The in vivo utilization of acetoacetate, D-(-)-3-hydroxybutyrate, and glucose for lipid synthesis in brain in the 18-day-old rat. Evidence for an acetyl-CoA bypass for sterol synthesis

Ketogenesis supports hepatic polyunsaturated fatty acid homeostasis via fatty acid elongation

Ketogenesis is a dynamic metabolic conduit supporting hepatic fat oxidation particularly when carbohydrates are in short supply. Ketone bodies may be recycled into anabolic substrates, but a physiological role for this process has not been identified. Here, we use mass spectrometry-based 13C-isotope tracing and shotgun lipidomics to establish a link between hepatic ketogenesis and lipid anabolism. Unexpectedly, mouse liver and primary hepatocytes consumed ketone bodies to support fatty acid biosynthesis via both de novo lipogenesis (DNL) and polyunsaturated fatty acid (PUFA) elongation. While an acetoacetate intermediate was not absolutely required for ketone bodies to source DNL, PUFA elongation required activation of acetoacetate by cytosolic acetoacetyl-coenzyme A synthetase (AACS). Moreover, AACS deficiency diminished free and esterified PUFAs in hepatocytes, while ketogenic insufficiency depleted PUFAs and increased liver triacylglycerols. These findings suggest that hepatic ketogenesis influences PUFA metabolism, representing a molecular mechanism through which ketone bodies could influence systemic physiology and chronic diseases.

The role of ATP citrate lyase in myelin formation and maintenance

Formation of myelin by Schwann cells is tightly coupled to peripheral nervous system development and is important for neuronal function and long-term maintenance. Perturbation of myelin causes a number of specific disorders that are among the most prevalent diseases affecting the nervous system. Schwann cells synthesize myelin lipids de novo rather than relying on uptake of circulating lipids, yet one unresolved matter is how acetyl CoA, a central metabolite in lipid formation is generated during myelin formation and maintenance. Recent studies have shown that glucose-derived acetyl CoA itself is not required for myelination. However, the importance of mitochondrially-derived acetyl CoA has never been tested for myelination in vivo. Therefore, we have developed a Schwann cell-specific knockout of the ATP citrate lyase (Acly) gene to determine the importance of mitochondrial metabolism to supply acetyl CoA in nerve development. Intriguingly, the ACLY pathway is important for myelin maintenance rather than myelin formation. In addition, ACLY is required to maintain expression of a myelin-associated gene program and to inhibit activation of the latent Schwann cell injury program.

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.

Neuroprotective Effects of Intermittent Fasting in the Aging Brain

BACKGROUND

A major risk factor for neurodegenerative disorders is old age. Nutritional interventions that delay aging, such as calorie restriction (CR) and intermittent fasting (IF), as well as pharmaceuticals that affect the pathways linking nutrition and aging processes, have been developed in recent decades and have been shown to alleviate the effects of aging on the brain.

SUMMARY

CR is accomplished by alternating periods of ad libitum feeding and fasting. In animal models, IF has been shown to increase lifespan and slow the progression and severity of age-related pathologies such as cardiovascular and neurodegenerative diseases and cancer. According to recent research, dietary changes can help older people with dementia retain brain function. However, the mechanisms underlying the neuroprotective effect of IF on the aging brain and related questions in this area of study (i.e., the potential of IF to treat neurodegenerative disorders) remain to be examined.

KEY MESSAGES

This review addresses the hypothesis that IF may have translational potential in protecting the aged brain while summarizing the research supporting the putative neuroprotective mechanisms of IF in animal models. Additionally, given the emerging understanding of the connection between aging and dementia, our investigations may offer a fresh perspective on the use of dietary interventions for enhancing brain function and preventing dementia in elderly individuals. Finally, the absence of guidelines regarding the application of IF in patients hampers its broad utilization in clinical practice, and further studies are needed to improve our knowledge of the long-term effects of IF on dementia before it can be widely prescribed. In conclusion, IF may be an ancillary intervention for preserving memory and cognition in elderly individuals.

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.

Gut Microbiota-Induced Changes in β-Hydroxybutyrate Metabolism Are Linked to Altered Sociability and Depression in Alcohol Use Disorder

Patients with alcohol use disorder (AUD) present with important emotional, cognitive, and social impairments. The gut microbiota has been recently shown to regulate brain functions and behavior but convincing evidence of its role in AUD is lacking. Here, we show that gut dysbiosis is associated with metabolic alterations that affect behavioral (depression, sociability) and neurobiological (myelination, neurotransmission, inflammation) processes involved in alcohol addiction. By transplanting the gut microbiota from AUD patients to mice, we point out that the production of ethanol by specific bacterial genera and the reduction of lipolysis are associated with a lower hepatic synthesis of β-hydroxybutyrate (BHB), which thereby prevents the neuroprotective effect of BHB. We confirm these results in detoxified AUD patients, in which we observe a persisting ethanol production in the feces as well as correlations among low plasma BHB levels and social impairments, depression, or brain white matter alterations.

