Inhibition of Gluconeogenesis by Butylmalonate in Perfused Rat Liver

Rapid fractionation of mitochondria from mouse liver and heart reveals in vivo metabolite compartmentation

The compartmentation and distribution of metabolites between mitochondria and the rest of the cell is a key parameter of cell signalling and pathology. Here, we have developed a rapid fractionation procedure that enables us to take mouse heart and liver from in vivo and within ~ 30 s stabilise the distribution of metabolites between mitochondria and the cytosol by rapid cooling, homogenisation and dilution. This is followed by centrifugation of mitochondria through an oil layer to separate mitochondrial and cytosolic fractions for subsequent metabolic analysis. Using this procedure revealed the in vivo compartmentation of mitochondrial metabolites and will enable the assessment of the distribution of metabolites between the cytosol and mitochondria during a range of situations in vivo.

The malate-aspartate shuttle (Borst cycle): How it started and developed into a major metabolic pathway

This article presents a personal and critical review of the history of the malate-aspartate shuttle (MAS), starting in 1962 and ending in 2020. The MAS was initially proposed as a route for the oxidation of cytosolic NADH by the mitochondria in Ehrlich ascites cell tumor lacking other routes, and to explain the need for a mitochondrial aspartate aminotransferase (glutamate oxaloacetate transaminase 2 [GOT2]). The MAS was soon adopted in the field as a major pathway for NADH oxidation in mammalian tissues, such as liver and heart, even though the energetics of the MAS remained a mystery. Only in the 1970s, LaNoue and coworkers discovered that the efflux of aspartate from mitochondria, an essential step in the MAS, is dependent on the proton-motive force generated by the respiratory chain: for every aspartate effluxed, mitochondria take up one glutamate and one proton. This makes the MAS in practice uni-directional toward oxidation of cytosolic NADH, and explains why the free NADH/NAD ratio is much higher in the mitochondria than in the cytosol. The MAS is still a very active field of research. Most recently, the focus has been on the role of the MAS in tumors, on cells with defects in mitochondria and on inborn errors in the MAS. The year 2019 saw the discovery of two new inborn errors in the MAS, deficiencies in malate dehydrogenase 1 and in aspartate transaminase 2 (GOT2). This illustrates the vitality of ongoing MAS research.

A computer model of gluconeogenesis and lipid metabolism in the perfused liver

A mathematical model of the perfused rat liver was developed to predict intermediate metabolite concentrations and fluxes in response to changes in various substrate concentrations in the perfusion medium. The model simulates gluconeogenesis in the liver perfused separately with lactate and pyruvate and the combination of these substrates with fatty acids (oleate). The model consists of key reactions representing gluconeogenesis, glycolysis, fatty acid metabolism, tricarboxylic acid cycle, oxidative phosphorylation, and ketogenesis. Michaelis-Menten-type kinetic expressions, with control by ATP/ADP, are used for many of the reactions. For key regulated reactions (fructose-1,6-bisphosphatase, phosphofructokinase, pyruvate carboxylase, pyruvate dehydrogenase complex, and pyruvate kinase), rate expressions were developed that incorporate allosteric effectors, specific substrate relationships (e.g., cooperative binding), and/or phosphorylation/dephosphorylation using in vitro enzyme activity data and knowledge of the specific mechanisms. The model was independently validated by comparing model predictions with 10 sets of experimental data from 7 different published works, with no parameter adjustments. The simulations predict the same trends, in terms of stimulation of substrate uptake by fatty acid addition, as observed experimentally. In general, the major metabolic indicators calculated by the model are in good agreement with experimental results. For example, the simulated glucose/pyruvate mass yield is 43% compared with the average of 45% reported in the literature. The model accurately predicts the specific time constants of the glucose response (2.5-4 min) and the dynamic behavior of substrate and product fluxes. It is expected that this model will be a useful tool for analyzing the complex relationships between carbohydrate and fat metabolism.

