Generation of extramitochondrial reducing power in gluconeogenesis

Purinergic ligands induce extracellular acidification and increased ATP turnover in HepG2 cells

Nucleosides and nucleotides at μM concentrations stimulated a 300% increase in acid secretion in HepG2 cells, which was quantitatively accounted for as increased export of lactate generated by glycogenolysis. Agonist selectivity encompassed nucleosides and nucleotides for all 5 natural nucleobases and, along with antagonist profiles, was inconsistent with a role for purinergic receptors in mediating this activity. Agonist catabolism did not contribute significantly to either low selectivity or lactate production. Lactate production was driven by an increase in ATP turnover of as much as 56%. For some agonists, especially adenosine, ATP turnover decreased precipitously at mM concentrations, correlating with known adenosine-stimulated apoptosis. We propose that nucleoside/nucleotide agonists induce a futile energy cycle via a novel mechanism, which results in increased ATP turnover and initiates a continuum of events that for some agonists culminates in apoptosis.

NADH inhibition of SIRT1 links energy state to transcription during time-restricted feeding

In mammals, circadian rhythms are entrained to the light cycle and drive daily oscillations in levels of NAD+, a cosubstrate of the class III histone deacetylase sirtuin 1 (SIRT1) that associates with clock transcription factors. Although NAD+ also participates in redox reactions, the extent to which NAD(H) couples nutrient state with circadian transcriptional cycles remains unknown. Here we show that nocturnal animals subjected to time-restricted feeding of a calorie-restricted diet (TRF-CR) only during night-time display reduced body temperature and elevated hepatic NADH during daytime. Genetic uncoupling of nutrient state from NADH redox state through transduction of the water-forming NADH oxidase from Lactobacillus brevis (LbNOX) increases daytime body temperature and blood and liver acyl-carnitines. LbNOX expression in TRF-CR mice induces oxidative gene networks controlled by brain and muscle Arnt-like protein 1 (BMAL1) and peroxisome proliferator-activated receptor alpha (PPARα) and suppresses amino acid catabolic pathways. Enzymatic analyses reveal that NADH inhibits SIRT1 in vitro, corresponding with reduced deacetylation of SIRT1 substrates during TRF-CR in vivo. Remarkably, Sirt1 liver nullizygous animals subjected to TRF-CR display persistent hypothermia even when NADH is oxidized by LbNOX. Our findings reveal that the hepatic NADH cycle links nutrient state to whole-body energetics through the rhythmic regulation of SIRT1.

Non-invasive detection of divergent metabolic signals in insulin deficiency vs. insulin resistance in vivo

The type 2 diabetic phenotype results from mixed effects of insulin deficiency and insulin resistance, but the relative contributions of these two distinct factors remain poorly characterized, as do the respective roles of the gluconeogenic organs. The purpose of this study was to investigate localized in vivo metabolic changes in liver and kidneys of contrasting models of diabetes mellitus (DM): streptozotocin (STZ)-treated wild-type Zucker rats (T1DM) and Zucker diabetic fatty (ZDF) rats (T2DM). Intermediary metabolism was probed using hyperpolarized (HP) [1-13C]pyruvate MRI of the liver and kidneys. These data were correlated with gene expression data for key mediators, assessed using rtPCR. Increased HP [1-13C]lactate was detected in both models, in association with elevated gluconeogenesis as reflected by increased expression of phosphoenolpyruvate carboxykinase. In contrast, HP [1-13C]alanine diverged between the two models, increasing in ZDF rats, while decreasing in the STZ-treated rats. The differences in liver alanine paralleled differences in key lipogenic mediators. Thus, HP [1-13C]alanine is a marker that can identify phenotypic differences in kidneys and liver of rats with T1DM vs. T2DM, non-invasively in vivo. This approach could provide a powerful diagnostic tool for characterizing tissue metabolic defects and responses to treatment in diabetic patients with ambiguous systemic manifestations.

Oral aversion to dietary sugar, ethanol and glycerol correlates with alterations in specific hepatic metabolites in a mouse model of human citrin deficiency

Mice carrying simultaneous homozygous mutations in the genes encoding citrin, the mitochondrial aspartate-glutamate carrier 2 (AGC2) protein, and mitochondrial glycerol-3-phosphate dehydrogenase (mGPD), are a phenotypically representative model of human citrin (a.k.a., AGC2) deficiency. In this study, we investigated the voluntary oral intake and preference for sucrose, glycerol or ethanol solutions by wild-type, citrin (Ctrn)-knockout (KO), mGPD-KO, and Ctrn/mGPD double-KO mice; all substances that are known or suspected precipitating factors in the pathogenesis of human citrin deficiency. The double-KO mice showed clear suppressed intake of sucrose, consuming less with progressively higher concentrations compared to the other mice. Similar observations were made when glycerol or ethanol were given. The preference of Ctrn-KO and mGPD-KO mice varied with the different treatments; essentially no differences were observed for sucrose, while an intermediate intake or similar to that of the double-KO mice was observed for glycerol and ethanol. We next examined the hepatic glycerol 3-phosphate, citrate, citrulline, lysine, glutamate and adenine nucleotide levels following forced enteral administration of these solutions. A strong correlation between the simultaneous increased hepatic glycerol 3-phosphate and decreased ATP or total adenine nucleotide content and observed aversion of the mice during evaluation of their voluntary preferences was found. Overall, our results suggest that the aversion observed in the double-KO mice to these solutions is initiated and/or mediated by hepatic metabolic perturbations, resulting in a behavioral response to increased hepatic cytosolic NADH and a decreased cellular adenine nucleotide pool. These findings may underlie the dietary predilections observed in human citrin deficient patients.

