Inhibition of oxidative phosphorylation by a Ca2+-induced diminution of the adenine nucleotide translocator

Ocean warming and acidification affect the transitional C:N:P ratio and macromolecular accumulation in the harmful raphidophyte Heterosigma akashiwo

Despite an increase in ocean warming and acidification that is expected to increase the number of harmful algal species worldwide, the population of the raphidophyte Heterosigma akashiwo has been reported to be reduced. However, how this species physically and metabolically modifies transitional C:N:P ratio and macromolecule accumulation is unknown. Considering 1st, 10th, and 20th culture generations under present (low-temperature; low-carbon-dioxide [LTLC] 21 °C; pCO₂ 400 ppm) and future (high-temperature; high-carbon-dioxide [HTHC] 25 °C; pCO₂ 1000 ppm) ocean conditions, we examined transitional C:N:P ratio and macromolecule level changes and performed transcriptome sequencing. The results showed that compared to 1st generation cells, 20th generation cells under HTHC conditions showed a large decrease in carbon quota (Q_C: 34%), nitrogen quota (Q_N: 36%), and phosphorus quota (Q_P: 32%), which were reflected in an overall reduction in DNA and RNA quantity. Decreased activation of photosynthetic, carbon fixation and lipid metabolic pathways coincided with changes in photosynthetic efficiency, carbon concentration, and lipid accumulation after long-term (20th generation) exposure to HTHC conditions. We observed that these variations in internal metabolic pathways were caused by external changes in temperature, which activated the (Ca⁺) signaling pathway, and external changes in pCO₂, which altered proton exchange pathways. Our results suggest that H. akashiwo in a temperate environment will undergo profound changes in C:N:P ratio and macromolecular properties, leading to programmed cell death, in the future.

A multimodal fluorescent probe for portable colorimetric detection of pH and it's application in mitochondrial bioimaging

Mitochondria, as vital energy supplying organelles, play important roles in cellular metabolism, which are closely related with mitochondrial pH (∼8.0). In this work, a novel multimodal fluorescent probe was employed for ratiometric and colorimetric detection of pH. The probe is designed to work by controlling benzothiazole phenol-hemicyanine system as the interaction site and hemicyanine connected by conjugate bonds as the mitochondrial targeting, which also could make the fluorescence of probe red-shifted. This system results in a perfect ratiometric fluorescent response, whose emission changed from red to blue under pH 2.0-10.0, having a broad linear range (pH = 3.0-10.0). And the marked colour change (light yellow to deep purple via naked eye under pH 2.0-11.0) could be used to construct the test strip colorimetry and smartphone APP detection method, realizing the fast, portable, and accurate detection of pH in vitro and environment. Besides, the probe owns the characteristics of easy loading, high selectivity and staining ability of mitochondria, and low cytotoxicity, thereby allowing imaging of pH values and real-time monitor the subcellular mitochondria pH changes caused by drugs in living cells. It thus could be used to monitor the organ-specific dynamics related to transitions between pathological and physiological states.

Modeling the Effects of Calcium Overload on Mitochondrial Ultrastructural Remodeling

Mitochondrial cristae are dynamic invaginations of the inner membrane and play a key role in its metabolic capacity to produce ATP. Structural alterations caused by either genetic abnormalities or detrimental environmental factors impede mitochondrial metabolic fluxes and lead to a decrease in their ability to meet metabolic energy requirements. While some of the key proteins associated with mitochondrial cristae are known, very little is known about how the inner membrane dynamics are involved in energy metabolism. In this study, we present a computational strategy to understand how cristae are formed using a phase-based separation approach of both the inner membrane space and matrix space, which are explicitly modeled using the Cahn–Hilliard equation. We show that cristae are formed as a consequence of minimizing an energy function associated with phase interactions which are subject to geometric boundary constraints. We then extended the model to explore how the presence of calcium phosphate granules, entities that form in calcium overload conditions, exert a devastating inner membrane remodeling response that reduces the capacity for mitochondria to produce ATP. This modeling approach can be extended to include arbitrary geometrical constraints, the spatial heterogeneity of enzymes, and electrostatic effects to mechanize the impact of ultrastructural changes on energy metabolism.

