Acetoacetate and D-(-)-beta-hydroxybutyrate as precursors for sterol synthesis by calf oligodendrocytes in suspension culture: extramitochondrial pathway for acetoacetate metabolism

The lipogenic enzyme acetoacetyl-CoA synthetase and ketone body utilization for denovo lipid synthesis, a review

Acetoacetyl-CoA synthetase (AACS) is the key enzyme in the anabolic utilization of ketone bodies (KBs) for denovo lipid synthesis, a process that bypasses citrate and ATP citrate lyase. This review shows that AACS is a highly regulated, cytosolic, and lipogenic enzyme and that many tissues can readily use KBs for denovo lipid synthesis. AACS has a low micromolar Km for acetoacetate, and supply of acetoacetate should not limit its activity in the fed state. In many tissues, AACS appears to be regulated in conjunction with the need for cholesterol, but in adipose tissue, it seems tied to fatty acid synthesis. KBs are readily utilized as substrates for lipid synthesis in lipogenic tissues, including liver, adipose tissue, lactating mammary gland, skin, intestinal mucosa, adrenals, and developing brain. In numerous studied cases, KBs served several-fold better than glucose as substrates for lipid synthesis, and when present, KBs suppressed the utilization of glucose for lipid synthesis. Here, it is hypothesized that a physiological role for the utilization of KBs for lipid synthesis is a metabolic process of lipid interconversion. Fatty acids are converted to KBs in liver, and then, the KBs are utilized to synthesize cholesterol and other long-chain fatty acids in liver and nonhepatic tissues. The conversion of fatty acids to cholesterol via the KBs may be a particularly important example of lipid interconversion. Utilizing KBs for lipid synthesis is glucose sparing and probably is important with low carbohydrate diets. Metabolic situations and tissues where this pathway may be important are discussed.

White matter disturbances in phenylketonuria: Possible underlying mechanisms

White matter pathologies, as well as intellectual disability, microcephaly, and other central nervous system injuries, are clinical traits commonly ascribed to classic phenylketonuria (PKU). PKU is an inherited metabolic disease elicited by the deficiency of phenylalanine hydroxylase. Accumulation of l-phenylalanine (Phe) and its metabolites is found in tissues and body fluids in phenylketonuric patients. In order to mitigate the clinical findings, rigorous dietary Phe restriction constitutes the core of therapeutic management in PKU. Myelination is the process whereby the oligodendrocytes wrap myelin sheaths around the axons, supporting the conduction of action potentials. White matter injuries are implicated in the brain damage related to PKU, especially in untreated or poorly treated patients. The present review summarizes evidence toward putative mechanisms driving the white matter pathology in PKU patients.

Simulated microgravity enhances oligodendrocyte mitochondrial function and lipid metabolism

The primary energy sources of mammalian cells are proteins, fats, and sugars that are processed by well-known biochemical mechanisms that have been discovered and studied in 1G (terrestrial gravity). Here we sought to determine how simulated microgravity (sim-µG) impacts both energy and lipid metabolism in oligodendrocytes (OLs), the myelin-forming cells in the central nervous system. We report increased mitochondrial respiration and increased glycolysis 24 hr after exposure to sim-µG. Moreover, examination of the secretome after 3 days' exposure of OLs to sim-µG increased the Krebs cycle (Krebs and Weitzman, ) flux in sim-µG. The secretome study also revealed a significant increase in the synthesis of fatty acids and complex lipids such as 1,2-dipalmitoyl-GPC (5.67); lysolipids like 1-oleoyl-GPE (4.48) were also increased by microgravity. Although longer-chain lipids were not observed in this study, it is possible that at longer time points OLs would have continued moving forward toward the synthesis of lipids that constitute myelin. For centuries, basic developmental biology research has been the pillar of an array of discoveries that have led to clinical applications; we believe that studies using microgravity will open new avenues to our understanding of the brain in health and disease-in particular, to the discovery of new molecules and mechanisms impossible to unveil while in 1G. © 2016 Wiley Periodicals, Inc.

