Oxidation Of Β-hydroxybutyrate Research Articles

The lipidosis of the liver of dairy cows: Part 1 - Role of insulin and the Growth Hormone-IGF-1 axis

Lipidosis of the liver of dairy cows is a metabolic disease known since many years and is caused by an uptake of nonesterified fatty acids (NEFA) into the liver cells, limited metabolism of NEFA (oxidation and production of β-hydroxybutyrate), and resynthesis in relation to a low efflux as triglyceride (TG). The pathogenesis of lipidosis includes a) an augmented release of NEFA by mobilisation of adipose tissue, b) uptake of NEFA into the liver cells, c) metabolism of NEFA and d) re-synthesis of triglyceride and e) an efflux of TG as very low density lipoprotein (VLDL). The steps a-e are postpartum modified by hormones as an increase of growth hormone, a pronounced insulin resistance in combination with a decreased insulin and of IGF-1 concentrations. These hormonal changes are related to an uncoupling of the growth hormone-IGF-1-axis with enhanced lipolysis and consequences mentioned above. These alterations are associated with inflammation, oxidative and endoplasmatic stress. The metabolic and hormonal alterations are the result of the selection of dairy cows primarily for milk production without adequate food intake with the consequence of lipidosis, ketosis and further health risks (production diseases).

In vivo assessment of β-hydroxybutyrate metabolism in mouse brain using deuterium (2 H) MRS.

To monitor the metabolic turnover of β-hydroxybutyrate (BHB) oxidation using 2 H-MRS in conjunction with intravenous administration of 2 H labeled BHB. Nine-month-old mice were infused with [3,4,4,4]-2 H4 -BHB (d4 -BHB; 3.11 g/kg) through the tail vein using a bolus variable infusion rate for a period of 90 min. The labeling of downstream cerebral metabolites from the oxidative metabolism of d4 -BHB was monitored using 2 H-MRS spectra acquired with a home-built 2 H surface coil on a 9.4T preclinical MR scanner with a temporal resolution of 6.25 min. An exponential model was fit to the BHB and glutamate/glutamine (Glx) turnover curves to determine rate constants of metabolite turnover and to aid in the visualization of metabolite time courses. Deuterium label was incorporated into Glx from BHB metabolism through the tricarboxylic acid (TCA) cycle, with an increase in the level of [4,4]-2 H2 -Glx (d2 -Glx) over time and reaching a quasi-steady state concentration of ∼0.6± 0.1 mM following 30 min of infusion. Complete oxidative metabolic breakdown of d4 -BHB also resulted in the formation of semi-heavy water (HDO), with a four-fold (10.1 to ∼42.1± 7.3 mM) linear (R2 = 0.998) increase in its concentration by the end of infusion. The rate constant of Glx turnover from d4 -BHB metabolism was determined to be 0.034 ± 0.004 min-1 . 2 H-MRS can be used to monitor the cerebral metabolism of BHB with its deuterated form by measuring the downstream labeling of Glx. The integration of 2 H-MRS with deuterated BHB substrate provides an alternative and clinically promising MRS tool to detect neurometabolic fluxes in healthy and disease conditions.

On the interdependence of ketone body oxidation, glycogen content, glycolysis and energy metabolism in the heart.

