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.     

Metabolic reorganization and signal transduction during estivation in the spadefoot toad

The maximal activities of 28 enzymes, representing multiple pathways of intermediary metabolism, were quantified in the brain, liver and skeletal muscle of spadefoot toads Scaphiopus couchii, comparing control toads with animals that had estivated for 2 months. Estivation-induced changes in brain enzyme activities were consistent with suppressed glycolysis and increased ketone body and amino acid catabolism. In liver, estivation resulted in reduced activities of eight enzymes representing carbohydrate, amino acid, ketone body and phosphagen metabolism, but the maximal activity of malic enzyme increased by 2.4-fold. Estivation led to a large-scale reorganization of skeletal muscle affecting most of the enzymes analyzed. Activities of enzymes of carbohydrate catabolism were generally elevated except for glycogen phosphorylase and hexokinase, whereas those of enzymes of fatty acid synthesis and ketone body metabolism were reduced. Increased glutamate dehydrogenase activities in both brain and muscle, as well as activities of other amino-acid-catabolizing enzymes in muscle, correlated with specific changes in the free amino acids pools in those tissues (reduced glutamine activity, increased glutamate, alanine and valine activities) that appear to be related to protein catabolism, for the purposes of elevating urea levels. The effects of estivation on signal transduction systems were also assessed. Total activities of protein kinases A and C (PKA and PKC) were largely unaltered in toad tissues during estivation (except for a 57% reduction in liver total PKC), but in seven organs there were strong reductions in the percentage of PKA present as the active catalytic subunit in estivating animals, and three contained a much lower percentage of membrane-bound active PKC during estivation. Activities of protein phosphatase types 1, 2A, 2B, and 2C were also frequently reduced during estivation. Overall, these results suggest that anuran estivation involves metabolic reorganization, including changing the maximal activities of key enzymes of intermediary metabolism as well as depressing the metabolic rate by suppressing signal transducing enzymes.     

Relative levels of hexokinase in isolated neuronal, astrocytic, and oligodendroglial fractions from rat brain

The levels of hexokinase, as well as those of the cytoplasmic glycolytic enzyme lactate dehydrogenase and the mitochondrial tricarboxylic acid cycle enzymes fumarase and citrate synthase, have been determined in whole rat brain and in neuronal, astrocytic, and oligodendroglial fractions isolated from rat brain. Compared with either whole brain or with isolated neurons or astrocytes, oligodendroglia are low in hexokinase content. This provides direct confirmation for the conclusion, based on an electron microscopic immunohistochemical method, that oligodendroglia, compared with other neural structures, contain relatively low levels of this key enzyme of glucose metabolism. Based on this confirmation, it is concluded that the electron-microscopic immunohistochemical procedure provides a valid indication of hexokinase content, and thus that other structures shown to stain weakly by the latter technique (e.g., dendritic terminals of cerebellar granule and Purkinje cells) are, indeed, low in hexokinase activity.     

Glucagon effects on brain carbohydrate and ketone body metabolism of rainbow trout.

The levels of glycogen in brain, lactate and acetoacetate in brain and plasma, glucose in plasma and the activities of brain key enzymes of glycogen metabolism (glycogen phosphorylase, GPase, glycogen synthetase, GSase), gluconeogenesis (fructose 1,6-bisphosphatase, FBPase), and glycolysis (6-phosphofructo 1-kinase, PFK) were evaluated in rainbow trout, Oncorhynchus mykiss, from 0.5 to 3 hr after intraperitoneal injection of 1 ml/kg(-1) body weight of saline alone (controls) or containing bovine glucagon at three different doses: 10, 50, and 100 ng/g(-1) body weight. The results obtained demonstrate, for the first time in a teleost fish, the existence of changes in brain carbohydrate and ketone body metabolism following peripheral glucagon treatment. A clear stimulation of brain glycogenolytic potential was observed after glucagon treatment, as judged by the time- and dose-dependent changes observed in brain glycogen levels (up to 88% decrease), and GPase (up to 30% increase) and GSase (up to 42% decrease) activities. In addition, clear time- and dose-dependent increased and decreased levels were observed in brain of glucagon-treated rainbow trout for lactate (up to 60% increase) and acetoacetate (up to 67% decrease), respectively. In contrast, no significant changes were observed after glucagon treatment in those parameters related to glycolytic/gluconeogenic capacity of rainbow trout brain. Altogether, these in vivo results suggest that glucagon may play a role (direct or indirect) in the regulation of carbohydrate and ketone body metabolism in brain of rainbow trout.     

