Rapid separation of particulate and soluble fractions from isolated cell preparations (digitonin and cell cavitation procedures)

Ethanol metabolism by HeLa cells transduced with human alcohol dehydrogenase isoenzymes: control of the pathway by acetaldehyde concentration

BACKGROUND

Human class I alcohol dehydrogenase 2 isoenzymes (encoded by the ADH1B locus) have large differences in kinetic properties; however, individuals inheriting the alleles for the different isoenzymes exhibit only small differences in alcohol elimination rates. This suggests that other cellular factors must regulate the activity of the isoenzymes.

METHODS

The activity of the isoenzymes expressed from ADH1B*1, ADH1B*2, and ADH1B*3 cDNAs was examined in stably transduced HeLa cell lines, including lines which expressed human low K(m) aldehyde dehydrogenase (ALDH2). The ability of the cells to metabolize ethanol was compared with that of HeLa cells expressing rat class I alcohol dehydrogenase (ADH) (HeLa-rat ADH cells), rat hepatoma (H4IIEC3) cells, and rat hepatocytes.

RESULTS

The isoenzymes had similar protein half-lives in the HeLa cells. Rat hepatocytes, H4IIEC3 cells, and HeLa-rat ADH cells oxidized ethanol much faster than the cells expressing the ADH1B isoenzymes. This was not explained by high cellular NADH levels or endogenous inhibitors; but rather because the activity of the β1 and β2 ADHs was constrained by the accumulation of acetaldehyde, as shown by the increased rate of ethanol oxidation by cell lines expressing β2 ADH plus ALDH2.

CONCLUSION

The activity of the human β2 ADH isoenzyme is sensitive to inhibition by acetaldehyde, which likely limits its activity in vivo. This study emphasizes the importance of maintaining a low steady-state acetaldehyde concentration in hepatocytes during ethanol metabolism.

Dynamics of glutathione and ophthalmate traced with 2H-enriched body water in rats and humans

We developed a LC-MS-MS assay of the (2)H labeling of free glutathione (GSH) and bound glutathione [GSSR; which includes all DTT-reducible forms, primarily glutathione disulfide (GSSG) and mixed disulfides with proteins] and ophthalmate (an index of GSH depletion) labeled from (2)H-enriched body water. In rats whose body water was 2.5% (2)H enriched for up to 31 days, GSH labeling follows a complex pattern because of different rates of labeling of its constitutive amino acids. In rats infused with [(13)C(2),(15)N-glycine]glutathione, the rate of appearance of plasma GSH was 2.1 micromol.min(-1).kg(-1), and the half-life of plasma GSH/GSSR was 6-8 min. In healthy humans whose body fluids were 0.5% (2)H enriched, the (2)H labeling of GSH/GSSR and ophthalmate can be precisely measured after 4 h, with GSH being more rapidly labeled than GSSR. Since plasma GSH/GSSR derives mostly from liver, this technique opens the way to 2) probe noninvasively the labeling pattern and redox status of the liver GSH system in humans and 2) assess the usefulness of ophthalmate as an index of GSH depletion.

Effect of transient and permanent permeability transition pore opening on NAD(P)H localization in intact cells

To study the effect of mitochondrial permeability transition pore (PTP) opening on NAD(P)H localization, intact cells were exposed to the Ca(2+) ionophore A23187. PTP opening, mitochondrial membrane potential, mitochondrial volume, and NAD(P)H localization were assessed by time-lapse laser confocal microscopy using the calcein-cobalt technique, tetramethylrhodamine methyl ester, MitoTracker, and NAD(P)H autofluorescence, respectively. Concomitant with PTP opening, NAD(P)H fluorescence increased outside mitochondria. These events occurred in all cells and were prevented by cyclosporin A. Mitochondrial membrane potential was not systematically collapsed, whereas mitochondrial volume did not change, confirming that A23187 induced transient PTP opening in a subpopulation of cells and suggesting that mitochondrial swelling did not immediately occur after PTP opening in intact cells. NAD(P)H autofluorescence remained elevated after PTP opening, particularly after membrane potential had been collapsed by an uncoupler. Extraction of nucleotide for NAD(P)H quantification confirmed that PTP opening led to an increase in NAD(P)H content. Because the oxygen consumption rate decreased, whereas the lactate/pyruvate ratio increased after PTP opening in intact cells, we conclude that PTP opening inhibits respiration and dramatically affects the cytosolic redox potential in intact cells.

