Synergistic inhibition of hepatic ketogenesis in the presence of insulin and a cAMP antagonist

The separate or combined effects of insulin and the cAMP antagonist, the Rp-diastereomer of adenosine cyclic 3',5'-phosphorothioate (Rp-cAMPS), were examined on fatty acid-stimulated ketogenesis in hepatocytes from normal fasted rats. Addition of 0.4 mM oleic acid or 0.4 mM octanoic acid resulted in a linear increase in ketone production measured over 60 min. When oleic acid was the substrate, incubation with 1 to 30 microns Rp-cAMPS alone or 0.1 to 10 nM insulin alone caused a variable decrease in the production of ketones which did not exceed an average value of 30% in any one experiment. The simultaneous addition of Rp-cAMPS and insulin resulted in a greater than additive inhibition which reached average values between 47-60% when the theoretical combined inhibitory effect of the insulin alone plus the Rp-cAMPS alone was less than 18%. No significant effects of either insulin or Rp-cAMPS, alone or in combination, were seen when octanoic acid was the substrate. These data imply that Rp-cAMPS can potentiate insulin inhibition of hepatic ketogenesis through inhibition of a cAMP-mediated process.

Pseudoketogenesis in the perfused rat heart

Ketogenesis is usually measured in vivo by dilution of tracers of (3R)-hydroxybutyrate or acetoacetate. We show that, in perfused working rat hearts, the specific activities of (3R)-hydroxybutyrate and acetoacetate are diluted by isotopic exchanges in the absence of net ketogenesis. We call this process pseudoketogenesis. When hearts are perfused with buffer containing 2.3 mM of [4-3H]- plus [3-14C]acetoacetate, the specific activities of [4-3H] and [3-14C]acetoacetate decrease while C-1 of acetoacetate becomes progressively labeled with 14C. This is explained by the reversibility of reactions catalyzed by mitochondrial 3-oxoacid-CoA transferase and acetoacetyl-CoA thiolase. After activation of labeled acetoacetate, the specific activity of acetoacetyl-CoA is diluted by unlabeled acetoacetyl-CoA derived from endogenous fatty acids or glucose. Acetoacetyl-CoA thiolase partially exchanges 14C between C-1 and C-3 of acetoacetyl-CoA. Finally, 3-oxoacid-CoA transferase liberates weakly labeled acetoacetate which dilutes the specific activity of extracellular acetoacetate. An isotopic exchange in the reverse direction is observed when hearts are perfused with unlabeled acetoacetate plus [1-14C]-, [13-14C]-, or [15-14C]palmitate; here also, acetoacetate becomes labeled on C-1 and C-3. Computations of specific activities of (3R)-hydroxybutyrate, acetoacetate, and acetyl-CoA yield minimal rates of pseudoketogenesis ranging from 19 to 32% of the net uptake of (3R)-hydroxybutyrate plus acetoacetate by the heart.

D-3-hydroxybutyrate metabolism in the perfused rat heart

The utilization of D-3-HB and the production of acetoacetate by the perfused rat heart were investigated over a wide range of DL-3-HB concentrations. The rate of D-3-HB utilization is concentration dependent, and shows saturation kinetics. The oxidized amount of D-3-HB when D-3-HB as a sole substrate, accounts at a maximum for 50% of the total oxygen consumption, which suggest the contribution of the endogenous substrate as fuel source along with D-3-HB. The proportion of the D-3-HB consumed that is oxidized rather than released as acetoacetate increases from 70% to 93% as the concentration of D-3-HB falls from 6.99 mM to 0.30 mM.

Effect of growth hormone on fatty acid oxidation: growth hormone increases the activity of 2,4-dienoyl-CoA reductase in mitochondria

