The steady-state kinetics of cytochrome c oxidation by cytochrome oxidase

Post-Subfunctionalization Functions of HIF-1αA and HIF-1αB in Cyprinid Fish: Fine-Tuning Mitophagy and Apoptosis Regulation Under Hypoxic Stress

Hypoxia-inducible factor 1 (HIF-1) is a crucial transcriptional factor that can restore oxygen balance in the body by regulating multiple vital activities. Two HIF-1α copies were retained in cyprinid fish after experiencing a teleost-specific genome duplication. How the "divergent collaboration" of HIF-1αA and HIF-1αB proceeds in regulating mitophagy and apoptosis under hypoxic stress in cells of cyprinid fish remains unclear. In this study, zebrafish HIF-1αA/B expression plasmids were constructed and transfected into the epithelioma papulosum cyprini cells and were subjected to hypoxic stress. HIF-1αA induced apoptosis through promoting ROS generation and mitochondrial depolarization when cells were subjected to oxygen deficiency. Conversely, HIF-1αB was primarily responsible for mitophagy induction, prompting ATP production to mitigate apoptosis. HIF-1αA did not induce mitophagy in the mitochondria and lysosomes co-localization assay but it was involved in the regulation of different mitophagy pathways. Over-expression of HIF-1αA increased the expression of bnip3, fundc1, Beclin1, and foxo3, suggesting it has a dual role in mitochondrial autophagy and cell death. Each duplicated copy also experienced functional divergence and target shifting in the regulation of complexes in the mitochondrial electron transport chain (ETC). Our findings shed light on the post-subfunctionalization function of HIF-1αA and HIF-1αB in zebrafish to fine-tune regulation of mitophagy and apoptosis following hypoxia exposure.

Hypoxic State of Cells and Immunosenescence: A Focus on the Role of the HIF Signaling Pathway

Hypoxia activates hypoxia-related signaling pathways controlled by hypoxia-inducible factors (HIFs). HIFs represent a quick and effective detection system involved in the cellular response to insufficient oxygen concentration. Activation of HIF signaling pathways is involved in improving the oxygen supply, promoting cell survival through anaerobic ATP generation, and adapting energy metabolism to meet cell demands. Hypoxia can also contribute to the development of the aging process, leading to aging-related degenerative diseases; among these, the aging of the immune system under hypoxic conditions can play a role in many different immune-mediated diseases. Thus, in this review we aim to discuss the role of HIF signaling pathways following cellular hypoxia and their effects on the mechanisms driving immune system senescence.

Lawrence K, Milej D. Assessing the Sensitivity of Multi-Distance Hyperspectral NIRS to Changes in the Oxidation State of Cytochrome C Oxidase in the Brain

Near-infrared spectroscopy (NIRS) measurements of tissue oxygen saturation (StO₂) are frequently used during vascular and cardiac surgeries as a non-invasive means of assessing brain health; however, signal contamination from extracerebral tissues remains a concern. As an alternative, hyperspectral (hs)NIRS can be used to measure changes in the oxidation state of cytochrome c oxidase (ΔoxCCO), which provides greater sensitivity to the brain given its higher mitochondrial concentration versus the scalp. The purpose of this study was to evaluate the depth sensitivity of the oxCCO signal to changes occurring in the brain and extracerebral tissue components. The oxCCO assessment was conducted using multi-distance hsNIRS (source-detector separations = 1 and 3 cm), and metabolic changes were compared to changes in StO₂. Ten participants were monitored using an in-house system combining hsNIRS and diffuse correlation spectroscopy (DCS). Data were acquired during carotid compression (CC) to reduce blood flow and hypercapnia to increase flow. Reducing blood flow by CC resulted in a significant decrease in oxCCO measured at r_SD = 3 cm but not at 1 cm. In contrast, significant changes in StO₂ were found at both distances. Hypercapnia caused significant increases in StO₂ and oxCCO at r_SD = 3 cm, but not at 1 cm. Extracerebral contamination resulted in elevated StO₂ but not oxCCO after hypercapnia, which was significantly reduced by applying regression analysis. This study demonstrated that oxCCO was less sensitive to extracerebral signals than StO₂.

Respiratory Supercomplexes Promote Mitochondrial Efficiency and Growth in Severely Hypoxic Pancreatic Cancer

Pancreatic ductal adenocarcinoma (PDAC) is characterized by extensive fibrosis and hypovascularization, resulting in significant intratumoral hypoxia (low oxygen) that contributes to its aggressiveness, therapeutic resistance, and high mortality. Despite oxygen being a fundamental requirement for many cellular and metabolic processes, and the severity of hypoxia in PDAC, the impact of oxygen deprivation on PDAC biology is poorly understood. Investigating how PDAC cells survive in the near absence of oxygen, we find that PDAC cell lines grow robustly in oxygen tensions down to 0.1%, maintaining mitochondrial morphology, membrane potential, and the oxidative metabolic activity required for the synthesis of key metabolites for proliferation. Disrupting electron transfer efficiency by targeting mitochondrial respiratory supercomplex assembly specifically affects hypoxic PDAC proliferation, metabolism, and in vivo tumor growth. Collectively, our results identify a mechanism that enables PDAC cells to thrive in severe, oxygen-limited microenvironments.

AMPK-mTOR Signaling and Cellular Adaptations in Hypoxia

Cellular energy is primarily provided by the oxidative degradation of nutrients coupled with mitochondrial respiration, in which oxygen participates in the mitochondrial electron transport chain to enable electron flow through the chain complex (I-IV), leading to ATP production. Therefore, oxygen supply is an indispensable chapter in intracellular bioenergetics. In mammals, oxygen is delivered by the bloodstream. Accordingly, the decrease in cellular oxygen level (hypoxia) is accompanied by nutrient starvation, thereby integrating hypoxic signaling and nutrient signaling at the cellular level. Importantly, hypoxia profoundly affects cellular metabolism and many relevant physiological reactions induce cellular adaptations of hypoxia-inducible gene expression, metabolism, reactive oxygen species, and autophagy. Here, we introduce the current knowledge of hypoxia signaling with two-well known cellular energy and nutrient sensing pathways, AMP-activated protein kinase (AMPK) and mechanistic target of rapamycin complex 1 (mTORC1). Additionally, the molecular crosstalk between hypoxic signaling and AMPK/mTOR pathways in various hypoxic cellular adaptions is discussed.

Metabolic Reprogramming of Cancer Cells during Tumor Progression and Metastasis

Cancer cells face various metabolic challenges during tumor progression, including growth in the nutrient-altered and oxygen-deficient microenvironment of the primary site, intravasation into vessels where anchorage-independent growth is required, and colonization of distant organs where the environment is distinct from that of the primary site. Thus, cancer cells must reprogram their metabolic state in every step of cancer progression. Metabolic reprogramming is now recognized as a hallmark of cancer cells and supports cancer growth. Elucidating the underlying mechanisms of metabolic reprogramming in cancer cells may help identifying cancer targets and treatment strategies. This review summarizes our current understanding of metabolic reprogramming during cancer progression and metastasis, including cancer cell adaptation to the tumor microenvironment, defense against oxidative stress during anchorage-independent growth in vessels, and metabolic reprogramming during metastasis.