Sequestration assays for mammalian autophagy

Macroautophagic activity is most directly and precisely measured by a cargo sequestration assay. Long-lived, cytosolic proteins that are degraded exclusively by the autophagic-lysosomal pathway, such as lactate dehydrogenase (LDH) are suitable as endogenous sequestration probes. Autophagic sequestration is measured as transfer of the protein from the soluble (cytosolic) to the sedimentable (organelle-containing) cell fraction, using leupeptin or other proteinase inhibitors to block inactivation and degradation of the protein inside autophagic vacuoles. A convenient separation method is electrodisruption of the cells, followed by sedimentation of the organelle fraction through a Nycodenz density cushion. A promising variant of the cargo assay is to use a protein probe that is processed by the autophagic-lysosomal pathway so as to generate an intravacuolar fragment. Because there is no cytosolic background, subcellular fractionation is unnecessary, allowing the use of the autophagic fragment assay to measure autophagic activity in whole cells. In hepatocytes, a small fragment, p10(BHMT), made by autophagic processing of the enzyme betaine:homocysteine methyltransferase, thus accumulates in an autophagy-dependent manner in the presence of leupeptin. Autophagic sequestration can also be measured by using exogenous cargo probes, such as radiolabeled di- and trisaccharides, which can be loaded into the cytosol of hepatocytes by reversible electrodisruption or mechanical stress. Raffinose is the preferable probe for measurement of autophagic activity, whereas sucrose (which can be hydrolyzed in amphisomes and lysosomes by added endocytosed invertase) and lactose (which is hydrolyzed in lysosomes by the endogenous beta-galactosidase) are useful for dissection of the various steps in the autophagic-lysosomal pathway and for studying autophagic-endocytic interactions. Furthermore, the intralysosomal hydrolysis of autophagocytosed lactose can be measured in whole cells (as formation of the hydrolysis product, galactose), thus providing a background-free assay (autophagic lactolysis) of the overall autophagic-lysosomal pathway.

High expression of thyroid hormone receptors and mitochondrial glycerol-3-phosphate dehydrogenase in the liver is linked to enhanced fatty acid oxidation in Lou/C, a rat strain resistant to obesity

Besides its well recognized role in lipid and carbohydrate metabolisms, glycerol is involved in the regulation of cellular energy homeostasis via glycerol-3-phosphate, a key metabolite in the translocation of reducing power across the mitochondrial inner membrane with mitochondrial glycerol-3-phosphate dehydrogenase. Here, we report a high rate of gluconeogenesis from glycerol and fatty acid oxidation in hepatocytes from Lou/C, a peculiar rat strain derived from Wistar, which is resistant to age- and diet-related obesity. This feature, associated with elevated cellular respiration and cytosolic ATP/ADP and NAD(+)/NADH ratios, was linked to a high expression and activity of mitochondrial glycerol-3-phosphate dehydrogenase. Interestingly, this strain exhibited high expression and protein content of thyroid hormone receptor, whereas circulating thyroid hormone levels were slightly decreased and hepatic thyroid hormone carrier MCT-8 mRNA levels were not modified. We propose that an enhanced liver thyroid hormone receptor in Lou/C may explain its unique resistance to obesity by increasing fatty acid oxidation and lowering liver oxidative phosphorylation stoichiometry at the translocation of reducing power into mitochondria.

Interrelation between the inhibition of glycolytic flux by silibinin and the lowering of mitochondrial ROS production in perifused rat hepatocytes

Silibinin, the most biologically active component of the polyphenolic extract from milk thistle seeds, is widely used to prevent many types of hepatobiliary disorders. Recent evidence suggests new applications for this ancient medication, notably for the treatment of type 2 diabetes owing to its antihyperglycemic properties. As we have lately demonstrated that silibinin lowered glucose production from various gluconeogenic substrates in perifused rat hepatocytes, the aim of this study was to examine the effect of silibinin on both oxidative glucose utilization and reactive oxygen species (ROS) generation since the release of ROS secondary to an increased mitochondrial metabolism may contribute to diabetic damage. We found that silibinin dose-dependently reduced glycolysis from carbohydrates in a cell perifusion system via an inhibitory effect targeted on pyruvate kinase activity. Furthermore, a dramatic effect upon oxidative phosphorylation was shown, as evidenced by a fall in ATP-to-ADP ratio, together with an increase in lactate-to-pyruvate ratio. The most attractive finding was that silibinin, at a concentration as low as 10 microM, fully mitigated the rise in metabolic flow-driven ROS formation. In addition, studies on isolated liver mitochondria revealed that this low dose of silibinin depressed ROS production linked to the electron transfer chain activity. From these results, one may tentatively suggest that interesting activities for silibinin, beyond its general antioxidant status, could be expected from its potential clinical use, especially in pathological conditions when mitochondrial ROS formation is severely enhanced.

The flavonoid silibinin decreases glucose-6-phosphate hydrolysis in perfused rat hepatocytes by an inhibitory effect on glucose-6-phosphatase

BACKGROUND/AIMS

The flavonoid silibinin has been reported to be beneficial in several hepatic disorders. Recent evidence also suggests that silibinin could be beneficial in the treatment of type 2 diabetes, owing to its anti-hyperglycemic properties. However, the mechanism(s) underlying these metabolic effects remains unknown.

METHODS

The effects of silibinin on liver gluconeogenesis were studied by titrating hepatocytes from starved rats with sub-saturating concentrations of various exogenous substrates in a perifusion system. Hepatocytes from fed rats were also used to investigate glycogenolysis from endogenous glycogen. The effect of silibinin on glucose-6-phosphatase kinetics was determined in intact and permeabilized rat liver microsomes.

RESULTS

Silibinin induced a dose-dependent inhibition of gluconeogenesis associated with a potent decrease in glucose-6-phosphate hydrolysis. This effect was demonstrated whatever the gluconeogenic substrates used, i.e. dihydroxyacetone, lactate/pyruvate, glycerol and fructose. In addition, silibinin decreased the glucagon-induced stimulation of both gluconeogenesis and glycogenolysis, this being associated with a reduction of glucose-6-phosphate hydrolysis. Silibinin inhibits glucose-6-phosphatase in rat liver microsomes in a concentration-dependent manner that could explain the decrease in glucose-6-phosphate hydrolysis seen in intact cells.

