Liposome-mitochondrial inner membrane fusion. Lateral diffusion of integral electron transfer components

Isolation and Phospholipid Enrichment of Muscle Mitochondria and Mitoplasts

The efficient ATP production in mitochondria relies on the highly specific organization of its double membrane. Notably, the inner mitochondrial membrane (IMM) displays a massive surface extension through its folding into cristae, along which concentrate respiratory complexes and oligomers of the ATP synthase. Evidence has accumulated to highlight the importance of a specific phospholipid composition of the IMM to support mitochondrial oxidative phosphorylation. Contribution of specific phospholipids to mitochondrial ATP production is classically studied by modulating the activity of enzymes involved in their synthesis, but the interconnection of phospholipid synthesis pathways often impedes the determination of the precise role of each phospholipid. Here, we describe a protocol to specifically enrich mitochondrial membranes with cardiolipin or phosphatidylcholine, as well as a fluorescence-based method to quantify phospholipid enrichment. This method, based on the fusion of lipid vesicles with isolated mitochondria, may further allow a precise evaluation of phospholipid contribution to mitochondrial functions.

Cardiolipin content controls mitochondrial coupling and energetic efficiency in muscle

Unbalanced energy partitioning participates in the rise of obesity, a major public health concern in many countries. Increasing basal energy expenditure has been proposed as a strategy to fight obesity yet raises efficiency and safety concerns. Here, we show that mice deficient for a muscle-specific enzyme of very-long-chain fatty acid synthesis display increased basal energy expenditure and protection against high-fat diet-induced obesity. Mechanistically, muscle-specific modulation of the very-long-chain fatty acid pathway was associated with a reduced content of the inner mitochondrial membrane phospholipid cardiolipin and a blunted coupling efficiency between the respiratory chain and adenosine 5'-triphosphate (ATP) synthase, which was restored by cardiolipin enrichment. Our study reveals that selective increase of lipid oxidative capacities in skeletal muscle, through the cardiolipin-dependent lowering of mitochondrial ATP production, provides an effective option against obesity at the whole-body level.

Clarifying the supercomplex: the higher-order organization of the mitochondrial electron transport chain

The oxidative phosphorylation electron transport chain (OXPHOS-ETC) of the inner mitochondrial membrane is composed of five large protein complexes, named CI-CV. These complexes convert energy from the food we eat into ATP, a small molecule used to power a multitude of essential reactions throughout the cell. OXPHOS-ETC complexes are organized into supercomplexes (SCs) of defined stoichiometry: CI forms a supercomplex with CIII₂ and CIV (SC I+III₂+IV, known as the respirasome), as well as with CIII₂ alone (SC I+III₂). CIII₂ forms a supercomplex with CIV (SC III₂+IV) and CV forms dimers (CV₂). Recent cryo-EM studies have revealed the structures of SC I+III₂+IV and SC I+III₂. Furthermore, recent work has shed light on the assembly and function of the SCs. Here we review and compare these recent studies and discuss how they have advanced our understanding of mitochondrial electron transport.

Deferoxamine-activated hypoxia-inducible factor-1 restores cardioprotective effects of sevoflurane postconditioning in diabetic rats

AIM

The cardioprotective effects of sevoflurane postconditioning (SpostC) are eliminated under diabetic conditions, and the underlying mechanism for this phenomenon remains unclear. Many studies have demonstrated that the hypoxia-inducible factor-1 (HIF-1) signalling pathway in the myocardium is impaired under diabetic conditions. This study was to investigate whether deferoxamine (DFO)-induced activation of HIF-1 signalling pathway can restore the cardioprotective effects of SpostC in diabetic rats.

METHODS

A model of myocardial ischaemia/reperfusion (I/R) injury was induced via ligation of the left anterior descending artery. SpostC was conducted by administering 1.0 MAC sevoflurane. After inducing the I/R injury, the following parameters were measured: myocardial infarct size, cardiac function, myocardial ultrastructure, mitochondrial respiratory function, respiratory chain enzyme activity, rate of reactive oxygen species (ROS) generation, and protein expression of HIF-1α, vascular endothelial growth factor (VEGF), cleaved caspase-3, Bcl-2 and Bax.

RESULTS

After DFO activated HIF-1 in the impaired myocardium of diabetic rats, SpostC significantly upregulated the protein expression of HIF-1α and its downstream mediator VEGF. This improved myocardial mitochondrial respiratory function and respiratory chain enzyme activity and reduced ROS generation as well as the protein expression of cleaved caspase-3 and Bax. As a result, myocardial infarct size decreased, and cardiac function and mitochondrial ultrastructure improved.

CONCLUSION

This study demonstrates for the first time that abolishment of the cardioprotective effects of SpostC in diabetic rats is associated with impairment of the HIF-1 signalling pathway and that DFO can activate HIF-1 to restore these cardioprotective effects of SpostC in diabetic rats.

The Enigma of the Respiratory Chain Supercomplex

Respiratory chain dysfunction plays an important role in human disease and aging. It is now well established that the individual respiratory complexes can be organized into supercomplexes, and structures for these macromolecular assemblies, determined by electron cryo-microscopy, have been described recently. Nevertheless, the reason why supercomplexes exist remains an enigma. The widely held view that they enhance catalysis by channeling substrates is challenged by both structural and biophysical information. Here, we evaluate and discuss data and hypotheses on the structures, roles, and assembly of respiratory-chain supercomplexes and propose a future research agenda to address unanswered questions.

Amazing structure of respirasome: unveiling the secrets of cell respiration

Respirasome, a huge molecular machine that carries out cellular respiration, has gained growing attention since its discovery, because respiration is the most indispensable biological process in almost all living creatures. The concept of respirasome has renewed our understanding of the respiratory chain organization, and most recently, the structure of respirasome solved by Yang's group from Tsinghua University (Gu et al. Nature 237(7622):639-643, 2016) firstly presented the detailed interactions within this huge molecular machine, and provided important information for drug design and screening. However, the study of cellular respiration went through a long history. Here, we briefly showed the detoured history of respiratory chain investigation, and then described the amazing structure of respirasome.

Increased myocardial vulnerability to ischemia-reperfusion injury in the presence of left ventricular hypertrophy

OBJECTIVE

Despite its high prevalence among patients suffering myocardial infarction, the significance of left ventricle hypertrophy for infarct size is not known. We asked whether infarct size might be increased by this condition, and whether any such increase might be associated with an increased mitochondrial damage following coronary occlusion.

