Permeability of isolated mitochondria to oxaloacetate

Mitochondrial Transport in Glycolysis and Gluconeogenesis: Achievements and Perspectives

Some metabolic pathways involve two different cell components, for instance, cytosol and mitochondria, with metabolites traffic occurring from cytosol to mitochondria and vice versa, as seen in both glycolysis and gluconeogenesis. However, the knowledge on the role of mitochondrial transport within these two glucose metabolic pathways remains poorly understood, due to controversial information available in published literature. In what follows, we discuss achievements, knowledge gaps, and perspectives on the role of mitochondrial transport in glycolysis and gluconeogenesis. We firstly describe the experimental approaches for quick and easy investigation of mitochondrial transport, with respect to cell metabolic diversity. In addition, we depict the mitochondrial shuttles by which NADH formed in glycolysis is oxidized, the mitochondrial transport of phosphoenolpyruvate in the light of the occurrence of the mitochondrial pyruvate kinase, and the mitochondrial transport and metabolism of L-lactate due to the L-lactate translocators and to the mitochondrial L-lactate dehydrogenase located in the inner mitochondrial compartment.

A critical tyrosine residue determines the uncoupling protein-like activity of the yeast mitochondrial oxaloacetate carrier

The mitochondrial Oac (oxaloacetate carrier) found in some fungi and plants catalyses the uptake of oxaloacetate, malonate and sulfate. Despite their sequence similarity, transport specificity varies considerably between Oacs. Indeed, whereas ScOac (Saccharomyces cerevisiae Oac) is a specific anion-proton symporter, the YlOac (Yarrowia lipolytica Oac) has the added ability to transport protons, behaving as a UCP (uncoupling protein). Significantly, we identified two amino acid changes at the matrix gate of YlOac and ScOac, tyrosine to phenylalanine and methionine to leucine. We studied the role of these amino acids by expressing both wild-type and specifically mutated Oacs in an Oac-null S. cerevisiae strain. No phenotype could be associated with the methionine to leucine substitution, whereas UCP-like activity was dependent on the presence of the tyrosine residue normally expressed in the YlOac, i.e. Tyr-ScOac mediated proton transport, whereas Phe-YlOac lost its protonophoric activity. These findings indicate that the UCP-like activity of YlOac is determined by the tyrosine residue at position 146.

Metabolite transport across the peroxisomal membrane

In recent years, much progress has been made with respect to the unravelling of the functions of peroxisomes in metabolism, and it is now well established that peroxisomes are indispensable organelles, especially in higher eukaryotes. Peroxisomes catalyse a number of essential metabolic functions including fatty acid beta-oxidation, ether phospholipid biosynthesis, fatty acid alpha-oxidation and glyoxylate detoxification. The involvement of peroxisomes in these metabolic pathways necessitates the transport of metabolites in and out of peroxisomes. Recently, considerable progress has been made in the characterization of metabolite transport across the peroxisomal membrane. Peroxisomes posses several specialized transport systems to transport metabolites. This is exemplified by the identification of a specific transporter for adenine nucleotides and several half-ABC (ATP-binding cassette) transporters which may be present as hetero- and homo-dimers. The nature of the substrates handled by the different ABC transporters is less clear. In this review we will describe the current state of knowledge of the permeability properties of the peroxisomal membrane.

Identification and kinetic characterization of HtDTC, the mitochondrial dicarboxylate-tricarboxylate carrier of Jerusalem artichoke tubers

