Paths of carbon in gluconeogenesis and lipogenesis: the role of mitochondria in supplying precursors of phosphoenolpyruvate

Gluconeogenesis in the kidney cortex. Effects of D-malate and amino-oxyacetate

1. Rat kidney-cortex slices incubated with d-malate alone formed very little glucose. d-Malate, however, augmented gluconeogenesis from l-lactate and inhibited gluconeogenesis from pyruvate and l-malate. 2. d-Malate had little effect on the rate of the tricarboxylic acid cycle with or without other substrates added. 3. d-Malate inhibited the activity of the l-malate dehydrogenase in a high-speed-supernatant fraction from kidney cortex. 4. It was concluded that d-malate inhibited either the operation of the cytoplasmic l-malate dehydrogenase or malate outflow from the mitochondria in the intact kidney-cortex cell. This supports the hypothesis of Lardy, Paetkau & Walter (1965) and Krebs, Gascoyne & Notton (1967) on the role of malate as carrier for carbon and reducing equivalents in gluconeogenesis. 5. Gluconeogenesis from l-lactate in kidney-cortex slices was strongly inhibited by a low concentration (0.1mm) of amino-oxyacetate, whereas glucose formation from pyruvate, malate, aspartate and several other compounds was only slightly affected. 6. High concentrations of l-aspartate largely reversed the inhibition of gluconeogenesis from l-lactate caused by amino-oxyacetate. 7. Amino-oxyacetate inhibited strongly the glutamate-oxaloacetate transaminase in the 30000g supernatant fraction of a kidney-cortex homogenate. The presence of l-aspartate decreased the inhibition of the transaminase by amino-oxyacetate. 8. Detritiation of l-[2-(3)H]aspartate was inhibited by 90% during an incubation of kidney-cortex slices with l-lactate and amino-oxyacetate. 9. Low concentrations (10mum) of artificial electron acceptors such as Methylene Blue and phenazine methosulphate abolished most of the inhibition of gluconeogenesis from l-lactate by amino-oxyacetate. This is interpreted as an activation of net malate outflow from the mitochondria by-passing the inhibited transfer of oxaloacetate. 10. These findings support the concept that transamination to aspartate is involved in the transfer of oxaloacetate from mitochondria to cytosol required in gluconeogenesis from l-lactate.

Gluconeogenesis in the kidney cortex. Flow of malate between compartments

1. Kidney-cortex slices from starved rats were incubated with l-[U-(14)C]lactate or l-[U-(14)C]malate plus unlabelled acetate and the specific radioactivity of the glucose formed was determined. In parallel experiments the specific radioactivity of the glucose formed from [1-(14)C]acetate plus unlabelled l-lactate and l-malate was determined. 2. By analytical methods the major products formed from the substrates were measured. The glucose formed was purified by paper chromatography for determination of specific radioactivity. 3. The specific radioactivity of the glucose formed from l-[U-(14)C]lactate agrees with predictions of a model based on interaction of the gluconeogenic and the oxidative pathways. 4. The specific radioactivity of the glucose formed from l-[U-(14)C]malate agrees with the predicted value if rapid malate exchange between the cytosol and mitochondria is assumed. 5. The rate of malate exchange between compartments was estimated to be rapid and at least several times the rate of glucose formation. 6. The specific radioactivity of the glucose formed from [1-(14)C]acetate plus unlabelled l-lactate or l-malate agrees with the predictions from the model, again assuming rapid malate exchange between compartments. 7. Malate exchange between compartments together with reversible malate dehydrogenase activity in the mitochondria and cytosol also tends to equilibrate isotopically the NADH pool in these compartments. (3)H from compounds such as l-[2-(3)H]lactate, which form NAD(3)H in the cytosol, appears in part in water; and (3)H from dl-beta-hydroxy[3-(3)H]butyrate, which forms NAD(3)H in the mitochondria, appears in part in glucose, largely on C-4.

