Fatty acids and glycerol or lactate are required to induce gluconeogenesis from alanine in isolated rabbit renal cortical tubules

Summary In isolated rabbit renal cortical tubules, glucose synthesis from 1 mM alanine is negligible, while the amino acid is metabolized to glutamine and glutamate. The addition of 0.5 mM octanoate plus 2 mM glycerol induces incorporation of [U-¹⁴C]Alnine into glucose and decreases glutamine synthesis, whereas oleate and palmitate in the presence of glycerol are less potent than octanoate. Gluconeogenesis is also significantly accelerated when glycerol is substituted by lactate. In view of an increase in¹⁴CO₂ fixation and elevation of both cytosolic and mitochondrial NADH/NAD⁺ ratios, the activation of glucose formation from alanine upon the addition of glycerol and octanoate is likely due to (i) stimulation of pyruvate carboxylation, (ii) increased availability of NADH for glyceraldehyde-3-phosphate dehydrogenase and (iii) elevation of mitochondrial redox state causing a diminished provision of ammonium for glutamine synthesis. The induction of gluconeogenesis in the presence of alanine, glycerol and octanoate is not related to cell volume changes. The results presented in this paper show the importance of free fatty acids and glycerol for regulation of renal gluconeogenesis from alanine. The possible physiological significance of the data is discussed.     

Gluconeogenic and lipogenic properties of isolated kidney tubules from the domestic fowl (Gallus domesticus)

Isolated kidney tubules synthesize glucose actively from fructose, lactate, glycerol and pyruvate and, to a lesser extent, from a variety of amino acids. Ethanol stimulated gluconeogenesis from pyruvate and inhibited it from lactate. The aminotransferase inhibitor, aminooxyacetate, greatly reduced synthesis from lactate but not from pyruvate. Quinolinate inhibited gluconeogenesis from both precursors, indicating an active role for cytosolic phosphoenolpyruvate carboxykinase (PEPCK) in the gluconeogenic pathway. Incorporation of lactate or glucose into triglycerides was relatively low, and since no fatty acid synthase (FAS) activity could be detected, probably represented chain elongation or reesterification.     

Location of three key enzymes of gluconeogenesis in baker's yeast

The subcellular location of hexose diphosphatase, phosphoenolpyruvate carboxykinase and pyruvate carboxylase in baker's yeast (Saccharomyces cerevisiae) was investigated by density gradient centrifugation of spheroplast lysates obtained by osmotic shock treatment of spheroplasts and centrifugation for 10000 g x min. On the evidence obtained from zonal separations these three enzymes of gluconeogenesis are most probably located in the soluble cytosol.List of Abbreviations HDPase Hexose diphosphatase - PEPCK Phosphoenolpyruvate carboxykinase - PC Pyruvate carboxylase     

Fatty acid effects on gluconeogenesis in goat, calf and guinea pig hepatocytes

1. Regulation of hepatic gluconeogenesis by fatty acid was studied in goat, calf and guinea pig hepatocytes. 2. Fatty acid effects on gluconeogenesis were dependent upon species; fatty acid and gluconeogenic substrate. 3. Oleate and octanoate inhibited gluconeogenesis from propionate in guinea pig hepatocytes and stimulated it in goat hepatocytes. 4. Oleate and octanoate markedly inhibited gluconeogenesis from lactate in guinea pig hepatocytes whereas octanoate, but not oleate, decreased glucose production from lactate in goat hepatocytes. 5. Effects of fatty acids on gluconeogenesis in calf hepatocytes were similar to goat hepatocytes suggesting control of gluconeogenesis is similar among ruminant species but differs from guinea pigs.     