Jerusalem artichoke (Helianthus tuberosus L.) tubers were reported to be tolerant to cold and freezing. The aim of this study was to perform a kinetic characterization of the mitochondrial dicarboxylate-tricarboxylate carrier (HtDTC) and to assess a possible involvement of this carrier in the cold tolerance of tubers. The HtDTC was purified from isolated mitochondria by sequential chromatography on hydroxylapatite/celite and Matrex Gel Orange A. SDS gel electrophoresis of the purified fraction showed a single polypeptide band with an apparent molecular mass of 31.6 kDa. A polyclonal antibody raised against the tobacco DTC cross-reacted with the purified protein on Western blot analysis. In gel trypsin, digestion of the purified HtDTC yielded peptides that exhibited strong amino acid sequence similarity to previously identified plant DTCs. Furthermore, using degenerate primers, a portion of the Htdtc cDNA was amplified and sequenced; this cDNA encoded for a protein with high sequence similarity to known plant homolog DTCs. When reconstituted in liposomes loaded with dicarboxylate (2-oxoglutarate, malate, malonate, succinate, and maleate) or tricarboxylate anions (citrate, trans-aconitate, and isocitrate), the purified HtDTC transported all these anions in exchange with external [14C]2-oxoglutarate. A kinetic characterization of HtDTC was performed: (a) the half-saturation constant Km and the Vmax at 25 degrees C of the 2-oxoglutarate/2-oxoglutarate exchange by reconstituted HtDTC were found to be 360 microM and 10.9 micromol/(min mg protein), respectively; (b) the activation energy Ea of the succinate/2-oxoglutarate exchange by the reconstituted HtDTC was found to be 50.7 kJ/mol constant between -5 and 35 degrees C. Similarly, the activation energy Ea of succinate respiration of isolated Jerusalem artichoke mitochondria, measured between -2 and 35 degrees C, was shown to be constant (65.3 kJ/mol). The physiological relevance of kinetic properties and temperature dependence of transport activities of HtDTC is discussed with respect to the cold tolerance ability of Jerusalem artichoke tubers.

Mitochondrial transport in proline catabolism in plants: the existence of two separate translocators in mitochondria isolated from durum wheat seedlings

Abiotic stresses, such as high salinity or drought, can cause proline accumulation in plants. Such an accumulation involves proline transport into mitochondria where proline catabolism occurs. By using durum wheat seedlings as a plant model system, we investigated how proline enters isolated coupled mitochondria. The occurrence of two separate translocators for proline, namely a carrier solely for proline and a proline/glutamate antiporter, is shown in a functional study in which we found the following: (1) Mitochondria undergo passive swelling in isotonic proline solutions in a stereospecific manner. (2) Externally added L: -proline (Pro) generates a mitochondrial membrane potential (Delta Psi) with a rate depending on the transport of Pro across the mitochondrial inner membrane. (3) The dependence of the rate of generation of Delta Psi on increasing Pro concentrations exhibits hyperbolic kinetics. Proline transport is inhibited in a competitive manner by the non-penetrant thiol reagent mersalyl, but it is insensitive to the penetrant thiol reagent N-ethylmaleimide (NEM). (4) No accumulation of proline occurs inside the mitochondria as a result of the addition of proline externally, whereas the content of glutamate increases both in mitochondria and in the extramitochondrial phase. (5) Glutamate efflux from mitochondria occurs at a rate which depends on the mitochondrial transport, and it is inhibited in a non-competitive manner by NEM. The dependence of the rate of glutamate efflux on increasing proline concentration shows saturation kinetics. The physiological role of carrier-mediated transport in the regulation of proline catabolism, as well as the possible occurrence of a proline/glutamate shuttle in durum wheat seedlings mitochondria, are discussed.

The role of mitochondrial transport in energy metabolism

Since mitochondria are closed spaces in the cell, metabolite traffic across the mitochondrial membrane is needed to accomplish energy metabolism. The mitochondrial carriers play this function by uniport, symport and antiport processes. We give here a survey of about 50 transport processes catalysed by more than 30 carriers with a survey of the methods used to investigate metabolite transport in isolated mammalian mitochondria. The role of mitochondria in metabolic pathways including ammoniogenesis, amino acid metabolism, mitochondrial shuttles etc. is also reported in more detail, mainly in the light of the existence of new transport processes.

Stoichiometry and compartmentation of NADH metabolism in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, reduction of NAD(+) to NADH occurs in dissimilatory as well as in assimilatory reactions. This review discusses mechanisms for reoxidation of NADH in this yeast, with special emphasis on the metabolic compartmentation that occurs as a consequence of the impermeability of the mitochondrial inner membrane for NADH and NAD(+). At least five mechanisms of NADH reoxidation exist in S. cerevisiae. These are: (1) alcoholic fermentation; (2) glycerol production; (3) respiration of cytosolic NADH via external mitochondrial NADH dehydrogenases; (4) respiration of cytosolic NADH via the glycerol-3-phosphate shuttle; and (5) oxidation of intramitochondrial NADH via a mitochondrial 'internal' NADH dehydrogenase. Furthermore, in vivo evidence indicates that NADH redox equivalents can be shuttled across the mitochondrial inner membrane by an ethanol-acetaldehyde shuttle. Several other redox-shuttle mechanisms might occur in S. cerevisiae, including a malate-oxaloacetate shuttle, a malate-aspartate shuttle and a malate-pyruvate shuttle. Although key enzymes and transporters for these shuttles are present, there is as yet no consistent evidence for their in vivo activity. Activity of several other shuttles, including the malate-citrate and fatty acid shuttles, can be ruled out based on the absence of key enzymes or transporters. Quantitative physiological analysis of defined mutants has been important in identifying several parallel pathways for reoxidation of cytosolic and intramitochondrial NADH. The major challenge that lies ahead is to elucidate the physiological function of parallel pathways for NADH oxidation in wild-type cells, both under steady-state and transient-state conditions. This requires the development of techniques for accurate measurement of intracellular metabolite concentrations in separate metabolic compartments.

