Competition between fatty acids and carbohydrate or ketone bodies as metabolic fuels for the isolated perfused heart

The ability of carbohydrate fuels (lactate, pyruvate, glucose) and the ketone bodies (acetoacetate, beta-hydroxybutyrate) to compete with fatty acids as fuels of respiration in the isolated Langendorf-perfused heart was studied. Oleate and octanoate were used as fatty acid fuels since oleate requires carnitine for entry into mitochondria, whereas octanoate does not. The two ketone bodies inhibited the oxidation of both oleate and octanoate implying an intramitochondrial site of action. Pyruvate, lactate, and lactate plus glucose inhibited oleate oxidation but not octanoate oxidation, indicating a mechanism of inhibition that involves the carnitine system. Pyruvate was a more potent inhibitor than lactate at equal concentrations, but the effect of lactate could be greatly increased by dichloroacetate, an inhibitor of pyruvate dehydrogenase kinase. The physiological and mechanistic implications of these observations are discussed.     

Activities of enzymes of ketone body metabolism in liver and muscle tissues of suckling rabbits

The concentration of ketone bodies in blood of suckling rabbits during the first 6 days following birth was higher than that found in the adult. In the liver the activities of the enzymes of ketone body synthesis were higher than in the adult during the same period. In the heart and leg muscle the activities of the enzymes of ketone body utilization were lower than those found in the adult. It is suggested that the capacity of the muscles of the developing rabbit to utilize ketone bodies is not greater than that of the adult and ketone bodies produced by the liver could contribute as fuel for oxidation and/or synthesis to the brain of the newborn rabbit.     

Role of ketone bodies in the diabetes-induced changes in hepatic mixed-function oxidase activities

In order to assess the role of ketone bodies in the diabetes-induced changes in hepatic mixed-function oxidase activity, rats rendered hyperketonaemic by dietary administration of medium chain triacylglycerols were compared with streptozotocin treated rats. Both groups of animals became hyperketonaemic but only the latter were hyperglycaemic. Treatment with streptozotocin or medium chain triacylglycerols gave rise to marked increases in the O-dealkylations of ethoxyresorufin, ethoxycoumarin and pentoxyresorufin, the p-hydroxylation of aniline and the N-demethylation of dimethylnitrosamine. It is concluded that the streptozotocin-induced changes in hepatic mixed-function oxidases are mediated, at least partly, by the high levels of ketone bodies.     

The effect of ketone bodies on nitrogen metabolism in skeletal muscle.

1. The ketone bodies, D-beta-hydroxybutyrate and acetoacetate, inhibit glycolysis thereby reducing pyruvate availability which leads to a marked inhibition of branched-chain amino acid metabolism and alanine synthesis in skeletal muscles from fasted mammalian and avian species. 2. The rate of glutamine release from skeletal muscles from fasted birds is increased at the expense of alanine in the presence of elevated concentrations of ketone bodies because of an increase in the availability of glutamate for glutamine synthesis. 3. Ketone bodies inhibit both protein synthesis and protein degradation in skeletal muscles from fasted mammalian and avian species in vitro. The mechanisms involved remain unknown. 4. Inhibition of amino acid metabolism and protein turnover in skeletal muscle by ketone bodies may be an important survival mechanism during adaptation to catabolic states such as prolonged fasting.     

Enzyme activities in plasma, kidney, liver, and muscle of five avian species

Activities of alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP), creatine phosphokinase (CPK), and lactate dehydrogenase (LDH) were determined in plasma, kidney, liver, and muscle from five species of captive birds. Few differences occurred in plasma activities between sexes but considerable differences occurred between species. All five enzymes were detected in each of the tissues sampled. Relative enzyme activities in liver, kidney, and muscle were similar for each species. CPK activity was much higher in muscle than in liver or kidney and, of the five enzymes studied, may be the best indicator of muscle damage. Most of the other enzymes were more evenly distributed among the three tissues, and no organ-specific enzyme could be identified for liver or kidney. Because of interspecific variations in plasma enzyme activities, it is important to establish baseline values for each species to ensure accurate interpretation of results.     