Calcium phosphate precipitation inhibits mitochondrial energy metabolism

Early studies have shown that moderate levels of calcium overload can cause lower oxidative phosphorylation rates. However, the mechanistic interpretations of these findings were inadequate. And while the effect of excessive calcium overload on mitochondrial function is well appreciated, there has been little to no reports on the consequences of low to moderate calcium overload. To resolve this inadequacy, mitochondrial function from guinea pig hearts was quantified using several well-established methods including high-resolution respirometry and spectrofluorimetry and analyzed using mathematical modeling. We measured key mitochondrial variables such as respiration, mitochondrial membrane potential, buffer calcium, and substrate effects for a range of mitochondrial calcium loads from near zero to levels approaching mitochondrial permeability transition. In addition, we developed a computer model closely mimicking the experimental conditions and used this model to design experiments capable of eliminating many hypotheses generated from the data analysis. We subsequently performed those experiments and determined why mitochondrial ADP-stimulated respiration is significantly lowered during calcium overload. We found that when calcium phosphate levels, not matrix free calcium, reached sufficient levels, complex I activity is inhibited, and the rate of ATP synthesis is reduced. Our findings suggest that calcium phosphate granules form physical barriers that isolate complex I from NADH, disrupt complex I activity, or destabilize cristae and inhibit NADH-dependent respiration.

Mitochondrial calcium handling has been studied for nearly half a century. As we understand it today, low concentrations (1–10 nmol/mg mitochondria) of calcium exert beneficial effects on energy transduction. And high concentrations (>500 nmol/mg mitochondria) lead to respiratory uncoupling and membrane damage. But relatively little is known about the effect of moderate calcium concentrations (10–500 nmol/mg mitochondria) on mitochondrial function. At these concentrations, mitochondrial membrane integrity remains intact and energized, while ATP synthesis becomes significantly impaired. Prior studies have postulated several possible mechanisms, but the precise consequence of calcium overload on mitochondrial ATP production remained obscure. In this study, we combine experimental and computational approaches to show that calcium phosphate precipitation, as opposed to matrix free calcium, inhibits respiratory function at complex I just enough to limit proton pumping during oxidative phosphorylation and decrease ATP synthesis rates.

The regulation of neuronal mitochondrial metabolism by calcium

Calcium signalling is fundamental to the function of the nervous system, in association with changes in ionic gradients across the membrane. Although restoring ionic gradients is energetically costly, a rise in intracellular Ca(2+) acts through multiple pathways to increase ATP synthesis, matching energy supply to demand. Increasing cytosolic Ca(2+) stimulates metabolite transfer across the inner mitochondrial membrane through activation of Ca(2+) -regulated mitochondrial carriers, whereas an increase in matrix Ca(2+) stimulates the citric acid cycle and ATP synthase. The aspartate-glutamate exchanger Aralar/AGC1 (Slc25a12), a component of the malate-aspartate shuttle (MAS), is stimulated by modest increases in cytosolic Ca(2+) and upregulates respiration in cortical neurons by enhancing pyruvate supply into mitochondria. Failure to increase respiration in response to small (carbachol) and moderate (K(+) -depolarization) workloads and blunted stimulation of respiration in response to high workloads (veratridine) in Aralar/AGC1 knockout neurons reflect impaired MAS activity and limited mitochondrial pyruvate supply. In response to large workloads (veratridine), acute stimulation of respiration occurs in the absence of MAS through Ca(2+) influx through the mitochondrial calcium uniporter (MCU) and a rise in matrix [Ca(2+) ]. Although the physiological importance of the MCU complex in work-induced stimulation of respiration of CNS neurons is not yet clarified, abnormal mitochondrial Ca(2+) signalling causes pathology. Indeed, loss of function mutations in MICU1, a regulator of MCU complex, are associated with neuromuscular disease. In patient-derived MICU1 deficient fibroblasts, resting matrix Ca(2+) is increased and mitochondria fragmented. Thus, the fine tuning of Ca(2+) signals plays a key role in shaping mitochondrial bioenergetics.