Metabolism and functions of lipids in myelin

Rapid conduction of nerve impulses requires coating of axons by myelin sheaths, which are lipid-rich and multilamellar membrane stacks. The lipid composition of myelin varies significantly from other biological membranes. Studies in mutant mice targeting various lipid biosynthesis pathways have shown that myelinating glia have a remarkable capacity to compensate the lack of individual lipids. However, compensation fails when it comes to maintaining long-term stability of myelin. Here, we summarize how lipids function in myelin biogenesis, axon-glia communication and in supporting long-term maintenance of myelin. We postulate that change in myelin lipid composition might be relevant for our understanding of aging and demyelinating diseases. This article is part of a Special Issue titled Brain Lipids.

Contribution of acetoacetate to the synthesis of cholesterol and fatty acids in regions of developing rat brain in vivo

(3)H(2)O and [3-(14)C]acetoacetate were injected i.p. into developing rats (5-50 days of age). After 2 h the brains were dissected into 6 parts. The incorporation of (3)H and (14)C into total fatty acids and into cholesterol in these 6 parts and in the spinal cord was measured. The data were analysed to evaluate the developmental patterns of the synthesis of fatty acids and cholesterol in various parts of the rat CNS and to compare the contribution of acetoacetate to these processes. Our results indicate (1) a large variation between CNS regions in the rates of lipid synthesis as well as in the developmental patterns; highest activities were found in the spinal cord during the third postnatal week, whereas the activities in cortical areas were much lower during all stages of development; (2) a constant ratio between the amounts of label incorporated into lipid fractions from [3-(14)C]acetoacetate and from (3)H(2)O, indicating that acetoacetate contributes to a similar extent to lipid synthesis in all parts of the developing rat CNS; (3) a similar preference in the use of acetoacetate for cholesterogenesis as compared to lipogenesis in all parts of the CNS of suckling rats; (4) a marked increase of this preference after weaning of the pups.

N-Acetylaspartate in the CNS: from neurodiagnostics to neurobiology

The brain is unique among organs in many respects, including its mechanisms of lipid synthesis and energy production. The nervous system-specific metabolite N-acetylaspartate (NAA), which is synthesized from aspartate and acetyl-coenzyme A in neurons, appears to be a key link in these distinct biochemical features of CNS metabolism. During early postnatal central nervous system (CNS) development, the expression of lipogenic enzymes in oligodendrocytes, including the NAA-degrading enzyme aspartoacylase (ASPA), is increased along with increased NAA production in neurons. NAA is transported from neurons to the cytoplasm of oligodendrocytes, where ASPA cleaves the acetate moiety for use in fatty acid and steroid synthesis. The fatty acids and steroids produced then go on to be used as building blocks for myelin lipid synthesis. Mutations in the gene for ASPA result in the fatal leukodystrophy Canavan disease, for which there is currently no effective treatment. Once postnatal myelination is completed, NAA may continue to be involved in myelin lipid turnover in adults, but it also appears to adopt other roles, including a bioenergetic role in neuronal mitochondria. NAA and ATP metabolism appear to be linked indirectly, whereby acetylation of aspartate may facilitate its removal from neuronal mitochondria, thus favoring conversion of glutamate to alpha ketoglutarate which can enter the tricarboxylic acid cycle for energy production. In its role as a mechanism for enhancing mitochondrial energy production from glutamate, NAA is in a key position to act as a magnetic resonance spectroscopy marker for neuronal health, viability and number. Evidence suggests that NAA is a direct precursor for the enzymatic synthesis of the neuron specific dipeptide N-acetylaspartylglutamate, the most concentrated neuropeptide in the human brain. Other proposed roles for NAA include neuronal osmoregulation and axon-glial signaling. We propose that NAA may also be involved in brain nitrogen balance. Further research will be required to more fully understand the biochemical functions served by NAA in CNS development and activity, and additional functions are likely to be discovered.

Lactate utilization by brain cells and its role in CNS development

We studied the role played by lactate as an important substrate for the brain during the perinatal period. Under these circumstances, lactate is the main substrate for brain development and is used as a source of energy and carbon skeletons. In fact, lactate is used actively by brain cells in culture. Neurons, astrocytes, and oligodendrocytes use lactate as a preferential substrate for both energy purposes and as precursor of lipids. Astrocytes use lactate and other metabolic substrates for the synthesis of oleic acid, a new neurotrophic factor. Oligodendrocytes mainly use lactate as precursor of lipids, presumably those used to synthesize myelin. Neurons use lactate as a source of energy and as precursor of lipids. During the perinatal period, neurons may use blood lactate directly to meet the need for the energy and carbon skeletons required for proliferation and differentiation. During adult life, however, the lactate used by neurons may come from astrocytes, in which lactate is the final product of glycogen breakdown. It may be concluded that lactate plays an important role in brain development.

