Effects of octanoate and oleate on energy metabolism in the perfused rat liver

A high-fat diet supplemented with medium-chain triglycerides ameliorates hepatic steatosis by reducing ceramide and diacylglycerol accumulation in mice

Non-alcoholic fatty liver disease (NAFLD) is projected to be the most common chronic liver disease worldwide and is closely linked to obesity, insulin resistance and type 2 diabetes. Currently, no pharmacological treatments are available to treat NAFLD, and lifestyle modification, including dietary interventions, is the only remedy. Therefore, we conducted a study to determine whether supplementation with medium-chain triglycerides (MCTs), containing a mixture of C8 and C10 (60/40), attenuates NAFLD in obese and insulin-resistant mice. To achieve that, we fed C57BL/6 male mice a high-fat diet (HFD) for 12 weeks to induce obesity and hepatic steatosis, after which obese mice were assigned randomly either to remain on the HFD or to transition to an HFD supplemented with MCTs (HFD + MCTs) or a low-fat diet (LFD) for 6 weeks as another dietary intervention model. Another group of mice was kept on an LFD throughout the study and used as a lean control group. Obese mice that transitioned to HFD + MCTs exhibited improvement in glucose and insulin tolerance tests, and the latter improvement was independent of changes in adiposity when compared with HFD-fed mice. Additionally, supplementation with MCTs significantly reduced hepatic steatosis, improved liver enzymes and decreased hepatic expression of inflammation-related genes to levels similar to those observed in obese mice transitioned to an LFD. Importantly, HFD + MCTs markedly lowered hepatic ceramide and diacylglycerol content and prevented protein kinase C-ε translocation to the plasma membrane. Our study demonstrated that supplementation with MCTs formulated mainly from C8 and C10 effectively ameliorated NAFLD in obese mice.

Transcriptomic Responses Induced in Muscle and Adipose Tissues of Growing Pigs by Intravenous Infusion of Sodium Butyrate

Simple Summary

The present study was constructed to determine the effects of short-term intravenous sodium butyrate (SB) administration on metabolism through transcriptomic responses in muscle and adipose tissue of pigs. We found that systemic butyrate displayed a discriminative metabolic regulation in muscle and adipose tissue. Intravenous SB infusion decreased amino acid metabolism pathways both in muscle and adipose tissues. Intravenous SB infusion increased glucose catabolism in muscle tissue and decreased glucose utilization in adipose tissue. Moreover, intravenous SB infusion decreased lipolysis in muscle tissue but increased lipolysis in adipose tissue. These findings support the direct role of the SCFA butyrate in the regulation of the metabolism.

Abstract

Butyrate has a central function in the regulation of energy metabolism as a metabolite of bacterial fermentation. This study evaluated the effects of intravenous sodium butyrate (SB) administration on the transcriptome of muscle and adipose tissue of pigs. Twelve crossbred barrows (Duroc × Landrace × Large White) were fitted with a medical polyethylene cannula via the internal jugular vein and were daily infused with 10 mL SB (200 mmol/L) or the same volume of physiological saline. Muscle transcriptome showed 11 DEGs related to carbohydrate metabolism, 28 DEGs related to lipid metabolism, and 10 DEGs related to amino acid metabolism. Among these, carbohydrate catabolic process-related genes (PPP1R3B, PRPS2, ALDOC), fatty acid synthase (FASN), and lipolysis-related genes (PLIN1) were upregulated, while the carbohydrate biosynthetic process-related genes (PCK1) and most amino acid metabolism-related genes were downregulated. Adipose transcriptome showed 12 DEGs related to carbohydrate metabolism, 27 DEGs related to lipid metabolism, and 10 DEGs related to amino acid metabolism. Among these, carbohydrate metabolism-related genes (IGF1, LEP, SLC2A4) and lipolysis-related genes (LPL) were upregulated, while lipolysis-related genes (ANGPTL4) and most amino acid metabolism-related genes were downregulated. The results suggest that short-term intravenous SB infusion could modulate the muscle and adipose tissue metabolism at the transcriptional level by decreasing amino acid metabolism pathways. Additionally, intravenous SB increased the glucose catabolism in muscle tissue and decreased the glucose utilization in adipose tissue. Intravenous SB increased the fatty acid synthesis, decreased the lipolysis in muscle tissue, and increased the lipolysis in adipose tissue. This suggests that systemic butyrate may display discriminative metabolic regulation in different tissues of barrows.

Role of Medium-Chain Fatty Acids in Healthy Metabolism: A Clinical Perspective

Medium-chain fatty acids (MCFAs) serve not only as an energy source but also regulate glucose and lipid metabolism. The unique transport and rapid metabolism of MCFAs provide additional clinical benefits over other substrates such as long-chain fatty acids (LCFAs) and have prompted interest in the use of MCFAs for treating metabolic and neurological disorders. This review focuses on the metabolic role of MCFAs in modulating cellular signaling and regulating key circulating metabolites and hormones. The potential of MCFAs in treating various metabolic diseases in a clinical setting has also been analyzed.