The effect of growth hormone on the beta-oxidation of saturated and unsaturated fatty acids was studied with mitochondria isolated from control rats, hypophysectomized rats, and hypophysectomized rats treated with growth hormone. Rates of respiration supported by polyunsaturated fatty acylcarnitines, in contrast to rates observed with palmitoylcarnitine or oleoylcarnitine, were slightly lower in hypophysectomized rats than in normal rats, but were higher in hypophysectomized rats treated with growth hormone. The effects were most pronounced with docosahexaenoylcarnitine, the substrate with the highest degree of unsaturation. Since uncoupling of mitochondria with 2,4-dinitrophenol resulted in lower rates of docosahexaenoylcarnitine-supported respiration, while substitution of ATP for ADP yielded higher rates, it appears that energy is required for the effective oxidation of polyunsaturated fatty acids. Growth hormone treatment of hypophysectomized rats caused a threefold increase in the activity of 2,4-dienoyl-CoA reductase or 4-enoyl-CoA reductase (EC 1.3.1.34) in mitochondria, but not in peroxisomes. The activities of other beta-oxidation enzymes remained virtually unchanged. Rates of acetoacetate formation from linolenoylcarnitine, but not from palmitoylcarnitine, were stimulated by glutamate in mitochondria from hypophysectomized rats and hypophysectomized rats treated with growth hormone. All data together lead to the conclusion that the mitochondrial oxidation of highly polyunsaturated fatty acids is limited by the availability of NADPH and the activity of 2,4-dienoyl-CoA reductase which is induced by growth hormone treatment.

Determination of ketone body kinetics using a D-(-)-3-hydroxy[4,4,4-2H3]butyrate tracer

In studies where D-(-)-3-hydroxy[4,4,4-2H3]butyrate is employed as isotopic tracer in vivo, we have described a selected ion monitoring, gas-liquid chromatography-mass spectrometry micromethod which measures [2H3] tracer enrichment in 3-hydroxybutyrate and acetoacetate from 300-microliters blood samples. For plasma samples in the physiologic range, intra- and interassay precisions for each ketone averaged better than +/- 1% and +/- 2%, respectively. The use of the method was validated by comparing kinetic data obtained with the above tracer with simultaneous flux data obtained with conventional D-(-)-3-hydroxy[3-14C]butyrate tracer in five fasted rats.

Differential effects of alanine on ketogenesis and triacylglycerol formation by isolated perfused livers from euthyroid and hyperthyroid rats

We examined the effects of infusion of alanine on hepatic concentration of glycero-3-phosphate (glycero-3-P) and output of triacylglycerol (TG) by isolated perfused livers from triiodothyronine (T3)-treated rats. It was expected that because of its gluconeogenic and antiketogenic properties, alanine might stimulate accumulation of glycero-3-P, which in turn might result in enhanced TG production by the hyperthyroid livers. The hepatic concentration of glycero-3-P is lower in livers from T3-treated rats than in euthyroid rats. Infusion of 1.83 and 4.23 mmol alanine/4 h did not alter the hepatic concentration of glycero-3-P and output of triacylglycerol by livers from T3-treated rats. However in these livers, the two concentrations of infused alanine increased the output of glucose and decreased the output of ketone bodies. In livers from euthyroid animals, the infusion of 1.83 mmol alanine/4 h had no effect, whereas 4.23 mmol/4 h decreased hepatic concentration of glycero-3-P. These concentrations of alanine did not alter the output of ketone bodies or TG, but progressively decreased the output of glucose by the euthyroid livers. Our data suggested that the decreased availability of glycero-3-P in livers from T3-treated rats is rate-limiting for TG production and correlated with the diminished output of TG, whereas in livers from euthyroid rats, the glycero-3-P concentration is sufficient to maintain maximal synthesis and secretion of TG. Furthermore, the effects of alanine on the output of ketone bodies or glucose appear to be independent of its effects on hepatic glycero-3-P.

Ketogenesis in the living rat followed by 13C NMR spectroscopy

The metabolic fate of 13C1-labeled butyrate in the liver of living rats has been studied by 13C NMR. The formation of the ketone bodies acetoacetate and beta-hydroxybutyrate was observed in vivo as well as resonances from glutamate, glutamine, and carbonate. The observed time course of these metabolites demonstrates the potential of the technique to measure enzyme kinetics in vivo and also to measure the enzyme capacity of a given organ to metabolize a substrate. The in vivo spectra were compared to in vitro spectra of the excised liver and perchloric acid extracts of the liver. Observation of the metabolites and monitoring of their time course in vivo would not have been possible without distinct improvements in the spectral resolution and the spatial localization of the radio-frequency field within the liver. As a novel approach, we have selected the carbonyl region of the 13C NMR spectra for identifying the ketone bodies and other oxidation products in the liver.