Role of glucocorticoids in activation of hepatic PEPCK gene transcription during exercise

The objective of these studies was to determine the molecular basis for the activation of phosphoenolpyruvate carboxykinase (PEPCK) gene transcription during prolonged submaximal exercise. Mice were fed a high-carbohydrate diet for 1 wk and exercised continuously by swimming for up to 120 min. The level of hepatic PEPCK mRNA increased progressively during exercise, reaching 510% above control, whereas transcription of the PEPCK gene increased 1,000%, before decreasing to control levels within 60 min of recovery. In transgenic mice carrying a chimeric gene consisting of the PEPCK promoter linked to a reporter gene for bovine growth hormone (bGH), PEPCK(-460)-bGH, the level of hepatic bGH mRNA increased by 490% in response to exercise, similar to the increase in the expression of the native PEPCK gene. However, in transgenic mice with a deletion of the glucocorticoid regulatory unit, PEPCK(-355)-bGH, bGH mRNA did not increase above control values. In transgenic mice with a block mutation in adenosine 3',5'-cyclic monophosphate (cAMP) regulatory regions -90/-82 and -250/-234, PEPCK cAMP response element 1 (CRE-1)/P3(1)-bGH, exercise increased bGH mRNA 260% above controls. Adrenalectomy (Adx) had no effect on PEPCK mRNA levels in nonexercised mice, whereas in adrenalectomized (Adx)-exercised mice, PEPCK mRNA increased only 80% above basal, and, in Adx mice injected with dexamethasone, PEPCK mRNA increased with exercise 570% above controls. Exercise was also associated with a large increase in transcription of the gene for the transcription factor CCAAT/enhancer-binding protein beta (C/EBP-beta) and a smaller rise in transcription of c-jun gene, both of which returned to control levels during recovery.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

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.

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.

Effect of mutants and inhibitors on mitochondrial transport systems in vivo in yeast

We have reported elsewhere (Wills, C. and Martin, T. (1984) Biochim. Biophys. Acta 782, 274-284) that one or more mitochondrial transport systems may be involved in the regulation of the inducible alcohol dehydrogenase of yeast, ADH-II. In order to investigate this phenomenon further, it was necessary to determine which of these systems operate in the cell in vivo. We give in this paper preliminary evidence that inhibitors of the malate-phosphate (n-butyl malonate), malate-citrate (hydroxycitrate) and malate-alpha-ketoglutarate (aminooxyacetate or cycloserine) transport systems all operate in vivo. While the demonstration of the in vivo inhibitory activity of n-butyl malonate and hydroxycitrate is entirely by physiological methods, that of the transaminase inhibitors aminooxyacetate and cycloserine depends in part on the isolation of mutants capable of growth on glycerol in minimal medium. On this medium these mutants depend on the malate-aspartate shuttle for growth, and as expected the transaminase inhibitors prevent their growth. Two of the mutants show an enhanced rate of mitochondrial glutamate uptake. A preliminary survey of the properties of the glycerol growth mutants is presented, showing that the probable mode of action of these mutants is an increase in the efficiency of the malate-aspartate shuttle.

Inhibition of glycogen synthesis in rat hepatocytes by medium Zn2+

In hepatocytes from fasted rats, Zn2+ in the range from 0 to 500 microM has relatively minor effects on gluconeogenesis from most substrates, or on ureagenesis from NH3. In hepatocytes from fed rats, Zn2+ does not affect glycogenolysis. In hepatocytes from fasted rats, in which glycogen is being actively synthesized using the substrate combination (Katz et al. (1976) Proc. Natl.Acad.Sci.USA 73,3433-3437) of glucose, lactate and glutamine (all 10mM), Zn2+ markedly inhibits glycogen synthesis, with total inhibition at 500 microM, and a half maximal effect in the range from 50 to 100 microM. Dipicolinate (pyridine 2,6-dicarboxylate), a zinc chelator, is about as effective as L-glutamine in activating glycogen synthesis with the substrate combination of dihydroxyacetone, lactate and glucose (all 10mM). This suggests the possible hypothesis that endogenous Zn2+ might control the rate of glycogen synthesis in vivo. However, alternate explanations such as metabolite accumulation are also possible, since dipicolinate causes inhibition of gluconeogenesis from L-lactate.