Cardiovascular risks impact human brain N-acetylaspartate in regionally specific patterns

Cardiovascular risk factors such as dyslipidemia and hypertension increase the risk for white matter pathology and cognitive decline. We hypothesize that white matter levels of N-acetylaspartate (NAA), a chemical involved in the metabolic pathway for myelin lipid synthesis, could serve as a biomarker that tracks the influence of cardiovascular risk factors on white matter prior to emergence of clinical changes. To test this, we measured levels of NAA across white matter and gray matter in the brain using echo planar spectroscopic imaging (EPSI) in 163 individuals and examined the relationship of regional NAA levels and cardiovascular risk factors as indexed by the Framingham Cardiovascular Risk Score (FCVRS). NAA was strongly and negatively correlated with FCVRS across the brain, but, after accounting for age and sex, the association was found primarily in white matter regions, with additional effects found in the thalamus, hippocampus, and cingulate gyrus. FCVRS was also negatively correlated with creatine levels, again primarily in white matter. The results suggest that cardiovascular risks are related to neurochemistry with a predominantly white matter pattern and some subcortical and cortical gray matter involvement. NAA mapping of the brain may provide early surveillance for the potential subclinical impact of cardiovascular and metabolic risk factors on the brain.

Intermittent metabolic switching, neuroplasticity and brain health

During evolution, individuals whose brains and bodies functioned well in a fasted state were successful in acquiring food, enabling their survival and reproduction. With fasting and extended exercise, liver glycogen stores are depleted and ketones are produced from adipose-cell-derived fatty acids. This metabolic switch in cellular fuel source is accompanied by cellular and molecular adaptations of neural networks in the brain that enhance their functionality and bolster their resistance to stress, injury and disease. Here, we consider how intermittent metabolic switching, repeating cycles of a metabolic challenge that induces ketosis (fasting and/or exercise) followed by a recovery period (eating, resting and sleeping), may optimize brain function and resilience throughout the lifespan, with a focus on the neuronal circuits involved in cognition and mood. Such metabolic switching impacts multiple signalling pathways that promote neuroplasticity and resistance of the brain to injury and disease.

Glut1 deficiency (G1D): epilepsy and metabolic dysfunction in a mouse model of the most common human phenotype

Brain glucose supplies most of the carbon required for acetyl-coenzyme A (acetyl-CoA) generation (an important step for myelin synthesis) and for neurotransmitter production via further metabolism of acetyl-CoA in the tricarboxylic acid (TCA) cycle. However, it is not known whether reduced brain glucose transporter type I (GLUT-1) activity, the hallmark of the GLUT-1 deficiency (G1D) syndrome, leads to acetyl-CoA, TCA or neurotransmitter depletion. This question is relevant because, in its most common form in man, G1D is associated with cerebral hypomyelination (manifested as microcephaly) and epilepsy, suggestive of acetyl-CoA depletion and neurotransmitter dysfunction, respectively. Yet, brain metabolism in G1D remains underexplored both theoretically and experimentally, partly because computational models of limited brain glucose transport are subordinate to metabolic assumptions and partly because current hemizygous G1D mouse models manifest a mild phenotype not easily amenable to investigation. In contrast, adult antisense G1D mice replicate the human phenotype of spontaneous epilepsy associated with robust thalamocortical electrical oscillations. Additionally, and in consonance with human metabolic imaging observations, thalamus and cerebral cortex display the lowest GLUT-1 expression and glucose uptake in the mutant mouse. This depletion of brain glucose is associated with diminished plasma fatty acids and elevated ketone body levels, and with decreased brain acetyl-CoA and fatty acid contents, consistent with brain ketone body consumption and with stimulation of brain beta-oxidation and/or diminished cerebral lipid synthesis. In contrast with other epilepsies, astrocyte glutamine synthetase expression, cerebral TCA cycle intermediates, amino acid and amine neurotransmitter contents are also intact in G1D. The data suggest that the TCA cycle is preserved in G1D because reduced glycolysis and acetyl-CoA formation can be balanced by enhanced ketone body utilization. These results are incompatible with global cerebral energy failure or with neurotransmitter depletion as responsible for epilepsy in G1D and point to an unknown mechanism by which glycolysis critically regulates cortical excitability.