Ketone body synthesis from leucine by adipose tissue from different sites in the rat

Leucine is catabolized to ketone bodies in adipose tissue, but the contribution of this output to overall ketone metabolism is not known. The intent of the present study was to determine the capacity of different adipose tissues to synthesize ketone bodies from leucine. The amino acid was readily converted into acetoacetate in epididymal, perirenal, and omental fat tissues. In rats fed ad libitum, the rate of acetoacetate synthesis in omental fat (about 2 mumol g tissue-1h-1) was at least 8 times higher than in epididymal or perirenal fat. In omental fat, the rates of acetoacetate formation from alpha-ketoisocaproic acid were 47-55% lower than from leucine at all concentrations examined. There was no significant synthesis of beta-hydroxybutyrate from leucine or alpha-ketoisocaproic acid. After oxidative decarboxylation, a greater proportion (about three-fourths) of leucine in omental fat was metabolized to acetoacetate than to CO2 production through the Krebs cycle. Although addition of glucose, pyruvate, or carnitine did not affect the production of acetoacetate, fasting for 24 h stimulated acetoacetate synthesis from leucine and alpha-ketoisocaproic acid in omental fat. The high rate of leucine conversion to acetoacetate in omental fat was related to high activities of leucine aminotransferase and branched-chain alpha-keto acid dehydrogenase. Moreover, protein content and cytochrome c oxidase activity of omental mitochondria were, respectively, 13 and 12 times higher than in epididymal mitochondria. In contrast, fat content of epididymal adipose tissue was 21 times that of omental adipose tissue. Epididymal depot consisted of 2.0% protein and 75.8% fat, whereas omental depot contains 17.2% protein and 3.6% fat, resembling that of liver and muscle. The results suggest that the high ketogenic capacity of omental fat stems in part from an augmented mitochondrial mass and high activity of branched-chain alpha-keto acid dehydrogenase.

The stimulation of hepatic gluconeogenesis by acetoacetate precursors. A role for the monocarboxylate translocator

The regulation of the gluconeogenic pathway from the 3-carbon precursors pyruvate, lactate, and alanine was investigated in the isolated perfused rat liver. Using pyruvate (less than 1 mM), lactate, or alanine as the gluconeogenic precursor, infusion of the acetoacetate precursors oleate, acetate, or beta-hydroxybutyrate stimulated the rate of glucose production and, in the case of pyruvate (less than 1 mM), the rate of pyruvate decarboxylation. alpha-Cyanocinnamate, an inhibitor of the monocarboxylate transporter, prevented the stimulation of pyruvate decarboxylation and glucose production due to acetate infusion. With lactate as the gluconeogenic precursor, acetate infusion in the presence of L-carnitine stimulated the rate of gluconeogenesis (100%) and ketogenesis (60%) without altering the tissue acetyl-CoA level usually considered a requisite for the stimulation of gluconeogenesis by fatty acids. Hence, our studies suggest that gluconeogenesis from pyruvate or other substrates which are converted to pyruvate prior to glucose synthesis may be limited or controlled by the rate of entry of pyruvate into the mitochondrial compartment on the monocarboxylate translocator.

Influence of thermal acclimation on glucose production and ketogenesis in isolated eel hepatocytes

Hepatocytes were isolated by collagenase perfusion of the liver from adult eels (Anguilla anguilla L.) acclimated to different temperatures. Whereas the relative weight of the liver increased in cold-acclimated fish, hepatocytes from 10- and 20 degrees C-acclimated animals did not differ in cellular weight, dry weight, or protein content. Endogenous rates of oxygen consumption and respiratory control ratios were independent of acclimation temperature. There was no effect of temperature on triacylglycerol content, but glycogen concentration was significantly higher in hepatocytes of cold-acclimated fish. Liver cells from cold-acclimated eels exhibited higher rates of glucose release and ketogenesis than those from warm-acclimated animals. It is concluded that the increase in acetoacetate production induced by cold acclimation results primarily from a higher rate of lipolysis. Cellular interactions between ketogenesis and gluconeogenesis are demonstrated and discussed.

