Lipogenesis from ketone bodies in rat brain. Evidence for conversion of acetoacetate into acetyl-coenzyme A in the cytosol

Ketogenesis supports hepatic polyunsaturated fatty acid homeostasis via fatty acid elongation

Therapeutic interventions targeting hepatic lipid metabolism in metabolic dysfunction-associated steatotic liver disease (MASLD) and steatohepatitis (MASH) remain elusive. Using mass spectrometry-based stable isotope tracing and shotgun lipidomics, we established a novel link between ketogenesis and MASLD pathophysiology. Our findings show that mouse liver and primary hepatocytes consume ketone bodies to support fatty acid (FA) biosynthesis via both de novo lipogenesis (DNL) and FA elongation. Analysis of 13 C-labeled FAs in hepatocytes lacking mitochondrial D-β-hydroxybutyrate dehydrogenase (BDH1) revealed a partial reliance on mitochondrial conversion of D-βOHB to acetoacetate (AcAc) for cytoplasmic DNL contribution, whereas FA elongation from ketone bodies was fully dependent on cytosolic acetoacetyl-CoA synthetase (AACS). Ketone bodies were essential for polyunsaturated FA (PUFA) homeostasis in hepatocytes, as loss of AACS diminished both free and esterified PUFAs. Ketogenic insufficiency depleted liver PUFAs and increased triacylglycerols, mimicking human MASLD, suggesting that ketogenesis supports PUFA homeostasis, and may mitigate MASLD-MASH progression in humans.

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.

The Novel Role of Mitochondrial Citrate Synthase and Citrate in the Pathophysiology of Alzheimer's Disease

Citrate synthase is a key mitochondrial enzyme that utilizes acetyl-CoA and oxaloacetate to form citrate in the mitochondrial membrane, which participates in energy production in the TCA cycle and linked to the electron transport chain. Citrate transports through a citrate malate pump and synthesizes acetyl-CoA and acetylcholine (ACh) in neuronal cytoplasm. In a mature brain, acetyl-CoA is mainly utilized for ACh synthesis and is responsible for memory and cognition. Studies have shown low citrate synthase in different regions of brain in Alzheimer's disease (AD) patients, which reduces mitochondrial citrate, cellular bioenergetics, neurocytoplasmic citrate, acetyl-CoA, and ACh synthesis. Reduced citrate mediated low energy favors amyloid-β (Aβ) aggregation. Citrate inhibits Aβ25-35 and Aβ1-40 aggregation in vitro. Hence, citrate can be a better therapeutic option for AD by improving cellular energy and ACh synthesis, and inhibiting Aβ aggregation, which prevents tau hyperphosphorylation and glycogen synthase kinase-3 beta. Therefore, we need clinical studies if citrate reverses Aβ deposition by balancing mitochondrial energy pathway and neurocytoplasmic ACh production. Furthermore, in AD's silent phase pathophysiology, when neuronal cells are highly active, they shift ATP utilization from oxidative phosphorylation to glycolysis and prevent excessive generation of hydrogen peroxide and reactive oxygen species (oxidative stress) as neuroprotective action, which upregulates glucose transporter-3 (GLUT3) and pyruvate dehydrogenase kinase-3 (PDK3). PDK3 inhibits pyruvate dehydrogenase, which decreases mitochondrial-acetyl-CoA, citrate, and cellular bioenergetics, and decreases neurocytoplasmic citrate, acetyl-CoA, and ACh formation, thus initiating AD pathophysiology. Therefore, GLUT3 and PDK3 can be biomarkers for silent phase of AD.

Metabolic and Cellular Compartments of Acetyl-CoA in the Healthy and Diseased Brain

The human brain is characterised by the most diverse morphological, metabolic and functional structure among all body tissues. This is due to the existence of diverse neurons secreting various neurotransmitters and mutually modulating their own activity through thousands of pre- and postsynaptic interconnections in each neuron. Astroglial, microglial and oligodendroglial cells and neurons reciprocally regulate the metabolism of key energy substrates, thereby exerting several neuroprotective, neurotoxic and regulatory effects on neuronal viability and neurotransmitter functions. Maintenance of the pool of mitochondrial acetyl-CoA derived from glycolytic glucose metabolism is a key factor for neuronal survival. Thus, acetyl-CoA is regarded as a direct energy precursor through the TCA cycle and respiratory chain, thereby affecting brain cell viability. It is also used for hundreds of acetylation reactions, including N-acetyl aspartate synthesis in neuronal mitochondria, acetylcholine synthesis in cholinergic neurons, as well as divergent acetylations of several proteins, peptides, histones and low-molecular-weight species in all cellular compartments. Therefore, acetyl-CoA should be considered as the central point of metabolism maintaining equilibrium between anabolic and catabolic pathways in the brain. This review presents data supporting this thesis.

Hepatic Mitochondrial Oxidative Metabolism and Lipogenesis Synergistically Adapt to Mediate Healthy Embryonic-to-Neonatal Transition in Chicken

During the normal embryonic-to-neonatal development, the chicken liver is subjected to intense lipid burden from high rates of yolk-lipid oxidation and also from the accumulation of the yolk-derived and newly synthesized lipids from carbohydrates. High rates of hepatic lipid oxidation and lipogenesis are also central features of non-alcoholic fatty liver disease (NAFLD) in both rodents and humans, but is associated with impaired insulin signaling, dysfunctional mitochondrial energetics and oxidative stress. However, these adverse effects are not apparent in the liver of embryonic and neonatal chicken, despite lipid burden. Utilizing comprehensive metabolic profiling, we identify that steady induction of hepatic mitochondrial tricarboxylic acid (TCA) cycle and lipogenesis are central features of embryonic-to-neonatal transition. More importantly, the induction of TCA cycle and lipogenesis occurred together with the downregulation of hepatic β-oxidation and ketogenesis in the neonatal chicken. This synergistic remodeling of hepatic metabolic networks blunted inflammatory onset, prevented accumulation of lipotoxic intermediates (ceramides and diacylglycerols) and reduced reactive oxygen species production during embryonic-to-neonatal development. This dynamic remodeling of hepatic mitochondrial oxidative flux and lipogenesis aids in the healthy embryonic-to-neonatal transition in chicken. This natural physiological system could help identify mechanisms regulating mitochondrial function and lipogenesis, with potential implications towards treatment of NAFLD.

