NADH/NAD+ interaction with NADH: ubiquinone oxidoreductase (complex I)

The quantitative data on the binding affinity of NADH, NAD(+), and their analogues for complex I as emerged from the steady-state kinetics data and from more direct studies under equilibrium conditions are summarized and discussed. The redox-dependency of the nucleotide binding and the reductant-induced change of FMN affinity to its tight non-covalent binding site indicate that binding (dissociation) of the substrate (product) may energetically contribute to the proton-translocating activity of complex I.

Redox-dependent change of nucleotide affinity to the active site of the mammalian complex I

A very potent and specific inhibitor of mitochondrial NADH:ubiquinone oxidoreductase (complex I), a derivative of NADH (NADH-OH) has recently been discovered (Kotlyar, A. B., Karliner, J. S., and Cecchini, G. (2005) FEBS Lett. 579, 4861-4866). Here we present a quantitative analysis of the interaction of NADH-OH and other nucleotides with oxidized and reduced complex I in tightly coupled submitochondrial particles. Both the rate of the NADH-OH binding and its affinity to complex I are strongly decreased in the presence of succinate. The effect of succinate is completely reversed by rotenone, antimycin A, and uncoupler. The relative affinity of ADP-ribose, a competitive inhibitor of NADH oxidation, is also shown to be significantly affected by enzyme reduction (KD of 30 and 500 microM for oxidized and the succinate-reduced enzyme, respectively). Binding of NADH-OH is shown to abolish the succinate-supported superoxide generation by complex I. Gradual inhibition of the rotenone-sensitive uncoupled NADH oxidase and the reverse electron transfer activities by NADH-OH yield the same final titration point (approximately 0.1 nmol/mg of protein). The titration of NADH oxidase appears as a straight line, whereas the titration of the reverse reaction appears as a convex curve. Possible models to explain the different titration patterns for the forward and reverse reactions are briefly discussed.

Synthesis and biological evaluation of the mitochondrial complex 1 inhibitor 2-[4-(4-fluorobutyl)benzylsulfanyl]-3-methylchromene-4-one as a potential cardiac positron emission tomography tracer

A series of fluorinated chromone analogs with IC50 values ranging from 9 to 133 nM for the mitochondrial complex 1 (MC-I) has been prepared. A structure-activity relationship (SAR) study of the most potent fluorinated chromone analog 10 demonstrated the linkage heteroatom preference of the side chain region of the molecule while maintaining potent MC-I inhibitory activity. Tissue distribution studies 30 min after [(18)F]10 administration to Sprague-Dawley (SD) rats demonstrated high uptake of the radiotracer from the blood pool into the myocardium (2.24% ID/g), kidney (1.93% ID/g), and liver (2.00% ID/g). After 2 h about 66% of the activity in the myocardium at 30 min had been retained, whereas approximately 70% had been cleared from the liver and kidney. MicroPET images of SD rats after [(18)F]10 administration allowed easy assessment of the myocardium through 60 min with minimal lung or liver interference.

Dehydromonocrotaline inhibits mitochondrial complex I. A potential mechanism accounting for hepatotoxicity of monocrotaline

Monocrotaline is a pyrrolizidine alkaloid present in plants of the Crotalaria species, which causes cytotoxicity and genotoxicity, including hepatotoxicity in animals and humans. It is metabolized by cytochrome P-450 in the liver to the alkylating agent dehydromonocrotaline. We evaluated the effects of monocrotaline and its metabolite on respiration, membrane potential and ATP levels in isolated rat liver mitochondria, and on respiratory chain complex I NADH oxidase activity in submitochondrial particles. Dehydromonocrotaline, but not the parent compound, showed a concentration-dependent inhibition of glutamate/malate-supported state 3 respiration (respiratory chain complex I), but did not affect succinate-supported respiration (complex II). Only dehydromonocrotaline dissipated mitochondrial membrane potential, depleted ATP, and inhibited complex I NADH oxidase activity (IC50=62.06 microM) through a non-competitive type of inhibition (K(I)=8.1 microM). Therefore, dehydromonocrotaline is an inhibitor of the activity of respiratory chain complex I NADH oxidase, an action potentially accounting for the well-documented monocrotaline's hepatotoxicity to animals and humans. The mechanism probably involves change of the complex I conformation resulting from modification of cysteine thiol groups by the metabolite.

Mitochondrial glycerol-3-P acyltransferase 1 is most active in outer mitochondrial membrane but not in mitochondrial associated vesicles (MAV)

Glycerol 3-phosphate acyltransferase-1 (GPAT1), catalyzes the committed step in phospholipid and triacylglycerol synthesis. Because both GPAT1 and carnitine-palmitoyltransferase 1 are located on the outer mitochondrial membrane (OMM) it has been suggested that their reciprocal regulation controls acyl-CoA metabolism at the OMM. To determine whether GPAT1, like carnitine-palmitoyltransferase 1, is enriched in both mitochondrial contact sites and OMM, and to correlate protein location and enzymatic function, we used Percoll and sucrose gradient fractionation of rat liver to obtain submitochondrial fractions. Most GPAT1 protein was present in a vesicular membrane fraction associated with mitochondria (MAV) but GPAT specific activity in this fraction was low. In contrast, highest GPAT1 specific activity was present in purified mitochondria. Contact sites from crude mitochondria, which contained markers for both endoplasmic reticulum (ER) and mitochondria, also showed high expression of GPAT1 protein but low specific activity, whereas contact sites isolated from purified mitochondria lacked ER markers and expressed highly active GPAT1. To determine how GPAT1 is targeted to mitochondria, recombinant protein was synthesized in vitro and its incorporation into crude and purified mitochondria was assayed. GPAT1 was rapidly incorporated into mitochondria, but not into microsomes. Incorporation was ATP-driven, and lack of GPAT1 removal by alkali and a chaotropic agent showed that GPAT1 had become an integral membrane protein after incorporation. These results demonstrate that two pools of GPAT1 are present in rat liver mitochondria: an active one, located in OMM and a less active one, located in membranes (ER-contact sites and mitochondrial associated vesicles) associated with both mitochondria and ER.

3D model of RNA polymerase and bidirectional transcription

In the in vitro mitochondrial (mt) transcription initiation system with mt RNA polymerase fraction and mt lysate, the transcription initiation products were shown to be synthesized bidirectionally from the only H-strand-promoter (HSP)/L-strand-promoter region (LSP) of the mitochondrial D-loop genome segment. These transcription products ranged between >100 and >800 bp with the purified mitochondrial RNA polymerase fraction, but were larger (>2030-4000 bp) in size with the mitochondrial lysate in both human and mouse. In this brief report, an in vitro reconstituted mitochondrial transcription system purified by affinity chromatography (heparin-Sepharose) from mouse hypotetraploid letter Ehrlich ascites tumor cell mitochondria was shown to initiate transcription bidirectionally from the mitochondrial D-loop region (HSP/LSP), as evidenced by in vitro generated transcription products. The in vitro generated transcription products were separated by sequencing gel. But this in vitro reconstituted transcription system was not studied beyond the D-loop region. A 3D model of the enzyme RNA polymerase was docked with both ATP and CTP.