Regulation of gluconeogenesis and lipogenesis. The regulation of mitochondrial pyruvate metabolism in guinea-pig liver synthesizing precursors for gluconeogenesis

1. The carboxylation of pyruvate to oxaloacetate by pyruvate carboxylase in guinea-pig liver mitochondria was determined by measuring the amount of (14)C from H(14)CO(3) (-) fixed into organic acids in the presence of pyruvate, ATP, Mg(2+) and P(i). The main products of pyruvate carboxylation were malate, fumarate and citrate. Pyruvate utilization, metabolite formation and incorporation of (14)C from H(14)CO(3) (-) into these metabolites in the presence and the absence of ATP were examined. The synthesis of phosphoenolpyruvate from pyruvate and bicarbonate is minimal during continued oxidation of pyruvate. Larger amounts of phosphoenolpyruvate are formed from alpha-oxoglutarate than from pyruvate. Addition of glutamate, alpha-oxoglutarate or fumarate did not appreciably increase formation of phosphoenolpyruvate when pyruvate was used as substrate. With alpha-oxoglutarate as substrate addition of fumarate resulted in increased formation of phosphoenolpyruvate, whereas addition of succinate inhibited phosphoenolpyruvate formation. In the presence of added oxaloacetate guinea-pig liver mitochondria synthesized phosphoenolpyruvate in amount sufficiently high to play an appreciable role in gluconeogenesis. 2. Addition of fatty acids of increasing carbon chain length caused a strong inhibition of pyruvate oxidation and phosphoenolpyruvate formation, and greatly promoted carbon dioxide fixation and malate, citrate and acetoacetate accumulation. The incorporation of (14)C from H(14)CO(3) (-), [1-(14)C]pyruvate and [2-(14)C]pyruvate into organic acids formed was examined. 3. It is concluded that guinea-pig liver pyruvate carboxylase contributes significantly to gluconeogenesis and that fatty acids and metabolites play an important role in its regulation.

The redistribution of carbon label by the reactions involved in glycolysis, gluconeogenesis and the tricarboxylic acid cycle in rat liver

A scheme is presented that shows how the reactions involved in gluconeogenesis, glycolysis and the tricarboxylic acid cycle are linked in rat liver. Equations are developed that show how label is redistributed in aspartate, glutamate and phosphopyruvate when it is introduced as specifically labelled pyruvate or glucose either at a constant rate (steady-state theory) or at a variable rate (non-steady-state theory). For steady-state theory the fractions of label introduced as specifically labelled pyruvate that are incorporated into glucose and carbon dioxide are also given, and for both theories the specific radioactivities of aspartate and glutamate relative to the specific radioactivity of the substrate. The theories allow for entry of label into the tricarboxylic acid cycle via both oxaloacetate and acetyl-CoA, for (14)CO(2) fixation and for loss of label from the tricarboxylic acid cycle in glutamate, but not for losses in citrate. They also allow for incomplete symmetrization of label in oxaloacetate due to incomplete equilibration with fumarate both in the extramitochondrial part of the cell and in the mitochondrion on entry of oxaloacetate into the tricarboxylic acid cycle. In the latter case failure both of oxaloacetate to equilibrate with malate and of malate to equilibrate with fumarate are considered.

Effects of guanidine derivatives on mitochondrial function. II. Reversal of guanidine-derivative inhibiton by free fatty acids

Long chain free fatty acids interfere with the inhibitory action of phenethylbiguanide and related compounds on mitochondrial respiration in vitro. This interference depends on binding of fatty acids to mitochondria and diminishes with decreasing chain length. Reversal of guanidine-derivative inhibition by fatty acids differs from that caused by dinitrophenol in that the effect of fatty acid is achieved without alteration in coupling or respiratory control. The binding of phenethylbiguanide to mitochondria is inhibited by both fatty acid and dinitrophenol. Serum albumin potentiates the inhibitory potency of guanidine derivatives, probably by removing endogenous mitochondrial free fatty acids.