Induction of ketone body enzymes in glial cells

Ketone bodies serve a dual function in developing brain. They are important sources of energy for metabolism and serve as precursors for lipid synthesis. Astrocytes have two to three times higher activity than oligodendroglia for one of the enzymes involved in ketone body metabolism, 3-ketoacid-CoA transferase. Both glial cell types have similar levels of activity for beta-hydroxybutyrate dehydrogenase. Glucocorticoids and dibutytyl cAMP produce a significant stimulation of activity of both enzymes in astrocytes and oligodendroglia. However, the most striking induction in activity of the two enzymes is in the presence of hydrocortisone and sodium butyrate. There is a three- to eightfold stimulation with these effectors in both astrocytes and oligodendroglia. Thus, in brain the expression of ketone body enzyme activities is finely regulated by hormones and by agents that increase cAMP levels.

Ketone body utilization for lipid synthesis in the murine sciatic nerve: alterations in the dysmyelinating trembler mutant

This work demonstrates that in vitro sciatic nerves of normal and trembler adult mice can use ketone bodies (beta-hydroxybutyrate and acetoacetate) and butyrate for lipid synthesis. In normal sciatic nerves, beta-hydroxybutyrate is incorporated in total lipids to a larger extent than acetoacetate (141% and 33%, respectively, of acetate incorporation), whereas for trembler sciatic nerves, these percentages are only 69% and 27%. Incorporation of ketone bodies is greater into sterols than into other lipids. Lipid metabolism of ketone bodies in trembler nerves is altered and could reflect a process similar to Wallerian degeneration: a dramatic decrease of sterol and free fatty acid synthesis and an increased synthesis of triglycerides. Moreover, differences seen in precursor incorporation into lipids between normal and trembler sciatic nerves suggest that their lipid metabolism is not the same.

Acetoacetate and glucose as lipid precursors and energy substrates in primary cultures of astrocytes and neurons from mouse cerebral cortex

Primary cultures of astrocytes and neurons derived from neonatal and embryonic mouse cerebral cortex, respectively, were incubated with [3-14C]acetoacetate or [2-14C]glucose. The utilization of glucose and acetoacetate, the production of lactate, D-3-hydroxybutyrate, and 14CO2, and the incorporation of 14C and of 3H from 3H2O into lipids and lipid fractions were measured. Both cell types used acetoacetate as an energy substrate and as a lipid precursor; lactate was the major product of glucose metabolism. About 60% of the acetoacetate that was utilized by neurons was oxidized to CO2, whereas this was only approximately 20% in the case of cultured astrocytes. This indicates that the rate at which 14C-labeled Krebs cycle intermediates exchange with pools of unlabeled intermediates is much higher in astrocytes than in neurons. Acetoacetate is a better precursor for the synthesis of fatty acids and cholesterol than glucose, presumably because it can be used directly in the cytosol for these processes; preferential incorporation into cholesterol was not observed in these in vitro systems. We conclude that ketone bodies can be metabolized both by the glial cells and by the neuronal cells of developing mouse brain.

Acetoacetate and glucose as substrates for lipid synthesis by rat brain oligodendrocytes and astrocytes in serum-free culture

We have compared glucose and acetoacetate as precursors for lipogenesis and cholesterogenesis by oligodendrocytes and astrocytes, using mixed glial cultures enriched in oligodendrocytes. In order to differentiate between metabolic processes in oligodendrocytes and those in astrocytes, the other major cell type present in the mixed culture, we carried out parallel incubations with cultures from which the oligodendrocytes had been removed by treatment with anti-galactocerebroside serum and guinea-pig complement. The following results were obtained: 1. Both oligodendrocytes and astrocytes in culture actively utilize acetoacetate as a precursor for lipogenesis and cholesterogenesis. 2. In both cell types, the incorporation of acetoacetate into fatty acids and cholesterol exceeds that of glucose by a factor of 5-10 when the precursors are present at concentrations of 1 mM and higher. 3. Glucose stimulates acetoacetate incorporation into fatty acids and cholesterol, whereas acetoacetate reduces the entry of glucose into these lipids. This suggests that glucose is necessary for NADPH generation, but that otherwise the two precursors contribute to the same acetyl-CoA pool. 4. Both with acetoacetate and with glucose as precursor, oligodendrocytes are more active in cholesterol synthesis than astrocytes. 5. Using incorporation of 3H2O as an indicator for total lipid synthesis, we estimated that acetoacetate contributes one third of the acetyl groups and glucose one twentieth when saturating concentrations of both substrates are present.