Short- and medium-chain fatty acids in energy metabolism: the cellular perspective

Short- and medium-chain fatty acids (SCFAs and MCFAs), independently of their cellular signaling functions, are important substrates of the energy metabolism and anabolic processes in mammals. SCFAs are mostly generated by colonic bacteria and are predominantly metabolized by enterocytes and liver, whereas MCFAs arise mostly from dietary triglycerides, among them milk and dairy products. A common feature of SCFAs and MCFAs is their carnitine-independent uptake and intramitochondrial activation to acyl-CoA thioesters. Contrary to long-chain fatty acids, the cellular metabolism of SCFAs and MCFAs depends to a lesser extent on fatty acid-binding proteins. SCFAs and MCFAs modulate tissue metabolism of carbohydrates and lipids, as manifested by a mostly inhibitory effect on glycolysis and stimulation of lipogenesis or gluconeogenesis. SCFAs and MCFAs exert no or only weak protonophoric and lytic activities in mitochondria and do not significantly impair the electron transport in the respiratory chain. SCFAs and MCFAs modulate mitochondrial energy production by two mechanisms: they provide reducing equivalents to the respiratory chain and partly decrease efficacy of oxidative ATP synthesis.

High expression of thyroid hormone receptors and mitochondrial glycerol-3-phosphate dehydrogenase in the liver is linked to enhanced fatty acid oxidation in Lou/C, a rat strain resistant to obesity

Besides its well recognized role in lipid and carbohydrate metabolisms, glycerol is involved in the regulation of cellular energy homeostasis via glycerol-3-phosphate, a key metabolite in the translocation of reducing power across the mitochondrial inner membrane with mitochondrial glycerol-3-phosphate dehydrogenase. Here, we report a high rate of gluconeogenesis from glycerol and fatty acid oxidation in hepatocytes from Lou/C, a peculiar rat strain derived from Wistar, which is resistant to age- and diet-related obesity. This feature, associated with elevated cellular respiration and cytosolic ATP/ADP and NAD(+)/NADH ratios, was linked to a high expression and activity of mitochondrial glycerol-3-phosphate dehydrogenase. Interestingly, this strain exhibited high expression and protein content of thyroid hormone receptor, whereas circulating thyroid hormone levels were slightly decreased and hepatic thyroid hormone carrier MCT-8 mRNA levels were not modified. We propose that an enhanced liver thyroid hormone receptor in Lou/C may explain its unique resistance to obesity by increasing fatty acid oxidation and lowering liver oxidative phosphorylation stoichiometry at the translocation of reducing power into mitochondria.

Effects of insulin and cytosolic redox state on glucose production pathways in the isolated perfused mouse liver measured by integrated 2H and 13C NMR

A great deal is known about hepatic glucose production and its response to a variety of factors such as redox state, substrate supply and hormonal control, but the effects of these parameters on the flux through biochemical pathways which integrate to control glucose production are less clear. A combination of 13C and [2H]water tracers and NMR isotopomer analysis were used to investigate metabolic fluxes in response to altered cytosolic redox state and insulin. In livers isolated from fed mice and perfused with a mixture of substrates including lactate/pyruvate (10:1, w/w), hepatic glucose production had substantial contributions from glycogen, PEP (phosphoenolpyruvate) and glycerol. Inversion of the lactate/pyruvate ratio (1:10, w/w) resulted in a surprising decrease in the contribution from glycogen and an increase in that from PEP to glucose production. A change in the lactate/pyruvate ratio from 10:1 to 1:10 also stimulated flux through the tricarboxylic acid cycle (2-fold), while leaving oxygen consumption and overall glucose output unchanged. When lactate and pyruvate were eliminated from the perfusion medium, both gluconeogenesis and tricarboxylic-acid-cycle flux were dramatically lower. Insulin lowered glucose production by inhibiting glycogenolysis at both low and high doses, but only at high levels of insulin did gluconeogenesis or tricarboxylic-acid-cycle flux tend towards lower values (P<0.1). Our data demonstrate that, in the isolated mouse liver, substrate availability and cellular redox state have a dramatic impact on liver metabolism in both the tricarboxylic acid cycle and gluconeogenesis. The tight correlation of these two pathways under multiple conditions suggest that interventions which increase or decrease hepatic tricarboxylic-acid-cycle flux will have a concomitant effect on gluconeogenesis and vice versa.

Short-term and long-term effects of fatty acids in rat hepatoma AS-30D cells: The way to apoptosis

Arachidonic acid and, to a smaller extent, oleic acid at micromolar concentrations decreased the mitochondrial membrane potential within AS-30D rat hepatoma cells cultivated in vitro and increased cell respiration. The uncoupling effect of both fatty acids on cell respiration was partly prevented by cyclosporin A, blocker of the mitochondrial permeability transition pore. Arachidonic acid increased the rate of reactive oxygen species (ROS) production, while oleic acid decreased it. Both fatty acids induced apoptotic cell death of AS-30D cells, accompanied by the release of cytochrome c from mitochondria to the cytosol, activation of caspase-3 and association of proapoptotic Bax protein with mitochondria; arachidonic acid being a more potent inducer than oleic acid. Trolox, a potent antioxidant, prevented ROS increase induced by arachidonic acid and protected the cells against apoptosis produced by this fatty acid. It is concluded that arachidonic and oleic acids induce apoptosis of AS-30D hepatoma cells by the mitochondrial pathway but differ in the mechanism of their action: Arachidonic acid induces apoptosis mainly by stimulating ROS production, whereas oleic acid may contribute to programmed cell death by activation of the mitochondrial permeability transition pore.