METHODS

Occlusion of the left descending artery in isolated, perfused hearts of SHR-SP (spontaneously hypertensive rat stroke-prone) (left ventricular hypertrophy) or Wistar-Kyoto (WKY) (control) rats was used, followed by reperfusion with or without exendin-4 (Exe-4), a glucagon-like peptide-1 receptor agonist. Infarct size relative to area-at-risk was determined. Separately, mitochondria were isolated after global ischemia. Activities of complexes III and IV and amounts of selected complex subunits and cytochromes a, b, c, and c1 were determined.

RESULTS

Infarct size (ischemia 35  min and 120  min reperfusion) was 65.8% (±3.3%) and 37.1% (±3.4%) in the SHR-SP and WKY hearts, respectively (P < 0.05). Exe-4 significantly decreased infarct size and hypercontracture in WKY, but not in SHR-SP, hearts. After ischemia 15  min in SHR-SP hearts, Exe-4 reduced the infarct (26.6%, ±3.8% to 9.3% ± 1.5%; P < 0.05). Mitochondria from postischemic SHR-SP hearts showed a reduction of complex III (368.1 ± 37.5 to 175.8 ± 23.0  nmoles/min × mg; P < 0.05) and complex IV (14.4 ± 0.22 to 5.8 ± 0.8 1/s × mg; P < 0.05) activities and decreased amounts of cytochromes a, b, and c.

CONCLUSION

Hearts from hypertensive (SHR-SP) rats with left ventricle hypertrophy appeared more vulnerable to ischemia-reperfusion injury, as supported by a more profound infarct development and an earlier loss of postconditioning by Exe-4. Mitochondrial complexes III and IV were identified among possible loci of this increased, hypertrophy-associated vulnerability.

Histones cause aggregation and fusion of lipid vesicles containing phosphatidylinositol-4-phosphate

In a previous article, we demonstrated that histones (H1 or histone octamers) interact with negatively charged bilayers and induce extensive aggregation of vesicles containing phosphatidylinositol-4-phosphate (PIP) and, to a lesser extent, vesicles containing phosphatidylinositol (PI). Here, we found that vesicles containing PIP, but not those containing PI, can undergo fusion induced by histones. Fusion was demonstrated through the observation of intervesicular mixing of total lipids and inner monolayer lipids, and by ultrastructural and confocal microscopy studies. Moreover, in both PI- and PIP-containing vesicles, histones caused permeabilization and release of vesicular aqueous contents, but the leakage mechanism was different (all-or-none for PI and graded release for PIP vesicles). These results indicate that histones could play a role in the remodeling of the nuclear envelope that takes place during the mitotic cycle.

Dietary lipid quality and mitochondrial membrane composition in trout: responses of membrane enzymes and oxidative capacities

To examine whether membrane fatty acid (FA) composition has a greater impact upon specific components of oxidative phosphorylation or on overall properties of muscle mitochondria, rainbow trout (Oncorhynchus mykiss) were fed two diets differing only in FA composition. Diet 1 was enriched in 18:1n-9 and 18:2n-6 while Diet 2 was enriched in 22:6n-3. The FA composition of mitochondrial phospholipids was strongly affected by diet. 22:6n-3 levels were twice as high (49%) in mitochondrial phospholipids of fish fed Diet 2 than in those fed Diet 1. 18:2n-6 content of the phospholipids also followed the diets, whereas 18:1n-9 changed little. All n-6 FA, most notably 22:5n-6, were significantly higher in fish fed Diet 1. Nonetheless, total saturated FA, total monounsaturated FA and total polyunsaturated FA in mitochondrial phospholipids varied little. Despite a marked impact of diet on specific FA levels in mitochondrial phospholipids, only non-phosphorylating (state 4) rates were higher in fish fed Diet 2. Phosphorylating rates (state 3), oxygen consumption due to flux through the electron transport chain complexes as well as the corresponding spectrophotometric activities did not differ with diet. Body mass affected state 4 rates and cytochrome c oxidase and F 0 F 1 ATPase activities while complex I showed a diet-specific effect of body mass. Only the minor FA that were affected by body mass were correlated with functional properties. The regulated incorporation of dietary FA into phospholipids seems to allow fish to maintain critical membrane functions even when the lipid quality of their diets varies considerably, as is likely in their natural environment.

Reaction mechanism of single subunit NADH-ubiquinone oxidoreductase (Ndi1) from Saccharomyces cerevisiae: evidence for a ternary complex mechanism

The flavoprotein rotenone-insensitive internal NADH-ubiquinone (UQ) oxidoreductase (Ndi1) is a member of the respiratory chain in Saccharomyces cerevisiae. We reported previously that bound UQ in Ndi1 plays a key role in preventing the generation of reactive oxygen species. Here, to elucidate this mechanism, we investigated biochemical properties of Ndi1 and its mutants in which highly conserved amino acid residues (presumably involved in NADH and/or UQ binding sites) were replaced. We found that wild-type Ndi1 formed a stable charge transfer (CT) complex (around 740 nm) with NADH, but not with NADPH, under anaerobic conditions. The intensity of the CT absorption band was significantly increased by the presence of bound UQ or externally added n-decylbenzoquinone. Interestingly, however, when Ndi1 was exposed to air, the CT band transiently reached the same maximum level regardless of the presence of UQ. This suggests that Ndi1 forms a ternary complex with NADH and UQ, but the role of UQ in withdrawing an electron can be substitutable with oxygen. Proteinase K digestion analysis showed that NADH (but not NADPH) binding induces conformational changes in Ndi1. The kinetic study of wild-type and mutant Ndi1 indicated that there is no overlap between NADH and UQ binding sites. Moreover, we found that the bound UQ can reversibly dissociate from Ndi1 and is thus replaceable with other quinones in the membrane. Taken together, unlike other NAD(P)H-UQ oxidoreductases, the Ndi1 reaction proceeds through a ternary complex (not a ping-pong) mechanism. The bound UQ keeps oxygen away from the reduced flavin.

Native E. coli inner membrane incorporation in solid-supported lipid bilayer membranes

Solid-supported bilayer lipid membranes (SBLMs) containing membrane protein have been generated through a simple lipid dilution technique. SBLM formation from mixtures of native Escherichia coli bacterial inner membrane (IM) vesicles diluted with egg phosphatidylcholine (egg PC) vesicles has been explored with dissipation enhanced quartz crystal microbalance (QCM-D), atomic force microscopy (AFM), attenuated total internal-reflection Fourier-transform infrared spectroscopy (ATR-FTIR), and fluorescence recovery after photobleaching (FRAP). QCM-D studies reveal that SBLM formation from vesicle mixtures ranging between 0% and 100% IM can be divided into two regimes. Samples with < or = 40% IM form SBLMs, while samples of greater IM fractions are dominated by vesicle adsorption. FRAP experiments showed that the bilayers formed from mixed vesicles with < or = 40% IM were fluid, and comprised a mixture of both egg PC and IM. ATR-FTIR measurements on SBLMs membranes formed with 30% IM confirm that protein is present. SBLM formation was also explored as a function of temperature by QCM-D and FRAP. For samples of 30% IM, QCM-D data show a decreased mass and viscoelasticity at elevated temperatures, and an increased fluidity is observed by FRAP measurements. These results suggest improved biomimetic characteristics can be obtained by forming and maintaining the system at, or close to, 37 degrees C.