Metabolite transport in isolated yeast mitochondria: fumarate/malate and succinate/malate antiports

In this study, we investigated the metabolite permeability of isolated coupled Saccharomyces cerevisiae mitochondria. The occurrence of a fumarate/malate antiporter activity was shown. The activity differs from that of the dicarboxylate carrier (which catalyses the succinate/malate antiport) in (a) kinetics (Km and Vmax values are about 27 microM and 22 nmol min(-1) mg protein(-1) and 70 microM and 4 nmol min(-1) mg protein(-1), respectively), (b) sensitivity to inhibitors, (c) Ki for the competitive inhibitor phenylsuccinate and (d) pH profiles.

Identification of the yeast mitochondrial transporter for oxaloacetate and sulfate

Saccharomyces cerevisiae encodes 35 members of the mitochondrial carrier family, including the OAC protein. The transport specificities of some family members are known, but most are not. The function of the OAC has been revealed by overproduction in Escherichia coli, reconstitution into liposomes, and demonstration that the proteoliposomes transport malonate, oxaloacetate, sulfate, and thiosulfate. Reconstituted OAC catalyzes both unidirectional transport and exchange of substrates. In S. cerevisiae, OAC is in inner mitochondrial membranes, and deletion of its gene greatly reduces transport of oxaloacetate sulfate, thiosulfate, and malonate. Mitochondria from wild-type cells swelled in isoosmotic solutions of ammonium salts of oxaloacetate, sulfate, thiosulfate, and malonate, indicating that these anions are cotransported with protons. Overexpression of OAC in the deletion strain increased greatly the [(35)S]sulfate/sulfate and [(35)S]sulfate/oxaloacetate exchanges in proteoliposomes reconstituted with digitonin extracts of mitochondria. The main physiological role of OAC appears to be to use the proton-motive force to take up into mitochondria oxaloacetate produced from pyruvate by cytoplasmic pyruvate carboxylase.

Purification and characterization of the reconstitutively active citrate carrier from maize mitochondria

The citrate carrier from maize (Zea mays) shoot mitochondria was solubilized with Triton X-100 and purified by sequential chromatography on hydroxyapatite and hydroxyapatite/celite in the presence of cardiolipin. SDS-gel electrophoresis of the purified fraction showed a single polypeptide band with an apparent molecular mass of 31 kD. When reconstituted into liposomes, the citrate carrier catalyzed a pyridoxal 5'-P-sensitive citrate/citrate exchange. It was purified 224-fold with a recovery of 50% and a protein yield of 0.22% with respect to the mitochondrial extract. In the reconstituted system the purified citrate carrier catalyzed a first-order reaction of citrate/citrate (0.065 min-1) or citrate/malate exchange (0.075 min-1). Among the various substrates and inhibitors tested, the reconstituted protein transported citrate, cis-aconitate, isocitrate, L-malate, succinate, malonate, glutarate, alpha-ketoglutarate, oxaloacetate, and alpha-ketoadipate and was inhibited by pyridoxal 5'-P, phenylisothiocyanate, mersalyl, and p-hydroxymercuribenzoate (but not N-ethylmaleimide), 1,2, 3-benzentricarboxylate, benzylmalonate, and butylmalonate. The activation energy of the citrate/citrate exchange was 66.5 kJ/mol between 10 degrees C and 35 degrees C; the half-saturation constant (Km) for citrate was 0.65 +/- 0.05 mM and the maximal rate (Vmax) of the citrate/citrate exchange was 13.0 +/- 1.0 micromol min-1 mg-1 protein at 25 degrees C.