CONCLUSION

The inhibitory effect of silibinin on both hepatic glucose-6-phosphatase and gluconeogenesis suggests that its use may be interesting in treatment of type 2 diabetes.

Prevention of cytosolic IAPs degradation: a potential pharmacological target in Huntington's Disease

Huntington's Disease (HD) is a neurodegenerative disorder caused by an abnormally expanded polyglutamine trait in the amino-terminal region of huntingtin. Pathogenic mechanisms involve a gained toxicity of mutant huntingtin and a potentially reduced neuroprotective function of the wild-type allele. Among the molecular abnormalities reported, HD cells are characterized by the presence of aggregates, transcriptional dysregulation, altered mitochondrial membrane potential and aberrant Ca++ handling. In addition, upon exposure to toxic stimuli, increased mitochondrial release of cytochrome C and activation of caspase-9 and caspase-3 are found in HD cells and tissue. Here we report that HTRA2 and Smac/DIABLO, two additional mitochondrial pro-apoptotic factors, are aberrantly released from brain-derived cells expressing mutant huntingtin. This event causes a reduction in levels of the cytosolic IAP1 (Inhibitor of Apoptosis Protein-1) and XIAP (X-linked inhibitor apoptosis) antiapoptotic IAP family members. Reduced IAP levels are also found in post-mortem HD brain tissue. Treatment with ucf101, a serine protease HTRA2 specific inhibitor, counteracts IAPs degradation in HD cells and increases their survival. These results point to the IAPs as potential pharmacological targets in Huntington's Disease.

Metformin inhibits mitochondrial permeability transition and cell death: a pharmacological in vitro study

Metformin, a drug widely used in the treatment of Type II diabetes, has recently received attention owing to new findings regarding its mitochondrial and cellular effects. In the present study, the effects of metformin on respiration, complex 1 activity, mitochondrial permeability transition, cytochrome c release and cell death were investigated in cultured cells from a human carcinoma-derived cell line (KB cells). Metformin significantly decreased respiration both in intact cells and after permeabilization. This was due to a mild and specific inhibition of the respiratory chain complex 1. In addition, metformin prevented to a significant extent mitochondrial permeability transition both in permeabilized cells, as induced by calcium, and in intact cells, as induced by the glutathione-oxidizing agent t-butyl hydroperoxide. This effect was equivalent to that of cyclosporin A, the reference inhibitor. Finally, metformin impaired the t-butyl hydroperoxide-induced cell death, as judged by Trypan Blue exclusion, propidium iodide staining and cytochrome c release. We propose that metformin prevents the permeability transition-related commitment to cell death in relation to its mild inhibitory effect on complex 1, which is responsible for a decreased probability of mitochondrial permeability transition.

Native, not nitrated, cytochrome c and mitochondria-derived hydrogen peroxide drive osteoclast apoptosis

Two unresolved aspects of the role of mitochondria-derived cytochrome c in apoptosis are whether there is a separate pool of cytochrome c within mitochondria that participates in the activation of apoptosis and whether a chemically modified cytochrome c drives apoptosis. These questions were investigated using osteoclasts, because they are rich in mitochondria and because osteoclast apoptosis is critical in bone metabolism regulation. H(2)O(2) production was increased during culture, preceding cytochrome c release; both processes occurred anterior to apoptosis. With the addition of a mitochondrial uncoupler, H(2)O(2) production and apoptosis were blocked, indicating the prominent role of mitochondria-derived H(2)O(2). Trapping H(2)O(2)-derived hydroxyl radical decreased apoptosis. Cytosolic cytochrome c was originated from a single mitochondrial compartment, supporting a common pool involved in respiration and apoptosis, and it was chemically identical to the native form, with no indication of oxidative or nitrative modifications. Protein levels of Bcl-2 and Bc-xL were decreased before apoptosis, whereas expression of wild-type Bcl-2 repressed apoptosis, confirming that cytochrome c release is critical in initiating apoptosis. Cytosolic cytochrome c participated in activating caspase-3 and -9, both required for apoptosis. Collectively, our data indicate that the mitochondria-dependent apoptotic pathway is one of the major routes operating in osteoclasts.

Rotenone inhibits the mitochondrial permeability transition-induced cell death in U937 and KB cells

The permeability transition pore (PTP) is a mitochondrial inner membrane Ca(2+)-sensitive channel that plays a key role in different models of cell death. Because functional links between the PTP and the respiratory chain complex I have been reported, we have investigated the effects of rotenone on PTP regulation in U937 and KB cells. We show that rotenone was more potent than cyclosporin A at inhibiting Ca(2+)-induced PTP opening in digitonin-permeabilized cells energized with succinate. Consistent with PTP regulation by electron flux through complex I, the effect of rotenone persisted after oxidation of pyridine nucleotides by duroquinone. tert-butyl hydroperoxide induced PTP opening in intact cells (as shown by mitochondrial permeabilization to calcein and cobalt), as well as cytochrome c release and cell death. All these events were prevented by rotenone or cyclosporin A. These data demonstrate that respiratory chain complex I plays a key role in PTP regulation in vivo and confirm the importance of PTP opening in the commitment to cell death.