The profound effects of microcystin on cardiac antioxidant enzymes, mitochondrial function and cardiac toxicity in rat

Deaths from microcystin toxication have widely been attributed to hypovolemic shock due to hepatic interstitial hemorrhage, while some recent studies suggest that cardiogenic complication is also involved. So far, information on cardiotoxic effects of MC has been rare and the underlying mechanism is still puzzling. The present study examined toxic effects of microcystins on heart muscle of rats intravenously injected with extracted MC at two doses, 0.16LD(50) (14 microg MC-LReq kg(-1) body weight) and 1LD(50) (87 microg MC-LReq kg(-1) body weight). In the dead rats, both TTC staining and maximum elevations of troponin I levels confirmed myocardial infarction after MC exposure, besides a serious interstitial hemorrhage in liver. In the 1LD(50) dose group, the coincident falls in heart rate and blood pressure were related to mitochondria dysfunction in heart, while increases in creatine kinase and troponin I levels indicated cardiac cell injury. The corresponding pathological alterations were mainly characterized as loss of adherence between cardiac myocytes and swollen or ruptured mitochondria at the ultrastructural level. MC administration at a dose of 1LD(50) not only enhanced activities and up-regulated mRNA transcription levels of antioxidant enzymes, but also increased GSH content. At both doses, level of lipid peroxides increased obviously, suggesting serious oxidative stress in mitochondria. Simultaneously, complex I and III were significantly inhibited, indicating blocks in electron flow along the mitochondrial respiratory chain in heart. In conclusion, the findings of this study implicate a role for MC-induced cardiotoxicity as a potential factor that should be considered when evaluating the mechanisms of death associated with microcystin intoxication in Brazil.

In vivo studies on the toxic effects of microcystins on mitochondrial electron transport chain and ion regulation in liver and heart of rabbit

This study examined the toxic effects of microcystins on mitochondria of liver and heart of rabbit in vivo. Rabbits were injected i.p. with extracted microcystins (mainly MC-RR and -LR) at two doses, 12.5 and 50 MC-LReq. microg/kg bw, and the changes in mitochondria of liver and heart were studied at 1, 3, 12, 24 and 48 h after injection. MCs induced damage of mitochondrial morphology and lipid peroxidation in both liver and heart. MCs influenced respiratory activity through inhibiting NADH dehydrogenase and enhancing succinate dehydrogenase (SDH). MCs altered Na(+)-K(+)-ATPase and Ca(2+)-Mg(2+)-ATPase activities of mitochondria and consequently disrupted ionic homeostasis, which might be partly responsible for the loss of mitochondrial membrane potential (MMP). MCs were highly toxic to mitochondria with more serious damage in liver than in heart. Damage of mitochondria showed reduction at 48 h in the low dose group, suggesting that the low dose of MCs might have stimulated a compensatory response in the rabbits.

Dietary fatty acid composition changes mitochondrial phospholipids and oxidative capacities in rainbow trout red muscle

Dietary conditioning of juvenile trout changed the acyl chain composition of mitochondrial phospholipids and the oxidative capacities of muscle mitochondria. Trout were fed three diets differing only in fatty acid (FA) composition. The highly unsaturated 22:6 n-3 (DHA) accounted for 0.4, 14, and 30% of fatty acids in Diets 1, 2 and 3. After 10 weeks of growth, the dietary groups differed markedly in FA composition of mitochondrial phospholipids, with significant dietary effects for virtually all FA. Mean mitochondrial DHA levels were 19, 40 and 33% in trout fed Diets 1, 2 and 3. Mitochondrial oxidative capacities changed with diet, while mitochondrial concentrations of cytochromes and of the adenylate nucleotide translocase (nmol mg(1) protein) did not. Mitochondria from fish fed Diet 1 had higher non-phosphorylating (state 4) rates at 5 degrees C than those fed other diets. When phosphorylating (state 3) rates differed between dietary groups, rates at 5 and 15 degrees C were higher for fish fed the more unsaturated diets. Stepwise multiple regressions indicated that FA composition could explain much (42-70%) of the variability of state 4 rates, particularly at 5 degrees C. At 15 degrees C, FA composition explained 16-42% of the variability of states 3 and 4 rates. Similar conclusions were obtained for the complete data set (trout fed diets 1, 2 and 3) and for the data from trout achieving similar growth rates (e.g. those fed Diets 1 and 2). Neither general characteristics of membrane FA, such as % saturates, unsaturation index, n-3, n-6 or n-3/n-6 nor levels of abundant unsaturated FA such as DHA or 18:1(n-9 + n-7), were systematically correlated with mitochondrial capacities even though they differed considerably between trout fed the different diets. Relatively minor FA (20:5n-3, 20:0, 18:2n-6, 18:3n-3, 18:0 and 15:0) showed better correlations with mitochondrial oxidative capacities. This supports the concept that acyl chain composition modulates mitochondrial capacities via interactions between membrane proteins and specific FA of particular phospholipid classes in their microenvironment.

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.

How might you compare mitochondria from different tissues and different species?

Mitochondria were isolated from the liver, kidney and mixed hindlimb skeletal muscle of three vertebrate species; the laboratory rat Rattus norvegicus, the bearded dragon lizard Pogona vitticeps, and the cane toad Bufo marinus. These vertebrate species are approximately the same body mass and have similar body temperatures. The content of cytochromes B, C, C1, and A were measured in these isolated mitochondria by oxidised-reduced difference spectra. Adenine nucleotide translocase (ANT) was measured by titration of mitochondrial respiration with carboxyactractyloside and the protein and phospholipid content of isolated mitochondria were also measured. Fatty acid composition of mitochondrial phospholipids was measured. Mitochondrial respiration was measured at 37 degrees C under states III and IV conditions as well as during oligomycin inhibition. Species differed in the ratios of different mitochondrial cytochromes. Muscle mitochondria differed from kidney and liver mitochondria by having a higher ANT content relative to cytochrome content. Respiration rates were compared relative to a number of denominators and found to be most variable when expressed relative to mitochondrial protein content and least variable when expressed relative to mitochondrial cytochrome A and ANT content. The turnover of cytochromes was calculated and found to vary between 1 and 94 electrons s(-1). The molecular activity of mitochondrial cytochromes was found to be significantly positively correlated with the relative polyunsaturation of mitochondrial membrane lipids.