The Regulatory Effects of Acetyl-CoA Distribution in the Healthy and Diseased Brain

Brain neurons, to support their neurotransmitter functions, require a several times higher supply of glucose than non-excitable cells. Pyruvate, the end product of glycolysis, through pyruvate dehydrogenase complex reaction, is a principal source of acetyl-CoA, which is a direct energy substrate in all brain cells. Several neurodegenerative conditions result in the inhibition of pyruvate dehydrogenase and decrease of acetyl-CoA synthesis in mitochondria. This attenuates metabolic flux through TCA in the mitochondria, yielding energy deficits and inhibition of diverse synthetic acetylation reactions in all neuronal sub-compartments. The acetyl-CoA concentrations in neuronal mitochondrial and cytoplasmic compartments are in the range of 10 and 7 μmol/L, respectively. They appear to be from 2 to 20 times lower than acetyl-CoA Km values for carnitine acetyltransferase, acetyl-CoA carboxylase, aspartate acetyltransferase, choline acetyltransferase, sphingosine kinase 1 acetyltransferase, acetyl-CoA hydrolase, and acetyl-CoA acetyltransferase, respectively. Therefore, alterations in acetyl-CoA levels alone may significantly change the rates of metabolic fluxes through multiple acetylation reactions in brain cells in different physiologic and pathologic conditions. Such substrate-dependent alterations in cytoplasmic, endoplasmic reticulum or nuclear acetylations may directly affect ACh synthesis, protein acetylations, and gene expression. Thereby, acetyl-CoA may regulate the functional and adaptative properties of neuronal and non-neuronal brain cells. The excitotoxicity-evoked intracellular zinc excess hits several intracellular targets, yielding the collapse of energy balance and impairment of the functional and structural integrity of postsynaptic cholinergic neurons. Acute disruption of brain energy homeostasis activates slow accumulation of amyloid-β_1-42 (Aβ). Extra and intracellular oligomeric deposits of Aβ affect diverse transporting and signaling pathways in neuronal cells. It may combine with multiple neurotoxic signals, aggravating their detrimental effects on neuronal cells. This review presents evidences that changes of intraneuronal levels and compartmentation of acetyl-CoA may contribute significantly to neurotoxic pathomechanisms of different neurodegenerative brain disorders.

Extracellular Redox Regulation of Intracellular Reactive Oxygen Generation, Mitochondrial Function and Lipid Turnover in Cultured Human Adipocytes

Background

Many tissues play an important role in metabolic homeostasis and the development of diabetes and obesity. We hypothesized that the circulating redox metabolome is a master metabolic regulatory system that impacts all organs and modulates reactive oxygen species (ROS) production, lipid peroxidation, energy production and changes in lipid turnover in many cells including adipocytes.

Methods

Differentiated human preadipocytes were exposed to the redox couples, lactate (L) and pyruvate (P), β–hydroxybutyrate (βOHB) and acetoacetate (Acoc), and the thiol-disulfides cysteine/ cystine (Cys/CySS) and GSH/GSSG for 1.5–4 hours. ROS measurements were done with CM-H₂DCFDA. Lipid peroxidation (LPO) was assessed by a modification of the thiobarbituric acid method. Lipolysis was measured as glycerol release. Lipid synthesis was measured as ¹⁴C-glucose incorporated into lipid. Respiration was assessed using the SeaHorse XF24 analyzer and the proton leak was determined from the difference in respiration with oligomycin and antimycin A.

Results

Metabolites with increasing oxidation potentials (GSSG, CySS, Acoc) increased adipocyte ROS. In contrast, P caused a decrease in ROS compared with L. Acoc also induced a significant increase in both LPO and lipid synthesis. L and Acoc increased lipolysis. βOHB increased respiration, mainly due to an increased proton leak. GSSG, when present throughout 14 days of differentiation significantly increased fat accumulation, but not when added later.

Conclusions

We demonstrated that in human adipocytes changes in the external redox state impacted ROS production, LPO, energy efficiency, lipid handling, and differentiation. A more oxidized state generally led to increased ROS, LPO and lipid turnover and more reduction led to increased respiration and a proton leak. However, not all of the redox couples were the same suggesting compartmentalization. These data are consistent with the concept of the circulating redox metabolome as a master metabolic regulatory system.

Beneficial effect of feeding a ketogenic diet to mothers on brain development in their progeny with a murine model of pyruvate dehydrogenase complex deficiency

Pyruvate dehydrogenase complex (PDC) deficiency is a major inborn error of oxidative metabolism of pyruvate in the mitochondria causing congenital lactic acidosis and primarily structural and functional abnormalities of the central nervous system. To provide an alternate source of acetyl-CoA derived from ketone bodies to the developing brain, a formula high in fat content is widely employed as a treatment. In the present study we investigated efficacy of a high-fat diet given to mothers during pregnancy and lactation on lessening of the impact of PDC deficiency on brain development in PDC-deficient female progeny.

Methods

A murine model of systemic PDC deficiency by interrupting the X-linked Pdha1 gene was employed in this study.

Results

Maternal consumption of a high-fat diet during pregnancy and lactation had no effect on number of live-birth, body growth, tissue PDC activity levels, as well as the in vitro rates of glucose oxidation and fatty acid biosynthesis by the developing brain of PDC-deficient female offspring during the postnatal age 35 days, as compared to the PDC-deficient progeny born to dams on a chow diet. Interestingly, brain weight was normalized in PDC-deficient progeny of high fat-fed mothers with improvement in impairment in brain structure deficit whereas brain weight was significantly decreased and was associated with greater cerebral structural defects in progeny of chow-fed mothers as compared to control progeny of mothers fed either a chow or high fat diet.