Differential action of thyroid hormones on the activity of certain enzymes in rat kidney and brain.

In rat kidney several mitochondrial and soluble enzyme activities are stimulated by thyroid hormones and the mitochondrial membrane fluidity is also increased. However, the ketone metabolism enzyme activities of D-3-hydroxybutyrate dehydrogenase and of 3-oxoacid CoA-transferase are not significantly affected by the hyperthyroid state and the ketone body concentration is not greatly changed. Therefore, in hyperthyroid rats the response of the kidney, as far as the ketone bodies and their metabolizing enzymes are concerned, is at variance with that of the liver and the heart. In the brain of young rats, age 8-9 weeks, the activities of the enzymes of ketone body metabolism and those responsible for other metabolic pathways are not influenced by the hyperthyroid state. In these animals, however, the activities of two enzymes, NAD-isocitrate dehydrogenase and pyruvate kinase, are still stimulated by 28 and 41%, respectively. This can be probably related to the higher energy requirement for definitive brain maturation in young hyperthyroid rats.     

Identification of the carboxy terminus as important for the isoform- specific subcellular targeting of glucose transporter proteins

Adhesion between Chlamydomonas reinhardtii gametes generates a rapid rise in cAMP levels which stimulates mating responses and zygotic cell fusion (Pasquale and Goodenough, 1987). We show here that sexual adhesion in vivo results in a twofold stimulation of flagellar adenylyl cyclase activity when the enzyme is subsequently assayed in vitro, a stimulation that is specifically blocked by Cd2+. A twofold stimulation is also elicited by the in vitro presentation of soluble cross-linking reagents (antisera and concanavalin A). In contrast, the 10-30-fold stimulation of the flagellar cyclase by in vitro exposure to 40 degrees C, first described by Zhang et al. (1991), is insensitive to Cd2+ but sensitive to such drugs as trifluoperizine and dibucaine. The capacity for twofold stimulation is displayed by the vegetative and gametic enzymes but is lost when gametes fuse to form zygotes; in contrast, the 10-fold stimulation is displayed by the gametic and zygotic enzymes but not the vegetative enzyme. The signal-defective mutant imp-3 fails to generate the normal mating-triggered cAMP production and can be rescued by exogenous dibutyryl cAMP. It displays normal basal rates of flagellar cyclase activity and a normal twofold stimulation by sexual adhesion and by soluble cross-linkers, but it is defective in 40 degrees C activation. The gametic cell-body adenylyl cyclase is stimulated when wild-type flagella, but not imp-3 flagella, undergo adhesive interactions in vivo, and it can be directly stimulated in vitro by cAMP presentation. We propose that the two levels of flagellar cyclase stimulation reflect either sequential steps in the activation of a single cyclase enzyme, with imp-3 blocked in the second step, or else the sequential activation of two different flagellar enzymes, with imp-3 defective in the second enzyme. We further propose that the cell- body enzyme is activated by the cAMP that is generated when flagellar cyclase activity is fully stimulated.     

The Development of Enzymes of Energy Metabolism in the Brain of a Precocial (Guinea Pig) and Non-Precocial (Rat) Species

Abstract: Key enzymes of ketone body metabolism (3-hydroxybutyrate de-hydrogenase, 3-oxo-acid: CoA transferase, acetoacetyl-CoA thiolase) and glucose metabolism (hexokinase, lactate dehydrogenase, pyruvate dehydrogenase, citrate synthase) have been measured in the brains of foetal, neonatal and adult guinea pigs and compared to those in the brains of neonatal and adult rats. The activities of the guinea pig brain ketone-body-metabolising enzymes remain relatively low in activity throughout the foetal and neonatal periods, with only slight increases occurring at birth. This contrasts with the rat brain, where three- to fourfold increases in activity occur during the suckling period (0–21 days post partum), followed by a corresponding decrease in the adult. The activities of the hexokinase (mitochondrial and cytosolic), pyruvate dehydrogenase, lactate dehydrogenase and citrate synthase of guinea pig brain show marked increases in the last 10–15 days before birth, so that at birth the guinea pig possesses activities of these enzymes similar to the adult state. This contrasts with the rat brain where these enzymes develop during the late suckling period (10–15 days after birth). The development of the enzymes of aerobic glycolytic metabolism correlate with the onset of neurological competence in the two species, the guinea pig being a "precocial" species born neurologically competent and the rat being a "non-precocial" species born neurologically immature. The results are discussed with respect to the enzymatic activities required for the energy metabolism of a fully developed, neurologically competent mammalian brain and its relative sensitivity to hypoxia.     