Mitochondria-targeted ratiometric fluorescent probe for real time monitoring of pH in living cells

Pyridinium functioned 7-hydroxy coumarin was presented as the first mitochondria-targeted ratiometric fluorescent probe CP for real time monitoring pH in living cells. Compared with commercially available mitochondrial trackers, CP possesses high specificity to mitochondria in living cells as well as good biocompatibility. Meanwhile, CP displays excellent pH sensitivity and anti-interference capability. Confocal image experiments confirm that CP can monitor mitochondrial pH changes associated with the mitochondrial acidification, cellular apoptosis and stress response efficiently in real time.

Highly sensitive cell imaging "Off-On" fluorescent probe for mitochondria and ATP

A smart Off-On molecular scaffold/fluorescent probe 1 has been designed and synthesized. The probe has shown considerable photostability, cell permeability, organelle specificity and selectivity for ATP. The multicolor live cell imaging experiments in HeLa cells showed high selectivity of probe 1 for mitochondria with fluorescence "turn-on" response. As a proof of concept and promising prospects for application in biological sciences probe 1 has been utilized to detect ATP sensitively in a partial aqueous medium and intracellularly in HeLa cells. The favorable interaction between triphosphate unit of ATP and piperazine N atoms of probe 1 is attributed to synergistic effects of H-bonding and electrostatic interactions that encouraged the CH-π and π→π stacking between anthracene and purine rings. Consequently, the observed enhanced "turn-on" emission and a naked-eye sensitive blue-green color in the medium is attributable to arrest in photoinduced electron transfer (PET) process.

Mitochondria-immobilized pH-sensitive off-on fluorescent probe

We report here a mitochondria-targetable pH-sensitive probe that allows for a quantitative measurement of mitochondrial pH changes, as well as the real-time monitoring of pH-related physiological effects in live cells. This system consists of a piperazine-linked naphthalimide as a fluorescence off–on signaling unit, a cationic triphenylphosphonium group for mitochondrial targeting, and a reactive benzyl chloride subunit for mitochondrial fixation. It operates well in a mitochondrial environment within whole cells and displays a desirable off–on fluorescence response to mitochondrial acidification. Moreover, this probe allows for the monitoring of impaired mitochondria undergoing mitophagic elimination as the result of nutrient starvation. It thus allows for the monitoring of the organelle-specific dynamics associated with the conversion between physiological and pathological states.

Role of mitochondrial Ca2+ in the regulation of cellular energetics

Calcium is an important signaling molecule involved in the regulation of many cellular functions. The large free energy in the Ca(2+) ion membrane gradients makes Ca(2+) signaling inherently sensitive to the available cellular free energy, primarily in the form of ATP. In addition, Ca(2+) regulates many cellular ATP-consuming reactions such as muscle contraction, exocytosis, biosynthesis, and neuronal signaling. Thus, Ca(2+) becomes a logical candidate as a signaling molecule for modulating ATP hydrolysis and synthesis during changes in numerous forms of cellular work. Mitochondria are the primary source of aerobic energy production in mammalian cells and also maintain a large Ca(2+) gradient across their inner membrane, providing a signaling potential for this molecule. The demonstrated link between cytosolic and mitochondrial Ca(2+) concentrations, identification of transport mechanisms, and the proximity of mitochondria to Ca(2+) release sites further supports the notion that Ca(2+) can be an important signaling molecule in the energy metabolism interplay of the cytosol with the mitochondria. Here we review sites within the mitochondria where Ca(2+) plays a role in the regulation of ATP generation and potentially contributes to the orchestration of cellular metabolic homeostasis. Early work on isolated enzymes pointed to several matrix dehydrogenases that are stimulated by Ca(2+), which were confirmed in the intact mitochondrion as well as cellular and in vivo systems. However, studies in these intact systems suggested a more expansive influence of Ca(2+) on mitochondrial energy conversion. Numerous noninvasive approaches monitoring NADH, mitochondrial membrane potential, oxygen consumption, and workloads suggest significant effects of Ca(2+) on other elements of NADH generation as well as downstream elements of oxidative phosphorylation, including the F(1)F(O)-ATPase and the cytochrome chain. These other potential elements of Ca(2+) modification of mitochondrial energy conversion will be the focus of this review. Though most specific molecular mechanisms have yet to be elucidated, it is clear that Ca(2+) provides a balanced activation of mitochondrial energy metabolism that exceeds the alteration of dehydrogenases alone.

