Hydrogen peroxide is not released following reaction of cyanide with several catalytically important derivatives of cytochrome c oxidase

Response of Heme Symmetry to the Redox State of Bovine Cytochrome c Oxidase

Second-derivative absorption spectroscopy was employed to monitor the response of effective symmetry of cytochromes a and a₃ to the redox and ligation states of bovine cytochrome c oxidase (CcO). The Soret band π → π* electronic transitions were used to display the changes in symmetry of these chromophores induced by the reduction of CcO inhibited by the exogenous ligands and during catalytic turnover. The second derivative of the difference absorption spectra revealed only a single Soret band for the oxidized cytochromes a and a₃ and cyanide-ligated oxidized cytochrome a₃. In contrast, two absorption bands were resolved in ferrous cytochrome a and ferrous cytochrome a₃ ligated with cyanide. A transition from one-band spectrum to two-band spectrum indicates the lowering of symmetry of these hemes due to the alteration of their immediate surroundings. It is suggested that the changes in polarity occurring in the vicinity of these cofactors are main reason for the split of the Soret band of both ferrous cytochrome a and cyanide-bound ferrous cytochrome a₃.

Redox signaling during hypoxia in mammalian cells

Hypoxia triggers a wide range of protective responses in mammalian cells, which are mediated through transcriptional and post-translational mechanisms. Redox signaling in cells by reactive oxygen species (ROS) such as hydrogen peroxide (H₂O₂) occurs through the reversible oxidation of cysteine thiol groups, resulting in structural modifications that can change protein function profoundly. Mitochondria are an important source of ROS generation, and studies reveal that superoxide generation by the electron transport chain increases during hypoxia. Other sources of ROS, such as the NAD(P)H oxidases, may also generate oxidant signals in hypoxia. This review considers the growing body of work indicating that increased ROS signals during hypoxia are responsible for regulating the activation of protective mechanisms in diverse cell types.

Functional and structural evaluation of bovine heart cytochrome c oxidase incorporated into bicelles

Bilayered long- and short-chain phospholipid assemblies, known as bicelles, have been widely used as model membranes in biological studies. However, to date, there has been no demonstration of structural or functional viability for the fundamental mitochondrial electron transport complexes reconstituted into or interacting with bicelles. In the present work, bicelles were formed from the mixture of long- and short-chain phospholipids, specifically 14:0 and 6:0 phosphatidylcholines (1,2-dimyristoyl-sn-glycero-3-phosphocholine, (DMPC) and 1,2-dihexanoyl-sn-glycero-3-phosphocholine, (DHPC)). Isolated from bovine heart, cytochrome c oxidase was successfully incorporated into bicelles. Bicelles and cytochrome c oxidase incorporated into bicelles ("proteobicelles") were characterized by absorption spectroscopy, dynamic light scattering, atomic force microscopy, sedimentation velocity and differential scanning calorimetry. It was demonstrated that at total concentration of phospholipids CL = 24 mM and the molar ratio (q) of long-chain DMPC over short-chain DHPC equal to 0.4, the diameter of bicelles formed at neutral pH is in the range of 30-60 nm with the thickness of bicelles of about 4 nm. Adding cytochrome c oxidase to bicelles unified the size of the resulting proteobicelles to about 160 nm. Cytochrome c oxidase in bicelles was fully reducible by artificial donors of electrons, exhibited "normal" reaction with external ligands, and was fully active. Both, sedimentation velocity analysis and temperature-induced denaturation indicated that enzyme in bicelles is monomeric. We concluded that cytochrome c oxidase in bicelles maintains its structural and functional integrity, and that bicelles can be used for more comprehensive investigation of cytochrome c oxidase and most likely other mitochondrial electron transfer complexes.

Mitochondrial ROS in cancer: initiators, amplifiers or an Achilles' heel?

