Glycolysis and its interaction with a gluconeogenic precursor in perfused rat liver

A non-recirculating, haemoglobin-free perfusion system was used to evaluate glycolysis in livers from fasted rats. Glycolysis was calculated from lactate and pyruvate production before and after infusion of glucose. In the absence of extracellular glucose glycolysis was small. Stimulation of glycolytic flux occured at extracellular glucose concentrations above 5 mM. Rates were proportional to glucose concentrations. Glycolysis was also investigated in the presence of extracellular lactate, which is a precursor of gluconeogenesis. High glucose concentrations (15-35 mM) were followed by lactate and pyruvate output suggesting that glycolysis was prevailing. Estimated flux rates, however, were significantly lower in the presence of the gluconeogenic precursor. Our results provide arguments that gluconeogenesis and glycolysis interact in order to regulate the peripheral glucose concentration.

Energy metabolism of isolated rat thymus cells

The energy metabolism of rat thymus cells has been investigated using preparations of isolated cells obtained by mechanical treatment of whole organs. The addition of glycolytic substrates such as glucose, pyruvate and lactate stimulates the endogenous respiration of these cells by 50%. On the other hand, succinate, glutamate and malate do not produce any effect. Oligomycin (10 mug/ml) inhibits both endogenous and glucose stimulated respiration by about 40%; 2, 4-DNP (50 muM) increases by 100% glucose induced respiration. The results obtained by using mitochondrial and glycolytic inhibitors as well as aminoxyacetic acid (AOA) and following pyridine nucleotides redox changes, support the idea that in thymus cells glucose is able to induce a great enhancement of O2 consumption both by raising the level of endogenous pyruvate and feeding the mitochondrial respiratory chain with cytosolic reducing equivalents, through an active malate-aspartate shuttle. Thymus cells exhibit a high Pasteur effect (74%). Both AOA and 2,4 DNP are able to stimulate aerobic lactate accumulation by 200% and 100% respectively, indicating that either the redox or phosphate potential do influence the rate of aerobic glycolysis in isolated thymus cells. Similar experiments are also reported on other cells with well known biochemical characteristics.

Alpha-Glycerophosphate dehydrogenase (insoluble) and lactic dehydrogenase activities in the skeletal muscles of two insects

The flight muscle preparations of the dragonfly Pantala flavescens and the aquatic beetle Cybister confusus showed extremely low levels of lactic dehydrogenase activity and high levels of alpha-glycerophosphate dehydrogenase (insoluble) activity. The activities of these two enzymes in the leg muscle of the beetle were approximately the same (1:1), but lactic dehydrogenase activity was several times higher than that in the flight muscles of both Insects. These results have been interpreted as indicating the high energy-yielding demands of the flight muscles during continuous sustained activity, while the leg muscles of the beetle which are involved in swimming activity derive their energy predominantly through anaerobic glycolysis.

Gylcerol-3-phosphate shuttle and its function in intermediary metabolism of hamster brown-adipose tissue

1. Brown adipose tissue of the hamster possesses high specific activities of soluble, cytoplasmic NAD-linked, as well as mitochondrial flavin-coupled, glycerol-3-phosphate dehydrogenases. The ratio of the two enzyme activities is high (close to 1), when compared with other tissues of the hamster. 2. In the presence of rotenone, NADH is oxidised very poorly by homogenates of brown adipose tissue. A high rate of oxidation is obtained upon further addition of dihydroxyacetone phosphate, which itself is negligible oxidised. When followed fluorimetrically glycerol 3-phosphate can also be observed to induce NADH oxidation, but only after a significant lag time. Similar results are obtained with isolated mitochondria plus high-speed supernatant. With high-speed supernatant alone, only dihydroxyacetone phosphate has any effect, whereas with isolated mitochondria neither dihydroxyacetone phosphate nor glycerol 3-phosphate induce any NADH disappearance. 3. Respiration induced by NADH plus dihydroxyacetone phosphate in homogenates equals 56% of the respiration induced by glycerol 3-phosphate alone. 4. Respiration induced by NADH plus dihydroxyacetone phosphate, as well as that induced by glycerol 3-phosphate, is inhibited by the same concentrations of inhibitors as are required for inhibition of the mitochondrial dehydrogenase i.e. EDTA, long-chain unsaturated fatty acids, long-chain fatty acyl CoA esters. 5. In isolated brown adipocytes in the presence of rotenone, norepinephrine significantly inhibits respiration induced by glycerol 3-phosphate. 6. The results obtained are discussed with respect to the role of glycerol 3-phosphate as an electron sink for cytosolic reducing equivalents to maintain a low level of extramitochondrial NADH. A means of maintaining a level of glycerol 3-phosphate adequate for triglyceride synthesis is also considered.

Mitochondrial oxygen affinity as a function of redox and phosphate potentials

1. The conditions under which mitochondria might catalyse a net reversal of oxidative phosphorylation are analysed. 2. Rat-liver mitochondria, incubated under such conditions, show a strongly diminished affinity for oxygen. 3. The velocity of respiration under these conditions is a hyperbolic function of the oxygen concentration. 4. The K-m for oxygen is less than 0.1 muM at low phosphate potential, irrespective of substrate, and 1-3 muM under reversal conditions. 5. The observed kinetics can be accounted for in a simple mechanism for cytochrome oxidase action.