Intestinal Inflammation as a Dysbiosis of Energy Procurement: New Insights into an Old Topic

Inflammatory bowel disease (IBD) coincides with profound shifts in microbiota and host metabolic energy supply and demand. The gastrointestinal epithelium is anatomically positioned to provide a selective barrier between the anaerobic luminal microbiota and host lamina propria, with the microbiota and epithelium participating in an intricate energy exchange necessary for homeostasis. Maintenance and restoration of the barrier requires high energy flux and places significant demands on available substrates to generate ATP. It is recently appreciated that components of the microbiota contribute significantly to a multitude of biochemical pathways within and outside of the mucosa. Decades-old studies have appreciated that byproducts of the microbiota provide essential sources of energy to the intestinal epithelium, especially the colon. More recent work has unveiled the existence of numerous microbial-derived metabolites that support energy procurement within the mucosa. It is now appreciated that disease-associated shifts in the microbiota, termed dysbiosis, places significant demands on energy acquisition within the mucosa. Here, we review the topic of host- and microbial-derived components that influence tissue energetics in health and during disease.

Microbiota-derived butyrate is an endogenous HIF prolyl hydroxylase inhibitor

The gut microbiota is essential for human health. Microbial supply of short-chain fatty acids (SCFAs), particularly butyrate, is a well-established contributor to gut homeostasis and disease resistance. Reaching millimolar luminal concentrations, butyrate is sequestered and utilized in the colon as the favored energy source for intestinal epithelia. Given the steep oxygen gradient across the anoxic lumen and the highly oxygenated lamina propria, the colon provides a particularly interesting environment to study oxygen sensing. Previous studies have shown that the transcription factor hypoxia-inducible factor (HIF) is stabilized in healthy colonic epithelia. Here we show that butyrate directly inhibits HIF prolyl hydroxylases (PHDs) to stabilize HIF. We find that butyrate stabilizes HIF in vitro despite eliminating β-oxidation and resultant oxygen consumption. Using recombinant PHD protein in combination with nuclear magnetic resonance and enzymatic biochemical assays, we identify butyrate to bind and function as a unique, noncompetitive inhibitor of PHDs relative to other SCFAs. Butyrate inhibited PHD with a noncompetitive Ki of 5.3 ± 0.5 mM, a physiologically relevant concentration. We also confirm that microbiota-derived butyrate is necessary to stabilize HIF in mice colonic tissue through antibiotic-induced butyrate depletion and reconstitution experiments. Our results suggest that the co-evolution of mammals and mutualistic microbiota has selected for butyrate to impact a critical gene regulation pathway that can be extended beyond the mammalian gut. As PHDs are a major target for drug development in the stabilization of HIF, butyrate holds great potential as a well-tolerated endogenous inhibitor with far-reaching therapeutic impact.

Cellular adaptation to hypoxia through hypoxia inducible factors and beyond

Molecular oxygen (O2) sustains intracellular bioenergetics and is consumed by numerous biochemical reactions, making it essential for most species on Earth. Accordingly, decreased oxygen concentration (hypoxia) is a major stressor that generally subverts life of aerobic species and is a prominent feature of pathological states encountered in bacterial infection, inflammation, wounds, cardiovascular defects and cancer. Therefore, key adaptive mechanisms to cope with hypoxia have evolved in mammals. Systemically, these adaptations include increased ventilation, cardiac output, blood vessel growth and circulating red blood cell numbers. On a cellular level, ATP-consuming reactions are suppressed, and metabolism is altered until oxygen homeostasis is restored. A critical question is how mammalian cells sense oxygen levels to coordinate diverse biological outputs during hypoxia. The best-studied mechanism of response to hypoxia involves hypoxia inducible factors (HIFs), which are stabilized by low oxygen availability and control the expression of a multitude of genes, including those involved in cell survival, angiogenesis, glycolysis and invasion/metastasis. Importantly, changes in oxygen can also be sensed via other stress pathways as well as changes in metabolite levels and the generation of reactive oxygen species by mitochondria. Collectively, this leads to cellular adaptations of protein synthesis, energy metabolism, mitochondrial respiration, lipid and carbon metabolism as well as nutrient acquisition. These mechanisms are integral inputs into fine-tuning the responses to hypoxic stress.

Cytochrome c phosphorylation: Control of mitochondrial electron transport chain flux and apoptosis

Cytochrome c (Cytc)¹is a cellular life and death decision molecule that regulates cellular energy supply and apoptosis through tissue specific post-translational modifications. Cytc is an electron carrier in the mitochondrial electron transport chain (ETC) and thus central for aerobic energy production. Under conditions of cellular stress, Cytc release from the mitochondria is a committing step for apoptosis, leading to apoptosome formation, caspase activation, and cell death. Recently, Cytc was shown to be a target of cellular signaling pathways that regulate the functions of Cytc by tissue-specific phosphorylations. So far five phosphorylation sites of Cytc have been mapped and functionally characterized, Tyr97, Tyr48, Thr28, Ser47, and Thr58. All five phosphorylations partially inhibit respiration, which we propose results in optimal intermediate mitochondrial membrane potentials and low ROS production under normal conditions. Four of the phosphorylations result in inhibition of the apoptotic functions of Cytc, suggesting a cytoprotective role for phosphorylated Cytc. Interestingly, these phosphorylations are lost during stress conditions such as ischemia. This results in maximal ETC flux during reperfusion, mitochondrial membrane potential hyperpolarization, excessive ROS generation, and apoptosis. We here present a new model proposing that the electron transfer from Cytc to cytochrome c oxidase is the rate-limiting step of the ETC, which is regulated via post-translational modifications of Cytc. This regulation may be dysfunctional in disease conditions such as ischemia-reperfusion injury and neurodegenerative disorders through increased ROS, or cancer, where post-translational modifications on Cytc may provide a mechanism to evade apoptosis.

Definition of the Interaction Domain and Electron Transfer Route between Cytochrome c and Cytochrome Oxidase

The reaction between cytochrome c (Cc) and cytochrome c oxidase (CcO) was studied using horse cytochrome c derivatives labeled with ruthenium trisbipyridine at Cys 39 (Ru-39-Cc). Flash photolysis of a 1:1 complex between Ru-39-Cc and bovine CcO at a low ionic strength resulted in the electron transfer from photoreduced heme c to Cu_A with an intracomplex rate constant of k₃ = 6 × 10⁴ s^-1. The K13A, K72A, K86A, and K87A Ru-39-Cc mutants had nearly the same k₃ value but bound much more weakly to bovine CcO than wild-type Ru-39-Cc, indicating that lysines 13, 72, 86, and 87 were involved in electrostatic binding to CcO, but were not involved in the electron transfer pathway. The Rhodobacter sphaeroides (Rs) W143F mutant (bovine W104) caused a 450-fold decrease in k₃ but did not affect the binding strength with CcO or the redox potential of Cu_A. These results are consistent with a computational model for Cc-CcO (Roberts and Pique ( 1999 ) J. Biol. Chem. 274 , 38051 - 38060 ) with the following electron transfer pathway: heme c → CcO-W104 → CcO-M207 → Cu_A. A crystal structure for the Cc-CcO complex with the proposed electron transfer pathway heme c → Cc-C14 → Cc-K13 → CcO-Y105 → CcO-M207 → Cu_A ( S. Shimada ( 2017 ) EMBO J. 36 , 291 - 300 ) is not consistent with the kinetic results because the K13A mutation had no effect on k₃. Addition of 40% ethylene glycol (as present during the crystal preparation) decreased k₃ significantly, indicating that it affected the conformation of the complex. This may explain the discrepancy between the current results and the crystallographic structure.

