Neuron-specific mitochondrial degeneration induced by hyperammonemia and octanoic acidemia

Changes in nuclear proteins of astrocytes as a result of acute ammonia or ethanol exposure

We have used an in vitro system to monitor the effects of high levels of ammonia and ethanol on glial cells. Nuclei were isolated and the protein profiles examined by two-dimensional gel electrophoresis. Acute exposure of rat astrocyte cell cultures to ammonia or ethanol resulted in changes in cellular morphology and the level of some nuclear proteins. Glial fibrillary acidic protein (GFAP) levels remained constant under both treatments. Several nuclear proteins were increased specifically. Only one protein was visually detected which was unique to treatment with ammonia or ethanol. This protein (p2a) appeared only in the presence of ammonia. There were no changes in previously observed astrocyte-associated proteins (Silverman et al. Neurochem Int.12, 513-518, 1988). Two proteins appeared de novo upon either treatment with either ammonia or ethanol. These latter proteins had a molecular weight and pI profile similar to the major class of nuclear stress proteins (hsp70). However, results from immunoblot experiments clearly demonstrated that hsp70 was not induced in astroycte cultures following exposure to ammonia or ethanol.

Ammonia potentiates methylmalonic acid-induced convulsions and TBARS production

Hyperammonemia is a common finding in children with methylmalonic acidemia, an inherited metabolic disease characterized by mental retardation, convulsions, and accumulation of methylmalonic acid (MMA). Although it has been suggested that MMA induces convulsions through succinate dehydrogenase (SDH) inhibition, very little is known about the contribution of hyperammonemia to the development of convulsions in these patients. In the present study we investigated the effects of ammonium ions on the convulsant action of MMA, MMA-induced inhibition of striatal succinate dehydrogenase, and the striatal content of thiobarbituric acid-reactive substances (TBARS). Adult rats were injected with ammonium acetate (1.5 mmol/kg, sc) or sodium acetate (1.5 mmol/kg, sc), followed 5 min later by buffered MMA (3 micromol/microl) or NaCl (4.5 micromol/microl) injected into the striatum. The animals were observed in an open field for the appearance of convulsive episodes. After 30 min of behavioral evaluation, the animals were sacrificed and had their striatal TBARS content measured. Ammonium acetate pretreatment caused no behavioral effects per se, but potentiated MMA-induced convulsions and increased basal TBARS content and MMA-induced TBARS production in the striatum. Ammonium chloride had no effect on basal succinate dehydrogenase activity and did not alter MMA-induced inhibition of SDH in vitro. These results suggest that ammonia potentiates MMA-induced behavioral effects through a mechanism that does not involve further succinate dehydrogenase inhibition, but may involve facilitation of MMA-induced oxidative damage and provide evidence that ammonia and MMA may have mutually additive toxicity.

Ammonia neurotoxicity: role of the mitochondrial permeability transition

Hepatic encephalopathy (HE) is an important cause of morbidity and mortality in patients with severe liver disease. Although the mechanisms responsible for HE remain elusive, ammonia is generally considered to be involved in its pathogenesis, and astrocytes are thought to be the principal target of ammonia neurotoxicity. Altered bioenergetics and oxidative stress are also thought to play a major role in this disorder. In this paper, we present data invoking the mitochondrial permeability transition (MPT) as a factor in the pathogenesis of HE/hyperammonemia. The MPT is a Ca2+-dependent, cyclosporin A (CsA) sensitive process due to the opening of a pore in the inner mitochondrial membrane that leads to a collapse of ionic gradients and ultimately to mitochondrial dysfunction. Many of the factors that facilitate the induction of the MPT are also known to be implicated in the mechanism of HE, including free radicals, Ca2+, nitric oxide, alkaline pH, and glutamine. We have recently shown that treatment of cultured astrocytes with 5 mM NH4Cl resulted in a dissipation of the mitochondrial membrane potential (delta(psi)m), which was sensitive to CsA. Similarly treated cultured neurons failed to show a loss of the delta(psi)m. Further support for the ammonia induction of the MPT was obtained by observing an increase in mitochondrial permeability to 2-deoxyglucose-6-phosphate, and a decrease in calcein fluorescence in astrocytes after ammonia treatment, both of which were also blocked by CsA. CsA was likewise capable of exerting a protective effect against hyperammonemia in mice. Taken together, our data suggest that the MPT represents an important component of the pathogenesis of HE and other hyperammonemic states.