Dietary enrichment with medium chain triglycerides (AC-1203) elevates polyunsaturated fatty acids in the parietal cortex of aged dogs: implications for treating age-related cognitive decline

Dogs demonstrate an age-related cognitive decline, which may be related to a decrease in the concentration of omega-3 polyunsaturated fatty acids (n-3 PUFA) in the brain. Medium chain triglycerides (MCT) increase fatty acid oxidation, and it has been suggested that this may raise brain n-3 PUFA levels by increasing mobilization of n-3 PUFA from adipose tissue to the brain. The goal of the present study was to determine whether dietary MCT would raise n-3 PUFA concentrations in the brains of aged dogs. Eight Beagle dogs were randomized to a control diet (n = 4) or an MCT (AC-1203) enriched diet (n = 4) for 2 months. The animals were then euthanized and the parietal cortex was removed for phospholipid, cholesterol and fatty acid determinations by gas-chromatography. Dietary enrichment with MCT (AC-1203) resulted in a significant increase in brain phospholipid and total lipid concentrations (P < 0.05). In particular, n-3 PUFA within the phospholipid, unesterified fatty acid, and total lipid fractions were elevated in AC-1203 treated subjects as compared to controls (P < 0.05). Brain cholesterol concentrations did not differ significantly between the groups (P > 0.05). These results indicate that dietary enrichment with MCT, raises n-3 PUFA concentrations in the parietal cortex of aged dogs.

Brainstem concentrations of cholesterol are not influenced by genetic ablation of the low-density lipoprotein receptor

PURPOSE

The low-density lipoprotein receptor (LDLr) mediates the uptake of LDL particles enriched with cholesterol, into several tissues. In contrast to other tissues, the brain is thought to obtain cholesterol solely by de novo synthesis, yet certain brain regions such as the brainstem are highly enriched with the LDLr. The goal of the present study was to assess the role of the LDLr in maintaining cholesterol concentrations in the brainstem of wildtype and LDLr knockout (LDLr-/-) mice. Cholesterol concentrations were also measured in the cortex, which served as a reference point, due to the lower expression of the LDLr, as compared to the brainstem.

METHODS

LDLr-/- and wildtype mice consumed an AIN-93G diet ad libitum until 7 weeks of age. After microwaving, the cortex and anterior brain stem were isolated for cholesterol analysis. Cholesterol was extracted into chloroform/methanol, derivatized in trimethylsilyl chloride and measured by gas chromatography/mass spectrometry.

RESULTS

Concentrations of cholesterol in the brainstem did not differ statistically between LDLr-/- (18.8 +/- 1.6 mg/g wet weight brain) and wildtype (19.1 +/- 2.0). Cortical cholesterol concentrations also did not differ statistically between LDLr-/- (11.0 +/- 0.4 mg/g wet weight brain) and wildtype (11.1 +/- 0.2) mice.

CONCLUSION

The LDLr is not necessary for maintaining cholesterol concentrations in the cortex or brainstem, suggesting that other mechanisms are sufficient to maintain brain cholesterol concentrations.

Partial blockage of sterol biosynthesis with a squalene synthase inhibitor in early postnatal Niemann-Pick type C npcnih null mice brains reduces neuronal cholesterol accumulation, abrogates astrogliosis, but may inhibit myelin maturation

Niemann-Pick C disease (NPC) is a fatal, neurovisceral genetic disorder. Cell culture studies showed that NPC1 or NPC2 mutations cause malfunctions in cellular cholesterol trafficking and lead to accumulation of cholesterol and other lipids in the late endo/lysosomes. Previous work showed that neuronal cholesterol accumulation occurs in the brains of young postnatal NPC1-/- mice. Here, to evaluate the potential of partial blockage of cholesterol biosynthesis as a therapy for the NPC disease, we first developed a simple method to monitor the relative rates of lipid biosynthesis in mice brains. We next administered squalene synthase inhibitor (SSI) CP-340868 to young mice. The results show that treating 8-day-old NPC1-/- mice with CP-340868 for 6 days significantly inhibits cholesterol biosynthesis in the mice brains. It reduces neuronal cholesterol accumulation, reduces GM3 ganglioside accumulation, and diminishes astrogliosis in the brain. These results suggest that neuronal cholesterol accumulation contributes to early pathogenesis in the NPC1-/- mice brains. The SSI treatment also reduced brain galactolipid content, suggesting that blocking endogenous cholesterol synthesis in the young mice brains may disrupt the normal myelin maturation processes. The methods described in the current work have general applicability for lipid metabolism studies in mice brains in various pathophysiological conditions.