The sterol composition of Trypanosoma cruzi changes after growth in different culture media and results in different sensitivity to digitonin-permeabilization

Respiration, oxidative phosphorylation. and the corresponding changes in membrane potential (deltapsi) of Trypanosoma cruzi epimastigotes grown either in liver infusion-tryptose (LIT) or brain heart infusion (BHI) culture medium were assayed in situ using digitonin to render their plasma membrane permeable to succinate, ADP, safranine O, and other small molecules. When the cells were permeabilized with 64 microM digitonin, a concentration previously used with epimastigotes, the ability of the cells grown in LIT medium to sustain oxidative phosphorylation was demonstrated by the detection of an oligomycin-sensitive decrease in mitochondrial membrane potential induced by ADP. In contrast, the cells grown in BHI medium were not able to sustain a stable membrane potential and did not respond to ADP addition. Analyses of oxygen consumption by these permeabilized cells indicated that the rate of basal respiration, which was similar in both cell types, was significantly decreased by 64 microM digitonin. Addition of ADP to the permeabilized cells grown in LIT medium promoted an oligomycin-sensitive transition from resting to phosphorylating respiration in contrast to the cells grown in BHI medium, whose respiration decreased steadily and did not respond either to ADP or CCCP. Titration of the cells grown in BHI medium with different digitonin concentrations indicated that their mitochondria have higher sensitivity to digitonin than those grown in LIT medium. Analysis of the sterol composition of epimastigotes grown in the two different media showed a higher percentage of cholesterol in total and mitochondrial extracts of epimastigotes grown in BHI medium as compared to those grown in LIT medium, suggesting the involvement of this sterol in their increased sensitivity to digitonin-permeabilization.

The relative importance of passive and P-glycoprotein mediated anthracycline efflux from multidrug-resistant cells

For the four anthracyclines idarubicin, daunorubicin, epirubicin and doxorubicin the passive and active efflux rates in intact multidrug resistant cells were compared. Although highly similar structurally, these anti-tumor agents differ in lipophilicity and membrane permeability (k). The method we used was based on the continuous measurement of the cellular efflux and determination of the ratio (RVp) of transport rates just before and just after inhibition of the active transport with verapamil (Vp). Hence, RVp - 1 should reflect the active transport rate relative to the passive transport rate. If cells were single, well-stirred compartments, RVp - 1 should equal Vmax/(k.Km), where Vmax is the maximal pumping rate and Km is the Michaelis constant. However, using the plasma membrane permeabilizing agent digitonin, we found an effective intracellular anthracycline store. Particularly, when the efflux was fast, e.g. with idarubicin or in intensively pumping cells, the intracellular transport began to control the cellular efflux. Under these conditions, k underestimated the true plasma membrane permeability (k0) and RVp - 1 underestimated Vmax/(k.Km). Based on the effects of digitonin on the efflux rates in pumping and nonpumping cells, we developed an index (RVp,corrected - 1) which should equal Vmax/(k0. Km). The term Vmax/(k0.Km) varied substantially between the drugs. It appears that differences in lipophilicity between the drugs do not affect passive efflux and pumping equally. This demonstrates that passive permeation plays a substantial and independent role in determining the drug resistance for these anthracyclines. The methods developed here enable dissection of this role from that of drug pumping and intracellular subcompartmentation.

Is beta-poly(L-malate) synthesis catalysed by a combination of beta-L-malyl-AMP-ligase and beta-poly(L-malate) polymerase?

beta-Poly(L-malate) is supposed to function in the storage and transport of histones, DNA polymerases and other nuclear proteins in the giant syncytical cells (plasmodia) of myxomycetes. Here we report on the biosynthesis of [14C]beta-poly(L-malate) from injected L-[14C]malate in the plasmodium of Physarum polycephalum. The effects of KCN, arsenate, adenosine 5'-(alpha, beta-methylene)triphosphate, adenosine 5'-(beta, gamma-methylene)triphosphate, guanosine 5'-(beta, gamma-methylene)triphosphate, desulfo coenzyme A and phenylarsinoxid on beta-poly(L-malate) synthesis were studied after their coinjection with L-[14C]malate. The synthesis was not affected by KCN or desulfo coenzyme A, but was blocked by arsenate and adenosine 5'-(alpha,beta-methylene)triphosphate. The plasmodium lysate catalysed an L-malate-dependent ATP-[32P]pyrophosphate exchange, but was devoid of beta-poly(L-malate) synthetic activity under all experimental conditions tested. The results suggested an extramitochondrial synthesis of beta-poly(L-malate), involving the polymerization of beta-L-malyl-AMP. It is assumed that the lack of synthesis in the lysate is caused by the inactivation of beta-poly(L-malate) polymerase involving a cell injury kinase pathway. Because injected guanosine 5'-(beta, gamma-methylene)triphosphate blocks the synthesis, the injury signal is likely to be GTP dependent.