Butyrate impairs energy metabolism in isolated perfused liver of fed rats

This study was designed to test the effects of short-chain fatty acids (SCFA) with an even number of carbon atoms on hepatic energy metabolism. The effect of the SCFA was evaluated by measuring liver ATP content and oxygen consumption. The ATP content was evaluated using (31)P nuclear magnetic resonance in isolated liver from fed rats. In addition, respiratory activity (VO(2)) was assessed using Clark electrodes. The livers were perfused with acetate, butyrate or a medium chain length fatty acid, octanoate, at a concentration of 0.05--5.0 mmol/L. The addition of each substrate enhanced the rate of the net ATP consumption (V(i)), establishing a new ATP steady state that required a perfusion time of > or = 20 min, dependent on the chain length and concentration of the fatty acid (FA). The initial V(i) was unchanged for acetate and the ATP level stabilized at 58% of the initial level. Both butyrate and octanoate induced a dose-dependent increase in V(i). This may reflect an ATP-consuming process for the intracellular pH regulation observed during the acidosis associated with the beta-oxidation pathway. At the new steady state, the ATP concentration was approximately 45% of the initial level for both FA. VO(2) was both rapidly and reversibly increased, and the change was a function of butyrate or octanoate concentration and of the chain length. K(m) values were similar for butyrate and octanoate. Because all of the effects were similar for butyrate and octanoate, in contrast to acetate, we suggest that the impairment of the energy metabolism by butyrate resulted from an increase in the FADH(2)/NADH ratio due to beta-oxidation. In conclusion, the difference in the hepatic oxidation pathways of two products of intestinal fermentation (acetate and butyrate) explains their different actions on energy metabolism.

Liposomes as fatty acids carriers in isolated rat liver: effect on energy metabolism and on isolated mitochondria activity

The effects of fatty acids (FA)-carrier, egg-lecithin liposomes (LIPO) as alternative to BSA, on ATP, glycogen and glucose contents in isolated perfused liver of fed rats were non-invasively studied using 31P/13C nuclear magnetic resonance (NMR). Oxidative phosphorylation was studied in isolated mitochondria from the same liver consecutively to the NMR experiments. ATP content decreased slowly and ATP turnover was similar during the perfusion with saline solution (KHB) or LIPO. However, LIPO induced an enhancement of respiratory control ratio in isolated mitochondria. Tissue glycogen and glucose content decreased when FA (linoleate or linolenate) were perfused with defatted BSA (3%) or LIPO (600 mg/l) whereas glucose excretion level was unchanged and lactate excretion tended to increase, reflecting changes in the cytosolic redox state and/or an enhancement of glycolysis. Addition of FA (0.5 or 1.5 mM) to LIPO caused a dramatic fall in liver ATP, a mitochondrial uncoupling and an impairment of the phosphorylation activity. Perfusion with FA (1.5 mM) carried by BSA significantly increased the ATP degradation without change of mitochondrial function. Owing to the higher affinity of BSA than LIPO for FA, these latter could be more easily released from complex LIPO-FA, increasing their uncoupling effect. Hence, the FA concentrations have to be largely decreased from the above currently used concentrations to avoid this effect. It will then be possible to minimize the effector action of FA and to study their more specific metabolic function as fuel. It was concluded that LIPO were appropriate carriers to study the different metabolic effects of FA.

Pyruvate and hydroxycitrate/carnitine may synergize to promote reverse electron transport in hepatocyte mitochondria, effectively 'uncoupling' the oxidation of fatty acids

In a recent pilot study, joint administration of pyruvate, hydroxycitrate (HCA), and carnitine to obese subjects was associated with a remarkable rate of body-fat loss and thermogenesis, strongly suggestive of uncoupled fatty-acid oxidation. Hepatocytes possess an uncoupling mechanism--reverse electron transport--that enables fasting ketogenesis to proceed independent of respiratory control. Electrons entering the respiratory chain at the coenzyme Q (CoQ) level via FAD-dependent acyl coA dehydrogenase, can be driven 'up' the chain by the electrochemical proton gradient to reduce NAD+; if these electrons are then shuttled to the cytoplasm, returning to the respiratory chain at the CoQ level, the net result is heat generation at the expense of the proton gradient, enabling the uncoupled flow of electrons to oxygen. Pyruvate's bariatric utility may stem from its ability to catalyze the rapid transport of high-energy electrons from mitochondria to the cytoplasm, thus stimulating electron shuttle mechanisms. By enabling rapid mitochondrial uptake of fatty acids and thus disinhibiting hepatocyte ketogenesis, HCA/carnitine should initiate reverse electron transport: concurrent amplification of electron shuttle mechanisms by pyruvate can be expected to accelerate this reverse electron transport, thereby decreasing the electrochemical proton gradient. As a result, hepatocytes may be able to convert fatty acids to CO2 and heat with little net generation of ATP. These considerations suggest that it may be feasible to render hepatocytes functionally equivalent to activated brown fat, such that stored fat can be selectively oxidized in the absence of caloric restriction. Other measures which enhance the efficiency of hepatocyte electron shuttle mechanisms may increase the efficacy of this strategy.