Quantitative proteomics suggests metabolic reprogramming during ETHE1 deficiency

Deficiency of mitochondrial sulfur dioxygenase (ETHE1) causes the severe metabolic disorder ethylmalonic encephalopathy, which is characterized by early-onset encephalopathy and defective cytochrome C oxidase because of hydrogen sulfide accumulation. Although the severe systemic consequences of the disorder are becoming clear, the molecular effects are not well defined. Therefore, for further elucidating the effects of ETHE1-deficiency, we performed a large scale quantitative proteomics study on liver tissue from ETHE1-deficient mice. Our results demonstrated a clear link between ETHE1-deficiency and redox active proteins, as reflected by downregulation of several proteins related to oxidation-reduction, such as different dehydrogenases and cytochrome P450 (CYP450) members. Furthermore, the protein data indicated impact of the ETHE1-deficiency on metabolic reprogramming through upregulation of glycolytic enzymes and by altering several heterogeneous ribonucleoproteins, indicating novel link between ETHE1 and gene expression regulation. We also found increase in total protein acetylation level, pointing out the link between ETHE1 and acetylation, which is likely controlled by both redox state and cellular metabolites. These findings are relevant for understanding the complexity of the disease and may shed light on important functions influenced by ETHE1 deficiency and by the concomitant increase in the gaseous mediator hydrogen sulfide. All MS data have been deposited in the ProteomeXchange with the dataset identifiers PXD002741 (http://proteomecentral.proteomexchange.org/dataset/PXD002741) and PXD002742 (http://proteomecentral.proteomexchange.org/dataset/PXD002741).

Effects of supplementation on food intake, body weight and hepatic metabolites in the citrin/mitochondrial glycerol-3-phosphate dehydrogenase double-knockout mouse model of human citrin deficiency

The C57BL/6:Slc23a13(-/-);Gpd2(-/-) double-knockout (a.k.a., citrin/mitochondrial glycerol 3-phosphate dehydrogenase double knockout or Ctrn/mGPD-KO) mouse displays phenotypic attributes of both neonatal intrahepatic cholestasis (NICCD) and adult-onset type II citrullinemia (CTLN2), making it a suitable model of human citrin deficiency. In the present study, we show that when mature Ctrn/mGPD-KO mice are switched from a standard chow diet (CE-2) to a purified maintenance diet (AIN-93M), this resulted in a significant loss of body weight as a result of reduced food intake compared to littermate mGPD-KO mice. However, supplementation of the purified maintenance diet with additional protein (from 14% to 22%; and concomitant reduction or corn starch), or with specific supplementation with alanine, sodium glutamate, sodium pyruvate or medium-chain triglycerides (MCT), led to increased food intake and body weight gain near or back to that on chow diet. No such effect was observed when supplementing the diet with other sources of fat that contain long-chain fatty acids. Furthermore, when these supplements were added to a sucrose solution administered enterally to the mice, which has been shown previously to lead to elevated blood ammonia as well as altered hepatic metabolite levels in Ctrn/mGPP-KO mice, this led to metabolic correction. The elevated hepatic glycerol 3-phosphate and citrulline levels after sucrose administration were suppressed by the administration of sodium pyruvate, alanine, sodium glutamate and MCT, although the effect of MCT was relatively small. Low hepatic citrate and increased lysine levels were only found to be corrected by sodium pyruvate, while alanine and sodium glutamate both corrected hepatic glutamate and aspartate levels. Overall, these results suggest that dietary factors including increased protein content, supplementation of specific amino acids like alanine and sodium glutamate, as well as sodium pyruvate and MCT all show beneficial effects on citrin deficiency by increasing the carbohydrate tolerance of Ctrn/mGPD-KO mice, as observed through increased food intake and maintenance of body weight.

Metabolomic analysis reveals hepatic metabolite perturbations in citrin/mitochondrial glycerol-3-phosphate dehydrogenase double-knockout mice, a model of human citrin deficiency

The citrin/mitochondrial glycerol-3-phosphate dehydrogenase (mGPD) double-knockout mouse displays phenotypic attributes of both neonatal intrahepatic cholestasis and adult-onset type II citrullinemia, making it a suitable model of human citrin deficiency. In the present study, we investigated metabolic disturbances in the livers of wild-type, citrin (Ctrn) knockout, mGPD knockout, and Ctrn/mGPD double-knockout mice following oral sucrose versus saline administration using metabolomic approaches. By using gas chromatography/mass spectrometry and capillary electrophoresis/mass spectrometry, we found three general groupings of metabolite changes in the livers of the double-knockout mice following sucrose administration that were subsequently confirmed using liquid chromatography/mass spectrometry or enzymatic methods: a marked increase of hepatic glycerol 3-phosphate, a generalized decrease of hepatic tricarboxylic acid cycle intermediates, and alterations of hepatic amino acid levels related to the urea cycle or lysine catabolism including marked increases in citrulline and lysine. Furthermore, concurrent oral administration of sodium pyruvate with sucrose ameliorated the hyperammonemia induced by sucrose, as had been shown previously, as well as almost completely normalizing the hepatic metabolite perturbations found. Overall, we have identified additional metabolic disturbances in double-KO mice following oral sucrose administration, and provided further evidence for the therapeutic use of sodium pyruvate in our mouse model of citrin deficiency.