[Hypoclorous acid modifies rat liver and heart enzymes of the pentose phosphate pathway and antioxidative defence in vitro]

Hypoclorous acid is an effective biological oxidant produced by activated neutrophils. HOCl plays a role of the major inflammation mediator in mammalian tissues. The aim of the present study was to investigate the mechanisms of hypochlorous acid-induced modification of antioxidant enzymes, which defence the cell under oxidative stress, and enzymes of the pentose phosphate pathway, which supply reducing equivalents in the cell. HOCl (100-1000 microM) in vitro inhibited considerably in a dose-dependent manner the activity of the enzymes of the pentose phosphate pathway in the rat liver postmitochondrial fraction. HOCI at a concentration of 100 nmol/mg protein inhibited transketolase activity by 65 +/- 5%, glucose-6-phosphate dehydrogenase--by 50 +/- 5% and 6-phosphogluconate dehydrogenase--by 55 +/- 5%. The activities of glutathione peroxidase and catalase slightly decreased. On the contrary, in the rat heart postmitochondrial fraction HOCl (100-1000 microM) inhibited considerably catalase, increased glutathione peroxidase activity and decreased significantly the activity of the key enzymes of the pentose phosphate pathway. The inhibition of the pentose phosphate pathway enzymes was accompanied by oxidation of intracellular reduced glutathione, oxidative protein modification (protein carbonyl group accumulation, mixed protein-glutathione disulphides and chloramine formation), and membrane lipid peroxidation. The sensitivity of rat heart cell components to oxidative damage by HOCl was higher in comparison with that of the liver.

Mode of Inhibitory Action of Δlac-Acetogenins, a New Class of Inhibitors of Bovine Heart Mitochondrial Complex I

We have revealed that Deltalac-acetogenins, a new class of inhibitors of bovine heart mitochondrial complex I (NADH-ubiquinone oxidoreductase), act differently from ordinary inhibitors such as rotenone and piericidin A [Ichimaru et al. (2005) Biochemistry 44, 816-825]. Since a detailed study of these unique inhibitors might provide new insight into the terminal electron transfer step of the enzyme, we further characterized their inhibitory action using the most potent Deltalac-acetogenin derivative (compound 1). Unlike ordinary complex I inhibitors, 1 had a dose-response curve for inhibition of the reduction of exogenous short-chain ubiquinones that was difficult to explain with a simple bimolecular association model. The inhibitory effect of 1 on ubiquinol-NAD(+) oxidoreductase activity (reverse electron transfer) was much weaker than that on NADH oxidase activity (forward electron transfer), indicating a direction-specific effect. These results suggest that the binding site of 1 is not identical to that of ubiquinone and the binding of 1 to the enzyme secondarily (or indirectly) disturbs the redox reaction of ubiquinone. Using endogenous and exogenous ubiquinone as an electron acceptor of complex I, we investigated the effect of 1 in combination with different ordinary inhibitors on the superoxide production from the enzyme. The results indicated that the level of superoxide production induced by 1 is significantly lower than that induced by ordinary inhibitors probably because of fewer electron leaks from the ubisemiquinone radical to molecular oxygen and that the site of inhibition by 1 is downstream of that by ordinary inhibitors. The unique inhibitory action of hydrophobic Deltalac-acetogenins may be closely associated with the dynamic function of the membrane domain of complex I.

Calpain 10: a mitochondrial calpain and its role in calcium-induced mitochondrial dysfunction

Calpains, Ca(2+)-activated cysteine proteases, are cytosolic enzymes implicated in numerous cellular functions and pathologies. We identified a mitochondrial Ca(2+)-inducible protease that hydrolyzed a calpain substrate (SLLVY-AMC) and was inhibited by active site-directed calpain inhibitors as calpain 10, an atypical calpain lacking domain IV. Immunoblot analysis and activity assays revealed calpain 10 in the mitochondrial outer membrane, intermembrane space, inner membrane, and matrix fractions. Mitochondrial staining was observed when COOH-terminal green fluorescent protein-tagged calpain 10 was overexpressed in NIH-3T3 cells and the mitochondrial targeting sequence was localized to the NH(2)-terminal 15 amino acids. Overexpression of mitochondrial calpain 10 resulted in mitochondrial swelling and autophagy that was blocked by the mitochondrial permeability transition (MPT) inhibitor cyclosporine A. With the use of isolated mitochondria, Ca(2+)-induced MPT was partially decreased by calpain inhibitors. More importantly, Ca(2+)-induced inhibition of Complex I of the electron transport chain was blocked by calpain inhibitors and two Complex I proteins were identified as targets of mitochondrial calpain 10, NDUFV2, and ND6. In conclusion, calpain 10 is the first reported mitochondrially targeted calpain and is a mediator of mitochondrial dysfunction through the cleavage of Complex I subunits and activation of MPT.

The peripheral stalk of the mitochondrial ATP synthase

The peripheral stalk of F-ATPases is an essential component of these enzymes. It extends from the membrane distal point of the F1 catalytic domain along the surface of the F1 domain with subunit a in the membrane domain. Then, it reaches down some 45 A to the membrane surface, and traverses the membrane, where it is associated with the a-subunit. Its role is to act as a stator to hold the catalytic alpha3beta3 subcomplex and the a-subunit static relative to the rotary element of the enzyme, which consists of the c-ring in the membrane and the attached central stalk. The central stalk extends up about 45 A from the membrane surface and then penetrates into the alpha3beta3 subcomplex along its central axis. The mitochondrial peripheral stalk is an assembly of single copies of the oligomycin sensitivity conferral protein (the OSCP) and subunits b, d and F6. In the F-ATPase in Escherichia coli, its composition is simpler, and it consists of a single copy of the delta-subunit with two copies of subunit b. In some bacteria and in chloroplasts, the two copies of subunit b are replaced by single copies of the related proteins b and b' (known as subunits I and II in chloroplasts). As summarized in this review, considerable progress has been made towards establishing the structure and biophysical properties of the peripheral stalk in both the mitochondrial and bacterial enzymes. However, key issues are unresolved, and so our understanding of the role of the peripheral stalk and the mechanism of synthesis of ATP are incomplete.

Subcomplexes of human ATP synthase mark mitochondrial biosynthesis disorders

OBJECTIVE

METHODS

We describe biochemically and clinically relevant aspects of mitochondrial ATP synthase, the enzyme that supplies most ATP for the cells energy demand.

RESULTS

Analyzing human Rho zero cells we could identify three subcomplexes of ATP synthase: F1 catalytic domain, F1 domain with bound natural IF1 inhibitor protein, and F1-c subcomplex, an assembly of F1 domain and a ring of F(O)-subunits c. Large amounts of F1 subcomplexes accumulated also in mitochondria of patients with specific mitochondrial disorders. By quantifying the F1 subcomplexes and other oxidative phosphorylation complexes in parallel, we were able to discriminate three classes of defects in mitochondrial biosynthesis, namely, mitochondrial DNA depletion, mitochondrial transfer RNA (tRNA) mutations, and mutations in the mitochondrial ATP6 gene.

INTERPRETATION

The relatively simple electrophoretic assay used here is a straightforward approach to differentiate between various types of genetic alterations affecting the biosynthesis of oxidative phosphorylation complexes and will be useful to guide molecular genetic diagnostics in the field of mitochondrial neuromuscular disorders.

Submitochondrial localization of the mitochondrial isoform of folylpolyglutamate synthetase in CCRF-CEM human T-lymphoblastic leukemia cells

Earlier studies from this laboratory showed that human folylpolyglutamate synthetase (FPGS) exists as cytosolic and mitochondrial (mFPGS) isoforms. Localization of mFPGS within mitochondria may help elucidate how the enzyme functions to maintain the mitochondrial folate pool. A human T-lymphoblastic leukemia CCRF-CEM cell lysate was fractionated by differential centrifugation into cytosolic and mitochondrial fractions. Activity assays for cytosol-and mitochondria-specific enzymes verified the purity and integrity of the fractions. Mitochondria were subfractionated with increasing concentrations of digitonin to successively extract the four submitochondrial compartments. Western analyses of the fractions using protein markers specific for each compartment suggest that mFPGS is distributed in the matrix and/or inner membrane compartments. Further support for an interaction of mFPGS with the inner mitochondrial membrane is provided by localization of about half of the mFPGS in the mitochondrial membrane fraction obtained by freeze-thaw of intact mitochondria; the remaining mFPGS is located in the soluble fraction. Resistance of about half of the mFPGS in whole mitochondria to alkaline carbonate extraction suggests that its interaction with the inner membrane is more similar to an integral, than a peripheral, membrane protein. The data suggest that human mFPGS is at least in part strongly associated with the inner mitochondrial membrane.