Why are some mitochondria more powerful than others: insights from comparisons of muscle mitochondria from three terrestrial vertebrates

We studied the molecular composition of muscle mitochondria to evaluate whether the contents of cytochromes or adenine nucleotide translocase (ANT) or phospholipid acyl compositions reflect differences in mitochondrial oxidative capacities. We isolated mitochondria from three vertebrates of similar size and preferred temperature, the rat (Rattus norvegicus), the cane toad (Bufo marinus) and the bearded dragon lizard (Pogona vitticeps). Mitochondrial oxidative capacities were higher in rats and cane toads than in bearded dragon, whether rates were expressed relative to protein, cytochromes or ANT. Inter-specific differences were least pronounced when rates were expressed relative to cytochrome A, a component of cytochrome C oxidase (CCO), or ANT. In mitochondria from rat and cane toad, cytochrome A was more abundant than C followed by B and then C(1), while in bearded dragon mitochondria, the cytochromes were present in roughly equal levels. Analysis of correlations between mitochondrial oxidative capacities and macromolecular components revealed that cytochrome A explained at least half of the intra- and inter-specific variability in substrate oxidation rates. ANT levels were an excellent correlate of state 3 rates while phospholipid contents were correlated with state 4 rates. As the % poly-unsaturation and the % 20:4n-6 in mitochondrial phospholipids were equivalent in toads and rats, and exceeded the levels in lizards, they may contribute to the inter-specific differences in oxidative capacities. We suggest that the numbers of CCO and ANT together with the poly-unsaturation of phospholipids explain the higher oxidative capacities in muscle mitochondria from rats and cane toads.

Photochemical activities of plant photosystem I particles reconstituted into phosphatidylglycerol liposomes

Phosphatidylglycerol (PG) is the only anionic phospholipid in photosynthetic membrane and the important component of photosystem I (PSI). In this study, the interaction of PG with PSI particle from spinach was investigated by using reconstitution method. The results from the properties of electron transport, fluorescence emission, turbidity, and protein secondary structures in PSI complex incorporated into PG liposomes revealed the existence of PSI-PG interactions. A stimulation and an inhibition of oxygen uptake in PSI particle at a low and higher PG/chlorophyll mass ratio, respectively, were observed. Moreover, an additional enhancement and depression of electron flow in the PSI-PG complexes were occurred in the reaction medium containing CaCl2 at concentrations below and above 5 mM, the aggregation threshold of the reconstituted membranes, respectively. The results demonstrated that the maintenance of the structural optimization was needed for a stimulation of electron transport at a low PG/PSI mass ratio, while a decay of this PSI activity at high PG/PSI ratio was the result of inhibition of the energy transfer from LHCI to PSI reaction center induced by the dissociation of LHCI-680.

Respiratory chain supercomplexes in plant mitochondria

Supercomplexes are defined associations of protein complexes, which are important for several cellular functions. This "quintenary" organization level of protein structure recently was also described for the respiratory chain of plant mitochondria. Except succinate dehydrogenase (complex II), all complexes of the oxidative phosphorylation (OXPOS) system (complexes I, III, IV and V) were found to form part of supercomplexes. Compositions of these supramolecular structures were systematically investigated using digitonin solubilizations of mitochondrial fractions and two-dimensional Blue-native (BN) polyacrylamide gel electrophoresis. The most abundant supercomplex of plant mitochondria includes complexes I and III at a 1:2 ratio (I1 + III2 supercomplex). Furthermore, some supercomplexes of lower abundance could be described, which have I2 + III4, V2, III2 + IV(1-2), and I1 + III2 + IV(1-4) compositions. Supercomplexes consisting of complexes I plus III plus IV were proposed to be called "respirasome", because they autonomously can carry out respiration in the presence of ubiquinone and cytochrome c. Plant specific alternative oxidoreductases of the respiratory chain were not associated with supercomplexes under all experimental conditions tested. However, formation of supercomplexes possibly indirectly regulates alternative respiratory pathways in plant mitochondria on the basis of electron channeling. In this review, procedures to characterize the supermolecular organization of the plant respiratory chain and results concerning supercomplex structure and function are summarized and discussed.

Control of oxidative phosphorylation during insect metamorphosis

The midgut of the tobacco hornworm ( Manduca sexta) is a highly aerobic tissue that is destroyed and replaced by a pupal epithelium at metamorphosis. To determine how oxidative phosphorylation is altered during the programmed death of the larval cells, top-down control analysis was performed on mitochondria isolated from the midguts of larvae before and after the commitment to pupation. Oxygen consumption and protonmotive force (measured as membrane potential in the presence of nigericin) were monitored to determine the kinetic responses of the substrate oxidation system, proton leak, and phosphorylation system to changes in the membrane potential. Mitochondria from precommitment larvae have higher respiration rates than those from postcommitment larvae. State 4 respiration is controlled by the proton leak and the substrate oxidation system. In state 3, the substrate oxidation system exerted 90% of the control over respiration, and this high level of control did not change with development. Elasticity analysis, however, revealed that, after commitment, the activity of the substrate oxidation system falls. This decline may be due, in part, to a loss of cytochrome c from the mitochondria. There are no differences in the kinetics of the phosphorylation system, indicating that neither the F1F0ATP synthase nor the adenine nucleotide translocase is affected in the early stages of metamorphosis. An increase in proton conductance was observed in mitochondria isolated from postcommitment larvae, indicating that membrane area, lipid composition, or proton-conducting proteins may be altered during the early stages of the programmed cell death of the larval epithelium.

