The effects of ammonium, inorganic phosphate and potassium ions on the activity of phosphofructokinases from muscle and nervous tissues of vertebrates and invertebrates

Functional Characterization of the SHIP1-Domains Regarding Their Contribution to Inositol 5-Phosphatase Activity

The Src homology 2 domain-containing inositol 5-phosphatase 1 (SHIP1) is a multidomain protein consisting of two protein-protein interaction domains, the Src homology 2 (SH2) domain, and the proline-rich region (PRR), as well as three phosphoinositide-binding domains, the pleckstrin homology-like (PHL) domain, the 5-phosphatase (5PPase) domain, and the C2 domain. SHIP1 is commonly known for its involvement in the regulation of the PI3K/AKT signaling pathway by dephosphorylation of phosphatidylinositol-3,4,5-trisphosphate (PtdIns(3,4,5)P3) at the D5 position of the inositol ring. However, the functional role of each domain of SHIP1 for the regulation of its enzymatic activity is not well understood. To determine the contribution of the individual domains to catalytic activity, the full-length protein was compared with truncated constructs lacking one or more domain(s), regarding the substrate turnover (kcat) and catalytic efficiency (kcat/Km) towards ci8-PtdIns(3,4,5)P3. With this approach, it was possible to verify the allosteric activation of SHIP1 mediated by the C2 domain as described previously, while the PHL domain seemed instead to have a negative effect regarding catalytic efficiency. The full-length SHIP1 clearly displayed the highest turnover and the second-highest catalytic efficiency, showing the role of the SH2 domain and PRR not only in protein-protein interactions but also in catalysis. The SH2 domain increased substrate turnover but negatively affected catalytic efficiency. The linker between the SH2 and the PHL domains decreased the turnover number but positively influenced the catalytic efficiency. The PRR increased both the substrate turnover and the protein's catalytic efficiency. The regression analysis of the Michaelis-Menten graph revealed SHIP1 to be an allosteric enzyme, with the PRR and the linker being the most involved domains in that regard. In summary, our data indicate a complex regulation of the enzymatic activity of SHIP1 by its individual domains. While the C2 domain and PRR at the carboxy-terminus have a positive effect on enzymatic activity, the SH2 and PHL domain at the amino-terminus inhibit catalytic efficiency.

Electro-metabolic coupling in atrial fibrillation: A deeper understanding of the metabolic driver

Atrial fibrillation (AF), the most common sustained heart rhythm abnormality, disrupts the normal link between electrical activity and atrial muscle contraction; this disruption is termed "excitation-contraction uncoupling". It weakens atrial contractions and contributes to the development and persistence of AF. In addition to electrical dysfunction, AF is increasingly recognized as a metabolic disorder. Metabolic remodeling may reportedly precede electrophysiological, contractile, and structural changes in AF. Both clinical observations and experimental studies have underscored the critical importance of metabolic homeostasis, and its disturbance is considered a key initial factor in the development of AF. Research in this field has progressed, and a consensus has emerged that metabolic status (energy flux) and electrophysiological signaling (ion flux) are interactively regulated, highlighting the concept of "electro-metabolic coupling." Their uncoupling or decompensation constitutes a common pathological basis of AF. Despite growing recognition of the importance of metabolic balance, the role of electro-metabolic coupling in AF remains unclear. Thus, this review aimed to discuss 1) a comprehensive understanding of electro-metabolic alterations post-AF, 2) the pivotal role of metabolic homeostasis in AF pathogenesis, and 3) the mutual regulation of electro-metabolic signaling, along with potential therapeutic strategies targeting these imbalances.

Lactate oxidation in Paracoccus denitrificans

Paracoccus denitrificans has a classical cytochrome-dependent electron transport chain and two alternative oxidases. The classical transport chain is very similar to that in eukaryotic mitochondria. Thus, P. denitrificans can serve as a model of the mammalian mitochondrion that may be more tractable in elucidating mechanisms of regulation of energy production than are mitochondria. In a previous publication we reported detailed studies on respiration in P. denitrificans grown aerobically on glucose or malate. We noted that P. denitrificans has large stores of lactate under various growth conditions. This is surprising because P. denitrificans lacks an NAD+-dependent lactate dehydrogenase. The aim of this study was to investigate the mechanisms of lactate oxidation in P. denitrificans. We found that the bacterium grows well on either d-lactate or l-lactate. Growth on lactate supported a rate of maximum respiration that was equal to that of cells grown on glucose or malate. We report proteomic, metabolomic, and biochemical studies that establish that the metabolism of lactate by P. denitrificans is mediated by two non-NAD+-dependent lactate dehydrogenases. One prefers d-lactate over l-lactate (D-iLDH) and the other prefers l-lactate (L-iLDH). We cloned and produced the D-iLDH and characterized it. The Km for d-lactate was 34 μM, and for l-lactate it was 3.7 mM. Pyruvate was not a substrate, rendering the reaction unidirectional with lactate being converted to pyruvate for entry into the TCA cycle. The intracellular lactate was ∼14 mM such that both isomers could be metabolized by the enzyme. The enzyme has 1 FAD per molecule and utilizes a quinone rather than NAD + as an electron acceptor. D-iLDH provides a direct entry of lactate reducing equivalents into the cytochrome chain, potentially explaining the high respiratory capacity of P. denitrificans in the presence of lactate.

Ammonia Inhalants Enhance Psychophysiological Responses and Performance During Repeated High Intensity Exercise

Purpose: Ammonia inhalants (NH3) are anecdotally used in competition by athletes for their purported stimulant effects. However, evidence on the efficacy of NH3 is conflicting, and little to no studies to date have investigated its effect on repeated exercise. The purpose of this study was to examine the effects of NH3 on psychophysiological responses and performance during repeated high-intensity exercise. Methods: In a counterbalanced crossover design, physically active females completed two repeated high-intensity sprint trials with a different treatment: Control (CON; water) or Ammonia Inhalants (NH3; 0.33 cc). For each trial, participants completed 3 × 15s Wingate anaerobic tests (WAnT) separated by 2 min of active recovery. Prior to each WAnT, participants took a single 3-s inhale of the corresponding treatment. After the succession of each WAnT, heart rate (HR) and rate of perceived exertion (RPE) were documented. Subjective feelings of alertness and "psyched up" energy were measured using a visual analog scale. Trials were separated by at least 48 hr. Results: The results indicate that over the WAnTs, mean power (p = .017) and peak power (p = .006) were significantly higher with NH3 compared to CON despite a lack of changes in fatigue index (p = .928). HR (p = .101) and RPE (p = .897) were not different with varying treatments. Perceived alertness (p = .010) and psyched-up energy (p = .002) were significantly higher with NH3 versus CON. Conclusion: These findings provide empirical support for the use of NH3 to improve repeated high-intensity exercise performance in females that may be underpinned by alterations in subjective alertness and energy.

Energy homeostasis is a conserved process: Evidence from Paracoccus denitrificans' response to acute changes in energy demand

Paracoccus denitrificans is a model organism for the study of oxidative phosphorylation. We demonstrate a very high respiratory capacity compared to mitochondria when normalizing to cytochrome aa3 content even in the absence of alternative terminal oxidases. To gain insight into conserved mechanisms of energy homeostasis, we characterized the metabolic response to K+ reintroduction. A rapid 3-4-fold increase in respiration occurred before substantial cellular K+ accumulation followed by a sustained increase of up to 6-fold that persisted after net K+ uptake stopped. Proton motive force (Δp) was slightly higher upon addition of K+ with ΔpH increasing and compensating for membrane potential (ΔΨ) depolarization. Blocking the F0F1-ATP synthase (Complex V) with venturicidin revealed that the initial K+-dependent respiratory activation was primarily due to K+ influx. However, the ability to sustain an increased respiration rate was partially dependent on Complex V activity. The 6-fold stimulation of respiration by K+ resulted in a small net reduction of most cytochromes, different from the pattern observed with chemical uncoupling and consistent with balanced input and utilization of reducing equivalents. Metabolomics showed increases in glycolytic and TCA cycle intermediates together with a decrease in basic amino acids, suggesting an increased nitrogen mobilization upon K+ replenishment. ATP and GTP concentrations increased after K+ addition, indicating a net increase in cellular potential energy. Thus, K+ stimulates energy generation and utilization resulting in an almost constant Δp and increased high-energy phosphates during large acute and steady state changes in respiration. The specific energy consuming processes and signaling events associated with this simultaneous activation of work and metabolism in P. denitrificans remain unknown. Nevertheless, this homeostatic behavior is very similar to that observed in mitochondria in tissues when cellular energy requirements increase. We conclude that the regulation of energy generation and utilization to maintain homeostasis is conserved across the prokaryote/eukaryote boundary.