Purification and characterization of calmodulin from porcine renal medulla

A calcium sensitive phosphodiesterase (PDE) activated by an endogenous calmodulin was identified in the cytosolic fraction of porcine renal medulla. The PDE and calmodulin were separated from each other by DEAE-cellulose column chromatography. Calmodulin was purified from a heat-treated supernatant by column chromatography with DEAE-cellulose and hydroxylapatite. The purified renal calmodulin has a molecular weight of 17,500, is heatstable, and has a pI of 4.2. Activation of the renal PDE by calmodulin was immediate and stoichiometric. The renal calmodulin and PDE cross react with bovine brain calmodulin and PDE, indicating a lack of tissue and species specificity. Thus, renal calmodulin is very similar to bovine brain calmodulin. However, renal calmodulin did not affect detergent-solubilized or membrane-bound renal adenylate cyclase or the antidiuretic hormone-stimulated activity of the enzyme. These results suggest that calmodulin may function in the renal medulla to regulate cAMP levels by stimulation of PDE but not adenylate cyclase. However, the ubiquitous distribution of calmodulin in eukaryotic cells and its effects on a number of other enzymes allow the possibility that calmodulin may have a role in renal function other than cAMP metabolism.     

Effects of food deprivation on 24 h-changes in brain and liver carbohydrate and ketone body metabolism of rainbow trout

Plasma glucose, lactate and acetoacetate, brain glycogen and acetoacetate, and liver acetoacetate, glycogen and lactate in fed rainbow trout exhibited daily changes. However, no daily changes were observed in the activities of the brain enzymes glycogen synthetase, 6-phosphofructo 1-kinase, and lactate dehydrogenase. Depending on the length of the previous fasting period most daily changes observed in the metabolic parameters of fed fish disappeared, except for liver acetoacetate levels, which displayed daily changes in both fed and fasted fish. These results suggest that feeding is an important factor regulating most daily changes in the brain and liver carbohydrate and ketone body metabolism of rainbow trout.     

Lipid, ketone body and oxidative metabolism in the African lungfish, Protopterus dolloi following 60 days of fasting and aestivation

The potential importance of lipids and ketone bodies as fuels in the African lungfish, Protopterus dolloi, and the role of oxidative metabolism, were examined under control, fasted and aestivated conditions. In aestivating but not fasting lungfish, the activities of citrate synthase (CS) and cytochrome c oxidase (CCO) (enzymes of oxidative metabolism) showed tissue-specific changes. Significant reductions in CS activity occurred in the kidney, heart, gill and muscle, and in CCO in the liver and kidney tissues. Aestivation, but not fasting, also had a tissue-specific effect on mitochondrial state 3 respiration rates (using succinate as a substrate), with a >50% reduction in the liver, yet no change within muscle mitochondria. There is no indication that enzymes involved in lipid catabolism are up-regulated during periods of fasting or aestivation; however, both 3-hydroxyacyl CoA dehydrogenase (HOAD) and carnitine palmitoyl CoA transferase (CPT) activities were sustained in the liver despite the approximately 42% reduction in CCO activity, potentially indicating lipid metabolism is of importance during aestivation. Lungfish are able to utilize both the d- and l-stereoisomers of the ketone body beta-hydroxybutyrate (beta-HB); however, beta-HB does not appear to be an important fuel source during aestivation or fasting as no changes were observed in beta-HB tissue levels. This study demonstrates that an important aspect of metabolic depression during aestivation in lungfish is the tissue-specific down regulation of enzymes of aerobic metabolism while maintaining the activities of enzymes in pathways that supply substrates for aerobic metabolism.     

ENZYMES OF FATTY ACID METABOLISM IN OVERWINTERING MATURE LARVAE OF THE PINE NEEDLE GALL MIDGE,THECODIPLOSIS JAPONENSIS*

Abstract In this study, overwintering larvae of pine needle gall midge, Thmodiplosis jaHnensis, were sampled at various dates in the winter of 1997 and profiles of some enzymes of fatty acid metabolism were studied. During overwintering, a decrease in total lipids in T. japonensis larvae suggested the use of fat reserves to maintain basal metabolism. Activities of two enzymes associated with fatty acid synthesis, i. e. malic enzyme and ATP‐dependent citrate lyase, decreased from December to mid‐January, then increased from the end of February, indicating a reduced potential for fatty acid synthesis during the winter. Enzymes for fatty acid oxidation, as indicated by the activities of hydroxyacyl‐CoA dehydrogenase, carnitine‐palmitoyl transferase and acetoacetyl‐CoA thiolase, showed different profiles. The potential for ketone body metabolism, as measured by p‐hydroxybutyrate dehydrogenase activity, decreased in the course of winter, indicating that ketone body as a metabolic fuel during overwintering is not important.     