In heart, glucose and glycolysis are important for anaplerosis and potentially therefore for d-β-hydroxybutyrate (βHB) oxidation. As a glucose store, glycogen may also furnish anaplerosis. We determined the effects of glycogen content on βHB oxidation and glycolytic rates, and their downstream effects on energetics, in the isolated rat heart. High glycogen (HG) and low glycogen (LG) containing hearts were perfused with 11mM [5-3 H]glucose and/or 4mM [14 C]βHB to measure glycolytic rates or βHB oxidation, respectively, then freeze-clamped for glycogen and metabolomic analyses. Free cytosolic [NAD+ ]/[NADH] and mitochondrial [Q+ ]/[QH2 ] ratios were estimated using the lactate dehydrogenase and succinate dehydrogenase reaction, respectively. Phosphocreatine (PCr) and inorganic phosphate (Pi ) concentrations were measured using 31 P-nuclear magnetic resonance spectroscopy. Rates of βHB oxidation in LG hearts were half that in HG hearts, with βHB oxidation directly proportional to glycogen content. βHB oxidation decreased glycolysis in all hearts. Glycogenolysis in glycogen-replete hearts perfused with βHB alone was twice that of hearts perfused with βHB and glucose, which had significantly higher levels of the glycolytic intermediates fructose 1,6-bisphosphate and 3-phosphoglycerate, and higher free cytosolic [NAD+ ]/[NADH]. βHB oxidation increased the Krebs cycle intermediates citrate, 2-oxoglutarate and succinate, the total NADP/H pool, reduced mitochondrial [Q+ ]/[QH2 ], and increased the calculated free energy of ATP hydrolysis (∆GATP ). Although βHB oxidation inhibited glycolysis, glycolytic intermediates were not depleted, and cytosolic free NAD remained oxidised. βHB oxidation alone increased Krebs cycle intermediates, reduced mitochondrial Q and increased ∆GATP . We conclude that glycogen facilitates cardiac βHB oxidation by anaplerosis. KEY POINTS: Ketone bodies (d-β-hydroxybutyrate, acetoacetate) are increasingly recognised as important cardiac energetic substrates, in both healthy and diseased hearts. As 2-carbon equivalents they are cataplerotic, causing depletion of Krebs cycle intermediates; therefore their utilisation requires anaplerotic supplementation, and intra-myocardial glycogen has been suggested as a potential anaplerotic source during ketone oxidation. It is demonstrated here that cardiac glycogen does indeed provide anaplerotic substrate to facilitate β-hydroxybutyrate oxidation in isolated perfused rat heart, and this contribution was quantified using a novel pulse-chase metabolic approach. Further, using metabolomics and 31 P-MR, it was shown that glycolytic flux from myocardial glycogen increased the heart's ability to oxidise βHB, and βHB oxidation increased the mitochondrial redox potential, ultimately increasing the free energy of ATP hydrolysis.

β-Hydroxybutyrate Oxidation Is Reduced and Hepatic Balance of Ketone Bodies and Free Fatty Acids Is Unaltered in Carnitine-Depleted, Pivalate-Treated Rats

These experiments were designed to determine whether carnitine depletion in the rat due to pivalate administration causes reduced β-hydroxybutyrate oxidation and alterations in hepatic balance of ketone bodies and free fatty acids, relative to control rats. Male rats were given 20 mmol/L sodium pivalate for 2 wk to induce a secondary carnitine deficiency. Control animals were given 20 mmol/L sodium bicarbonate in their drinking water. In the ketone utilization experiment, rats were food-deprived for 24 h and infused with the sodium salt of β-hydroxybutyrate to maintain total plasma ketone concentrations between 6.0 and 10.0 mmol/L. After a bolus of 3-hydroxy[3-14C]butyrate, the recovery of expired 14CO2 collected during the ensuing 100 main was significantly lower in the pivalate-treated rats than in the controls (P < 0.05). In the second experiment, blood samples from pivalate-treated and control rats were collected from the abdominal aorta, portal vein and hepatic vein to determine the hepatic balance (hepatic input — hepatic output) of ketone bodies and free fatty acids. No significant differences were seen between the two treatment groups for either ketone bodies or free fatty acids. We conclude that the higher plasma ketone concentrations seen in food-deprived, pivalate-treated rats were due to a lower rate of ketone utilization. No evidence of pivalate influence upon ketone production was found.

Ketone body oxidation increases cardiac endothelial cell proliferation.

Blood vessel formation is dependent on metabolic adaption in endothelial cells. Glucose and fatty acids are essential substrates for ATP and biomass production; however, the metabolism of other substrates remains poorly understood. Ketone bodies are important nutrients for cardiomyocytes during starvation or consumption of carbohydrate‐restrictive diets. This raises the question whether cardiac endothelial cells would not only transport ketone bodies but also consume some of these to achieve their metabolic needs. Here, we report that cardiac endothelial cells are able to oxidize ketone bodies and that this enhances cell proliferation, migration, and vessel sprouting. Mechanistically, this requires succinyl‐CoA:3‐oxoacid‐CoA transferase, a key enzyme of ketone body oxidation. Targeted metabolite profiling revealed that carbon from ketone bodies got incorporated into tricarboxylic acid cycle intermediates as well as other metabolites fueling biomass production. Elevation of ketone body levels by a high‐fat, low‐carbohydrate ketogenic diet transiently increased endothelial cell proliferation in mouse hearts. Notably, in a mouse model of heart hypertrophy, ketogenic diet prevented blood vessel rarefication. This suggests a potential beneficial role of dietary intervention in heart diseases.