Mitochondria uncoupling by a long chain fatty acyl analogue

Mitochondria uncoupling by fatty acids in vivo is still questionable, being confounded by their dual role as substrates for oxidation and as putative genuine uncouplers of oxidative phosphorylation. To dissociate between substrate and the uncoupling activity of fatty acids in oxidative phosphorylation, the uncoupling effect was studied here using a nonmetabolizable long chain fatty acyl analogue. beta,beta'-Methyl-substituted hexadecane alpha,omega-dioic acid (MEDICA 16) is reported here to induce in freshly isolated liver cells a saturable oligomycin-insensitive decrease in mitochondrial proton motive force with a concomitant increase in cellular respiration. Similarly, MEDICA 16 induced a saturable decrease in membrane potential, proton gradient, and proton motive force in isolated liver and heart mitochondria accompanied by an increase in mitochondrial respiration. Uncoupling by MEDICA 16 in isolated mitochondria was partially suppressed by added atractyloside. Hence, fatty acids may act as genuine uncouplers of cellular oxidative phosphorylation by interacting with specific mitochondrial proteins, including the adenine nucleotide translocase.

On isolated hepatocytes mitochondrial swelling induced in hypoosmotic medium does not affect the respiration rate

In isolated hepatocytes incubated in hypoosmotic media, a large increase in the mitochondrial volume is not directly involved in the activation of respiration. Moreover, results of the quantification of the various bioenergetic parameters are not in accordance with an activation of the respiratory chain as previously proposed (Halestrap, A.P. (1989) Biochim. Biophys. Acta, 973, 355-382), but point more to an inhibition of respiration. The same respiration rate is obtained in hypoosmolar incubation media in vitro and in situ for a higher overall thermodynamic driving force over the electron transport chain.

Effect of fatty acids on energy coupling processes in mitochondria

Long-chain fatty acids are natural uncouplers of oxidative phosphorylation in mitochondria. The protonophoric mechanism of this action is due to transbilayer movement of undissociated fatty acid in one direction and the passage of its anion in the opposite direction. The transfer of the dissociated form of fatty acid can be, at least in some kinds of mitochondrion, facilitated by adenine nucleotide translocase. Apart from dissipating the electrochemical proton gradient, long-chain fatty acids decrease the activity of the respiratory chain by mechanism(s) not fully understood. In intact cells and tissues fatty acids operate mostly as excellent respiratory substrates, providing electrons to the respiratory chain. This function masks their potential uncoupling effect which becomes apparent only under special physiological or pathological conditions characterized by unusual fatty acid accumulation. Short- and medium-chain fatty acids do not have protonophoric properties. Nevertheless, they contribute to energy dissipation because of slow intramitochondrial hydrolysis of their activation products, acyl-AMP and acyl-CoA. Long-chain fatty acids increase permeability of mitochondrial membranes to alkali metal cations. This is due to their ionophoric mechanism of action. Regulatory function of fatty acids with respect to specific cation channels has been postulated for the plasma membrane of muscle cells, but not demonstrated in mitochondria. Under cold stress, cold acclimation and arousal from hibernation the uncoupling effect of fatty acids may contribute to increased thermogenesis, especially in the muscle tissue. In brown adipose tissue, the special thermogenic organ of mammals, long-chain fatty acids promote operation of the unique natural uncoupling protein, thermogenin. As anionic amphiphiles, long-chain fatty acids increase the negative surface charge of biomembranes, thus interfering in their enzymic and transporting functions.

Effects of fatty acid oxidation on glucose utilization by isolated hepatocytes

We have studied the inhibitory action of long- and short-chain fatty acids on hepatic glucose utilization in hepatocytes isolated from fasted rats. The rates of hepatic glucose phosphorylation and glycolysis were determined from the tritiated products of [2-3H] and [6-3H]glucose metabolism, respectively. The difference between these was taken as an estimate of the 'cycling' between glucose and glucose-6-phosphate. In the presence of 40 mM glucose this cycling was estimated at 0.68 mumol/min/g wet wt. Glucose phosphorylation was unaffected during palmitate and hexanoate oxidation to ketone bodies but glycolysis was inhibited. The rate of glucose cycling was increased during this phase to 1.25 mumol/min/g. Following the complete metabolism of the fatty acids, glycolysis was reinstated and cycling rates returned to control levels. Hepatic glucose cycling appears to be an important component of the glucose/fatty acid cycle.