FAMIN Is a Multifunctional Purine Enzyme Enabling the Purine Nucleotide Cycle

Mutations in FAMIN cause arthritis and inflammatory bowel disease in early childhood, and a common genetic variant increases the risk for Crohn's disease and leprosy. We developed an unbiased liquid chromatography-mass spectrometry screen for enzymatic activity of this orphan protein. We report that FAMIN phosphorolytically cleaves adenosine into adenine and ribose-1-phosphate. Such activity was considered absent from eukaryotic metabolism. FAMIN and its prokaryotic orthologs additionally have adenosine deaminase, purine nucleoside phosphorylase, and S-methyl-5′-thioadenosine phosphorylase activity, hence, combine activities of the namesake enzymes of central purine metabolism. FAMIN enables in macrophages a purine nucleotide cycle (PNC) between adenosine and inosine monophosphate and adenylosuccinate, which consumes aspartate and releases fumarate in a manner involving fatty acid oxidation and ATP-citrate lyase activity. This macrophage PNC synchronizes mitochondrial activity with glycolysis by balancing electron transfer to mitochondria, thereby supporting glycolytic activity and promoting oxidative phosphorylation and mitochondrial H⁺ and phosphate recycling.

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An unbiased LC-MS screen reveals FAMIN as a purine nucleoside enzyme

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FAMIN combines adenosine phosphorylase with ADA-, PNP-, and MTAP-like activities

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FAMIN enables a purine nucleotide cycle (PNC) preventing cytoplasmic acidification

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The FAMIN-dependent PNC balances the glycolysis-mitochondrial redox interface

Neuronal lactate levels depend on glia-derived lactate during high brain activity in Drosophila

Lactate/pyruvate transport between glial cells and neurons is thought to play an important role in how brain cells sustain the high-energy demand that neuronal activity requires. However, the in vivo mechanisms and characteristics that underlie the transport of monocarboxylates are poorly described. Here, we use Drosophila expressing genetically encoded FRET sensors to provide an ex vivo characterization of the transport of monocarboxylates in motor neurons and glial cells from the larval ventral nerve cord. We show that lactate/pyruvate transport in glial cells is coupled to protons and is more efficient than in neurons. Glial cells maintain higher levels of intracellular lactate generating a positive gradient toward neurons. Interestingly, during high neuronal activity, raised lactate in motor neurons is dependent on transfer from glial cells mediated in part by the previously described monocarboxylate transporter Chaski, providing support for in vivo glia-to-neuron lactate shuttling during neuronal activity.

Fermentation Revisited: How Do Microorganisms Survive Under Energy-Limited Conditions?

During fermentation F_OF₁ hydrolyzes ATP, coupling proton transport to proton-motive force (pmf) generation. Despite that, pmf generated by ATP hydrolysis does not satisfy the energy budget of a fermenting cell. However, pmf can also be generated by extrusion of weak organic acids such as lactate and by hydrogen cycling catalyzed by hydrogenases (Hyds). Here we highlight recent advances in our understanding of how the transport of weak organic acids and enzymes contributes to pmf generation during fermentation. The potential impact of these processes on metabolism and energy conservation during microbial fermentation have been overlooked and they not only expand on Mitchell's chemiosmotic theory but also are of relevance to the fields of microbial biochemistry and human and animal health.

Exercise testing-based algorithms to diagnose McArdle disease and MAD defects

OBJECTIVE

As exercise intolerance and exercise-induced myalgia are commonly encountered in metabolic myopathies, functional screening tests are commonly used during the diagnostic work-up. Our objective was to evaluate the accuracy of isometric handgrip test (IHT) and progressive cycle ergometer test (PCET) to identify McArdle disease and myoadenylate deaminase (MAD) deficiency and to propose diagnostic algorithms using exercise-induced lactate and ammonia variations.

METHODS

A prospective sample of 46 patients underwent an IHT and a PCET as part of their exercise-induced myalgia and intolerance evaluation. The two diagnostics tests were compared against the results of muscle biopsy and/or the presence of mutations in PYGM. A total of 6 patients had McArdle disease, 5 a complete MAD deficiency (MAD absent), 12 a partial MAD deficiency, and 23 patients had normal muscle biopsy and acylcarnitine profile (disease control).

RESULTS

The two functional tests could diagnose all McArdle patients with statistical significance, combining a low lactate variation (IHT: <1 mmol/L, AUC = 0.963, P < .0001; PCET: <1 mmol/L, AUC = 0.990, P < .0001) and a large ammonia variation (IHT: >100 μmol/L, AUC = 0.944, P = .0005; PCET: >20 μmol/L, AUC = 1). PCET was superior to IHT for MAD absent diagnosis, combining very low ammonia variation (<10 μmol/L, AUC = 0.910, P < .0001) and moderate lactate variation (>1 mmol/L).

CONCLUSIONS

PCET-based decision tree was more accurate than IHT, with respective generalized squared correlations of 0.796 vs 0.668. IHT and PCET are both interesting diagnostic tools to identify McArdle disease, whereas cycle ergometer exercise is more efficient to diagnose complete MAD deficiency.

The Interplay between Feedback and Buffering in Cellular Homeostasis

Buffering, the use of reservoirs of molecules to maintain concentrations of key molecular species, and negative feedback are the primary known mechanisms for robust homeostatic regulation. To our knowledge, however, the fundamental principles behind their combined effect have not been elucidated. Here, we study the interplay between buffering and negative feedback in the context of cellular homeostasis. We show that negative feedback counteracts slow-changing disturbances, whereas buffering counteracts fast-changing disturbances. Furthermore, feedback and buffering have limitations that create trade-offs for regulation: instability in the case of feedback and molecular noise in the case of buffering. However, because buffering stabilizes feedback and feedback attenuates noise from slower-acting buffering, their combined effect on homeostasis can be synergistic. These effects can be explained within a traditional control theory framework and are consistent with experimental observations of both ATP homeostasis and pH regulation in vivo. These principles are critical for studying robustness and homeostasis in biology and biotechnology.

Effects of arginine supplementation during early gestation (day 1 to 30) on litter size and plasma metabolites in gilts and sows

Two experiments were conducted, under typical commercial swine production conditions, to determine effects of dietary arginine supplementation during early gestation on the performance of gilts and sows. In Exp. 1, between d 1 and 30 of gestation, 62 Landrace gilts and 113 sows consumed a corn- and soybean meal-based diet supplemented with 1.3% -arginine HCl or 2.2% -alanine. Total numbers of piglets born ( < 0.05) and born alive ( < 0.01) per litter and litter birth weights of piglets born ( < 0.05) and born alive ( < 0.05) were increased in the arginine group compared with the control. In Exp. 2, 155 multiparous Landrace sows received 1.3% -arginine HCl supplementation between d 1 and 14 (T2; = 41), d 15 and 30 (T3; = 40), or d 1 and 30 (T4; = 37), whereas the control group received 2.2% -alanine supplementation between d 1 and 30 (T1; = 37). Blood samples were randomly obtained from 6 sows per group on d 1, 14, and 28 of gestation to determine plasma concentrations of AA and related metabolites. Total numbers of piglets born ( = 0.084) and born alive ( = 0.080) per litter tended to be higher for sows supplemented with arginine between d 1 and 14 of gestation (T2) than for control sows (T1). Concentrations of arginine and nitric oxide metabolites were greater ( < 0.05) in T4 compared with T1 and T3 on d 14 of gestation and were also greater in T4 compared with T1 and T2 on d 28 of gestation. Plasma concentrations of spermidine ( < 0.001) were increased in T3 and T4 compared with T1 and T2 on d 28. These results indicate that dietary arginine supplementation during early gestation improves the reproductive performance of gilts and sows, possibly via nitric oxide and polyamine-dependent mechanisms.