β-Hydroxybutyrate Oxidation in Exercise Is Impaired by Low-Carbohydrate and High-Fat Availability.

Purpose: In this study, we determined ketone oxidation rates in athletes under metabolic conditions of high and low carbohydrate (CHO) and fat availability.Methods: Six healthy male athletes completed 1 h of bicycle ergometer exercise at 75% maximal power (WMax) on three occasions. Prior to exercise, participants consumed 573 mg·kg bw−1 of a ketone ester (KE) containing a 13C label. To manipulate CHO availability, athletes undertook glycogen depleting exercise followed by isocaloric high-CHO or very-low-CHO diets. To manipulate fat availability, participants were given a continuous infusion of lipid during two visits. Using stable isotope methodology, β-hydroxybutyrate (βHB) oxidation rates were therefore investigated under the following metabolic conditions: (i) high CHO + normal fat (KE+CHO); (ii) high CHO + high fat KE+CHO+FAT); and (iii) low CHO + high fat (KE+FAT).Results: Pre-exercise intramuscular glycogen (IMGLY) was approximately halved in the KE+FAT vs. KE+CHO and KE+CHO+FAT conditions (both p < 0.05). Blood free fatty acids (FFA) and intramuscular long-chain acylcarnitines were significantly greater in the KE+FAT vs. other conditions and in the KE+CHO+FAT vs. KE+CHO conditions before exercise. Following ingestion of the 13C labeled KE, blood βHB levels increased to ≈4.5 mM before exercise in all conditions. βHB oxidation was modestly greater in the KE+CHO vs. KE+FAT conditions (mean diff. = 0.09 g·min−1, p = 0.03; d = 0.3), tended to be greater in the KE+CHO+FAT vs. KE+FAT conditions (mean diff. = 0.07 g·min−1; p = 0.1; d = 0.3) and were the same in the KE+CHO vs. KE+CHO+FAT conditions (p < 0.05; d < 0.1). A moderate positive correlation between pre-exercise IMGLY and βHB oxidation rates during exercise was present (p = 0.04; r = 0.5). Post-exercise intramuscular βHB abundance was markedly elevated in the KE+FAT vs. KE+CHO and KE+CHO+FAT conditions (both, p < 0.001; d = 2.3).Conclusion: βHB oxidation rates during exercise are modestly impaired by low CHO availability, independent of circulating βHB levels.

β-Hydroxybutyrate Oxidation Promotes the Accumulation of Immunometabolites in Activated Microglia Cells.

Metabolic regulation of immune cells has arisen as a critical set of processes required for appropriate response to immunological signals. While our knowledge in this area has rapidly expanded in leukocytes, much less is known about the metabolic regulation of brain-resident microglia. In particular, the role of alternative nutrients to glucose remains poorly understood. Here, we use stable-isotope (13C) tracing strategies and metabolomics to characterize the oxidative metabolism of β-hydroxybutyrate (BHB) in human (HMC3) and murine (BV2) microglia cells and the interplay with glucose in resting and LPS-activated BV2 cells. We found that BHB is imported and oxidised in the TCA cycle in both cell lines with a subsequent increase in the cytosolic NADH:NAD+ ratio. In BV2 cells, stimulation with LPS upregulated the glycolytic flux, increased the cytosolic NADH:NAD+ ratio and promoted the accumulation of the glycolytic intermediate dihydroxyacetone phosphate (DHAP). The addition of BHB enhanced LPS-induced accumulation of DHAP and promoted glucose-derived lactate export. BHB also synergistically increased LPS-induced accumulation of succinate and other key immunometabolites, such as α-ketoglutarate and fumarate generated by the TCA cycle. Finally, BHB upregulated the expression of a key pro-inflammatory (M1 polarisation) marker gene, NOS2, in BV2 cells activated with LPS. In conclusion, we identify BHB as a potentially immunomodulatory metabolic substrate for microglia that promotes metabolic reprogramming during pro-inflammatory response.

Evidence for Intramyocardial Disruption of Lipid Metabolism and Increased Myocardial Ketone Utilization in Advanced Human Heart Failure.