Sensors and regulators of intracellular pH

Protons dictate the charge and structure of macromolecules and are used as energy currency by eukaryotic cells. The unique function of individual organelles therefore depends on the establishment and stringent maintenance of a distinct pH. This, in turn, requires a means to sense the prevailing pH and to respond to deviations from the norm with effective mechanisms to transport, produce or consume proton equivalents. A dynamic, finely tuned balance between proton-extruding and proton-importing processes underlies pH homeostasis not only in the cytosol, but in other cellular compartments as well.

Glucose-stimulated oscillations in free cytosolic ATP concentration imaged in single islet beta-cells: evidence for a Ca2+-dependent mechanism

Normal glucose-stimulated insulin secretion is pulsatile, but the molecular mechanisms underlying this pulsatility are poorly understood. Oscillations in the intracellular free [ATP]/[ADP] ratio represent one possible mechanism because they would be expected to cause fluctuations in ATP-sensitive K(+) channel activity and hence oscillatory Ca(2+) influx. After imaging recombinant firefly luciferase, expressed via an adenoviral vector in single human or mouse islet beta-cells, we report here that cytosolic free ATP concentrations oscillate and that these oscillations are affected by glucose. In human beta-cells, oscillations were observed at both 3 and 15 mmol/l glucose, but the oscillations were of a longer wavelength at the higher glucose concentration (167 vs. 66 s). Mouse beta-cells displayed oscillations in both cytosolic free [Ca(2+)] and [ATP] only at elevated glucose concentrations, both with a period of 120 s. To explore the causal relationship between [Ca(2+)] and [ATP] oscillations, the regulation of each was further investigated in populations of MIN6 beta-cells. Incubation in Ca(2+)-free medium lowered cytosolic [Ca(2+)] but increased [ATP] in MIN6 cells at both 3 and 30 mmol/l glucose. Removal of external Ca(2+) increased [ATP], possibly by decreasing ATP consumption by endoplasmic reticulum Ca(2+)-ATPases. These results allow a model to be constructed of the beta-cell metabolic oscillator that drives nutrient-induced insulin secretion.

Nutrient-secretion coupling in the pancreatic islet beta-cell: recent advances

Insulin secretion from pancreatic islet beta-cells is a tightly regulated process, under the close control of blood glucose concentrations, and several hormones and neurotransmitters. Defects in glucose-triggered insulin secretion are ultimately responsible for the development of type II diabetes, a condition in which the total beta-cell mass is essentially unaltered, but beta-cells become progressively "glucose blind" and unable to meet the enhanced demand for insulin resulting for peripheral insulin resistance. At present, the mechanisms by which glucose (and other nutrients including certain amino acids) trigger insulin secretion in healthy individuals are understood only in part. It is clear, however, that the metabolism of nutrients, and the generation of intracellular signalling molecules including the products of mitochondrial metabolism, probably play a central role. Closure of ATP-sensitive K+(K(ATP)) channels in the plasma membrane, cell depolarisation, and influx of intracellular Ca2+, then prompt the "first phase" on insulin release. However, recent data indicate that glucose also enhances insulin secretion through mechanisms which do not involve a change in K(ATP) channel activity, and seem likely to underlie the second, sustained phase of glucose-stimulated insulin secretion. In this review, I will discuss recent advances in our understanding of each of these signalling processes.