Conclusion

The findings provide for the first time experimental support for beneficial effects of a ketogenic diet during the prenatal and early postnatal periods on the brain development of PDC-deficient mammalian progeny.

Propofol compared with isoflurane inhibits mitochondrial metabolism in immature swine cerebral cortex

Anesthetics used in infants and children are implicated in the development of neurocognitive disorders. Although propofol induces neuroapoptosis in developing brain, the underlying mechanisms require elucidation and may have an energetic basis. We studied substrate utilization in immature swine anesthetized with either propofol or isoflurane for 4 hours. Piglets were infused with 13-Carbon-labeled glucose and leucine in the common carotid artery to assess citric acid cycle (CAC) metabolism in the parietal cortex. The anesthetics produced similar systemic hemodynamics and cerebral oxygen saturation by near-infrared spectroscopy. Compared with isoflurane, propofol depleted ATP and glycogen stores. Propofol decreased pools of the CAC intermediates, citrate, and α-ketoglutarate, while markedly increasing succinate along with decreasing mitochondrial complex II activity. Propofol also inhibited acetyl-CoA entry into the CAC through pyruvate dehydrogenase, while promoting glycolytic flux with marked lactate accumulation. Although oxygen supply appeared similar between the anesthetic groups, propofol yielded a metabolic phenotype that resembled a hypoxic state. Propofol impairs substrate flux through the CAC in the immature cerebral cortex. These impairments occurred without systemic metabolic perturbations that typically accompany propofol infusion syndrome. These metabolic abnormalities may have a role in the neurotoxity observed with propofol in the vulnerable immature brain.

On the catalytic mechanism of human ATP citrate lyase

ATP citrate lyase (ACL) catalyzes an ATP-dependent biosynthetic reaction which produces acetyl-coenzyme A and oxaloacetate from citrate and coenzyme A (CoA). Studies were performed with recombinant human ACL to ascertain the nature of the catalytic phosphorylation that initiates the ACL reaction and the identity of the active site residues involved. Inactivation of ACL by treatment with diethylpyrocarbonate suggested the catalytic role of an active site histidine (i.e., His760), which was proposed to form a phosphohistidine species during catalysis. The pH-dependence of the pre-steady-state phosphorylation of ACL with [γ-(33)P]-ATP revealed an ionizable group with a pK(a) value of ~7.5, which must be unprotonated for the catalytic phosphorylation of ACL to occur. Mutagenesis of His760 to an alanine results in inactivation of the biosynthetic reaction of ACL, in good agreement with the involvement of a catalytic histidine. The nature of the formation of the phospho-ACL was further investigated by positional isotope exchange using [γ-(18)O(4)]-ATP. The β,γ-bridge to nonbridge positional isotope exchange rate of [γ-(18)O(4)]-ATP achieved its maximal rate of 14 s(-1) in the absence of citrate and CoA. This rate decreased to 5 s(-1) when citrate was added, and was found to be 10 s(-1) when both citrate and CoA were present. The rapid positional isotope exchange rates indicated the presence of one or more catalytically relevant, highly reversible phosphorylated intermediates. Steady-state measurements in the absence of citrate and CoA showed that MgADP was produced by both wild type and H760A forms of ACL, with rates at three magnitudes lower than that of k(cat) for the full biosynthetic reaction. The ATPase activity of ACL, along with the small yet significant positional isotope exchange rate observed in H760A mutant ACL (~150 fold less than wild type), collectively suggested the presence of a second, albeit unproductive, phosphoryl transfer in ACL. Mathematical analysis and computational simulation suggested that the desorption of MgADP at a rate of ~7 s(-1) was the rate-limiting step in the biosynthesis of AcCoA and oxaloacetate.

Chemistry and biochemistry of (-)-hydroxycitric acid from Garcinia

(-)-Hydroxycitric acid [(-)-HCA] is the principal acid of fruit rinds of Garcinia cambogia, Garcinia indica, and Garcinia atroviridis. (-)-HCA was shown to be a potent inhibitor of ATP citrate lyase (EC 4.1.3.8), which catalyzes the extramitochondrial cleavage of citrate to oxaloacetate and acetyl-CoA: citrate + ATP + CoA --> acetyl-CoA + ADP + P(i) + oxaloacetate. The inhibition of this reaction limits the availability of acetyl-CoA units required for fatty acid synthesis and lipogenesis during a lipogenic diet, that is, a diet high in carbohydrates. Extensive animal studies indicated that (-)-HCA suppresses the fatty acid synthesis, lipogenesis, food intake, and induced weight loss. In vitro studies revealed the inhibitions of fatty acid synthesis and lipogenesis from various precursors. However, a few clinical studies have shown controversial findings. This review explores the literature on a number of topics: the source of (-)-HCA; the discovery of (-)-HCA; the isolation, stereochemistry, properties, methods of estimation, and derivatives of (-)-HCA; and its biochemistry, which includes inhibition of the citrate cleavage enzyme, effects on fatty acid synthesis and lipogenesis, effects on ketogenesis, other biological effects, possible modes of action on the reduction of food intake, promotion of glycogenesis, gluconeogenesis, and lipid oxidation, (-)-HCA as weight-controlling agent, and some possible concerns about (-)-HCA, which provides a coherent presentation of scattered literature on (-)-HCA and its plausible mechanism of action and is provocative of further research.