Evidence that stimulation of gluconeogenesis by fatty acid is mediated through thermodynamic mechanisms

We have studied the stimulatory effects of palmitate on the rate of glucose synthesis from lactate in isolated hepatocytes. Control of the metabolic flow was achieved by modulating the activity of enolase using graded concentrations of fluoride. Unexpectedly, palmitate stimulated gluconeogenesis even when enolase was rate-limiting. This stimulation was also observed when the activities of phosphoenolpyruvate carboxykinase and aspartate aminotransferase were modulated using graded concentrations of quinolinate and aminooxyacetate, respectively. Linear force-flow relationships were found between the rate of gluconeogenesis and indicators of cellular energy status (i.e. mitochondrial membrane and redox potentials and cellular phosphorylation potential). These findings suggest that the fatty acid stimulation of glucose synthesis is in part mediated through thermodynamic mechanisms.     

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

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

Substrate-dependent effect of 1-34 human parathyroid hormone fragment, dibutyryl cAMP and cAMP on gluconeogenesis in rabbit renal tubules

In the presence of 0.5 mM extracellular Ca2+ concentration both 1-34 human parathyroid hormone fragment (0.5 micrograms/ml) as well as 0.1 mM dibutyryl cAMP stimulated gluconeogenesis from lactate in renal tubules isolated from fed rabbits. However, these two compounds did not affect glucose synthesis from pyruvate as substrate. When 2.5 mM Ca2+ was present the stimulatory effect of the hormone fragment on gluconeogenesis from lactate was not detected but dibutyryl cAMP increased markedly the rate of glucose formation from lactate, dihydroxyacetone and glutamate, and inhibited this process from pyruvate and malate. Moreover, dibutyryl cAMP was ineffective in the presence of either 2-oxoglutarate or fructose as substrate. Similar changes in glucose formation were caused by 0.1 mM cAMP. As concluded from the 'crossover' plot the stimulatory effect of dibutyryl cAMP on glucose formation from lactate may result from an acceleration of pyruvate carboxylation due to an increase of intramitochondrial acetyl-CoA, while an inhibition by this compound of gluconeogenesis from pyruvate is likely due to an elevation of mitochondrial NADH/NAD+ ratio, resulting in a decrease of generation of oxaloacetate, the substrate of phosphoenolpyruvate carboxykinase. Dibutyryl cAMP decreased the conversion of fracture 1,6-bisphosphate to fructose 6-phosphate in the presence of both substrates which may be secondary to an inhibition of fructose 1,6-bisphosphatase.     

Induction of rat kidney gluconeogenesis during acute liver intoxication by carbon tetrachloride.

1. Glucose production from L-lactate was completely inhibited 24h after carbon tetrachloride treatment in liver from 48h-starved rats. The activities of phosphoenolpyruvate carboxykinase, fructose diphosphatase and glucose 6-phosphatase were decreased by this treatment in fed and starved rats, whereas lactate dehydrogenase activity was only decreased in fed animals. 2. The production of glucose by renal cortical slices from fed rats previously treated with carbon tetrachloride was enhanced when L-lactate, pyruvate and glutamine but not fructose were used as glucose precursors. Renal phosphoenolpyruvate carboxykinase activity was increased in this condition. 3. This increase was counteracted by cycloheximide or actinomycin D, suggesting that the effect was due to the synthesis de novo of the enzyme. 4. The pattern of hepatic gluconeogenic metabolites in treated animals was characterized by an increase in lactate, pyruvate, malate and citrate as well as a decrease in glucose 6-phosphate, suggesting an impairment of liver gluconeogenesis in vivo. 5. In contrast, the profile of renal metabolites suggested that gluconeogenesis was operative in the treated rats, as indicated by the marked increase in the content of phosphoenolpyruvate, 2-phosphoglycerate, 3-phosphoglycerate and glucose 6-phosphate. 6. It is postulated that renal gluconeogenesis could contribute to the maintenance of glycaemia in carbon tetrachloride-treated rats.     