Multifactorial Effects on Different Types of Brain Cells Contribute to Ammonia Toxicity

Effects of ammonia on astrocytes play a major role in hepatic encephalopathy, acute liver failure and other diseases caused by increased arterial ammonia concentrations (e.g., inborn errors of metabolism, drug or mushroom poisoning). There is a direct correlation between arterial ammonia concentration, brain ammonia level and disease severity. However, the pathophysiology of hyperammonemic diseases is disputed. One long recognized factor is that increased brain ammonia triggers its own detoxification by glutamine formation from glutamate. This is an astrocytic process due to the selective expression of the glutamine synthetase in astrocytes. A possible deleterious effect of the resulting increase in glutamine concentration has repeatedly been discussed and is supported by improvement of some pathologic effects by GS inhibition. However, this procedure also inhibits a large part of astrocytic energy metabolism and may prevent astrocytes from responding to pathogenic factors. A decrease of the already low glutamate concentration in astrocytes due to increased synthesis of glutamine inhibits the malate-aspartate shuttle and energy metabolism. A more recently described pathogenic factor is the resemblance between NH₄⁺ and K⁺ in their effects on the Na⁺,K⁺-ATPase and the Na⁺,K⁺, 2 Cl^- and water transporter NKCC1. Stimulation of the Na⁺,K⁺-ATPase driven NKCC1 in both astrocytes and endothelial cells is essential for the development of brain edema. Na⁺,K⁺-ATPase stimulation also activates production of endogenous ouabains. This leads to oxidative and nitrosative damage and sensitizes NKCC1. Administration of ouabain antagonists may accordingly have therapeutic potential in hyperammonemic diseases.

Diagnostic Algorithm for Glycogenoses and Myoadenylate Deaminase Deficiency Based on Exercise Testing Parameters: A Prospective Study

Aim

Our aim was to evaluate the accuracy of aerobic exercise testing to diagnose metabolic myopathies.

Methods

From December 2008 to September 2012, all the consecutive patients that underwent both metabolic exercise testing and a muscle biopsy were prospectively enrolled. Subjects performed an incremental and maximal exercise testing on a cycle ergometer. Lactate, pyruvate, and ammonia concentrations were determined from venous blood samples drawn at rest, during exercise (50% predicted maximal power, peak exercise), and recovery (2, 5, 10, and 15 min). Biopsies from vastus lateralis or deltoid muscles were analysed using standard techniques (reference test). Myoadenylate deaminase (MAD) activity was determined using p-nitro blue tetrazolium staining in muscle cryostat sections. Glycogen storage was assessed using periodic acid-Schiff staining. The diagnostic accuracy of plasma metabolite levels to identify absent and decreased MAD activity was assessed using Receiver Operating Characteristic (ROC) curve analysis.

Results

The study involved 51 patients. Omitting patients with glycogenoses (n = 3), MAD staining was absent in 5, decreased in 6, and normal in 37 subjects. Lactate/pyruvate at the 10th minute of recovery provided the greatest area under the ROC curves (AUC, 0.893 ± 0.067) to differentiate Abnormal from Normal MAD activity. The lactate/rest ratio at the 10th minute of recovery from exercise displayed the best AUC (1.0) for discriminating between Decreased and Absent MAD activities. The resulting decision tree achieved a diagnostic accuracy of 86.3%.

Conclusion

The present algorithm provides a non-invasive test to accurately predict absent and decreased MAD activity, facilitating the selection of patients for muscle biopsy and target appropriate histochemical analysis.

Region-specific causal mechanism in the effects of ammonia on cerebral glucose metabolism in the rat brain

Ammonia, which is considered to be the main agent responsible for hepatic encephalopathy, inhibits oxidative glucose metabolism in the brain. However, the effects of ammonia on cerebral glucose metabolism in different brain regions remains unclear. To clarify this issue, we added ammonia directly to fresh rat brain slices and measured its effects on glucose metabolism. Dynamic positron autoradiography with [(18)F]2-fluoro-2-deoxy-D-glucose and 2-(4-iodophenyl)-3-(4-nitrophenyl)-5-(2,4-disulfophenyl)-2H-tetrazolium (WST-1) colorimetric assay revealed that ammonia significantly increased the cerebral glucose metabolic rate and depressed mitochondrial function, as compared to the unloaded control in each of the brain regions examined (cerebral cortex, striatum, and cerebellum), reflecting increased glycolysis that compensates for the decrease in aerobic metabolism. Pre-treatment with (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801), a N-methyl-D-aspartate (NMDA) receptor antagonist, significantly attenuated these changes induced by ammonia in cerebellum, but not in cerebral cortex or striatum. The addition of ammonia induced an increase in cyclic guanosine monophosphate (cGMP) levels in cerebellum, but not in cerebral cortex or striatum, reflecting the activation of the NMDA receptor-nitric oxide-cGMP pathway. These results suggested that NMDA receptor activation is responsible for the impairment of glucose metabolism induced by ammonia specifically in cerebellum.

Ammonia toxicity and its prevention in inherited defects of the urea cycle

The urea cycle is the final pathway for removal of surplus nitrogen from the body, and the major route in humans for detoxification of ammonia. The full complement of enzymes is expressed only in liver. Inherited deficiencies of urea cycle enzymes lead to hyperammonaemia, which causes brain damage. Severe defects present with hyperammonaemic crises in neonates. Equally devastating episodes may occur in previously asymptomatic adults with mild defects, most often X-linked ornithine transcarbamylase (OTC) deficiency. Several mechanisms probably contribute to pathogenesis. Treatment aims to reduce plasma ammonia quickly, reduce production of waste nitrogen, dispose of waste nitrogen using alternative pathways to the urea cycle and replace arginine. These therapies have increased survival and probably improve the neurological outcome. Arginine, sodium benzoate, sodium phenylbutyrate and, less often, sodium phenylacetate are used. Long-term correction is achieved by liver transplantation. Gene therapy for OTC deficiency is effective in animals, and work is ongoing to improve persistence and safety.

Mechanisms of and defense against acute ammonia toxicity in the aquatic Chinese soft-shelled turtle, Pelodiscus sinensis

The objective of this study was to elucidate the mechanisms of acute ammonia toxicity in the aquatic Chinese soft-shelled turtle, Pelodiscus sinensis, and to examine how this turtle defended against a sublethal dose of NH(4)Cl injected into its peritoneal cavity. The ammonia and glutamine contents in the brains of turtles that succumbed within 3h to an intraperitoneal injection with a lethal dose (12.5 micromolg(-1) turtle) of NH(4)Cl were 21 and 4.4 micromolg(-1), respectively. Since the brain glutamine content increased to 8 micromolg(-1) at hour 6 and recovered thereafter in turtles injected with a sub-lethal dose of NH(4)Cl (7.5 micromolg(-1) turtle), it can be concluded that increased glutamine synthesis and accumulation was not the major cause of acute ammonia toxicity in P. sinensis. Indeed, the administration of l-methionine S-sulfoximine (MSO; 82 microgg(-1) turtle), a glutamine synthetase (GS) inhibitor, prior to the injection of a lethal dose of NH(4)Cl had no significant effect on the mortality rate. Although the prior administration of MSO led to an extension of the time to death, it was apparently a result of its effects on glutamate dehydrogenase and glutamate formation, instead of glutamine synthesis and accumulation, in the brain. By contrast, a prior injection with MK801 (1.6 microgg(-1) turtle), a NMDA receptor antagonist, reduced the 24h mortality of turtles injected with a lethal dose of NH(4)Cl by 50%. Thus, acute ammonia toxicity in P. sinensis was probably a result of glutamate dysfunction and the activation of NMDA receptors. NMDA receptor activation could also be exacerbated through membrane depolarization caused by the extraordinarily high level of ammonia (21 micromolg(-1) brain) in the brain of turtles that succumbed to a lethal dose of NH(4)Cl. One hour after the injection with a sub-lethal dose of NH(4)Cl, the brain of P. sinensis exhibited an extraordinarily high tolerance of ammonia (16 micromolg(-1) brain). The transient nature of ammonia accumulation indicates that P. sinensis could ameliorate ammonia toxicity through the suppression of endogenous ammonia production and/or the excretion of exogenous ammonia. Despite being ureogenic and ureotelic, only a small fraction of the exogenous ammonia was detoxified to urea. A major portion of ammonia was excreted unchanged, resulting in an apparent ammonotely in the experimental turtles. Since there were increases in total essential free amino acid contents in the brain, liver and muscle, it can be deduced that a suppression of amino acid catabolism had occurred, reducing the production of endogenous ammonia and hence alleviating the possibility of ammonia intoxication.

