oxide (NO), carbon monoxide (CO), hydrogen sulfide (H(2)S) and hydrogen cyanide (HCN) all readily inhibit oxygen consumption by mitochondrial cytochrome oxidase. This inhibition is responsible for much of their toxicity when they are applied externally to the body. However, recently these gases have all been implicated, to greater or lesser extents, in normal cellular signalling events. In this review we analyse the chemistry of this inhibition, comparing and contrasting mechanism and discussing physiological consequences. The inhibition by NO and CO is dependent on oxygen concentration, but that of HCN and H(2)S is not. NO and H(2)S are readily metabolised by oxidative processes within cytochrome oxidase. In these cases the enzyme may act as a physiological detoxifier of these gases. CO oxidation is much slower and unlikely to be as physiologically important. The evidence for normal physiological levels of these gases interacting with cytochrome oxidase is equivocal, in part because there is little robust data about their steady state concentrations. A reasonable case can be made for NO, and perhaps CO and H(2)S, inhibiting cytochrome oxidase in vivo, but endogenous levels of HCN seem unlikely to be high enough.

Nitric oxide (NO) increase at fertilization in sea urchin eggs upregulates fertilization envelope hardening

Previous studies indicate that the nitric oxide (NO) increase at fertilization in sea urchin eggs is Ca(2+)-dependent and attributed to the late Ca(2+) rise. However, its role in fertilization still remains unclear. Simultaneous measurements of the activation current, by a single electrode voltage clamp, and NO, using the NO indicator DAF-FM, showed that the NO increase occurred at the time of peak current (t(p)) which corresponds to peak [Ca(2+)](i), suggesting that NO is not related to any other ionic changes besides [Ca(2+)](i). We measured O(2) consumption by a polarographic method to examine whether NO regulated a respiratory burst for protection as reported in other biological systems. Our results suggested NO increased O(2) consumption. The fluorescence of reduced pyridine nucleotides, NAD(P)H was measured in controls and when the NO increase was eliminated by PTIO, a NO scavenger. Surprisingly, PTIO decreased the rate of the fluorescence change and the late phase of increase in NAD(P)H was eliminated. PTIO also suppressed the production of H(2)O(2) and caused weak and high fertilization envelope (FE). Our results suggest that NO increase upregulates NAD(P)H and H(2)O(2) production and consolidates FE hardening by H(2)O(2).