Ketone bodies and two-compartment tumor metabolism: Stromal ketone production fuels mitochondrial biogenesis in epithelial cancer cells

We have previously suggested that ketone body metabolism is critical for tumor progression and metastasis. Here, using a co-culture system employing human breast cancer cells (MCF7) and hTERT-immortalized fibroblasts, we provide new evidence to directly support this hypothesis. More specifically, we show that the enzymes required for ketone body production are highly upregulated within cancer-associated fibroblasts. This appears to be mechanistically controlled by the stromal expression of caveolin-1 (Cav-1) and/or serum starvation. In addition, treatment with ketone bodies (such as 3-hydroxy-butyrate, and/or butanediol) is sufficient to drive mitochondrial biogenesis in human breast cancer cells. This observation was also validated by unbiased proteomic analysis. Interestingly, an MCT1 inhibitor was sufficient to block the onset of mitochondrial biogenesis in human breast cancer cells, suggesting a possible avenue for anticancer therapy. Finally, using human breast cancer tumor samples, we directly confirmed that the enzymes associated with ketone body production (HMGCS2, HMGCL and BDH1) were preferentially expressed in the tumor stroma. Conversely, enzymes associated with ketone re-utilization (ACAT1) and mitochondrial biogenesis (HSP60) were selectively associated with the epithelial tumor cell compartment. Our current findings are consistent with the “two-compartment tumor metabolism” model. Furthermore, they suggest that we should target ketone body metabolism as a new area for drug discovery, for the prevention and treatment of human cancers.     

The AMP-Activated Protein Kinase Is Involved in the Regulation of Ketone Body Production by Astrocytes

The possible role of the AMP-activated protein kinase (AMPK), a highly conserved stress-activated kinase, in the regulation of ketone body production by astrocytes was studied. AMPK activity in rat cortical astrocytes was three times higher than in rat cortical neurons. AMPK in astrocytes was shown to be functionally active. Thus, incubation of astrocytes with 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR), a cell-permeable activator of AMPK, stimulated both ketogenesis from palmitate and carnitine palmitoyltransferase I. This was concomitant to a decrease of intracellular malonyl-CoA levels and an inhibition of acetyl-CoA carboxylase/fatty acid synthesis and 3-hydroxy-3-methylglutaryl-CoA reductase/cholesterol synthesis. Moreover, in microdialysis experiments AICAR was shown to stimulate brain ketogenesis markedly. The effect of chemical hypoxia on AMPK and the ketogenic pathway was studied subsequently. Incubation of astrocytes with azide led to a remarkable drop of fatty acid beta-oxidation. However, activation of AMPK during hypoxia compensated the depression of beta-oxidation, thereby sustaining ketone body production. This effect seemed to rely on the cascade hypoxia --> increase of the AMP/ATP ratio --> AMPK stimulation --> acetyl-CoA carboxylase inhibition --> decrease of malonyl-CoA concentration --> carnitine palmitoyltransferase I deinhibition --> enhanced ketogenesis. Furthermore, incubation of neurons with azide blunted lactate oxidation, but not 3-hydroxybutyrate oxidation. Results show that (a) AMPK plays an active role in the regulation of ketone body production by astrocytes, and (b) ketone bodies produced by astrocytes during hypoxia might be a substrate for neuronal oxidative metabolism.     

Novel aspects of the activities and subcellular distribution of enzymes of ketone body metabolism in the liver and kidney of the goldfish, Carassius auratus

The metabolic organization of ketone body metabolism of liver and kidney of the goldfish Carassius auratus was assessed by measuring maximal activities, subcellular distribution, and stereoisomer preference of ketone body enzymes. These determinations indicate that the organization of ketone body metabolism in liver and kidney of goldfish differs from that of mammals in some respects. All the enzymes of ketone body metabolism were present in liver and kidney of goldfish, with the exception of hydroxymethylglutaryl-CoA (HMG-CoA) synthetase, which was not detected in liver. Two forms of beta-hydroxybutyrate dehydrogenase (betaHBDH) with different stereospecificity for beta-hydroxybutyrate (D- and L-beta-hydroxybutyrate) were detectable in liver and kidney. All of the ketone body enzymes measured in liver were mainly in the mitochondrial fraction, with the exception of D- and L-betaHBDH, which were cytosolic. In kidney, HMG-CoA synthase, together with HMG-CoA lyase and acetoacetyl CoA thiolase (AcoAT), were found mainly in the mitochondrial fraction. L-betaHBDH was mainly cytosolic in kidney, but by contrast with liver, D-betaHBDH was mainly found in the mitochondria, and SKT was distributed in both the mitochondrial and cytosolic compartments. J. Exp. Zool. 286:434-439, 2000.     