Ammonia induces the mitochondrial permeability transition in primary cultures of rat astrocytes

Ammonia is a toxin that has been strongly implicated in the pathogenesis of hepatic encephalopathy (HE), and the astrocyte appears to be the principal target of ammonia toxicity. The specific neurochemical mechanisms underlying HE, however, remain elusive. One of the suggested mechanisms for ammonia toxicity is impaired cellular bioenergetics. Because there is evidence that the mitochondrial permeability transition (MPT) is associated with mitochondrial dysfunction, we determined whether the MPT might be involved in the bioenergetic alterations related to ammonia toxicity. Accordingly, we examined the mitochondrial membrane potential (Deltapsi(m)) in cultured astrocytes and neurons using laser-scanning confocal microscopy after loading the cells with the voltage-sensitive dye JC-1. We found that ammonia induced a dissipation of the Deltapsi(m) in a time- and concentration-dependent manner. These findings were supported by flow cytometry using the voltage-sensitive dye tetramethylrhodamine ethyl ester (TMRE). Cyclosporin A, a specific inhibitor of the MPT, completely blocked the ammonia-induced dissipation of the Deltapsi(m). We also found an increase in the mitochondrial permeability to 2-deoxyglucose in astrocytes that had been exposed to 5 mM NH(4)Cl, further supporting the concept that ammonia induces the MPT in these cells. Pretreatment with methionine sulfoximine, an inhibitor of glutamine synthetase, blocked the ammonia-induced collapse of Deltapsi(m), suggesting a role of glutamine in this process. Over a 24-hr period, ammonia had no effect on the Deltapsi(m) in cultured neurons. Collectively, our data indicate that ammonia induces the MPT in cultured astrocytes, which may be a factor in the mitochondrial dysfunction associated with HE and other hyperammonemic states.

Differential inhibition by hyperammonemia of the electron transport chain enzymes in synaptosomes and non-synaptic mitochondria in ornithine transcarbamylase-deficient spf-mice: restoration by acetyl-L-carnitine

Sparse-fur (spf) mouse is the ideal animal model to study the neuropathology of congenital ornithine transcarbamylase (OTC) deficiency. Our current hypothesis implies that an ammonia-induced depletion of energy metabolism in the spf mouse, could be due to a reduction in the activities of the enzymes of the electron transport chain and a treatment with acetyl-L-carnitine could normalize this abnormality. We also hypothesized that there might be a differential degree of inhibition in synaptosomal and non-synaptic mitochondria, for the enzymes of the electron transport chain, caused by congenital hyperammonemia. We have therefore measured the activities of NADH-cytochrome C oxidoreductase, succinate cytochrome C oxidoreductase and cytochrome C oxidase in synaptosomes and non-synaptic mitochondria, isolated from spf mice and CD-1 controls with and without acetyl-L-carnitine treatment. Our results indicate a significant reduction (19-34%) in the activities of these complexes in synaptosomes in untreated spf mice, whereas in non-synaptic mitochondria, there was a tendency for the activities to decrease. Acetyl-L-carnitine treatment enhanced these activities (15-64%) for all the three enzyme complexes and its effect was more prominent on succinate cytochrome C oxidoreductase activity (64%). These studies point out that: (a) ammonia-induced disturbances in the energy metabolism could be more pronounced in neuronal mitochondria, and (b) the effect of acetyl-L-carnitine on the restoration of cerebral ATP in hyperammonemia could be through an enhancement of the activities of various electron transport chain enzymes.