Import and processing of heart mitochondrial cyclophilin D

Cyclophilins are a family of cyclosporin-A-binding proteins which catalyse rotation about prolyl peptide bonds. A mitochondrial isoform in mammalian cells, cyclophilin D, is a component of the permeability transition pore that is formed by the adenine nucleotide translocase and the voltage-dependent anion channel at contact sites between the inner and outer membrane. This study investigated the submitochondrial location of cyclophilin D by following the fate of radiolabelled protein following import. Precursor [(35)S]cyclophilin D was expressed in vitro from a PCR-generated cDNA. The precursor was imported by rat heart mitochondria and processed in a single step to a 21-kDa protein that was identical (SDS/PAGE) to an in vitro expressed mature protein and a cyclophilin D purified from rat heart mitochondria. No further modification of the mature protein could be demonstrated. Fractionation of mitochondria following import established that cyclophilin D locates only to the matrix. It is concluded that cyclophilin D binding to the permeability transition pore must occur at the inner face of the mitochondrial inner membrane.

Fluxes of nicotinamide adenine dinucleotides through mitochondrial membranes in human cultured cells

We report on the loss of mitochondrial nicotinamide adenine dinucleotides in human cultured cells along with cell culture and acidification of the culture medium. This was established both by the direct measurement of the decrease in the mitochondrial NAD content and by the alteration of the oxidative properties of the mitochondria. In situ, this loss could be reversed in less than 2 h by changing the culture medium or by readjusting the pH of the medium at physiological pH values. By studying the oxidative properties of intact, but NAD-depleted, mitochondria in digitonin-permeabilized cells, we found that a rapid influx of NAD could replenish the mitochondrial NAD pool. This allowed the restoration of an active NAD+-dependent substrate oxidation. Depletion of mitochondrial NAD in cells grown under quiescent conditions was further confirmed by fluorimetric measurement of mitochondrial NAD, as was the influx of NAD+ into the mitochondrial matrix. These data constitute the first evidence of rapid fluxes of NAD through mitochondrial membranes in animal cells. They also point to the possible confusion between a loss of mitochondrial NAD and a defect of respiratory chain complex I in the context of screening procedures for respiratory chain disorder in human.

Expression of human mitochondrial NADP-dependent isocitrate dehydrogenase during lymphocyte activation

In the process of identifying genes involved in optimization of lymphocyte activation, we have cloned the human mitochondrial NADP-dependent isocitrate dehydrogenase (mNADP-IDH) cDNA. The cDNA and its deduced amino acid (AA) sequence had a high degree of homology with those of the porcine and bovine. The heart and muscle had the highest constitutive expression of the gene. The expression of steady-state mRNA in the resting T and B lymphocytes was low but was induced after mitogen stimulation. The mRNA levels peaked around 48 h and remained elevated at 72 h. At the protein level, the mitochondrial but not cytosolic NADP-IDH activity was augmented after the mitogen stimulation. There was no cell cycle-dependent fluctuation of mNADP-IDH expression in synchronized Jurkat cells. In T and B cells, rapamycin (RAPA) could repress the mitogen-stimulated mNADP-IDH expression, although most of the early or late phase activation-related genes including a G-protein beta subunit-related gene H12.3 were not affected by the drug. The restricted expression of the gene in certain tissues and the activation-related expression in lymphocytes suggest that this gene might be necessary for optimal functions in heart, muscle, and the activated lymphocytes.

Molecular cloning of the cDNA of mouse mitochondrial NADP-dependent isocitrate dehydrogenase and the expression of the gene during lymphocyte activation

The current report documents the molecular cloning of the mouse mitochondrial NADP-dependent isocitrate dehydrogenase (mNADP-IDH) cDNA. The cDNA was 1,863 bp in length and contained one open reading frame encoding a 523-residue polypeptide with a predicted molecular weight of 58 kDa. The cDNA and the deduced amino acid (AA) sequence of the mouse mNADP-IDH had a high degree of homology with those of porcine, bovine, alfalfa, and yeast. The recombinant mNADP-IDH expressed in Escherichia coli had active enzymatic function, as well as an expected molecular weight. The heart had the highest constitutive expression of the steady-state mNADP-IDH mRNA, followed by the kidney, while the expression of the gene in other tissues was low. The enzymatic activity of different tissues was in agreement with their mNADP-IDH mRNA levels. The resting lymphocytes had low constitutive expression of the gene, but the steady-state mRNA could be induced 48 h after mitogen stimulation. At the protein level, the resting lymphocytes had low enzymatic activity of mNADP-IDH, but the activity was augmented fivefold after mitogen stimulation. The cytosolic NADP-IDH, on the contrary, remained low or undetectable before and after the mitogen stimulation. Based on our current findings as well as the known roles of the mNADP-IDH in anabolism and in the isocitrate shuttle, it is conceivable that the mNADP-IDH is necessary for optimizing proliferation in lymphocytes.