Lipogenesis in rat and guinea-pig isolated epididymal fat-cells

Fat-cells were prepared from rat and guinea-pig epididymal adipose tissue and compared on the basis of the intracellular distributions and activities of enzymes and with respect to their utilization of various U-(14)C-labelled substrates for lipogenesis. 1. Compared with the rat, guinea-pig extramitochondrial enzyme activities differed in that aconitate hydratase, alanine aminotransferase, ATP-citrate lyase, lactate dehydrogenase, NAD-malate dehydrogenase, NADP-malate dehydrogenase and phosphoenolpyruvate carboxykinase activities were appreciably lower, whereas aspartate aminotransferase, glucose 6-phosphate dehydrogenase, NADP-isocitrate dehydrogenase and 6-phosphogluconate dehydrogenase activities were appreciably higher. Mitochondrial activities of citrate synthase, NADP-isocitrate dehydrogenase and pyruvate carboxylase were appreciably lower, whereas mitochondrial activities of aspartate aminotransferase, glutamate dehydrogenase, NAD-malate dehydrogenase and phosphoenolpyruvate carboxykinase were higher in the guinea pig compared with the rat. 2. In general guinea-pig fat-cells incorporated acetate and lactate into fatty acids more readily than rat fat-cells, whereas rat fat-cells incorporated glucose and pyruvate more readily than guinea-pig fat-cells. 3. Acetate stimulated the incorporation of glucose into fatty acids in rat fat-cells, but had no appreciable effect upon this process in guinea-pig fat-cells. Acetate greatly decreased the incorporation of lactate into fatty acids in cells from both species. 4. Lactate/pyruvate ratios produced by incubation of guinea-pig cells with glucose+insulin were very low compared with those found with rat cells under the same conditions. 5. With glucose (+insulin) or with glucose+acetate (+insulin) as substrates guinea-pig cells produced enough NADPH by the hexose monophosphate pathway to satisfy the NADPH requirements of lipogenesis. In rat fat-cells under the same conditions, hexose monophosphate-pathway NADPH provision was not sufficient to meet the requirements of lipogenesis. 6. These results are discussed, particularly in relationship to the disposition of cytosolic reducing equivalents in the cells.     

The activities of fructose 1,6-diphosphatase, phosphofructokinase and phosphoenolpyruvate carboxykinase in white muscle and red muscle

1. The activities of fructose 1,6-diphosphatase were measured in extracts of muscles of various physiological function, and compared with the activities of other enzymes including phosphofructokinase, phosphoenolpyruvate carboxykinase and the lactate-dehydrogenase isoenzymes. 2. The activity of phosphofructokinase greatly exceeded that of fructose diphosphatase in all muscles tested, and it is concluded that fructose diphosphatase could not play any significant role in the regulation of fructose 6-phosphate phosphorylation in muscle. 3. Fructose-diphosphatase activity was highest in white muscle and low in red muscle. No activity was detected in heart or a deep-red skeletal muscle, rabbit semitendinosus. 4. The lactate-dehydrogenase isoenzyme ratio (activities at high and low substrate concentration) was measured in various muscles because a low ratio is characteristic of muscles that are more dependent on glycolysis for their energy production. As the ratio decreased the activity of fructose diphosphatase increased, which suggests that highest fructose-diphosphatase activity is found in muscles that depend most on glycolysis. 5. There was a good correlation between the activities of fructose diphosphatase and phosphoenolpyruvate carboxykinase in white muscle, where the activities of these enzymes were similar to those of liver and kidney cortex. However, the activities of pyruvate carboxylase and glucose 6-phosphatase were very low in white muscle, thereby excluding the possibility of gluconeogenesis from pyruvate and lactate. 6. It is suggested that the presence of fructose diphosphatase and phosphoenolpyruvate carboxykinase in white muscle may be related to operation of the alpha-glycerophosphate-dihydroxyacetone phosphate and malate-oxaloacetate cycles in this tissue.     

A selective system for hepatoma cells producing gluconeogenic enzymes.