Acetoacetate is a cholesterogenic precursor for myelinating rat brain and spinal cord. Incorporation of label from [3-14C]acetoacetate, [14C]glucose and 3H2O

Rat pups, 3 weeks old, were injected i.p. with combinations of 3H2O and either [3-14C]acetoacetate or [14C]glucose. 3H/14C incorporation ratios were measured in lipid fractions of homogenates and myelin prepared from whole brain and spinal cord. Spinal cord synthesized at least twice as much fatty acids and 3-fold more sterols than whole brain. Both tissues used acetoacetate preferentially for sterol synthesis, whereas label from [14C]glucose was distributed between fatty acids and sterols in the same way as 3H from 3H2O. The relative contributions of acetoacetate to sterol synthesis in whole tissue and in the purified myelin fraction were about the same, both for the cerebrum and for the spinal cord.

Competition among oxidizable substrates in brains of young and adult rats. Whole homogenates

The rates of conversion into 14CO2 of D-(-)-3-hydroxy[3-14C]butyrate, [3-14C]acetoacetate, [6-14C]glucose and [U-14C]glutamine were measured in the presence and absence of unlabelled alternative oxidizable substrates in whole homogenates from the brains of young and adult rats. The addition of unlabelled glutamine resulted in decreased 14CO2 production from [6-14C]glucose in brain homogenates from both young and adult rats. In contrast, glucose had no effect on [U-14C]glutamine oxidation. In suckling animals, both 3-hydroxybutyrate and acetoacetate decreased the rate of oxidation of [6-14C]glucose, but in adults only 3-hydroxybutyrate had an effect, and to a lesser degree. The addition of unlabelled glucose markedly enhanced the rates of oxidation of both ketone bodies in adult brain tissue and had little or no effect in the young. The rate of production of 14CO2 from [U-14C]glutamine was increased by the addition of unlabelled ketone bodies in brain homogenates from young, but not from adult rats. In the converse situation, unlabelled glutamine added to 14C-labelled ketone bodies diminished 14CO2 production in young rats, but had no effect in adult animals. These results revealed a complex age-dependent pattern of interaction in which certain substrates apparently competed with each other, whereas an enhanced rate of 14CO2 production was found with others.

Effect of pH on binding of agonists and antagonists to rat heart muscarinic receptors

The pH-dependence of antagonist and agonist binding to rat heart muscarinic receptors was investigated at 25 degrees C, in the absence and in the presence of GTP. The small inhibitory effect observed at the lowest pH investigated (pH 6.0) on [N-methyl-3H]methscopolamine chloride and [methyl-3H]oxotremorine-M acetate binding indicated that one or more amino acid residues of the receptor had to be deprotonated for optimal binding affinity. The low pK value of these residues (between 5 and 6) prevented their identification. The binding of scopolamine (pK 7.6) was favoured by a positive charge in the titratable amine, but binding with a lower affinity remained possible charge in the titratable amine, but binding with a lower affinity remained possible without this charge. GTP did not affect antagonist binding at any pH, but converted more than 90% of agonist binding sites into a low affinity conformation. In the absence of GTP, we observed a time- and pH-dependent conversion of the super-high- and high-affinity receptors to a low-affinity GTP-insensitive state. This conversion was markedly accelerated at high pH (above pH 8.0). In the presence of GTP, a positive charge on the titratable amine of pilocarpine (pK 7.05) and oxotremorine (pK 8.60) was required for binding. These results support the view that antagonist (e.g. methscopolamine) binding to receptors was largely facilitated by hydrophobic interactions, whereas agonist binding to low-affinity sites was mainly driven by ionic interactions.