Effect of nitrogen limitation on the ergosterol production by fed-batch culture of Saccharomyces cerevisiae

The diversity and content of available nitrogen sources in the growth medium both are very important in the accumulation of ergosterol in the yeast cell membrane. Growth on the good nitrogen sources such as ammonia can harvest more yeast cells than on poor ones, but ergosterol content in those yeast cells is relatively lower. Ergosterol content, one of the most variable parameters in ergosterol production by yeast cultivation, is greatly influenced by nitrogen limitation. The aim of our work was to study how the nitrogen sources affected the membrane ergosterol content and increase the total ergosterol yield. On the premise of keeping high ergosterol content in yeast cell, the ergosterol yield was enhanced by increasing the yeast biomass. Direct feed back control of glucose using an on-line ethanol concentration monitor was introduced to achieve high cell density. Ammonia, which acted as nitrogen source, was added to adjust pH during fermentation process, but its addition needed careful control. Cultivation in 5 L bioreactor was carried out under following conditions: culture temperature 30+/-1 degrees C, pH 5.5+/-0.1, agitation speed 600 rpm, controlling ethanol concentration below 1% and controlling ammonium ion concentration below 0.1 mol/L. Under these conditions the yeast dry weight reached 95.0+/-2.6 g/L and the ergosterol yield reached 1981+/-34 mg/L.

Perturbations in Mouse Embryo Development and Viability Caused by Ammonium Are More Severe after Exposure at the Cleavage Stages1

The presence of ammonium in culture medium has a detrimental effect on embryo physiology and biochemistry; however, the stage at which the embryo is most sensitive to this effect is unknown. The aim of this study was to determine the exact stage at which the embryo is most vulnerable to ammonium by exposing the preimplantation embryo to 300 muM ammonium either at the precompaction stage (between the zygote and two-cell or the two-cell to eight-cell) or at the postcompaction stage (between the eight-cell and blastocyst). This study determined that exposure of embryos to ammonium at the precompaction stage from either the zygote to two-cell stage or from the two-cell to the eight-cell stage did not affect the rate of development to the blastocyst stage; however, the resultant blastocysts had decreased cell numbers and inner cell mass cells. Furthermore, these blastocysts had increased levels of cellular apoptosis and perturbed levels of Slc2a3 expression and glucose uptake. Transfer of these blastocysts revealed that, while implantation was not affected, the number of fetuses was reduced by culture with ammonium at the precompaction stage and fetal development was delayed, as observed by reduced crown-rump length and maturity. In contrast, the later stage embryo was more resistant to the negative effects of ammonium, with only Slc2a3 expression and fetal maturity affected. This raises the possibility that the later stage embryo is more able to protect itself from in vitro-derived stress and that the majority of in vitro-induced damage to mouse embryos is inflicted at the early stages of development.

High-cell-density fermentation for ergosterol production by Saccharomyces cerevisiae

The direct feedback control of glucose using an on-line ethanol concentration monitor for ergosterol production by high-cell-density fermentation was investigated and the fermentation parameters (e.g., pH, dissolved oxygen, ethanol concentration, oxygen uptake rate, carbon dioxide evolution rate and respiratory quotient) were analyzed. Controlling glucose feeding rate in accordance with ethanol concentration and adjusting pH with ammonia during the fermentation process were effective fed-batch methods for ergosterol production. The fermentation parameters well described the variation of the whole fermentation process. Cultivation in a 5 l fermentor was carried out under the following conditions: culture temperature, 30 degrees C; pH, 5.5; agitation speed, 600 rpm; fermentation time, 60 h; controlling ethanol concentration below 1% and keeping respiratory quotient (RQ) at approximately 1.0. Under these conditions, the yeast dry weight reached 120 g/l and the ergosterol yield reached 1500 mg/l.

Muscle glycogen inharmoniously regulates glycogen synthase activity, glucose uptake, and proximal insulin signaling

Insulin-stimulated glucose uptake and incorporation of glucose into skeletal muscle glycogen contribute to physiological regulation of blood glucose concentration. In the present study, glucose handling and insulin signaling in isolated rat muscles with low glycogen (LG, 24-h fasting) and high glycogen (HG, refed for 24 h) content were compared with muscles with normal glycogen (NG, rats kept on their normal diet). In LG, basal and insulin-stimulated glycogen synthesis and glycogen synthase activation were higher and glycogen synthase phosphorylation (Ser(645), Ser(649), Ser(653), Ser(657)) lower than in NG. GLUT4 expression, insulin-stimulated glucose uptake, and PKB phosphorylation were higher in LG than in NG, whereas insulin receptor tyrosyl phosphorylation, insulin receptor substrate-1-associated phosphatidylinositol 3-kinase activity, and GSK-3 phosphorylation were unchanged. Muscles with HG showed lower insulin-stimulated glycogen synthesis and glycogen synthase activation than NG despite similar dephosphorylation. Insulin signaling, glucose uptake, and GLUT4 expression were similar in HG and NG. This discordant regulation of glucose uptake and glycogen synthesis in HG resulted in higher insulin-stimulated glucose 6-phosphate concentration, higher glycolytic flux, and intracellular accumulation of nonphosphorylated 2-deoxyglucose. In conclusion, elevated glycogen synthase activation, glucose uptake, and GLUT4 expression enhance glycogen resynthesis in muscles with low glycogen. High glycogen concentration per se does not impair proximal insulin signaling or glucose uptake. "Insulin resistance" is observed at the level of glycogen synthase, and the reduced glycogen synthesis leads to increased levels of glucose 6-phosphate, glycolytic flux, and accumulation of nonphosphorylated 2-deoxyglucose.

Effect of hypercapnia on changes in blood pH, plasma lactate and ammonia due to exercise

The present study examined the effects of hypercapnia on changes in blood pH, plasma lactate and ammonia due to exhaustive exercise. Six male subjects underwent exercise of increasing intensity until exhaustion: (1) breathing air = MAX (maximal exercise), or (2) under hypercapnia (HC: 21% O(2), 6% CO(2)) that had been maintained from 60 min before to 30 min after exercise = HC; and (3) exercise of the same intensity as HC in air = SUB (submaximal exercise). Arterialized blood was drawn from a superficial vein. Blood pH in HC was significantly lower than in MAX or SUB at rest, at the end of exercise and throughout recovery (P<0.05). Plasma lactate and ammonia concentration in HC was significantly lower than in MAX (P<0.05), and similar to that in SUB at the end of exercise and throughout recovery. Respiratory acidosis resulting from hypercapnia shifted the linear lactate to blood pH relationship during exhaustive exercise below that at normocapnia (P<0.001). The reduced slope of linear blood pH to ammonia relationship under hypercapnia (P<0.001) is attributed to lactic acidosis that is less, due to the lesser work intensity at the end of exhaustion, than that of normocapnia. From these results we conclude that (1) hypercapnia-induced respiratory acidosis promoted the decrease in blood pH due to lactate production throughout recovery; (2) plasma lactate concentration at maximal exercise was lowered under hypercapnia; (3) plasma ammonia concentration at maximal exercise was reduced, probably due to a less intense lactic acidosis.

Chronic and acute ammonia toxicity in mudskippers, Periophthalmodon schlosseri and Boleophthalmus boddaerti: brain ammonia and glutamine contents, and effects of methionine sulfoximine and MK801

The objective of this study was to elucidate if chronic and acute ammonia intoxication in mudskippers, Periophthalmodon schlosseri and Boleophthalmus boddaerti, were associated with high levels of ammonia and/or glutamine in their brains, and if acute ammonia intoxication could be prevented by the administration of methionine sulfoximine [MSO; an inhibitor of glutamine synthetase (GS)] or MK801 [an antagonist of N-methyl D-aspartate type glutamate (NMDA) receptors]. For P. schlosseri and B. boddaerti exposed to sublethal concentrations (100 and 8 mmol l(-1) NH4Cl, respectively, at pH 7.0) of environmental ammonia for 4 days, brain ammonia contents increased drastically during the first 24 h, and they reached 18 and 14.5 micromol g(-1), respectively, at hour 96. Simultaneously, there were increases in brain glutamine contents, but brain glutamate contents were unchanged. Because glutamine accumulated to exceptionally high levels in brains of P. schlosseri (29.8 micromol g(-1)) and B. boddaerti (12.1 micromol g(-1)) without causing death, it can be concluded that these two mudskippers could ameliorate those problems associated with glutamine synthesis and accumulation as observed in patients suffering from hyperammonemia. P. schlosseri and B. boddaerti could tolerate high doses of ammonium acetate (CH3COONH4) injected into their peritoneal cavities, with 24 h LC50 of 15.6 and 12.3 micromol g(-1) fish, respectively. After the injection with a sublethal dose of CH3COONH4 (8 micromol g(-1) fish), there were significant increases in ammonia (5.11 and 8.36 micromol g(-1), respectively) and glutamine (4.22 and 3.54 micromol g(-1), respectively) levels in their brains at hour 0.5, but these levels returned to normal at hour 24. By contrast, for P. schlosseri and B. boddaerti that succumbed within 15-50 min to a dose of CH3COONH4 (15 and 12 micromol g(-1) fish, respectively) close to the LC50 values, the ammonia contents in the brains reached much higher levels (12.8 and 14.9 micromol g(-1), respectively), while the glutamine level remained relatively low (3.93 and 2.67 micromol g(-1), respectively). Thus, glutamine synthesis and accumulation in the brain was not the major cause of death in these two mudskippers confronted with acute ammonia toxicity. Indeed, MSO, at a dosage (100 microg g(-1) fish) protective for rats, did not protect B. boddaerti against acute ammonia toxicity, although it was an inhibitor of GS activities from the brains of both mudskippers. In the case of P. schlosseri, MSO only prolonged the time to death but did not reduce the mortality rate (100%). In addition, MK801 (2 microg g(-1) fish) had no protective effect on P. schlosseri and B. boddaerti injected with a lethal dose of CH3COONH4, indicating that activation of NMDA receptors was not the major cause of death during acute ammonia intoxication. Thus, it can be concluded that there are major differences in mechanisms of chronic and acute ammonia toxicity between brains of these two mudskippers and mammalian brains.

