Transport of 2-oxoisocaproate in isolated hepatocytes and liver mitochondria.

The transport of 2-oxoisocaproate into isolated hepatocytes and liver mitochondria of rat was studied using [U-14C]2-oxoisocaproate and the silicone oil filtration procedure. 2-Oxoisocaproate uptake by hepatocytes was composed of: rapid adsorption, unmediated diffusion and carrier-mediated transport. The carrier-mediated transport was strongly inhibited by 4,4'-diisothiocyano-2,2'-stilbenedisulphonic acid and p-chloromercuribenzoate, was less sensitive to alpha-cyano-4-hydroxycinnamate and insensitive to p-chloromercuriphenylsulphonate. Other 2-oxo acids: pyruvate, 2-oxoisovalerate and 2-oxo-3-methylvalerate, were also inhibitory. The kinetic parameters of the carrier-mediated transport were Km 30.6 mM and Vmax 23.4 nmol/min per mg wet wt, at 37 degrees C. It is concluded that at its low, physiological, concentration, 2-oxoisocaproate penetrates the hepatocyte membrane mainly by unmediated diffusion. The uptake of 2-oxoisocaproate by isolated liver mitochondria was partly inhibited by alpha-cyano-4-hydroxycinnamate, the inhibitor of mitochondrial monocarboxylate carrier. The remaining uptake was linearly dependent on 2-oxoisocaproate concentration and represented unmediated diffusion. The carrier-mediated transport exhibited the following kinetic parameters: Km 0.47 mM, Vmax 1.0 nmol/min per mg protein at 6 degrees C; and Km 0.075 mM and Vmax about 8 nmol/min per mg protein at 37 degrees C.

Purification and functional characterisation of the pyruvate (monocarboxylate) carrier from baker's yeast mitochondria (Saccharomyces cerevisiae).

Isolated yeast mitochondria were subjected to solubilization by Triton X-114 and the detergent extract was subsequently chromatrographed on dry hydroxyapatite. Purification of the yeast monocarboxylate (pyruvate) carrier was achieved by affinity chromatography on immobilized 2-cyano-4-hydroxycinnamate, as described previously for bovine heart mitochondria (Bolli, R., Nalecz K.A. and Azzi, A. (1989) J. Biol. Chem. 264 18024-18030). The final preparation contained two polypeptides of apparent molecular mass 26 and 50 kDa. The yeast carrier appeared to be less abundant, but more active, than the analogous protein from higher eukaryotes. The carrier was able to catalyse the pyruvate / pyruvate and pyruvate / acetoacetate exchange reactions, both reactions being sensitive to cyanocinnamate and its derivatives, to phenylpyruvate and to mersalyl and p-chloromercuribenzoate. In the pyruvate / acetoacetate exchange reaction (200 mM internal acetoacetate, enzymatic assay), the Km value for external pyruvate was found to be 0.8 mM and the Vmax 135 mumol/min per mg protein. Among other substrates of the yeast carrier, all transported with similar affinity and identical maximal velocity against acetoacetate, we identified 2-oxoisocaproate, 2-oxoisovalerate and 2-oxo-3-methylvalerate. Lactate was not translocated by this carrier with a measurable rate, neither were di- or tricarboxylates.

The role of subunit III in bovine cytochrome c oxidase. Comparison between native, subunit III-depleted and Paracoccus denitrificans enzymes.

In order to obtain information on the role of subunit III in the function and aggregation state of cytochrome c oxidase, the kinetics of ferrocytochrome c oxidation by the bovine cytochrome c oxidase depleted of its subunit III were studied and compared with those of the oxidase isolated from P. denitrificans which contains only two subunits. The aggregation state of both enzymes dispersed in dodecyl maltoside was also compared. The two-subunit oxidase from P. denitrificans gave linear Eadie-Hofstee plots and the enzyme resulted to be monomeric (Mr = 82 000) both, in gel filtration and sucrose gradient centrifugation studies. The bovine heart subunit III depleted enzyme, under conditions when the P. denitrificans cytochrome c oxidase was in the form of monomers, was found to be dimeric by sucrose gradient centrifugation analysis. At lower enzyme concentrations monomers were, however, detected by gel filtration. Depletion of subunit III was accompanied by the loss of small polypeptides (VIa, VIb and VIIa) and of almost all phospholipid (1-2 molecules were left per molecule of enzyme). The electron-transfer activity of the subunit III-depleted enzyme showed a monophasic Eadie-Hofstee plot, which upon addition of phospholipids became non-linear, similar to that of the control bovine cytochrome c oxidase. One of the roles of subunit III may be that of stabilising the dimers of cytochrome c oxidase. Lack of this subunit and loss of phospholipid is accompanied by a change in the kinetics of electron transfer, which might be the consequence of enzyme monomerisation.