Phosphorylation of recombinant human ATP:citrate lyase by cAMP-dependent protein kinase abolishes homotropic allosteric regulation of the enzyme by citrate and increases the enzyme activity. Allosteric activation of ATP:citrate lyase by phosphorylated sugars

Recombinantly expressed human ATP:citrate lyase was purified from E. coli, and its kinetic behavior was characterized before and after phosphorylation. Cyclic AMP-dependent protein kinase catalyzed the incorporation of only 1 mol of phosphate per mole of enzyme homotetramer, and glycogen synthase kinase-3 incorporated an additional 2 mol of phosphate into the phosphorylated protein. Isoelectric focusing revealed that all of the phosphates were incorporated into only one of the four enzyme subunits. Phosphorylation resulted in a 6-fold increase in V(max) and the conversion of citrate dependence from sigmoidal, displaying negative cooperativity, to hyperbolic. The phosphorylated recombinant enzyme is more similar to the enzyme isolated from mammalian tissues than unphosphorylated enzyme with respect to the K(m) for citrate, CoA, and ATP, and the specific activity. Fructose 6-phosphate was found to be a potent activator (60-fold) of the unphosphorylated recombinant enzyme, with half-maximal activation at 0.16 mM, which results in a decrease in the apparent K(m) for citrate and ATP, as well as an increase in the V(max) of the reaction. Thus, human ATP:citrate lyase activity is regulated in vitro allosterically by phosphorylated sugars as well as covalently by phosphorylation.

Developmental expression of GLUT3 glucose transporter in the rat brain

The ontogeny of the GLUT3 glucose transporter gene and protein expression was studied in rat brain. Northern blot analysis using total RNA from rat brains at different developmental stages revealed that the levels of GLUT3 mRNA were very low during the embryonic stage and increased towards the postnatal stage. Immunohistochemistry using a specific antibody showed that the expression of GLUT3 protein was barely detectable in the embryonic stage, but was clearly detected on the plasma membrane of neuronal cells from 10 days after birth to the adult. Expression of GLUT3 mRNA and protein in the cerebral neuronal cell cultures was also examined during the maturation of neurons. GLUT3 glucose transporter of primary neuronal cultured cerebral cortical neurons was only detected in mature neurons after they were cultured for 14 days. These results indicate that GLUT3 plays an important role in glucose homeostasis postnatally in neurons of the rat brain.

Utilization of ketone bodies by chick brain and spinal cord during embryonic and postnatal development

Lipid synthesis from acetoacetate and 3-hydroxybutyrate was studied in chick embryo from 15 to 21 days and in chick neonate from 1 to 21 days. Embryonic spinal cord showed higher ability than brain to incorporate acetoacetate into total lipids, although a sharp decrease was found at hatching. 3-Hydroxybutyrate incorporation into total lipids was also higher in spinal cord than in brain, especially during the embryonic period. Phospholipids were the main lipids formed in both tissues from both precursors. An appreciable percentage of radioactivity was also recovered as free cholesterol, especially during the embryonic phase. The developmental patterns of amino acid synthesis from acetoacetate and 3-hydroxybutyrate were similar in both tissues: a clear increase after hatching was followed by a decrease at day 4 of neonatal life. Acetoacetate was a better substrate for amino acid synthesis than 3-hydroxybutyrate during the embryonic development in both tissues. Oxidation of both precursors to CO2 strongly decreased between 15 and 21 days of embryonic development both in brain and spinal cord.

Metabolism of lactate in the rat brain during the early neonatal period

The metabolism of lactate in isolated cells from early neonatal rat brain has been studied. In these circumstances, lactate was mainly oxidized to CO2, although a significant portion was incorporated into lipids (78% sterols, 4% phosphatidylcholine, 2% phosphatidylethanolamine, and 1% phosphatidylserine). The rate of lactate incorporation into CO2 and lipids was higher than those found for glucose and 3-hydroxybutyrate. Lactate strongly inhibited glucose oxidation through the pyruvate dehydrogenase-catalyzed reaction and the tricarboxylic acid cycle while scarcely affecting glucose utilization by the pentose phosphate pathway. Lipogenesis from glucose was strongly inhibited by lactate without relevant changes in the rate of glycerol phosphate synthesis. These results suggest that lactate inhibits glucose utilization at the level of the pyruvate dehydrogenase-catalyzed reaction, which may be a mechanism to spare glucose for glycerol and NADPH synthesis. The effect of 3-hydroxybutyrate inhibiting lactate utilization only at high concentrations of 3-hydroxybutyrate suggests that before ketogenesis becomes active, lactate may be the major fuel for the neonatal brain. (-)-Hydroxycitrate and aminooxyacetate markedly inhibited lipogenesis from lactate, suggesting that the transfer of lactate carbons through the mitochondrial membrane is accomplished by the translocation of both citrate and N-acetylaspartate.

Cloning and expression of a human ATP-citrate lyase cDNA

A full-length cDNA clone of 4.3 kb encoding the human ATP-citrate lyase enzyme has been isolated by screening a human cDNA library with the recently isolated rat ATP-citrate lyase cDNA clone [Elshourbagy et al. (1990) J. Biol. Chem. 265, 1430]. Nucleic-acid sequence data indicate that the cDNA contains the complete coding region for the enzyme, which is 1105 amino acids in length with a calculated molecular mass of 121,419 Da. Comparison of the human and rat ATP-citrate lyase cDNA sequences reveals 96.3% amino acid identity throughout the entire sequence. Further sequence analysis identified the His765 catalytic phosphorylation site, the ATP-binding site, as well as the CoA binding site. The human ATP-citrate lyase cDNA clone was subcloned into a mammalian expression vector for expression in African green monkey kidney cells (COS) and Chinese hamster ovary cells (CHO) cells. Transfected COS cells expressed detectable levels of an enzymatically active recombinant ATP-citrate lyase enzyme. Stable, amplified expression of ATP-citrate lyase in CHO cells as achieved by using coamplification with dihydrofolate reductase. Resistant cells expressed high levels of enzymatically active ATP-citrate lyase (3 pg/cell/d). Site-specific mutagenesis of His765----Ala diminishes the catalytic activity of the expressed ATP-citrate lyase protein. Since catalysis of ATP-citrate lyase is postulated to involve the formation of phosphohistidine, these results are consistent with the pattern of earlier observations of the significance of the histidine residue in catalysis of the human ATP-citrate lyase.