Brain uptake and metabolism of ketone bodies in animal models

As a consequence of the high fat content of maternal milk, the brain metabolism of the suckling rat represents a model of naturally occurring ketosis. During the period of lactation, the rate of uptake and metabolism of the two ketone bodies, beta-hydroxybutyrate and acetoacetate is high. The ketone bodies enter the brain via monocarboxylate transporters whose expression and activity is much higher in the brain of the suckling than the mature rat. beta-Hydroxybutyrate and acetoacetate taken up by the brain are efficiently used as substrates for energy metabolism, and for amino acid and lipid biosynthesis, two pathways that are important for this period of active brain growth. Ketone bodies can represent about 30-70% of the total energy metabolism balance of the immature rat brain. The active metabolism of ketone bodies in the immature brain is related to the high activity of the enzymes of ketone body metabolism. Thus, the use of ketone bodies by the immature rodent brain serves to spare glucose for metabolic pathways that cannot be fulfilled by ketones such as the pentose phosphate pathway mainly. The latter pathway leads to the biosynthesis of ribose mandatory for DNA synthesis and NADPH which is not formed during ketone body metabolism and is a key cofactor in lipid biosynthesis. Finally, ketone bodies by serving mainly biosynthetic purposes spare glucose for the emergence of various functions such as audition, vision as well as more integrated and adapted behaviors whose appearance during brain maturation seems to critically relate upon active glucose supply and specific regional increased use.

Why is carbon from some polyunsaturates extensively recycled into lipid synthesis?

We summarize here the evidence indicating that carbon from alpha-linolenate and linoleate is readily recycled into newly synthesized lipids. This pathway consumes the majority of these fatty acids that is not beta-oxidized as a fuel. Docosahexaenoate undergoes less beta-oxidation and carbon recycling than do alpha-linolenate or linoleate, but is it still actively metabolized by this pathway? Among polyunsaturates, arachidonate appears to undergo the least beta-oxidation and carbon recycling, an observation that may help account for the resistance of brain membranes to loss of arachidonate during dietary deficiency of n-6 polyunsaturates. Preliminary evidence suggests that de novo lipid synthesis consumes carbon from alpha-linolenate and linoleate in preference to palmitate, but this merits systematic study. Active beta-oxidation and carbon recycling of 18-carbon polyunsaturates does not diminish the importance of being able to convert alpha-linolenate and linoleate to long-chain polyunsaturates but suggests that a broad perspective is required in studying the metabolism of polyunsaturates in general and alpha-linolenate and linoleate in particular.

Carbon recycling into de novo lipogenesis is a major pathway in neonatal metabolism of linoleate and alpha-linolenate

Recent reports indicate that recycling of the beta-oxidized carbon skeleton of linoleate and alpha-linolenate into newly synthesized cholesterol and fatty acids in the brain is quantitatively significant in both suckling rats and pre- and postnatally in rhesus monkeys. The recycling appears to occur via ketones which are not only readily produced from these 18 carbon polyunsaturates but are also the main lipogenic precursors for the developing mammalian brain. Since the neonatal rat brain appears not to acquire cholesterol or long chain saturated or monounsaturated fatty acids from the circulation, ketones and ketogenic precursors seem to be crucial for normal brain synthesis of these lipids. Cholesterol is plentiful in brain membranes and it has also been discovered to be the essential lipid adduct of the 'hedgehog' family of proteins, the appropriate expression of which determines normal embryonic tissue patterning and neurological development. Insufficient cholesterol or inappropriate expression of 'sonic hedgehog' has major adverse neurodevelopmental consequences typified in humans by Smith-Lemli-Optiz syndrome. Hence, we propose that the importance of alpha-linolenate and linoleate for normal neural development arises not only from being precursors to longer chain polyunsaturates incorporated into neuronal membranes but, perhaps equally importantly, by being ketogenic precursors needed for in situ brain lipid synthesis.

Neonatal polyunsaturated fatty acid metabolism

The importance of n-6 and n-3 polyunsaturated fatty acids (PUFA) in neonatal development, particularly with respect to the developing brain and retina, is well known. This review combines recent information from basic science and clinical studies to highlight recent advances in knowledge on PUFA metabolism and areas where research is still needed on infant n-6 and n-3 fatty acid requirements. Animal, cell culture, and infant studies are consistent in demonstrating that synthesis of 22:6n-3 involves C24 PUFA and that the amounts of 18:2n-6 and 18:3n-3 influence PUFA metabolism. Studies to show that addition of n-6 fatty acids beyond delta6-desaturase alters n-6 fatty acid metabolism with no marked increase in tissue 20:4n-6 illustrate the limitations of analyses of tissue fatty acid compositions as an approach to study the effects of diet on fatty acid metabolism. New information to show highly selective pathways for n-6 and n-3 fatty acid uptake in brain, and efficient pathways for conservation of 22:6n-3 in retina emphasizes the differences in PUFA metabolism among different tissues and the unique features which allow the brain and retina to accumulate and maintain high concentrations of n-3 fatty acids. Further elucidation of the delta6-desaturases involved in 24:5n-6 and 22:6n-3 synthesis; the regulation of fatty acid movement between the endoplasmic reticulum and peroxisomes; partitioning to acylation, desaturation and oxidation; and the effects of dietary and hormonal factors on these pathways is needed for greater understanding of neonatal PUFA metabolism.