Differences in Isolated Mitochondria Are Insufficient to Account for Respiratory Depression during Diapause inArtemia franciscanaEmbryos

In response to cues signifying the approach of winter, adult Artemia franciscana produce encysted embryos that enter diapause. We show that respiration rates of diapause embryos collected from the field (Great Salt Lake, Utah) are reduced up to 92% compared with postdiapause embryos when measured under conditions of normoxia and full hydration. However, mitochondria isolated from diapause embryos exhibit rates of state 3 and state 4 respiration on pyruvate that are equivalent to those from postdiapause embryos with active metabolism; a reduction in these rates (15%-27%) is measured with succinate for two of three collection years. Respiratory control ratios for diapause mitochondria are comparable to or higher than those from postdiapause embryos. The P : O flux ratios are statistically identical. Our calculations suggest that respiration of intact, postdiapause embryos is operating close to the state 3 oxygen fluxes measured for isolated mitochondria. Cytochrome c oxidase (COX) activity is 53% lower in diapause mitochondria during one collection year; the minimal impact of this COX reduction on mitochondrial respiration appears to be due to the 31% excess COX capacity in A. franciscana mitochondria. Transmission electron micrographs of embryos reveal mitochondria that are well differentiated and structurally similar in both states. As inferred from the similar amounts of mitochondrial protein extractable, tissue contents of mitochondria in diapause and postdiapause embryos are equivalent. Thus, metabolic depression during diapause cannot be fully explained by altered properties of isolated mitochondria. Rather, mechanisms for active inhibition or substrate limitation of mitochondrial metabolism in vivo may be operative.

Energy metabolism during insect flight: biochemical design and physiological performance

Flying insects achieve the highest known mass-specific rates of O(2) consumption in the animal kingdom. Because the flight muscles account for >90% of the organismal O(2) uptake, accurate estimates of metabolic flux rates (J) in the muscles can be made. In steady state, these are equal to the net forward flux rates (v) at individual steps and can be compared with flux capacities (V(max)) measured in vitro. In flying honeybees, hexokinase and phosphofructokinase, both nonequilibrium reactions in glycolysis, operate at large fractions of their maximum capacities (i.e., they operate at high v/V(max)). Phosphoglucoisomerase is a reversible reaction that operates near equilibrium. Despite V(max) values more than 20-fold greater than the net forward flux rates during flight, a close match is found between the V(max) required in vivo (estimated using the Haldane relationship) to maintain near equilibrium and this net forward flux rate and the V(max) measured in vitro under simulated physiological conditions. Rates of organismal O(2) consumption and difference spectroscopy were used to estimate electron transfer rates per molecule of respiratory chain enzyme during flight. These are much higher than those estimated in mammalian muscles. Current evidence indicates that metabolic enzymes in honeybees do not display higher catalytic efficiencies than the homologous enzymes in mammals, and the high electron transfer rates do not appear to be the result of higher enzyme densities per unit cristae surface area. A number of possible mechanistic explanations for the higher rates of electron transfer are proposed.

Ca2+-reversible inhibition of the mitochondrial megachannel by ubiquinone analogues

Ubiquinone 0 and decylubiquinone have been reported to inhibit the mitochondrial permeability transition pore (PTP) [Fontaine, E., Ichas, F. and Bernardi, P. (1998) J. Biol. Chem. 273, 25734-257401, offering a new clue to its molecular composition. In patch-clamp experiments on rat liver mitochondria we have observed that these compounds also inhibit the previously described mitochondrial megachannel (MMC), confirming its identification as the PTP. Inhibition can be reversed by increasing [Ca2+], in analogy to the behavior observed with several other disparate PTP/MMC inhibitors. To rationalize the ability of Ca2+ to overcome inhibition by various quite different compounds we propose that it acts via the phospholipid bilayer.

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.

Novel fluorescence membrane fusion assays reveal GTP-dependent fusogenic properties of outer mitochondrial membrane-derived proteins

We have shown that fusion of small unilamellar vesicles (SUV) with outer mitochondrial membranes occurs at physiological pH [Cortese et al., 1991, J. Cell Biol., Vol. 113, 1331-1340]. The proteins driving this process could be involved in mitochondrial membrane fusion, which is presently poorly understood. In this study, we release from rat liver mitochondria a soluble protein fraction (SF) that increases fusion at neutral pH measured by membrane fusion assays (MFAs). Since this fusogenic activity was specifically enhanced by GTP, we separate SF by GTP affinity chromatography into: i) a flow-through subfraction (G1) containing numerous proteins with low GTP affinity; and ii) a subfraction (G2) which may contain GTP-binding proteins. A novel array of MFAs is developed to study the fusogenic properties of these fractions, measuring the merging of membranes (membrane-mixing) or the mixing of intravesicular aqueous contents (content-mixing). The MFAs use: a) SUV/large unilamellar vesicles, lacking mitochondrial membranes; b) SUV/mitochondria, reconstituting membrane-mitochondrial interactions; and c) mitochondria/mitochondria, mimicking mitochondrial fusion. The results indicate that: i) G1 contains GTP-independent, in vitro fusogenic proteins that are not sufficient to induce mitochondrial fusion; and ii) G2 contains GTP-dependent proteins that stimulate mitochondrial fusion at neutral pH. The MFAs described here could be used to monitor the isolation of active proteins from these subfractions and to define the mechanism of intermitochondrial membrane fusion.

The respiratory chain in yeast behaves as a single functional unit

Inhibitor titrations using antimycin have been used to study the pool behavior of ubiquinone and cytochrome c in the respiratory chain of the yeast Saccharomyces cerevisiae. If present in a homogeneous pool, these carriers should be able to diffuse freely through or along the membrane respectively and accept and subsequently donate electrons to an infinite number of the respective respiratory complex. However, we show that under physiological conditions neither ubiquinone nor cytochrome c exhibits pool behavior, implying that the respiratory chain in yeast is one functional unit. Pool behavior can be introduced for both small carriers by adding chaotropic agents to the reaction medium. We conclude that these agents disrupt the interaction between the respiratory complexes, thereby causing them to become randomly arranged in the membrane. In such a situation, ubiquinone and cytochrome c become mobile carriers, shuttling between the large respiratory complexes. Furthermore, we conclude from the respiratory activities found for different substrates that the respiratory units in yeast vary in composition with respect to the ubiquinone reducing enzyme. All units contain the cytochrome chain, supplemented with either succinate dehydrogenase or the internal or the external NADH dehydrogenase. This implies that when only one substrate is available, only a certain fraction of the cytochrome chain is used in respiration. The molecular organization of the respiratory chain in yeast is compared with that of higher eukaryotes and to the electron transfer systems of photosynthetic membranes. Differences between the organization of the respiratory chain of yeast and that of higher eukaryotes are discussed in terms of the ability of yeast to radically alter its metabolism in response to change of the available carbon source.