Glucose homeostasis and hypothalamic-pituitary-adrenocortical axis during development in rats

Glucoprivation represents a model stress in which activation of different stress responses at different ages can be monitored both in vivo and in vitro. Physiological data indicate rat brain contains a liver/pancreas-type glucose sensor, yet no biochemical or immunocytochemical evidence exists for such a sensor. Young rats appear to lack normal hypothalamic glucose-sensing ability and do not show typical secretory patterns of corticotropin-releasing factor, adrenocorticotropic hormone, or corticosterone after experimentally induced glucoprivation. However, they hypersecrete catecholamines and glucagon (compared with adults) and thrive on fuel sources other than glucose that are abundant after birth. High steroid levels during the first 24 h after birth may be critical for inducing gluconeogenic enzymes and promoting differentiation of tissues like pancreas. Neonatal rats also have unique control systems to combat the damaging effects of other stresses like hypoxia; these systems may disappear in adults. Thus the definition of stress may change during development, and the compensatory mechanisms employed to combat stress change from neonatal to adult life and are intricately related to the metabolic needs of the animal.

Acetyl-CoA carboxylase in rat brain. I. Activities in homogenates and isolated fractions

Acetyl-CoA carboxylase (ACC) catalyzes the rate-limiting and/or first committed step in fatty acid biosynthesis. Because fatty acids must be synthesized as components of the galactolipids and phospholipids in myelin, high specific activities of ACC would be expected in brain during myelination and in the myelinating cells, the oligondendroglia, in particular. Under reaction conditions where ACC was linear with time and protein concentration, we found specific activities of 1.7 and 3.1 nmol/min/mg protein in supernatants from forebrains and brainstems, respectively, of 20-day-old rats. In both regions, ACC declined during development, particularly after the age of 20 days. To separate forebrain into discrete fractions containing cells, membrane vesicles, and other components, without destroying the ACC, it was necessary to modify the published methods by adding citrate to the isolation medium and by omitting trypsin. A fraction which sedimented over 1.2 M sucrose showed the highest specific activities and recoveries of ACC. This fraction was rich in small cells, many of which immunostained with antibodies against galactocerebroside and carbonic anhydrase, both of which are localized in oligodendrocytes and immature glial cells. The cells in this fraction also immunostained with antibodies against ACC. The results are consistent with the hypothesis that ACC is an oligodendrocyte-associated enzyme, although it probably is not exclusive to cells of that type.

Acetoacetyl-CoA synthetase specific activity and concentration in rat tissues

An antibody against acetoacetyl-CoA synthetase purified from rat liver was raised in rabbits. Utilizing the binding of antibody-antigen complexes to a nitrocellulose membrane, a sensitive enzyme-linked immunosorbent assay was developed to estimate the enzyme concentration in rat tissues. The enzyme concentration (microgram immunoreactive protein/mg protein) in rat liver cytosol was increased about 3-, 1.8- and 7-fold by feeding rats diets containing 5% cholestyramine, 0.2% ML-236B (compactin), and 5% cholestyramine plus 0.2% ML-236B for 4 days, respectively, and decreased about 1.8-fold by fasting the animals or 1.3-fold by feeding them a diet containing 5% cholesterol. Changes in the enzyme activity were almost parallel to those in the enzyme concentration, suggesting the physiological role of this enzyme in cholesterol biosynthesis. Immunoblotting of the hepatic cytosol also confirmed that the increase in enzyme concentration on cholestyramine and/or ML-236B feeding was due to an increase in an enzyme protein the same as the purified enzyme and not the isozymic protein. Among various rat tissues examined, the concentrations of immunologically crossreactive enzyme were higher in lipogenic tissues, such as brain, adipose tissue and liver, than in other tissues. The enzymes in these three tissues were identical in molecular weight determined by gel filtration and immunoblotting.

Apparent inhibition of ATP citrate lyase by L-glutamate in vitro is due to the presence of glutamine synthetase

Preincubation in assay mixture for 30 min at 37 degrees C of ATP citrate lyase from rat brain and liver results in 65-70% inhibition in the presence of 10 mM L-glutamate. This inhibition is specific since none of the known brain metabolites of glutamate shows this effect. ATP and ammonium sulphate-suspended, commercially purified malate dehydrogenase are both important in the generation of inhibition; citrate and NADH are not. The ATP citrate lyase activity in desalted crude extracts and 11% polyethylene glycol-precipitated fractions is inhibited but the enzyme purified by dye affinity chromatography is unaffected. Such purification reveals the presence of a factor responsible for the generation of the inhibition shown to be of Mr 380,000. These lines of evidence implicate endogenous glutamine synthetase, and the involvement of this enzyme is established by the use of its inhibitor L-methionine sulphoximine and by the addition of purified glutamine synthetase to restore the glutamate inhibition of purified ATP citrate lyase. The phenomenon probably arises from the production by glutamine synthetase of ADP, a known product inhibitor of ATP citrate lyase. Therefore contrary to previous reports elsewhere, L-glutamate has no role in the regulation of brain ATP citrate lyase and thus the supply of cytoplasmic acetyl groups for biosynthesis.

A biochemical explanation of phenyl acetate neurotoxicity in experimental phenylketonuria

The in vivo formation of [1-14C]acetyl-coenzyme A from D-[3-14C]3-hydroxybutyrate in the brain of the suckling rat was not affected by postnatal exposure to phenyl acetate. However, utilization of the generated acetyl-coenzyme A was significantly inhibited in certain metabolic reactions, namely synthesis of fatty acids and of sterols, but not in others as the Krebs cycle reactions that lead to the production of dicarboxylic amino acids. The incorporation of D-[U-14C]glucosamine into N-acetylneuraminic acid bound to glycoproteins was appreciably diminished in the rat pup previously exposed to maternal phenylketonuria induced by phenyl acetate. During the period of very rapid development of the brain, interference by phenyl acetate and/or its metabolites with certain critical biosynthetic pathways that require acetyl-coenzyme A would significantly contribute to retarded maturation of the brain that occurs in phenylketonuria.