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

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

Reye's and Reye-like syndromes, drug-related diseases? (causative agents, etiology, pathogenesis, and therapeutic approaches)

In the literature the separation between RS and RLS is confusing and makes it difficult to plan an appropriate preventive action or to develop new therapeutic approaches. We suggest that the generalized damage and encephalopathy seen in both RS and RLS may be due to a wide variety of causative agents that contribute to a common derangement, principally involving mitochondrial oxidative pathway. Fasting status and infections increase the catabolism and the subsequent flux of metabolites from peripheral tissues to the liver (FA and amino acids); cytokines (TNF, IL-1, and IL-6), in particular, mediate this effect during infection and experimental endotoxemia. Some drugs and other toxic compounds induce functional and morphological liver mitochondrial derangement. Oxidative metabolism is impaired, with subsequent stimulation of alternative pathways of oxidation, following production of unusual toxic acyl CoAs and dicarboxylic acids. Toxic compounds accumulate in the liver, deranging its functions and causing energy depletion, and are also released in the circulation from which they reach other tissues, including the brain. Neurons and astrocytes in the brain may be affected differently: Neurons suffer from the lack of energy and the effect of toxic compounds arriving from the bloodstream, and astrocytes may be directly affected by the beta-oxidation derangement. Very important may be genetic predisposition, which, by making the patient more sensitive to a particular causative agent, may facilitate the onset of RS and RLS. The therapeutic approach is, presently, mainly symptomatic, directed as it is to counteracting each alteration shown, depending by the clinical gravity. Other pharmacological approaches are only studied experimentally, like carnitine supplementation and PGE2 administration, or theoretically envisaged, like monoclonal antibody therapy directed at LPS or at pro-inflammatory cytokines or treatment with interferon-alpha.

Brain mitochondrial citrate synthase and glutamate dehydrogenase: differential inhibition by fatty acyl coenzyme A derivatives

Organic acidemia is found in several metabolic encephalopathies (e.g., hepatic and valproate encephalopathies, Reye's syndrome, and hereditary organic acidemias). Although fatty acids are known to be neurotoxic, the underlying mechanisms have not been fully elucidated. It has been hypothesized that one mechanism underlying fatty acid neurotoxicity is the selective inhibition of rate-limiting and/or regulated tricarboxylic acid (TCA) cycle and related enzymes by fatty acyl-coenzyme A (CoA) derivatives. To test the hypothesis, this study has examined the effects of several fatty acyl-CoAs on citrate synthase (CS) and glutamate dehydrogenase (GDH) in brain mitochondria. At levels higher than 100 microM, butyryl-CoA (BCoA; a short-chain acyl-CoA; IC50 approximately 640 microM), octanoyl-CoA (OCoA; a medium-chain acyl-CoA; IC50 approximately 380 microM), n-decanoyl-CoA (DCoA; a medium-chain acyl-CoA; IC50 approximately 436 microM), and palmitoyl-CoA (PCoA; a long-chain acyl-CoA; IC50 approximately 340 microM) inhibited brain mitochondrial CS activity in a concentration-related manner. However, these fatty acyl-CoAs were less effective inhibitors (IC50 values for OCoA, DCoA, and PCoA being approximately 1260, 420, and 720 microM, respectively) of brain mitochondrial GDH activity. Compared to the other three acyl-CoAs investigated, BCoA was a very poor inhibitor of GDH. These results demonstrate that fatty acyl-CoAs are inhibitors of brain mitochondrial CS and GDH activities only at pathological/toxicological levels. Thus, the fatty acyl-CoA inhibition of brain mitochondrial CS and GDH is unlikely to assume major pathophysiological and/or pathogenetic importance.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in the cytoplasmic (lactate dehydrogenase) and plasma membrane (acetylcholinesterase) marker enzymes in the synaptic and nonsynaptic mitochondria derived from rats with moderate hyperammonemia