Utilization and preferred metabolic pathway of ketone bodies for lipid synthesis by isolated rat hepatoma cells

Morris hepatoma 7777 cells freshly isolated from highly malignant tumors grown in the hindlimb of buffalo rats actively convert ketone bodies to cholesterol and fatty acids. On the basis of results obtained with (-)-hydroxycitrate, an inhibitor of the ATP citrate lyase enzyme, the metabolic pathway for acetoacetate conversion to lipids is exclusively cytoplasmic, whereas that for 3-hydroxybutyrate involves both extra- and intramitochondrial compartments. Subcellular distribution studies indicated accumulation and compartmentation of 3-hydroxybutyryl CoA primarily in the cytoplasm of hepatoma cells incubated with either ketone body. In contrast, the compartmentation of acetoacetyl CoA is dependent on whether the substrate is acetoacetate or 3-hydroxybutyrate. With acetoacetate, the acetoacetyl CoA is entirely cytoplasmic, whereas with 3-hydroxybutyrate, it is equally divided between the intra- and extramitochondrial compartments. The results are discussed in terms of the known and proposed metabolic pathways for lipid synthesis from ketone bodies, particularly that from 3-hydroxybutyrate.

In vivo measurement of fatty acids and cholesterol synthesis using D2O and mass isotopomer analysis

The synthesis of palmitate, stearate, and cholesterol in liver and nervous tissues (brain, cord, and nerve) of Sprague-Dawley rats was determined using deuterated water (D2O) and mass isotopomer analysis. Rats were given 4% deuterium in their drinking water after each receiving an intraperitoneal priming dose. Animals were killed at 1, 2, 4, and 8 wk for deuterium enrichment in body water and determination of mass isotopomer distribution in lipids from various tissues. In 1 wk, the enrichment in the body water reached a plateau of 2.6%, which is 65% of that in the drinking water. We observed the maximum incorporation number (N) in all lipids to be higher than those previously observed, being 22, 24, and 30 for liver palmitate, stearate, and cholesterol, respectively, and N may vary among tissues. Using a single exponential model, we found the half-time (t1/2) and the plateau levels of the newly synthesized lipids of the nervous tissues (t1/2 values ranging from 5 to 28 days) to be different from those of the liver (t1/2 values < or = 4 days) in this relatively long-term study. Mass isotopomer distribution analysis and D2O can be used not only to quantitate the replacement rate of many lipids in various compartments but may also be used to elucidate the tissue-specific synthetic pathways from N.

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.

Effect of valproate on lipogenesis in neonatal rat brain

The effect of valproate on lipogenesis in brain slices from early neonatal rats was studied. The rate of lipid synthesis from lactate and 3-hydroxybutyrate, but not from glucose, was decreased significantly by 1 mM valproate. Separation by high performance liquid chromatography of brain lipids showed that valproate inhibited the synthesis of major phospholipids (phosphatidylcholine, phosphatidylethanolamine and phosphatidylserine) from lactate and major sterols (desmosterol, cholesterol and lanosterol) from lactate and 3-hydroxybutyrate. Valproate did not affect sterol synthesis but slightly enhanced phospholipid synthesis from glucose. However, the ratio of phosphatidylserine/phosphatidylethanolamine synthesis was decreased from lactate, glucose and 3-hydroxybutyrate, suggesting that valproate changes phospholipid composition of brain structures. These changes may contribute to the pharmacological action of the drug.

The origin of palmitic acid in brain of the developing rat

A rat milk substitute containing lower amounts of palmitic and oleic acid in the triacylglycerols in comparison to natural rat milk was fed to artificially reared rat pups from day 7 after birth to day 14. Pups reared by their mother served as controls. Free trideuterated (D3) palmitic acid [(C2H3)(CH2)14COOH, 98 atom % D] and free perdeuterated (D31) palmitic acid [C15(2)H31COOH, 99 atom % D] in equal quantity were mixed into the triacylglycerols of the milk substitute in an amount equal to 100% of the palmitic acid in the triacylglycerols. A control milk substitute contained unlabeled free palmitic acid in an amount equal to 100% of the palmitic acid in the triacylglycerols of the milk substitute. The objective was to determine if palmitic acid in the diet contributed significantly to the palmitic acid content of developing brain and other organs. The methyl esters of the fatty acids were analyzed by gas chromatography and the palmitic acid methyl ester was examined by fast atom bombardment mass spectrometry. The proportion of deuterated methyl palmitate as a percentage of total palmitate was determined; 32% of the palmitic acid in liver and 12% of the palmitic acid in lung were trideuterated and perdeuterated palmitic acid in approximately equal amounts. The brain, by contrast, did not contain the deuterated palmitic acid moiety. Quantitation of palmitic acid and total fatty acids revealed a significant accumulation in organs in the interval from 7 to 14 days of age. Under our experimental conditions, labeled palmitic acid does not enter the brain. Consequently, we conclude that the developing brain produces all required palmitic acid by de novo synthesis.