Membrane-Anchored Cyclic Peptides as Effectors of Mitochondrial Oxidative Phosphorylation

The echinocandins are membrane-anchored, cyclic lipopeptides (CLPs) with antifungal activity due to their ability to inhibit a glucan synthase located in the plasma membrane of fungi such as Candida albicans. A hydrophobic tail of an echinocandin CLP inserts into a membrane, placing a six-amino acid cyclic peptide near the membrane surface. Because processes critical for the function of the electron transfer complexes of mitochondria, such as proton uptake and release, take place near the surface of the membrane, we have tested the ability of two echinocandin CLPs, caspofungin and micafungin, to affect the activity of electron transfer complexes in isolated mammalian mitochondria. Indeed, caspofungin and micafungin both inhibit whole chain electron transfer in isolated mitochondria at low micromolar concentrations. The effects of the CLPs are fully reversible, in some cases simply via the addition of bovine serum albumin to bind the CLPs via their hydrophobic tails. Each CLP affects more than one complex, but they still exhibit specificity of action. Only caspofungin inhibits complex I, and the CLP inhibits liver but not heart complex I. Both CLPs inhibit heart and liver complex III. Caspofungin inhibits complex IV activity, while, remarkably, micafungin stimulates complex IV activity nearly 3-fold. Using a variety of assays, we have developed initial hypotheses for the mechanisms by which caspofungin and micafungin alter the activities of complexes IV and III. The dication caspofungin partially inhibits cytochrome c binding at the low-affinity binding site of complex IV, while it also appears to inhibit the release of protons from the outer surface of the complex, similar to Zn(2+). Anionic micafungin appears to stimulate complex IV activity by enhancing the transfer of protons to the O2 reduction site. For complex III, we hypothesize that each CLP binds to the cytochrome b subunit and the Fe-S subunit to inhibit the required rotational movement of the latter.

Cytochrome redox states and respiratory control in mouse and beef heart mitochondria at steady-state levels of hypoxia

Mitochondrial control of cellular redox states is a fundamental component of cell signaling in the coordination of core energy metabolism and homeostasis during normoxia and hypoxia. We investigated the relationship between cytochrome redox states and mitochondrial oxygen consumption at steady-state levels of hypoxia in mitochondria isolated from beef and mouse heart (BHImt, MHImt), comparing two species with different cardiac dynamics and local oxygen demands. A low-noise, rapid spectrophotometric system using visible light for the measurement of cytochrome redox states was combined with high-resolution respirometry. Monophasic hyperbolic relationships were observed between oxygen consumption, JO2, and oxygen partial pressure, Po2, within the range <1.1 kPa (8.3 mmHg; 13 μM). P50j (Po2 at 0.5·Jmax) was 0.015 ± 0.0004 and 0.021 ± 0.003 kPa (0.11 and 0.16 mmHg) for BHImt and MHImt, respectively. Maximum oxygen consumption, Jmax, was measured at saturating ADP levels (OXPHOS capacity) with Complex I-linked substrate supply. Redox states of cytochromes aa3 and c were biphasic hyperbolic functions of Po2. The relationship between cytochrome oxidation state and oxygen consumption revealed a separation of distinct phases from mild to severe and deep hypoxia. When cytochrome c oxidation increased from fully reduced to 45% oxidized at 0.1 Jmax, Po2 was as low as 0.002 kPa (0.02 μM), and trace amounts of oxygen are sufficient to partially oxidize the cytochromes. At higher Po2 under severe hypoxia, respiration increases steeply, whereas redox changes are small. Under mild hypoxia, the steep slope of oxidation of cytochrome c when flux remains more stable represents a cushioning mechanism that helps to maintain respiration high at the onset of hypoxia.

Product-controlled steady-state kinetics between cytochrome aa(3) from Rhodobacter sphaeroides and equine ferrocytochrome c analyzed by a novel spectrophotometric approach

Cytochrome c oxidase (CcO) catalyzes the reduction of molecular oxygen to water using ferrocytochrome c (cyt c(2+)) as the electron donor. In this study, the oxidation of horse cyt c(2+) by CcO from Rhodobacter sphaeroides, was monitored using stopped-flow spectrophotometry. A novel analytic procedure was applied in which the spectra were deconvoluted into the reduced and oxidized forms of cyt c by a least-squares fitting method, yielding the reaction rates at various concentrations of cyt c(2+) and cyt c(3+). This allowed an analysis of the effects of cyt c(3+) on the steady-state kinetics between CcO and cyt c(2+). The results show that cyt c(3+) exhibits product inhibition by two mechanisms: competition with cyt c(2+) at the catalytic site and, in addition, an interaction at a second site which further modulates the reaction of cyt c(2+) at the catalytic site. These results are generally consistent with previous reports, indicating the reliability of the new procedure. We also find that a 6×His-tag at the C-terminus of the subunit II of CcO affects the binding of cyt c at both sites. The approach presented here should be generally useful in spectrophotometric studies of complex enzyme kinetics. This article is part of a Special Issue entitled: 17th European Bioenergetics Conference (EBEC 2012).

Formamide probes a role for water in the catalytic cycle of cytochrome c oxidase

Formamide is a slow-onset inhibitor of mitochondrial cytochrome c oxidase that is proposed to act by blocking water movement through the protein. In the presence of formamide the redox level of mitochondrial cytochrome c oxidase evolves over the steady state as the apparent electron transfer rate from cytochrome a to cytochrome a(3) slows. At maximal inhibition cytochrome a and cytochrome c are fully reduced, whereas cytochrome a(3) and Cu(B) remain fully oxidized consistent with the idea that formamide interferes with electron transfer between cytochrome a and the oxygen reaction site. However, transient kinetic studies show that intrinsic rates of electron transfer are unchanged in the formamide-inhibited enzyme. Formamide inhibition is demonstrated for another member of the heme-oxidase family, cytochrome c oxidase from Bacillus subtilis, but the onset of inhibition is much quicker than for mitochondrial oxidase. If formamide inhibition arises from a steric blockade of water exchange during catalysis then water exchange in the smaller bacterial oxidase is more open. Subunit III removal from the mitochondrial oxidase hastens the onset of formamide inhibition suggesting a role for subunit III in controlling water exchange during the cytochrome c oxidase reaction.

Changes in mitochondrial electron transport chain activity during insect metamorphosis

The midgut of the tobacco hornworm (Manduca sexta) is a highly aerobic tissue that is destroyed by programmed cell death during larval-pupal metamorphosis. The death of the epithelium begins after commitment to pupation, and the oxygen consumption of isolated midgut mitochondria decreases soon after commitment. To assess the role of the electron transport chain in this decline in mitochondrial function, the maximal activities of complexes I-IV of the respiratory chain were measured in isolated midgut mitochondria. Whereas there were no developmental changes in the activity of complex I or III, activities of complexes II and IV [cytochrome c oxidase (COX)] were higher in mitochondria from precommitment than postcommitment larvae. This finding is consistent with a higher rate of succinate oxidation in mitochondria isolated from precommitment larvae and reveals that the metamorphic decline in mitochondrial respiration is due to the targeted destruction or inactivation of specific sites within the mitochondria, rather than the indiscriminate destruction of the organelles. The COX turnover number (e- x s(-1) x cytochrome aa3(-1)) was greater for the enzyme from precommitment than postcommitment larvae, indicating a change in the enzyme structure and/or its lipid environment during the early stages of metamorphosis. The turnover number of COX in the intact mitochondria (in organello COX) was also lower in postcommitment larvae. In addition to changes in the protein or membrane phospholipids, the metamorphic decline in this rate constant may be a result of the observed loss of endogenous cytochrome c.