Gluconeogenesis is a liver-specific pathway which permits the synthesis of phosphorylated sugars from oxaloacetate, pyruvate, amino acids, or trioses. The absolute requirement for glucose or an alternative hexose which characterizes most mammalian cells probably reflects an inablility to perform gluconeogenesis rather than to generate sufficient energy by respiration alone. Cells of diverse histogenetic origins have been tested in glucose-free medium, supplemented with oxaloacetate or with dihydroxyacetone. The only cells able to grow are well-differentiated hepatoma cells which produce the relevant gluconeogenic enzymes: phosphoenolpyruvate carboxykinase, fructose diphosphatase, and triokinase. Reconstruction experiments demonstrate that glucose-free media permit the selective growth of cells producing gluconeogenic enzymes. These media should be useful for analysis of reexpression of differentiated functions in somatic cell hybrids and for the isolation of mutants.     

Gluconeogenesis in isolated liver cells of the eel,Anguilla japonica

Summary The pathway of gluconeogenesis from pyruvate, lactate and alanine was investigated in isolated liver cells of the eel. Amino-oxyacetate, a transaminase inhibitor, inhibited gluconeogenesis not only from lactate, but also from pyruvate by 60%.d-Malate did not inhibit gluconeogenesis from either of the substrates (Table 1 A).The effects of various amino acids on gluconeogenesis were investigated. Leucine accelerated gluconeogenesis from pyruvate or alanine (Table 2). Leucine promoted the incorporation of¹⁴C-pyruvate into glutamate and aspartate, and increased the glutamate content. The specific activity of¹⁴C-aspartate was increased markedly by leucine (Table 5).From the investigation of subcellular distribution of enzymes unique to gluconeogenesis, it was found that pyruvate carboxylase was located almost exclusively in the mitochondrial fraction, and that phophoenolpyruvate carboxykinase and aspartate transaminase were located in both the mitochondrial and the cytosolic fractions (Table 7).From these results it is concluded that the oxaloacetate-aspartate pathway is a major route in gluconeogenesis from any of the substrates in the eel liver.Abbreviations AOA amino-oxyacetate - PEP phosphoenolpyruvate     

Propionate metabolism and its regulation by fatty acids in ovine hepatocytes

Propionate metabolism was studied in ovine hepatocytes. The main products of metabolism were CO2, glucose, L-lactate and pyruvate. The fatty acids, butyrate and palmitate inhibited propionate oxidation; butyrate inhibited but palmitate slightly stimulated gluconeogenesis from propionate. Butyrate and palmitate also inhibited lactate and pyruvate production from both endogenous substrates and from propionate.     

The appearance of gluconeogenesis at birth in sheep. Activation of the pathway associated with blood oxygenation.

1. Measurements of pyruvate carboxylase, mitochondrial phosphoenolpyruvate carboxykinase (GTP), hexose bisphosphatase and glucose 6-phosphatase in developing sheep liver showed substantial activities of all enzymes in the foetus, especially towards the end of gestation. Cytosol phosphoenolpyruvate carboxykinase (GTP) in livers of mid-term foetuses was only 10% of the activity at birth. 2. All enzymes except pyruvate carboxylase showed 1.5-2-fold increases after birth. 3. Gluconeogenesis form [14C]actate could not be detected in chronically cannulated sheep foetuses at any developmental stage and was not initiated by the infusion of adrenaline or glucagon. 4. An active pathway of gluconeogenesis was evident in vivo within 2 min after natural birth or within 4 min after Caesarian delivery of term lambs, and was delayed in prematurely delivered lambs until breathing was established and the blood fully oxygenated. 5. It is proposed that oxygen availability initiates gluconeogenesis in the newborn lamb.     