Synthesis and hypoglycemic activity of benzamidophenyl-alkanoic acid derivates: new inhibitors of gluconeogenesis

A series of omega-[2-(N-alkylbenzamido)-phenyl]-alkanoic acids was synthesized and tested for its effects on blood glucose concentration in fasted rats and on gluconeogenesis from lactate and pyruvate in isolated perfused rat livers. The compounds led to a dose-dependent and reversible inhibition of gluconeogenesis, with 4-[2-(N-methyl-3-trifluoromethylbenzamido)-phenyl]-butanoic acid leading to a 50% inhibition at 0.02 mM. The compounds lowered blood glucose in fasted rats. No correlation between hypoglycemic effect and inhibition of gluconeogenesis could be detected, however.

Fatty acid, 3-beta-hydroxysterol, and ketone synthesis in the perfused rat liver. Effects of (--)-hydroxycitrate and oleate

The effects of oleate and hydroxycitrate on the rate of long-chain fatty acid and 3-beta-hydroxysterol synthesis were measured in perfused rat livers. Metabolite measurements show that in livers from fed animals inhibition of fatty acid synthesis by oleate or hydroxycitrate is associated with an increase in the tissue content of glucose 6-phosphate and fructose 6-phosphate, and a diminution in glycolytic intermediates from fructose diphosphate to phosphoenolpyruvate. Oleate also causes an increase in the tissue content of long-chain fatty acyl-CoA and citrate. The increase in long-chain fatty acyl-CoA is larger in livers from starved as compared to fed rats, while the increase in citrate is larger in livers from fed as compared to starved rats. However, the increase in the citrate content of livers from fed rats occurs in a range where it causes no further activation of acetyl-CoA carboxylase in vitro. Ketogenesis by livers from fed rats perfused without free fatty acids is strongly inhibited by hydroxycitrate. However, ketogenesis is not inhibited by hydroxycitrate when livers from starved rats are perfused with oleate, and ketogenesis is increased somewhat by hydroxycitrate when livers from fed rats are perfused with oleate. These results are interpreted in terms of an extramitochondrial pathway of ketogenesis which operates in carbohydrate-fed animals. The intramitochondrial pathway predominates in starved animals, or when the concentration of fatty acids is high, or both. Other interpretations, which cannot be ruled out at present, are also considered.

Use of endogenous triglycerides to support gluconeogenesis in the perfused isolated rat liver

The carbon balances in isolated perfused rat liver during gluconeogenesis from L-alanine and sodium L-lactate indicate that assuming the substrate unaccounted for were fully oxidized the energy yielded was not sufficient to support the observed rates of glucose synthesis. This observation indicates that endogenous substrates must also be oxidized. The possibility that endogenous fatty acid oxidation was the source of the energy needed to support glucose synthesis was investigated by measuring the rate of 14CO2 formation from tracer quantities of added [U-14C] palmitate. Short pulses of L-alanine or sodium L-lactate infusion produced an increased rate of 14CO2 production paralleled by increases in oxygen uptake indicating that more endogenous fuel is being mobilized. That the rate of 14CO2 output is an expression of fatty acid mobilization was supported by experiments demonstrating that the addition of octanoate to dilute the fatty acid pool produced an immediate fall in the rate of 14CO2 output. On the other hand, the administration of glucose produced no changes in oxygen uptake or 14CO2 output. However, lactate even in the presence of glucose induced a rise in 14CO2 production which occurred in parallel with the enhancement in oxygen uptake. It is concluded that mobilization of hepatic endogenous fatty acid is a metabolic event intimately associated with enhancement of gluconeogenesis. Consequently the control of the different steps of this process may indirectly control gluconeogenesis.