Transglutaminase activity is present in highly purified nonsynaptosomal mouse brain and liver mitochondria

Several active transglutaminase (TGase) isoforms are known to be present in human and rodent tissues, at least three of which, namely, TGase 1, TGase 2 (tissue transglutaminase), and TGase 3, are present in the brain. TGase activity is known to be present in the cytosolic, nuclear, and extracellular compartments of the brain. Here, we show that highly purified mouse brain nonsynaptosomal mitochondria and mouse liver mitochondria and mitoplast fractions derived from these preparations possess TGase activity. Western blotting and experiments with TGase 2 knock-out (KO) mice ruled out the possibility that most of the mitochondrial/mitoplast TGase activity is due to TGase 2, the TGase isoform responsible for the majority of the activity ([14C]putrescine-binding assay) in whole brain and liver homogenates. The identity of the mitochondrial/mitoplast TGase(s) is not yet known. Possibly, the activity may be due to one of the other TGase isoforms or perhaps to a protein that does not belong to the classical TGase family. This activity may play a role in regulation of mitochondrial function both in normal physiology and in disease. Its nature and regulation deserve further study.

Effect of denaturants on multisite and unisite ATP hydrolysis by bovine heart submitochondrial particles with and without inhibitor protein

The effect of guanidinium hydrochloride (GdnHCl) on multisite and unisite ATPase activity by F0F1 of submitochondrial particles from bovine hearts was studied. In particles without control by the inhibitor protein, 50 mM GdnHCl inhibited multisite hydrolysis by about 85%; full inhibition required around 500 mM. In the range of 500-650 mM, GdnHCl enhanced the rate of unisite catalysis by promoting product release; it also increased the rate of hydrolysis of ATP bound to the catalytic site without GdnHCl. GdnHCl diminished the affinity of the enzyme for aurovertin. The effects of GdnHCl were irreversible. The results suggest that disruption of intersubunit contacts in F0F1 abolishes multisite hydrolysis and stimulates of unisite hydrolysis. Particles under control by the inhibitor protein were insensitive to concentrations of GdnHCl that induce the aforementioned alterations of F0F1 free of inhibitor protein, indicating that the protein stabilizes the global structure of particulate F1.

Reactivation of the alternative oxidase of Yarrowia lipolytica by nucleoside monophosphates

The study of the effect of nucleoside phosphates on the activity of cyanide-resistant oxidase in the mitochondria and submitochondrial particles of Yarrowia lipolytica showed that adenosine monophosphate (5'-AMP, AMP) did not stimulate the respiration of intact mitochondria. The incubation of mitochondria at room temperature (25 degrees C) for 3-5 h or their treatment with ultrasound, phospholipase A, and the detergent Triton X-100 at a low temperature inactivated the cyanide-resistant alternative oxidase. The inactivated alternative oxidase could be reactivated with AMP. The reactivating effect of AMP was enhanced by azolectin. Some other nucleoside phosphates also showed reactivating ability in the following descending order: AMP = GMP > GDP > GTP > MP > IMP. The apparent K(m) values for AMP in reactivation of the alternative oxidase of submitochondrial particles or mitochondria treated with Triton X-100 and incubated at 25 degrees C were calculated. Physiological aspects of activation of the alternative oxidase are discussed in connection with the impairment of electron transfer through the cytochrome pathway.

Heart mitochondrial nitric oxide synthase is upregulated in male rats exposed to high altitude (4,340 m)

Male rats exposed for 21 days to high altitude (4,340 m) responded with arrest of weight gain and increased hematocrit and testosterone levels. High altitude significantly (58%) increased heart mitochondrial nitric oxide (NO) synthase (mtNOS) activity, whereas heart cytosolic endothelial NOS (eNOS) and liver mtNOS were not affected. Western blot analysis found heart mitochondria reacting only with anti-inducible NOS (iNOS) antibodies, whereas the postmitochondrial fraction reacted with anti-iNOS and anti-eNOS antibodies. In vitro-measured NOS activities allowed the estimation of cardiomyocyte capacity for NO production, a value that increased from 57% (sea level) to 79 nmol NO.min(-1).g heart(-1) (4,340 m). The contribution of mtNOS to total cell NO production increased from 62% (sea level) to 71% (4340 m). Heart mtNOS activity showed a linear relationship with hematocrit and a biphasic quadratic association with estradiol and testosterone. Multivariate analysis showed that exposure to high altitude linearly associates with hematocrit and heart mtNOS activity, and that testosterone-to-estradiol ratio and heart weight were not linearly associated with mtNOS activity. We conclude that high altitude triggers a physiological adaptive response that upregulates heart mtNOS activity and is associated in an opposed manner with the serum levels of testosterone and estradiol.

[Functional disorders of FOF1-ATPase in submitochondrial particles obtained from platelets of patients with a diagnosis of probable Alzheimer's disease]

INTRODUCTION

Recent studies indicate that decreased energy generation by mitochondria is a feature that is common across neurodegenerative diseases.

PATIENTS AND METHODS

In order to obtain direct evidence that mitochondrial functioning is altered, we measured the hydrolytic activity of F0F1-ATPase and its capacity to generate a stable proton gradient in submitochondrial particles in 29 patients diagnosed with probable Alzheimer's disease (AD). Submitochondrial particles were obtained from platelets of patients with a diagnosis of probable AD and from clinically healthy controls.

RESULTS

Data revealed that the hydrolytic activity of F0F1-ATPase increases significantly in patients with probable AD (41.7+/-4.3 nmol PO4 min-1[mg protein]-1, n=29) as compared to the control subjects (29.1+/-1.9 nmol PO4 min-1 [mg protein]-1, n=29). It is important to note that, in the male population with probable AD, we found that hydrolytic activity of F0F1-ATPase increased as cerebral deterioration progressed. We also detected a lower pH gradient in the submitochondrial particles of patients with probable AD (0.28+/-0.08 pH units, n=25) as compared to the controls (0.5+/-0.1 pH units, n=20).

CONCLUSIONS

Overall, these data point to an alteration in the functioning of the enzyme.