The activities of the cytoplasmic and plasma membrane marker enzymes: lactate dehydrogenase (LDH) and acetylcholinesterase (AChE), respectively, were measured in the cerebral homogenates, in the synaptic and nonsynaptic mitochondrial fractions, and in the postmitochondrial supernatants derived from rats in which a 3-d, moderately hyperammonemic condition (no more than 120% increases in blood ammonia) was produced by repeated administration of ammonium acetate (simple hyperammonemia, SHA) or a hepatotoxin, thioacetamide (TAA) (hepatic encephalopathy, HE). As measured in the homogenate and postmitochondrial supernatants, neither of the enzyme activities was affected by SHA or HE. SHA and HE increased the synaptic mitochondrial LDH activity by respectively 53 and 24%, but reduced this enzyme activity in nonsynaptic mitochondria by 19%. Both conditions stimulated the synaptic and nonsynaptic mitochondrial AChE activity by 30-40%. By contrast, the only significant change produced in these fractions by in vitro treatment with a toxic (3 mM) concentration of ammonium chloride was a slight decrease of LDH activity in nonsynaptic mitochondria and postmitochondrial supernatants. It is concluded that moderate hyperammonemia modifies subsequent separation of both cerebral classes of mitochondria from the cytosolic and plasma membrane components. This modification is likely to reflect subtle hyperammonemia-related changes in the physicochemical properties of the two mitochondrial classes and/or other subcellular components.

Astrocyte volume regulation and ATP and phosphocreatine concentrations after exposure to salicylate, ammonium, and fatty acids

Cellular volume regulation following swelling in hypo-osmotic phosphate-buffered saline (PBS) and ATP and phosphocreatine concentrations of cells incubated in iso-osmotic or hypo-osmotic PBS were measured in primary cultured rat cerebral astrocytes exposed for 30 min to NH4Cl, salicylate, hexanoate, octanoate, and/or dodecanoate. These compounds have been implicated in the pathogenesis of cerebral edema in Reye's Syndrome. NH4Cl (0.10 - 10 mM) had no effect on astrocyte volume regulation or ATP concentration. Salicylate significantly reduced ATP concentrations at 3.0 mM and 10 mM but had no effect on volume regulation. Hexanoate (10 mM and 30 mM) decreased astrocyte ATP content by over 80% while octanoate (10 mM) reduced ATP content by more than 50%. Concentrations of these fatty acids at or below 3.0 mM had no effect on ATP content. Volume regulation was inhibited by 3.0 mM hexanoate and 3.0 mM octanoate but not lower concentrations. Dodecanoate (0.1-3.0 mM) decreased cellular ATP content by 33-51% in iso-osmotic PBS solutions. Phosphocreatine content was reduced by exposure to salicylate or octanoate at concentrations which had no effect on ATP content. These results indicate that astrocyte energy metabolism and volume regulation may be compromised by agents associated with cerebral edema in Reye's Syndrome. Analysis of the dose-dependence of these effects suggests that inhibition of astrocyte energy metabolism is not sufficient to affect volume regulation.

Beta-oxidation of [1-14C]palmitic acid by mouse astrocytes in primary culture: effects of agents implicated in the encephalopathy of Reye's syndrome