The failing human heart is characterized by metabolic abnormalities, but these defects remains incompletely understood. In animal models of heart failure there is a switch from a predominance of fatty acid utilization to the more oxygen-sparing carbohydrate metabolism. Recent studies have reported decreases in myocardial lipid content, but the inclusion of diabetic and nondiabetic patients obscures the distinction of adaptations to metabolic derangements from adaptations to heart failure per se. We performed both unbiased and targeted myocardial lipid surveys using liquid chromatography-mass spectroscopy in nondiabetic, lean, predominantly nonischemic, advanced heart failure patients at the time of heart transplantation or left ventricular assist device implantation. We identified significantly decreased concentrations of the majority of myocardial lipid intermediates, including long-chain acylcarnitines, the primary subset of energetic lipid substrate for mitochondrial fatty acid oxidation. We report for the first time significantly reduced levels of intermediate and anaplerotic acyl-coenzyme A (CoA) species incorporated into the Krebs cycle, whereas the myocardial concentration of acetyl-CoA was significantly increased in end-stage heart failure. In contrast, we observed an increased abundance of ketogenic β-hydroxybutyryl-CoA, in association with increased myocardial utilization of β-hydroxybutyrate. We observed a significant increase in the expression of the gene encoding succinyl-CoA:3-oxoacid-CoA transferase, the rate-limiting enzyme for myocardial oxidation of β-hydroxybutyrate and acetoacetate. These findings indicate increased ketone utilization in the severely failing human heart independent of diabetes mellitus, and they support the role of ketone bodies as an alternative fuel and myocardial ketone oxidation as a key metabolic adaptation in the failing human heart.

Pathogenesis of A−β+ Ketosis-Prone Diabetes

A−β+ ketosis-prone diabetes (KPD) is an emerging syndrome of obesity, unprovoked ketoacidosis, reversible β-cell dysfunction, and near-normoglycemic remission. We combined metabolomics with targeted kinetic measurements to investigate its pathophysiology. Fasting plasma fatty acids, acylcarnitines, and amino acids were quantified in 20 KPD patients compared with 19 nondiabetic control subjects. Unique signatures in KPD—higher glutamate but lower glutamine and citrulline concentrations, increased β-hydroxybutyryl-carnitine, decreased isovaleryl-carnitine (a leucine catabolite), and decreased tricarboxylic acid (TCA) cycle intermediates—generated hypotheses that were tested through stable isotope/mass spectrometry protocols in nine new-onset, stable KPD patients compared with seven nondiabetic control subjects. Free fatty acid flux and acetyl CoA flux and oxidation were similar, but KPD had slower acetyl CoA conversion to β-hydroxybutyrate; higher fasting β-hydroxybutyrate concentration; slower β-hydroxybutyrate oxidation; faster leucine oxidative decarboxylation; accelerated glutamine conversion to glutamate without increase in glutamate carbon oxidation; and slower citrulline flux, with diminished glutamine amide–nitrogen transfer to citrulline. The confluence of metabolomic and kinetic data indicate a distinctive pathogenic sequence: impaired ketone oxidation and fatty acid utilization for energy, leading to accelerated leucine catabolism and transamination of α-ketoglutarate to glutamate, with impaired TCA anaplerosis of glutamate carbon. They highlight a novel process of defective energy production and ketosis in A−β+ KPD.

Obligate Role for Ketone Body Oxidation in Neonatal Metabolic Homeostasis

To compensate for the energetic deficit elicited by reduced carbohydrate intake, mammals convert energy stored in ketone bodies to high energy phosphates. Ketone bodies provide fuel particularly to brain, heart, and skeletal muscle in states that include starvation, adherence to low carbohydrate diets, and the neonatal period. Here, we use novel Oxct1(-/-) mice, which lack the ketolytic enzyme succinyl-CoA:3-oxo-acid CoA-transferase (SCOT), to demonstrate that ketone body oxidation is required for postnatal survival in mice. Although Oxct1(-/-) mice exhibit normal prenatal development, all develop ketoacidosis, hypoglycemia, and reduced plasma lactate concentrations within the first 48 h of birth. In vivo oxidation of (13)C-labeled β-hydroxybutyrate in neonatal Oxct1(-/-) mice, measured using NMR, reveals intact oxidation to acetoacetate but no contribution of ketone bodies to the tricarboxylic acid cycle. Accumulation of acetoacetate yields a markedly reduced β-hydroxybutyrate:acetoacetate ratio of 1:3, compared with 3:1 in Oxct1(+) littermates. Frequent exogenous glucose administration to actively suckling Oxct1(-/-) mice delayed, but could not prevent, lethality. Brains of newborn SCOT-deficient mice demonstrate evidence of adaptive energy acquisition, with increased phosphorylation of AMP-activated protein kinase α, increased autophagy, and 2.4-fold increased in vivo oxidative metabolism of [(13)C]glucose. Furthermore, [(13)C]lactate oxidation is increased 1.7-fold in skeletal muscle of Oxct1(-/-) mice but not in brain. These results indicate the critical metabolic roles of ketone bodies in neonatal metabolism and suggest that distinct tissues exhibit specific metabolic responses to loss of ketone body oxidation.