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

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

Interrelationships and metabolic effects of fatty acids in the perfused rat liver at hyperthermic temperatures

Livers of fasted rats were perfused for 70 min at 37 degrees-43 degrees C in the presence or absence of acetate, octanoate or palmitate. Hepatic biosynthetic capacity was assessed by measuring rates of gluconeogenesis, ureogenesis, ketogenesis and O2 consumption. In the presence of each fatty acid, gluconeogenesis, ureogenesis and oxygen consumption were maintained at 37 degrees and 42 degrees C. At 43 degrees, the rate of glucose formation decreased markedly and rates of ureogenesis and oxygen consumption were distinctly lower. As the temperature was increased from 37 degrees to 43 degrees C without fatty acids, i.e. albumin only, there was a progressive decrease in the rate of gluconeogenesis while the ratio of net C3 utilized to glucose formed, increased successively. The values of this ratio in the presence of palmitate or octanoate at 43 degrees were smaller than those for albumin or acetate, but higher than the figure of 2 for complete conversion of C3 units to glucose. Although fatty acid was added in equimolar amounts of C2 units, total ketone formation was influenced significantly by chain length. Hepatic ketogenesis was similar at 37 degrees with albumin, palmitate, or acetate, but was stimulated significantly by octanoate at 37 degrees and 42 degrees C. At 42 degrees, ketone formation increased in the presence of palmitate. At 43 degrees C, ketogenesis with palmitate or octanoate decreased, while that with acetate or albumin was maintained at the same lower rates. The ratio of 3-hydroxybutyrate to acetoacetate in the perfusate was increased with palmitate at the end of perfusion at 37 degrees and 42 degrees C or octanoate at 42 degrees and 43 degrees C. Thus, long (palmitate)- and medium (octanoate)- but not short (acetate)-chain fatty acids enhance not only beta-oxidation, but influence the redox state of hepatic mitochondria with an increase in the state of reduction of the pyridine nucleotides. Such a shift in the redox state would be operable in the perfused liver even at 43 degrees C and may be responsible for improved conversion of lactate to glucose when medium- or long-chain fatty acids are present at hyperthermic temperatures.     

The characteristics and site of inhibition of gluconeogenesis in rat liver cells by bacterial endotoxin. Stimulation of phosphofructokinase-1.

The characteristics and site of inhibition of gluconeogenesis by endotoxin were investigated in liver cells isolated from control and endotoxin-treated rats. Endotoxin treatment was associated with inhibition (40-50%) of gluconeogenesis from lactate plus pyruvate over a range of concentrations of substrate and of oleate and with or without glucose or glucagon. Similar inhibition was observed with asparagine, proline, glutamine, alanine and a substrate mixture, but not with glycerol, glyceraldehyde, dihydroxyacetone or endogenous substrates. There was no change in cellular ATP content or in the rates of ketogenesis or ureogenesis from asparagine, proline or glutamine. Other effects on isotopic fluxes, metabolite contents, enzyme activities and control coefficients were consistent with the suggestion that the effects of endotoxin on gluconeogenesis are exerted at the level of phosphofructokinase-1, and not at phosphoenolpyruvate carboxykinase, pyruvate kinase, pyruvate carboxylase or glucokinase.     

Modulation of the sympathetic nerve action on carbohydrate and ketone body metabolism by fatty acids, glucagon und insulin in perfused rat liver