Biochemical aspects of bovine ketosis

1. The concentrations of acetoacetate, beta-hydroxybutyrate and metabolites related to gluconeogenesis were determined in biopsy samples of the livers of ketotic, normal lactating and normal non-lactating cows. Key enzymes of gluconeogenesis in the liver were also assayed. 2. Significant decreases were found in the ketotic liver in the concentrations of glucogenic amino acids (glutamate, glutamine, alanine) and of glucogenic oxo acids (alpha-oxoglutarate, pyruvate, oxaloacetate). 3. The beta-hydroxybutyrate/acetoacetate concentration ratios were generally much higher than in rat liver. 4. The concentration of total fat was sevenfold higher in the ketotic liver, and that of glucose plus glycogen fourfold lower than in normal liver. 5. The blood of ketotic cows showed a marked rise in the concentration of free fatty acids. 6. The activities of pyruvate carboxylase, propionyl-CoA carboxylase, phosphopyruvate carboxylase and fructose 1,6-diphosphatase showed no clear-cut differences between normal and ketotic animals. 7. Glucose injection promptly relieved the ketotic condition with respect to both the clinical and biochemical signs. The fall in the concentrations of the ketone bodies in the blood was preceded by a fall in the concentrations of free fatty acids and glycerol. 8. The findings are taken to be consistent with the concept that an increased rate of gluconeogenesis, causing a decrease in the concentration of oxaloacetate, is a major causal factor in ketogenesis.

The permeability of mitochondria to oxaloacetate and malate

1. A spectrophotometric assay of the rates of penetration of oxaloacetate and l-malate into mitochondria is described. The assay is based on the measurement of the oxidation of intramitochondrial NADH by oxaloacetate and of the reduction of intramitochondrial NAD(+) by malate. 2. The rate of entry of both oxaloacetate and l-malate into mitochondria is restricted, as shown by the fact that disruption of the mitochondrial structure can increase the rate of interaction between the dicarboxylic acids and intramitochondrial NAD(+) and NADH by between 100- and 1000-fold. 3. The rates of entry of oxaloacetate and malate into liver, kidney and heart mitochondria increased by up to 50-fold on addition of a source of energy, either ascorbate plus NNN'N'-tetramethyl-p-phenylenediamine aerobically, or ATP anaerobically. 4. In the absence of a source of energy the changes in the concentrations of intramitochondrial NAD(+) and NADH brought about by the addition of l-malate or oxaloacetate were followed by parallel changes in the concentrations of NADP(+) and NADPH, indicating the presence in the mitochondria of an energy-independent transhydrogenase system. 5. The results are discussed in relation to the hypothesis that malate acts as a carrier of reducing equivalents between mitochondria and cytoplasm.

Histological findings in the livers of patients with neonatal intrahepatic cholestasis caused by citrin deficiency

AIM

To characterize the histological features of the livers of patients with neonatal intrahepatic cholestasis caused by citrin deficiency (NICCD), we studied specimens from 30 patients diagnosed with NICCD by genetically analyzing the SLC25A13 gene.

METHODS

Liver biopsy specimens were subjected to hematoxylin-eosin, Azan, and Berlin-blue staining.

RESULTS

Most specimens showed varying degrees of fibrosis. The degree of inflammation varied among the specimens, with half showing moderate or severe inflammatory changes. Fat deposition in hepatocytes was observed in almost all of the specimens, and severe fatty liver was noted in 20 (67%) of them. There was a mixture of two types of hepatocytes with macrovesicular or microvesicular fat droplets, and cholestasis was observed at a rate of 77%. Hemosiderin deposition, mostly mild and localized in periportal hepatocytes and macrophages in portal areas, was observed in 57% of the specimens.

CONCLUSION

A combination of mixed macrovesicular and microvesicular fatty hepatocytes and the above-described findings, such as fatty liver, cholestasis, necroinflammatory reaction and iron deposition, are almost never observed in other liver diseases in infants and adults. We believe that NICCD is a disease with characteristic hepatopathological features.

Cytosolic aspartate aminotransferase, a new partner in adipocyte glyceroneogenesis and an atypical target of thiazolidinedione

We show that cytosolic aspartate aminotransferase (cAspAT) is involved in adipocyte glyceroneogenesis, a regulated pathway that controls fatty acid homeostasis by promoting glycerol 3-phosphate formation for fatty acid re-esterification during fasting. cAspAT activity, as well as the incorporation of [(14)C]aspartate into the neutral lipid fraction of 3T3-F442A adipocytes was stimulated by the thiazolidinedione (TZD) rosiglitazone. Conversely, the ratio of fatty acid to glycerol released into the medium decreased. Regulation of cAspAT gene expression was specific to differentiated adipocytes and did not require any peroxisome proliferator-activated receptor gamma (PPARgamma)/retinoid X receptor-alpha direct binding. Nevertheless, PPARgamma is indirectly necessary for both cAspAT basal expression and TZD responsiveness because they are, respectively, diminished and abolished by ectopic overexpression of a dominant negative PPARgamma. The cAspAT TZD-responsive site was restricted to a single AGGACA hexanucleotide located at -381 to -376 bp whose mutation impaired the specific RORalpha binding. RORalpha ectopic expression activated the cAspAT gene transcription in absence of rosiglitazone, and its protein amount in nuclear extracts is 1.8-fold increased by rosiglitazone treatment of adipocytes. Finally, the amounts of RORalpha and cAspAT mRNAs were similarly increased by TZD treatment of human adipose tissue explants, confirming coordinated regulation. Our data identify cAspAT as a new member of glyceroneogenesis, transcriptionally regulated by TZD via the control of RORalpha expression by PPARgamma in adipocytes.