Cytosolic aspartate aminotransferase encoded by the AAT2 gene is targeted to the peroxisomes in oleate-grown Saccharomyces cerevisiae

Fatty acid beta-oxidation in peroxisomes requires the continued uptake of fatty acids or their derivatives into peroxisomes and export of beta-oxidation products plus oxidation of NADH to NAD. In an earlier study we provided evidence for the existence of an NAD(H) redox shuttle in which peroxisomal malate dehydrogenase plays a pivotal role. In analogy to the NAD(H)-redox-shuttle systems in mitochondria we have investigated whether a malate/aspartate shuttle is operative in peroxisomes. The results described in this paper show that peroxisomes of oleate-grown Saccharomyces cerevisiae contain aspartate aminotransferase (AAT) activity. Whereas virtually all cellular AAT activity was peroxisomal in oleate-grown cells, we found that in glucose-grown cells most of the AAT activity resided in the cytosol. We demonstrate that the gene AAT2 codes for the cytosolic and peroxisomal AAT activities. Disruption of the AAT2 gene did not affect growth on oleate. Furthermore beta-oxidation of palmitate was normal. These results indicate that AAT2 is not essential for the peroxisomal NAD(H) redox shuttle.

Effects of acute hyperammonemia in vivo on oxidative metabolism in nonsynaptic rat brain mitochondria

The effects of hyperammonemia induced in vivo by injecting rats with ammonium acetate on oxidative phosphorylation, malate-aspartate shuttle, some related enzyme activities and metabolite levels in brain mitochondria were studied ex vivo. Rats were found to be either ammonia-sensitive (showing convulsions) or ammonia-resistant (without convulsions) after intraperitoneal injection of ammonium acetate (7 mmol/kg). Ammonium acetate administration to ammonia-sensitive rats led to inhibition of State 3 rates of brain mitochondria utilizing pyruvate, glutamate, isocitrate, and succinate as substrates and to decreased respiratory control index. In brain mitochondria isolated from ammonia-resistant animals, the ammonia-induced effect on such State 3 rates was not observed. In brain mitochondria from hyperammonemic rats without convulsions, a small increase in the activity of malate dehydrogenase was observed; glutamate dehydrogenase, succinate dehydrogenase, and aspartate aminotransferase were not affected. In brain mitochondria from rats with ammonia-induced convulsions, the activities of malate dehydrogenase and succinate dehydrogenase were reduced significantly. Ammonium acetate injection to rats was associated with a 5-fold increase in the brain mitochondrial ammonium ion content and a decrease (ca. 50%) in brain mitochondrial glutamate and aspartate; brain mitochondrial malate and 2-oxoglutarate levels remained unchanged. The rate of the malate-aspartate shuttle in brain mitochondria of hyperammonemic rats was decreased by 20% as compared to corresponding rate in control rats. We conclude that acute administration of ammonium acetate induces serious disturbances in the electron-transport chain, interferences of the malate-aspartate shuttle, alterations of the levels of shuttle intermediates and inhibition of the activities of malate and succinate dehydrogenases in brain mitochondria.

Carrier-mediated transport controls hydroxyproline catabolism in heart mitochondria from spontaneously hypertensive rat

In this study we have investigated hydroxyproline transport in rat heart mitochondria and, in particular, in heart left ventricle mitochondria isolated from both spontaneously hypertensive and Wistar-Kyoto rats. Hydroxyproline uptake by mitochondria, where its catabolism takes place, occurs via a carrier-mediated process as demonstrated by the occurrence of both saturation kinetics and the inhibition shown by phenylsuccinate and the thiol reagent mersalyl. In any case, hydroxyproline transport was found to limit the rate of mitochondrial hydroxyproline catabolism. A significant change in Vmax and Km values was found in mitochondria from hypertensive/hypertrophied rats in which the Km value decreases and the Vmax value increases with respect to normotensive rats, thus accounting for the increase of hydroxyproline metabolism due to its increased concentration in a hypertrophic/hypertensive state.

Mechanisms of toxicity of 3'-azido-3'-deoxythymidine. Its interaction with adenylate kinase