Glycemia, ketonemia, and brain enzymes of ketone body utilization in suckling and adult rats undernourished from intrauterine life

The effect of undernutrition from the 16th day of pregnancy up to 70th day of life on blood glucose and ketone bodies and on several brain mitochondrial enzymes related to energy metabolism or biosynthetic function was investigated. Undernutrition in perinatal period was established by means of a food restriction to pregnant rats and, later, to the lactating mother; undernourished postweaned rats received half the diet consumed by the controls. Body and brain weight from undernourished rats was less than controls throughout the entire period studied. Glycemia and ketonemia were also always lower than controls. Cytochrome c oxidase, citrate synthase, 3-hydroxybutyrate dehydrogenase, 3-oxoacid coenzyme A transferase, and acetoacetyl-coenzyme A thiolase activities during the suckling period were in most stages lower than controls; subsequently, activities in undernourished rats reached or surpassed the control values. These results could explain the "catch up" phenomenon in several ultrastructural parameters found by other authors in undernourished postweaned rats.

The metabolism of D- and L-3-hydroxybutyrate in developing rat brain

The incorporation of L- and D-3-hydroxybutyrate into rat brain protein, lipid, and amino acids during development was studied. L-3-Hydroxybutyrate was found to label brain protein and amino acids in addition to sterols and fatty acids throughout the first 32 postnatal days. Age related changes in L- and D-3-hydroxybutyrate labeling of protein and amino acids were similar. Whereas L-3-hydroxybutyrate incorporation into brain lipids rose sharply between 6-15 days of age, D-3-HOB incorporation into the lipid fraction gradually increased from birth through the age of 15 days. Incorporation by both isomers into lipid was greatest during the third week of suckling and then declined when the animals were weaned. At 15 days of age, the distribution of L-3-hydroxybutyrate into glutamate, glutamine + aspartate, and gamma-aminobutyrate was similar to that obtained with D-3-hydroxybutyrate. L-3-Hydroxybutyrate was poorly oxidized to CO2 by brain slices and mitochondria. Oxidation capacity was maximal from 15-21 days of age for both isomers. The activity of L-3-hydroxybutyrl-CoA ligase increased between 6-28 days of age, and its increase is well correlated with the developmental pattern of L-3-hydroxybutyrate incorporation and mitochondrial oxidation. L-3-Hydroxybutyrate was not detected in the blood of palmitate-injected pups or fasted adult animals. These results suggest that although L-3-hydroxybutyrate can be utilized for the synthesis of brain components during development, its negligible blood concentration precludes a significant contribution to either tissue synthesis or energy balance during the suckling period.

Temporal changes in plasma levels and metabolism of ketone bodies by liver and brain after ethanol and/or starvation in C57BL/6J mice

The effects of ethanol and starvation on ketone body production and utilization were investigated. In the first experiment, adult C57BL/6J mice were divided into four groups: (i) control (fed); (ii) starvation (up to 31 h); (iii) ethanol (acute 5 g/kg i.p.); (iv) ethanol (ETOH) + starvation. Plasma ketone body (KB) concentrations in control mice remained constant at approx. 0.37 mM. The levels of KBs in starved mice began to increase at about 7 h and rose to a peak of 2.5 mM at about 24 h, then fell to 1.8 mM at 31 h. The levels in mice treated with ETOH began to rise soon after injection, reached 1.5 mM at 10 h, and returned to control levels by 15 h. Although there was no difference in elevated levels of KBs between two groups of mice treated with ETOH plus starvation and ETOH alone at 7-10 h, the level continued to rise steadily to 2.0 mM through 31 h in the former group. At 10 h post ETOH, mice either fed ad lib. or fasted had increased hepatic capacity to synthesize acetoacetate (AcAc) from palmitate; this effect was prolonged and enhanced by continued fasting for 24 h. In the brain, the rate of AcAc oxidation was twice that for beta-hydroxybutyrate (beta OHB) and glucose. Neither ETOH nor starvation affected energy production from KB and glucose. AcAc was also utilized for fatty acid synthesis and the rate of synthesis was stimulated by ETOH at 10 h after injection. The rate of lipogenesis from beta OHB accounted for less than 10% of that from AcAc. Together these experiments demonstrate that ETOH increases both hepatic ketone production and plasma KB levels for at least 10 h. ETOH alone led to elevated KB levels long before the rise due to starvation. In brain, at 10 h, an increased capacity to utilize AcAc for lipogenesis was found. The results indicate that ETOH through the production of KBs could provide an important source of energy and lipid precursors for the brain of mice.

Pathways of acetyl CoA production for lipogenesis from acetoacetate, beta-hydroxybutyrate, pyruvate and glucose in neonatal rat lung

The rate of fatty acid synthesis from acetoacetate (AcAc) is 2-3 times greater than from glucose in developing rat lung. To determine the reason for this difference, we investigated the pathways of lipogenesis from [3-14C] AcAc, [3-14C] beta-hydroxybutyrate (beta OHB), [U-14C] glucose or [2-14C] pyruvate in minced lung tissue of 3- to 4-day-old rats. The addition of (-)hydroxycitrate, an inhibitor of ATP-citrate lyase, inhibited fatty acid synthesis from glucose, pyruvate, and beta OHB by 88%, 70% and 60%, respectively, but had no effect on that from AcAc. Benzene 1,2,3-tricarboxylate, an inhibitor of tricarboxylate translocase, inhibited fatty acid synthesis from all substrates by at least 50%. Incubation with aminooxyacetate, an inhibitor of aspartate aminotransferase, had no effect on lipid synthesis from glucose, pyruvate or AcAc, but increased lipid synthesis from beta OHB. Results indicate that for lipid synthesis in the neonatal lung, acetyl CoA from AcAc is derived predominantly from a cytoplasmic pathway involving AcAcCoA synthetase and AcAcCoA thiolase, whereas citrate is the major route of acetyl group transfer from glucose. Lipogenesis from beta OHB involves both the cytoplasmic and citrate pathways.