Complex I regulates mutant mitochondrial aldehyde dehydrogenase activity and voluntary ethanol consumption in rats

Animals selectively bred for a desirable trait retain wanted genes but exclude genes that may counteract the expression of the former. The possible interactions between selected and excluded genes cannot be readily studied in transgenic or knockout animals but may be addressed by crossing animals bred for opposite traits and studying the F2 offspring. Ninety-seven percent of Wistar-derived rats selectively bred for their voluntary low-alcohol consumption display a mutated nuclear allele of aldehyde dehydrogenase Aldh22 that encodes an enzyme with a low affinity for NAD+, whereas rats bred for high-alcohol consumption do not present the Aldh22 allele. This enzyme is inserted into mitochondria, where NADH-ubiquinone oxidoreductase (complex I) regenerates NAD+. The possible influence of complex I on ALDH2 activity and voluntary ethanol intake was investigated. Homozygous Aldh22/Aldh22 rats derived from a line of high-drinker F0 females (and low-drinker F0 males) showed a markedly higher ethanol consumption (3.9=/-0.5 g x kg(-1) x day(-1)) than homozygous animals derived from a line of low-drinker F0 females (and high-drinker F0 males) (1.8+/-0.4 g x kg(-1) x day(-1)). Mitochondria of F2 rats derived from high alcohol-consuming females were more active in oxidizing substrates that generate NADH for complex I than were mitochondria derived from low alcohol-consuming females, leading in the former to higher rates of acetaldehyde metabolism and to a reduced aversion to ethanol. This is the first demonstration that maternally derived genes can either allow or counteract the phenotypic expression of a mutated gene in the context of alcohol abuse or alcoholism

Guanidine-induced dissociation of mitochondrial F1-ATPase

The effect of guanidine hydrochloride on ATPase activity, gel filtration, turbidity, and the fluorescence emission intensity of mitochondrial F1-ATPase was examined. Purified F1 from bovine heart mitochondria was slowly inactivated at low denaturant concentration, and inactivation was associated with delta and epsilon subunit dissociation. delta and epsilon subunits were bound together to form a stable and soluble heterodimer. In parallel, appearance of turbidity was observed. This was caused by the formation of alpha3beta3gamma non-covalent aggregates, as analyzed by SDS-PAGE. Short periods of exposition of the F1 complex to high concentrations of guanidine hydrochloride (0.8-3 M) again induced deltaepsilon dissociation as a heterodimer and the formation of an inactive alpha3beta3gamma subcomplex. This eventually dissociated progressively into single subunits caused by partial unfolding, as evidenced through changes of the protein intrinsic fluorescence emission. Our results suggest that the delta and epsilon subunits are loosely bound to alpha3beta3gamma , and play an important role in determining structural stability to isolated mitochondrial F1-ATPase.

Synthesis and mitochondrial complex I inhibition of dihydroxy-cohibin A, non-THF annonaceous acetogenin analogue

To elucidate the inhibitory action of acetogenins, we synthesized an acetogenin derivative which possesses tetraol in place of the tetrahydrofuran ring and examined its inhibitory activity against bovine heart mitochondrial complex I. Our results indicate that these hydroxy groups are an essential structural factor though it is not effective as bis-THF hydroxy groups combination.

Proton-translocating ATP-synthase of Paracoccus denitrificans: ATP-hydrolytic activity

Tightly coupled membranes of Paracoccus denitrificans catalyze oxidative phosphorylation but are incapable of ATP hydrolysis. The conditions for observation and registration of the venturicidin-sensitive ATPase activity of subbacterial particles derived from this organism are described. The ATP hydrolytic activity does not appear after prolonged incubation in the presence of pyruvate kinase and phosphoenol pyruvate (to remove ADP), EDTA (to remove Mg2+) and/or inorganic phosphate, whereas the activity dramatically increases after energization of the membranes. ATP hydrolysis by -activated ATPase is coupled with electric potential formation. Inorganic phosphate prevents and azide promotes a decline of the enzyme activity during ATP hydrolysis. The addition of uncouplers results in rapid and complete inactivation of ATPase. The -dependent ATPase activity increases upon dilution of the membranes. The results are discussed as evidence for the presence of distinct ATP-synthase and ATP-hydrolase states of F(o)F(1) complex in the coupling membranes (Vinogradov, A. D. (1999) Biochemistry (Moscow), 64, 1219-1229). The proposal is made that part of the free energy released from oxidoreduction in the respiratory chain is used to maintain active conformation of the energy-transducing proteins.

Functional transitions of F0F1-ATPase mediated by the inhibitory peptide IF1 in yeast coupled submitochondrial particles

The mechanism of inhibition of yeast F(0)F(1)-ATPase by its naturally occurring protein inhibitor (IF1) was investigated in submitochondrial particles by studying the IF1-mediated ATPase inhibition in the presence and absence of a protonmotive force. In the presence of protonmotive force, IF1 added during net NTP hydrolysis almost completely inhibited NTPase activity. At moderate IF1 concentration, subsequent uncoupler addition unexpectedly caused a burst of NTP hydrolysis. We propose that the protonmotive force induces the conversion of IF1-inhibited F(0)F(1)-ATPase into a new form having a lower affinity for IF1. This form remains inactive for ATP hydrolysis after IF1 release. Uncoupling simultaneously releases ATP hydrolysis and converts the latent form of IF1-free F(0)F(1)-ATPase back to the active form. The relationship between the different steps of the catalytic cycle, the mechanism of inhibition by IF1 and the interconversion process is discussed.

Oxidatively damaged proteins of heart mitochondrial electron transport complexes

Protein modifications, such as carbonylation, nitration and formation of lipid peroxidation adducts, e.g. 4-hydroxynonenal (HNE), are products of oxidative damage attributed to reactive oxygen species (ROS). The mitochondrial respiratory chain Complexes I and III have been shown to be a major source of ROS in vitro. Additionally, modifications of the respiratory chain Complexes (I-V) by nitration, carbonylation and HNE adduct decrease their enzymatic activity in vitro. However, modification of these respiratory chain complex proteins due to in vivo basal level ROS generation has not been investigated. In this study, we show a basal level of oxidative damage to specific proteins of adult bovine heart submitochondrial particle (SMP) complexes, and find that most of these proteins are localized in the mitochondrial matrix. We postulate that electron leakage from respiratory chain complexes and subsequent ROS formation may cause damage to specific complex subunits and contribute to long-term accumulation of mitochondrial dysfunction.

Synthesis and Inhibitory Action of Novel Acetogenin Mimics with Bovine Heart Mitochondrial Complex I†

Studies on the inhibition mechanism of acetogenins, the most potent inhibitors of complex I, are useful to elucidate the structural and functional features of the terminal electron-transfer step of this enzyme. We synthesized acetogenin mimics that possess two alkyl tails without a gamma-lactone ring, named Deltalac-acetogenin, and examined their inhibitory action on bovine heart mitochondrial complex I. Unexpectedly, the Deltalac-acetogenin carrying two n-undecanyl groups (compound 3) elicited very potent inhibition comparable to that of bullatacin. The inhibitory potency of compound 3 markedly decreased with shortening the length of either or both alkyl tails, indicating that symmetric as well as hydrophobic properties of the inhibitor are important for the inhibition. Both acetylation and deoxygenation of either or both of two OH groups adjacent to the tetrahydrofuran (THF) rings resulted in a significant decrease in inhibitory potency. These structural dependencies of the inhibitory action of Deltalac-acetogenins are in marked contrast to those of ordinary acetogenins. Double-inhibitor titration of steady-state complex I activity showed that inhibition of compound 3 and bullatacin are not additive, though the inhibition site of both inhibitors is downstream of iron-sulfur cluster N2. Our results indicate that the mode of inhibitory action of Deltalac-acetogenins differs from that of ordinary acetogenins. Therefore, Deltalac-acetogenins can be regarded as a novel type of inhibitor acting on the terminal electron-transfer step of complex I.

Localization of an alkyl-acetyl-glycerol-CDP-choline: cholinephosphotransferase activity in submitochondrial fractions of Tetrahymena pyriformis

Tetrahymena pyriformis contains platelet-activating factor (PAF) as a minor lipid, which is biosynthesized de novo. A dithiothreitol-insensitive CDP-choline:cholinephosphotransferase (AAG-CPT), which utilizes alkyl-acetyl-glycerol as a substrate, had been detected in both the mitochondrial and microsomal fractions of the protozoan. In the present report, localization of this enzyme in submitochondrial fractions was studied. Cell fractionation was evaluated with enzyme and morphological markers. In this respect, succinate dehydrogenase, NADPH:cytochrome c reductase, glucose-6-phosphatase, alkaline phosphatase, monoaminoxidase, and cytochrome c oxidase activities were investigated. In the presence of antimycin A, mitochondrial activity of NADPH-cytochrome c reductase, was increased, while the microsomal one was reduced. Cardiolipin was distributed in the inner mitochondrial membrane. Alkaline phosphatase was found exclusively in the cytosol of the protozoan. The main portion of the dithiothreitol-insensitive AAG-CPT was localized in the inner mitochondrial membrane. Our data indicate that mitochondria are able to produce PAF, which might be associated with their function.