Significance and redox state of SH groups in pyruvate carrier isolated from bovine heart mitochondria.

The role and properties of -SH groups of purified pyruvate (monocarboxylate) carrier were investigated. After isolation, this protein has all -SH groups in the oxidized state. Upon reduction, the carrier can be labelled with eosin-5-maleimide. The shift in apparent Mr after the labelling points to the presence of at least two cysteine residues. Pyruvate uptake in the reconstituted system is inhibited by both permeable (eosin-5-maleimide at 1 mM concentration) and impermeable (mersalyl, p-chloromercuribenzoate) -SH group reagents. Phenylarsine oxide inhibits pyruvate transport only slightly (20%), but the inhibition is enhanced after preincubation with the substrate.

The monocarboxylate carrier from bovine heart mitochondria: partial purification and its substrate-transporting properties in a reconstituted system.

The monocarboxylate (pyruvate) carrier from bovine heart mitochondria was extracted from submitochondrial particles with Triton X-114 in the presence of cardiolipin. By a single hydroxylapatite chromatography step a 125-fold purification of the carrier protein could be achieved. High pyruvate/pyruvate-exchange activity was recovered, when the protein was reconstituted into phospholipid vesicles. No transport activity was observed, when the isolation occurred in the absence of phospholipids. The 2-cyano-4-hydroxycinnamate sensitive pyruvate exchange reaction was strongly temperature sensitive and dependent on the amount of protein reconstituted. Other 2-ketoacids caused competitive inhibition of the pyruvate uptake. Inhibitors of other mitochondrial carries, however, had very low or no effect on the monocarboxylate exchange. The influence of different -SH group reagents on the measured pyruvate/pyruvate-exchange in the reconstituted system was similar to the one observed with intact mitochondria. It is concluded that the described procedures for extraction, purification and reconstitution of the mitochondrial monocarboxylate carrier conserved the functional properties of the protein.

Extraction, partial purification and functional reconstitution of two mitochondrial carriers transporting keto acids: 2-oxoglutarate and pyruvate.

Bovine heart submitochondrial particles were treated with a medium containing Triton X-114 and cardiolipin. The extract was subjected to hydroxyapatite chromatography. Only a few major polypeptides of similar molecular masses were found in the eluate, as shown by electrophoresis in an SDS-polyacrylamide gel stained with silver. The eluate was reconstituted into liposomes and was shown to catalyse two different transport activities: 2-oxoglutarate-2-oxoglutarate exchange sensitive to phthalonate and phenylsuccinate and pyruvate-pyruvate exchange sensitive to 2-cyano-4-hydroxycinnamate. Since both activities were found to have characteristics similar to those described for intact mitochondria, it was concluded that at least two of the polypeptides found in the hydroxyapatite eluate correspond to the two mitochondrial carriers.

The interconversion between monomeric and dimeric bovine heart cytochrome c oxidase.

Monomers and dimers of bovine heart cytochrome c oxidase (EC 1.9.3.1.) were separated by gel filtration chromatography on Ultrogel AcA 34 or by sucrose gradient centrifugation. Factors influencing the interconversion of the two aggregation states of this enzyme were analyzed. At very low ionic strength, in the presence of dodecyl maltoside, monomers were the main species. Salts appeared to stabilize the dimeric form, divalent cations being more efficient than monovalent. High enzyme concentrations favoured the formation of dimers, also at low ionic strength. The type of detergent had a strong influence on the monomer-dimer interconversion; in Triton X-100 and dodecyl maltoside (at high ionic strength) cytochrome c oxidase was homogenously dispersed in its dimeric form, while in Tween-80 gel filtration showed only large particles eluting in the void volume. In cholate monomers and aggregates were observed but no dimers. The aggregation state had an influence on the steady state kinetics of the ferrocytochrome c oxidase activity. Monomers showed linear Eadie-Hofstee plots, whilst the dimeric and aggregated enzyme gave nonlinear Eadie-Hofstee plots. Ionic strength, enzyme concentration and type of detergent were affecting the enzyme's kinetics in a way consistent with the molecular form obtained by the gel filtration or sedimentation analysis. The data support a negative cooperative mechanism for the interaction of cytochrome c with the dimeric enzyme, as proposed earlier (K.A. Nalecz et al., (1983) Biochem. Biophys. Res. Commun., 114, 822-828).