Adult-onset type II citrullinemia and idiopathic neonatal hepatitis caused by citrin deficiency: involvement of the aspartate glutamate carrier for urea synthesis and maintenance of the urea cycle

Citrin is a mitochondrial aspartate glutamate carrier primarily expressed in the liver, heart, and kidney. We found that adult-onset type II citrullinemia is caused by mutations in the SLC25A13 gene that encodes for citrin. In this report, we describe the frequency of SLC25A13 mutations, the roles of citrin as a member of the urea cycle and as a member of the malate-aspartate shuttle, the relationship between its functions and symptoms of citrin deficiency, and therapeutic issues.

Effects of inhibitors of gluconeogenesis on weak organic acid uptake in rat renal tubules

Using inhibitors of gluconeogenesis (phenylpyruvate, alpha-cyano-4-hydroxycinnamate, quinolinate, D-malate, aminooxyacetate), we analysed mechanisms by which the gluconeogenic substrates, lactate and pyruvate, as well as a short-chain fatty acid, acetate, stimulate the uptake of a weak organic acid, fluorescein, in the rat kidney. We have shown that these inhibitors modified both the rate of glucose production from lactate and pyruvate in the renal cortex fragment suspension and the stimulatory effects of the metabolic substrates on fluorescein uptake in superficial proximal tubules in the renal cortex slices. The peculiarities of the effects of lactate and pyruvate on the uptake were correlated with the partial divergence of the pathways of gluconeogenesis from these precursors. The linkage of the weak organic acid uptake with gluconeogenesis is interpreted in terms of the hypothesis that the uptake is controlled by the cytoplasmic pyridine nucleotide redox potential, which is maintained with the participation of certain processes involved in glucose synthesis.

Operation and energy dependence of the reducing-equivalent shuttles during lactate metabolism by isolated hepatocytes

The participation and energy dependence of the malate-aspartate shuttle in transporting reducing equivalents generated from cytoplasmic lactate oxidation was studied in isolated hepatocytes of fasted rats. Both lactate removal and glucose synthesis were inhibited by butylmalonate, aminooxyacetate or cycloserine confirming the involvement of malate and aspartate in the transfer of reducing equivalents from the cytoplasm to mitochondria. In the presence of ammonium ions the inhibition of lactate utilization by butylmalonate was considerably reduced, yet the transfer of reducing equivalents into the mitochondria was unaffected, indicating a substantially lesser role for butylmalonate-sensitive malate transport in reducing-equivalent transfer when ammonium ions were present. Ammonium ions had no stimulatory effect on uptake of sorbitol, a substrate whose oxidation principally involves the alpha-glycerophosphate shuttle. The role of cellular energy status (reflected in the mitochondrial membrane electrical potential (delta psi) and redox state), in lactate oxidation and operation of the malate-aspartate shuttle, was studied using a graded concentration range of valinomycin (0-100 nM). Lactate oxidation was strongly inhibited when delta psi fell from 130 to 105 mV whereas O2 consumption and pyruvate removal were only minimally affected over the valinomycin range, suggesting that the oxidation of lactate to pyruvate is an energy-dependent step of lactate metabolism. Our results confirm that the operation of the malate-aspartate shuttle is energy-dependent, driven by delta psi. In the presence of added ammonium ions the removal of lactate was much less impaired by valinomycin, suggesting an energy-independent utilization of lactate under these conditions. The oxidizing effect of ammonium ions on the mitochondrial matrix apparently alleviates the need for energy input for the transfer of reducing equivalents between the cytoplasm and mitochondria. It is concluded that, in the presence of ammonium ions, the transport of lactate hydrogen to the mitochondria is accomplished by malate transfer that is not linked to the electrogenic transport of glutamate across the inner membrane, and, hence, is clearly distinct from the butylmalonate-sensitive, energy-dependent, malate-aspartate shuttle.

Carbon flux through tricarboxylic acid cycle in rat renal tubules

Our aim was to delineate the effect(s) of chronic metabolic acidosis on renal TCA-cycle metabolism. Renal tubules isolated from control and chronically acidotic rats were incubated at pH 7.4 with either 2 mM [2,3-13C]pyruvate or [2-13C]acetate. GC-MS and/or 13C-NMR were utilized to monitor the flux of 13C through pyruvate dehydrogenase, pyruvate carboxylase and the TCA-cycle. With either, precursor acidosis was associated with significantly decreased formation of 13C-labelled citrate, malate, aspartate and alanine and increased formation of glucose, lactate and acetyl-CoA as compared with the control. The results indicate that adaptation of renal metabolism to chronic metabolic acidosis is associated with diminished flux through citrate synthetase and concomitantly increased flux through pyruvate carboxylase. The data suggest that depletion of TCA-cycle intermediates and enhanced ammoniagenesis in the kidney of chronically acidotic rats may be regulated at the site of mitochondrial citrate-condensing enzyme.

The characterization of perfluorosuccinate as an inhibitor of gluconeogenesis in isolated rat hepatocytes

The effects on metabolism of the fluorinated dicarboxylic acid, perfluorosuccinate, were examined in hepatocytes from fasted rats. Perfluorosuccinate (5 mM) inhibited gluconeogenesis from lactate by 80% and from pyruvate by 40%. Significant inhibition (up to 30%) occurred at a concentration of perfluorosuccinate of 50 microM. Cellular ATP levels were not affected by perfluorosuccinate, nor was the rate of formation of ketone bodies from palmitate, although the ratio [3-hydroxybutyrate]/[acetoacetate] was increased up to 5-fold relative to the control. An increased concentration of cellular L-malate was measured in the presence of perfluorosuccinate but this did not reflect inhibition of malate transport between the mitochondrial and cytoplasmic compartments. In addition, ethanol oxidation by hepatocytes was inhibited 25% by 1 mM perfluorosuccinate. Ureogenesis from ammonia was relatively insensitive to inhibition by perfluorosuccinate. In cytoplasmic extracts of rat liver, the activities of phosphoenolpyruvate carboxykinase and aspartate aminotransferase were inhibited 40-50% and 23%, respectively, by 1 mM perfluorosuccinate. The observed metabolic effects of perfluorosuccinate are consistent with inhibition of the activities of phosphoenolpyruvate carboxykinase and aspartate aminotransferase within the cytoplasm.