Effects of starvation on oxidative metabolism and insulin release by isolated mouse pancreatic islets

Pancreatic islets were isolated from either 60-h-starved or fed mice and subsequently incubated in order to determine the insulin release in response to various secretagogues, rates of glucose, leucine or glutamine oxidation or the acetoacetate production from leucine. It was found that in contrast to findings with islets isolated from fed mice 16.7 mm glucose, 10 mM leucine and 10 mm α-ketoisocaproic acid did not stimulate the insulin release of islets isolated from starved mice. Moreover, the insulin release in response to leucine plus glutamine or glucose plus glutamine was decreased after starvation although these values were higher than those obtained for glutamine addition alone. Theophylline, however, restored partly the impaired insulin response to glucose and completely that to leucine or α-ketoisocaproic acid. Starvation was found to inhibit the islet glucose oxidation rate but the addition of theophylline was without effect irrespective of whether the islets were prepared from fed or starved mice. On the contrary, islet leucine oxidation was increased after starvation and again theophylline did not affect the islet leucine oxidation rate. Likewise, the islet acetoacetate production was increased after starvation. The glutamine oxidation rates were not affected by starvation, either when tested alone or together with glucose or leucine. It is concluded that although the starvation-induced impairment of glucose-stimulated insulin release may well be explained by an influence on the oxidative metabolism other factors are also involved as regards leucine-stimulated insulin release.

On the renal tubular damage in hereditary tyrosinemia and on the formation of succinylacetoacetate and succinylacetone

Phenylalanine and homogentisate increase the concentration of succinylacetoacetate and succinylacetone both in serum and urine in patients with hereditary tyrosinemia and therefore increase the excretion of 5-aminolevulinate. Both phenylalanine and homogentisate cause a tubular proteinuria which is in agreement with our hypothesis that their metabolites maleylacetoacetate and fumarylacetoacetate are the toxic compounds in hereditary tyrosinemia. The patient with the highest excretion of succinylacetoacetate and succinylacetone has the slightest tubular proteinuria whereas the one with the lowest excretion of these compounds has the more pronounced tubular proteinuria. It is suggested that this is caused by a difference in the ability to reduce the presumed toxic compounds fumarylacetoacetate and maleylacetoacetate, i.e. the precursors of succinylacetoacetate.

Effects of branched chain alpha-ketoacids on the metabolism of isolated rat liver cells. I. Regulation of branched chain alpha-ketoacid metabolism

alpha-Ketoisocaproate (ketoleucine) is shown to be metabolized to ketone bodies rapidly by isolated rat liver cells. Acetoacetate is the major end product and maximum rates were observed with 2 mM substrate. Studies with 2-tetradecylglycidic acid (an inhibitor of long chain fatty acid oxidation) showed that ketogenesis from alpha-ketoisocaproate and from endogenous fatty acids were additive. With alpha-ketoisocaproate present as soole substrate at 2 mM, leucine production was less than 10% of alpha-ketoisocaproate uptake and only 30% of the acetyl coenzyme A generated was oxidized in the citric acid cycle. Metabolism of alpha-ketoisocaproate was inhibited by fatty acids, alpha-ketoisovalerate, alpha-keto-beta-methylvalerate, and pyruvate. Oxidation of acetyl-CoA generated from alpha-ketoisocaproate was suppressed by oleate and by pyruvate, but was enhanced by lactate. Metabolism between the different branched chain alpha-ketoacids was mutually competitive. When alpha-ketoisocaproate (2 mM) was added in the presence of high pyruvate concentrations (4.4 mM), flux through pyruvate dehydrogenase was decreased, and the proportion of total pyruvate dehydrogenase in the active form (PDHa) also fell. With lactate as substrate, PDHa was only 25% of total activity and was little affected by addition of alpha-ketoisocaproate. These data suggest that enhanced oxidation of acetyl-CoA from alpha-ketoisocaproate by lactate addition is caused by a low activity of pyruvate dehydrogenase combined with increased flux through the citric acid cycle in response to the energy requirements for gluconeogenesis. However, acetyl-CoA generation from pyruvate is apparently insufficiently inhibited by alpha-ketoisocaproate to cause a diversion of acetyl-CoA formed during alpha-ketoisocaproate metabolism from ketone body formation to oxidation in the citric acid cycle. Measurements of the cell contents of CoASH, acetyl-CoA, acid-soluble acyl-CoA, and acid-insoluble fatty acyl-CoA indicated that when the branched chain alpha-ketoacids were added as sole substrate, their oxidation was limited at a step distal to the branched chain alpha-ketoacid dehydrogenase. Acid-soluble acyl-CoA derivatives were depleted after oleate addition in the presence of alpha-ketoisocaproate, suggesting an inhibition of the branched chain alpha-ketoacid dehydrogenase by the elevation of the mitochondrial NADH/NAD+ ratio observed during fatty acid oxidation. This effect was not observed in the presence of oleate and 2-tetradecylglycidic acid.