Mitochondria cooperate with their host cells by contributing to bioenergetics, metabolism, biosynthesis, and cell death or survival functions. Reactive oxygen species (ROS) generated by mitochondria participate in stress signalling in normal cells but also contribute to the initiation of nuclear or mitochondrial DNA mutations that promote neoplastic transformation. In cancer cells, mitochondrial ROS amplify the tumorigenic phenotype and accelerate the accumulation of additional mutations that lead to metastatic behaviour. As mitochondria carry out important functions in normal cells, disabling their function is not a feasible therapy for cancer. However, ROS signalling contributes to proliferation and survival in many cancers, so the targeted disruption of mitochondria-to-cell redox communication represents a promising avenue for future therapy.

Ferricytochrome c protects mitochondrial cytochrome c oxidase against hydrogen peroxide-induced oxidative damage

An excess of ferricytochrome c protects purified mitochondrial cytochrome c oxidase and bound cardiolipin from hydrogen peroxide-induced oxidative modification. All of the peroxide-induced changes within cytochrome c oxidase, such as oxidation of Trp(19,IV) and Trp(48,VIIc), partial dissociation of subunits VIa and VIIa, and generation of cardiolipin hydroperoxide, no longer take place in the presence of ferricytochrome c. Furthermore, ferricytochrome c suppresses the yield of H(2)O(2)-induced free radical detectable by electron paramagnetic resonance spectroscopy within cytochrome c oxidase. These protective effects are based on two mechanisms. The first involves the peroxidase/catalase-like activity of ferricytochrome c, which results in the decomposition of H(2)O(2), with the apparent bimolecular rate constant of 5.1±1.0M(-1)s(-1). Although this value is lower than the rate constant of a specialized peroxidase, the activity is sufficient to eliminate H(2)O(2)-induced damage to cytochrome c oxidase in the presence of an excess of ferricytochrome c. The second mechanism involves ferricytochrome c-induced quenching of free radicals generated within cytochrome c oxidase. These results suggest that ferricytochrome c may have an important role in protection of cytochrome c oxidase and consequently the mitochondrion against oxidative damage.

Hypoxia-induced changes in pulmonary and systemic vascular resistance: where is the O2 sensor?

Pulmonary arteries (PA) constrict in response to alveolar hypoxia, whereas systemic arteries (SA) undergo dilation. These physiological responses reflect the need to improve gas exchange in the lung, and to enhance the delivery of blood to hypoxic systemic tissues. An important unresolved question relates to the underlying mechanism by which the vascular cells detect a decrease in oxygen tension and translate that into a signal that triggers the functional response. A growing body of work implicates the mitochondria, which appear to function as O2 sensors by initiating a redox-signaling pathway that leads to the activation of downstream effectors that regulate vascular tone. However, the direction of this redox signal has been the subject of controversy. Part of the problem has been the lack of appropriate tools to assess redox signaling in live cells. Recent advancements in the development of redox sensors have led to studies that help to clarify the nature of the hypoxia-induced redox signaling by reactive oxygen species (ROS). Moreover, these studies provide valuable insight regarding the basis for discrepancies in earlier studies of the hypoxia-induced mechanism of redox signaling. Based on recent work, it appears that the O2 sensing mechanism in both the PA and SA are identical, that mitochondria function as the site of O2 sensing, and that increased ROS release from these organelles leads to the activation of cell-specific, downstream vascular responses.

Hypoxic pulmonary vasoconstriction: redox events in oxygen sensing

Recently, the mitochondria have become the focus of attention as the site of O(2) sensing underlying hypoxic pulmonary vasoconstriction (HPV). However, two disparate models have emerged to explain how mitochondria react to a decrease in Po(2). One model proposes that a drop in Po(2) decreases the rate of mitochondrial reactive oxygen species (ROS) generation, resulting in a decrease in oxidant stress and an accumulation of reducing equivalents. The resulting shift of the cytosol to a reduced state causes the inhibition of voltage-dependent potassium channels, membrane depolarization, and the influx of calcium through voltage-gated (L-type) calcium channels. A second and opposing model suggests that hypoxia triggers a paradoxical increase in a mitochondrial-induced ROS signal. The resulting shift of the cytosol to an oxidized state triggers the release of intracellular calcium stores, recruitment of calcium channels in the plasma membrane, and activation of contraction. This article summarizes the potential involvement of a mitochondria-induced ROS signal in these two very different models.