Gluconeogenesis in perfused chicken kidney. Effects of feeding and starvation

1. Starvation for 48 hr doubled the rate of gluconeogenesis from lactate and pyruvate in perfused chicken kidney, but did not change the rate of production of glucose from malate, succinate, or alpha-ketoglutarate. 2. Amino-oxyacetate and D-malate inhibited the production of glucose from lactate and from pyruvate by 55% in each case. Quinolinate reduced the production of glucose from lactate and from pyruvate by 50% in both fed and starved chickens, but had no effect on the production of glucose from intermediates in the citric acid cycle. 3. Starvation increased the rate of formation of mitochondrial phosphoenolpyruvate from pyruvate, but had no effect on the rate of formation of mitochondrial phosphoenolpyruvate from malate.

Metabolic effects induced by long-term feeding of medium-chain triglycerides in the rat

Energy intake, weight gain, carcass composition, plasma hormones and fuels, hepatic metabolites and the activities of phosphoenolpyruvate carboxykinase (PEPCK), malic enzyme, and glucose 6-phosphate dehydrogenase (G6P-DH) were examined in adult rats during a 44-day period of low fat, high carbohydrate (LF) feeding or of consumption of one or two high (70% metabolizable energy) fat diets composed of 63% (metabolizable energy) long-chain (LCT) or medium-chain (MCT) triglycerides. Energy intake was similar in the LCT and MCT groups but was less than that of LF group. The weight gain of rats fed MCT diet was 30% less than that of rats fed LF or LCT diets. Energy retention was less when the diet provided MCT than LCT or LF, and that resulted in a 60% decrease in the daily lipids deposition. Plasma glucose, free fatty acids, glycerol, and insulin/glucagon ratio were similar in the three groups. Blood ketone body (KB) concentrations in rats fed the high fat diets were extremely elevated, particularly in the MCT group, but declined throughout the experiment and by the 44th day hyperketonemia decreased by 50% but remained higher than in the LF diet. The blood beta-hydroxybutyrate/acetoacetate (B/A) ratio remained slightly elevated in rats fed the high fat diets. Similar changes were observed in liver KB concentration and in the B/A ratio. Liver lactate/pyruvate ratio elevated in the LCT and MCT groups at the initiation of the diets decreased by 50% at the end of the experiment. The consumption of high fat diets led to a 1.5-fold increase in liver PEPCK activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of oxamate with the gluconeogenic pathway in rat liver

Oxamate, a structural analog of pyruvate, known as a potent inhibitor of lactic dehydrogenase, lactic dehydrogenase, produces an inhibition of gluconeogenic flux in isolated perfused rat liver or hepatocyte suspensions from low concentrations of pyruvate (less than 0.5 mM) or substrates yielding pyruvate. The following observations indicate that oxamate inhibits flux through pyruvate carboxylase: accumulation of substrates and decreased concentration of all metabolic intermediates beyond pyruvate; decreased levels of aspartate, glutamate, and alanine; and enhanced ketone body production, which is a sensitive indicator of decreased mitochondrial free oxaloacetate levels. The decreased pyruvate carboxylase flux does not seem to be the result of a direct inhibitory action of oxamate on this enzyme but is secondary to a decreased rate of pyruvate entry into the mitochondria. This assumption is based on the following observations: Above 0.4 mM pyruvate, no significant inhibitory effect of oxamate on gluconeogenesis was observed. The competitive nature of oxamate inhibition is in conflict with its effect on isolated pyruvate carboxylase which is noncompetitive for pyruvate. Fatty acid oxidation was effective in stimulating gluconeogenesis in the presence of oxamate only at concentrations of pyruvate above 0.4 mM. Since only at low pyruvate concentrations its entry into the mitochondria occurs via the monocarboxylate translocator, from these observations it follows that pyruvate transport across the mitochondrial membrane, and not its carboxylation, is the first nonequilibrium step in the gluconeogenic pathway. In the presence of oxamate, fatty acid oxidation inhibited gluconeogenesis from lactate, alanine, and low pyruvate concentrations (less than 0.5 mM), and the rate of transfer of reducing equivalents to the cytosol was significantly decreased. Whether fatty acids stimulate or inhibit gluconeogenesis appears to correlate with the rate of flux through pyruvate carboxylase which ultimately seems to rely on pyruvate availability. Unless adequate rates of oxaloacetate formation are maintained, the shift of the mitochondrial NAD couple to a more reduced state during fatty acid oxidation seems to decrease mitochondrial oxaloacetate resulting in a decreased rate of transfer of carbon and reducing power to the cytosol.