The metabolism of 4-methyl-2-oxopentanoate in rat pancreatic islets

1. Radioactively labelled 4-methyl-2-oxopentanoate was taken up by isolated pancreatic islets in a concentration- and pH-dependent manner and led to the intracellular accumulation of labelled amino acid and to a decrease in the intracellular pH. Uptake of 4-methyl-2-oxopentanoate did not appear to be either electrogenic or Na+-dependent. The islet content of 2-oxo acid radioactivity was not affected by either 2-cyano-3-hydroxy-cinnamate (10mM) or pyruvate (10mM), although both these substances inhibited the oxidation of [U-14C]4-methyl-2-oxopentanoate by islet tissue. 2. 4-Methyl-2-oxopentanoate markedly stimulated islet-cell respiration, ketone-body formation and biosynthetic activity. The metabolism of endogenous nutrients by islets appeared to be little affected by the compound. 3. Studies with the 3H- and 14C-labelled substrate revealed that 4-methyl-2-oxopentanoate was incorporated by islets into CO2, water, acetoacetate, L-leucine and to a lesser extent into islet protein and lipid. Carbon atoms C-2, C-3 and C-4 of the acetoacetate produced were derived from the carbon skeleton of the 4-methyl-2-oxopentanoate, but the acetoacetate carboxy group was derived from the incorporation of CO2. These results, and consideration of the relative rates of 14CO2 and acetoacetate formation from 1-14C-labelled as opposed to U-14C-labelled 4-methyl-2-oxopentanoate, led to the conclusion that the pathway of catabolism of this 2-oxo acid in pancreatic islets is identical with that described in other tissues. The amination of 4-methyl-2-oxopentanoate by islets was attributed to the presence of a branched-chain amino acid aminotransferase (EC 2.6.1.42) activity in the tissue. Although glutamate dehydrogenase activity was demonstrated in islet tissue, the reductive amination of 2-oxoacids did not seem to be of importance in the formation of leucine from 4-methyl-2-oxopentanoate. 4. The results of experiments with respiratory inhibitors and uncouplers, and the finding that 14CO2 production and islet respiration were linked in a 1:1 stoicheiometry suggested that 4-methyl-2-oxopentanoate catabolism was coupled to mitochondrial oxidative phosphorylation. The catabolism of 4-methyl-2-oxopentanoate in islet tissue appeared to be regulated at the level of the initial 2-oxo acid dehydrogenase (EC 1.2.1.25) reaction.

The effect of rumen epithelial development on metabolic activities and ketogenesis by the tissue in vitro

1. An investigation was made on oxygen consumption, glucose and lactate uptake and ketogenesis from butyrate by rumen epithelium in vitro from lambs at various stages of development. 2. Oxygen uptake was decreased by about 35% and glucose uptake by about 90% between 2 weeks and 1/2 year of age. 3. The uptake of L-lactate and the utilization of butyrate as a substrate for respiration were increased during epithelial development. 4. The production of D(-)-3-hydroxybutyrate and acetoacetate from butyrate by the epithelium was largely increased between 4 to 10 weeks of age, independently of rumen fermentation. 5. A synergistic effect of glucose on the production of D(-)-3-hydroxybutyrate and on total ketone bodies from butyrate by the epithelium was observed. It accounted to 40-80% over butyrate depending on the stage of epithelial development.