Ammonia-mediated LTP inhibition: effects of NMDA receptor antagonists and L-carnitine

Because hyperammonemia is thought to contribute to the pathogenesis of hepatic encephalopathy, we examined the effects of ammonia on ATP levels, neuronal morphology, and synaptic function in rat hippocampal slices. Although ammonia did not alter ATP levels supported by 10 mM glucose, ammonia significantly depressed ATP levels in the presence of 3.3 mM glucose or 10 mM pyruvate, suggesting effects on respiratory energy metabolism. Ammonia also impaired synaptic function and neuronal integrity sustained by pyruvate. In 10 mM glucose, ammonia inhibited the induction and maintenance of long-term potentiation (LTP) in a concentration-dependent fashion. These inhibitory effects of ammonia were overcome by L-carnitine. DL-APV, an antagonist of NMDA receptors, also diminished the effects of ammonia on ATP levels and LTP induction, indicating that ammonia impairs neuronal function via altered metabolism and untimely NMDA receptor activation. These results suggest that L-carnitine and NMDA receptor antagonists have the potential to preserve neuronal function during hyperammonemia.

Five tropical air-breathing fishes, six different strategies to defend against ammonia toxicity on land

Most tropical fishes are ammonotelic, producing ammonia and excreting it as NH3 by diffusion across the branchial epithelia. Hence, those air-breathing tropical fishes that survive on land briefly or for an extended period would have difficulties in excreting ammonia when out of water. Ammonia is toxic, but some of these air-breathing fishes adopt special biochemical adaptations to ameliorate the toxicity of endogenous ammonia accumulating in the body. The amphibious mudskipper Periophthalmodon schlosseri, which is very active on land, reduces ammonia production by suppressing amino acid catabolism (strategy 1) during aerial exposure. It can also undergo partial amino acid catabolism, leading to the accumulation of alanine (strategy 2) to support locomotory activities on land. In this case, alanine formation is not an ammonia detoxification process but reduces the production of endogenous ammonia. The snakehead Channa asiatica, which exhibits moderate activities on land although not truly amphibious, accumulates both alanine and glutamine in the muscle, with alanine accounting for 80% of the deficit in reduction in ammonia excretion during air exposure. Unlike P. schlosseri, C. asiatica apparently cannot reduce the rates of protein and amino acid catabolism and is incapable of utilizing partial amino acid catabolism to support locomotory activities on land. Unlike alanine formation, glutamine synthesis (strategy 3) represents an ammonia detoxification mechanism that, in effect, removes the accumulating ammonia. The four-eyed sleeper Bostrichyths sinensis, which remains motionless during aerial exposure, detoxifies endogenous ammonia to glutamine for storage. The slender African lungfish Protopterus dolloi, which can aestivate on land on a mucus cocoon, has an active ornithine-urea cycle and converts endogenous ammonia to urea (strategy 4) for both storage and subsequent excretion. Production of urea and glutamine are energetically expensive and appear to be adopted by fishes that remain relatively inactive on land. The Oriental weatherloach Misgurnus anguillicaudatus, which actively burrows into soft mud during drought, manipulates the pH of the body surface to facilitate NH3 volatilization (strategy 5) and develops high ammonia tolerance at the cellular and subcellular levels (strategy 6) during aerial exposure. Hence, with regard to excretory nitrogen metabolism, modern tropical air-breathing fishes exhibit a variety of strategies to survive on land, and they represent a spectrum of specimens through which we may examine various biochemical adaptations that would have facilitated the invasion of the terrestrial habitat by fishes during evolution.

Dogmas and controversies in the handling of nitrogenous wastes: ammonia tolerance in the oriental weatherloach Misgurnus anguillicaudatus

The oriental weatherloach Misgurnus anguillicaudatus is an extremely ammonia-tolerant fish. Many ammonia-protection mechanisms have been reported in this fish. Six strategies used by this fish to deal with the problem of excess ammonia are described. The fish can (1) reduce ammonia production through reduction in protein and/or amino acid catabolism; (2) reduce ammonia production and obtain energy through partial amino acid catabolism leading to alanine formation; (3) detoxify ammonia to glutamine; (4) tolerate very high ammonia levels in its tissues; (5) get rid of ammonia as NH(3) gas and, probably, (6) possesses background K(+) channels that are impermeable to NH(4)(+). The effects of extracellular ammonia on the contraction performance of the heart from this fish were found to be the same as in rainbow trout, an ammonia-sensitive fish. It suggests that the hearts of most, if not all, fish species are protected against ammonia. MK-801, an NMDA receptor blocker, was found to have a protective effect against ammonia intoxication in the oriental weatherloach, which suggests that the NMDA receptor, as in mammals, is involved in ammonia toxicity.

Neuron-glial trafficking of NH4+ and K+: separate routes of uptake into glial cells of bee retina

Ammonium (NH4+ and/or NH3) and K+ are released from active neurons and taken up by glial cells, and can modify glial cell behaviour. Study of these fluxes is most advanced in the retina of the honeybee drone, which consists essentially of identical neurons (photoreceptors) and identical glial cells (outer pigment cells). In isolated bee retinal glial cells, ammonium crosses the membrane as NH4+ on a Cl- cotransporter. We have now investigated, in the more physiological conditions of a retinal slice, whether the NH4+-Cl- cotransporter can transport K+ and whether the major K+ conductance can transport NH4+. We increased [NH4+] or [K+] in the superfusate and monitored uptake by recording from the glial cell syncytium or from interstitial space with microelectrodes selective for H+ or K+. In normal superfusate solution, ammonium acidified the glial cells but, after 6 min superfusion in low [Cl-] solution, ammonium alkalinized them. In the same low [Cl-] conditions, the rise in intraglial [K+] induced by an increase in superfusate [K+] was unchanged, i.e. no K+ flux on the Cl- cotransporter was detected. Ba2+ (5 mm) abolished the glial depolarization induced by K+ released from photoreceptors but did not reduce NH4+uptake. We estimate that when extracellular [NH4+] is increased, 62-100% is taken up by the NH4+-Cl- cotransporter and that when K+ is increased, 77-100% is taken up by routes selective for K+. This separation makes it possible that the glial uptake of NH4+ and of K+, and hence their signalling roles, might be regulated separately.

Sub-lethal plasma ammonia accumulation and the exercise performance of salmonids

The proposal that plasma ammonia accumulation might impair the swimming performance of fish was first made over a decade ago, and has now proven to be the case for a number of salmonid species. The first experimental evidence was indirect, when a negative linear relationship between plasma ammonia concentrations and maximum sustainable swimming speed (U(crit)) was found following the exposure of brown trout (Salmo trutta) to sub-lethal concentrations of copper in soft acidic water. Since then, negative linear relationships between plasma ammonia concentration and U(crit) have been demonstrated following exposure of brown trout, rainbow trout (Oncorhynchus mykiss) and coho salmon (Oncorhynchus kisutch) to elevated water ammonia. For brown trout, the relationships between plasma ammonia and U(crit) were remarkably similar following either exposure to elevated water ammonia or to sub-lethal copper. This indicates that the impairment of swimming performance resulting from exposure to sub-lethal concentrations of heavy metals may be attributable in large part to an accumulation of endogenous ammonia. The negative relationship between plasma ammonia concentration and U(crit) was similar in size-matched rainbow and brown trout but, under similar regimes of ammonia exposure, rainbow trout were able to maintain a significantly lower plasma ammonia concentration, revealing inter-specific differences in ammonia permeability and/or transport. One primary mechanism by which ammonia accumulation may impair exercise performance is a partial depolarisation of membrane potential in tissues such as the brain and white muscle. This may prejudice the co-ordination of swimming movements and reduce or abolish the development of muscle tension, thus, compromising swimming efficiency and performance at the top end of the range.