Biosynthesis of phospholipids and sphingolipids from acetoacetate and glucose in different regions of developing brain in vivo

The incorporation of 14C-label from subcutaneously injected [3-14C]acetoacetate and [U-14C]glucose into phospholipids and sphingolipids in different regions of developing rat brain was determined. In all regions, phosphatidylcholine was the lipid synthesized most readily from either substrate. The percentages of radioactivity in other phospholipids and most sphingolipids remained relatively constant throughout postnatal development. An exceptional increase in the percentage of radioactivity incorporated into cerebroside, coinciding with a decrease of incorporation into phosphatidylcholine, was first noted on day 12 and continued until a maximal level was reached between days 18 and 20 of postnatal age. These developmental changes in preferential synthesis of lipids were associated with increased demands for phospholipids and cerebroside during the early and late postnatal stages, respectively. There was no difference in accumulation of radioactivity from acetoacetate, expressed as dpm of [14C]acetoacetate recovered in phospholipids plus sphingolipids per g of tissue, among all brain regions during the first 5 days of life. During active myelination (12 to 20 days of age); however, the amount of 14C-label was highest in brain stem, ranging from 1.9- to 2.3-fold greater than values for cerebrum and thalamus. The region with the next highest accumulation was cerebellum, followed by midbrain. During the same period, brain stem was likewise the most active site of accumulation of radioactivity from 14C-labeled glucose. Higher amounts of [14C]acetoacetate label accumulated in lipids of brain stem and cerebellum, relative to midbrain, thalamus, and cerebrum, coincide with evidence that active myelination begins in the hindbrain and proceeds rostrally toward the forebrain. Ketone bodies could therefore serve as a potential source of phospholipids and sphingolipids for brain growth and maturation.

The enzymes of acetyl-CoA metabolism in differentiating cholinergic (s-20) and noncholinergic (NIE-115) neuroblastoma cells

Dibutyryl cyclic AMP and butyrate inhibited growth of S-20 (cholinergic) and NIE-115 (adrenergic) neuroblastoma clones. Both these drugs resulted in a parallel increase of choline acetyltransferase and ATP-citrate lyase activities in S-20 neuroblastoma cells. On the other hand, the increase in tyrosine hydroxylase activity in NIE-115 caused by these drugs was not accompanied by a significant change in ATP-citrate lyase activity. Both dibutyryl cyclic AMP and butyrate caused a decrease in fatty acid synthetase activity in both cell lines. The activities of pyruvate dehydrogenase, citrate synthase, choline acetyltransferase, and lactate dehydrogenase in both S-20 and NIE-115 cells were not significantly influenced by the drugs. ATP-citrate lyases from S-20 and NIE-115 had similar kinetic and immunological properties, and their subunits had the same molecular weight as the rat liver enzyme. These data indicate that the differential regulation of ATP-citrate lyase activity in cholinergic and adrenergic cells does not result from the existence of different molecular forms of the enzyme in these cell lines. They also provide further evidence to support the hypothesis that ATP-citrate lyase activity increases during maturation of normal cholinergic neurons and decreases in noncholinergic cells of the brain.

Purification and some properties of ATP-citrate lyase from rat brain

ATP-citrate lyase has been purified from rat brain by a new procedure which yields an enzyme of specific activity of 21 U/mg protein (37 degrees C) (2050-fold purification). Purity (by sodium dodecyl sulfate-gel electrophoresis) of the preparation was comparable to that of rat liver ATP-citrate lyase of similar specific activity. Both brain and liver ATP-citrate lyase have the same electrophoretic mobility, as well as the same immunoreactivity against specific rabbit anti-rat liver ATP-citrate lyase antibody. These data indicate that rat brain ATP-citrate lyase is similar or identical to that present in rat liver. Intraperitoneally injected 32Pi was incorporated into the structural phosphate of ATP-citrate lyase in rat liver but not into the rat brain enzyme.

Changes in lipogenic capacity and activities of ketolytic and lipogenic enzymes in brain regions of developing rats

Oxidation of ketone bodies (KBs) generates acetyl coenzyme A (AcCoA), which can be further incorporated into fatty acid. We have determined the rates of lipogenesis from ketone bodies in developing rats and their relation to the activities of enzymes involved in the production of cytoplasmic AcCoA via different pathways in brain regions. In the cerebrum (Cbr), rates of fatty acid synthesis from [3-14C]acetoacetate ([3-14C]AcAc) were high during the early postnatal period but decreased rapidly thereafter until weaning. Although similar developmental patterns of synthesis characterized the cerebellum (Cbl), midbrain (Mb), brain stem (Bs), and thalamus (Th), maximal rates were highest in the Cbr and lowest in the Th. In all regions, synthetic rates were higher throughout the entire suckling period than in adulthood. There were not appreciable differences in synthetic rates among brain regions of adult rats. The developmental changes in rates AcAc incorporation into fatty acids were closely related to AcAcCoA synthetase activity, but not to activities of ATP-citrate lyase or AcCoA synthetase. During the early postnatal stage enhanced rates of lipogenesis were accompanied by increased activities of AcAcCoA synthetase in all regions, with the highest activity occurring in the Cbr. The sequence of reactions coupling AcAcCoA synthetase and AcAcCoA thiolase in cytoplasm may be an important pathway for generation of AcCoA from KBs for fatty acid synthesis in all regions of the developing brain. This interpretation is strengthened by evidence of concomitant increases in the activities of fatty acid synthetase and AcCoA carboxylase.