Decrease of phosphorylating oxidation and increase of heat producing NADH oxidation in rat liver mitochondria during life-span prolongation of rats by calorie-restricted diet

The influence of calorie-restricted diet, initiated at weaning, on some of the oxidative processes in liver homogenates and isolated mitochondria of 2-, 3-, 4-, 24-, 35- and 45-month-old male Wistar rats was studied in comparison with control ad libitum-fed 1–2 day-old rats and 0.5-, 1-, 2-, 3-, 4- and 24-month-old rats. It was shown that a calorie-restricted diet (at 37% of the ad libitum calorific level) did not change the rate of succinate oxidation coupled with oxidative phosphorylation in homogenates, but resulted in a decrease of succinate, glutamate plus malate and β-hydroxybutyrate oxidation and cytochrome c-oxidase activity in isolated mitochondria without any uncoupling of oxidative phosphorylation or change in cytochrome content in the mitochondria. On the other hand, a significant increase in mitochondrial rotenone-insensitive NADH oxidation and a higher level mass/body mass ratio in rats under the calorie-restricted diet was established. It may be considered that the activation of a heat-producing mechanism is a very important physiological function in such a condition.

An NADH-linked luciferase assay for glycogen: The preparation of glycogen-free, viable human eccrine sweat glands

A glycogen assay based on bacterial NADH luciferase is described. It is free of tissue interference. The detection limit is 0.12 nmol glycogen, and the coefficient of variation is 5.5%. A method of depleting human eccrine sweat glands while retaining their viability is described. This depends on their incubation in 10 −5 m acetylcholine and 1 m m pyruvate. This method may be applicable to other tissues. The evidence for the viability of glycogen-depleted human eccrine sweat glands is reported and includes tissue contents of ATP and the rates of oxidation of glucose, pyruvate, β-hydroxybutyrate, and palmitate.

Influence of Diet on the Production of a “Lipid-Depleting” Factor in Obese Parabiotic Rats

When one member of a parabiosed pair of rats is made obese its partner experiences a specific loss of body fat. In this study we determined whether production of the “lipid-depleting” factor in obese rats was diet specific. One member in each of 30 pairs was tube fed 200% normal intake of high carbohydrate (CHO), high fat corn oil or high fat medium-chain triglyceride diet. Their partners and both members of five control pairs ate CHO diet ad libitum. After 27 d of 200% feeding, in vitro hepatic, adipose and hypothalamic metabolism were measured. Composition of the diet used to induce obesity did not change the response of partners. All non-tube-fed partners had normal food intakes, body protein, hepatic fatty acid synthesis (FAS) and esterification (FAE), palmitate, glucose and β-hydroxybutyrate oxidation. Adipose FAS and FAE were depressed, body fat was halved. Hypothalamic metabolism was not changed. A lipid-depleting agent originating in obese rats specifically inhibits adipose lipogenesis in their partners. Production of this factor does not appear to be influenced by dietary energy source.