Differential substrate oxidation by dissociated brain cells and homogenates during development

The rates of oxidation of 3-hydroxy[3-14C]butyrate, [3-14C]acetoacetate and [6-14C]glucose were compared by using two different preparations of brain from the same animals (i.e. whole homogenates and dissociated brain cells) at various ages during development. In homogenates the rates of oxidation of 3-hydroxy[3-14C]butyrate and [3-14C]acetoacetate were high in young rats and low in adults, and were significantly higher at most ages during development than those obtained for intact cells. In contrast, rates of [6-14C]glucose oxidation by homogenates and intact cells were essentially the same at early ages; however, the rate by homogenates did not change throughout development, whereas that by intact cells increased severalfold by adulthood. In adult animals the initial glucose concentration affected the rate of glucose oxidation in homogenates, but not in intact cells. These data suggest a role for the intact cell membrane in the regulation of alternative substrate utilization by brain cells and that this process changes during development. However, the data may reflect selective differences in the cellular and subcellular components in these two preparations.

Activity of acetoacetyl--CoA thiolase and regulation of ketone body metabolism in the brain of the developing chick

The acetoacetyl-CoA thiolase activity was estimated in subcellular fractions isolated from the cerebral hemispheres, the optic lobes and the cerebellum of the chick between the 20th day of embryonic life and the 30th day of postnatal maturation. Acetoacetyl-CoA thiolase is located both in mitochondria and microsomes of the chick brain. Mitochondrial enzyme activity remains high between the 20th day of embryonic life and the 2nd or the 4th day after hatching, depending on the considered brain area. It then decreases until 30 days after hatching. Cytoplasmic thiolase activity remains unchanged during pre- and postnatal development in the cerebral hemispheres; it increases during the same time in the optic lobes. In the cerebellum, cytoplasmic thiolase activity develops in the same way as in mitochondria. The regulation of ketone body utilization by the developing chick brain widely differs from that by the mammalian brain. In the chick, 3-hydroxybutyrate is nearly the single ketone body utilized by the brain. It is converted into acetyl-CoA in the mitochondria and cytoplasm of the chick brain through two exactly parallel pathways involving the participation of the same enzymes.

On the phospholipid metabolism of glial cell primary cultures: cell characterization and their utilization of 1-alkyl-glycerophosphoethanolamine

Primary cultures prepared from newborn rat brain were grown for 16 or 17 days in culture. Addition of brain extract from newborn rats to the medium stimulated the maturation of astrocytes and the development of oligodendrocytes. Both cell types were characterized by morphology and by immunohistochemistry. The phospholipid composition of these cells was estimated. Incubations were performed with 1-[3H]alkyl-sn-glycerophosphoethanolamine in varying concentrations for 3 h. About one-third of the substrate supplied was internalized by the cells. Several enzymic reactions were observed. The acylating enzyme system was the most active one--a Km was determined with 5 nmol intracellular 1-alkyl-sn-glycerophosphoethanolamine/mg cell protein. Plasmalogen formation was rather low. 1-Alkyl-sn-glycerol, a hydrolysis product, was found in small amounts. Some radioactivity was also incorporated into the phosphatidylcholine fraction.

Utilization of ketone bodies and glucose by established neural cell lines

The rates of utilization of [3-14C]-acetoacetate, [3-14C]-3-hydroxybutyrate, and [6-14C]-glucose were measured in four established cell lines from neuroblastoma of rat (B103) and mouse (N4TG1) and from rat astrocytoma (RGC6) and mouse oligodendroglia (G2620). The rates of incorporation of acetoacetate into lipid were 3-5 times higher than glucose in all cell lines. The incorporation of 3-hydroxybutyrate was similar to that of glucose. Thin-layer chromatography of the total lipid extracts showed the same relative rates of use of these substrates for synthesis of various phospholipids and neutral lipids. The rates of incorporation into neutral lipids and phosphatidylcholine were essentially linear for 12 hr; however, that into phosphatidylethanolamine was markedly higher in the second 6 hr interval than in the first. In all cases, the greatest percentage of label (35-50%) appeared in the phosphatidylcholine fraction. The distribution of label from each of the three substrates among the various lipids was similar in the glial cells, but there were marked differences in distribution of the two ketone bodies in the neuroblastoma lines. These cells also synthesized lipids that migrated to the same area on the chromatogram as cholesterol esters and free fatty acids. In three of the four cell lines the rates of oxidation were highest for glucose, intermediate for acetoacetate, and lowest for 3-hydroxybutyrate. The ratios of the rate of incorporation to the rate of oxidation were higher for ketone bodies (3.32 for 3-hydroxybutyrate and 5.29 for acetoacetate) than for glucose (0.41). This indicates that in these cells ketone bodies are directed toward lipid synthesis rather than oxidation, and glucose is preferentially used as an energy source.