Changes in activity of some enzymes involved in glucose utilization and formation in developing rat liver

1. The activities of some enzymes involved in both the utilization of glucose (pyruvate kinase, ATP citrate lyase, NADP-specific malate dehydrogenase, glucose 6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase and NADP-specific isocitrate dehydrogenase, all present in the supernatant fraction of liver homogenates) and the formation of glucose by gluconeogenesis (glucose 6-phosphatase in the whole homogenate and fructose 1,6-diphosphatase, phosphopyruvate carboxylase, NAD-specific malate dehydrogenase and fumarase in the supernatant fraction) have been determined in rat liver around birth and in the postnatal period until the end of weaning. 2. The activities of those enzymes involved in the conversion of glucose into lipid are low during the neonatal period and increase with weaning. NADP-specific malate dehydrogenase first appears and develops at the beginning of the weaning period. 3. The marked increase in cytoplasmic phosphopyruvate carboxylase activity at birth is probably the major factor initiating gluconeogenesis at that time. 4. The results are discussed against the known changes in dietary supplies and the known metabolic patterns during the period of development.

Gluconeogenesis from amino acids in neonatal rat liver

1. The utilization of amino acids for gluconeogenesis by rat liver develops in postnatal life, reaching maximum activity at the fifth day. 2. The activity of aspartate transaminase shows a similar trend in postnatal development and the increased activity appears to be due to the soluble enzyme. 3. The activity of alanine transaminase is low in foetal and postnatal rat liver and increases in activity at about the twentieth day. 4. Aspartate, glutamate and alanine make a major contribution to gluconeogenesis in the postnatal rat liver.

A continuous recording technique for the measurement of carbon dioxide, and its application to mitochondrial oxidation and decarboxylation reactions

1. A technique is described for continuously recording the concentration of carbon dioxide in mitochondrial suspensions. 2. The oxidation of palmitoylcarnitine by rat-liver mitochondria inhibits the oxidation of pyruvate and isocitrate, and stimulates the carboxylation of pyruvate. 3. These effects of palmitoylcarnitine oxidation are reversed by the uncoupling agent pentachlorophenol. 4. The effects of palmitoylcarnitine oxidation and pyruvate oxidation on the acylation of mitochondrial coenzyme A and reduction of nicotinamide nucleotides were measured. 5. Control mechanisms are discussed for the interactions between palmitoylcarnitine oxidation and the tricarboxylic acid cycle in rat-liver mitochondria.

Generation of extramitochondrial reducing power in gluconeogenesis

1. Kidney-cortex slices incubated with pyruvate formed glucose and lactate in relatively large and approximately equimolar quantities. The formation of these products involves two exclusively cytoplasmic NADH(2)-requiring reductions, catalysed by lactate dehydrogenase and triose phosphate dehydrogenase. From the rates of glucose and lactate formation it can be calculated that over 1000mu-moles of NADH(2) must have been produced in the cytoplasm/g. dry wt. of tissue/hr. 2. When lactate is a gluconeogenic precursor the required NADH(2) is generated in the cytoplasm, but, when a substrate more highly oxidized than glucose, such as pyruvate, is the precursor, there is no direct cytoplasmic source of NADH(2). Quantitative data on the fate of pyruvate are in accord with the conclusion that the NADH(2) was primarily formed intramitochondrially by the dehydrogenases of cell respiration, with pyruvate as the major substrate. 3. Similar observations and conclusions apply to experiments with mouse-liver slices incubated with pyruvate, serine or aspartate. 4. Addition of ethanol, which increases the formation of NADH(2) in the cytoplasm, increased the formation from pyruvate of lactate but not of glucose. 5. In view of the low permeability of mitochondria for NAD and NADH(2) it must be postulated that special carrier mechanisms transfer the reducing equivalents of intramitochondrially generated NADH(2) to the cytoplasm. Reasons are given in support of the assumption that the malate-oxaloacetate system acts as the carrier. 6. Various aspects of the generation of reducing power and its transfer from mitochondria to cytoplasm are discussed.