Effect of palmitate concentration on the relative contributions of the beta-oxidation pathway and citric acid cycle to total O2 consumption of isolated rat hepatocytes

The relative contributions of beta-oxidation and citric acid cycle activity to total O2 consumption during fatty acid oxidation were examined in isolated hepatocytes. When hepatocytes were incubated with palmitate alone, a rise in fatty acid concentration induced an increase in O2 uptake that reflected a large stimulation of beta-oxidation and an accompanying smaller inhibition of citric acid cycle oxidation. In the presence of lactate, successive increments in palmitate concentration over the range from 0 to 1.0 mM stimulated glucose synthesis and brought about a concomitant incremental stimulation of both beta-oxidation and citric acid cycle flux. However, above 1.5 mM palmitate, additional increments in fatty acid concentration depressed gluconeogenesis and citric acid cycle activity but induced a further stimulation of beta-oxidation. These findings demonstrate that, during fatty acid oxidation, the rate of citric acid cycle turnover is more closely linked to the rate of glucose synthesis than is the rate of beta-oxidation. This may be relevant to observations that the stimulation of hepatic O2 consumption, induced by fatty acid oxidation, is much greater than can be explained in terms of the ATP-demand arising from exposure of hepatocytes to fatty acid.

The contribution of the leak of protons across the mitochondrial inner membrane to standard metabolic rate

This paper presents and assesses the hypothesis that the proton leak across the mitochondrial inner membrane is an important contributor to standard metabolic rate, and that increases in the amount of mitochondrial inner membrane may be important in causing changes in proton leak and in the standard metabolic rate. The standard metabolic rate of an animal is known to be a function of body mass, phylogeny and thyroid status, and is largely attributed to the metabolically active internal organs. The total area of mitochondrial inner membrane in these organs correlates well with standard metabolic rate over a wide range of body masses in both ectotherms and endotherms. In hepatocytes isolated from rats, proton leak across the mitochondrial inner membrane accounts for about 30% of the resting oxygen consumption, and the distribution of control over respiration suggests that changes in mitochondrial inner membrane surface area will be accompanied by significant changes in the proton leak. This change in the leak will result in significant changes in resting oxygen consumption, but changes in ATP demand may also have a role to play in determining resting respiration rate. Extrapolation of these results to other tissues and other animals suggests that the hypothesis has the potential to explain a substantial proportion of the variation in standard metabolic rate with body mass, phylogeny and thyroid status. However, in most cases the quantitative contribution of proton leak compared to cellular ATP turnover has yet to be experimentally determined.

Role of peroxisomal fatty acid beta-oxidation in ethanol metabolism

The contribution of peroxisomal fatty acid beta-oxidation to ethanol metabolism was examined in deermice hepatocytes. Addition of 1 mM oleate to hepatocytes isolated from fasted alcohol dehydrogenase (ADH)-positive deermice in the presence of 4-methylpyrazole or to hepatocytes from fasted or fed ADH-negative deermice produced only a slight and statistically not significant increase in ethanol oxidation. Lactate (10 mM), which is not a peroxisomal substrate, showed a greater effect on ethanol oxidation. There was also a lack of oleate effect on the oxidation of ethanol by hepatocytes of ADH-positive deermice. Furthermore, in ADH-negative deermice, the catalase inhibitor azide (0.1 mM) did not inhibit the increase in ethanol oxidation by oleate and lactate. The rate of oleate oxidation by hepatocytes from fasted ADH-negative deermice was much lower than that of ethanol. These results indicate that in deermice hepatocytes, peroxisomal fatty acid oxidation does not play major role in ethanol metabolism.

On the mechanism of the so-called uncoupling effect of medium- and short-chain fatty acids

Octanoate applied to rat liver mitochondria respiring with glutamate plus malate or succinate (plus rotenone) under resting-state (State 4) conditions stimulates oxygen uptake and decreases the membrane potential, both effects being sensitive to oligomycin but not to carboxyatractyloside. Octanoate also decreases the rate of pyruvate carboxylation under the same conditions, this effect being correlated with the decrease of intramitochondrial content of ATP and increase of AMP. The decrease of pyruvate carboxylation and the change of mitochondrial adenine nucleotides are both reversed by 2-oxoglutarate. Fatty acids of shorter chain length have similar effects, though at higher concentrations. Addition of octanoate in the presence of fluoride (inhibitor of pyrophosphatase) produces intramitochondrial accumulation of pyrophosphate, even under conditions when oxidation of octanoate is prevented by rotenone. In isolated hepatocytes incubated with lactate plus pyruvate, octanoate also increases oxygen uptake and produces a shift in the profile of adenine nucleotides similar to that observed in isolated mitochondria. It decreases the 'efficiency' of gluconeogenesis, as expressed by the ratio between an increase of glucose production and an increase of oxygen uptake upon addition of gluconeogenic substrates (lactate plus pyruvate), and increases the reduction state of mitochondrial NAD. These effects taken together are not compatible with uncoupling, but point to intramitochondrial hydrolysis of octanoyl-CoA and probably also shorter chain-length acyl-CoAs. This mechanism probably functions as a 'safety valve' preventing a drastic decrease of intramitochondrial free CoA under a large supply of medium- and short-chain fatty acids.