Preferential utilization of ketone bodies for the synthesis of myelin cholesterol in vivo

1. The distribution of radioactivity among lipid classes of myelin and other subcellular brain fractions of young rats (18-21 days) was determined after in vivo injection of (3-(14)C-labelled ketone bodies, [U-(14)C] glucose or [2-(14)C] glucose. 2. The incorporation ratios (sterol/fatty acids) were 0.67, 1.48, 0.25, 0.62 and 0.54 for whole brain, myelin, mitochondria, microsomes and synaptosomes, respectively, with (3-(14)C)-labelled ketone bodies as substrate and 0.37, 0.89, 0.19, 0.34 and 0.29 with [U-(14)C] glucose as substrate. These data show that, both in whole brain and in subcellular brain fractions, acetyl groups derived from ketone bodies are used for sterol synthesis to a large extent than acetyl groups originating from glucose. 3. The specific radioactivity of cholesterol is much higher in myelin than in whole brain or in the other brain fractions, particularly after administration of labelled ketone bodies as substrate. 4. The incorporation patterns of acetoacetate and D-3-hydroxybutyrate were very similar, indicating that both ketone bodies contribute acetyl groups for lipid synthesis via the same metabolic route. 5. Our data suggest that a direct metabolic path from ketone bodies towards cholesterol exists - possibly via acetoacetyl-CoA formation in the cytosol of brain cells - and that this process is most active in oligodendrocytes.

Ketone-body metabolism in glioma and neuroblastoma cells

We have examined the metabolism of ketone bodies in neuroblastoma C1300 and glioma C6 cells, two established lines of neural origin. The three ketone body-metabolizing enzymes are present in cells of both lines in the relative proportions normally found in brain (D-3-hydroxybutyrate dehydrogenase less than acetoacetyl-CoA thiolase less than 3-ketoacid CoA-transferase), the activities of the first two are higher in glioma cells than in neuroblastoma, and that of the third is 2-fold higher in neuroblastoma cells than in glioma cells. The specific activity of 3-ketoacid CoA-transferase (EC 2.8.3.5) in both cell lines increased as the cultures achieved confluence, then decreased. Ketone bodies and especially acetoacetate are preferred substrates for synthesis of neural lipids in cells of both lines. The incorporation of glucose carbon into lipids is significantly reduced in cells of both lines in the presence of ketone bodies. Addition of acetoacetate but not DL-3-hydroxybutyrate to the culture medium resulted in a significant increase in the activity of 3-ketoacid CoA-transferase and also in the rate of acetoacetate oxidation in neuroblastoma cells but not glioma cells. These findings indicate that specific differences exist in the capacity of these two cell lines to metabolize ketone bodies and also that substrate-level regulation of the ketone body-metabolizing pathway exists. These two lines therefore provide a potentially useful system in which the mechanisms of regulation of these enzymes may be examined.

beta-Hydroxybutyrate as a precursor to the acetyl moiety of acetylcholine

Rat brain cortex slices were incubated with 10 mM-glucose and trace amounts of [6-3H]glucose and [3-14C]beta-hydroxybutyrate. The effects of (-)-hydroxycitrate, an inhibitor of ATP-citrate lyase; methylmalonate, an inhibitor of beta-hydroxybutyrate dehydrogenase; and increasing concentrations of unlabeled acetoacetate were examined. The incorporation of label into lactate, citrate, malate, and acetylcholine (ACh) was measured and 3H:14C ratios calculated. Incorporation of [14C]beta-hydroxybutyrate into lactate was limited because of the low activity of gluconeogenic enzymes in brain, whereas incorporation of 14C label into Krebs cycle intermediates and ACh was higher than in previous experiments with [3H-,14C]-glucose. (-)-Hydroxycitrate (5.0 mM) reduced incorporation of [3H]glucose and [14C]beta-hydroxybutyrate into ACh. In contrast, slices incubated with methylmalonate (1 mM) showed a decrease in 14C incorporation without appreciably affecting glucose metabolism. The effects of high concentrations of methylmalonate were nonselective and yielded a generalized decrease in metabolism. Acetoacetate (1 mM) also produced a decreased 14C incorporation into ACh and its precursors. At 10 mM, acetoacetate reduced 3H and 14C incorporation into ACh without substantially affecting total ACh content. From the results, it is suggested that in adult rats beta-hydroxybutyrate can contribute to the acetyl moiety of ACh, possibly via the citrate cleavage pathway, though it is quantitatively less important than glucose and pyruvate. This contribution of ketone bodies could become significant should their concentration become abnormally high or glucose metabolism be reduced.