Increased extracellular brain glutamate in acute liver failure: decreased uptake or increased release?

Glutamatergic dysfunction has been suggested to play an important role in the pathogenesis of hepatic encephalopathy (HE) in acute liver failure (ALF). Increased extracellular brain glutamate concentrations have consistently been described in different experimental animal models of ALF and in patients with increased intracranial pressure due to ALF. High brain ammonia levels remain the leading candidate in the pathogenesis of HE in ALF and studies have demonstrated a correlation between ammonia and increased concentrations of extracellular brain glutamate both clinically and in experimental animal models of ALE Inhibition of glutamate uptake or increased glutamate release from neurons and/or astrocytes could cause an increase in extracellular glutamate. This review analyses the effect of ammonia on glutamate release from (and uptake into) both neurons and astrocytes and how these pathophysiological mechanisms may be involved in the pathogenesis of HE in ALF.

Neurobiology of ammonia

Hyperammonemia resulting from inherited urea cycle enzyme deficiencies or liver failure results in severe central nervous system dysfunction including brain edema, convulsions and coma. Neuropathologic evaluation in these disorders reveals characteristic alterations of astrocyte morphology ranging from cell swelling (acute hyperammonemia) to Alzheimer Type II astrocytosis (chronic hyperammonemia). Having no effective urea cycle, brain relies on glutamine synthesis for the removal of excess ammonia and the enzyme responsible, glutamine synthetase, has a predominantly astrocytic localization. Accumulation of ammonia in brain results in a redistribution of cerebral blood flow and metabolism from cortical to sub-cortical structures. In addition to changes in astrocyte morphology, increased brain ammonia concentrations result in alterations in expression of key astrocyte proteins including glial fibrillary acidic protein, glutamate and glycine transporters and "peripheral-type" (mitochondrial) benzodiazepine receptors. Such changes result in alterations of astrocytic volume and increased extracellular concentrations of excitatory and inhibitory substances. In addition, the ammonium ion has direct effects on excitatory-inhibitory transmission via distinct mechanisms involving cellular chloride extrusion and postsynaptic receptor function. Acute ammonia exposure leads to activation of NMDA receptors and their signal transduction pathways. Chronic hyperammonemia also results in increased concentrations of neuroactive L-tryptophan metabolites including serotonin and quinolinic acid. Therapy in hyperammonemic syndromes continues to rely on ammonia-lowering strategies via peripheral mechanisms (reduction of ammonia production in the gastrointestinal tract, increased ammonia removal by muscle).

A model for regulation of the Mg(2+)-stimulated acto-myosin-ATPase activity: inhibition of the formation of actin-myosin complex and the Mg( 2+)-stimulated acto-myosin-ATPase activity by IMP and AMP

Previously, we showed that the decrease in force output during continuous isometric contractions in rat skeletal muscle was related to an increase in the concentration of IMP. In this paper we report on additional experiments in which the effect of IMP on the Mg(2+)-stimulated acto-myosin-ATPase activity of isolated actin and myosin is measured at 35 degrees C. The results show that 1) the binding of actin to myosin is co-operative (Hill coefficient = 3.82); 2) in the presence of IMP or AMP the Mg(2+)-stimulated acto-myosin-ATPase activity is inhibited up to 60% at 10 mM; 3) in the presence of IMP or AMP not only the Mg(2+)-stimulated acto-myosin-ATPase activity decreases, but also K(50). From these results we conclude that IMP and AMP may be considered as uncompetitive inhibitors. Our results suggest that IMP and AMP can prevent an 'energy crisis' during exhaustive exercise of short duration by down-regulating the contractile machinery.

Ammonium in nervous tissue: transport across cell membranes, fluxes from neurons to glial cells, and role in signalling

Most, but not all, animal cell membranes are permeable to NH3, the neutral, minority form of ammonium which is in equilibrium with the charged majority form NH4+. NH4+ crosses many cell membranes via ion channels or on membrane transporters, and cultured mammalian astrocytes and glial cells of bee retina take up NH4+ avidly, in the latter case on a Cl(-)-cotransporter selective for NH4+ over K+. In bee retina, a flux of ammonium from neurons to glial cells is an essential component of energy metabolism, which involves a flux of alanine from glial cells to neurons. In mammalian brain, both glutamate and ammonium are taken up preferentially by astrocytes and form glutamine. Glutamine is transferred to neurons where it is deamidated to re-form glutamate; the maintenance of this cycle appears to require a substantial flux of ammonium from neurons to astrocytes. In addition to maintaining the glial cell content of fixed N (a "bookkeeping" function), ammonium is expected to participate in the regulation of glial cell metabolism (a signalling function): it will increase conversion of glutamate to glutamine, and, by activating phosphofructokinase and inhibiting the alpha-ketoglutarate dehydrogenase complex, it will tend to increase the formation of lactate.

A Cl(-) cotransporter selective for NH(4)(+) over K(+) in glial cells of bee retina

There appears to be a flux of ammonium (NH(4)(+)/NH(3)) from neurons to glial cells in most nervous tissues. In bee retinal glial cells, NH(4)(+)/NH(3) uptake is at least partly by chloride-dependant transport of the ionic form NH(4)(+). Transmembrane transport of NH(4)(+) has been described previously on transporters on which NH(4)(+) replaces K(+), or, more rarely, Na(+) or H(+), but no transport system in animal cells has been shown to be selective for NH(4)(+) over these other ions. To see if the NH(4)(+)-Cl(-) cotransporter on bee retinal glial cells is selective for NH(4)(+) over K(+) we measured ammonium-induced changes in intracellular pH (pH(i)) in isolated bundles of glial cells using a fluorescent indicator. These changes in pH(i) result from transmembrane fluxes not only of NH(4)(+), but also of NH(3). To estimate transmembrane fluxes of NH(4)(+), it was necessary to measure several parameters. Intracellular pH buffering power was found to be 12 mM. Regulatory mechanisms tended to restore intracellular [H(+)] after its displacement with a time constant of 3 min. Membrane permeability to NH(3) was 13 microm s(-1). A numerical model was used to deduce the NH(4)(+) flux through the transporter that would account for the pH(i) changes induced by a 30-s application of ammonium. This flux saturated with increasing [NH(4)(+)](o); the relation was fitted with a Michaelis-Menten equation with K(m) approximately 7 mM. The inhibition of NH(4)(+) flux by extracellular K(+) appeared to be competitive, with an apparent K(i) of approximately 15 mM. A simple standard model of the transport process satisfactorily described the pH(i) changes caused by various experimental manipulations when the transporter bound NH(4)(+) with greater affinity than K(+). We conclude that this transporter is functionally selective for NH(4)(+) over K(+) and that the transporter molecule probably has a greater affinity for NH(4)(+) than for K(+).

Energy metabolism of synaptosomal subpopulations from different neuronal systems of rat hippocampus: effect of L-acetylcarnitine administration in vivo

The maximum rate (Vmax) of some enzyme activities related to glycolysis, Krebs' cycle, acetylcholine catabolism and amino acid metabolism were evaluated in different types of synaptosomes obtained from rat hippocampus. The enzyme characterization was performed on two synaptosomal populations defined as "large" and "small" synaptosomes, supposed to originate mainly from the granule cell glutamatergic mossy fiber endings and small cholinergic nerve endings mainly arising from septohippocampal fiber synapses, involved with cognitive processes. Thus, this is an unique model of pharmacological significance to study the selective action of drugs on energy metabolism of hippocampus and the sub-chronic i.p. treatment with L-acetylcarnitine at two different dose levels (30 and 60 mg x kg(-1), 5 day a week, for 4 weeks) was performed. In control animals, the results indicate that these two hippocampal synaptosomal populations differ for the potential catalytic activities of enzymes of the main metabolic pathways related to energy metabolism. This energetic micro-heterogeneity may cause their different behaviour during both physiopathological events and pharmacological treatment, because of different sensitivity of neurons. Therefore, the micro-heterogeneity of brain synaptosomes must be considered when the effect of a pharmacological treatment is to be evaluated. In fact, the in vivo administration of L-acetylcarnitine affects some specific enzyme activities, suggesting a specific molecular trigger mode of action on citrate synthase (Krebs' cycle) and glutamate-pyruvate-transaminase (glutamate metabolism), but mainly of "small" synaptosomal populations, suggesting a specific synaptic trigger site of action. These observations on various types of hippocampal synaptosomes confirm their different metabolic machinery and their different sensitivity to pharmacological treatment.