Effect of L-alanine infusion on gluconeogenesis and ketogenesis in the rat in vivo

1. In 48 h-starved 6-week-old rats the 14C incorporation in vivo into blood glucose from a constant-specific-radioactivity pool of circulating [14c]actateconfirmed that lactate is the preferred gluconeogenic substrate. 2. Increasing the blood [alanine] to that occurrring in the fed state increased 14C incorporation into blood glucose 2.3-fold from [14c]alanine and 1.7-fold from [14c]lactate. 3. When the blood [alanine] was increased to that in the fed state, the 14C incorporation into liver glycogen from circulating [14c]alanine or [14c]lactate increased 13.5- and 1.7-fold respectively. 4. The incorporation of 14C into blood acetoacetate and 3-hydroxybutyrate from a constant-specific-radioactivity pool of circulating [14c]oleate was virtually abolished by increasing the blood [alanine] to that existing in the fed state. However, the [acetoacetate] remained unchanged, whereas [3-hydroxybutyrate] decreased, although less rapidly than did its radiochemical concentration. 5. It is concluded that during starvation in 6-week-old rats, the blood [alanine] appears to influence ketogenesis for circulating unesterfied fatty acids and inversely affects gluconeogenesis from either lactate or alanine. A different pattern of gluconeogenesis may exist for alanine and lactate as evidenced by comparative 14C incorporation into liver glycogen and blood glucose.

Relative utilization of fatty acids for synthesis of ketone bodies and complex lipids in the liver of developing rats

The regulation of hepatic ketogenesis, as related to the metabolism of fatty acids through oxidative and synthetic pathways, was studied in developing rats. [1-14C] palmitate was used as a substrate to determine the proportions of free fatty acids utilized for the production of ketone bodies, CO2 and complex lipids. Similar developmental patterns of hepatic ketogenesis were obtained by measuring the production of either [14C] acetoacetate from exogenous [1-14C] palmitate or the sum of unlabeled acetoacetate and beta-hydroxybutyrate from endogenous fatty acids. The production of total ketone bodies was low during the late fetal stage and at birth, but increased rapidly to a miximum value within 24 hr after brith. The maximal ketogenic capacity appeared to be maintained for the first 10 days of life. 14CO2 production from [1-14C] palmitate increased by two- to fourfold during the suckling period, from its initial low rate seen at birth. The capacity for synthesis of total complex lipids was low at birth and had increased by day 3 to a maximal value, which was comparable to that of adult fed rats. The high lipogenic capacity lasted throughout the remaining suckling period. When ketogenesis was inhibited by 4-pentenoic acid, the rate of synthesis of complex lipids did not increase despite an increase in unutilized fatty acids. During the mid-suckling period, approximately equal amounts of [1-14C] palmitate were utilized for the synthesis of ketone plus CO2 and for complex lipid synthesis. By contrast, in adult fed rats, the incorporation of fatty acids into complex lipids was four times higher than that of ketone plus CO2. These observations suggest that stimulated hepatic ketogenesis in suckling rats results from the rapid oxidation of fatty acids and consequent increased production of acetyl CoA, but not from impaired capacity for synthesis of complex lipids.

Effects of glucagon and insulin on lipolysis and ketogenesis in sheep

The hepatic and portal productions of acetoacetate and beta-hydroxybutyrate and lipolysis were studied in normal and insulin-controlled alloxan-diabetic sheep. Since hyperinsulinemia is associated with glucagon administration, the latter group of sheep were used to maintain constant plasma insulin levels. After control values were obtained glucagon was infused intraportally at 90 mug/hr for two hours. The ketone body production by portal drained viscera was not significantly affected by glucagon. In alloxanized sheep, glucagon significantly (P less than 0.01) increased net hepatic production of acetoacetate (from -0.54 +/- 0.08 to 0.46 +/- 0.07 g/hr). Lipolysis also increased. However, in the normal sheep, hyperinsulinemia prevented any stimulatory effect of glucagon on hepatic ketogenesis and lipolysis. Therefore, while glucagon appears capable of stimulating ketogenesis andlipolysis, these effects are readily suppressed by insulin.

Degradation of homogentisate by strains of Bacillus and Moraxella

The metabolic pathways whereby strains of Moraxella and Bacillus degrade homogentisate (2,5-dihydroxyphenylacetate) are delineated. The Moraxella (strain OA3) is shown to degrade homogentisate via the pathway previously described in liver: homogentisate is cleaved by a 1,2-dioxygenase (E.C 1.13.11.5) yielding maleylacetoacetate which is isomerized by a GSH-dependent isomerase to fumarylacetoacetate before hydrolysis to acetoacetate and fumarate. A strain of Bacillus (B11c) is shown to catabolize homogentisate via a previously undescribed version of the above sequence: homogentisate is cleaved by a 1,2-dioxygenase (E.C 1.13.11.5) yielding maleylacetoacetate which is hydrolyzed directly to acetoacetate and maleate.