Long term culture of bovine oligodendroglia isolated with a Percoll gradient

Oligodendroglia were isolated from calf central nervous system (CNS) white matter by trypsinization in phosphate buffered saline and separation by centrifugation through Percoll. Using antisera to phenotypic markers and double labelling experiments we were able to identify essentially all cells in the cultures. The cells obtained were: (1) viable; (2) had intact plasma membranes and well preserved organelles, ribosomes and mitochondria; and (3) were greater than or equal to 95% oligodendroglia 16-20 h after isolation as determined by ability to bind antigalactocerebroside antibodies (anti-GalC). Oligodendroglia could be cultured for several weeks to months. Oligodendroglia established and maintained processes which bound anti-GalC. Myelin basic protein could be demonstrated in the cytoplasm of 40-60% of oligodendroglia cell bodies but not in the processes.

Fatty acid and cholesterol synthesis from specifically labeled leucine by isolated rat hepatocytes

Hepatocytes isolated from female rats meal-fed a high-glucose diet were incubated in Krebs-Henseleit bicarbonate medium containing 16.5 mM glucose, 3H2O, and 14C-labeled amino acids (-)-Hydroxycitrate depressed the incorporation of 3H2O and [14C] alanine into fatty acids and cholesterol. Incorporation of [U-14C]leucine into lipids was not affected but incorporation of 3H2O into lipids was decreased significantly by (-)-hydroxycitrate. (-)-Hydroxycitrate depressed the incorporation of radioactivity from [2-14C]leucine into fatty acids and cholesterol by 61 and 38%, respectively, and stimulated the incorporation of radioactivity from [4,5-3H]leucine 35 and 28%. As [2-14C]leucine labels the acetyl-CoA pool and [4,5-3H]leucine labels the acetoacetate pool, it was concluded that mitochondrial 3-hydroxy-3-methylglutaryl-CoA is not incorporated intact into cholesterol, and that acetoacetate can be activated effectively in the liver cytosol for support of cholesterol and fatty acid synthesis.

Ketone bodies, glucose and glutamine as lipogenic precursors in human diploid fibroblasts

Incorporation of [14C] from acetoacetate, D(-)- and L(+)-3-hydroxybutyrate, glucose, glutamine, acetate and palmitate in cellular lipids were studied in cultures in human diploid fibroblasts (HDF). The results showed that acetoacetate was 2-10 times more effective as a lipogenic precursor than was either D- or L-3-hydroxybutyrate. Its extent of incorporation into lipids was 2- to 8-fold more than the other precursors examined under conditions when the overall rates of nonsaponifiable and saponifiable lipogenesis as measured by 3H2O incorporation were essentially unchanged. Acetoacetate supported both saponifiable and nonsaponifiable lipid synthesis with half-saturation values (Km app.) of 185 microM and 30 microM, respectively. Glucose stimulated acetoacetate incorporation into lipids whereas, conversely, acetoacetate inhibited [14C]glucose incorporation into lipids. The presence of low density lipoproteins (LDL) cholesterol (40 microgram cholesterol/mL) inhibited the incorporation of [14C] from acetoacetate 56% into nonsaponifiable lipids; the inhibition was consistently higher (75%) when [14C]glucose or glutamine were the precursors. The loss of 3-hydroxy-3-methyl-glutaryl CoA (HMG CoA) reductase activity upon addition of LDL-cholesterol was greater than the suppression of [14C] incorporation from acetoacetate or glucose into nonsaponifiable lipids. In the presence of glucose, [14C]acetoacetate was incorporated into 3-beta OH sterols (digitonin precipitable) 7.7 +/- 1.1 times more effectively than was [14C]glucose. The results suggest that HDF would be a suitable model to investigate the effects of various precursors of HMG CoA on the rate of cholesterol biosynthesis.