Two Sites of Interaction of Anions with Cytochrome a in Oxidized Bovine Cytochrome c Oxidase

An interaction between cytochrome a in oxidized cytochrome c oxidase (CcO) and anions has been characterized by EPR spectroscopy. Those anions that affect the EPR g = 3 signal of cytochrome a can be divided into two groups. One group consists of halides (Cl-, Br-, and I-) and induces an upfield shift of the g = 3 signal. Nitrogen-containing anions (CN-, NO2-, N3-, NO3-) are in the second group and shift the g = 3 signal downfield. The shifts in the EPR spectrum of CcO are unrelated to ligand binding to the binuclear center. The binding properties of one representative from each group, azide and chloride, were characterized in detail. The dependence of the shift on chloride concentration is consistent with a single binding site in the isolated oxidized enzyme with a Kd of approximately 3 mm. In mitochondria, the apparent Kd was found to be about four times larger than that of the isolated enzyme. The data indicate it is the chloride anion that is bound to CcO, and there is a hydrophilic size-selective access channel to this site from the cytosolic side of the mitochondrial membrane. An observed competition between azide and chloride is interpreted by azide binding to three sites: two that are apparent in the x-ray structure plus the chloride-binding site. It is suggested that either Mg2+ or Arg-438/Arg-439 is the chloride-binding site, and a mechanism for the ligand-induced shift of the g = 3 signal is proposed.

Proton involvement in the transition from the "peroxy" to the ferryl intermediate of cytochrome c oxidase

In the absence of any external electron donor, the "peroxy" intermediate of cytochrome c oxidase (CcO-607) is converted to the ferryl form (CcO-580) and subsequently to oxidized enzyme. The rate of conversion of CcO-607 to the CcO-580 form is pH dependent between pH 3.0 and pH 7.6. A plot of the logarithm of the rate constant for this conversion is a linear function of pH with a slope of -0.92, implying the involvement of a single proton in the transition. Upon rapidly lowering the pH from 8.1 to 5.8, the uptake of one proton was observed by direct pH measurement, and the kinetics of proton uptake coincide with the spectral conversion of CcO-607 to CcO-580. We interpret the slow endogenous decay of CcO-607 to CcO-580 to be the result of proton transfer to a deprotonated group generated in the binuclear cavity during CcO-607 formation. This group is not freely accessible to protons from the medium, and its pK(a) is probably higher than 9.0.