Effects of endosulfan and its metabolites on rat liver mitochondrial respiration and enzyme activities in vitro

Endosulfan (E) is an organochloric insecticide, which is quickly metabolized and eliminated from the body system. Toxic effects of E and its metabolites have been reported. The influence of E and its metabolites, viz. endosulfan sulfate (ES), endosulfan diol (ED) and endosulfan lactone (EL), has been examined on rat liver mitochondria in vitro. Endosulfan stimulated state-4 respiration at lower concentrations and inhibited it at higher ones, whereas state-3 respiration was inhibited at all the concentrations used, i.e. 5–100 μg/ml. A maximal 25-fold activation of latent Mg 2+-ATPase was achieved at a concentration that caused maximal stimulation of state-4 respiration. Activities of the respiratory chain-linked enzymes were inhibited at levels which corresponded to the concentrations of endosulfan used in vitro. Both the respiratory control ratio (RCR) and the ADP:O ratio fell sharply at endosulfan concentrations above 10 μg/ml. ES and ED exerted similar effects on mitochondrial oxidation of β-hydroxybutyrate, but at more than double the concentration of the parent compound, while EL proved least effective. The effects of the latter compound on mitochondrial enzyme activities were negligible. Our results suggest that endosulfan possesses dual properties, that of an uncoupler of oxidative phosphorylation and of an inhibitor of the electron transport chain, and that the in vivo cytotoxic/insecticidal effects of endosulfan and its metabolites might, therefore, be the consequence of impaired mitochondrial bioenergetics.

GLUTAMINE IS THE PREFERRED OXIDATIVE SUBSTRATE FOR SMALL INTESTINE OF SUCKLING RAT

We determined the rate of oxidation of glutamine and β-hydroxybutyrate (βHβ), the preferred substrates in adult intestine, in developing rat. βHβ oxidation is low during the suckling period and increases four-fold at the time of weaning. In contrast glutamine oxidation is high during the suckling period and decreases at the time of weaning. Acetyl-CoA, an intermediate product of ketone oxidation is oxidized by intestinal mitochondria of suckling rat pups at 1/8 the rate of postwean indicating that tricarboxylic acid (TCA) cycle activity may be rate-limiting in βHβ oxidation to CO2 during the suckling period. A high mitochondrial (mito) [NADH]/[NAD] inhibits isocitrate dehydrogenase, a rate limiting step of the TCA cycle. The mito [NADH]/[NAD] decreases four fold at the time of weaning which is consistent with an increase in TCA cycle activity at this time. Since glutamine oxidation enters the TCA cycle in the form of α-ketoglutarate, the inhibition of isocitrate dehyrogenase by an increase in mito [NADH]/[NAD] would not affect glutamine oxidation. These data suggest that an increase in mito [NADH]/[NAD] results in inhibition of oxidation of substrates which enter the TCA cycle at the level of acetyl-CoA and results in the utilization of glutamine as a primary substrate during the suckling period.

Oxidation of Alanine and β-Hydroxybutyrate in Late Gestation by Zinc-Restricted Rats

The day 21 response of the Long-Evans pregnant rat to marginal dietary zinc intake was investigated in this experiment. Day 20 and 21 food intakes of zinc-restricted dams were below the control group amounts and indicated that dietary nitrogen and energy did not meet gestation requirements. Intravenous injections of tracer doses of [14C]alanine provided evidence for decreased oxidation of the carbon chain of alanine when zinc-restricted were compared to control dams. The plasma pool size of β-hydroxybutyrate and the rate of 14CO2 production following injection of the radioactively labeled ketone body indicated that substantial increases in the oxidation of β-hydroxybutyrate were induced by restricted zinc feeding. Maternal plasma concentrations of alanine were marginally reduced, β-hydroxybutyrate increased, and glucose unchanged by the suboptimal zinc diet. Litter weights at day 21 were moderately depressed (- 11%) by maternal zinc-restriction, suggesting that the chronic effects of mild dietary zinc deficiency are most apparent near term in this animal model.

Effect of fasting on the metabolism in rat aorta.

The oxidation of 14C-labelled glucose, beta-hydroxybutyrate and palmitate to CO2 and the incorporation of 14C-labelled amino acid into alkali-soluble protein were studied in aorta from fed and fasted male Sprague-Dawley rats, weighting about 200 g. Substrate oxidation and amino acid incorporation were measured during incubation of rat aorta in vitro for 2-3 h. After fasting for 6 h there was a slight but significant increase in the plasma concentration of beta-hydroxybutyrate. Blood glucose was lowered after 12 h while an increase in the plasma concentration of free fatty acids was found after 24 h. A decrease in the oxidation of glucose in rat aorta was found after fasting for 12 h and with prolonged fasting a further decrease in the aortic glucose oxidation was found. After fasting for 4 days the oxidation of beta-hydroxybutyrate and the incorporation of 14C-leucine and 14C-phenylalanine into protein were reduced in rat aorta while the oxidation of palmitate was not altered. The effects of fasting on substrate oxidation and amino acid incorporation in rat aorta, found in this study are similar to the known effects of diabetes on vascular metabolism.