Cellular metabolite distribution and the control of gluconeogenesis in the perfused isolated rat liver

Glucose production was measured in isolated rat livers perfused with 100 ml of blood-free recirculating medium. The gluconeogenic rate using L-alanine as substrate was only 55% of that obtained with L-lactate. The steady-state concentration of gluconeogenic and tricarboxylic acid cycle intermediates were measured in freeze clamped biopsies. Livers perfused with L-lactate displayed higher concentrations of malate, alpha-glycerophosphate and beta-hydroxybutyrate probably as a result of a higher state of reduction of the nicotinamide system. Hexose-phosphate intermediates were also increased when L-lactate was the substrate. Phosphoenolpyruvate and 3-phosphoglycerate were considerably elevated when L-alanine was the glucose precursor. Livers perfused with L-lactate displayed higher cytosolic concentration of all the tricarboxylic acid cycle intermediates except oxaloacetate while glutamate was slightly and aspartate considerably higher when alanine was the substrate. In the mitochondrial compartment the pattern of distribution tended to be the opposite; that is, livers perfused with L-lactate showed lower concentrations of all the intermediates except alpha-ketoglutarate. The mitochondrial: cytosolic metabolite gradients of all the intermediates whose distribution was studied were higher in livers perfused with L-alanine. The relevance of these findings to the observed differences in the gluconeogenic fluxes are discussed.

Relationship of energy production to gluconeogenesis in renal cortical tubules

Isolated tubules prepared by collagenase treatment of rat renal cortex retained their ultrastructural integrity and responded to added lactate and succinate with an increase in gluconeogenesis and respiration. Inhibition of the mitochondrial respiratory chain with rotenone, or energy conservation sites with oligomycin caused a marked reduction in respiration and ATP content thereby completely inhibiting net gluconeogenesis. Dissociation of gluconeogenesis from respiration was accomplished with quinolinic acid and hydrazine, inhibitors of gluconeogenesis. At 5 times 10(-3) M quinolinic acid, gluconeogenesis from succinate was inhibited approximately 50% and from lactate nearly 100%. This concentration of quinolinic acid did not affect oxygen uptake or the ATP content of tubules in the presence or absence of substrate. Hydrazine at 10(-3) M resulted in approximately 75% inhibition of glucose formation from succinate and complete inhibition from lactate without interfering with respiration or ATP content. The increased mitochondrial energy generation, as manifested by accelerated respiration was independent of gluconeogenesis. The unchanging cell ATP concentration with a higher respiratory rate upon addition of exogenous substrate bespeaks increased ATP turnover. ATP utilization for the substrate-induced enhancement of gluconeogenesis could not account for the increment in ATP hydrolysis.

The basic requirements for the function of the isolated cell free perfused rat kidney

We have attempt to define experimental conditions which would overcome or minimize some of the well known functional limitations of isolated single pass kidney preparations. Rat kidneys were perfused with a Krebs-Henseleit solution containing the gelatine derivative Haemaccel as colloid. Perfusion was initiated in situ via the mesenteric artery. Arterial flow rate was measured continuously from the very onset of perfusion. Effective perfusion pressure was recorded distal to the perfusion capillary in the aorta. Aliquots of the venous effluate and of an arterial bypass solution were drawn through an O-2 electrode for the calculation of Q-o-2. First it was shown that the often observed initial vasoconstriction of the preparation which occurs immediately after cannulation of the kidney can be eliminated by rapid disconnection of the autonomic nerve supply. A more delayed gradual increase of renal resistance, which we observed after 30 min could be prevented by using sterile perfusion solutions. Using glucose as the only substrate fuel, fractional Na-reabsorption decreased to 65% 3 hrs after the onset of perfusion (T Na equals 27.3 muEq/g with min). When a substrate enriched sterile solution was used containing pyruvate, lactate, oxaloacetate, and glutamate, Na conservation of the isolated kidney could be maintained at a higher level. Fractional Na-reabsorption levelled off and was still 88% after 3 hrs (T Na equals 64.4 muEq/g with min). The results demonstrate that the transport function of the isolated kidney preparation critically depends on the supply with substrate hydrogen. Thus, the present system meets the basic requirements necessary for further micropuncture evaluation of renal function under the condition of isolated single pass perfusion.