Substrate-dependent utilization of the glycerol 3-phosphate or malate/aspartate redox shuttles by Ehrlich ascites cells

The rate of transfer of reducing equivalents from cytoplasm to mitochondria has been examined in Ehrlich ascites tumour cells incubated in the presence of lactate. The flux of reducing equivalents was determined from the rate of metabolism of reduced intermediates that are oxidized within the cytosol. The magnitude of the flux of reducing equivalents was dependent on both the concentration of added lactate and the presence of carbohydrate. The rate of flux was twice as great in the presence of glucose and four times as high when glucose and lactate were added together as when lactate was the only added substrate. Fructose was less effective than glucose in stimulating reducing equivalent flux. In the presence of glucose or fructose, there was a substantial accumulation of hexose phosphates, dihydroxyacetone phosphate and glycerol 3-phosphate. Rotenone, an inhibitor of NADH dehydrogenase, and amino-oxyacetate, which inhibits the malate/aspartate shuttle, were powerful suppressors of reducing equivalent flux from lactate as sole substrate, but were much less potent in the presence of carbohydrate. Antimycin substantially inhibited reducing equivalent flux from all combinations of added substrates, consistent with its ability to block oxidation of reducing equivalents transferred by both the malate/aspartate and glycerol 3-phosphate shuttles. The glycerol 3-phosphate shuttle represents around 80% of the maximum total observed activity but is active only while glycolytic intermediates are present to provide the necessary substrates of the shuttle. This Ehrlich ascites cell line has an essentially similar total reducing equivalent shuttle capacity to that of isolated hepatocytes.

Regulation of argininosuccinate synthetase level by corticosteroid and pancreatic hormones during perinatal period

The urea cycle takes place in the hepatocyte of ureothelic animals. The conversion of ammonia into urea involves five reactions. The first 2 take place in the matrix of the mitochondria, the last 2 occur in the cytosol. Argininosuccinate synthetase (AS) is the third reaction of the urea cycle. It catalyses the condensation of citrulline and aspartate into argininosuccinate. We have previously reported that rat AS activity was present in the cytosol and the outer membrane of the mitochondria. We have shown that, at the activity level, the colocation of AS was changing during fetal and neonatal development and was under the control of corticosteroid and pancreatic hormones. However, an unresolved issue was whether both AS had the same specific activity and that their location was changing during ontogenesis or that the specific activities of mitochondrial and cytosolic enzymes were different and/or modified during this period. In the present report, we compared the compartmentalization of AS activity and protein level in the fetus, the new-born and the adult rat and the role of corticosteroid and pancreatic hormones. Specific activities of both AS remained unchanged during ontogenesis. Glucocorticoids induced an increase in mitochondrial AS while glucagon appeared to induce a concomitant decrease in the level of mitochondrial AS and an increase in cytosolic AS.

Demonstration of argininosuccinate synthetase activity associated with mitochondrial membrane: characterization and hormonal regulation

Argininosuccinate synthetase (AS) is the third enzyme in ureogenesis, it catalyses the reaction of condensation of citrulline and aspartate into argininosuccinate. In the present report, we described the first characterization of AS within the outer membrane of rat liver mitochondria. Mitochondria-associated AS displayed the same kinetic characteristics as the cytoplasmic enzyme, but was found to be thermostable while cytoplasmic AS was not. The evolution of the co-location of AS was analyzed during ontogenesis. Total AS activity increased throughout rat fetal development. Simultaneously, the subcellular distribution of the enzyme has changed. AS activity was mainly mitochondrial in fetal and new-born liver liver and cytoplasmic in adult rat liver. The variation in subcellular distribution of AS may be due to the dramatic changes in hormonal levels that occur during this period. The role of corticosteroid and pancreatic hormones was studied. During fetal period, corticosteroid hormones induced an increase in mitochondria-associated AS activity. This was prevented by insulin. Glucagon did not modify total AS activity but reduced mitochondrial AS activity, meanwhile, a comparable increase in cytoplasmic AS activity was observed. One may hypothesize that glucagon may participate in the transfer of mitochondrial enzyme into the cytosol.

Isolation and characterization of mitochondria from human B lymphoblastoid cell lines

Mitochondria were isolated from detergent-treated Epstein-Barr virus-transformed human lymphocytes to examine their potential use in the study of the functional expression of genetic disorders of the respiratory chain. The increase of cytochrome c oxidase activity in the mitochondrial fraction indicated a 6-fold purification of intact mitochondria. Polarographic and spectrophotometric studies revealed that the isolated mitochondria were functionally well preserved. Furthermore, the isolated mitochondria supported an active in organello protein synthesis, which was dependent on the presence of a respiratory substrate generating ATP and was essentially abolished by chloramphenicol or by a specific respiratory chain inhibitor, such as antimycin. Thus, B lymphoblastoid cell lines constitute a valuable source of mitochondria to investigate mitochondrial functions in patients affected by respiratory chain disorders.

Studies on the intracellular distributions of soluble epoxide hydrolase and of catalase by digitonin-permeabilization of hepatocytes isolated from control and clofibrate-treated mice

Digitonin permeabilization of hepatocytes from control and clofibrate-treated (0.5% by mass, 10 days) male C57bl/6 mice was used to study the intracellular distributions of soluble ('cytosolic') epoxide hydrolase and of catalase. The following conclusions were drawn. (1) About 60% of the total soluble epoxide hydrolase activity in control mouse hepatocytes is situated in the cytosol. (2) The rest is not mitochondrial, but probably peroxisomal. (3) Of the total catalase activity in control mouse hepatocytes, 5-10% is found in the cytosol. (4) Treatment of mice with clofibrate increases the total hepatocyte activity of soluble epoxide hydrolase 4-fold, but does not influence the relative distribution of this enzyme between cytosol and peroxisomes. (5) The total catalase activity is increased 3.5-fold by clofibrate treatment and 15-35% of this activity is shifted from the peroxisomes to the cytosol.