Investigation of the role and mechanism of IF1 and STF1 proteins, twin inhibitory peptides which interact with the yeast mitochondrial ATP synthase

Inhibition of the yeast F(0)F(1)-ATP synthase by the regulatory peptides IF1 and STF1 was studied using intact mitochondria and submitochondrial particles from wild-type cells or from mutants lacking one or both peptides. In intact mitochondria, endogenous IF1 only inhibited uncoupled ATP hydrolysis and endogenous STF1 had no effect. Addition of alamethicin to mitochondria readily made the mitochondrial membranes permeable to nucleotides, and bypassed the kinetic control exerted on ATP hydrolysis by the substrate carriers. In addition, alamethicin made the regulatory peptides able to cross mitochondrial membranes. At pH 7.3, F(0)F(1)-ATPase, initially inactivated by either endogenous IF1 or endogenous STF1, was completely reactivated hours or minutes after alamethicin addition, respectively. Previous application of a membrane potential favored the release of endogenous IF1 and STF1. These observations showed that IF1 and STF1 can fully inhibit ATP hydrolysis at physiological concentrations and are sensitive to the same effectors. However, ATP synthase has a much lower affinity for STF1 than for IF1, as demonstrated by kinetic studies of ATPase inhibition in submitochondrial particles by externally added IF1 and STF1 at pHs ranging from 5.5 to 8.0. Our data do not support previously proposed effects of STF1, like the stabilization of the IF1-F(0)F(1) complex or the replacement of IF1 on its binding site in the presence of the proton-motive force or at high pH, and raise the question of the conditions under which STF1 could regulate ATPase activity in vivo.

The Ala16Val genetic dimorphism modulates the import of human manganese superoxide dismutase into rat liver mitochondria

A genetic dimorphism encodes for either alanine (Ala) or valine (Val) in the mitochondrial targeting sequence (MTS) of human manganese superoxide dismutase (MnSOD) and has been reported to modulate the risk of some cancers, neurodegenerative diseases and severe alcoholic liver disease. Although functional consequences of this dimorphism on MnSOD activity have not been assessed, computer models predict a partial alpha-helix structure for the Ala-MnSOD/MTS, but a beta-sheet structure for the Val-variant, which could hamper mitochondrial import. To investigate this hypothesis, we studied the in-vitro import of chimaeric proteins composed of either one of the MnSOD/MTS fused to the mouse dihydrofolate reductase (DHFR) protein, and the import of the two human MnSOD precursor variants into rat liver mitochondria. Compared to Ala-proteins, the Val-MnSOD/MTS-DHFR precursor and Val-MnSOD precursor were both partly arrested within the inner mitochondrial membrane. The Ala-MnSOD precursor generated 30-40% more of the active, matricial, processed MnSOD homotetramer than the Val-MnSOD precursor. These results show that the Ala-MnSOD/MTS allows efficient MnSOD import into the mitochondrial matrix, while the Val-variant causes partial arrest of the precursor within the inner membrane and decreased formation of the active MnSOD tetramer in the mitochondrial matrix.

Structural and functional organization of Complex I in the mitochondrial respiratory chain

Metabolic flux control analysis of NADH oxidation in bovine heart submitochondrial particles revealed high flux control coefficients for both Complex I and Complex III, suggesting that the two enzymes are functionally associated as a single enzyme, with channelling of the common substrate, Coenzyme Q. This is in contrast with the more accepted view of a mobile diffusable Coenzyme Q pool between these enzymes. Dilution with phospholipids of a mitochondrial fraction enriched in Complexes I and III, with consequent increased theoretical distance between complexes, determines adherence to pool behavior for Coenzyme Q, but only at dilution higher than 1:5 (protein:phospholipids), whereas, at lower phospholipid content, the turnover of NADH cytochrome c reductase is higher than expected by the pool equation.

Mammalian mitochondrial endonuclease G. Digestion of R-loops and localization in intermembrane space

Mammalian mitochondria contain strong nuclease activity. Endonuclease G (endoG), which predominantly resides in mitochondria, accounts for a large part of this nuclease activity. It has been proposed to act as an RNase H-like nuclease on RNA.DNA hybrids (R-loops) in the D-loop region where the origins of mitochondrial replication are mapped, providing RNA primers for mtDNA replication. However, in contrast with this proposed activity, endoG has recently been shown to translocate to nuclei on apoptotic stimulation and act as a nuclease without sequence specificity. To clarify the role of endoG in mtDNA replication, we examined its submitochondrial localization and its ability to cleave R-loops. At low concentration, it preferentially produces double-stranded breaks in R-loops, but does not act as an RNase H-like nuclease. In addition, it exists in the mitochondrial intermembrane space, but not in the matrix where mtDNA replication occurs. These results do not support the involvement of endoG in mtDNA replication. Based on the fact that guanine tracts, which are preferential targets of endoG, tend to form triplex structures and that endoG produces double-stranded breaks in R-loops, we propose that three-stranded DNA may be the preferred substrate of endoG.

Cross-linking of the endogenous inhibitor protein (IF1) with rotor (gamma, epsilon) and stator (alpha) subunits of the mitochondrial ATP synthase

The location of the endogenous inhibitor protein (IF1) in the rotor/stator architecture of the bovine mitochondrial ATP synthase was studied by reversible cross-linking with dithiobis(succinimidylpropionate) in soluble F1I and intact F1F0I complexes of submitochondrial particles. Reducing two-dimensional electrophoresis, Western blotting, and fluorescent cysteine labeling showed formation of alpha-IF1, IF1-IF1, gamma-IF1, and epsilon-IF1 cross-linkages in soluble F1I and in native F1F0I complexes. Cross-linking blocked the release of IF1 from its inhibitory site and therefore the activation of F1I and F1F0I complexes in a dithiothreitol-sensitive process. These results show that the endogenous IF1 is at a distance < or = 12 angstroms to gamma and epsilon subunits of the central rotor of the native mitochondrial ATP synthase. This finding strongly suggests that, without excluding the classical assumption that IF1 inhibits conformational changes of the catalytic beta subunits, the inhibitory mechanism of IF1 may involve the interference with rotation of the central stalk.

The modulation of mitochondrial nitric-oxide synthase activity in rat brain development

Different mitochondrial nitric-oxide synthase (mtNOS) isoforms have been described in rat and mouse tissues, such as liver, thymus, skeletal muscle, and more recently, heart and brain. The modulation of these variants by thyroid status, hypoxia, or gene deficiency opens a broad spectrum of mtNOS-dependent tissue-specific functions. In this study, a new NOS variant is described in rat brain with an M(r) of 144 kDa and mainly localized in the inner mitochondrial membrane. During rat brain maturation, the expression and activity of mtNOS were maximal at the late embryonic stages and early postnatal days followed by a decreased expression in the adult stage (100 +/- 9 versus 19 +/- 2 pmol of [(3)H]citrulline/min/mg of protein, respectively). This temporal pattern was opposite to that of the cytosolic 157-kDa nNOS protein. Mitochondrial redox changes followed the variations in mtNOS activity: mtNOS-dependent production of hydrogen peroxide was maximal in newborns and decreased markedly in the adult stage, thus reflecting the production and utilization of mitochondrial matrix nitric oxide. Moreover, the activity of brain Mn-superoxide dismutase followed a developmental pattern similar to that of mtNOS. Cerebellar granular cells isolated from newborn rats and with high mtNOS activity exhibited maximal proliferation rates, which were decreased by modifying the levels of either hydrogen peroxide or nitric oxide. Altogether, these findings support the notion that a coordinated modulation of mtNOS and Mn-superoxide dismutase contributes to establish the rat brain redox status and participate in the normal physiology of brain development.