Parathyroid hormone induction of creatine kinase activity and DNA synthesis is mimicked by phospholipase C, diacylglycerol and phorbol ester

Parathyroid hormone (PTH), which increases cAMP levels, also induces an increase in the activity of the brain isozyme of creatine kinase and in DNA synthesis in osteoblast-enriched bone cell cultures by a cAMP-independent mechanism. The following results lead us to the conclusion that PTH induction of brain isozyme of creatine kinase activity and DNA synthesis occurs by activation of membranal phospholipid metabolism leading to increased protein kinase C activity and Ca2+ mobilization, a mechanism demonstrated for several growth factors and other hormones. (1) Binding of membranal phospholipids by agents such as gentamycin or antiphospholipid antibodies abolishes the stimulation by PTH of creatine kinase activity and DNA synthesis but not of cAMP production. (2) Treatment of cell cultures with exogenous phospholipase C increases brain isozyme of creatine kinase activity and DNA synthesis, but not cAMP production; these stimulations are also blocked by serum containing anti-phospholipid antibodies. PTH has no additional effect on stimulation of creatine kinase activity by phospholipase C (and only a slight effect on DNA synthesis). (3) A synthetic diacylglycerol (1-oleyl-2-acetyl glycerol) or phorbol ester (phorbol 12-myristate 13-acetate) or Ca2+ ionophore, A23187 induces creatine kinase activity and DNA synthesis in the cultures. However, this effect is not blocked by antiphospholipid sera and PTH has no additional effect. (4) Inhibition of protein kinase C activity by drugs reported to inhibit the enzyme (retinoic acid, quercetin) abolishes the stimulation of brain isozyme of creatine kinase activity and of DNA synthesis by PTH.     

Versatility of microglial bioenergetic machinery under starving conditions

Microglia are highly dynamic cells in the brain. Their functional diversity and phenotypic versatility brought microglial energy metabolism into the focus of research. Although it is known that microenvironmental cues shape microglial phenotype, their bioenergetic response to local nutrient availability remains unclear.In the present study effects of energy substrates on the oxidative and glycolytic metabolism of primary – and BV-2 microglial cells were investigated. Cellular oxygen consumption, glycolytic activity, the levels of intracellular ATP/ADP, autophagy, mTOR phosphorylation, apoptosis and cell viability were measured in the absence of nutrients or in the presence of physiological energy substrates: glutamine, glucose, lactate, pyruvate or ketone bodies.All of the oxidative energy metabolites increased the rate of basal and maximal respiration. However, the addition of glucose decreased microglial oxidative metabolism and glycolytic activity was enhanced. Increased ATP/ADP ratio and cell viability, activation of the mTOR and reduction of autophagic activity were observed in glutamine-supplemented media. Moreover, moderate and transient oxidation of ketone bodies was highly enhanced by glutamine, suggesting that anaplerosis of the TCA-cycle could stimulate ketone body oxidation.It is concluded that microglia show high metabolic plasticity and utilize a wide range of substrates. Among them glutamine is the most efficient metabolite. To our knowledge these data provide the first account of microglial direct metabolic response to nutrients under short-term starvation and demonstrate that microglia exhibit versatile metabolic machinery. Our finding that microglia have a distinct bioenergetic profile provides a critical foundation for specifying microglial contributions to brain energy metabolism.     