Quantitation of ketogenesis in periportal and pericentral regions of the liver lobule

A method has been devised to quantitate rates of ketogenesis (acetoacetate + beta-hydroxybutyrate production) in discrete regions of the liver lobule based on changes in NADH fluorescence. In perfused livers from fasted rats, ketogenesis was inhibited nearly completely with either 2-bromoctanoate (600 microM) or 2-tetradecylglycidic acid (25 microM). During inhibition of ketogenesis, a linear relationship (r = 0.90) was observed between decreases in NADH fluorescence detected from the liver surface and decreases in ketone body production. NADH fluorescence was monitored subsequently from individual regions of the liver lobule by placing microlight guides on periportal and pericentral regions of the liver lobule visible on the liver surface. Rates of ketogenesis in sublobular regions were calculated from regional decreases in NADH fluorescence and changes in the rate of ketone body formation by the whole liver during infusion of inhibitors. In the presence of bromoctanoate, ketogenesis was reduced 80% and local rates of ketogenesis were decreased 31 +/- 4 mumol/g/h in periportal areas and 28 +/- 3 mumol/g/h in pericentral regions. Similar results were observed with tetradecylglycidic acid. Therefore, it was concluded that submaximal rates of ketogenesis from endogenous, mainly long-chain fatty acids are nearly equal in periportal and pericentral regions of the liver lobule in liver from fasted rats. Rates of ketogenesis and NADH fluorescence were strongly correlated during fatty acid infusion. Infusion of 250 microM oleate increased NADH fluorescence maximally by 8 +/- 1% over basal values in periportal regions and 17 +/- 4% in pericentral areas. Local rates of ketogenesis, calculated from these changes in fluorescence, increased 35 +/- 6 mumol/g/h in periportal areas and 55 +/- 5 mumol/g/h in pericentral regions. Thus, oleate stimulated ketogenesis nearly 60% more in pericentral than in periportal regions of the liver lobule.

Quantification of fatty acid activation of the uncoupling protein in brown adipocytes and mitochondria from the guinea-pig

Brown adipocytes from cold-adapted guinea-pigs (C-cells) are more sensitive to uncoupling by exogenous palmitate than are cells from warm-adapted animals (W-cells) with much less uncoupling protein. Half-maximal respiratory stimulation of C-cells requires 80 nM free palmitate. Noradrenaline-stimulated lipolysis is not rate-limiting for the respiration of either C-cells or W-cells. Half-maximal stimulation of fatty acid oxidation by mitochondria from warm-adapted guinea-pigs (W-mitochondria) and cold-adapted guinea-pigs (C-mitochondria) both require 12 nM free palmitate. Palmitate uncouples C-mitochondria much more readily than M-mitochondria, paralleling its action on the adipocytes. The uncoupling is partially saturable, about 100 nM free palmitate being required for half-maximal response of C-mitochondria. W- and C-mitochondria show identical binding characteristics for palmitate. The respiratory increase of mitochondria is calculated as a function of bound palmitate. After correcting for the residual uncoupling protein present in W-mitochondria, palmitate is estimated to be almost ineffective as an uncoupler of brown fat mitochondria in the absence of the protein. It is concluded that fatty acids display characteristics required of a necessary and sufficient physiological activator of the uncoupling protein.

Regulation of respiration and ATP synthesis in higher organisms: hypothesis

The present view on the regulation of respiration and ATP synthesis in higher organisms implies only Michaelis-Menten type kinetics and respiratory control as regulatory principles. Recent experimental observations, suggesting further regulatory mechanisms at respiratory chain complexes, are reviewed. A new hypothesis is presented implying regulation of respiration and ATP synthesis in higher organisms mainly via allosteric modification of respiratory chain complexes, in particular of cytochrome c oxidase. The allosteric effectors, e.g., metabolites, cofactors, ions, hormones, and the membrane potential are suggested to change the activity and the coupling degree of cytochrome c oxidase by binding to specific sites at nuclear coded subunits. Recent results on the structure and activity of cytochrome c oxidase, supporting the hypothesis, are reviewed.

Mechanism of the stimulation of respiration by fatty acids in rat liver

The mechanism of stimulation of hepatic respiration by fatty acids was studied in isolated rat hepatocytes. Stimulation of respiration by fatty acids varied from about 35% to about 105% depending on chain length. The stimulatory effect of octanoate (1 mM) or oleate (0.5 mM) was prevented by oligomycin (2 micrograms/ml). With carboxyatractyloside (100 microM) and ouabain (2 mM) the stimulation of respiration was partially inhibited (by 50-70 and 50-60%, respectively). From these results it can be concluded that the increased rate of respiration after addition of fatty acids is coupled to ATP synthesis. A large part (50-60%) of this ATP is utilized by the (Na+ + K+)-ATPase.