Factor B is essential for ATP synthesis by mitochondria

Factor B is a water-soluble protein, which is required for the coupled activity of the mitochondrial ATP synthase complex. Specific removal of factor B from well-coupled bovine heart submitochondrial particles (SMP) results in uncoupling and the loss of ATP-driven membrane potential formation and reverse electron transfer from succinate to NAD. Addition of recombinant human factor B (molecular mass 20,341 Da) to factor B-depleted SMP (AE-SMP) restores these properties [G.I. Belogrudov, and Y. Hatefi, (2002) J. Biol. Chem. 277, 6097-6103]. This paper shows that extraction and purification of ATP synthase complex (complex V) from bovine heart mitochondria results in extensive loss of factor B. Addition of recombinant human factor B to AE-SMP completely restores the lost oxidative phosphorylation and ATP-32P(i) exchange activities of the particles and increases the ATP-32P(i) exchange activity of complex V by 2.5-fold. These results further indicate that factor B is an essential component of the mammalian ATP synthase complex.

The human silent information regulator (Sir)2 homologue hSIRT3 is a mitochondrial nicotinamide adenine dinucleotide-dependent deacetylase

The yeast silent information regulator (Sir)2 protein links cellular metabolism and transcriptional silencing through its nicotinamide adenine dinucleotide (NAD)-dependent histone deacetylase activity. We report that mitochondria from mammalian cells contain intrinsic NAD-dependent deacetylase activity. This activity is inhibited by the NAD hydrolysis product nicotinamide, but not by trichostatin A, consistent with a class III deacetylase. We identify this deacetylase as the nuclear-encoded human Sir2 homologue hSIRT3, and show that hSIRT3 is located within the mitochondrial matrix. Mitochondrial import of hSIRT3 is dependent on an NH2-terminal amphipathic alpha-helix rich in basic residues. hSIRT3 is proteolytically processed in the mitochondrial matrix to a 28-kD product. This processing can be reconstituted in vitro with recombinant mitochondrial matrix processing peptidase (MPP) and is inhibited by mutation of arginines 99 and 100. The unprocessed form of hSIRT3 is enzymatically inactive and becomes fully activated in vitro after cleavage by MPP. These observations demonstrate the existence of a latent class III deacetylase that becomes catalytically activated upon import into the human mitochondria.

Reactive oxygen species affect mitochondrial electron transport complex I activity through oxidative cardiolipin damage

The aim of this study was to investigate the influence of reactive oxygen species (ROS) on the activity of complex I and on the cardiolipin content in bovine heart submitochondrial particles (SMP). ROS were generated through the use of xanthine/xanthine oxidase (X/XO) system. Treatment of SMP with X/XO resulted in a large production of superoxide anion, detected by acetylated cytochrome c method, which was blocked by superoxide dismutase (SOD). Exposure of SMP to ROS generation resulted in a marked loss of complex I activity and to parallel loss of mitochondrial cardiolipin content. Both these effects were completely abolished by SOD+catalase. Exogenous added cardiolipin was able to almost completely restore the ROS-induced loss of complex I activity. No restoration was obtained with other major phospholipid components of the mitochondrial membrane such as phosphatidylcholine and phosphatidylethanolamine, nor with peroxidized cardiolipin. These results demonstrate that ROS affect the mitochondrial complex I activity via oxidative damage of cardiolipin which is required for the functioning of this multisubunit enzyme complex. These results may prove useful in probing molecular mechanisms of ROS-induced peroxidative damage to mitochondria, which have been proposed to contribute to those pathophysiological conditions characterized by an increase in the basal production of reactive oxygen species such as aging, ischemia/reperfusion and chronic degenerative diseases.

Inhibition of mitochondrial calcium-independent phospholipase A2 (iPLA2) attenuates mitochondrial phospholipid loss and is cardioprotective

Calcium-independent phospholipase A(2) (iPLA(2)) is the predominant phospholipase A(2) present in myocardium, and its pathophysiological role in acute myocardial infarction has been suggested by the rapid increase in membrane-associated iPLA(2) activity during myocardial ischaemia and reperfusion (I/R). We therefore examined iPLA(2) in mitochondrial fractions prepared from Langendorff-perfused adult rabbit hearts. Our studies indicate that iPLA(2)beta is present in rabbit heart mitochondrial inner membranes with no apparent translocation during ischaemia, I/R or preconditioning. Mitochondrion-associated iPLA(2) was catalytically competent and exhibited 2-, 3- and 2.5-fold increases in measured iPLA(2) activity following ischaemia, I/R and preconditioning, respectively, when compared with the activity of iPLA(2) measured in mitochondria from control hearts. Mitochondrial phospholipids are essential for maintaining the ordered structure and function of the organelle. I/R resulted in a rapid overall decrease in phosphatidylcholine and phosphatidylethanolamine glycerophospholipid species, as determined by electrospray ionization MS, that was partially alleviated by pretreatment of hearts with the iPLA(2)-specific inhibitor, bromoenol lactone (BEL). Pretreatment of I/R hearts with 10 microM BEL significantly reduced the infarct size almost to that of continuously perfused hearts and was cardioprotective only when administered prior to ischaemia. Cardioprotection by BEL was reversed by the simultaneous perfusion of 100 microM 5-hydroxydecanoate, implicating the mitochondrial K(ATP) channel in BEL-mediated protection from I/R. Preconditioning also significantly reduced the infarct size in response to I/R but protection was lost by concurrent perfusion of 10 microM arachidonic acid. Taken together, these data strongly implicate mitochondria-associated iPLA(2) in the signal transduction of myocardial I/R injury.

Regulation of mitochondrial carbamoyl-phosphate synthetase 1 activity by active site fatty acylation

In addition to its role in reversible membrane localization of signal-transducing proteins, protein fatty acylation could play a role in the regulation of mitochondrial metabolism. Previous studies have shown that several acylated proteins exist in mitochondria isolated from COS-7 cells and rat liver. Here, a prominent fatty-acylated 165-kDa protein from rat liver mitochondria was identified as carbamoyl-phosphate synthetase 1 (CPS 1). Covalently attached palmitate was linked to CPS 1 via a thioester bond resulting in an inhibition of CPS 1 activity at physiological concentrations of palmitoyl-CoA. This inhibition corresponds to irreversible inactivation of CPS 1 and occurred in a time- and concentration-dependent manner. Fatty acylation of CPS 1 was prevented by preincubation with N-ethylmaleimide and 5'-p-fluorosulfonylbenzoyladenosine, an ATP analog that reacts with CPS 1 active site cysteine residues. Our results suggest that fatty acylation of CPS 1 is specific for long-chain fatty acyl-CoA and very likely occurs on at least one of the essential cysteine residues inhibiting the catalytic activity of CPS 1. Inhibition of CPS 1 by long-chain fatty acyl-CoAs could reduce amino acid degradation and urea secretion, thereby contributing to nitrogen sparing during starvation.