Location of ubiquinone homologues in liposome membranes studied by fluorescence anisotropy of diphenyl-hexatriene and trimethylammonium-diphenyl-hexatriene

The measurements of diphenyl-hexatriene (DPH) and trimethylammonium-diphenyl-hexatriene (TMA-DPH) fluorescence anisotropy in dipalmitoylphosphatidylcholine (DPPC) and egg yolk lecithin (EYL) liposomes containing different concentrations of various ubiquinone (UQ) homologues have been performed. UQ-4 induced the highest DPH anisotropy increase in DPPC liposomes, whereas for higher UQ homologues the anisotropy was lowered with the increase of UQ side-chain length. These differences were less pronounced in EYL liposomes. It was concluded that at a higher content in the membranes (3-4 mol%), the short-chain ubiquinones are arranged parallel to lipid fatty acid chains, whereas long-chain homologues are progressively removed from the lipid acyl chains into the midplane region of the membrane. At the lower (1-2 mol%) concentrations, long-chain quinones seem to be evenly distributed within the membrane, especially in EYL membranes. UQ-10 in EYL liposomes perturbed TMA-DPH to a similar extend as the short-chain ubiquinones indicating that UQ-10 penetrates the interface regions of the membrane where its redox reactions occur. The localization and physical state of UQ-10 in native membranes is discussed.

Temperature-induced changes in fluorescence properties as a probe of porphyrin microenvironment in lipid membranes. 2. The partition of hematoporphyrin and protoporphyrin in mitochondria

The temperature dependence of hematoporphyrin and protoporphyrin fluorescence quantum yields (phi F) was studied after delivery to whole mitochondria or isolated inner (IMM) and outer (OMM) mitochondrial membranes, obtained from liver of Wistar rats. These studies are very sensitive to variations of the porphyrin lipid environment. Before incorporation, the porphyrins were dissolved in 0.01 M sodium phosphate, 0.15 M NaCl, pH 7.4) NaCl/Pi (only hematoporphyrin) or dispersed into liposomes of dipalmitoylphosphoglycerocholine (Pam2GroPCho), sometimes enriched with cholesterol or cardiolipin. Whole mitochondria show higher incorporation capacity of hematoporphyrin and protoporphyrin than isolated IMM and OMM, probably because additional, energy-sensitive transport mechanisms for the porphyrin uptake occur in intact organelles. A small decrease in protoporphyrin uptake is observed in OMM in comparison with IMM; in contrast, the decrease in hematoporphyrin uptake by OMM is rather significant. A comparison between the results obtained with IMM, OMM and whole mitochondria show that both porphyrins, when released to the intact organelles, preferentially localize in the IMM, irrespective of the lipid carrier used. NaCl/Pi-dissolved hematoporphyrin probably interacts with some membrane proteins, due to the similarity of the Arrhenius plots with those obtained for liposome-entrapped human serum albumin/hematoporphyrin complexes which were used as models to mimic hematoporphyrin-membrane protein binding sites. Liposomal hematoporphyrin and protoporphyrin bind to lipid domains. Hematoporphyrin accumulates in specific, localized lipid regions, perhaps in the boundary lipids area surrounding some inner-mitochondrial carriers; protoporphyrin accomodates in more rigid, lipid areas. On these bases, the higher photoactivity of hematoporphyrin, previously observed in mitochondria, in comparison with protoporphyrin, can be easily explained. Formation of linear dimers/aggregates, endowed with higher phi F than that of the monomers, are postulated to occur for both porphyrins only in the inner mitochondrial membrane.

Persistence of cytochrome c binding to membranes at physiological mitochondrial intermembrane space ionic strength

We have shown that cytochrome c (cyt c) diffuses primarily in three dimensions in the intermembrane space (IMS) of intact mitochondria at physiological ionic strength (I). Recently, we found that a small percentage (11.2 +/- 2.1%) of endogenous cyt c remains bound to inner mitochondrial membranes (IMM) at high, physiological I (I = 150 mM), even after extensive washing with solutions at physiological I, overnight dialysis, changes in medium osmolarity, or further purification of IMM at high I using self-generating Percoll gradients. Measurements of heme c/heme a ratios, and electron transport (ET) reactions in which cyt c participates, confirmed the presence of a low amount of this I-resistant, membrane-bound form of cyt c (MB-cyt c), that had one third of the ET activity of electrostatically-bound cyt c (EB-cyt c), and which could not account for maximal ET rates. The amount of MB-cyt c was significantly increased above endogenous MB-cyt c by exposing KCl-washed IMM to increasing concentrations of exogenous cyt c. Also, subjecting large unilamellar vesicles (LUV) to successive cycles of cyt c binding/high I KCl-washes gave progressive increases in MB-cyt c. These protocols allowed in vitro characterization of MB-cyt c. The I at which binding takes place affects the affinity of cyt c for membranes, and oxidized cyt c had a greater intrinsic affinity for IMM or SUV than reduced cyt c. MB-cyt c appears to be bound partially by hydrophobic interactions since MB-cyt c was detected on negatively charged (asolectin) LUV and also on neutral, zwitterionic (phosphatidylcholine) LUV at high I. Consistent with the concentration-dependent changes in MB-cyt c, decreasing the IMS-volume of intact mitochondria (i.e., increasing th endogenous IMS-cyt c concentration) by metabolic or osmotic means increased the amount of MB-cyt c. After cyt c was delivered into the IMS by liposome-mediated low pH-induced fusion, resonance energy transfer showed a time-dependent cyt c-membrane proximity which was consistent with slow exchange of soluble IMS-entrapped cyt c molecules with a population bound to membranes at I = 150 mM. We conclude that, even though the majority of functional IMS-cyt c diffuses in three dimensions, a small portion remains firmly bound on the surface of the IMM under I conditions that are physiological for intact mitochondria. The occurrence of MB-cyt c may reflect an intrinsic conformational flexibility in cyt c, that allows a degree of membrane penetration and the formation of hydrophobic interactions which stabilize the membrane-bound form. The persistence of cyt c-membrane interactions under physiological I conditions indicates that cyt c-mediated ET in the IMS involves both fast (3D-diffusion) and slow (2D-diffusion) pathways for electron transfer.

Photosensitization of mitochondria by liposome-bound porphyrins

We have compared the photodynamic activities of hematoporphyrin (HP) and protoporphyrin (PP) on isolated rat liver mitochondria by measuring the decline of the respiratory control ratio (RCR) after irradiation at 365 nm. Before addition to the respiratory medium, the dyes were dissolved in phosphate-buffered saline (PBS) or incorporated into unilamellar liposomes of dipalmitoyl-phosphatidylcholine (DPPC), sometimes enriched with cholesterol (Chol) or cardiolipin (Card), which are naturally present in mitochondrial membranes. Chol and especially Card strongly increase the porphyrin uptake by mitochondria. In all experimental conditions, PP is taken up by mitochondria to a higher extent than HP. Nevertheless, under conditions giving the same amount of mitochondria-bound dye, HP is a more efficient photosensitizer than PP. As the efficiency of singlet oxygen production has been shown to be equivalent for the two porphyrins in monomeric state, the resulting photobiological effects are explained in terms of different localization of HP and PP in the mitochondrial membranes. In particular, HP preferentially localizes in the protein-rich polar domains of the inner mitochondrial membrane, whereas PP dissolves in the lipid regions of the membranes.