beta-Oxidation of [1-14C]palmitic acid was examined in homogenates of astrocytes cultured from neonatal mouse brain. Under optimal reaction conditions (< or = 50 micrograms protein, 10 min at 37 degrees C), oxidation increased as a function of palmitate concentration (15 microM to 2 mM) and reached a maximum rate of 1.98 +/- 0.29 nmol/min/mg protein (mean +/- SEM) at 0.2 mM substrate. Eadie-Hofstee analysis of data from four experiments yielded apparent values for Vmax of 1.87 nmol/min/mg protein, and for Km, 35-40 microM. There were no dramatic changes in the oxidation rate in cells between 10 and 36 days in culture. During the 10-min assays, less than 0.05% of the radioactivity was converted to 14CO2 by the astrocytes; water-soluble products accounted for 1-2% of the total substrate added. Studies with KCN indicated that 60-70% of the total activity occurred in the mitochondria. We have been studying the structural and functional changes associated with the cerebral encephalopathy of Reye's syndrome (RS). Three-week-old astrocytes exposed to serum from RS children for the final 7 days of culture exhibited minor mitochondrial pleomorphism and had increased numbers of other intracellular organelles. Examination of the effects of agents implicated in RS indicated that oxidation of [1-14C]palmitate was not altered by Na+ salicylate (1-3 mM), but was inhibited by the industrial surfactant, Toximul MP-8 (> or = 10 micrograms/ml), 4-pentenoic acid (> or = 0.1 microM), or with 4 days' exposure to ammonia (10 nM). The latter treatment also resulted in an increase in protein synthesis, cell volume, and malondialdehyde formation. These results suggest that some of the "toxins" implicated in RS inhibit fatty-acid oxidation in the astrocytes and produce other lipid-related abnormalities that could be related to encephalopathy.

Neurotoxicity of ammonia and fatty acids: differential inhibition of mitochondrial dehydrogenases by ammonia and fatty acyl coenzyme A derivatives

In several metabolic encephalopathies, hyperammonemia and organic acidemia are consistently found. Ammonia and fatty acids (FAs) are neurotoxic: previous workers have shown that ammonia and FAs can act singly, in combination, or synergistically, in inducing coma in experimental animals. However, the biochemical mechanisms underlying the neurotoxicity of ammonia and FAs have not been fully elucidated. FAs are normally converted to their corresponding CoA derivatives (CoAs) once they enter cells and it is known that these fatty acyl CoAs can alter intermediary metabolism. The present study was initiated to determine the effects of ammonia and fatty acyl CoAs on brain mitochondrial dehydrogenases. At a pathophysiological level (2 mM), ammonia is a potent inhibitor of brain mitochondrial alpha-ketoglutarate dehydrogenase complex (KGDHC). Only at toxicological levels (10-20 mM) does ammonia inhibit brain mitochondrial NAD(+)- and NADP(+)- linked isocitrate dehydrogenase (NAD-ICDH, NADP-ICDH), and NAD(+)-linked malate dehydrogenase (MDH) and liver mitochondrial NAD-ICDH. Butyryl- (BCoA), octanoyl- (OCoA), and palmitoyl (PCoA) CoA were potent inhibitors of brain mitochondrial KGDHC, with IC50 values of 11, 20, and 25 microM, respectively; moreover, the inhibitory effect of fatty acyl CoAs and ammonia were additive. At levels of 250 microM or higher, both OCoA (IC50 = 1.15 mM) and PCoA (IC50 = 470 microM) inhibit brain mitochondrial NADP-ICDH; only at higher levels (0.5-1 mM) does BCoA inhibit this enzyme (by 30-45%). Much less sensitive than KGDHC and NADP-ICDH, brain mitochondrial NAD-ICDH is only inhibited by 1 mM BCoA, OCoA, and PCoA by 22%, 35%, and 44%, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Fatty acid oxidation and ketogenesis by astrocytes in primary culture