The dependence of glucose formation from lactate on the adenosine triphosphate content in the isolated perfused rat liver

Bilateral intercollicular lesions in the chick abolish or depress notonly calling, but also those phases of behavior when calling would have been occurring. These include: long bouts of excited feeding immediately after food is made available;examining and pecking moving targets and novel objects; persistent scanning, and inhibitionof other behaviour in a novel environment. Deaf birds behave precisely like controls,so that possible auditory deficits are not involved. During calling phases significantvisual stimuli are treated as if they were startling or conspicuous. Conversely,continuousexamination of a stimulus causes calling to diminish or disappear even though responsecontinues; a brief period when the stimulus is not seen causes calling to begin againwhen it is once more perceived. In addition to the increased effectiveness of relevantvisual stimuli, motor facilitation is usual in calling phases, as is inhibition ofirrelevant responses. Emotioanl behaviour in man and other mammals is composed tocalling phases in the chick

Formation of acetoacetate from 3-hydroxy-3-methylglutarate by rat liver and isolation of a mitochondrial coenzyme A-transferase activity involved

1. Formation of acetoacetate from 3-hydroxy-3-methylglutarate was observed in the perfused rat liver. Production of 3.5mumol of acetoacetate/h per g of tissue was obtained. 2. Formation of acetoacetate was catalysed mainly by the mitochondrial fraction of the homogenized liver, at a rate of 62nmol/h per mg of protein. 3. Experiments with hydroxy-[3-(14)C]methylglutarate demonstrated that the acetoacetate formed was derived mainly from this compound. 4. A mitochondrial transferase activity catalysing the transfer of a CoA molecule from succinyl-CoA (3-carboxypropionyl-CoA) to hydroxymethylglutarate was shown. The K(m) value for hydroxymethylglutarate was 5x10(-3)m.

Ketogenesis from butyrate and acetate by the caecum and the colon of rabbits

1. When studied in vitro, tissue from the caecum and the proximal colon of rabbits converted butyrate into ketone bodies. The conversion was similar to that observed with liver slices. The ketogenic activity was associated with the mucosa rather than the muscle of the gut wall and, in the colon, diminished as the distance from the caecal-colonic junction increased. 2. Tissue from the wall of the ileum, caecum, proximal colon and distal colon was also shown to metabolize [1-(14)C]butyrate to carbon dioxide. 3. Enzyme assays showed that in both liver tissue and caecal mucosa the activity of hydroxymethylglutaryl-CoA synthase was more than ten times that of acetoacetyl-CoA deacylase. Labelling experiments in vitro gave confirmation of the hydroxymethylglutaryl-CoA pathway. 4. The significance of the conversion of butyrate into ketone bodies is discussed.

Studies on spermatogenesis in rats. IV. Rates of oxidation of palmitate and pyruvate by various testicular cell populations

Measurements are reported on the rates of oxidation of 14C-1-palmitate, 14C-1-pyruvate, and 14C-2-pyruvate by cell suspensions obtained from testes of normal rats of varying ages and from hypophysectomized regressed rats. Highest conversion rates of labeled pyruvate to CO2 were observed in testes from 24-day-old rats, in which all germinal cells except spermatids and spermatozoa were present. Cell suspensions from testes of adult rats, containing predominantly spermatids, had relatively low rates of palmitate and pyruvate oxidation. These rates were increased in cell suspensions from testes of regressed hypophysectomized rats towards those observed in testicular cell preparations from immature rats. The predominant cell types in testes from hypophysectomized, regressed rats are spermatogonia and early spermatocytes, although early stage spermatids are also present in lesser numbers. The ketogenic enzyme capacity was greatest in particulate preparations obtained from testes of normal 14-day-old rats, in which the predominant germinal cells present are spermatogonia. The activity of succinyl-CoA: 3-oxoacid CoA-transferase was also highest in these preparations. Cell suspensions from testes of 14-day-old rats incorporated significant amounts of labeled palmitate and pyruvate into acetoacetate, whereas cell suspensions from testes of other groups of animals examined did not. The data are discussed in relation to factors controlling rates of fatty acid oxidation in various germinal epithelial cells. It is concluded that spermatocytes have highest rates of pyruvate oxidation, but that both spermatogonia and spermatocytes have relatively high rates of palmitate oxidation. Since spermatogonia also were shown to contain the relatively highest ketogenic enzymic capacity, and since these cells had previously been observed to have lowest levels of carnitine acetyltransferase (CAT), it may be deduced that high CAT activity is not required for fatty acid oxidation or ketogenesis by testicular cells.