Mitochondrial cytochrome oxidase produces nitric oxide under hypoxic conditions: implications for oxygen sensing and hypoxic signaling in eukaryotes

Eukaryotic cells respond to low-oxygen concentrations by upregulating hypoxic nuclear genes (hypoxic signaling). Although it has been shown previously that the mitochondrial respiratory chain is required for hypoxic signaling, its underlying role in this process has been unclear. Here, we find that yeast and rat liver mitochondria produce nitric oxide (NO) at dissolved oxygen concentrations below 20 microM. This NO production is nitrite (NO2-) dependent, requires an electron donor, and is carried out by cytochrome c oxidase in a pH-dependent fashion. Mitochondrial NO production in yeast is influenced by the YHb flavohemoglobin NO oxidoreductase, stimulates expression of the hypoxic nuclear gene CYC7, and is accompanied by an increase in protein tyrosine nitration. These findings demonstrate an alternative role for the mitochondrial respiratory chain under hypoxic or anoxic conditions and suggest that mitochondrially produced NO is involved in hypoxic signaling, possibly via a pathway that involves protein tyrosine nitration.

The electric field induced by light can explain cellular responses to electromagnetic energy: a hypothesis of mechanism

When cells are irradiated with visible and near-infrared wavelengths a variety of stimulatory effects are observed in their metabolism. To explain the observed light effects, researchers try to identify the chromophores that are involved in the processes. However, the mechanism of light absorption by a chromophore does not explain many of the experimental observations and therefore the primary mechanism for cellular light responses remains unproven. In addition to the ability of photons to produce electronic excitation in chromophores, light induces a wave-like alternating electric field in a medium that is able to interact with polar structures and produce dipole transitions. These dipole transitions are analyzed in the present article at different cellular and biochemical levels, leading to the proposal that the primary mechanism for the observed light effects is related to the light-induced electric field.

A possible restriction of ferro- and ferricyanide oxidoreductase activities of rat liver mitochondria by the outer membrane

In this work, various ferro-ferricyanide oxidoreductase activities of rat liver mitochondria were studied to find conditions under which the outer membrane might restrict the flux of these highly charged non-biological anions. When the isotonic low ionic strength medium was supplemented with 25mM KCl, a several-fold increase in the succinate-ferricyanide reductase activity of mitochondria and in the rate of external NADH oxidation in the presence of ferrocyanide was observed. Mitochondrial respiration with 5mM ferrocyanide was almost completely inhibited after consumption of 3.8-18.5% of the dissolved oxygen, depending on the medium and the presence of 2,4-dinitrophenol. These and other experimental data together with mathematical modeling of the redox-state equilibrium suggest that the measured activities might be restricted by two factors: first, the permeability of the outer mitochondrial membrane and second, a strong influence of the ionic strength of incubation media on the intermembrane space redox reactions.

Expression, purification, and characterization of the CuA–cytochrome c domain from subunit II of the Bacillus subtilis cytochrome caa3 complex in Escherichia coli

Cytochrome caa3 from Bacillus subtilis is a member of the heme-copper oxidase family of integral membrane enzymes that includes mitochondrial cytochrome c oxidase. Subunit II of cytochrome caa3 has an extra 100 amino acids at its C-terminus, relative to its mitochondrial counterpart, and this extension encodes a heme C binding domain. Cytochrome caa3 has many of the properties of the complex formed between mitochondrial cytochrome c and mitochondrial cytochrome c oxidase. To examine more closely the interaction between cytochrome c and the oxidase we have cloned and expressed the Cu(A)-cytochrome c portion of subunit II from the cytochrome caa3 complex of B. subtilis. We are able to express about 2000 nmol, equivalent to 65 mg, of the Cu(A)-cytochrome c protein per litre of Escherichia coli culture. About 500 nmol is correctly targeted to the periplasmic space and we purify 50% of that by a combination of affinity chromatography and ammonium sulfate fractionation. The cytochrome c containing sub-domain is well-folded with a stable environment around the heme C center, as its mid-point potential and rates of reduction are indistinguishable from values for the cytochrome c domain of the holo-enzyme. However, the Cu(A) site lacks copper leading to an inherent instability in this sub-domain. Expression of B. subtilis cytochrome c, as exemplified by the Cu(A)-cytochrome c protein, can be achieved in E. coli, and we conclude that the cytochrome c and Cu(A) sub-domains behave independently despite their close physical and functional association.

The Redox State of Cytochrome Oxidase in Brain in Vivo: An Historical Perspective

Recent evidence suggests that cytochrome oxidase is partially reduced under resting conditions in the brain. Previous data, recorded over the past 30 years from intact brain using optical methods in the visible wavelength range, are consistent with this observation. These older data, while not conclusive in themselves, support the overall conclusions. The historical perspective on the experiments and controversies illustrates a number of useful principles. The first is that new methods tend to produce new observations that may be difficult to reproduce due to the uniqueness of the instrumentation. The second is that any new and different observations cannot be assimilated without an acceptable theoretical framework and, without assimilation can have little impact. Finally, the mechanisms which might explain why cytochrome oxidase may be more reduced than previously thought are still not fully developed and, therefore, the physiological significance of such reduction is not known.

Influence of structure, pH and membrane potential on proton movement in cytochrome oxidase

Cytochrome c oxidase (CcO) reconstituted into phospholipid vesicles and subject to a membrane potential, exhibits different characteristics than the free enzyme, with respect to effects of mutations, pH, inhibitors, and native structural differences between CcO from different species. The results indicate that the membrane potential influences the conformation of CcO and the direction of proton movement in the exit path. The importance of the protein structure above the hemes in proton exit, back leak and respiratory control is discussed.

Cytochrome c sorption-desorption effects on the external NADH oxidation by mitochondria: experimental and computational study

The rupture of the outer mitochondrial membrane is known to be critical for cell death, but the mechanism, specifically its redox-signaling aspects, still needs to be studied in more detail. In this work, the external NADH oxidation by rat liver mitochondria was studied under the outer membrane rupture induced by the mitochondria hypotonic treatment or the inner membrane permeability transition. The saturation of the oxidation rate was observed as a function of mitochondrial protein concentration. This effect was shown to result from cytochrome c binding to the mitochondrial membranes. At a relatively high concentration of mitochondria, the oxidation rate was strongly activated by 4 mm Mg(2+) due to cytochrome c desorption from the membranes. A minimal kinetic model was developed to explain the main phenomena of the external NADH oxidation modulated by cytochrome c and Mg(2+) in mitochondria with the ruptured outer membrane. The computational behavior of the model closely agreed with the experimental data. We suggest that the redox state of the released cytochrome c, considered by other authors to be important for apoptosis, may strongly depend on its oxidation by the fraction of mitochondria with the ruptured outer membrane and on the cytoplasmic cytochrome c reductase activity.