Recent experiments from our laboratory have indicated that the inhibitory effect of 3'-azido-3'-deoxythymidine (AZT) on oxidative phosphorylation may occur directly, in addition to being brought about by its inhibition of mtDNA replication. We report here studies on the effect of AZT on adenylate kinase, an enzyme crucial to oxidative phosphorylation. AZT decreased the aromatic residues fluorescence of rabbit muscle adenylate kinase, indicating binding of AZT to the enzyme. Of three other enzymes studied as controls, AZT bound only to those that possessed ATP/ADP binding sites. Up to concentrations of 15 microM, AZT was a more potent effector of fluorescence quenching than were ATP, ADP, AMP, and the AZT control, deoxythymidine. AZT strongly inhibited adenylate kinase in the direction of ATP synthesis (Ki, 8 microM), the inhibition being of the partial competitive type, whereas deoxythymidine inhibition, also partially competitive, was much weaker (Ki, 90 microM). When measured in the direction of ADP synthesis, AZT failed to demonstrate any inhibition at concentrations up to 10 microM. Experiments on isolated intact rat liver mitochondria with the enzyme activity measured in both directions confirmed the isolated enzyme results. Respiratory control by these mitochondria was not affected by AZT. The finding of AZT affinity for ATP/ADP binding sites may open new avenues of approach to the study of AZT toxicity.

Malate dehydrogenase isoenzymes: cellular locations and role in the flow of metabolites between the cytoplasm and cell organelles

Malate dehydrogenases belong to the most active enzymes in glyoxysomes, mitochondria, peroxisomes, chloroplasts and the cytosol. In this review, the properties and the role of the isoenzymes in different compartments of the cell are compared, with emphasis on molecular biological aspects. Structure and function of malate dehydrogenase isoenzymes from plants, mammalian cells and ascomycetes (yeast, Neurospora) are considered. Significant information on evolutionary aspects and characterisation of functional domains of the enzymes emanates from bacterial malate and lactate dehydrogenases modified by protein engineering. The review endeavours to give up-to-date information on the biogenesis and intracellular targeting of malate dehydrogenase isoenzymes as well as enzymes cooperating with them in the flow of metabolites of a given pathway and organelle.

Ornithine/phosphate antiport in rat kidney mitochondria. Some characteristics of the process

[14C]Ornithine uptake by rat kidney mitochondria has been investigated according to the stop inhibitor method by using praseodimium chloride as an inhibitor. The existence of an ornithine/Pi exchange was found occurring with 1:1 stoichiometry. Both uptake and efflux follow first-order kinetics with a k of 2.4 min-1. Uptake increases with increasing pH. The activation energy for the process is 58.6 kJ/mol and Q10 is 2.6. Ornithine/Pi exchange is electrical and energy-dependent, as suggested by the sensitivity of the process to the ionophores valinomycin and nigericin. Measurements both of proton movement across the mitochondrial membrane and of membrane potential strongly suggest that ornithine uptake into mitochondria is driven by the electrochemical proton gradient via the dependent ornithine/Pi translocator and delta pH-dependent Pi carrier.

Carrier thiols are targets of Photofrin II photosensitization of isolated rat liver mitochondria

To gain further insight into the ability of Photofrin II to photosensitize mitochondrial translocators, and to ascertain whether mitochondrial thiols are specific targets of Photofrin II, the activity of phosphate carrier was measured in isolated rat liver mitochondria irradiated with 365 nm light in the presence of Photofrin II. Photodynamic treatment decreased the maximum rate of phosphate uptake, without changing the phosphate affinity for its own carrier. The ability of the thiol reagent mersalyl (an inhibitor of phosphate, dicarboxylate and oxodicarboxylate carriers) to protect these carriers against Photofrin II photosensitization was also tested. Protection was observed, indicating the involvement of carrier thiols in mitochondrial photosensitization.

Haematoporphyrin derivative (Photofrin II) photosensitization of isolated mitochondria: inhibition of ADP/ATP translocator

To gain further insight into the mechanism by which irradiation of mitochondria in the presence of haematoporphyrin derivative (Photofrin II) (PF II) causes impairment of mitochondrial oxidative phosphorylation, the rate of ADP/ATP exchange via the ADP/ATP translocator was measured fluorometrically is isolated rat liver mitochondria. In accord with noncompetitive inhibition, PF II photosensitization decreases the maximum rate of exchange Vmax (20.8 and 9.6 nmol ATP effluxed min-1 x mg protein in the control and after 2 min irradiation, respectively) without changing the ADP affinity for the carrier (Km = 5 microM in both cases). Comparison of the rate of oxygen uptake by mitochondria stimulated by either ADP or by the uncoupler carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone (FCCP) confirms that the adenine nucleotide carrier is a major target of photodynamic action which causes oxidative phosphorylation impairment.