Anabolic regulation of gluconeogenesis by insulin in isolated rat hepatocytes

The role of substrate availability in the regulation of gluconeogenesis in isolated rat hepatocytes was studied using [U-14C]alanine as a tracer in the presence of different concentrations of L-alanine in the incubation medium. At low alanine concentrations (0.5 mM) insulin decreased the 14C incorporation into the glucose pool and increased the incorporation of tracer carbons into the protein and lipid pools and into CO2. The net radioactivity lost from the glucose pool was only a small percentage of the total increase in the activity of the protein, lipid, CO2, or glycogen pools, supporting the notion that the effect of insulin in diminishing gluconeogenesis is secondary to its effects on pathways using pyruvate. At higher concentrations of alanine (2.5, 5.0, and 10.0 mM) in the incubation medium insulin increased the movement of alanine carbons into protein and glucose. This suggests that at higher substrate concentrations the ability of the liver to synthesize proteins is overwhelmed and the pyruvate carbons are forced into the gluconeogenesis pathway. These results were further confirmed by using [U-14C]lactate. The increases in observed specific activity of glucose following insulin administration would not be possible if insulin acted by affecting the activity of any enzyme directly involved in the formation or utilization of pyruvate, most of which have been proposed as sites of insulin action. Data presented show that insulin "inhibits" gluconeogenesis by affecting a change in substrate availability.

The contribution of hepatic metabolism to diet-induced thermogenesis

In order to examine the possible contribution of the liver to diet-induced thermogenesis, we examined the metabolism of hepatocytes from rats that had been fed a varied choice of highly palatable human food items ("cafeteria feeding"). Liver cells derived from cafeteria-fed rats that had been fasted for 20 hours showed marked increases in rates of respiration and gluconeogenesis in the presence of glycerol or sorbitol. These cells were also much less sensitive to the inhibitory effects of rotenone than were hepatocytes of control animals. hepatocytes from fasted cafeteria-fed rats also demonstrated a substantially enhanced rate of fatty acid oxidation and ketogenesis, which did not appear to be correlated with cellular demands for adenosine triphosphate (ATP). This apparent fall in metabolic efficiency was confirmed by calorimetric studies, which indicated augmented cellular heat production. These changes in hepatic metabolism, associated with cafeteria-feeding, suggest that the liver may have a significant role in diet-induced thermogenesis.

Fatty acid biosynthesis in liver and adipose tissue from dogs

Production of CO2, fatty acids and glycerol from glucose and acetate was measured in slices of liver and adipose tissue taken from mature dogs. Acetate was the predominant carbon source for de novo fatty acid synthesis in both tissues. Fatty acid synthesis occurred at greater rates in adipose tissue than in liver. Glucose provided carbon for glycerol synthesis production in adipose tissue. Results support the concept that adipose tissue, and not liver, is the principal anatomical site for fatty acid synthesis in dogs.

The interaction of carbaryl with the metabolism of isolated hepatocytes: II. Effect on gluconeogenesis

The effects of 1-naphthyl-N-methylcarbamate (carbaryl) upon glucose production from several precursors (lactate, glycerol, alanine, fructose and pyruvate) and on activities of gluconeogenic enzymes (glucose-6-phosphatase, lactate dehydrogenase and aspartate aminotransferase) in isolated rat hepatocytes was studied. The results show that carbaryl inhibits lactate-gluconeogenesis at all concentrations of substrate studied. Gluconeogenesis from 10 mM fructose or 10 mM pyruvate or 10 mM alanine is also inhibited by carbaryl 1 mM. However, glycerol-gluconeogenesis is unaffected. Concentrations of carbaryl at 0.01 and 0.1 mM did not significantly modify lactic dehydrogenase activity, but at 1.0 mM this activity was reduced by 38% in relation to the dimethylsulphoxide-treated group. The synthetic activity of glucose-6-phosphatase is enhanced by carbaryl, but the increase is only significant for 1 mM carbaryl. In the study of aspartate aminotransferase activities two fractions, cytoplasmic and mitochondrial, are differentiated; and, it is observed that both fractions are inhibited by 0.1 and 1.0 mM carbaryl. The results indicate that carbaryl produces major decreases of the glucose production by hepatic cells, and suggest that the carbaryl-induced hyperglycemia in the fasted animal would be due to deficiencies in the peripheral utilization of the glucose.

Gluconeogenesis in isolated chicken hepatocytes: effect of fatty acids, beta-hydroxybutrate, ethanol, and various pyruvate/lactate ratios

The effect of fatty acids beta-hydroxybutyrate, ethanol, and different pyruvate/lactate ratios on gluconeogenesis in isolated chicken hepatocytes was investigated. Glucogenesis was significantly affected by a change in the oxidation-reduction (pyruvate/lactate) ratio, and this effect was greater than could be accounted for by the additive effects of these substrates. Substituting lactate with nongluconeogenic substrates, such as beta-hydroxybutyrate or ethanol, increased the formation of glucose by 80 and 200%, respectively, demonstrating the beneficial effect of the increased reducing equivalents in the hepatocytes. Oleic acid per se had no effect but when added complexed with albumin, it had a negative effect on gluconeogenesis.

The tricarboxylic acid cycle in Dictyostelium discoideum. A model of the cycle at preculmination and aggregation

A preliminary model of tricarboxylic acid-cycle activity in Dictyostelium discoideum is presented. Specific-radioactivity labelling patterns of intra- and extra-mitochondrial pools are simulated by this model and compared with the experimental data. The model arrived at by this method shows the following features. (1) The cycle flux rate is approx. 0.4 mM/min. (2) Both fumarate and malate are compartmentalized at approx. 1:5 between cycle pools and non-cycle pools. These may represent mitochondrial and cytoplasmic pools. Citrate is compartmentalized at 1:10. Succinate appears to exist in three compartments, two of which become labelled by [14C]glutamate and only one by [14C]aspartate (3) Two pools of aspartate with two associated pools of oxaloacetate are necessary for simulation. (4) Exchange between the cycle and non-cycle pools of both citrate and fumarate occurs at very low rates of about 0.003 mM/min, whereas exchange between the malate pools is about 0.004 mM/min. The exchange reaction glutamate in equilibrium 2-oxoglutarate runs at approx. 15 times the cycle flux. (5) A reaction catalysed by "malic" enzyme is included in the model, as this reaction is necessary for complete oxidation of amino acid substrates. (6) Calculation of the ATP yield from the model is consistent with earlier estimates of ATP turnover if the activity of adenylate kinase is considered.