Calcium- and ADP-magnesium-induced respiratory uncoupling in isolated cardiac mitochondria: influence of cyclosporin A

This study was designed to determine the effect of calcium and ADP-Mg on the oxidative phosphorylation in isolated cardiac mitochondria. The influence of cyclosporin A was also evaluated. The mitochondria were extracted from rat ventricles. Their oxidative phosphorylations were determined in two respiration media with different free Ca2+ concentrations. Respiration was determined with palmitoylcarnitine and either ADP or ADP-Mg. With elevated free Ca2+ concentrations and ADP-Mg, the transition state III to state IV respiration did not occurred. The ADP:O ratio was reduced. The phenomenon was not observed in the other experimental conditions (low free Ca2+ concentration with either ADP- or ADP-Mg or elevated free Ca2+ concentration with ADP-). Uncoupling was allied with a constant AMP production, which maintained an elevated ADP level in the respiration medium and prevented the return to state IV respiration. It was also observed in a respiration medium devoid of free Ca2+ when the mitochondria were pre-loaded with Ca2+. Uncoupling was inhibited by cyclosporin A. Furthermore, the Krebs cycle intermediates released from 14C-palmitoylcarnitine oxidation revealed that succinate was increased by elevated free Ca2+ and ADP-Mg. Succinate is a FAD-linked substrate with low respiration efficiency. Its accumulation could account for the decreased ADP:O ratio. The Ca2+- and ADP-Mg-induced uncoupling might be partly responsible for the mechanical abnormalities observed during low-flow ischemia.

Inhibition and uncoupling of oxidative phosphorylation by nonsteroidal anti-inflammatory drugs: study in mitochondria, submitochondrial particles, cells, and whole heart

The effects of the anti-inflammatory drugs diclofenac, piroxicam, indomethacin, naproxen, nabumetone, nimesulide, and meloxicam on mitochondrial respiration, ATP synthesis, and membrane potential were determined. Except for nabumetone and naproxen, the other drugs stimulated basal and uncoupled respiration, inhibited ATP synthesis, and collapsed membrane potential in mitochondria incubated in the presence of either glutamate + malate or succinate. Plots of membrane potential versus ATP synthesis (or respiration) showed proportional variations in both parameters, induced by different concentrations of nimesulide, meloxicam, piroxicam, or indomethacin, but not by diclofenac. The activity of the adenine nucleotide translocase was blocked by diclofenac and nimesulide; diclofenac also slightly inhibited mitochondrial ATPase activity. Naproxen did not affect any of the mitochondrial parameters measured. Nabumetone inhibited respiration, ATP synthesis, and membrane potential in the presence of glutamate + malate, but not with succinate. NADH oxidation in submitochondrial particles also was inhibited by nabumetone. Nabumetone inhibited O2 uptake in intact cells and in whole heart, whereas the other five drugs stimulated respiration. These observations revealed that in situ mitochondria are an accessible target. Except for diclofenac, a negative inotropic effect on cardiac contractility was induced by the drugs. The data indicated that nimesulide, meloxicam, piroxicam, and indomethacin behaved as mitochondrial uncouplers, whereas nabumetone exerted a specific inhibition of site 1 of the respiratory chain. Diclofenac was an uncoupler too, but it also affected the adenine nucleotide translocase and the H+-ATPase.