The oxidation of the fatty acids octanoate and palmitate to CO2 and the ketone bodies acetoacetate and D-(-)-3-hydroxybutyrate was examined in astrocytes that were prepared from cortex of 2-day-old rat brain and grown in primary culture to confluence. Accumulation of acetoacetate (by mass) in the culture medium of astrocytes incubated with octanoate (0.3-0.5 mM) was 50-90 nmol C2 units h-1 mg of protein-1. A similar rate was obtained using radiolabeled tracer methodology with [1-14C]octanoate as labeled substrate. The results from the radiolabeled tracer studies using [1-14C]- and [7-14C]octanoate and [1-14C]-, [13-14C]-, and [15-14C]palmitate indicated that a substantial proportion of the omega-terminal four-carbon unit of these fatty acids bypassed the beta-ketothiolase step of the beta-oxidation pathway and the 3-hydroxy-3-methylglutaryl (HMG)-CoA cycle of the classic ketogenic pathway. The [14C]acetoacetate formed from the 1-14C-labeled fatty acids, obligated to pass through the acetyl-CoA pool, contained 50% of the label at carbon 3 and 50% at carbon 1. By contrast, the [14C]acetoacetate formed from (omega-1)-labeled fatty acids contained 90% of the label at carbon 3 and 10% at carbon 1, whereas that formed from the (omega-3)-labeled fatty acid contained 20% of the label at carbon 3 and 80% at carbon 1. These results indicate that acetoacetate is primarily formed either by the action of 3-oxo-acid-CoA transferase (EC 2.8.3.5) or acetoacetyl-CoA deacylase (EC 3.1.2.11) or both on acetoacetyl-CoA and not by the action of the mitochondrial HMG-CoA cycle involving HMG-CoA lyase (EC 4.1.3.4), which was readily detected, and HMG-CoA synthase (EC 4.1.3.5), which was barely measurable.

Influence of ammonia, octanoate, quinolinate and hypoxic conditions on NAD(P)H fluorescence of hippocampal slices

Acute exposure of rat hippocampal slices to high concentrations of the neurotoxicants, ammonia, octanoate and quinolinate, was monitored by microscope fluorimetry and NAD(P)H absorption. High concentrations of ammonia or octanoate, or a combination of both, reduced the fluorescence signals under aerobic conditions to a similar extent as observed in the case of CN(?)-induced model hypoxia. Quinolinate alone did not produce any effect, nor was it effective in the presence of ammonia or octanoate. Ammonia or octanoate or a combination of both neurotoxicants being present, administration of 2-oxoglutarate or valine was found to prevent efficiently changes in fluorimetric response in a protective manner. The results are interpreted as being supportive of the notion that these compounds may entail a mechanism of protective potency. Glutamate being substituted for glucose of the normal incubation medium, addition of ammonia produced higher oxidation states of nicotinamide nucleotides (K(m app) = 16.9 mM; SD = 3.1; n = 7) only when excessive ammonia concentrations were chosen. Hence, it is conjectured that glutamate uptake and metabolization are not essentially reduced by ammonia levels even in the case of severe hyperammonaemia.

Fine structural changes in the rat arcuate nucleus by forced running stress

The forced running stress in male rats induced some fine structural changes in the arcuate nucleus. By inducing stress for 2 days, the number of multilamellar astrocytic wrappings and endoplasmic reticular (ER) whorls increased significantly. After a stress of 12 days, half of the rats remained inactive for several weeks, and the other half regained their normal activity. In the inactive group, the ER whorls were partially degenerated at 2 weeks after the stress. These degenerative changes remained for 14 weeks after the stress with the increased lysosomes and the disorganization of r-ER. These findings may suggest that long-term stress induces degenerative changes in the arcuate nucleus neurons and that some of these changes persist with some aging-like morphological changes.

Activities of pyruvate dehydrogenase, enzymes of citric acid cycle, and aminotransferases in the subcellular fractions of cerebral cortex in normal and hyperammonemic rats