Ketone bodies and two-compartment tumor metabolism: Stromal ketone production fuels mitochondrial biogenesis in epithelial cancer cells

We have previously suggested that ketone body metabolism is critical for tumor progression and metastasis. Here, using a co-culture system employing human breast cancer cells (MCF7) and hTERT-immortalized fibroblasts, we provide new evidence to directly support this hypothesis. More specifically, we show that the enzymes required for ketone body production are highly upregulated within cancer-associated fibroblasts. This appears to be mechanistically controlled by the stromal expression of caveolin-1 (Cav-1) and/or serum starvation. In addition, treatment with ketone bodies (such as 3-hydroxy-butyrate, and/or butanediol) is sufficient to drive mitochondrial biogenesis in human breast cancer cells. This observation was also validated by unbiased proteomic analysis. Interestingly, an MCT1 inhibitor was sufficient to block the onset of mitochondrial biogenesis in human breast cancer cells, suggesting a possible avenue for anticancer therapy. Finally, using human breast cancer tumor samples, we directly confirmed that the enzymes associated with ketone body production (HMGCS2, HMGCL and BDH1) were preferentially expressed in the tumor stroma. Conversely, enzymes associated with ketone re-utilization (ACAT1) and mitochondrial biogenesis (HSP60) were selectively associated with the epithelial tumor cell compartment. Our current findings are consistent with the “two-compartment tumor metabolism” model. Furthermore, they suggest that we should target ketone body metabolism as a new area for drug discovery, for the prevention and treatment of human cancers.     

ACETOACETATE AND d-(-)-BETA-HYDROXYBUTYRATE AS PRECURSORS FOR STEROL SYNTHESIS BY CALF OLIGODENDROCYTES IN SUSPENSION CULTURE: EXTRAMITOCHONDRIAL PATHWAY FOR ACETOACETATE METABOLISM

Abstract— Oligodendroglia prepared from calf cerebral white matter were incubated with [3^-14C]aceto-acetate (AcAc), d -[3^-14C](-)-beta-hydroxybutyrate (OHbut) or ³H₂O. Addition to the medium of the ATP-rate oxaloacetate lyase inhibitor (-)-hydroxycitrate diminished sterol labelling from OHbut and ³H₂O, but caused 4-fold stimulation of sterol labelling from AcAc. The discrepancy between results with the two ketone bodies indicates the existence in oligodendroglia of an extrarnitochondrial pathway for conversion of AcAc but not OHbut to acetyl CoA. Acetoacetyl CoA synthetase, the first enzyme in this pathway, was more than 20-fold enriched in oligodendroglia over whole brain.     

Nutritional and hormonal regulation of mRNA abundance for arginine biosynthetic enzymes in kidney

Argininosuccinate synthetase and argininosuccinate lyase catalyze the synthesis of arginine from citrulline in kidney and also serve as components of the urea cycle in liver of ureotelic animals. Dietary and hormonal regulation of mRNAs encoding these enzymes have been well studied in liver but not in kidney. Messenger RNAs for these enzymes are localized within the renal cortex. Starvation and extreme variations in dietary protein content (0% vs 60% casein) produced 2.6- to 3.5-fold increases in mRNA abundance for these two enzymes in rat kidney. Argininosuccinate lyase mRNA was not induced by dibutyryl cAMP, dexamethasone, or a combination of the two agents. In contrast, argininosuccinate synthetase mRNA was induced 2-fold by dibutyryl cAMP but was unresponsive to dexamethasone. Thus, diet and hormones regulate levels of these mRNAs in rat kidney, but the responses are both qualitatively and quantitatively distinct from the responses previously reported for rat liver.     

The metabolic organization of a primitive air-breathing fish, the Florida gar (Lepisosteus platyrhincus)

The metabolic organization of the air-breathing Florida gar, Lepisosteus platyrhincus, was assessed by measuring the maximal activities of key enzymes in several metabolic pathways in selected tissues, concentrations of plasma metabolites including nonesterified fatty acids (NEFA), free amino acids (FAA) and glucose as well as tissue FAA levels. In general, L. platyrhincus has an enhanced capacity for carbohydrate metabolism as indicated by elevated plasma glucose levels and high activities of gluconeogenic and glycolytic enzymes. Based upon these properties, glucose appears to function as the major fuel source in the Florida gar. The capacity for lipid metabolism in L. platyrhincus appears limited as plasma NEFA levels and the activities of enzymes involved in lipid oxidation are low relative to many other fish species. L. platyrhincus is capable of oxidizing both D- and L-beta-hydroxybutyrate, with tissue-specific preferences for each stereoisomer, yet the capacity for ketone body metabolism is low compared with other primitive fishes. Based on enzyme activities, the metabolism of the air-breathing organ more closely resembles that of the mammalian lung than a fish swim bladder. The Florida gar sits phylogenetically and metabolically in an intermediate position between the "primitive" elasmobranchs and the "advanced" teleosts. The apparently unique metabolic organization of the gar may have evolved in the context of a bimodal air-breathing environmental adaptation.