Dependence of mitochondrial and cytosolic adenine nucleotides on oxygen partial pressure in isolated hepatocytes. Application of a new rapid high pressure filtration technique for fractionation

By using a new rapid high pressure filtration technique, mitochondrial and cytosolic ATP and ADP contents were determined in isolated hepatocytes at different oxygen partial pressures. At 670 mmHg, subcellular adenine nucleotide contents and ATP/ADP ratios were comparable with values obtained with the digitonin fractionation technique. However at lower oxygen partial pressure ADP appears to be rephosphorylated during digitonin fractionation whereas with high pressure filtration fractionation rephosphorylation of ADP is avoided due to shorter fractionation times. Cytosolic and mitochondrial ATP/ADP ratios decrease if oxygen partial pressure is lowered. However the absolute values of ATP/ADP ratios depend critically on the incubation conditions. Thus incubation of hepatocytes in an oxystat system, where oxygen partial pressure is maintained constant by infusing oxygen-saturated medium and the hepatocyte suspension is continuously stirred, yields much higher subcellular and overall ATP/ADP ratios than incubation in Erlenmeyer flasks gassed with different gas mixtures and shaken in a water bath. This is ascribed to limited diffusion of oxygen from the medium into the cell if the suspension is not mixed thoroughly by stirring. The strong dependence of subcellular ATP/ADP ratios on incubation conditions indicates that oxygen may be one rate-controlling factor for oxidative phosphorylation in the intact cell.

Intractable unphysiologically low adenylate energy charge values in synaptosome fractions: an explanatory hypothesis based on the fraction's heterogeneity

Synaptosomes prepared and incubated in a variety of ways from rat cerebra exhibited intractable, unphysiologically low adenylate energy charge values (approximately 0.37-0.60), low total adenine nucleotide contents (approximately 8-10 nmol/mg protein), and much higher adenylate kinase apparent Keq values (approximately 3-8) as compared to intact brain tissue (values of approximately 0.90, 25 nmol/mg, and 0.74, respectively). Synaptosomes prepared from mouse, dog, and chicken cerebra had values essentially identical to those from rat. When incubated under oxygen in a physiological salt solution containing glucose, synaptosomes metabolized more glucose to lactic acid than to CO2, and the addition of 100 microM veratridine caused a two- to threefold stimulation of O2 uptake, lactate accumulation, and CO2 output. It is known that synaptosome fractions contain a substantial number (at least 30-45% by volume) of cytoplasm-containing particles devoid of mitochondria (henceforth termed "cytosolic particles"), and that approximately 80% of brain hexokinase is bound to the outer mitochondrial membrane. For the cytosolic particles, lacking oxidative phosphorylation, to maintain their "in vivo" ATP turnover would require about a 19-fold increase in the glycolytic rate, which is not possible due to limiting amounts of hexokinase, and thus these particles are postulated to be responsible for the high level of aerobic lactate accumulation and the intractable low energy charge values found in synaptosome fractions. The mitochondria-containing particles are postulated to have a normal energy charge, a submaximal glycolytic rate, and minimal lactate production, on the basis of the capacity of veratridine to stimulate synaptosomal O2 uptake and CO2 and lactate output. Calculations based on this "two populations of particles" hypothesis indicate that for synaptosome fractions in general, (1) the cytosolic particles contain approximately 35-64% of the total adenine nucleotides and maintain an energy charge of approximately 0.12; (2) the cytosolic particles and mitochondria-containing particles have adenylate kinase apparent Keq values of approximately 0.21-1.66 and 0.74, respectively, revealing that the higher apparent Keq values of the synaptosome fractions probably are not real departures from equilibrium: and (3) approximately 31-45% of synaptosome fraction protein is contained in debris, which, when taken into account, yields total adenine nucleotide contents in the cytosolic particles and mitochondria-containing particles of approximately 15-24 and approximately 11-19 nmol/mg of particle protein, respectively.

A method for determining the intracellular distribution of enzymes in yeast provides no evidence for the association of hexokinase with mitochondria

A simple procedure based upon the principle discovered by Dürr et al. (Arch. Microbiol. (1975) 105, 319-327) was used to measure the intracellular distribution of enzymes in S. cerevisiae grown under both glucose repression and derepression. No substantial hexokinase activity was found to be associated with cellular organelles. The result does not support the hypotheses that reversible binding of hexokinase to mitochondria is important in regulation of glycolysis and cell growth.

Cadmium-induced alteration of the energy level in isolated hepatocytes

The incubation of isolated hepatocytes with 10-100 microM cadmium (Cd) decreased the ATP/ADP-ratio, the ATP/AMP-ratio and the adenylate energy charge and enhanced the lactate/pyruvate-ratio in a time- and concentration-dependent manner. Likewise, the cellular oxygen-consumption was decreased at early incubation times, when the cell membrane was still intact, as judged by the Trypan Blue-exclusion test. Similarly, Cd-concentrations above 0.3 nmol/mg mitochondrial protein inhibited the succinate- and malate/pyruvate-stimulated respiration of isolated mitochondria. This critical concentration was also reached in mitochondria of hepatocytes, when exposed to 25 microM Cd for less than 30 min. We therefore suggest that the inhibition of cellular respiration and the breakdown of cellular energy generation is mediated by a severe disturbance of mitochondrial respiratory functions due to Cd. This effect seems to be an early event in Cd-toxicity in isolated hepatocytes.