Identification of a mitochondrial Na+/H+ exchanger

The electroneutral exchange of protons for Na+ and K+ across the mitochondrial inner membrane contributes to organellar volume and Ca2+ homeostasis. The molecular nature of these transporters remains unknown. In this report, we characterize a novel gene (YDR456w; renamed NHA2) in Saccharomyces cerevisiae whose deduced protein sequence is homologous to members of the mammalian Na+/H+ exchanger gene family. Fluorescence microscopy showed that a Nha2-green fluorescent protein chimera colocalizes with 4',6-diamidino-2-phenylindole staining of mitochondrial DNA. To assess the function of Nha2, we deleted the NHA2 gene by homologous disruption and found that benzamil-inhibitable, acid-activated 22Na+ uptake into mitochondria was abolished in the mutant strain. It also showed retarded growth on nonfermentable carbon sources and severely reduced survival during the stationary phase of the cell cycle compared with the parental strain, consistent with a defect in aerobic metabolism. Sequence comparisons revealed that Nha2 has highest identity to a putative Na+/H+ exchanger homologue (KIAA0267; renamed NHE6) in humans. Northern blot analysis demonstrated that NHE6 is ubiquitously expressed but is most abundant in mitochondrion-rich tissues such as brain, skeletal muscle, and heart. Fluorescence microscopy showed that a NHE6-green fluorescent protein chimera also accumulates in mitochondria of transfected HeLa cells. These data indicate that NHA2 and NHE6 encode homologous Na+/H+ exchangers and suggest they may be important for mitochondrial function in lower and higher eukaryotes, respectively.

The mitochondrial membrane permeability transition induced by inorganic phosphate or inorganic arsenate. A comparative study

The membrane permeability transition (MPT) induced by Ca2+ and Pi or Asi was studied in rat kidney mitochondria. Membrane potential, Ca2+ transport and swelling were used to monitor the MPT. Asi promoted a faster and more extensive collapse of membrane potential, Ca2+ release and swelling than Pi. The MPT induced by Pi was fully blocked by Mg(2+)+ADP, spermine+ADP, Mg(2+)+ cyclosporin A (CSA), and ADP+CSA. In contrast, the MPT induced by Asi was only prevented, although not completely, by CSA+Mg2+ or ADP+CSA. Asi, but not Pi, was able to cause collapse of membrane potential in the presence of Sr2+. Carboxyatractyloside (CAT) produced collapse of membrane potential at a lower concentration in the presence of Asi+Ca(2+)+ADP than with Pi+Ca(2+)+ADP. The addition of Pi+Ca2+ to [14C]-ADP loaded mitochondria brought about a greater ADP release than Asi+Ca2+. The ADP release was CAT-sensitive with Pi but it was only partially blocked by Asi. The diminution of external pH did not inhibit the MPT induced by Pi or Asi. The results of this study suggest that the adenine nucleotide translocase does not have an essential role in the MPT induced by Asi+Ca2+.

Inhibition by Ca2+ of the hydrolysis and the synthesis of ATP in Ehrlich ascites tumour mitochondria: relation to the Crabtree effect

Phosphorylation of ADP and hydrolysis of ATP by isolated mitochondria from Ehrlich ascites tumour cells is greatly reduced when the mitochondria have been preloaded with Ca2+ (50 nmol/mg protein or more). Translocation of ADP is diminished in Ca(2+)-loaded mitochondria. However, ATPase in toluene-permeabilized mitochondria and in inside-out submitochondrial particles is also strongly inhibited by micromolar concentrations of Ca2+, indicating that, independently of adenine nucleotide transport, F1Fo-ATPase is also affected. ATP hydrolysis by submitochondrial particles depleted of the inhibitory subunit of F1Fo-ATPase (the Pullman-Monroy protein inhibitor) is insensitive to Ca2+; however, this sensitivity is restored when the particles are supplemented with the inhibitory subunit isolated from beef heart mitochondria. In view of the previous observations that glucose elicits in Ehrlich ascites tumour cells an increase of cytoplasmic free Ca2+ (Teplova, V.V., Bogucka, K., Czyz, A., Evtodienko, Yu.V., Duszyński, J. and Wojtczak, L. (1993) Biochem. Biophys. Res. Commun. 196, 1148-1154) and that this calcium is then taken up by mitochondria, resulting in a strong inhibition of coupled respiration (Evtodienko, Yu.V., Teplova, V.V., Duszyński, J., Bogucka, K. and Wojtczak, L. (1994) Cell Calcium 15, 439-446), the present results are discussed in terms of the mechanism of the Crabtree effect in tumour cells.