ATP citrate lyase in cholinergic nerve endings

The activity of ATP-citrate lyase in homogenates of five selected rat brain regions varied from 2.93 to 6.90 nmol/min/mg of protein in the following order: cerebellum less than hippocampus less than parietal cortex less than striatum less than medulla oblongata and that of the choline acetyltransferase from 0.15 to 2.08 nmol/min/mg of protein in cerebellum less than parietal cortex less than hippocampus = medulla oblongata less than striatum. No substantial differences were found in regional activities of lactate dehydrogenase, pyruvate dehydrogenase, citrate synthase or acetyl-CoA synthase. High values of relative specific activities for both choline acetyltransferase and ATP-citrate lyase were found in synaptosomal and synaptoplasmic fractions from regions with a high content of cholinergic nerve endings. There are significant correlations between these two enzyme activities in general cytocol (S3), synaptosomal (B) and synaptoplasmic (Bs) fractions from the different regions (r = 0.92-0.99). These data indicate that activity of ATP-citrate lyase in cholinergic neurons is several times higher than that present in glial and noncholinergic neuronal cells.

The contribution of citrate to the synthesis of acetyl units in synaptosomes of developing rat brain

The activities of pyruvate dehydrogenase, citrate synthase, and choline acetyltransferase in rat brain synaptosomes increased during ontogenesis by 3 and 14 times, respectively. Activity of ATP-citrate lyase decreased by 26% during the same period. Pyruvate consumption by synaptosomes from 1-day-old animals was 40% lower than that found in older rats; however, citrate efflux from intrasynaptosomal mitochondria in immature synaptosomes was over twice as high as that in mature ones. The rates on production of synaptoplasmic acetyl-CoA by ATP-citrate lyase were 1.03, 1.40, and 0.49 nmol/min/mg protein in 1-, 10-day-old, and adult rats, respectively. 3-Bromopyruvate (0.5 mM) inhibited pyruvate consumption by 70% and caused a complete block of citrate utilization by citrate lyase in every age group. Parameters of citrate metabolism in cerebellar synaptosomes were the same as those in cerebral ones. These data indicate that production of acetyl-CoA from citrate in synaptoplasm may be regulated either by adaptative, age-dependent changes in permeability and carrier capacity of the mitochondrial membrane or by the inhibition of synthesis of intramitochondrial acetyl-CoA. ATP-citrate lyase activity is not a rate-limiting factor in this process. Metabolic fluxes of pyruvate to cytoplasmic citrate and acetyl-CoA are presumably the same in both cholinergic and noncholinergic nerve endings. The significance of citrate release from intrasynaptosomal mitochondria as a regulatory step in acetylcholine synthesis is discussed.

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.

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.

Effects of lactation on L-leucine metabolism in the rat. Studies in vivo and in vitro

1. The turnover rate of L-[1-14C]leucine was increased by 35% in lactating rats compared with virgin rats. Starvation or removal of pups (24 h) returned the value to that of the virgin rat. 2. Incorporation of L-[U-14C]leucine into lipid and protein of mammary glands of lactating rats in vivo increased 7-fold and 6-fold respectively compared with glands of virgin rats. Lactation caused no change in the incorporation of L-[U-14C]leucine into hepatic lipid and protein. 3. The production of 14CO2 from L[l-14C]leucine (in the presence of glucose) was similar in isolated acini from glands of fed (chow) and starved lactating rats. Feeding with a 'cafeteria' diet caused a slight decrease, and removal of pups a large decrease, in the oxidative decarboxylation of leucine. 4. Oxidation of L-[2-14C]leucine to 14CO2 was increased about 3-fold in acini from starved lactating rats or lactating rats fed on a 'cafeteria' diet compared with rats fed on a chow diet. Insulin decreased the formation of 14CO2 in all three situations. 5. Incorporation of L-[U-14C]- and [2-14C]-leucine into lipid was decreased in acini from starved lactating rats and lactating rats fed on a 'cafeteria' diet. Insulin tended to increase the conversion of [2-14C]leucine into lipid, but this was significant only in the case of the acini from 'cafeteria'-fed rats. 6. Experiments with (-)-hydroxycitrate indicate that the major route for conversion of leucine carbon into lipid in acini is via citrate translocation from the mitochondria. 7. The physiological implications of these findings are discussed.

NADP-dependent dehydrogenases in rat liver parenchyma. III. The description of a liponeogenic area on the basis of histochemically demonstrated enzyme activities and the neutral fat content during fasting and refeeding

The activities of glucose-6-phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase(6PGDH), malic enzyme (ME) and isocitrate dehydrogenase (ICDh) were investigated with optimized histochemical methods (Rieder it al 1978), and the activity of 3-hydroxybutyrate dehydrogenase (3HBDH) and neutral fat content with conventional techniques in the liver of male rats under the following experimental dietary conditions: (A) Fasting for 0, 12 and 84h; (B) 84-h fasting followed by refeeding with a low-fat, high-carbohydrate diet for 6 h and for 2, 3, 5, 7, 11 and 14 nights; (C) refeeding with standard diet for 5 nights; (D) low-fat high-carbohydrate diet for 7 an 14 nights. The activities of G6PDH, 6PGDH and ME decreased slightly during fasting primarily in zone 1 and increased dramatically on refeeding with a low-fat, high-carbohydrate diet. This activity increase was confined mainly to zone 3 during the first 3 days and was accompanied by a deposition of neutral fats that began in zone 3 and progressed to zone 1. Neutral for accumulation was maximal after 3 nights, with a uniform accumulation of large droplets in all the hepatocytes; this was followed by a release that started in zone 3 and proceeded in a periportal direction. On the other hand, G6PDH, 6PGDH and ME attained their maximum activities after 5 amd 7 nights of low-fat diet, the activities being nearly homogeneously distributed over the liver acinus in a few cases. Subsequently the activities fill mainly in zone 1, causing the activity patterns and levels to approach those of the animals in group (D). In contrast to this, the activity of ICDH increased during fasting principally in zone 1, so that the otherwise steep activity gradient in favor of zone 3 lessened. Refeeding led at first to a fall of activity below the initial value, but later the normal distribution pattern was restored. The activity of 3HBDH showed a behavior similar to that of ICDH. The findings are discussed with reference to the functional heterogeneity of the liver parenchyma, and the existence of a liponeogenic area in zone 3 is proposed.