Effect of externally added carnitine on the synthesis of acetylcholine in rat cerebral cortex cells.

Acetylcholine synthesis from radiolabelled glucose was monitored in cerebral cortex cells isolated from brains of suckling and adult rats. Acetylcholine synthesis was found much higher in suckling animals, both in the absence and presence of acetylcholinesterase (acetylcholine hydrolase, EC 3.1.1.7) inhibitor, paraoxon. Together with choline (20 microM), carnitine was found to stimulate acetylcholine synthesis in a synergistic way in cortex cells from adult rats (18%). Choline, however, was incapable of reversing an inhibitory effect exerted by carnitine on acetylcholine synthesis in cortex cells from suckling animals. Distribution of carnitine derivatives was found significantly different in the cells from young and old animals, the content of acetylcarnitine decreased with age with a corresponding increase of free carnitine. The observed differences in carnitine effect on acetylcholine synthesis suggested that high acetylcarnitine in cells capable of beta-oxidation might be correlated with the lower level of acetylcholine synthesis.

Modulation of absence seizures by branched-chain amino acids: correlation with brain amino acid concentrations.

The occurrence of absence seizures might be due to a disturbance of the balance between excitatory and inhibitory neurotransmissions in the thalamo-cortical loop. In this study, we explored the consequences of buffering the glutamate content of brain cells on the occurrence and duration of seizures in Genetic Absence Epilepsy Rats from Strasbourg (GAERS), a genetic model of generalized non-convulsive epilepsy. Branched-chain amino acids (BCAAs) and alpha-ketoisocaproate (alpha-KIC), the ketoacid of leucine were repeatedly shown to have a critical role in brain glutamate metabolism. Thus, GAERS were injected by intraperitoneal (i.p.) or intracerebroventricular (i.c.v.) route with these compounds, then the effects on seizures were evaluated on the electroencephalographic recording. We also measured the concentration of amino acids in thalamus and cortex after an i.p. injection of leucine or alpha-KIC. Intracerebroventricular injections of leucine or alpha-KIC did not influence the occurrence of seizures, possibly because the substances reached only the cortex. BCAAs and alpha-KIC, injected intraperitoneally, increased the number of seizures whereas they had only a slight effect on their duration. Leucine and alpha-KIC decreased the concentration of glutamate in thalamus and cortex without affecting GABA concentrations. Thus, BCAAs and alpha-KIC, by decreasing the effects of glutamatergic neurotransmission could facilitate those of GABAergic neurotransmission, which is known to increase the occurrence of seizures in GAERS.

Modulation of pentylenetetrazol-induced seizure activity by branched-chain amino acids and alpha-ketoisocaproate.

Branched-chain amino acids, and mainly leucine act as nitrogen donors in the cerebral glutamate-glutamine cycle, thereby reducing brain excitability. Rats equipped with cortical electrodes received 300 mg/kg of leucine, isoleucine, valine or the ketoacid of leucine, alpha-ketoisocaproate at 2 h before the induction of seizures by 40 mg/kg pentylenetetrazol. Control groups received saline or a commercial mixture of amino acids, Vamine(R). Leucine and isoleucine increased the latency to absence-like and tonic-clonic seizures but did not influence the duration of the tonic-clonic seizure. Vamine(R), valine and alpha-ketoisocaproate had no effect. These data are consistent with the role of leucine in buffering brain glutamate concentration.

Monocarboxylate carrier from bovine heart mitochondria: reconstitution in a functionally active state.

A method has been developed for functional reconstitution of the isolated pyruvate carrier. Optimal conditions were reached after removal of deoxycholate with 27 mg Triton X-100 per gram of moist Amberlite XAD-2 beads and 7 micrograms protein/mg of phospholipid. Uptake of pyruvate into proteoliposomes and sensitivity of the carrier towards inhibitors resemble those in intact mitochondria.

Carnitine--a known compound, a novel function in neural cells.

Carnitine (4-N-trimethylammonium-3-hydroxybutyric acid) seems to fulfill in the brain a different role than in peripheral tissues. Carnitine is accumulated by neural cells in a sodium-dependent way. The existence of a novel transporter in plasma membrane, specific to compounds with a polar group in the beta-position with respect to carboxyl group, has been postulated. The presence of a carnitine carrier in the inner mitochondrial membrane has been proven and the protein has been purified. It is postulated that its major role in adult brain would be translocation of acetyl moieties from mitochondria into the cytoplasm for acetylcholine synthesis. The latter process is stimulated by carnitine and choline in a synergistic way in cells utilizing glucose as the main energetic substrate. Carnitine metabolism in neural cells leads to accumulation of different acyl derivatives of carnitine. Palmitoylcarnitine can influence directly the activity of protein kinase C. An involvement of carnitine in a decrease of palmitate pool used for palmitoylation of regulatory proteins has been postulated.