Control of oxidative phosphorylation in AS-30D hepatoma mitochondria

1. The distribution of control of the rate of state 3 respiration of AS-30D hepatoma mitochondria was determined. 2. The ATP/ADP carrier (flux control coefficient, Ci = 0.70) and the ATP synthase (Ci = 0.19-0.32) were the only steps that exerted significant control on the phosphorylating flux supported by either glutamate+malate, pyruvate+malate, or succinate+rotenone. This is in contrast to liver mitochondria where the control is distributed between several steps. 3. It is suggested that this pattern of control of phosphorylation in hepatoma mitochondria is a consequence of a lower content of adenine nucleotides or a higher content of Mg2+.

Ischemic injury to rat forebrain mitochondria and cellular calcium homeostasis

The three-vessel occlusion model of Kameyama et al. (Kameyama, M., Suzuki, J., Shirane, R. and Ogawa, A. (1985) Stroke 16, 489-493) was adapted with modifications to induce complete reversible rat forebrain ischemia. A fast and simple procedure for the isolation and purification of rat brain mitochondria, which provides high yield, is described. Mitochondria isolated from ischemic brain (12-30 min ischemia) exhibited decreases in State 3 respiratory rates of approx. 70% with NAD-linked respiratory substrates. Less effect was observed with succinate and rotenone. The State 4 respiratory activity remained near control levels except at 15 min of ischemia (25% increase) with NAD-linked substrates. Similarly, with succinate and rotenone, an approx. 30% increase in State 4 activity was observed at 20 min of ischemia. Consequently, the respiratory control indices (RCIs) were decreased. Both the respiratory rates and RCIs could be restored to near control levels upon the addition of EGTA(EDTA) or ruthenium red to the assay mixture. Analysis employing fura-2 as a Ca2+ probe, indicated a great decrease in the first order rate constant for Ca2+ uptake of ischemic mitochondria and a significant increase in Ca2+ homeostasis with an increase in the cytosolic Ca2+ concentration which results in excessive association of Ca2+ on the mitochondrial membrane and an inhibition of the respiratory chain-linked oxidative phosphorylation and Ca(2+)-transport activity of forebrain mitochondria. These deficits are proportional to the duration of ischemia.

Mitochondrial nucleotide translocase from skeletal muscle of halothane sensitive pigs: an electrophoretic study

ATP translocation into mitochondria isolated from halothane-sensitive pig (HP) muscle was dramatically reduced compared with normal pigs (NP). To determine if this was due to a decreased amount of ATP translocase in the mitochondrial membranes, or a structural modification of this protein, an electrophoretic study was undertaken. Total proteins and purified translocase preparations from (NP) and (HP) mitochondria were analyzed by SDS gel electrophoresis. In the two types of mitochondria no significant differences were observed either in the amount of ATP translocase or in the molecular weight. Also, neither nonequilibrium pH gradient gel electrophoresis nor the analysis of peptides produced by limited proteolysis revealed any structural difference between the two types of protein. On the basis of these results, the depressed translocase activity observed in (HP) mitochondria cannot be explained by a reduced amount of the nucleotide translocase, nor a structural alteration of this protein. Possible inhibition of (HP) translocase activity by Ca2+ accumulation or by other mechanisms is discussed.