Mitochondrial energy metabolism is regulated via nuclear-coded subunits of cytochrome c oxidase

A new mechanism on regulation of mitochondrial energy metabolism is proposed on the basis of reversible control of respiration by the intramitochondrial ATP/ADP ratio and slip of proton pumping (decreased H+/e- stoichiometry) in cytochrome c oxidase (COX) at high proton motive force delta p. cAMP-dependent phosphorylation of COX switches on and Ca2+-dependent dephosphorylation switches off the allosteric ATP-inhibition of COX (nucleotides bind to subunit IV). Control of respiration via phosphorylated COX by the ATP/ADP ratio keeps delta p (mainly delta psi(m)) low. Hormone induced Ca2+-dependent dephosphorylation results in loss of ATP-inhibition, increase of respiration and delta p with consequent slip in proton pumping. Slip in COX increases the free energy of reaction, resulting in increased rates of respiration, thermogenesis and ATP-synthesis. Increased delta psi(m) stimulates production of reactive oxygen species (ROS), mutations of mitochondrial DNA and accelerates aging. Slip of proton pumping without dephosphorylation and increase of delta p is found permanently in the liver-type isozyme of COX (subunit VIaL) and at high intramitochondrial ATP/ADP ratios in the heart-type isozyme (subunit VIaH). High substrate pressure (sigmoidal v/s kinetics), palmitate and 3,5-diiodothyronine (binding to subunit Va) increase also delta p, ROS production and slip but without dephosphorylation of COX.

Mitochondrial function in flying honeybees (Apis mellifera): respiratory chain enzymes and electron flow from complex III to oxygen

The biochemical bases for the high mass-specific metabolic rates of flying insects remain poorly understood. To gain insights into mitochondrial function during flight, metabolic rates of individual flying honeybees were measured using respirometry, and their thoracic muscles were fixed for electron microscopy. Mitochondrial volume densities and cristae surface densities, combined with biochemical data concerning cytochrome content per unit mass, were used to estimate respiratory chain enzyme densities per unit cristae surface area. Despite the high content of respiratory enzymes per unit muscle mass, these are accommodated by abundant mitochondria and high cristae surface densities such that enzyme densities per unit cristae surface area are similar to those found in mammalian muscle and liver. These results support the idea that a unit area of mitochondrial inner membrane constitutes an invariant structural unit. Rates of O(2) consumption per unit cristae surface area are much higher than those estimated in mammals as a consequence of higher enzyme turnover rates (electron transfer rates per enzyme molecule) during flight. Cytochrome c oxidase, in particular, operates close to its maximum catalytic capacity (k(cat)). Thus, high flux rates are achieved via (i) high respiratory enzyme content per unit muscle mass and (ii) the operation of these enzymes at high fractional velocities.

Definition of the interaction domain for cytochrome c on cytochrome c oxidase. I. Biochemical, spectral, and kinetic characterization of surface mutants in subunit ii of Rhodobacter sphaeroides cytochrome aa(3)

To determine the interaction site for cytochrome c (Cc) on cytochrome c oxidase (CcO), a number of conserved carboxyl residues in subunit II of Rhodobacter sphaeroides CcO were mutated to neutral forms. A highly conserved tryptophan, Trp(143), was also mutated to phenylalanine and alanine. Spectroscopic and metal analyses of the surface carboxyl mutants revealed no overall structural changes. The double mutants D188Q/E189N and D151Q/E152N exhibit similar steady-state kinetic behavior as wild-type oxidase with horse Cc and R. sphaeroides Cc(2), showing that these residues are not involved in Cc binding. The single mutants E148Q, E157Q, D195N, and D214N have decreased activities and increased K(m) values, indicating they contribute to the Cc:CcO interface. However, their reactions with horse and R. sphaeroides Cc are different, as expected from the different distribution of surface lysines on these cytochromes c. Mutations at Trp(143) severely inhibit activity without changing the K(m) for Cc or disturbing the adjacent Cu(A) center. From these data, we identify a Cc binding area on CcO with Trp(143) and Asp(214) close to the site of electron transfer and Glu(148), Glu(157), and Asp(195) providing electrostatic guidance. The results are completely consistent with time-resolved kinetic measurements (Wang, K., Zhen, Y., Sadoski, R., Grinnell, S., Geren, L., Ferguson-Miller, S., Durham, B., and Millett, F. (1999) J. Biol. Chem. 274, 38042-38050) and computational docking analysis (Roberts, V. A., and Pique, M. E. (1999) J. Biol. Chem. 274, 38051-38060).

Definition of the interaction domain for cytochrome c on cytochrome c oxidase. Ii. Rapid kinetic analysis of electron transfer from cytochrome c to Rhodobacter sphaeroides cytochrome oxidase surface mutants

The reaction between cytochrome c (Cc) and Rhodobacter sphaeroides cytochrome c oxidase (CcO) was studied using a cytochrome c derivative labeled with ruthenium trisbipyridine at lysine 55 (Ru-55-Cc). Flash photolysis of a 1:1 complex between Ru-55-Cc and CcO at low ionic strength results in electron transfer from photoreduced heme c to Cu(A) with an intracomplex rate constant of k(a) = 4 x 10(4) s(-1), followed by electron transfer from Cu(A) to heme a with a rate constant of k(b) = 9 x 10(4) s(-1). The effects of CcO surface mutations on the kinetics follow the order D214N > E157Q > E148Q > D195N > D151N/E152Q approximately D188N/E189Q approximately wild type, indicating that the acidic residues Asp(214), Glu(157), Glu(148), and Asp(195) on subunit II interact electrostatically with the lysines surrounding the heme crevice of Cc. Mutating the highly conserved tryptophan residue, Trp(143), to Phe or Ala decreased the intracomplex electron transfer rate constant k(a) by 450- and 1200-fold, respectively, without affecting the dissociation constant K(D). It therefore appears that the indole ring of Trp(143) mediates electron transfer from the heme group of Cc to Cu(A). These results are consistent with steady-state kinetic results (Zhen, Y., Hoganson, C. W., Babcock, G. T., and Ferguson-Miller, S. (1999) J. Biol. Chem. 274, 38032-38041) and a computational docking analysis (Roberts, V. A., and Pique, M. E. (1999) J. Biol. Chem. 274, 38051-38060).

Lipid peroxidation and alterations to oxidative metabolism in mitochondria isolated from rat heart subjected to ischemia and reperfusion

Ischemia-reperfusion injury to cardiac myocytes involves membrane damage mediated by oxygen free radicals. Lipid peroxidation is considered a major mechanism of oxygen free radical toxicity in reperfused heart. Mitochondrial respiration is an important source of these reactive oxygen species and hence a potential contributor to reperfusion injury. We have examined the effects of ischemia (30 min) and ischemia followed by reperfusion (15 min) of rat hearts, on the kinetic parameters of cytochrome c oxidase, on the respiratory activities and on the phospholipid composition in isolated mitochondria. Mitochondrial content of malonyldialdheyde (MDA), an index of lipid peroxidation, was also measured. Reperfusion was accompanied by a significant increase in MDA production. Mitochondrial preparations from control, ischemic and reperfused rat heart had equivalent Km values for cytochrome c, although the maximal activity of the oxidase was 25 and 51% less in ischemic and reperfused mitochondria than that of controls. These changes in the cytochrome c oxidase activity were associated to parallel changes in state 3 mitochondrial respiration. The cytochrome aa3 content was practically the same in these three types of mitochondria. Alterations were found in the mitochondrial content of the major phospholipid classes, the most pronounced change occurring in the cardiolipin, the level that decreased by 28 and by 50% as function of ischemia and reperfusion, respectively. The lower cytochrome c oxidase activity in mitochondria from reperfused rat hearts could be almost completely restored to the level of control hearts by exogenously added cardiolipin, but not by other phospholipids nor by peroxidized cardiolipin. It is proposed that the reperfusion-induced decline in the mitochondrial cytochrome c oxidase activity can be ascribed, at least in part, to a loss of cardiolipin content, due to peroxidative attack of its unsaturated fatty acids by oxygen free radicals. These findings may provide an explanation for some of the factors that lead to myocardial reperfusion injury.