Characterization of growth plate mitochondria

The growth plate chondrocyte plays a central role in growth plate function. The purpose of this study was to characterize the respiratory and calcium transport properties of isolated mammalian growth plate chondrocytes and mitochondria obtained from these cells and to quantitate the mitochondrial weight and volume fraction in each zone of the growth plate. A new method was developed for isolation of mitochondria from chondrocyte suspensions. Isolated chondrocyte mitochondria demonstrated an eightfold increase in oxygen consumption in response to calcium and a two- to threefold increase in oxygen consumption in response to adenosine diphosphate. Similar responses were observed in chondrocytes treated with digitonin. The mitochondrial protein content of the growth plate and hyaline cartilage chondrocytes is significantly less than hepatocytes. Conversely, the chondrocyte mitochondrial cytochrome aa3 content is similar to mitochondria from a wide variety of sources. A zonal analysis of the growth plate demonstrates an increase in the mitochondrial weight (protein) fraction from the reserve to the hypertrophic zone whereas the mitochondrial volume fraction decreases from the reserve to the hypertrophic zone. The findings of this study emphasize the dependence of chondrocytes on glycolysis as a prime energy source and support the concept that chondrocyte mitochondria have become specialized in the process of matrix calcification.

Studies on the pathway from ornithine to proline in cultured skin fibroblasts with reference to the defect in hyperornithinaemia with hyperammonaemia and homocitrullinuria

The reduction in the conversion of ornithine to proline by fibroblasts from a patient with hyperornithinaemia with hyperammonaemia and homocitrullinuria cannot be explained by a reduced uptake of exogenous ornithine, an altered total intracellular ornithine content, or reduced conversion of gamma-glutamate semialdehyde to proline. However, neither could the postulated defect in mitochondrial ornithine uptake be demonstrated using the digitonin method. Increasing the ornithine concentration in the medium increased the incorporation of 14C label from ornithine into protein in both the patient's and control cells. In the patient's cells the apparent Km for ornithine was ten times that of the controls, although the Vmax values were comparable. This result parallels the clinical response to ornithine supplementation.

Characterization of isolated rat-liver cells made permeable with filipin

When isolated rat-liver cells were incubated for 1 min at 37 degrees C with filipin at a concentration of 50 microM, the plasma membrane became permeable to sucrose, inulin, glycerol 3-phosphate and other low-molecular-weight compounds. Upon removal of the filipin and subsequent incubation of the cells at 37 degrees C there was a gradual leakage of lactate dehydrogenase from the cells. However, the leakage of lactate dehydrogenase could be prevented for about 10 min by including glutathione and ATP in the incubation medium. The filipin-treated cells were able to metabolize phosphorylated sugars. The conversion of fructose 1,6-bisphosphate to fructose 6-phosphate, glucose-6-phosphate and glucose was inhibited by AMP but not by high concentrations of fructose 1,6-bisphosphate. The results indicate that filipin-treated cells can be used to study the kinetic parameters of enzymes in their macromolecular environment in situ.

Mitochondrial and cytosolic ATP/ADP ratios in isolated hepatocytes. A comparison of the digitonin method and the non-aqueous fractionation procedure

The ratio of ATP content/ADP content in the mitochondrial matrix was found to be 2.07 +/- 0.21 and 2.26 +/- 0.22 as determined with six different preparations of isolated hepatocytes subfractionated with the digitonin and non-aqueous-fractionation procedures, respectively. In contrast, the mitochondrial matrix ATP/ADP determined with isolated haemoglobin-free perfused liver by using the non-aqueous-fractionation procedure was about 0.2, whereas the cytosolic values obtained with isolated cells and with the intact organ were similar. It is concluded that the relatively higher ATP/ADP ratio in the mitochondrial matrix of isolated hepatocytes represents a biochemical difference due to properties of the model rather than a methodological artifact.

The relationship between intramitochondrial N-acetylglutamate and activity of carbamoyl-phosphate synthetase (ammonia). The effect of glucagon

1. The relationship between urea synthesis, intracellular N-acetylglutamate and the capacity of rat-liver mitochondria to synthesize citrulline was investigated. 2. Treatment of rats with glucagon prior to killing results not only in an increased intramitochondrial ATP concentration and an increased capacity of the mitochondria to synthesize citrulline, but also in an increased concentration of intramitochondrial N-acetylglutamate. 3. Comparison of the rate of citrulline synthesis in mitochondria from glucagon-treated and from control rats, incubated under different conditions, shows that the increased N-acetylglutamate concentration after glucagon treatment is at least in part responsible for the observed increased capacity of the mitochondria to synthesize citrulline. 4. Ureogenic flux in isolated hepatocytes under different incubation conditions correlated with the intracellular concentration of N-acetylglutamate and with the capacity of the mitochondria to synthesize citrulline. 5. When isolated hepatocytes were incubated with NH3, ornithine, lactate and oleate, intracellular N-acetylglutamate increased about eightfold in the first 10 min; during this period the rate of urea synthesis increased considerably. 6. It is concluded that the concentration of intramitochondrial N-acetylglutamate plays an important role in the short-term control of flux through the urea cycle under different nutritional and hormonal conditions.