Antiketonemic and Antiketogenic Actions of Carnitine in Vivo and in Vitro in Rats

The effect of carnitine on ketone and lipid metabolism in intact rats and in isolated tissues was investigated. DL-Carnitine (200 mg/100 g body weight) administered orally lowered by approximately 40–65% plasma concentrations of ketone bodies in suckling rats and fasted adult rats. It also suppressed hyperketonemia induced by corn oil feeding in suckling and adult rats by approximately 30–60%. Carnitine did not change β-hydroxybutyrate oxidation in heart and kidney mitochondria. Incubation of carnitine with tissue slices of cerebral cortex, kidney and diaphragm did not affect 14CO2 production or lipid synthesis from DL-[3-14C]-β-hydroxybutyrate. The compound reduced the urinary concentration and excretion of ketone bodies in rats fed corn oil but had no effect in rats not treated with corn oil. Although DL-carnitine at concentrations ranging from 0.2 to 20 mm stimulated ketone production in hepatocytes, at a higher concentration (i.e., 40 mm) it produced a sharp depression in ketogenesis. These results indicate that carnitine does not stimulate ketone utilization by extrahepatic tissues. The antiketonemic effect of carnitine in rats treated with a high dose of the compound most likely results from inhibition of hepatic ketone production. An excessive increase in free carnitine by exogenous administration could upset the equilibrium for conversion of acylcarnitine to acylCoA in mitochondria and hence suppress β-oxidation and ketogenesis.carnitine hyperketonemia ketone utilization ketogenesis

2′,3′-Cyclic nucleotide 3′-phosphohydrolase in rat liver mitochondrial membranes

2′,3′-Cyclic nucleotide 3′-phosphohydrolase (nucleoside-2′:3′-cyclic-phosphate 2′-nucleotidohydrolase, EC 3.1.4.37) activity has been demonstrated in rat liver mitochondria. The enzyme was localized in both the outer and inner mitochondrial membranes but was absent from the intermembrane space and matrix. The mitochondrial (cyclic nucleotide) phosphohydrolase was activated by freezing and thawing and by treatment with digitonin or detergents. It is suggested that (cyclic nucleotide) phosphohydrolase is an integral membrane protein which is buried to a significant degree within the membrane. Atractyloside was found to be a noncompetitive inhibitor of the enzyme both in intact mitochondria and in preparations of the mitochondrial membranes. The enzyme substrate, 2′,3′-cyclic adenosine monophosphate, had no effect on the oxidation of exogenous β-hydroxybutyrate or succinate by intact mitochondria. These findings suggest that 2′,3′-cyclic nucleotide 3′phosphohydrolase is more widely distributed than was previously thought and that the enzyme may play a fundamental role in membranes, independent of their specialized structure or functions.

Focal differences in bioenergetic metabolism of atherosclerosis-susceptible and -resistant pigeon aortas

To test the hypothesis of altered energy production in aortic sites predisposed to lesion formation, both coupled oxidative phosphorylation (evaluated from P/O ratios) and control of the energy-linked (ATP) NADH transhydrogenase were examined in muscular cushions at the celiac artery bifurcation. Comparisons were made between atherosclerosis-susceptible White Carneau (WC) and atherosclerosis-resistant Show Racer (SR) pigeons at ages up to early atherogenesis in WC (1 day to 6 months of development). From 1 day to 12 weeks of age, mitochondria from cushions of both breeds tightly coupled phosphorylation to the oxidation of either β-hydroxybutyrate or succinate. However, by 6 months of age, WC celiac cushions had significantly ( P < 0.05) lower P/O ratios than the SR cushions with either substrate, suggesting uncoupled respiratory-chain phosphorylation. Aortic submitochondrial A-particles from celiac cushions of susceptible pigeons showed no ATP regulation of NADH transhydrogenation at 6 weeks and subsequent ages. This defect is believe to enhance lipid biosynthesis and may be involved in exacerbation of the biochemical arteriopathy in WC. These findings are discussed in terms of relationships between oxidative energy production, lipid accumulation, hypoxic stress, and atherosclerotic involvement.