Fatty acid oxidation and ketogenesis by astrocytes in primary culture

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

In vivo utilization of D(-)-3-hydroxybutyrate by chick brain and spinal cord

The in vivo utilization of D-3-hydroxy[3-14C]butyrate for oxidation in the whole animal and for lipid and amino acid synthesis in brain and spinal cord of overnight-fasted 15-day-old chicks has been measured. Appreciable amounts of injected 3-hydroxy[3-14C]butyrate were expired as 14CO2 one hour after injection, the total amount of which increased with increasing dosages. Lipid synthesis was high in both brain and spinal cord. Free cholesterol and phospholipids were the main lipids labeled in both tissues, increasing with time after injection up to 120 min. The incorporation of radioactivity into triglycerides, esterified cholesterol and free fatty acids was not time-dependent. Increased concentrations of 3-hydroxybutyrate gave rise to higher synthetic rates both in brain and spinal cord. The rate of amino acid synthesis was slightly higher in brain than in spinal cord. Glutamate was always the major amino acid formed.

Milk-substitutes comparable to rat's milk; their preparation, composition and impact on development and metabolism in the artificially reared rat

1. Procedures are described to prepare nutritionally adequate rat milk-substitutes by modifying commercially available processed cow's milk, rich in carbohydrate and low in protein and fat compared with rat's milk. 2. Premilk formulas, prepared as intermediates in the preparation of rat milk-substitutes, are rich in protein but low in their concentration of fat, carbohydrate, and minerals when compared with rat's milk. 3. Premilks were supplemented with lactose, vitamins, minerals, fat as oil mixtures, certain amino acids and other constituents to yield rat milk-substitutes which resemble the known composition of rat's milk in their properties and composition. 4. Detailed analyses of the milk-substitutes show them to be comparable to rat's milk in energy content, pH, osmolarity, the concentration of the macronutrients, fat, protein and carbohydrate, and the major minerals. 5. Rat pups were artificially reared from postnatal day 4 or 5 until days 16-18 by fitting them with gastric cannulas through which the milk-substitutes could be infused automatically. 6. The nutritional impact of the milk-substitutes was assessed by a comparison of growth and metabolic characteristics for artificially reared rats with age-matched sucking rats reared by their mother. 7. Indices which were taken to be appropriate included (a) body-weight gain; (b) the concentration in blood of protein, amino acids, ketone bodies, carnitine, glucose, galactose, lactate, insulin, and the electrolytes calcium, sodium, potassium and chloride; (c) the turnover of glucose and 3-hydroxybutyrate; (d) the concentration in brain of protein, cholesterol, cerebroside sulphate and the activities of the enzymes pyruvate dehydrogenase (EC 1.2.4.1), 3-oxo-acid-CoA transferase (EC 2.8.3.5) and acetoacetyl-CoA ligase (EC 6.2.1.16). 8. The studies suggest that milk-substitutes approximating to rat's milk in composition promote acceptable metabolism in the artificially reared rat pup.

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.

In vivo utilization of [3-14C]acetoacetate for lipid and amino acid synthesis in the 15-day-old chick

1. The utilization of different concentrations of acetoacetate for the in vivo synthesis of lipids and amino acids has been studied in brain, spinal cord, liver, small intestine and kidney of 15-day-old chicks. Both lipid and amino acid synthesis increased in an almost linear fashion with precursor concentration in the five tissues mentioned. 2. Lipid synthesis was very high in spinal cord, followed in decreasing order by brain, small intestine, liver and kidney. At the highest concentration assayed (48 mM) the main lipids synthesized were triglycerides in liver (75%) and kidney (52%) and cholesterol in brain (47%), spinal cord (47%) and small intestine (42%). 3. Amino acid synthesis from acetoacetate did not vary markedly among the five organs, although brain and spinal cord showed higher rates at the maximal concentrations of precursor. Glutamate was always the main amino acid formed.