Release of Ca2+ from heart and kidney mitochondria by peripheral-type benzodiazepine receptor ligands

1. The effect of the benzodiazepines Ro5-4864, AHN 086 and clonazepam on the release of Ca2+ from rat heart and kidney mitochondria was studied. 2. The peripheral-type benzodiazepines Ro5-4864 and AHN 086 induced Ca2+ release which was blocked by Mg2+ whereas the central-type benzodiazepine clonazepam was ineffective. 3. An associated collapse of membrane potential and swelling were also induced by AHN 086 in the presence of Ca2+. 4. However, no oxidation of pyridine nucleotides or increased rate or respiration were observed. 5. Release of Sr2+ was induced by AHN 086 in the absence of inorganic phosphate but not in its presence. 6. These data are discussed in the context of the current hypotheses on the mechanism of mitochondrial Ca2+ release.

Calcium inhibition of the ATP in equilibrium with [32P]Pi exchange and of net ATP synthesis catalyzed by bovine submitochondrial particles

A previous communication (Fagian, M. M., Pereira da Silva, L. and Vercesi, A. E. (1986) Biochim. Biophys. Acta 852, 262-268) indicated that intramitochondrial calcium inhibits oxidative phosphorylation by decreasing the availability of adenine nucleotides to both the ADP/ATP translocase and the F0F1-ATP synthase complex. In this work we analyzed the interactions of calcium-nucleotide and magnesium-nucleotide complexes with the ATP synthase during catalysis of ATP in equilibrium with [32P]Pi exchange and net synthesis of ATP by submitochondrial particles. Concerning the ATP in equilibrium with [32P]Pi exchange reaction, calcium was ineffective as divalent cation when assayed alone. Furthermore, the addition of calcium increased the magnesium concentration required for half-maximal activation of the exchange, without changing Vmax. With respect to net ATP synthesis, the inhibition by calcium was shown to be due to formation of the CaADP- complex, which competes with MgADP- for the active site of the F0F1-ATP synthase. Moreover, ATP hydrolysis was competitively inhibited by CaATP2-, showing that calcium is able to interact with the enzyme in both forward and backward reactions in the same manner. That high calcium concentrations are required for significant inhibition of ATP synthesis indicates that this inhibition is relevant under conditions in which cytosolic calcium concentrations rise to pathological levels. Therefore, this mechanism may be responsible, in part, for the decrease in cellular ATP content that has been observed to occur when calcium accumulates in the cytosol.

Impairment of ATP-linked reactions in mitochondria isolated from skeletal muscle of halothane-sensitive pigs

Oxygen consumption was depressed in mitochondria isolated from halothane sensitive pig (HP) muscle. The calculation of the respiratory control ratio (RCR) indicated that mitochondria were more affected at the site-I level of the respiratory chain. Calcium accumulation in these mitochondria was not altered when driven by the oxidation of succinate. This process was abolished when linked to ATP as a source of energy. ATP transport was completely inhibited in (HP) mitochondria.

Inhibition of oxidative phosphorylation by Ca2+ or Sr2+: a competition with Mg2+ for the formation of adenine nucleotide complexes

Intramitochondrial Sr2+, similar to Ca2+, inhibits oxidative phosphorylation in intact rat-liver mitochondria. Both Ca2+ and Sr2+ also inhibit the hydrolytic activity of the ATPase in submitochondrial particles. Half-maximal inhibition of ATPase activity was attained at a concentration of 2.5 mM Ca2+ or 5.0 mM Sr2+ when the concentration of Mg2+ in the medium was 1.0 mM. The inhibition of ATPase activity by both cations was strongly decreased by increasing the Mg2+ concentration in the reaction medium. In addition, kinetical data and the determination of the concentration of MgATP, the substrate of the ATPase, in the presence of different concentrations of Ca2+ or Sr2+ strongly indicate that these cations inhibit ATP hydrolysis by competing with Mg2+ for the formation of MgATP. On the basis of a good agreement between these results with submitochondrial particles and the results of titrations of oxidative phosphorylation with carboxyatractyloside or oligomycin in mitochondria loaded with Sr2+ it can be concluded that intramitochondrial Ca2+ or Sr2+ inhibits oxidative phosphorylation in intact mitochondria by decreasing the availability of adenine nucleotides to both the ADP/ATP carrier and the ATP synthase.