Modulation of cytochrome c-mediated extramitochondrial NADH oxidation by contact site density

Data presented in previous reports suggest that in rat liver mitochondria a "bi-trans-membrane" electron transport pathway is present which promotes the transfer of reducing equivalents directly from cytosolic NADH to molecular oxygen inside the mitochondria. Here we show that the oxidation of external NADH is stimulated by atractylate + ADP and greatly inhibited by glycerol. These two conditions have been documented to promote the increase and the decrease respectively of the frequency of "contact sites" between the two mitochondrial membranes. NADH oxidation is not affected at all by glycerol and atractylate + ADP when TMPD and endogenous cytochrome c are utilized as electron carriers. The results obtained are consistent with the proposal that the bi-trans-membrane electron transport chain might be localized at the level of respiratory contact sites having the function of promoting the oxidation of the surplus amount of cytosolic NADH. This electron transport pathway has been suggested to play a decisive role in the early stages of apoptosis [Biochem. Biophys. Res. Commun. 246, 556-561, 1998].

Noninvasive assessment of changes in cytochrome-c oxidase oxidation in human subjects during visual stimulation

In this study the authors used a whole-spectrum near-infrared spectroscopy approach to noninvasively assess changes in hemoglobin oxygenation and cytochrome-c oxidase redox state (Cyt-Ox) in the occipital cortex during visual stimulation. The system uses a white light source (halogen lamp). The light reflected from the subject's head is spectrally resolved by a spectrograph and dispersed on a cooled charge-coupled device camera. The authors showed the following using this approach: (1) Changes in cerebral hemoglobin oxygenation (increase in concentration of oxygenated hemoglobin, decrease in concentration of deoxygenated hemoglobin) in the human occipital cortex during visual stimulation can be assessed quantitatively. (2) The spectral changes during functional activation cannot be completely explained by changes in hemoglobin oxygenation solely; Cyt-Ox has to be included in the analysis. Only if Cyt-Ox is considered can the spectral changes in response to increased brain activity be explained. (3) Cytochrome-c oxidase in the occipital cortex of human subjects is transiently oxidized during visual stimulation. This allows us to measure vascular and intracellular energy status simultaneously.

Intracellular signaling by reactive oxygen species during hypoxia in cardiomyocytes

Cardiomyocytes suppress contraction and O2 consumption during hypoxia. Cytochrome oxidase undergoes a decrease in Vmax during hypoxia, which could alter mitochondrial redox and increase generation of reactive oxygen species (ROS). We therefore tested whether ROS generated by mitochondria act as second messengers in the signaling pathway linking the detection of O2 with the functional response. Contracting cardiomyocytes were superfused under controlled O2 conditions while fluorescence imaging of 2, 7-dichlorofluorescein (DCF) was used to assess ROS generation. Compared with normoxia (PO2 approximately 107 torr, 15% O2), graded increases in DCF fluorescence were seen during hypoxia, with responses at PO2 = 7 torr > 20 torr > 35 torr. The antioxidants 2-mercaptopropionyl glycine and 1,10-phenanthroline attenuated these increases and abolished the inhibition of contraction. Superfusion of normoxic cells with H2O2 (25 microM) for >60 min mimicked the effects of hypoxia by eliciting decreases in contraction that were reversible after washout of H2O2. To test the role of cytochrome oxidase, sodium azide (0.75-2 microM) was added during normoxia to reduce the Vmax of the enzyme. Azide produced graded increases in ROS signaling, accompanied by graded decreases in contraction that were reversible. These results demonstrate that mitochondria respond to graded hypoxia by increasing the generation of ROS and suggest that cytochrome oxidase may contribute to this O2 sensing.

Cellular respiration during hypoxia. Role of cytochrome oxidase as the oxygen sensor in hepatocytes

We previously reported that hepatocytes exhibit a reversible suppression of respiration during prolonged hypoxia (PO2 = 20 torr for 3-5 h). Also, isolated bovine heart cytochrome c oxidase undergoes a reversible decrease in apparent Vmax when incubated under similar conditions. This study sought to link the hypoxia-induced changes in cytochrome oxidase to the inhibition of respiration seen in intact cells. Hepatocytes incubated at PO2 = 20 torr exhibited decreases in respiration and increases in [NAD(P)H] after 2-3 h that were reversed upon reoxygenation (PO2 = 100 torr). Respiration during hypoxia was also inhibited when N,N,N',N'-tetramethyl-p-phenylenediamine (0.5 mM) and ascorbate (5 mM) were used to reduce cytochrome c, suggesting that cytochrome oxidase was partially inhibited. Similarly, liver submitochondrial particles revealed a 44% decrease in the apparent Vmax of cytochrome oxidase after hypoxic incubation. In hepatocytes loaded with tetramethylrhodamine ethyl ester (10 nM) to quantify mitochondrial membrane potential, acute hypoxia (<30 min) produced no change in fluorescence, consistent with the absence of an acute change in respiration. However, fluorescence increased during acute reoxygenation after prolonged hypoxia, suggesting an increase in potential. The control exhibited by NADH over mitochondrial respiration was not altered during hypoxia. Thus, changes in the Vmax of cytochrome oxidase during prolonged hypoxia correlate with the changes in respiration and mitochondrial potential. This suggests that the oxidase functions as an oxygen sensor in the intact hepatocyte.

Energy metabolism, enzymatic flux capacities, and metabolic flux rates in flying honeybees

Honeybees rely primarily on the oxidation of hexose sugars to provide the energy required for flight. Measurement of VCO2 (equal to VO2, because VCO2/VO2 = 1.0 during carbohydrate oxidation) during flight allowed estimation of steady-state flux rates through pathways of flight muscle energy metabolism. Comparison of Vmax values for flight muscle hexokinase, phosphofructokinase, citrate synthase, and cytochrome c oxidase with rates of carbon and O2 flux during flight reveal that these enzymes operate closer to Vmax in the flight muscles of flying honeybees than in other muscles previously studied. Possible mechanistic and evolutionary implications of these findings are discussed.

Molecular oxygen modulates cytochrome c oxidase function

This study sought to determine whether molecular oxygen interacts with cytochrome c oxidase to modify its catalytic activity. Such an interaction could explain the observation that mitochondria incubated under low O2 concentrations exhibit a reversible suppression of State 3 respiration. Oxidized bovine heart cytochrome c oxidase was incubated in oxygen concentrations of <50 microM for 4 h. The enzyme exhibited a reversible decrease in Vmax after incubation, compared with control enzyme incubated at higher oxygen concentrations. This change was accompanied by a small increase in the apparent Km of the enzyme for both cytochrome c and oxygen, although the optical absorption spectra of oxidized, cycling, or reduced enzyme were not affected. Spectroscopy studies after 4 h of incubation revealed that heme a3 was 33% reduced during cycling at [O2] = 25 microM whereas enzyme at [O2] = 135 microM was only 18% reduced, suggesting that the site of inhibition occurred at the electron transfer step between heme a3 and O2. These results provide a mechanistic explanation for the observation that intact cells or mitochondria exhibit a reversible inhibition of respiration during prolonged exposure to [O2] <25 mM, by demonstrating that the catalytic activity of cytochrome c oxidase function is similarly inhibited, possibly through an allosteric effect of molecular O2 on the enzyme.