Fatty acid oxidation, oxidative phosphorylation and ultrastructure of mitochondria in the diabetic rat liver. Hepatic factors in diabetic ketosis

Oxidative phosphorylation, maximum rate of palmitate xidation and ultrastructure of mitochondria (MT) were tudied in livers of ketotic, nonketotic and insulin-treated liabetic rats. Significant changes in oxidative phosphorylaion were found only in ketotic diabetes. Respiratory control atios were decreased for all of the substrates studied and vere primarily due to increased state 4 respiration with lalmitylcarnitine and 2-oxoglutarate, and lowered state 3 espiration in the case of succinate. ADP/O ratio was only partially lowered with 2-step oxidation of 2-oxoglutarate. The naximum rate of palmitate oxidation to acetoacetate was ncreased 90 per cent in MT of rats with ketotic diabetes ind 50 per cent in MT from animals with nonketotic diabetes. Accompanying these changes were ultrastructural alteritions of MT. The average size of MT in the diabetic rat liver was 50 per cent larger in cut sections. MT within hepacocytes in ketotic diabetes were distorted, enlarged, and had irregular shapes and contours. Cristae tended to be deranged; when isolated, crescentic MT were predominant, In the insulin-treated diabetic rats, these functional and morphological changes were not observed. The results imply that in diabetic ketosis, increased availability of free fatty acid(s) (FFA) in hepatocytes together with increased capacity of mitochondrial FA oxidation produces more acetyl CoA which in turn results in an overproduction of ketone bodies. Since a significant increase in state 4 respiration of MT isolation from ketotic diabetic rats was observed, FA oxidation may proceed independently of ADP availability because of “loose” coupling of respiration to phosphorylation.

Factors controlling ketogenesis by rat liver mitochondria

Factors controlling the rates of ketogenesis by intact rat liver mitochondria have been investigated. High rates of ketone body formation were obtained with (−)-palmitoylcarnitine (20–120 μM) as substrate, but much lower rates were observed when pyruvate (0.33–1.66 mM) or (−)-acetylcarnitine (0.33–1.00 mM) was substrate. Concentrations of CoA-SH, acetyl-CoA, and long-chain acyl-CoA have been determined in mitochondria incubated with each of these substrates in the absence of metabolic inhibitors. In general, rates of ketogenesis increased as CoA-SH levels fell. Although acetyl-CoA concentrations increased in mitochondria incubated in the presence of low concentrations of (−)-palmitoylcarnitine (below 40 μM), they decreased when higher concentrations of (−)-palmitoylcarnitine were employed. This lowering of acetyl-CoA levels occurred concomitantly with an increase in concentrations of long-chain acyl-CoA and a decrease in CoA-SH levels.In soluble mitochondrial fractions obtained after sonication, CoA-SH addition inhibited acetoacetate formation. The ratio of [acetyl-CoA]/[CoA-SH] and the concentrations of CoA-SH were shown to be of greater importance in the regulation of ketogenesis than was the concentration of acetyl-CoA. Additional factors controlling rates of ketogenesis are discussed in relation to data presented. For example, the [acetyl-CoA]/[CoA-SH] ratio was considerably elevated when pyruvate or (−)-acetylcarnitine was substrate, but at such ratios the rates of ketogenesis were far lower than when (−)-palmitoylcarnitine was the substrate. It was calculated that the "apparent Km" of acetoacetyl-CoA for ketone body formation in intact rat liver mitochondria was approximately 10−9 M when (−)-palmitoylcarnitine was the substrate but it was significantly higher when (−)-acetylcarnitine and pyruvate were substrates.