Mechanism of carnitine transport catalyzed by carnitine carrier from rat brain mitochondria.

The transport mechanism of reconstituted carnitine carrier purified from rat brain mitochondria was studied kinetically. Short and medium chain acyl carnitine derivatives had much higher affinity to the carnitine carrier in comparison with long chain acyl carnitine derivatives, therefore both homologous (carnitine/carnitine) and heterologous (carnitine/acetylcarnitine) antiports were analysed. A complete set of half-saturation constants was established for various substrate concentrations on both the external and the internal side of the membrane. Bisubstrate initial velocity analyses of the exchange reaction resulted in a kinetic pattern which is consistent with a sequential antiport mechanism. This type of mechanism implies formation of a ternary complex of the carrier with one internal and one external substrate molecule before the transport reaction occurs.

Effect of palmitoylcarnitine on the cellular differentiation, proliferation and protein kinase C activity in neuroblastoma nb-2a cells.

Palmitoylcarnitine is synthesized through the action of palmitoylcarnitine transferase I--an enzyme specifically inhibited by etomoxir. An increase of the intracellular content of palmitoylcarnitine in neuroblastoma NB-2a cells after administration of carnitine was correlated with an inhibition of cell proliferation and a concomitant promotion of differentiation processes. The activity of protein kinase C was measured in vivo, with cells permeabilized through the use of streptolysin O and a peptide substrate. Palmitoylcarnitine inhibited the phorbol ester stimulated reaction of the peptide phosphorylation in a concentration dependent way. The degree of protein kinase C inhibition was correlated with intracellular increase of the palmitoylcarnitine content, pointing to this compound as a natural modulator of protein kinase C activity.

[The 1999 Nobel Prize for physiology or medicine]. / Nagroda Nobla 1999 w dziedzinie fizjologii lub medycyny.

It has been awarded to Günter Blobel, for a so-called signal hypothesis. In this research Günter Blobel has proven that information about the final localization of a protein in the cell is given in its sequence. Such a targeting signal can be either cut off (presequence) or can be localized within the final sequence of a protein. The mechanisms responsible for secretion, the movement of proteins to or from nucleus, and guiding of proteins to mitochondria, chloroplasts and peroxisomes have been described. In particular, proteins involved in interactions with the signal sequences were reviewed. The channels responsible for transfer of polipeptides through the membranes were also presented. A contribution of Günter Blobel and his coworkers in the foundation and development of a new field of research on protein guidance has been emphasized.

The aggregation state of bovine heart cytochrome c oxidase and its kinetics in monomeric and dimeric form.

The monomeric and dimeric forms of bovine cytochrome c oxidase (EC 1.9.3.1) were obtained from gel filtration chromatography on Ultrogel AcA 34 and analyzed. Both species contained all 12-13 subunits described for this enzyme. In the dimer 320 molecules [3H]dodecyl-beta-D-maltoside were bound per heme aa3 and in the monomer 360 molecules per heme aa3. The monomers contained 10 mol of tightly bound phospholipid/mol heme aa3 and the dimers 14. Sedimentation coefficients of 15.5-18 S for the dimer and 9.6 S for the monomer were calculated from sucrose density centrifugation analysis and analytical centrifugation. By the laser beam light-scattering technique a Stokes radius of 70 A for the dimeric detergent-lipid-protein complex was measured. From those parameters and the densitometric determined partial specific volumes of the detergent and the enzyme, the molecular weights of 400,000 for the protein moiety of the dimer and 170,000-200,000 for the monomer were calculated. Under very low ionic strength conditions the monomer/dimer equilibrium was found to be dependent on the protein concentration. At low enzyme concentrations (10(-9) M) monomers were predominant, whereas at concentrations above 5 X 10(-6) M the amounts of dimers and higher aggregates were more represented. The cytochrome c oxidase activity, measured spectrophotometrically and analyzed by Eadie-Hofstee plot, was biphasic as a function of cytochrome c concentration for the dimeric enzyme. Pure monomers gave monophasic kinetics. The data, fitting with a homotropic negative cooperative mechanism for the dimer of cytochrome c oxidase, are discussed and compared with other described mechanisms.