Surface plasmon resonance studies of complex formation between cytochrome c and bovine cytochrome c oxidase incorporated into a supported planar lipid bilayer. II. Binding of cytochrome c to oxidase-containing cardiolipin/phosphatidylcholine membranes

Complex formation between horse heart cytochrome c (cyt c) and bovine cytochrome c oxidase (cco) incorporated into a supported planar egg phosphatidylcholine membrane containing varying amounts of cardiolipin (CL) (0-20 mol%) has been studied under low (10 mM) and medium (160 mM) ionic strength conditions by surface plasmon resonance (SPR) spectroscopy. Both specific and nonspecific modes of cyt c binding are observed. The dissociation constant of the specific interaction between cyt c and cco increases from approximately 6.5 microM at low ionic strength to 18 microM at medium ionic strength, whereas the final saturation level of bound protein is independent of salt concentration and corresponds to approximately 53% of the total cco molecules present in the membrane. This suggests a 1:1 binding stoichiometry between the two proteins. The nonspecific binding component is governed by electrostatic interactions between cyt c and the membrane lipids and results in a partially ionic strength-reversible protein-membrane association. Thus, hydrophobic interactions between cyt c and the membrane, which are the predominant mode of binding in the absence of cco, are greatly suppressed. Both the amount of nonspecifically bound protein and the binding affinity can be varied over a broad range by changing the ionic strength and the extent of CL incorporation into the membrane. Under conditions approximating the physiological state in the mitochondrion (i.e., 20 mol% CL and medium ionic strength), 1-1.5 cyt c molecules are bound to the lipid phase per molecule of cco, with a dissociation constant of 0.1 microM. The possible physiological significance of these observations is discussed.

Activation-dependent and biphasic electromagnetic field effects: model based on cooperative enzyme kinetics in cellular signaling

Experiments on filed exposure effects of extremely-low-frequency electric and magnetic fields (EMFs) on biological systems have shown that, in many cases, the biological-functional status is of fundamental importance for an effective interaction. For example, studies of calcium uptake regulation in cells of the immune system, particularly in T lymphocytes, have revealed that, depending on the degree of cellular activation, either stimulatory, inhibitory, or no field exposure effects are observed for identical field parameters. A brief summary of the experimental findings is given, and a theoretical approach is presented that accounts in a qualitative manner for EMF exposure effects 1) that depend on the degree of cellular activation and 2) that exhibit a biphasic response behavior (stimulation/ inhibition). In the model, biochemical stimulation of the cell results in activation of specific signaling pathways that regulate calcium dynamics in the cell (calcium release from intracellular calcium stores and capacitative calcium entry). We assume that, controlled by these pathways, a specific EMF-sensitive enzyme system becomes activated. The activated enzyme, in turn, exhibits a feedback control on the signal processes, thus leading to a modulation of calcium entry. This modulation may affect other cellular processes that are calcium dependent (e.g., DNA synthesis). Magnetic field exposure is assumed to alter the kinetics of a specific step within the enzyme-reaction cycle in accord with the radical-pair mechanism, although the formulism is not restricted to this specific example. Results show that inclusion of cooperative steps within the enzyme-reaction cycle provides a theoretical basis that enables a simple description of a biphasic response behavior to EMF exposure.

Nature of respiratory stimulation in hyperthyroidism: the redox behaviour of cytochrome c

Hyperthyroid mitochondria show an increased Km and Vmax in the high affinity phase of cytochrome oxidase kinetics. During inhibitor titrations, cytochrome c shows a different redox behaviour in hyperthyroid with respect to protonophore-treated euthyroid mitochondria. The observations are discussed in terms of a different regulation of electron input and output into the respiratory chain during slip and leak types of uncoupling. In hyperthyroid mitochondria during inhibitor titrations, the pattern of the relationship between uncoupler-induced extra-respiration and membrane potential is highly non-linear. The complex nature of the respiratory stimulation in hyperthyroid mitochondria is discussed.

Inhibition of cytochrome c oxidase in turnover by nitric oxide: mechanism and implications for control of respiration

Binding of nitric oxide (NO) to isolated cytochrome c oxidase in turnover was investigated by static and kinetic spectroscopic methods. These studies indicate that cytochrome c oxidase rapidly binds NO when the enzyme enters turnover. Our results show that NO binds to ferrocytochrome a3, competing with oxygen for this binding site. However, the main features of the binding process, in particular the rapid onset of inhibition, cannot be fully explained on this basis. We suggest, therefore, that there is a second binding site for NO, which has lower affinity but nevertheless plays an important role in the inhibitory process. A likely possibility is that CuB+ constitutes this second binding site. The fast onset of inhibition observed in the presence of NO, along with the dependence on the oxygen concentration, suggests that under physiological conditions, where the oxygen concentration is low, nanomolar concentrations of NO can effectively act as a regulator of the mitochondrial respiratory chain.

Altered kinetics of cytochrome c oxidase in a patient with severe mitochondrial encephalomyopathy

Deficiency of cytochrome c oxidase activity was established in a girl born to consanguineous parents. She showed symptoms of dysmaturity, generalized hypotonia, myoclonic seizures and progressive respiratory failure, leading to death on the seventh day of life. Structural abnormalities of the central nervous system consisted of severe cerebellar hypoplasia and optic nerve atrophy. Biochemical analysis of a muscle biopsy specimen demonstrated deficiency of cytochrome c oxidase activity. Cultured fibroblasts from this patient also showed a selective decrease in the activity of cytochrome c oxidase, excluding a muscle-specific type of deficiency. Further investigations in cultured fibroblasts revealed that synthesis, assembly and stability of both the mitochondrial and the nuclear subunits of the enzyme were entirely normal. The steady-state concentration of cytochrome c oxidase in the fibroblasts of the patient was also normal, suggesting that the kinetic properties of the enzyme were altered. Analysis of the kinetic parameters of cytochrome c oxidase demonstrated an aberrant interaction between cytochrome c oxidase and its substrate, cytochrome c, most likely because of a mutation in one of the nuclear subunits of the enzyme.