Relative importance of physiological precursors for ketogenesis in the nutritional homeostasis of the development of the embryonic chick

This paper investigates the relative importances of specific amino acids, and, in particular, branched chain amino acids and their keto derivatives as possible ketogenic precursors in suspensions of liver cells isolated from chick embryos. Addition of the branched chain keto acids stimulated ketogenesis. The order of potency was alpha-ketoisocaproic acid greater than alpha-ketoisovaleric acid greater than DL-alpha-keto-beta-methyl-n-valeric acid. The relative order of effectiveness for branched chain keto acids was maintained at all comparable concentrations, and in each case maximum rates were observed with concentrations of 1-2 mM. In contrast to the stimulation of ketogenesis by their keto derivatives, branched chain amino acids were ineffective as precursors for ketogenesis. Of the other amino acids (utilised at concentrations present in chick embryo plasma) only Tyr, Lys, Phe and Arg produced significant increases in ketone body formation above the endogenous rate. At these physiological concentrations, the effectiveness of the amino acids were in the order of Tyr greater than Lys = Phe greater than Arg. The interactions between three groups of ketogenic precursor (fatty acids, amino acids and keto amino acids, all at physiological concentrations), produced rates of ketogenesis that were purely additive. These results indicate that high concentrations of hydroxybutyrate and acetoacetate found in plasma of developing chick embryos may arise from hepatic metabolism of several distinct precursors. The relative importance of each category of precursor may vary with the precise developmental status of animals.

Acetoacetyl-CoA ligase activity in the isolated rat hepatocyte: effects of 25-hydroxycholesterol and high density lipoprotein

The activity of acetoacetyl-CoA (AcAc-CoA) ligase (E.C.6.2.1.16) in hepatocytes from rats was shown to be the same as the activity in homogenates of their livers. In hepatocytes treated with 25-hydroxycholesterol, AcAc-CoA ligase, 3-hydroxy-3-methyl-glutaryl-CoA (HMG-CoA) reductase and rates of sterol synthesis were substantially decreased. Hepatocytes treated with high density lipoprotein (HDL) exhibited a 2 to 4 fold induction of HMG-CoA reductase activity; however an accompanying increase in AcAc-CoA ligase activity and the rate of cholesterol synthesis was not observed. We conclude (a) that increases in the activity of HMG-CoA reductase when mediated by HDL in hepatocytes do not result in a corresponding change in the capacity for sterol synthesis and (b) that changes in the activity state of HMG-CoA reductase can be dissociated from that of AcAc-CoA ligase.

Capacity for substrate utilization in oxidative metabolism by neurons, astrocytes, and oligodendrocytes from developing brain in primary culture

Neuron, astrocyte, and oligodendrocyte cultures which were established from developing rat brain were examined for their utilization of glucose, ketone bodies, and free fatty acids by oxidative processes. 14CO2 production was measured in these cells from [1-14C] or [6-14C]glucose; [1-14C]octanoate and [1-14C], [6-14C], or [16-14C]palmitate; and [3-14C]acetoacetate and D(-)-3-hydroxy[3-14C]butyrate. Pyruvate dehydrogenase (EC 1.2.4.1.) and 3-oxoacid-CoA transferase (EC 2.8.3.5) activities were found at high levels in each of the cell populations. Astrocytes and oligodendrocytes produced much more 14CO2 from [1-14C]glucose than from [6-14C]glucose, indicating substantial hexose monophosphate shunt activity. This process was not as active in neurons. All three cell populations readily utilized the ketone bodies for oxidative metabolism at rates 7-9 times greater than they utilized glucose. Only astrocytes were able to utilize fatty acids for 14CO2 production, and the rate of utilization was greater than that of the ketone bodies. We found that the metabolic patterns of these brain cells which were derived from the developing brain complement the nature of the diet of the suckling animal which is rich in fat and low in carbohydrate. They readily utilized the ketone bodies or fatty acids and spared glucose for processes that metabolites of fat cannot fulfill.

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.

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.

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.

Cholesterol biosynthesis and its regulation in dissociated cell cultures of fetal rat brain: developmental changes and the role of 3-hydroxy-3-methylglutaryl coenzyme A reductase

Primary cultures of cells dissociated from fetal rat brain were utilized to define the developmental changes in cholesterol biosynthesis and the role of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase in the regulation of these changes. Cerebral hemispheres of fetal rats of 15-16 days of gestation were dissociated mechanically into single cells and grown in the surface-adhering system. Cholesterol biosynthesis, studied as the rate of incorporation of [14C]acetate into digitonin-precipitable sterols, was shown to exhibit two distinct increases in synthetic rates, a prominent increase after 6 days in culture and a smaller one after 14 days in culture. Parallel measurements of HMG-CoA reductase activity also demonstrated two discrete increases in enzymatic activity, and the quantitative and temporal aspects of these increases were virtually identical to those for cholesterol synthesis. These data indicate that cholesterol biosynthesis undergoes prominent alterations with maturation and suggest that these alterations are mediated by changes in HMG-CoA reductase activity. The timing of the initial prominent peak in both cholesterol biosynthesis and HMG-CoA reductase activity at 6 days was found to be the same as the timing of the peak in DNA synthesis, determined as the rate of incorporation of [3H]thymidine into DNA. The second, smaller peak in reductase activity and sterol biosynthesis at 14 days occurred at the time of the most rapid rise in activity of the oligodendroglial enzyme, 2':3'-cyclic nucleotide 3'-phosphohydrolase (CNP). These latter observations suggest an intimate relationship of the sterol biosynthetic pathway with cellular proliferation and with oligodendroglial differentiation in developing mammalian brain.