Rat liver was perfused in situ via the portal vein without recirculation: 1) Nerve stimulation (20 Hz, 2 ms, 20 V) increased glucose output and shifted lactate uptake to output; the alterations were diminished by oleate but not octanoate. 2) Glucagon (1nM) stimulated glucose output maximally also in the presence of the fatty acids, so that nerve stimulation could not increase it further. The hormone also enhanced lactate uptake and nerve stimulation counteracted this effect. The counteraction was diminished by oleate but not octanoate. 3) Insulin (100nM) slightly lowered glucose output and had no effect on lactate balance. It antagonized the increase of glucose output by nerve stimulation, but left the shift of lactate uptake to release unaffected. These events were not influenced by the fatty acids. 4) Nerve stimulation decreased ketone body production from oleate and octanoate. 5) Glucagon increased ketogenesis from oleate, but not octanoate. In the presence of glucagon nerve stimulation also lowered ketogenesis. This decrease was diminished in the presence of oleate. 6) Insulin lowered ketogenesis from oleate but not octanoate. In the presence of insulin nerve stimulation decreased ketogenesis; the relative change was independent of the fatty acids. The complex interactions between fatty acids, glucagon and insulin in the modulation of sympathetic nerve actions can be summarized as follows: Oleate, which enters the mitochondria via the carnitine system, but not octanoate, which enters independently from this system, as well as insulin but not glucagon effectively modulated the nerve actions on carbohydrate metabolism. Glucagon but not insulin modulated the nerve effects on ketogenesis from oleate but not octanoate. The regulatory interactions between substrates, hormones and nerves can best be explained on the basis of the model of metabolic zonation.     

Inhibition of carbon dioxide fixation by lead acetate in rat liver mitochondria.

These studies were undertaken to determine the mechanism by which intravenously administered lead salts inhibit hepatic gluconeogenesis. Within 1 h after the intravenous administration of lead acetate (10 mg), there is 97% inhibition of CO2 fixation in isolated rat liver mitochondria. This effect is concentration-dependent. The induction of phosphoenolpyruvate carboxykinase activity observed with starvation was also inhibited by intravenously administered lead acetate, but the activities of pyruvate kinase, glucose 6-phosphate dehydrogenase and pyruvate carboxylase were unaffected, as was the oxidation of palmitate and palmitoyl-CoA by mitochondria from Pb2+-treated animals. The addition of reduced glutathione to mitochondria from Pb2+-treated animals had no effect on the inhibited CO2 fixation. ATP concentrations in mitochondria from Pb2+-treated animals are decreased and the dose-response relationships for the effect of Pb2+ on CO2 fixation and ATP concentrations correspond. We conclude that the decrease in mitochondrial ATP in Pb2+-treated animals is probably responsible for the marked inhibition ov CO2 fixation, and hence the impairment of gluconeogenesis from alanine, lactate and pyruvate observed by others.     

Control of gluconeogenesis in rat liver cells. I. Kinetics of the individual enzymes and the effect of glucagon

Control of gluconeogenesis from lactate was studied by titrating rat liver cells with lactate and pyruvate in a ratio of 10:1 in a perifusion system. At different steady states of glucose formation, the concentration of key gluconeogenic intermediates was measured and plotted against gluconeogenic flux (J glucose). Complete saturation was observed only in the plot relating J glucose to the extracellular pyruvate concentration. Measurement of pyruvate distribution in the cell showed that the mitochondrial pyruvate translocator operates close to equilibrium at high lactate and pyruvate concentrations. It can therefore be concluded that pyruvate carboxylase limits maximal gluconeogenic flux. Addition of glucagon did not cause a shift in the plots relating J glucose to glucose 6-phosphate, dihydroxyacetone phosphate, 3-phosphoglycerate, and phosphoenolpyruvate. It can thus be concluded that glucagon does not affect the kinetic parameters of the enzymes involved in the conversion of phosphoenolpyruvate to glucose. Addition of glucagon led to a shift in the curves relating J glucose to the concentration of cytosolic oxalacetate and extracellular pyruvate. The shift in the curve relating J glucose to oxalacetate is due to glucagon-induced inhibition of pyruvate kinase. The stimulation of gluconeogenesis by glucagon can be accounted for almost completely by inhibition of pyruvate kinase. There was almost no stimulation by glucagon of pyruvate carboxylation. In the absence of glucagon, control on gluconeogenesis from lactate is distributed among different steps including pyruvate carboxylase and pyruvate kinase. Assuming that in the presence of glucagon all pyruvate kinase flux is inhibited, the control of gluconeogenesis in the presence of the hormone is confined exclusively to pyruvate carboxylase.