Acid proteolytic activities in mouse liver and muscle tissues after treatment with protease inhibitor leupeptin

The activities of acid proteolytic enzymes were assayed in the liver and muscular tissues of mice (Mus musculus) 1, 6 and 24 hr after the administration of a protease inhibitor leupeptin (i.p., 15.5 mg/kg body wt). Leupeptin administration induced a strong inhibition of cathepsin B and a moderate inhibition of cathepsin C and acid autolytic rate in mouse liver 1 hr after injection. Thereafter the inhibition reduced and disappeared during 24 hr. The activity of cathepsin D was increased in liver 6 and 24 hr after injection. The activity of beta-glucuronidase was not affected by the leupeptin treatment. The administration of leupeptin did not affect the rate of acid autolysis and the activities of cathepsin C and D in cardiac and skeletal muscles. A slight increase in cathepsin B activity was observed 1 hr after leupeptin treatment in calf muscles. The cause of both tissue and enzyme specific changes after leupeptin treatment is discussed.     

Lipogenesis from ketone bodies in the perfused rat liver: effects of acetate and ethanol

The interactions between acetate or ethanol metabolism, lipogenesis, and ketone body utilization have been studied in isolated livers from fed rats perfused with 15 mM glucose and 10 mM acetate or ethanol. The contribution of acetate to ketogenesis is constant; on the other hand, the contribution of ethanol to ketogenesis increases with time, presumably because of the accumulation of acetate in the perfusate. Ketogenesis is decreased in the presence of ethanol (but not acetate), while ketone body utilization is not affected by ethanol or acetate. Acetate contributes one third and ethanol contributes one half of the carbon incorporated into fatty acids and 3-beta-hydroxysterols. Only a small fraction (less than 5%) of the incorporation of acetate or ethanol into fatty acids and sterols occurs via transient incorporation into ketone bodies.     

Dual effects of calcium on the oxidation of fatty acids to ketone bodies in liver mitochondria

The addition of calcium ions (Ca²⁺) to rat liver mitochondria, under conditions of rapid accumulation of 10–40 nmol Ca²⁺/mg protein, inhibited the oxidation of long and medium chain fatty acids to ketone bodies, whereas higher quantities of Ca²⁺ activated the process. The mitochondrial NADH:NAD ratio exhibited corresponding depression and elevation. Both inhibitory and stimulatory actions of Ca²⁺ were operative in liver mitochondria from fed and fasted rats and appear to be localized in the mitochondrial inner membranematrix region. These observations may signify involvement of Ca²⁺ in the regulation of fatty acid oxidation and ketogenesis.     

Enzyme activities of plasma and selected tissues in rainbow trout Salmo gairdneri Richardson

This paper describes the determination in rainbow trout of the normal levels in the plasma of eight enzymes known to be significant in animal pathology. Some relationships between plasma enzyme levels and the enzyme activities in selected tissues are discussed. For LDH which is ubiquitously distributed, we chose, by way of saturation tests, the optimum concentrations of pyruvate and NADH in the assay medium. The LDH 'isoenzyme ratio' was determined for heart tissue, liver, and white muscle. When blood was withdrawn from caudal vessels, we observed a net increase of plasma enzyme activities, mainly of CPK and LDH, which was demonstrated to originate from surrounding muscle. Thus cardiac sampling was the only suitable way of obtaining blood for this kind of study. Slightly haemolyzed blood appeared suitable for enzyme determinations except for Alk Pase which is about 50 times more concentrated in erythrocytes than in plasma. CPK was highly concentrated in the heart and the muscle, GOT was concentrated in the heart while the liver appeared to be a valuable source of GDH (as well as the kidney) and GPT.     

The effect of ketone bodies on protein turnover in isolated skeletal muscle from the fed and fasted chick

1. The addition of 4 mM acetoacetate or DL-beta-hydroxybutyrate to the incubation medium decreased the rate of protein synthesis without influencing the rate of protein degradation in extensor digitorum communis (EDC) muscles from fed chicks and decreased the rates of protein synthesis and degradation in muscles from fasted chicks. 2. Ketone bodies markedly decreased intracellular concentrations of glutamine in EDC muscles from fed chicks by increasing glutamine oxidation. 3. The addition of 0.5 mM glutamine to incubation media containing 1.0 mM glutamine reversed the ketone body-induced decrease in intracellular glutamine concentration to the control value and blocked the inhibiting effect of ketone bodies on protein synthesis in skeletal muscles from fed chicks. 4. The addition of 5 mM pyruvate blocked the ability of ketone bodies to increase glutamine oxidation and prevented the associated decrease in intracellular glutamine concentration and the rate of protein synthesis in EDC muscles from fed chicks. 5. These results suggest that ketone bodies can act directly on skeletal muscle to inhibit the rate of protein synthesis in muscles from fed chicks by decreasing intracellular glutamine concentration by increasing its oxidation.