Anion transport in rat brain mitochondria: fumarate uptake via the dicarboxylate carrier

Penetration of fumarate into rat brain mitochondria has been investigated, as required in brain ammoniogenesis. Mitochondria swell in ammonium fumarate and this swelling is increased by both Pi and malate. According to a carrier mediated process, fumarate translocation, which occurs in exchange with intramitochondrial malate or Pi shows saturation characteristics. By photometrically investigating the kinetics of fumarate/malate, fumarate/Pi and malate/Pi exchanges, different Km values were obtained (10, 22 and 250 microM, respectively), whereas no significant difference was found for Vmax values (40 nmol NAD(P)+ reduced/min X mg protein). This suggests that fumarate and malate share a single carrier to enter mitochondria, namely the dicarboxylate carrier. Both comparison made of the Vmax values and inhibition studies exclude a fumarate translocation via either the tricarboxylate carrier, whose occurrence in brain is here demonstrated, or oxodicarboxylate carrier. Kinetic investigation of the dicarboxylate translocator shows the existence of thiol group/s and metal ion/s at or near the substrate binding sites. The experimental findings are discussed in the light of fumarate uptake in vivo in brain ammoniogenesis.

Fumarate permeation in rat liver mitochondria: fumarate/malate and fumarate/phosphate translocators

Fumarate permeation in isolated rat liver mitochondria was demonstrated by measuring malate and phosphate efflux caused by fumarate added externally to the mitochondrial suspension. The existence of two specific fumarate translocators, fumarate/malate and fumarate/phosphate, is shown here. These carriers are distinguished in the light of different kinetic parameters (Km values are 50 microM and 150 microM, and Vmax values are 17 and 40 nmoles/min X mg mitochondrial protein, respectively) and of differing sensitivity to non-penetrant compounds. Fumarate was found to cause oxaloacetate efflux from mitochondria by means of an indirect process which involves the cooperation of both fumarate/malate and malate/oxaloacetate translocators. Results are discussed in the light of the physiological role played by fumarate translocation in both ureogenesis and aminoacid metabolism.

Oxaloacetate permeation in rat kidney mitochondria: pyruvate/oxaloacetate and malate/oxaloacetate translocators

The mechanism of oxaloacetate efflux from rat kidney mitochondria has been investigated in view of its possible role both in gluconeogenesis and in transferring cytosolic reducing equivalents into mitochondria. Thus reconstruction of the malate/oxaloacetate shuttle made possible by the oxaloacetate carrier has been made. Moreover the existence of a separate translocator able to allow a bidirectional alpha-cyanocinnamate-insensitive pyruvate/oxaloacetate exchange has been ascertained. This carrier is specific of gluconeogenetic organs in particularly of kidney, where it shows a marked affinity for pyruvate (Km = 0.45 mM and Vmax = 38 nmoles oxaloacetate effluxed/min X mg mitochondrial protein at 20 degrees C). Some features of both pyruvate/oxaloacetate and malate/oxaloacetate exchanges are also described.

Carrier mediated GABA translocation into rat brain mitochondria

GABA added to rat brain mitochondria causes oxidation of intramitochondrial NAD(P)H as well as inducing glutamate efflux from the mitochondrial matrix. The rate of NAD(P)H oxidation shows saturation characteristics, depends on GABA transport across the mitochondrial membrane and is inhibited by non-penetrant compounds and by the metal-complexing agent bathophenanthroline. These results show the existence of a specific GABA carrier. Inhibition studies strongly suggest the existence of two separate binding sites, namely the GABA binding site and the dicarboxylates binding site, as well as suggest the presence of a metal ion (ions) at GABA binding site. The occurrence of a GABA/GLUTAMATE antiport is proposed which allows a cyclical route to account for GABA synthesis and degradation in brain.

Selective permeability of rat liver mitochondria to purified malate dehydrogenase isoenzymes in vitro

1. The mitochondrial malate dehydrogenase from rat liver has been purified to a state of homogeneity as judged by starch-gel electrophoresis and the cytoplasmic isoenzyme has been obtained in a partically purified state. 2. Inhibition of the isoenzymes by sulphite has been studied. 3. In mitochondria loaded with sulphite, the catalytic activity of the (partially inhibited) internal malate dehydrogenase has been measured by addition of oxaloacetate to the suspension medium and observation of the consequent decrease in fluorescence of NADH. 4. Addition of mitochondrial malate dehydrogenase to suspensions of mitochondria loaded with sulphite resulted in an increase in the level of intramitochondrial enzymic activity as measured by the above technique. Addition of the cytoplasmic isoenzyme did not result in such an increase. 5. These results show that mitochondria in suspension are permeable to the mitochondrial malate dehydrogenase but not to the cytoplasmic isoenzyme. 6. This conclusion has been confirmed by direct measurement of a decrease of enzyme activity in solution and an increase inside the mitochondria after incubation of organelles in solutions containing mitochondrial malate dehydrogenase. No such effect was observed with the cytoplasmic isoenzyme. 7. Some features of the permeation process have been studied.