Cerebral oxygen supply and utilization during infant cardiac surgery

The survival of infants with congenital heart disease has improved dramatically. However, the incidence of neurological injury in infants surviving cardiac surgery remains considerable. These neurological sequelae are attributable at least in part to hypoxia-ischemia/reperfusion, which inevitably accompanies infant heart surgery with deep hypothermia, cardiopulmonary bypass, and circulatory arrest. To begin to identify mechanisms of brain injury during infant cardiac surgery, we used near-infrared spectroscopy to study the relationship between cerebral intravascular (hemoglobin) and mitochondrial (cytochrome aa3) oxygenation in 63 infants (aged 1 day to 9 months) undergoing deep hypothermic repair of congenital heart defects, throughout the intraoperative period. Moreover, we assessed the effect of postnatal age on these changes. The cerebral concentration of oxidized cytochrome aa3 decreased from the onset of deep hypothermic cardiopulmonary bypass, despite apparent abundant intravascular oxygenation manifested by a simultaneous increase in the cerebral concentration of oxyhemoglobin. During this interval infants older than 2 weeks had a greater decrease in oxidized cytochrome aa3 than did infants 2 weeks old or younger. During deep hypothermic circulatory arrest, cerebral levels of oxidized cytochrome aa3 remained depressed while those of oxyhemoglobin declined. With reperfusion following circulatory arrest, the recovery of oxidized cytochrome aa3 was delayed, despite a rapid recovery of intravascular oxygenation (HbO2). After rewarming and 60 minutes of reperfusion, only 46% of infants recovered to the baseline level of cerebral oxidized cytochrome aa3. These findings demonstrate a paradoxical dissociation of changes in intravascular and mitochondrial oxygenation during hypothermic cardiopulmonary bypass; a pronounced decrease of mitochondrial oxygenation is established during induction of hypothermia and a delay in recovery of mitochondrial oxygenation occurs following circulatory arrest. These effects were more pronounced in infants older than 2 weeks than in younger infants. The data suggest potentially deleterious impairments of intrinsic mitochondrial function or of delivery of intravascular oxygen to the mitochondrion or both, effects previously undetected and apparently influenced by cerebral maturation.

Structure and function of a molecular machine: cytochrome c oxidase

Cytochrome c is responsible for over 90% of the dioxygen consumption in the living cell and contributes to the build-up of a proton electrochemical gradient derived by the vectorial transfer of electrons between cytochrome c and molecular oxygen. The metal ions found in cytochrome oxidases play a crucial role in these processes and have been extensively studied. In this review we present and discuss some of the relevant spectroscopic and kinetic properties of the prosthetic groups of cytochrome c oxidase.

Design of a ruthenium-cytochrome c derivative to measure electron transfer to the initial acceptor in cytochrome c oxidase

A ruthenium-labeled cytochrome c derivative was prepared to meet two design criteria: the ruthenium group must transfer an electron rapidly to the heme group, but not alter the interaction with cytochrome c oxidase. Site-directed mutagenesis was used to replace His39 on the backside of yeast C102T iso-1-cytochrome c with a cysteine residue, and the single sulfhydryl group was labeled with (4-bromomethyl-4' methylbipyridine) (bis-bipyridine)ruthenium(II) to form Ru-39-cytochrome c (cyt c). There is an efficient pathway for electron transfer from the ruthenium group to the heme group of Ru-39-cyt c comprising 13 covalent bonds and one hydrogen bond. Electron transfer from the excited state Ru(II*) to ferric heme c occurred with a rate constant of (6.0 +/- 2.0) x 10(5) s-1, followed by electron transfer from ferrous heme c to Ru(III) with a rate constant of (1.0 +/- 0.2) x 10(6) s-1. Laser excitation of a complex between Ru-39-cyt c and beef cytochrome c oxidase in low ionic strength buffer (5 mM phosphate, pH7) resulted in electron transfer from photoreduced heme c to CuA with a rate constant of (6 +/- 2) x 10(4) s-1, followed by electron transfer from CuA to heme a with a rate constant of (1.8 +/- 0.3) x 10(4) s-1. Increasing the ionic strength to 100 mM leads to bimolecular kinetics as the complex is dissociated. The second-order rate constant is (2.5 +/- 0.4) x 10(7) M-1s-1 at 230 mM ionic strength, nearly the same as that of wild-type iso-1-cytochrome c.

Integral cytochrome-c oxidase. Preparation and progress towards a three-dimensional crystallization

A new rapid procedure for the preparation of monodispersed highly active cytochrome-c oxidase from bovine heart is described. The crucial step is the separation of cytochrome-c oxidase from cytochrome-c reductase by selective solubilization in the non-ionic detergents Triton X-100 or lauryl beta-D-maltoside. The enzyme is purified by subsequent anion-exchange chromatography. The preparation is finished within two days yielding approximately 60% of the oxidase present in mitochondria. The enzyme has a heme alpha/protein ratio of 9.7 +/- 0.5 nmol/mg, approximately equal to the theoretical value of 9.77 nmol/mg based on a molecular mass of 204.696 kDa for the protein monomer. SDS/PAGE of the preparation reveals the presence of the well-known thirteen protein components. Quantitative Edman degradation of the enzyme exclusively releases the known ten N-terminal residues; three of the thirteen protein components are blocked at the N-terminus. The preparation is highly active with maximal turnover numbers of approximately 600 s-1, identical to the maximal activity found in the mitochondrial membrane under these conditions. No g = 12 signal and no adventitious copper signal are observed in the EPR spectrum. The enzyme exhibits a fast monophasic reaction with cyanide. Determination of the metal contents of the enzyme indicates the stoichiometric presence of three copper ions besides two iron, one magnesium and one zinc ion in relation to the 94 sulfur atoms of the protein monomer. Gel-filtration experiments show a monodispersed dimeric association to form a complex of approximately 500 kDa. The phosphorus content 44 +/- 6.8 atoms/dimer, results from 59% cardiolipin, 23% phosphatidylethanolamine and 18% phosphatidylcholine, indicating a stable lipid shell, different from other previously described preparations. Crystals have been obtained from these preparations and are investigated for their suitability for X-ray work.

Effects of electromagnetic fields on molecules and cells

Evidence suggests that cell processes can be influenced by weak electromagnetic fields (EMFs). EMFs appear to represent a global interference or stress to which a cell can adapt without catastrophic consequences. There may be exceptions to this observation, however, such as the putative role of EMFs as promoters in the presence of a primary tumor initiator. The nature of the response suggests that the cell is viewing EMFs as it would another subtle environmental change. The age and state of the cell can profoundly affect the EMF bioresponse. There is no evidence that direct posttranscription effects occur as a result of EMF exposure. Although transcription alterations occur, no apparent disruption in routine physiological processes such as growth and division is immediately evident. What is usually observed is a transient perturbation followed by an adjustment by the normal homeostatic machinery of the cells. DNA does not appear to be significantly altered by EMF. If EMF exposure is associated with an increased risk of cancer, the paucity of genotoxic effects would support the suggestion that the fields act in tumor promotion rather than initiation. The site(s) and mechanisms of interaction remain to be elaborated. Although there are numerous studies and hypotheses that suggest the membrane represents the primary site of interaction, there are also several different studies showing that in vitro systems, including cell-free systems, are responsive to EMFs. The debate about potential hazards or therapeutic value of weak electromagnetic fields will continue until the mechanism of interaction has been clarified.

The effect of the protonmotive force on the redox state of mitochondrial cytochromes

In the absence of kinetic limitations, as determined either by high substrate concentrations or by absence of respiratory chain inhibitors, we have observed that: (a) the relationship between the percentage reduction of the cytochromes and the protonmotive force is linear in the case of cytochrome c and biphasic in the case of cytochrome b, (b) the redox state of cytochrome c depends only on the membrane potential and not on the total proton motive force and (c) the alkalinization of the matrix enhances the extent of cytochrome c reduction because of the marked inhibitory effect on the cytochrome oxidase activity. Thus, although the redox states of the b, c and aa3 mitochondrial cytochromes depend on the protonmotive force, the quantitative correlation between the two parameters and the relative effects of the electrical and chemical components of the force differ among the various cytochromes.