Aldolase and dehydrogenase activities in Spirillum bengal

Spirillum bengal is unable to grow on sugars, but can utilize different organic acids as carbon sources. Dehydrogenase activities were tested with different substrates and were found highest with lactate, glutamate, acetate, succinate and malate. A low aldolase activity was also detectable.

Regulation of glucose formation from lactate and pyruvate in isolated tubules of chicken kidney

1. Isolated kidney tubules from chicken have been used to study the actions of ethanol, ouabain and aminooxyacetate on glucose formation from lactate and pyruvate. 2. In kidney tubules from well-fed chickens the rate of glucose production from lactate was higher than from pyruvate. Ethanol (10 mM) and ouabain (0.1 mM) were found to increase glucose formation from pyruvate but not from lactate. 3. It is concluded that in the presence of ethanol the fluxes of pyruvate through pyruvate dehydrogenase are in favour of the pyruvate carboxylase reaction restricted. 4. Glucose formation from lactate is decreased by aminooxyacetate (0.1 mM) and ouabain (0.1 mM). 5. Aminooxyacetate inhibited glucose formation from lactate, although chicken phosphoenolpyruvate carboxykinase is located intramitochondrially. 6. The results indicate that the effect of aminooxyacetate like that of ouabain is caused by the restricted formation of pyruvate.

Lactate and pyruvate metabolism in isolated renal tubules of normal dogs

The kinetics of lactate and pyruvate (1 and 5 mM in each case) metabolism was studied in isolated dog renal tubules. Utilization of these two substrates and the production of glucose, pyruvate, or lactate, and alanine were determined. The rates of lactate and pyruvate utilization and of glucose production were constant during 60 min of incubation. Glucose production from pyruvate was less than that from lactate. Addition of albumin to the incubation medium greatly inhibited lactate and pyruvate utilization at both substrate concentrations. It stimulated, however, glucose production from 1 mM, but not 5 mM, lactate or pyruvate. These effects were found to be due to the presence of fatty acids in the albumin solution used. In the absence of fatty acids, glucose production represented 35 to 40% of lactate uptake, but represented less than 20% of pyruvate uptake. Fatty acids markedly enhanced the percentage of transformation of lactate and pyruvate into glucose, and that of pyruvate into lactate. Alanine represented 20% or less of lactate and pyruvate uptake. These results suggest that fatty acids have a regulatory influence on lactate and pyruvate dog kidney metabolism.

Evidence for the participation of aspartate aminotransferase in hepatic glucose synthesis in the suckling newborn rat

Inhibition of liver aspartate aminotransferase by L-2-amino-4-methoxy-trans-3-butenoic acid in the suckling newborn rat causes a decrease in all gluconeogenic precursors from phosphoenolpyruvate to glucose and an accumulation of lactate but not of pyruvate. This suggests that the aspartate shuttle is operative and confirms the quantitative importance of lactate as a gluconeogenic precursor at this time during development.

A study of regulation of gluconeogenesis and the supply of cytosolic reducing equivalents for lactate formation in rat kidney-cortical-tubule fragments incubated with pyruvate

1. Tubule fragments were isolated after treatment of rat kidney cortex with collagenase. The formation of glucose and lactate on incubation with 5mM-pyruvate was then measured under various conditions. 2. When tubule fragments were isolated from fed rats in the absence of Ca2+ and then incubated with various Ca2+ concentrations, an incubation period of 15--30 min was necessary to establish a metabolic steady state. Under these conditions glucose formation was increased by Ca2+, adrenaline or 3':5'-cyclic AMP to a greater extent than was lactate formation. Data show that appreciable lactate formation could not have resulted from glycolytic metabolism of glucose formed by gluconeogenesis during incubation. 3. When tubule fragments were isolated from fed rats in the presence of 1.27 mM-Ca2+ and adjustments made to the Ca2+ concentration at the commencement of incubation, metabolic steady state was rapidly established. Under these conditions lactate formation was almost insensitive to Ca2+ concentration (0.16--4.5 mM), whereas glucose formation varied with Ca2+ concentration in a sigmoidal manner. 3':5'-Cyclic AMP decreased this sigmoidicity. 4. Ca2+ depletion of the tissue before incubation appeared to change permanently the relationship between extracellular Ca2+ concentration and the measured rates of metabolic processes. 5. Under conditions of metabolic steady state, glucose formation by tubule fragments from fed rats was less sensitive than lactate formation to inhibition by 3-mercaptopicolinate or 2-n-butylmalonate. Lactate formation by tubule fragments prepared from 48 h-starved rats was more sensitive to these inhibitors. 6. Estimates were made of the rate of futile cycling of C3 species through pyruvate kinase. This was greater in the starved than in the fed state, was decreased by 3':5'-cyclic AMP in both the fed and the starved state, but was unaffected by Ca2+. 7. These results suggested that formation of lactate and glucose is less tightly linked in kidney cortex than in liver. A considerable amount of the supply of reducing equivalents for lactate formation did not appear to be associated with an energy-dependent translocation from mitochondria to cytosol involving a pyruvate leads to oxaloacetate leads to phosphoenolpyruvate leads to pyruvate cycle.