The site of production of superoxide radical in mitochondrial Complex I is not a bound ubisemiquinone but presumably iron-sulfur cluster N2

The mitochondrial respiratory chain is a powerful source of reactive oxygen species, considered as the pathogenic agent of many diseases and of aging. We have investigated the role of Complex I in superoxide radical production in bovine heart submitochondrial particles and found, by combined use of specific inhibitors of Complex I and by Coenzyme Q (CoQ) extraction from the particles, that the one-electron donor in the Complex to oxygen is a redox center located prior to the binding sites of three different types of CoQ antagonists, to be identified with a Fe-S cluster, most probably N2 on the basis of several known properties of this cluster. Short chain CoQ analogs enhance superoxide formation, presumably by mediating electron transfer from N2 to oxygen. The clinically used CoQ analog, idebenone, is particularly effective in promoting superoxide formation.

Distribution of protein disulphide isomerase in rat liver mitochondria

Here we report the localization of protein disulphide isomerase (PDI) in the mitochondrial compartments, comparing it with that of thioredoxin reductase. The latter enzyme is present mostly in the matrix, whereas PDI is located at the level of the outer membrane. We characterize the different submitochondrial fractions with specific marker enzymes. PDI, whether isolated from whole mitochondria or from purified outer membranes, exhibits the same electrophoretic mobility, indicating identical molecular masses. Moreover, immunoblot analysis with monoclonal anti-PDI antibody shows immunoreactivity only with the microsomal PDI, indicating the specificity of the mitochondrial isoform. The significance of these findings is discussed with reference to the potential role of PDI and thioredoxin reductase in regulating the mitochondrial functions dependent on the thiol-disulphide transition.

Gamma-lactone-Functionalized antitumoral acetogenins are the most potent inhibitors of mitochondrial complex I

To study the relevance of the terminal alpha,beta-unsaturated gamma-methyl-gamma-lactone moiety of the antitumoral acetogenins of Annonaceae for potent mitochondrial complex I inhibition, we have prepared a series of semisynthetic acetogenins with modifications only in this part of the molecule, from the natural rolliniastatin-1 (1) and cherimolin-1 (2). Some of the hydroxylated derivatives (1b, 1d and 1e) in addition to two infrequent natural beta-hydroxy gamma-methyl gamma-lactone acetogenins, laherradurin (3) and itrabin (4), are more potent complex I inhibitors than any other known compounds.

Cardiac creatine kinase metabolite compartments revealed by NMR magnetization transfer spectroscopy and subcellular fractionation

In the perfused rat heart NMR inversion transfer revealed the existence of a compartment of ATP not exchanging through creatine kinase (CK), as demonstrated by an apparent discrepancy between the forward (F(f)) and reverse (F(r)) CK flux if this compartment was neglected in the analysis [Joubert et al. (2000) Biophys. J. 79, 1-13]. To localize this compartment, CK fluxes were measured by inversion of PCr (inv-PCr) or gamma ATP (inv-ATP), and the distribution of metabolites between mitochondria and cytosol was studied by subcellular fractionation. Physiological conditions were designed to modify the concentration and distribution of CK metabolites (control, adenylate depletion, inhibition of respiration, KCl arrest). Depending on cardiac activity, mitochondrial ATP (mito-ATP) assessed by fractionation varied from 11% to 30% of total ATP. In addition, the apparent flux discrepancy increased together with mito-ATP (F(f)/F(r) ranged from 0.85 to 0.50 in inv-PCr and from 1.13 to 1.88 in inv-ATP). Under conditions masking the influence of the ATP-P(i) exchange on CK flux, the ATP compartment could be directly quantified by the apparent flux discrepancy; its size was similar to that of mito-ATP measured by fractionation. Thus NMR inversion technique is a potential tool to assess metabolite compartmentation in the whole organ.

New evidence for the multiplicity of ubiquinone- and inhibitor-binding sites in the mitochondrial complex I

Determination of the number of ubiquinone- and inhibitor-binding sites in the mitochondrial complex I (NADH:ubiquinone oxidoreductase) is a controversial question with a direct implication for elaborating a suitable model to explain the bioenergetic mechanism of this complicated enzyme. We have used combinations of both selective inhibitors and common ubiquinone-like substrates to demonstrate the multiplicity of the reaction centers in the complex I in contrast with competition studies that have suggested the existence of a unique binding site for ubiquinone. Our results provide new evidence for the existence of at least two freely exchangeable ubiquinone-binding sites with different specificity for substrates, as well as for a different kinetic interaction of inhibitors with the enzyme.

Properties and submitochondrial localization of pig and rat renal phosphate-activated glutaminase

Two pools of phosphate-activated glutaminase (PAG) were separated from pig and rat renal mitochondria. The partition of enzyme activity corresponded with that of the immunoreactivity and also with the postembedding immunogold labeling of PAG, which was associated partly with the inner membrane and partly with the matrix. The outer membrane was not labeled. PAG in intact mitochondria showed enzymatic characteristics that were similar to that of the membrane fraction and also mimicked that of the polymerized form of purified pig renal PAG. PAG in the soluble fraction showed properties similar to that of the monomeric form of purified enzyme. It is indicated that the pool of PAG localized inside the inner mitochondrial membrane is dormant due to the presence of high concentrations of the inhibitor glutamate. Thus the enzymatically active PAG is assumed to be localized on the outer face of the inner mitochondrial membrane. The activity of this pool of PAG appears to be regulated by compounds in the cytosol, of which glutamate may be most important.

Mitochondrial and submitochondrial localization of human deoxyguanosine kinase

Deoxyguanosine kinase and thymidine kinase 2 are responsible for catalysing the first step in the salvage of deoxynucleosides in mitochondria. These enzymes also play an important role in activating several antiviral and anticancer nucleoside analogs, which may lead to unwanted side-effects when the resulting nucleotides are incorporated into the mitochondrial genome. We studied deoxyguanosine kinase in submitochondrial fractions from human placental mitochondria. It was localized in the mitochondrial matrix fraction by Western blotting using a purified polyclonal antibody. This antibody was also used in an immunohistochemical in situ experiment with human embryonic kidney 293 cells, in which the deoxyguanosine kinase antibody colocalized with a mitochondrion-specific fluorescent probe and there was no significant cytosolic staining.

In vivo functional analysis of the human mitochondrial DNA polymerase POLG expressed in cultured human cells

The human gene POLG encodes the catalytic subunit of mitochondrial DNA polymerase, but its precise roles in mtDNA metabolism in vivo have not hitherto been documented. By expressing POLG fusion proteins in cultured human cells, we show that the enzyme is targeted to mitochondria, where the Myc epitope-tagged POLG is catalytically active as a DNA polymerase. Long-term culture of cells expressing wild-type POLG-myc revealed no alterations in mitochondrial function. Expression of POLG-myc mutants created dominant phenotypes demonstrating important roles for the protein in mtDNA maintenance and integrity. The D198A amino acid replacement abolished detectable 3'-5' (proofreading) exonuclease activity and led to the accumulation of a significant load (1:1700) of mtDNA point mutations during 3 months of continuous culture. Further culture resulted in the selection of cells with an inactivated mutator polymerase, and a reduced mutation load in mtDNA. Transient expression of POLG-myc variants D890N or D1135A inhibited endogenous mitochondrial DNA polymerase activity and caused mtDNA depletion. Deletion of the POLG CAG repeat did not affect enzymatic properties, but modestly up-regulated expression. These findings demonstrate that POLG exonuclease and polymerase functions are essential for faithful mtDNA maintenance in vivo, and indicate the importance of key residues for these activities.