The mobility of a fluorescent ubiquinone in model lipid membranes. Relevance to mitochondrial electron transport

The diffusion and location of a functional, fluorescent ubiquinone molecule, NBDHA-Q, were determined as a function of temperature using microscopic observation, fluorescence recovery after photobleaching and fluorescence spectroscopy in protein-free, pure-lipid dimyristoylphosphatidylcholine and dimyristoylphosphatidylcholine/cholesterol multibilayers. The data reveal that in a liquid-crystalline membrane (1) ubiquinone is highly mobile, (2) ubiquinone uniformly diffuses laterally with the same diffusion coefficient (3.10(-8) cm2/s at 25 degrees C) as the phospholipids in which it resides, (3) the diffusion coefficients of ubiquinone and phospholipid both decrease at the exothermic phase transition of the phospholipid, (4) cholesterol affects the diffusion coefficients of ubiquinone and phospholipids to the same degree, (5) cholesterol induces a lateral phase separation progressively excluding ubiquinone from cholesterol-containing domains. These data suggest that ubiquinone does not reside at the membrane surface or in the mid-plane for any appreciable length of time. Rather, the data indicate that ubiquinone is highly mobile laterally and transversely, spending the majority of its time in the acyl chain region of the membrane, where its lateral and transverse diffusion is limited by the lateral diffusion and the transverse microviscosity gradient of the phospholipids and where its lateral location can be affected by the presence of cholesterol. In addition, based upon a comparison of the diffusion coefficients for ubiquinone, phospholipids and mitochondrial redox complexes, we hypothesize that no significant portion of the ubiquinone pool remains bound to redox complexes for any significant length of time relative to that for electron transport as resolvable by fluorescence recovery after photobleaching.

Influence of tension reduction and peripheral dissection on histologic, biochemical and bioenergetic profiles, and kinetics of skeletal muscle fast-to-slow transformation

Seven goat latissimus dorsi muscles were submitted to a progressive electrostimulation program through intramuscular electrodes (Medtronic SP5528). Group 1 (n = 3) consisted of muscles stimulated in situ, and group 2 (n = 4), of muscles dissected distally and reinserted on the chest wall with a reduced tension. In group 1, complete fiber switch from type II to I occurred within 60-100 days after the beginning of stimulation, as demonstrated by myosin isoforms and lactate dehydrogenase (LDH) isozymes pattern. Respiratory chain oxidases first increased within 30-70 days after stimulation, then progressively decreased to stabilized values, higher than the basal ones. Total LDH activity showed progressive decrease to one-fifth of the initial value. Morphological analysis confirmed the structural integrity and physical reinforcement of the muscles. In group 2, respiratory chain oxidases showed initial increase followed first by a fast reduction to values less than the starting ones, and then by a slow secondary increase between day 40 and 90. LDH activity displayed a sharp decrease between day 15 and 36. Myosin as well as LDH isoforms showed progressive conversion. This kinetic study suggests a three-phase adaptative evolution of the goat latissimus dorsi submitted to increased workload (group 1): a fast increase (phase I) in oxidative capacities is followed by the development of an efficient contractile machinery (phase II), with subsequent adaptation (phase III) of the terminal chemosmotic enzymes involved in energy production.+2

Heart mitochondrial creatine kinase revisited: the outer mitochondrial membrane is not important for coupling of phosphocreatine production to oxidative phosphorylation

The state of mitochondrial creatine kinase (CKmi-mi) in intact dog heart mitochondria and mitoplasts and the mechanism of its functional coupling with the oxidative phosphorylation system have been reinvestigated under different osmotic conditions and ionic compositions of the medium. It has been established that in a medium which mimics the cardiac cell cytoplasma, dissociation of CKmi-mi from the membrane of mitoplasts increases when the mitoplasts are swollen due to hypoosmotic treatment. It was shown by EPR that hypoosmotic treatment results in the enhancement of the mobility of phospholipids in the membrane bilayer. It has been also shown that when CKmi-mi is detached from the inner membrane in intact mitochondria in isotonic KCl solution, the effects of the coupling between CKmi-mi and oxidative phosphorylation via ATP/ADP translocase disappear in spite of the presence of CKmi-mi in the intermembrane space and intactness of the outer mitochondrial membrane. Therefore, this coupling cannot be explained by the "compartmented coupling" mechanism or "dynamic adenine nucleotide compartmentation" in the intermembrane space due to diffusion limitation for adenine nucleotides through the outer mitochondrial membrane, as has been supposed by several authors (F.N. Gellerich et al. (1987) Biochim. Biophys. Acta 890, 117-126; S.P.J. Brooks and C.H. Suelter (1987) Arch. Biochem. Biophys. 253, 122-132). The data obtained show that the displacement of the enzyme from the membrane results in significantly increased sensitivity of the coupled processes of aerobic phosphocreatine synthesis to inhibition by the product, phosphocreatine. Thus, all results show that under physiological osmotic and ionic conditions CKmi-mi remains firmly attached to the inner mitochondrial membrane and effectively coupled with ATP/ADP translocase due to intimate dynamic interaction between those proteins.

Lipid-protein interactions. The mitochondrial complex III-phosphatidylcholine-water system

Bovine heart mitochondrial complex III (ubiquinol-cytochrome-c reductase) has been reconstituted into phosphatidylcholine bilayers and the effect of varying lipid/protein ratios on the structure and function of the protein has been examined. Electron microscopy, differential scanning calorimetry and Arrhenius plots of enzyme activity provide evidence that the protein is incorporated in an active conformation into pure phosphatidylcholine bilayers. At low lipid/protein ratios (e.g. 80:1 molar ratio) the protein exists in the form of aggregates. As the lipid proportion is increased, electron microscopy reveals the gradual formation of lipid bilayers; structures with the appearance of closed vesicles are seen at or above 300:1 phospholipid/protein molar ratios. Changes in enzyme activity as a function of lipid contents reveal a progressive increase in activity as more lipid is added, with a tendency to reach a saturation point. From the experimental data, a kinetic model is proposed, according to which the protein has an indefinite number of unspecific, independent and identical binding sites for phospholipids, the latter acting as essential enzyme activators. Varying lipid/protein ratios induce structural changes in complex III; visible spectra indicate changes in the polarity of the heme group environment, while Fourier-transform infrared spectroscopy suggests a change in the secondary structure of the protein as the lipid proportion is increased.