Cardiac accumulation of citrate during brief myocardial ischaemia and reperfusion in the pig in vivo

Citrate is a key intermediate in energy metabolism and an inhibitor of phosphofructokinase of the glycolytic pathway. During myocardial ischaemia glycolysis is the main source of cardiac ATP. The aim of the present study was to determine if myocardial ischaemia and reperfusion alter cardiac tissue levels of citrate. Open-chest, anaesthetized pigs were subjected to 10 min of regional myocardial ischaemia by occlusion of the left anterior descending coronary artery, with and without reperfusion, and to 10 min of global ischaemia by circulatory arrest. Citrate, amino acids, glucose and NH3 were measured in biopsies. Ischaemia, whether regional or global, caused a 60-70% increase in tissue levels of citrate. During 1 min of reperfusion following regional ischaemia the level of citrate increased 460%, to approximately 600 nmol g-1 wet weight. The level of glutamate decreased by 20-33% (corresponding to 1300-2200 nmol g-1 wet weight), indicating net consumption of this amino acid during ischaemia. The level of aspartate decreased 50% indicating conversion of aspartate to oxaloacetate for the synthesis of citrate. Theoretically, the accumulation of myocardial citrate during brief ischaemia and early reperfusion is large enough to significantly inhibit phosphofructokinase activity and could therefore affect the ability of the myocardium to increase the glycolytic rate in response to ischaemia. This could, however, be partly compensated by the metabolism of myocardial glutamate.

Effects of hyperammonaemia on brain function

Neuropsychiatric symptoms of hyperammonaemia include alterations of mood and personality, cognitive impairment, ataxia, convulsions and coma. The nature and severity of CNS dysfunction depend upon the aetiology and degree of hyperammonaemia, its acuteness of onset and the age of the patient. Neuropathological studies reveal Alzheimer type II astrocytosis in the adult hyperammonaemic patient, whereas hyperammonaemia in the infant resulting from congenital urea cycle disorders or Reye syndrome is accompanied by cerebral atrophy, neuronal loss and cerebral oedema. Several electrophysiological and biochemical mechanisms have been proposed to explain the deleterious effects of ammonia on CNS function. Such mechanisms include direct effects of the ammonium ion on excitatory and inhibitory neurotransmission and a deficit in cerebral energy metabolism due to ammonia-induced inhibition of alpha-ketoglutarate dehydrogenase. In addition, ammonia has been shown to interfere with normal processes of uptake, storage and release of various neurotransmitters. Ammonia disrupts monoamine storage, inhibits the high-affinity uptake of glutamate by both astrocytic and neuronal elements and activates 'peripheral-type' benzodiazepine receptors leading to the potential synthesis of neuroactive steroids in brain. On the basis of these actions, it has been proposed that ammonia disrupts neuron-astrocyte trafficking of amino acids and monoamines in brain. The increased formation of brain glutamine in hyperammonaemic syndromes could be responsible for the phenomenon of brain oedema in these disorders. Therapies aimed at either decreasing ammonia production in the gastrointestinal tract or increasing ammonia removal by liver or skeletal muscle are the mainstay in the prevention and treatment of the CNS consequences of hyperammonaemia. New therapeutic approaches aimed at correction of the neurotransmitter and cerebral energy deficits in these syndromes could hold promise for the future.

Effects of simulated upper gastrointestinal hemorrhage on ammonia and related amino acids in blood and brain of chronic portacaval-shunted rats

Gastrointestinal (GI) hemorrhage during compromised liver function is known to precipitate portal-systemic encephalopathy (PSE). Hypothetically, the induced hyperammonemia depletes cerebral glutamate pools. To investigate this hypothesis, rats were studied 14 days after portacaval shunt (PCS) or sham surgery (SHAM). Rats received 3 mL bovine erythrocytes or saline at t = 0, 1, 2, and 3h via a previously placed gastrostomy catheter. At t = 0, 2, 4, 6 and 8h arterial blood and at t = 8h cerebral cortex were sampled for determination of ammonia and amino acids. Control rats (NORM) were sampled without previous surgery. Repeated intragastric blood administration increased the already elevated arterial ammonia levels in PCS rats further. This resulted in higher cerebral cortex ammonia and glutamine levels after blood administration. Despite the accumulation of ammonia and glutamine, cerebral cortex glutamate concentrations remained unaltered. Yet, PCS rats became more encephalopathic after blood gavages, suggesting that there is not a clear-cut relation between cerebral cortex glutamate concentrations and degree of PSE. Interestingly, cerebral cortex concentrations of GABA, tyrosine and phenylalanine were markedly increased. Whether these observations are pathogenetically related to PSE remains to be established. The present model of simulated GI hemorrhage in PCS rats seems to be a suitable, clinically valid model for future research regarding hepatic encephalopathy.

Contraction-induced changes in acetyl-CoA carboxylase and 5'-AMP-activated kinase in skeletal muscle

The concentration of malonyl-CoA, a negative regulator of fatty acid oxidation, diminishes acutely in contracting skeletal muscle. To determine how this occurs, the activity and properties of acetyl-CoA carboxylase beta (ACC-beta), the skeletal muscle isozyme that catalyzes malonyl-CoA formation, were examined in rat gastrocnemius-soleus muscles at rest and during contractions induced by electrical stimulation of the sciatic nerve. To avoid the problem of contamination of the muscle extract by mitochondrial carboxylases, an assay was developed in which ACC-beta was first purified by immunoprecipitation with a monoclonal antibody. ACC-beta was quantitatively recovered in the immunopellet and exhibited a high sensitivity to citrate (12-fold activation) and a Km for acetyl-CoA (120 microM) similar to that reported for ACC-beta purified by other means. After 5 min of contraction, ACC-beta activity was decreased by 90% despite an apparent increase in the cytosolic concentration of citrate, a positive regulator of ACC. SDS-polyacrylamide gel electrophoresis of both homogenates and immunopellets from these muscles showed a decrease in the electrophoretic mobility of ACC, suggesting that phosphorylation could account for the decrease in ACC activity. In keeping with this notion, citrate activation of ACC purified from contracting muscle was markedly depressed. In addition, homogenization of the muscles in a buffer free of phosphatase inhibitors and containing the phosphatase activators glutamate and MgCl2 or treatment of immunoprecipitated ACC-beta with purified protein phosphatase 2A abolished the decreases in both ACC-beta activity and electrophoretic mobility caused by contraction. The rapid decrease in ACC-beta activity after the onset of contractions (50% by 20 s) and its slow restoration to initial values during recovery (60-90 min) were paralleled temporally by reciprocal changes in the activity of the alpha2 but not the alpha1 isoform of 5'-AMP-activated protein kinase (AMPK). In conclusion, the results suggest that the decrease in ACC activity during muscle contraction is caused by an increase in its phosphorylation, most probably due, at least in part, to activation of the alpha2 isoform of AMPK. They also suggest a dual mechanism for ACC regulation in muscle in which inhibition by phosphorylation takes precedence over activation by citrate. These alterations in ACC and AMPK activity, by diminishing the concentration of malonyl-CoA, could be responsible for the increase in fatty acid oxidation observed in skeletal muscle during exercise.

Effects of hypoxia on enzyme activities in skeletal muscle of rats of different ages. An attempt at pharmacological treatment

The activities of enzymes related to energy metabolism in the gastrocnemius and soleus muscles in young-adult (4 months), mature (12 months) and senescent (24 months) rats were compared after 72 h of continuous exposure to normobaric hypoxia or normoxia after alpha-adrenergic antagonist nicergoline or saline solution had been given intraperitoneally for 30 consecutive days. The maximum rates (Vmax) of the following enzyme activities in the crude extract and/or the mitochondrial fraction of each muscle specimen were evaluated: (1) for the anaerobic glycolytic pathway: hexokinase, phosphofructokinase, pyruvate kinase and lactate dehydrogenase; (2) for the tricarboxylic acid cycle; citrate synthase and malate dehydrogenase; (3) for the electron transfer chain; cytochrome oxidase; and (4) for the NAD+/NADH redox state: total NADH cytochrome c reductase. The significant differences between the enzyme activities at different ages or under different experimental conditions in the two tissue preparations of the two muscles were determined by ANOVA. MCA and ETA were used to evaluate the net effects of the experimental conditions. Ageing did not seem to affect the soleus and gastrocnemius muscles in the same way. Changes were seen only in the glycolytic pathway enzymes in the crude extract from the gastrocnemius muscle. In the soleus muscle changes in enzyme activities as a function of ageing were also found in the mitochondrial fraction. We also found that hypoxia caused greater changes in 12-month-old rats than in those of other ages (especially in the enzyme activities of the gastrocnemius muscle). Finally out data show that only in certain cases was the pharmacological treatment able to modify the influence of hypoxic conditions on the levels of enzyme activities, regardless of the age of animals.