Aminooxyacetate inhibits gluconeogenesis by isolated chicken hepatocytes

Although the pathway for glucose synthesis from lactate in avian liver is not thought to involve transamination steps, inhibitors of transamination (aminooxyacetate and L-2-amino-4-methoxy-trans-3-butenoic acid) block lactate gluconeogenesis by isolated chicken hepatocytes. Inhibition of glucose synthesis from lactate by aminooxyacetate is accompanied by a large increase in the lactate-to-pyruvate ratio. Oleate largely relieves inhibition by aminooxyacetate and lowers the lactate-to-pyruvate ratio. In parallel studies with rat hepatocytes, oleate did not overcome aminooxyacetate inhibition of glucose synthesis. The ratios of lactate used to glucose formed were greater than 2 with both rat and chicken hepatocytes, were increased by aminooxyacetate, and were restored toward 2 by oleate. Thus, in the absence of oleate, lactate is oxidized to provide the energy needed to meet the metabolic demand of chicken hepatocytes. Excess cytosolic reducing equivalents generated by the oxidation of lactate to pyruvate are transferred from the cytosol to the mitosol by the malate-aspartate shuttle. Aminooxyacetate inhibits the shuttle and, consequently, glucose synthesis for want of pyruvate.

Effect of bicarbonate and oxaloacetate on malate oxidation by spinach leaf mitochondria

Mitochondria isolated from spinach leaves oxidized malate by both a NAD+-linked malic enzyme and malate dehydrogenase. In the presence of sodium arsenite the accumuation of oxaloacetate and pyruvate during malate oxidation was strongly dependent on the malate concentration, the pH in the reaction medium and the metabolic state condition. Bicarbonate, especially at alkaline pH, inhibited the decarboxylation of malate by the NAD+-linked malic enzyme in vitro and in vivo. Analysis of the reaction products showed that with 15 mM bicarbonate, spinach leaf mitochondria excreted almost exclusively oxaloacetate. The inhibition by oxaloacetate of malate oxidation by spinach leaf mitochondria was strongly dependent on malate concentration, the pH in the reaction medium and on the metabolic state condition. The data were interpreted as indicating that: (a) the concentration of oxaloacetate on both sides of the inner mitochondrial membrane governed the efflux and influx of oxaloacetate; (b) the NAD+/NADH ratio played an important role in regulating malate oxidation in plant mitochondria; (c) both enzymes (malate dehydrogenase and NAD+-linked malic enzyme) were competing at the level of the pyridine nucleotide pool, and (d) the NAD+-linked malic enzyme provided NADH for the reversal of the reaction catalyzed by the malate dehydrogenase.

Aerobic lactate synthesis by cardiac muscle

Cardiac lactate production under aerobic conditions is absolutely dependent upon the availability of extracellular pyruvate. In the steady state, aerobic lactate output is largely independent of cardiac work load, but increases slightly when octanoate is included in addition to pyruvate in the perfusion fluid. Transient episodes of supra-normal lactate production are seen after sudden increases in cardiac work output, and also after transitions from octanoate to pyruvate in the perfusion media. These pulses of lactate production are invariably associated with the slow activation of pyruvate dehydrogenase in response to a sudden change in cardiac metabolic state, and they are abolished by pre-perfusion with dichloracetate, which converts pyruvate dehydrogenase into the fully active form. A second, additional component of the lactate pulses is sensitive to pre-perfusion with the transaminase inhibitor aminooxyacetate. The size of the second component is markedly dependent upon the precise protocol adopted for the experiment, and these variations suggest that the second component is associated with a major redistribution of cardiac Krebs' cycle intermediates and amino acids following the initial exposure to pyruvate-containing media. Steadystate aerobic lactate production is insensitive to both dichloroacetate and aminooxyacetate, and is thought to result from a direct exchange of malate for oxaloacetate across the heart mitochondrial membranes.