Electron microscopy and image analysis of the complexes I and V of the mitochondrial respiratory chain

The results of Section IV can be summarized in a simple ATP synthase model. This model implies that either the alpha or the beta subunits must be closer to the membrane. The work of Gao and Bauerlein (1987) indicates that the alpha subunits are closer to the membrane. Although the overall structure is more or less clear, important questions need to be clarified. First, the number and the arrangement of the subunits in the F0 part must be known. Second, the exact shape of F1, and particularly the shape of the large subunits needs to be elucidated. On the basis of fluorescence resonance energy transfer measurements by McCarty and Hammes (1987), a model was presented showing large oblong subunits. Such 'banana-shaped' subunits, which are also presented in the many phantasy models (e.g. Walker et al., 1982), are very unlikely in view of the electron microscopical results, although the large subunits do not need to be exactly spherical. The third and most interesting central question is on the changes in the structure that take place during the different steps in the synthesis of ATP. It can now be taken as proven that the energy transmitted to the ATP synthase is used to induce a conformational change in the latter enzyme, in such a way as to bring about the energy-requiring dissociation of already synthesized ATP (Penefsky, 1985 and reviewed in Slater, 1987). But the way in which the three parts of the ATP synthase are involved is completely unknown. It is rather puzzling that such a long distance exists between the catalytic sites, which are on the interface of the alpha and beta subunits and the F0 part where the proton movements occur, which, according to Mitchell's theory (1961), is the driving force for the synthesis of ATP. Perhaps alternative mechanisms such as the collision hypothesis formulated by Herweijer et al. (1985) are more realistic in describing the mechanism of ATP synthesis. It would bring the complexes I and V close together, not only in the artificial way treated in this paper, but in a useful way for energy conversion.

Properties of mitochondria from Penicillium cyclopium and their response to calcium and other divalent cations

The respiratory properties of isolated mitochondria from P. cyclopium were studied with particular attention to their response to calcium ions. The results obtained indicate concentration dependent stimulation of NADH oxidation by calcium ions. Similar effects could also be obtained with other divalent cations.

Structural and functional consequences of galactolipids on thylakoid membrane organization

Photosynthetic membranes of higher plant chloroplasts are composed primarily of polar, but uncharged, galactolipids unlike most mammalian membranes which contain large amounts of phosphatidylcholine. It is unclear what role(s) the galactolipids play in maintaining the differentiated thylakoid membranes, or in stabilizing the photosynthetically active enzyme complexes. Some of the membrane complexes show no lipid selectivity for maintaining structural or functional integrity. Others are poisoned or dissociated in the presence of high concentrations of a trace lipid class. The efficiency of energy transfer and the reconstitution of protein complexes into liposomes are dependent on the lipid class employed. The lipids are asymmetrically arranged along and across the thylakoid membranes but not as distinctly as the proteins.

Specific inhibition of ATP-ADP translocase in cardiac mitoplasts by antibodies against mitochondrial creatine kinase

Mitochondrial creatine kinase was purified from rat hearts and used to produce antibodies in chicken and rabbits. Antibodies were purified to a high degree of homogeneity by an affinity chromatography method. Chicken antibodies against mitochondrial creatine kinase inhibited this enzyme in rat-heart mitochondrial inner membrane and matrix preparation, and simultaneously blocked oxidative phosphorylation. Under these conditions respiratory chain activities remained unchanged, but adenine nucleotide translocase was inhibited. Removal of mitochondrial creatine kinase from the membrane by pretreatment with 0.15 M KCl and 20 mM ADP completely abolished the effect of antibodies against mitochondrial creatine kinase on oxidative phosphorylation. Noninhibitory antibodies from rabbit with high affinity to rat mitochondrial creatine kinase inhibited neither creatine kinase activity nor oxidative phosphorylation. These data show close and specific spatial arrangement of mitochondrial creatine kinase and adenine nucleotide translocase in mitochondria. It is supposed that there is a fixed orientation of these proteins in the cardiolipin domain in the membrane and that their interaction may occur by a frequent collision due to their lateral movement.

Is ubiquinone diffusion rate-limiting for electron transfer?

The different possible dispositions of the electron transfer components in electron transfer chains are discussed: random distribution of complexes and ubiquinone with diffusion-controlled collisions of ubiquinone with the complexes, random distribution as above, but with ubiquinone diffusion not rate-limiting, diffusion and collision of protein complexes carrying bound ubiquinone, and solid-state assembly. Discrimination among these possibilities requires knowledge of the mobility of the electron transfer chain components. The collisional frequency of ubiquinone-10 with the fluorescent probe 12-(9-anthroyl)stearate, investigated by fluorescence quenching, is 2.3 X 10(9) M-1 sec-1 corresponding to a diffusion coefficient in the range of 10(-6) cm2/sec (Fato, R., Battino, M., Degli Esposti, M., Parenti Castelli, G., and Lenaz, G., Biochemistry, 25, 3378-3390, 1986); the long-range diffusion of a short-chain polar Q derivative measured by fluorescence photobleaching recovery (FRAP) (Gupte, S., Wu, E. S., Höchli, L., Höchli, M., Jacobson, K., Sowers, A. E., and Hackenbrock, C. R., Proc. Natl. Acad. Sci. USA 81, 2606-2610, 1984) is 3 X 10(-9) cm2/sec. The discrepancy between these results is carefully scrutinized, and is mainly ascribed to the differences in diffusion ranges measured by the two techniques; it is proposed that short-range diffusion, measured by fluorescence quenching, is more meaningful for electron transfer than long-range diffusion measured by FRAP, or microcollisions, which are not sensed by either method. Calculation of the distances traveled by random walk of ubiquinone in the membrane allows a large excess of collisions per turnover of the respiratory chain. Moreover, the second-order rate constants of NADH-ubiquinone reductase and ubiquinol-cytochrome c reductase are at least three orders of magnitude lower than the second-order collisional constant calculated from the diffusion of ubiquinone. The activation energies of either the above activities or integrated electron transfer (NADH-cytochrome c reductase) are well above that for diffusion (found to be ca. 1 kcal/mol). Cholesterol incorporation in liposomes, increasing bilayer viscosity, lowers the diffusion coefficients of ubiquinone but not ubiquinol-cytochrome c reductase or succinate-cytochrome c reductase activities. The decrease of activity by ubiquinone dilution in the membrane is explained by its concentration falling below the Km of the partner enzymes. It is calculated that ubiquinone diffusion is not rate-limiting, favoring a random model of the respiratory chain organization.(ABSTRACT TRUNCATED AT 400 WORDS)