Modifications by chronic intermittent hypoxia and drug treatment on skeletal muscle metabolism

The energy metabolism was evaluated in gastrocnemius muscle from 3-month-old rats subjected to either mild or severe 4-week intermittent normobaric hypoxia. Furthermore, 4-week treatment with CNS-acting drugs, namely, alpha-adrenergic (delta-yohimbine), vasodilator (papaverine, pinacidil), or oxygen-increasing (almitrine) agents was performed. The muscular concentration of the following metabolites was evaluated: glycogen, glucose, glucose 6-phosphate, pyruvate, lactate, lactate-to-pyruvate ratio; citrate, alpha-ketoglutarate, succinate, malate; aspartate, glutamate, alanine; ammonia; ATP, ADP, AMP, creatine phosphate. Furthermore the Vmax of the following muscular enzymes was evaluated: hexokinase, phosphofructokinase, pyruvate kinase, lactate dehydrogenase; citrate synthase, malate dehydrogenase; total NADH cytochrome c reductase; cytochrome oxidase. The adaptation to chronic intermittent normobaric mild or severe hypoxia induced alterations of the components in the anaerobic glycolytic pathway [as supported by the increased activity of lactate dehydrogenase and/or hexokinase, resulting in the decreased glycolytic substrate concentration consistent with the increased lactate production and lactate-to-pyruvate ratio] and in the mitochondrial mechanism [as supported by the decreased activity of malate dehydrogenase and/or citrate synthase resulting in the decreased concentration of some key components in the tricarboxylic acid cycle]. The effect of the concomitant pharmacological treatment suggests that the action of CNS-acting drugs could be also related to their direct influence on the muscular biochemical mechanisms linked to energy transduction.

Age-related alterations of skeletal muscle metabolism by intermittent hypoxia and TRH-analogue treatment

The characteristics of the energy metabolism were evaluated in the gastrocnemius muscle from 3- and 24-month-old rats in normoxia or subjected to either mild or severe chronic (4 weeks) intermittent normobaric hypoxia. Furthermore, 4-week treatment with saline or the TRH-analogue posatireline was performed. The muscular concentration of the following metabolites related to the energy metabolism was evaluated: glycogen, glucose, glucose 6-phosphate, pyruvate, lactate, lactate-to-pyruvate ratio; citrate, alpha-ketoglutarate, succinate, malate; aspartate, glutamate, alanine; ammonia; ATP, ADP, AMP, creatine phosphate; energy charge potential. Furthermore the maximum rate of the following muscular enzymes was evaluated: hexokinase, phosphofructokinase, pyruvate kinase, lactate dehydrogenase; citrate synthase, malate dehydrogenase; total NADH cytochrome c reductase; cytochrome oxidase. The age-related decrease in muscular glucose 6-phosphate, pyruvate and alanine concentrations and increase in citrate concentration were consistent with the age-related decreased hexokinase and increased citrate synthase activities. Ageing was characterized by a decrease in muscular creatine phosphate concentration, while the energy mediators and the energy charge potential were unchanged. The chronic (4 weeks) intermittent normobaric mild and severe hypoxia-induced alterations of the components in the anaerobic glycolytic pathway, tricarboxylic acid cycle and energy storage, that were magnified in the skeletal muscle from the oldest animals. The effect of the chronic treatment with the TRH-analogue posatireline suggests that the action of central nervous system-acting drugs could also be related to their direct influence on the muscular biochemical mechanisms related to the energy transduction.

Parkinson-like disease by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) toxicity in Macaca fascicularis: synaptosomal metabolism and action of dihydroergocriptine

The maximal rates (Vmax) of some enzyme activities related to synaptosomal energy metabolism were studied in different types of synaptosomes from cerebellar cortex of Macaca Fascicularis (Cynomolgus monkey). Different synaptosomal populations, namely "large" and "small" synaptosomes, were isolated from the anterior lobule of the cerebellar cortex of monkeys treated p.o. with dihydroergocriptine at the dose of 12 mg/kg/day before and during the induction of a Parkinson's-like syndrome by MPTP administration (i.v., 0.3 mg/kg/day for 5 days). The enzymes were chosen according to their regulatory role and as markers of the following metabolic pathways: (a) glycolysis ((hexokinase, phosphofructokinase, lactate dehydrogenase), (b) Krebs' (TCA) cycle (citrate synthase, malate dehydrogenase), (c) amino acid, glutamate metabolism (glutamate dehydrogenase, glutamate-pyruvate- and glutamate-oxaloacetate-transaminases), (d) acetylcholine catabolism (acetylcholinesterase) and (e) ATPases, i.e. Na(+)-K(+)-ATPase, Mg(2+)-ATP synthetase, Mg(2+)-ATPase, Ca(2+)-Mg(2+)-ATPase and Ca(2+)-ATPase Low and High affinity for Ca2+. The MPTP administration modified the activities of citrate synthase, malate dehydrogenase, Na(+)-K(+)-ATPase, acetylcholinesterase and glutamate-oxaloacetate transaminase only on selected types of synaptosomes. Pharmacological treatment by dihydroergocriptine was able to recovery at the steady-state levels the activities of these enzymes, thus demonstrating a partial protective effect on these biochemical parameters.

Response of rat cerebral glycolytic enzymes to hyperammonemic states

Activity levels of enzymes of glycolytic pathway viz., hexokinase (EC.2.7.1.1), phosphofructokinase (EC.2.7.1.11), aldolase (EC.4.1.2.13), glyceraldehyde-3-phosphate dehydrogenase (EC.1.2.1.12), enolase (EC.4.2.1.11), pyruvate kinase (EC.2.7.1.40) and lactate dehydrogenase (EC.1.1.1.27) were estimated in cerebral cortex, cerebellum and brainstem of the rats treated with subacute and acute doses of ammonium acetate and compared with those of control animals. In general, the activities of all the enzymes except for hexokinase and lactate dehydrogenase, were elevated in all the three regions of the brain. The results suggests an enhanced rate of glycolysis in brain in hyperammonemic states and strengthens the role of ammonium ion in stimulating certain enzymes of the glycolytic pathway.

[The hyperammonemia of exercise; value of automatic measurement]

Ammonia blood level (NH3) was measured during maximum exercise performed on cycloergometer, in 11 patients. NH3 measurements through an automatic chemical method (whole blood) were compared to those of the enzymatic reference method (plasma). Automatic analysis made it possible to quickly obtain [NH3] values, that were highly correlated with those of the enzymatic method (P < 0.001). Plasma [NH3] may be calculated from the following equation, where it is expressed versus [NH3] obtained with the automatic method (c anal): [NH3] mumol.l-1 = 0.795. [c] anal.microgram.dl-1 + 5.446.

Muscle phosphofructokinase deficiency in a myopathic child with severe mental retardation and aplasia of cerebellar vermis

Muscle phosphofructokinase (PFK) deficiency in man is responsible for at least two forms of myopathy; one is characterized by painful contractures of muscles and typically occurs in adults, whereas the other is often disabling and typically occurs in childhood, with psychomotor and growth retardation. In this investigation, a young myopathic patient with severe mental retardation and aplasia of the cerebellar vermis presented with muscular hypotrophy of the limbs, generalized hypotonia, convergent strabismus and marked pain during passive movement. Biopsy of quadriceps femoris muscle showed variation in the fiber size with sarcoplasmic areas positive for periodic acid-Schiff stain. Histochemical qualitative reaction for PFK showed no staining of muscle fibers; ultrastructural studies showed abnormal accumulation of glycogen granules in both intermyofibrillar and subsarcolemmal areas. While some enzyme activities in the muscular crude extract were significantly lower than in controls, direct assay of PFK revealed no activity, thus demonstrating that the child's myopathy was due to the lack of PFK activity.