Nitroprusside induces cardiomyocyte death: interaction with hydrogen peroxide

We examined the hypothesis that sodium nitroprusside (SNP) produces cell death in cardiomyocytes through generation of H(2)O(2). Embryonic chick cardiomyocytes in culture were treated with SNP, and cell viability was assessed by trypan blue, MTT assay, and fluorescent activated cell sorting (FACS) analysis. SNP for 24 h induced a significant (P < 0.001) dose-dependent loss of cell viability. On MTT assay, the half-maximal effective concentration was 0.53 mM (confidence interval 0.45-0.59 mM). SNP-treated cardiomyocytes displayed characteristic microscopic features of apoptosis: reduced cell size, nuclear disintegration, and membrane bleb formation. FACS analysis demonstrated SNP-induced apoptosis as well as cell changes consistent with necrosis. The proportion of cells with nuclear changes of apoptosis, identified by propidium iodide (PI) staining of permeabilized cells, increased significantly (P < 0.05) after 0.5 mM SNP for 24 h. The proportion of apoptotic cells, characterized by dual staining of intact cardiomyocytes with fluorescein diacetate and PI, was significantly (P < 0.05) increased after treatment with 0.5 mM SNP for 24 h. SNP metabolism and NO production was suggested by the significant (P < 0.05) increase in nitrite generation in the media with 0.5 mM SNP compared with control. SNP-mediated H(2)O(2) production was implicated in the mechanism of SNP-induced cell death. First, SNP produced a significant (P < 0.05) increase in H(2)O(2) detected in the media after 6 or 24 h of SNP treatment. Second, catalase completely blocked the reduction of cell viability induced by 0.1 mM SNP and significantly (P < 0.05) blunted the effect of 0.5 mM SNP. In contrast, the iron chelator deferoxamine did not alter SNP-induced loss of cell viability. FACS analysis showed that the combination of low concentrations of H(2)O(2) (10(-8) M) that did not alter cell viability augmented SNP-induced apoptosis. In contrast, the amount of necrotic cell death was unchanged by the combination of H(2)O(2) and SNP. H(2)O(2) plus SNP produced a dramatic alteration in cell structure with greater membrane bleb formation, shrunken cells, and more intense cytosolic acridine orange staining and nuclear fragmentation than either agent alone. These data indicate the vulnerability of cardiomyocytes to SNP and suggest the involvement of H(2)O(2) in the pathogenesis of SNP-induced cardiomyocyte cell death. Establishing apoptosis as a component of the type of cell death induced by SNP permitted the recognition that SNP-induced apoptosis was increased by chronic treatment with low (subtoxic) concentrations of H(2)O(2).

Cellular oxygen sensing by mitochondria: old questions, new insight

Hypoxia elicits a variety of adaptive responses at the tissue level, at the cellular level, and at the molecular level. A physiological response to hypoxia requires the existence of an O(2) sensor coupled to a signal transduction system, which in turn activates the functional response. Although much has been learned about the signaling systems activated by hypoxia, no consensus exists regarding the nature of the underlying O(2) sensor or whether multiple sensors exist. Among previously considered mechanisms, heme proteins have been suggested to undergo allosteric modification in response to O(2) binding or release at different PO(2) levels. Other studies suggest that ion channels may change conductance as a function of PO(2), allowing them to signal the onset of hypoxia. Still other studies suggest that NADPH oxidase may decrease its generation of reactive O(2) species (ROS) during hypoxia. Recent data suggest that mitochondria may function as O(2) sensors by increasing their generation of ROS during hypoxia. These oxidant signals appear to act as second messengers in the adaptive responses to hypoxia in a variety of cell types. Such observations contribute to a growing awareness that mitochondria do more than just generate ATP, in that they initiate signaling cascades involved in adaptive responses to hypoxia and that they participate in the control of cell death pathways.

Mass spectrometric determination of dioxygen bond splitting in the "peroxy" intermediate of cytochrome c oxidase

The "peroxy" intermediate (P form) of bovine cytochrome c oxidase was prepared by reaction of the two-electron reduced mixed-valence CO complex with (18)O(2) after photolytic removal of CO. The water present in the reaction mixture was recovered and analyzed for (18)O enrichment by mass spectrometry. It was found that approximately one oxygen atom ((18)O) per one equivalent of the P form was present in the bulk water. The data show that the oxygen-oxygen dioxygen bond is already broken in the P intermediate and that one oxygen atom can be readily released or exchanged with the oxygen of the solvent water.

Redox state of peroxy and ferryl intermediates in cytochrome c oxidase catalysis

The redox states of the "peroxy" (P) and "ferryl" (F) intermediates formed during reoxidation of reduced bovine cytochrome c oxidase have been probed by reduction with both ferrocytochrome c and acetylpyridine NADH under anaerobic conditions using optical spectroscopy. The reduction of the P and F forms revealed that both are in very similar redox states. One-electron reduction of either the P or F form yields an optical spectrum primarily due to oxidized enzyme implying that the heme iron of cytochrome a3 is in the ferryl state in both forms. The F and P forms were found to be 1 and less than 1.3 oxidizing equiv, respectively, above the oxidized enzyme. The slightly higher oxidation state in the P form is interpreted as being due to an optically undetectable redox center presumably located in the binuclear cavity.