Interrelations between ureogenesis and gluconeogenesis in isolated hepatocytes. The role of antion transport and the competition for energy

1. Glucose synthesis from lactate plus pyruvate and from lactate plus alanine was measured in the presence or absence of 1mM-oleate or 2mM-octanoate at low (2mM) or high (8mM) concentrations of NH4Cl. 2. Both fatty acids alone or with 2mM-NH4Cl doubled glucose production from lactate plus pyruvate. Glucose synthesis from lactate plus alanine, in the presence of oleate, was decreased 16% by 2mM-NH4Cl. 3. In the presence of fatty acids, 8mM-NH4Cl decreased gluconeogenesis by 60-65% from both lactate plus pyruvate and lactate plus alanine. This inhibition was correlated with a high accumulation of aspartate and a drastic decrease in 2-oxoglutarate and malate in the cells. 4. In the presence of 2mM- or 8 mM-NH4Cl, oleate and glucogenic precursors, the addition of 2.5mM-ornithine stimulated urea synthesis. 5. This was paralleled by a decrease of 16% in glucose synthesis from lactate plus pyruvate in the presence of 2mM-NH4Cl and had no effect at 8mM-NH4Cl. In the system producing glucose from lactate plus alanine, ornithine completely reversed the inhibition caused by 2mM-NH4Cl and only partly that by 8mM-NH4Cl. 6. Gluconeogenesis from pyruvate was also inhibited by 2mM-NH4Cl in the presence of oleate or ethanol. This way due to the decrease of malate, which is the C4 precursor of glucose in this system. 7. The limitation of gluconeogenesis by 2-oxoglutarate and malate concentrations in the liver cell and the competition for energy between glucose and urea synthesis is discussed.

Experimental phenylketonuria: metabolic studies in rat liver

The "in vivo" effects of L-phenylalanine on the gluconeogenic pathway in the liver of fasted rats with experimentally induced phenylketonuria-like characteristics have been investigated. Significant increases of the fructose 6-phosphate, glucose 6-phosphate and glucose concentrations were observed. The study of the effect of L-phenylalanine on the cytoplasmic and mitochondrial redox state and energy charge showed an increase in the mitochondrial NAD+/NADH ratio while the energy charge was virtually unchanged. The effects of phenylalanine and its metabolic derivatives (phenylacetate, phenylethylamine, phenyl-lactate, o-hydroxyphenylacetate and phenylpyruvate) on the activity of lactate dehydrogenase (EC 1.1.1.27), malate dehydrogenase (EC 1.1.1.37) and 3-hydroxybutyrate dehydrogenase (EC 1.1.1.30) in rat liver have been also investigated. Phenylpyruvate inhibited the lactate dehydrogenase activity with a Ki of 5.3 mM. Phenylpyruvate also inhibited both the mitochondrial (Ki = 4 mM) and cytoplasmic (Ki = 5 mM) malate dehydrogenase activities. Phenylpyruvate, phenylacetate and o-hydroxyphenylacetate inhibited the 3-hydroxybutyrate dehydrogenase activity with Ki values of 0.7, 6.0 and 9.5 mM respectively.

Effect of dichloroacetate on gluconeogenesis in isolated rat hepatocytes

The effect of dichloroacetate on rates of gluconeogenesis was studied in isolated parenchymal cells obtained from the livers of normal fasted rats. Dichloroacetate significantly inhibited glucose formation from endogenous substrates and from added precursors (e.g., lactate, pyruvate, or glycerate) which enter the gluconeogenic pathway prior to the level of glyceraldehyde-3-phosphate dehydrogenase (GPDH). In contrast, dichloroacetate did not significantly affect glucose synthesis from precursors (e.g., fructose, or glycerol) which enter beyond the GPDH-catalyzed step. Lactate production from fructose of glycerol was unaffected by dichloroacetate. Inhibition of gluconeogenesis occurred regardless of the apparent effects of dichloroacetate on the redox state of the cytosol. Dichloroacetate produced variable effects on the lactate-pyruvate substate pair, while it consistently produced a more oxidized state in the beta-hydroxybutyrate--acetoacetate couple. Unlike uncoupling agents, dichloroacetate reduced glucose synthesis without stimulating respiration or altering total adenine nucleotide levels or ATP/ADP ratios. Dichloroacetate did not affect the metabolism of lactate or pyruvate to CO2 or glycogen. It did, however, significantly inhibit conversion by the cells of added lactate to pyruvate and glucose or of added pyruvate to lactate and glucose.

Inhibition of lipogenesis by halothane in isolated rat liver cells

1. Halothane at clinically effective concentrations [2.5 and 4% (v/v) of the gas phase of the incubation flask] was found to inhibit significantly lipogenesis from endogenous substrates, e.g., glycogen, or from added lactate plus pyruvate. This was accompanied by a decrease in the ratio of the free [NAD+]/[NADH] of the mitochondrion and the cytoplasm, as shown by the [3-hydroxybutyrate]/[acetoacetate] ratio and the [lactate]/[pyruvate] ratio. 2. Acetoacetate or pyruvate decreased the inhibitory effect of halothane and restored lipogenesis to control rates. They were reduced rapidly by 3-hydroxybutyrate dehydrogenase or lactate dehydrogenase respectively, with the concomitant oxidation of NADH and the generation of NAD+. 3. These results suggest that the mechanism by which halothane inhibits lipogenesis from glycogen or lactate is by inhibition of the oxidation of NADH; this results in inhibition of flux of carbon through pyruvate dehydrogenase and a shortage of acetyl-CoA for fatty acid synthesis. Thus when NADH acceptors are added in the presence of halothane, the concentration of mitochondrial NAD+ is raised so that the flux of carbon through pyruvate dehydrogenase increases and lipogenesis is restored.