Lipids of nervous tissue: composition and metabolism

As indicated in the Introduction, the many significant developments in the recent past in our knowledge of the lipids of the nervous system have been collated in this article. That there is a sustained interest in this field is evident from the rather long bibliography which is itself selective. Obviously, it is not possible to summarize a review in which the chemistry, distribution and metabolism of a great variety of lipids have been discussed. However, from the progress of research, some general conclusions may be drawn. The period of discovery of new lipids in the nervous system appears to be over. All the major lipid components have been discovered and a great deal is now known about their structure and metabolism. Analytical data on the lipid composition of the CNS are available for a number of species and such data on the major areas of the brain are also at hand but information on the various subregions is meagre. Such investigations may yet provide clues to the role of lipids in brain function. Compared to CNS, information on PNS is less adequate. Further research on PNS would be worthwhile as it is amenable for experimental manipulation and complex mechanisms such as myelination can be investigated in this tissue. There are reports correlating lipid constituents with the increased complexity in the organization of the nervous system during evolution. This line of investigation may prove useful. The basic aim of research on the lipids of the nervous tissue is to unravel their functional significance. Most of the hydrophobic moieties of the nervous tissue lipids are comprised of very long chain, highly unsaturated and in some cases hydroxylated residues, and recent studies have shown that each lipid class contains characteristic molecular species. Their contribution to the properties of neural membranes such as excitability remains to be elucidated. Similarly, a large proportion of the phospholipid molecules in the myelin membrane are ethanolamine plasmalogens and their importance in this membrane is not known. It is firmly established that phosphatidylinositol and possibly polyphosphoinositides are involved with events at the synapse during impulse propagation, but their precise role in molecular terms is not clear. Gangliosides, with their structural complexity and amphipathic nature, have been implicated in a number of biological events which include cellular recognition and acting as adjuncts at receptor sites. More recently, growth promoting and neuritogenic functions have been ascribed to gangliosides. These interesting properties of gangliosides wIll undoubtedly attract greater attention in the future.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

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.

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.

Cytosolic 3-hydroxy-3-methyl glutaryl coenzyme a synthase in rat brain: properties and developmental change

The properties and developmental change in the activity of cytosolic 3-hydroxy-3-methyl glutaryl coenzyme A (HMG-CoA) synthase in brain was examined and whether or not HMG-CoA lyase is present in cytosol and mitochondria from brain was determined. Although mitochondrial fractions contained significant HMG-CoA lyase activity, the enzyme activity was not detected in brain cytosol. The synthase activity was present in both mitochondrial and cytosolic fraction. The HMG-CoA synthesis by brain cytosol was optimal at pH 8.0 and did not require Mg2+ or exogenous acetoacetyl CoA. This indicates that brain cytosol can synthesize sufficient quantity of acetoacetyl CoA from acetyl CoA to be utilized for HMG-CoA synthesis. Our results also showed that the specific activity (nmol acetyl CoA incorporated/mg protein) of HMG-CoA synthase in brain cytosol was high (between 2-11 days of postnatal age) when the cholesterol content of brain is increasing rapidly, and the activity declined slowly thereafter. This suggests that in brain, cytosolic enzyme HMG-CoA synthase plays a role in the regulation of cholesterol synthesis.

Ketone-body utilization by homogenates of adult rat brain

The regulation of ketone-body metabolism and the quantitative importance of ketone bodies as lipid precursors in adult rat brain has been studied in vitro. Utilization of ketone bodies and of pyruvate by homogenates of adult rat brain was measured and the distribution of 14C from [3-14C]ketone bodies among the metabolic products was analysed. The rate of ketone-body utilization was maximal in the presence of added Krebs-cycle intermediates and uncouplers of oxidative phosphorylation. The consumption of acetoacetate was faster than that of D-3-hydroxybutyrate, whereas, pyruvate produced twice as much acetyl-CoA as acetoacetate under optimal conditions. Millimolar concentrations of ATP in the presence of uncoupler lowered the consumption of ketone bodies but not of pyruvate. Indirect evidence is presented suggesting that ATP interferes specifically with the mitochondrial uptake of ketone bodies. Interconversion of ketone bodies and the accumulation of acid-soluble intermediates (mainly citrate and glutamate) accounted for the major part of ketone-body utilization, whereas only a small part was oxidized to CO2. Ketone bodies were not incorporated into lipids or protein. We conclude that adult rat-brain homogenates use ketone bodies exclusively for oxidative purposes.