Changes in enzyme pattern of Ehrlich ascites tumor cells following serial cultivation in media with increased (hypertonic) NaCl content

Adaptation of Ehrlich ascites tumor cells to serial cultivation in media with progressively elevated (hypertonic) NaCl content ("high NaCl"-tolerant cells) has resulted in progressive increases of the cellular activities of NAD-dependent glycerol-3-phosphate dehydrogenase (EC 1.1.1.8), NAD-dependent malate dehydrogenase (EC 1.1.1.37), glutamate--oxalacetate transaminase (EC 2.6.1.1), NAD (P)-dependent glutamate dehydrogenase (EC 1.4.1.3), NADP-dependent isocitrate dehydrogenase (EC 1.1.1.42). The activities of glutamate-pyruvate transaminase (EC 2.6.1.2.) and of glycolytic enzymes as phospho-fructokinase (EC 2.7.1.11), glyceraldehydephosphate dehydrogenase (EC 1.2.1.12) and lactate dehydrogenase (EC 1.1.1.27) were only slightly and not in progressive manner (in response to the progressive increase of the environmental NaCl concentration) affected. These changes are discussed with respect to a metabolic pattern of these "high NaCl"-tolerant cells which is compatible with increased energy requirements, especially for active cation transport. It is suggested that these increased cellular enzyme activities reflect an increased transfer of reducing equivalents across mitochondrial membranes (via the "glycerophosphate cycle and the malate-aspartate shuttle") and possibly a stimulated lipid metabolism. These alterations in the level of enzyme activities must be regarded asan adaptive cellular response to the "high NaCl" environment, since readaptation to growth in regular isotonic media resulted in a reversion to the enzyme pattern characteristic of the parent cells.

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.

Pathways of ethanol metabolism in perfused rat liver

The primary pathway of hepatic ethanol metabolism involves alcohol dehydrogenase. Hydrogen generated from ethanol metabolism enters the mitochondrial space most likely as malate over a substrate shuttle mechanism, and is subsequently oxidized by the mitochondrial respiratory chain. The rate-limiting step in this overall multicompartmental process is the rate of reduced cofactor (NADH) reoxidation by the respiratory chain. Since the electron flux in the respiratory chain is controlled by the ADP supply, alcohol dehydrogenase-dependent ethanol metabolism can be activated by perturbations which circumvent the rate-limiting step, such as artificial electron acceptors, gluconeogenic precursors, and uncoupling agents. Moreover, an ATP utilizing process is responsible for the stimulation of ethanol metabilism observed following chronic pretreatment with ethanol. In perfused rat liver catalase also participates in ethanol metabolism to a lesser extent than alcohol dehydrogenase. Quantitative assessments indicate that the predominant ethanol oxidase at low ethanol concentrations (less than 20 mM) is a alcohol dehydrogenase; however, at higher ethanol concentrations, a significant portion of total ethanol metabolism (up to 50%) is mediated by catalase-hydrogen peroxide complex. This pathway is limited by the rate of generation of hydrogen peroxide in the hepatocyte, and can be stimulated with substrates for intraperoxisomal hydrogen peroxide generation such as glycolate, urate and D-amino acids. Considerable evidence implicates catalase-hydrogen peroxide complex in the mechanism of NADPH-dependent microsomal ethanol oxidation.

Changes in the activity of citrate lyase, malic enzyme and acetyl-CoA synthetase in rat liver after short-term and long-term feeding with ethanol

1. The effect of short-term and long-term feeding (0–80 d) with a liquid diet containing ethanol on the activity of rat hepatic enzymes related to lipogenesis has been evaluated. Carbohydrates were isoenergetically substituted for ethanol in the control animals.

2. The maximum concentration of triglycerides in the livers was reached after about 30 d, when it was almost three times as high as in the control animals. The activity of malic enzyme (EC1·1·1·40) and ATP citrate lyase (EC4·1·3·8) decreased significantly in the ethanol group, compared with the control rats, within 10 d and remained low during the rest of the experiment (80 d). After 20 d, the acetyl-CoA synthetase (EC6·2·1·1) activity increased significantly in the livers of the ethanol-fed rats but fell subsequently to values similar to those in the livers of the control rats. Thus, despite a pronounced increase in the amount of triglyceride in the livers of rats on a liquid diet containing ethanol, there was a dramatic decrease in the activity of the enzymes (malic enzyme and citrate lyase) involved in lipogenesis.

3. The almost unchanged activity of acetyl-CoA synthetase shows that the utilization of acetate, produced when ethanol is oxidized, is not stimulated by long-term feeding with ethanol. The involvement of citrate lyase in various postulated shuttles for the transport of reducing equivalents across the mitochondrial membrane and the role of malic enzyme in the microsomal ethanol-oxidizing system are discussed.