Inhibition of p-nitroanisole O-demethylation in perfused rat liver by oleate

p-Nitroanisole O-demethylation in perfused livers from fasted, phenobarbital-treated rats was rapidly and reversibly inhibited by sodium oleate (0.3 to 0.6 mM). Xylitol partially reversed this inhibitory effect. The inhibition was not mediated by a direct effect of oleate on microsomal components since concentrations of oleate ranging up to 1.0 mM did not affect p-nitroanisole O-demethylation by isolated microsomes. Infusion of 0.6 mM oleate did not alter the measured intracellular NAD+/NADH ratio but did cause a significant increase in the intracellular NADP+/NADPH ratio. A significant decrease in the ATP/ADP ratio was also observed. Oleoyl CoA inhibited p-nitroanisole O-demethylation in microsomes (Ki about 30 microM), and both oleoyl CoA and palmitoyl CoA inhibited the energy-linked nicotinamide nucleotide transhydrogenase in submitochondrial particles (Ki about 1 microM). Thus, inhibition of mixed-function oxidation in the intact liver by oleate is most likely mediated by oleoyl CoA. Oleoyl CoA inhibits mixed-function oxidation in the intact liver by acting directly on cytochrome P-450 and by decreasing generation of NADPH via inhibition of key enzymes of the citric acid cycle and the energy-linked transhydrogenase.

Influence of fatty acids on energy metabolism. 1. Stimulation of oxygen consumption, ketogenesis and CO2 production following addition of octanoate and oleate in perfused rat liver

Changes in metabolic rates (oxygen consumption, ketogenesis, 14CO2 production from labelled fatty acids, glycolysis) following the addition of octanoate or oleate were studied in isolated livers from fed and starved rats perfused with Krebs-Henseleit bicarbonate buffer in a non-recirculating system. The following results were obtained. The infusion of fatty acids caused a large increase in the rate of oxygen consumption. The effect was greater with octanoate than with oleate and was half-maximal with fatty acid concentrations (free plus albumin bound) around 0.1 mM. The effects of oleate were only partially suppressed when the perfusate contained albumin concentrations near the physiological range. When fatty acids were oxidized at high rates, the glycolytic rate was diminished by 50%. The increase in oxygen consumption could not be explained fully by the increased ATP demand for fatty acid metabolism or by a compensation for the diminished extramitochondrial ATP generation. In the presence of phenylalkyl oxirane carboxylic acid, an inhibitor of the transport of long-chain acyl-CoA derivates into the mitochondria, ketogenesis and 14CO2 production from labelled oleate were strongly inhibited, whereas the increase in oxygen consumption was only slightly affected. In the presence of antimycin A, the increase in oxygen consumption due to fatty acids was totally abolished. Following pretreatment of rats with ciprofibrate (induction of enzymes for peroxisomal beta-oxidation of long-chain fatty acids), ketogenesis (but not 14CO2 production) from oleate was enhanced threefold. The increase in oxygen consumption, however, was not affected. In conclusion, the increase in hepatic oxygen consumption due to addition of fatty acids reflects a mitochondrial process; it is, in part, independent of the ATP demand of the cell. An uncoupling-like effect of fatty acids on the respiratory chain and its possible physiological significance in ketogenesis are discussed.

Influence of fatty acids on energy metabolism. 2. Kinetics of changes in metabolic rates and changes in subcellular adenine nucleotide contents and pH gradients following addition of octanoate and oleate in perfused rat liver

The uncoupling-like effect of fatty acids [ Scholz , R., Schwabe , U., and Soboll , S. (1984) Eur. J. Biochem. 141, 223-230] was further substantiated in experiments with perfused rat livers by two ways: firstly the kinetics of changes in metabolic rates (oxygen consumption, ketogenesis, fatty acid oxidation) were analysed; secondly subcellular contents of adenine nucleotides and pH gradients across the mitochondrial membrane were determined following fractionation of freeze-fixed and dried tissues in non-aqueous solvents. The following results were obtained. The relaxation kinetics of the increase in oxygen consumption following fatty acid infusion revealed two components, a rapid one with a half-time around 10 s and a slow one with a half-time of more than 100 s. The rapid component was similar to the kinetics of fatty acid oxidation (ketogenesis and 14CO2 production from labelled fatty acids) whereas the half-time of the slow component was in the range of half-times observed with the increase in oxygen consumption following addition of carbonylcyanide p-trifluoromethoxyphenylhydrazone. In the presence of fatty acids, the cytosolic ATP concentrations and ATP/ADP ratios decreased, whereas the corresponding parameters for the mitochondrial space were either increased (oleate) or decreased (octanoate). The effects of oleate were dependent on the albumin concentrations in the perfusate. The normally large difference between cytosolic and mitochondrial ATP/ADP ratios became smaller. Similar observations were obtained with uncoupling agents. The pH gradient across the mitochondrial membrane as calculated from the subcellular distribution of 5,5 dimethyl[2-14C]oxazolidine-2,4-dione was inversed following the addition of both carbonylcyanide p-trifluoromethoxyphenylhydrazone and fatty acids, i.e. the mitochondrial matrix became more acidic than the cytosol. The pH gradient was not affected when oleate was added in the presence of high albumin concentrations. The data support the hypothesis that the increase in hepatic oxygen consumption due to octanoate or oleate is, in part, caused by a mechanism similar to uncoupling of oxidative phosphorylation. This mechanism seems not to be an artifact of isolated systems; it may be of physiological importance for processes in which reducing equivalents are removed independently of the ATP demand of the hepatocyte.