Activity levels of pyruvate dehydrogenase, enzymes of citric acid cycle, aspartate and alanine aminotransferases were estimated in mitochondria, synaptosomes and cytosol isolated from brains of normal rats and those injected with acute and subacute doses of ammonium acetate. In mitochondria isolated from animals treated with acute dose of ammonium acetate, there was an elevation in the activities of pyruvate, isocitrate and succinate dehydrogenases while the activities of malate dehydrogenase (malate----oxaloacetate), aspartate and alanine aminotransferases were suppressed. In subacute conditions a similar profile of change was noticed excepting that there was an elevation in the activity of alpha-ketoglutarate dehydrogenase in mitochondria. In the synaptosomes isolated from animals administered with acute dose of ammonium acetate, there was an increase in the activities of pyruvate, isocitrate, alpha-ketoglutarate and succinate dehydrogenases while the changes in the activities of malate dehydrogenase, aspartate and alanine amino transferases were suppressed. In the subacute toxicity similar changes were observed in this fraction except that the activity of malate dehydrogenase (oxaloacetate----malate) was enhanced. In the cytosol, pyruvate dehydrogenase and other enzymes of citric acid cycle except malate dehydrogenase were enhanced in both acute and subacute ammonia toxicity though their activities are lesser than that of mitochondria. In this fraction malate dehydrogenase (oxaloacetate----malate) was enhanced while activities of malate dehydrogenase (malate----oxaloacetate), aspartate and alanine aminotransferases were suppressed in both the conditions. Based on these results it is concluded that the decreased activities of malate dehydrogenase (malate----oxaloacetate) in mitochondria and of aspartate aminotransferase in mitochondria and cytosol may be responsible for the disruption of malate-aspartate shuttle in hyperammonemic state.(ABSTRACT TRUNCATED AT 250 WORDS)

Enzymes of fatty acid beta-oxidation in developing brain

Developmental profiles were determined for the activities of eight enzymes involved in fatty acid beta-oxidation in rat brain. The enzymes studied were the palmitoyl-CoA, octanoyl-CoA, butyryl-CoA, glutaryl-CoA, and 3-hydroxyacyl-CoA dehydrogenases, the enoyl-CoA hydratase (crotonase), and the C4- and C10-thiolases. With the exception of the thiolases, all of the activities (expressed on the basis of brain weight) increased during the postnatal period of brain maturation. The activity of octanoyl-CoA dehydrogenase was elevated markedly compared to that of palmitoyl-CoA dehydrogenase at all developmental stages and in all brain regions in the rat. A similar relationship between these enzymes was observed in various regions of adult human brain. Comparisons of the activities of the beta-oxidation enzymes in human brain versus human skeletal muscle and in cultured neural cell lines (neuroblastoma and glioma) versus cultured skin fibroblasts revealed that the elevated activity of octanoyl-CoA dehydrogenase relative to palmitoyl-CoA dehydrogenase was specific to the neural tissues. This relationship was particularly evident when the enzyme activities were normalized to the activity of crotonase. The data support previous findings with radiochemical tracers, indicating that the brain is capable of utilizing fatty acids as substrates for oxidative energy metabolism. The relatively high activity of the medium-chain fatty acyl-CoA dehydrogenase in neural tissue may represent an adaptive mechanism to protect the brain from the known encephalopathic effects of octanoate and other medium-chain fatty acids that readily cross the blood-brain barrier.

Effects of prolonged exposure to ammonia on fluid-phase, receptor-mediated, and adsorptive (non specific) endocytosis in cultured neuroblastoma cells. A flow-cytometry and cytochemical study

The effect of prolonged exposure to ammonia on fluid-phase, receptor-mediated, and adsorptive (non specific) endocytosis in cultured neuroblastoma (Neuro-2a) cells were studied using fluorescein-labeled dextran, concanavalin A conjugated with fluorescein isothiocyanate, and cationized ferritin as tracers. Ammonia treatment increased the rate of endocytosis of cationized ferritin as well as the number of cell elements involved in the process. Moreover, the number of cytoplasmic components containing acid phosphatase activity was also found to increase following ammonia treatment. In contrast, flow-cytometric analyses showed that, under experimental conditions, exposure to ammonia did not alter the intralysosomal pH and had little effect on the fluid-phase and receptor-mediated endocytosis of fluorescein-labeled dextran and concanavalin-A fluorocrome, respectively.