The reduction of diamide by rat liver mitochondria and the role of glutathione

Diamide is reduced by mitochondria utilizing endogenous substrates with Vmax. 20nmol/min per mg of protein and Km 75micrometer. The reaction is inhibited by: (a) thiol-blocking reagents (N-ethylmaleimide, p-hydroxymercuribenzoate, mersalyl and 2,6-dichlorophenol-indophenol);(b) respiratory inhibitors (arsenicals, malonate and antimycin, but not cyanide or oligomycin; inhibition by antimycin is reversed by ATP); (c) uncouplers (carbonyl cyanide p-trifluoromethoxyphenylhydrazone, 2,4-dinitrophenol and valinomycin with K+; inhibition by the first of these uncouplers is not reversed by cyanide); (d) reagents affecting energy conservation (Ca2+, increasing pH, phosphate; phosphate inhibition is augmented by catalytic ADP or ATP and augmentation is abolished by respiratory inhibitors). Concentrations of mitochondrial glutathione are high when diamide reduction is uninhibited, but low after adding one of the above inhibitors such that the reduction rate is roughly proportional to the glutathione concentration. Endogenous ATP concentrations are lower in the presence of diamide than without, but the difference is abolished by respiratory inhibitors. With oligomycin added, however, ATP concentrations are higher in the presence of diamide and this positive increment is decreased by antimycin, N-ethylmaleimide and malonate. In the presence of diamide and an uncoupler, the mitochondrial glutathione content does not fall if various reducible substrates are present, although the inhibition of diamide reduction is not relieved. Some of these substrates prevent the fall in reduced glutathione concentration found with diamide and phosphate. They also relieve the inhibition of diamide reduction and the relief is sensitive to butylmalonate. The inhibition of diamide reduction by N-ethylmaleimide, mersalyl or p-hydroxymercuribenzoate is not relieved by reducible substrates, but the latter mitigate the fall in the concentration of glutathione. Inhibitors of carriers of tricarboxylic acid-cycle intermediates also inhibit reduction of diamide. The reduced glutathione concentration remains high when they are added singly, but falls when two of them are combined. It is proposed that diamide may enter the matrix as a protonated adduct formed with the thiol groups of mitochondrial carriers and then be reduced in the matrix by glutathione, which is regenerated via NADH, energy-dependent transhydrogenase and NADP+-specific glutathione reductase. Some of the high-energy equivalents required for the transhydrogeneration may be generated by the substrate phosphorylation step of the tricarboxylic acid cycle.

Selective permeability of rat liver mitochondria to purified aspartate aminotransferases in vitro

1. A method was devised to allow determination of intramitochondrial aspartate amino-transferase activity in suspensions of intact mitochondria. 2. Addition of purified rat liver mitochondrial aspartate aminotransferase to suspensions of rat liver mitochondria caused an apparent increase in the intramitochondrial enzyme activity. No increase was observed when the mitochondria were preincubated with the purified cytoplasmic isoenzyme. 3. These results suggest that mitochondrial aspartate aminotransferase, but not the cytoplasmic isoenzyme, is able to pass from solution into the matrix of intact rat liver mitochondria in vitro. 4. This system may provide a model for studies of the little-understood processes by which cytoplasmically synthesized components are incorporated into mitochondria in vivo.

Two cases of phosphoenolpyruvate carboxykinase deficiency

Two children are described who suffered from hypoglycemia and liver impairment. Assays of gluconeogenic enzymes in liver samples taken immediately after death demonstrated a deficiency of phosphoenolpyruvate carboxykinase, a key enzyme of gluconeogenesis. Post mortem examination demonstrated massive fat deposition in liver and kidney and to a lesser extent in other tissues. The fatty changes in liver and kidney could be explained by the absence of phosphoenolpyruvate carboxykinase, which would cause an alteration in the mitochondrial-cytosolic processes related to gluconeogenesis.

The role of intermediates in mitochondrial fatty acid oxidation

1. Rat liver mitochondria oxidizing [16-14C]palmitoylcarnitine accumulate saturated long-chain thiester intermediates which may be detected by radio-g.1.c.2. Time-courses of intermediate accumulation display no product-precursor relationships and the end product, measured as [14C]citrate, is produced without a detectable initial lag. 3. A short pulse of [16-14C]palmitoylcarnitine followed by unlabelled palmitoylcarnitine showed that the observed intermediates(at least in the greater part)were not the direct precursors of [14C]citrate. 4. The quantity of saturated intermediates depended on the total accumulated flux of acyl units through the pathway provided that some mitochondrial CoA and unused substrate remained. 5. In the presence of rotenone and carnitine, 2-unsaturated, 3-unsaturated and 3-hydroxy intermediates were formed as well as saturated intermediates...