Evidence that peroxisomal acyl-CoA synthetase is located at the cytoplasmic side of the peroxisomal membrane

1. Subfractionation by isopycnic density-gradient centrifugation in self-generating Percoll gradients of peroxisome-rich fractions prepared by differential centrifugation confirmed the presence of acyl-CoA synthetase in peroxisomes. Peroxisomes did not contain nicotinamide or adenine nucleotides other than CoA. 2. The gradient fractions most enriched in peroxisomes were pooled and the peroxisomes sedimented by centrifugation, resulting in a 50-fold-purified peroxisomal preparation as revealed by marker enzyme analysis. 3. Palmitate oxidation by intact purified peroxisomes was CoA-dependent, whereas palmitoyl-CoA oxidation was not, demonstrating that the peroxisomal CoA was available for the thiolase reaction, located in the peroxisomal matrix, but not for acyl-CoA synthetase. This suggests that the latter enzyme is located at the cytoplasmic side of the peroxisomal membrane. 4. Additional evidence for this location of peroxisomal acyl-CoA synthetase was as follows. Mechanical disruption of purified peroxisomes resulted in the release of catalase from the broken organelles, but not of acyl-CoA synthetase, indicating that the enzyme was membrane-bound. Acyl-CoA synthetase was not latent, despite the fact that at least one of its substrates appears to have a limited membrane permeability, as evidenced by the presence of CoA in purified peroxisomes. Finally, Pronase, a proteinase that does not penetrate the peroxisomal membrane, almost completely inactivated the acyl-CoA synthetase of intact peroxisomes.

Hepatic fatty acid oxidation and ketogenesis after clofibrate treatment

The effect of clofibrate treatment on hepatic ketogenic capacity was studied in rats. Ketogenesis from octanoate and oleate was increased 2- and 4,5-fold, respectively, in hepatocytes from fed, treated rats. In contrast to controls ketogenic rates did not increase upon starvation. While ketogenesis from oleate was higher in fed, treated animals than in fasted controls, endogenous ketogenesis was lower and increased upon starvation. Ketogenesis from octanoate and oleate was stimulated approx. 2-fold in homogenates from treated animals. Labeled pyruvate and succinate oxidation was unaltered. [1-14C]Oleate oxidation was severely inhibited by cyanide, both in homogenates from controls and treated animals. Clofibrate caused a 3-fold increase in hepatic carnitine levels. Catalase and glutamate dehydrogenase activities were also increased by the drug. Cytochrome c oxidase did not change. Despite their increased ketogenic capacity hepatocytes from treated rats esterified as much oleate as controls. The increased oxidation was matched by an increased oleate uptake. Plasma ketones were increased 2-fold in fasted, treated animals. Plasma free fatty acids were unaffected. It is concluded that the enhanced ketogenic capacity induced by clofibrate is the result of an increase in mitochondrial beta-oxidation, an increase in the activity of carnitine palmitoyltransferase and possibly of the observed increases in hepatic carnitine content and fatty acid uptake.

Effect of porphyrinogenic agents on protein synthesis and bilirubin formation by the isolated perfused rat liver

We established an isolated rat liver perfusion system for the study of heme catabolism. The liver of rats fasted for 48 h is perfused with an erythrocyte-free medium. Ultrastructural analysis shows integrity of all subcellular organelles with the exception of minor alterations in the rough endoplasmic reticulum. The perfused liver synthesizes serum proteins at a constant rate for 5 h. Albumin is secreted at a mean rate of 17 +/- 2 mg/h per 100 g liver, hemopexin at 5.0 +/- 0.7, haptoglobin at 3.2 +/- 0.6 and transferrin at 5.1 +/- 0.8 mg/h per 100 g liver. The mean ratio of ATP : ADP is 3.5 +/- 0.1, and that of lactate: pyruvate 27 +/- 6. The rate of conversion of heme into bilirubin is comparable to that reported for in vivo studies. A minimal effect on protein synthesis is observed after administration of the porphyrinogenic agents, allylisopropylacetamide (AIA) and 3,5-diethoxycarbonyl-1,4 dihydrocollidine (DDC). Pretreatment of the rats with the iron chelator, Desferal, causes a 3-4-fold increase in hemopexin but not in albumin and transferrin synthesis. A striking 2-3-fold enhancement of bile bilirubin production follows treatment with DDC and Desferal, but not with AIA. The amount of bilirubin formed from heme added to the perfusate is reduced by AIA and DDC and enhanced by Desferal treatment. It is proposed that unavailability of iron in a certain hepatic tissue pool causes protoporphyrin IX accumulation which may serve as an alternate source for bilirubin production.