Evidence that metformin exerts its anti-diabetic effects through inhibition of complex 1 of the mitochondrial respiratory chain

Although metformin is widely used for the treatment of non-insulin-dependent diabetes, its mode of action remains unclear. Here we provide evidence that its primary site of action is through a direct inhibition of complex 1 of the respiratory chain. Metformin(50 microM) inhibited mitochondrial oxidation of glutamate+malate in hepatoma cells by 13 and 30% after 24 and 60 h exposure respectively, but succinate oxidation was unaffected. Metformin also caused time-dependent inhibition of complex 1 in isolated mitochondria, whereas in sub-mitochondrial particles inhibition was immediate but required very high metformin concentrations (K(0.5),79 mM). These data are compatible with the slow membrane-potential-driven accumulation of the positively charged drug within the mitochondrial matrix leading to inhibition of complex 1. Metformin inhibition of gluconeogenesis from L-lactate in isolated rat hepatocytes was also time- and concentration-dependent, and accompanied by changes in metabolite levels similar to those induced by other inhibitors of gluconeogenesis acting on complex 1. Freeze-clamped livers from metformin-treated rats exhibited similar changes in metabolite concentrations. We conclude that the drug's pharmacological effects are mediated, at least in part, through a time-dependent, self-limiting inhibition of the respiratory chain that restrains hepatic gluconeogenesis while increasing glucose utilization in peripheral tissues. Lactic acidosis, an occasional side effect, canal so be explained in this way.

Effects of naturally occurring polyols and urea on mitochondrial F0F1ATPase

We show that urea inhibits the ATPase activity of MgATP submitochondrial particles (MgATP-SMP) with Ki = 0.7 M, probably as a result of direct interaction with the structure of F0F1-ATPase. Counteracting compounds (sorbitol, mannitol or inositol), despite slightly (10-20%) inhibiting the ATPase activity, also protect the F0F1-ATPase against denaturation by urea. However, this protection was only observed at low urea concentrations (less than 1.5 M), and in the presence of three polyols, the Ki for urea shift from 0.7 M to 1.2 M. Urea also increases the initial activation rate of latent MgATP-SMP in a dose-dependent-manner. However, when the particles (0.5 mg/ml) were preincubated in the presence of 1 M, 2 M or 3 M urea, a decrease in the activation level occurred after 1 h, 30 and 10 min, respectively. At high MgATP-SMP concentration (3 mg/ml) a decrease in activation was observed after 2 h, 1 h and 20 min, respectively. These data indicate that the effect of urea on the activation of MgATP-SMP depends on time, urea and protein concentrations. It was also observed that polyols suppress the activation of latent MgATP-SMP in a dose-dependent manner, and protect the particles against urea denaturation during activation. We suppose that a decrease in membrane mobility promoted by interactions of polyols with phospholipids around the F0F1-ATPase may also increase the compactation of protein structure, explaining the inhibition of natural inhibitor protein of ATPase (IF1) release and the activation of the enzyme.

Radiation inactivation analysis of H(+)-pyrophosphatase from submitochondrial particles of etiolated mung bean seedlings

Radiation inactivation analysis was employed to determine the functional masses of enzymatic activity and proton translocation of H(+)-pyrophosphatase from submitochondrial particles of etiolated mung bean seedlings. The activities of H(+)-pyrophosphatase decayed as a simple exponential function with respect to radiation dosage. D(37) values of 6.9+/-0.3 and 7.5+/-0.5 Mrad were obtained for pyrophosphate hydrolysis and its associated proton translocation, yielding molecular masses of 170+/-7 and 156+/-11 kDa, respectively. In the presence of valinomycin and 50 mM KCl, the functional size of H(+)-pyrophosphatase of tonoplast was decreased, while that of submitochondrial particles remained the same, indicating that they are two distinct types of proton pump using PP(i) as their energy source.

The effect of reactive oxygen species generated from the mitochondrial electron transport chain on the cytochrome c oxidase activity and on the cardiolipin content in bovine heart submitochondrial particles

The effect of reactive oxygen species (ROS), produced by the mitochondrial respiratory chain, on the activity of cytochrome c oxidase and on the cardiolipin content in bovine heart submitochondrial particles (SMP) was studied. ROS were produced by treatment of succinate-respiring SMP with antimycin A. This treatment resulted in a large production of superoxide anion, measured by epinephrine method, which was blocked by superoxide dismutase (SOD). Exposure of SMP to mitochondrial mediated ROS generation, led to a marked loss of cytochrome c oxidase activity and to a parallel loss of cardiolipin content. Both these effects were completely abolished by SOD+catalase. Added cardiolipin was able to almost completely restore the ROS-induced loss of cytochrome c oxidase activity. No restoration was obtained with peroxidized cardiolipin. These results demonstrate that mitochondrial mediated ROS generation affects the activity of cytochrome c oxidase via peroxidation of cardiolipin which is needed for the optimal functioning of this enzyme complex. These results may prove useful in probing molecular mechanism of ROS-induced peroxidative damage to mitochondria which have been proposed to contribute to aging, ischemia/reperfusion and chronic degenerative diseases.

Angular dependences of perpendicular and parallel mode electron paramagnetic resonance of oxidized beef heart cytochrome c oxidase

Cytochrome c oxidase catalyzes the reduction of oxygen to water with a concomitant conservation of energy in the form of a transmembrane proton gradient. The enzyme has a catalytic site consisting of a binuclear center of a copper ion and a heme group. The spectroscopic parameters of this center are unusual. The origin of broad electron paramagnetic resonance (EPR) signals in the oxidized state at rather low resonant field, the so-called g' = 12 signal, has been a matter of debate for over 30 years. We have studied the angular dependence of this resonance in both parallel and perpendicular mode X-band EPR in oriented multilayers containing cytochrome c oxidase to resolve the assignment. The "slow" form and compounds formed by the addition of formate and fluoride to the oxidized enzyme display these resonances, which result from transitions between states of an integer-spin multiplet arising from magnetic exchange coupling between the five unpaired electrons of high spin Fe(III) heme a(3) and the single unpaired electron of Cu(B). The first successful simulation of similar signals observed in both perpendicular and parallel mode X-band EPR spectra in frozen aqueous solution of the fluoride compound of the closely related enzyme, quinol oxidase or cytochrome bo(3), has been reported recently (Oganesyan et al., 1998, J. Am. Chem. Soc. 120:4232-4233). This suggested that the exchange interaction between the two metal ions of the binuclear center is very weak (|J| approximately 1 cm(-1)), with the axial zero-field splitting (D approximately 5 cm(-1)) of the high-spin heme dominating the form of the ground state. We show that this model accounts well for the angular dependences of the X-band EPR spectra in both perpendicular and parallel modes of oriented multilayers of cytochrome c oxidase derivatives and that the experimental results are inconsistent with earlier schemes that use exchange coupling parameters of several hundred wavenumbers.

Kinetic mechanism of ATP synthesis catalyzed by mitochondrial Fo x F1-ATPase

Initial rates of succinate-dependent ATP synthesis catalyzed by submitochondrial particles from bovine heart substoichiometrically coupled with oligomycin were found to have hyperbolic dependencies on contents of Mg x ADP, free Mg2+, and phosphate. The results suggest that Mg x ADP complex and free phosphate are true substrates of the enzyme; and an unordered ternary complex of Fo x F1-ATPase, Mg x ADP, and phosphate is generated during the catalysis. The presence of free Mg2+ is required for the reaction. Mg2+ was a noncompetitive activator of ATP synthesis relative to Mg x ADP and a competitive activator relative to phosphate. The decrease in steady-state values of Deltamu(H)+ (by the inhibition of succinate oxidase with malonate) results in the decreased value of Vmax and in a slight decrease in Km for the substrates and Mg2+ without changes in affinity for the substrates. Based on these results, a kinetic scheme of ATP synthesis is proposed.