Activities of 3-oxo acid-CoA transferase and acetoacetyl-CoA synthetase in brain and liver of the developing chick

The 3-oxo acid-CoA transferase and acetoacetyl-CoA synthetase activities were estimated in subcellular fractions isolated from the cerebral hemispheres, the optic lobes, the cerebellum and the liver of the chick between the twentieth day of embryonic life and the thirtieth day of postnatal development. In the 3 areas of chick brain, the transferase activity increases during the whole postnatal development. Transferase is located both in mitochondria and microsomes unlike in the rat brain where this enzyme is specifically mitochondrial. There is no detectable activity of synthetase in the chick brain. These differences in enzyme localization imply that acetoacetate is converted into acetoacetyl-CoA only by 3-oxo acid-CoA transferase in the mitochondria and cytoplasm of the chick brain, whereas, in the rat brain, this reaction is performed by 3-oxo acid-CoA transferase in mitochondria and by acetoacetyl-CoA synthetase in the cytosol. There is no detectable activity either for transferase or for synthetase in the chick liver.

Variations of 3-hydroxybutyrate dehydrogenase activity in brain and liver mitochondria of the developing chick

1. The 3-hydroxybutyrate dehydrogenase activity was estimated in the crude mitochondrial fraction isolated from the cerebral hemispheres, the optic lobes, the cerebellum and the liver of the chick between the 20th day of embryonic life and the 30th day of postnatal maturation. 2. The optimal conditions of liberation and of determination of 3-hydroxybutyrate dehydrogenase activity were studied in the mitochondrial fraction isolated from chick cerebral hemispheres and liver. 3. The subcellular distribution of the enzyme in the chick brain and liver is very different from that in the rat. 3-Hydroxybutyrate dehydrogenase is completely mitochondrial in the rat brain and liver whereas in the chick brain and liver, it is located in mitochondrial and microsomal fractions; moreover, a third component can even be found in the soluble fraction of chick liver. 4. The 3-hydroxybutyrate dehydrogenase activity reaches the same value in the three areas of 20-day-old chick embryo brain. Between this stage and the 4th day after hatching, it increases to reach the same peak in the three areas. This peak however, appears at different stages according to the considered brain area. At 30 days after hatching, the enzyme activity is higher in the cerebellum than in the cerebral hemispheres and optic lobes. 5. The activity of hepatic 3-hydroxybutyrate dehydrogenase is 10 to 20 times lower than in the brain. It does not significantly change between 1 day before and 4 days after hatching and increases 2-fold between 4 and 30 days after hatching. 6. The variations of 3-hydroxybutyrate dehydrogenase activity in chick brain indicate correlations of this enzyme activity with development, particularly related to the nutritional state of the chicks. The fairly important differences in the activity of 3-hydroxybutyrate dehydrogenase in the liver of the chick and the rat enable us to come to a better understanding of the regulation of the concentration of the different ketone bodies in the blood of the chick and the rat. Moreover, the presence of the microsomal component of 3-hydroxybutyrate dehydrogenase in chick brain probably originates in the low concentration of acetoacetate in chick blood.

Coenzyme A-acetylating enzymes in Alzheimer's disease: possible cholinergic 'compartment' of pyruvate dehydrogenase

In the mammalian central cholinergic system the precise mechanism for the production of acetyl-CoA used in acetylcholine synthesis has not yet been identified. As a possible means of investigating this problem the relationship between the activities of several enzymes which can synthesize acetyl-CoA and the cholinergic defect of Alzheimer's disease has been examined. Small, but significant reductions in the activities of pyruvate dehydrogenase, ATP-citrate lyase and acetoacetyl-CoA thiolase were found in post mortem brain tissue from cases of Alzheimer's disease, and the decrease in pyruvate dehydrogenase appeared to be related to the extent of the cholinergic defect (as indicated by loss of choline acetyltransferase). Furthermore, the regional distribution of choline acetyltransferase was similar to that of pyruvate dehydrogenase but not to the distribution of the other enzymes investigated in normal human brain tissue. These observations tend to support a recent suggestion that there may be a particular form of pyruvate dehydrogenase associated with cholinergic neurones.