Validity of the digitonin method for metabolite compartmentation in isolated hepatocytes

ATP Redux

The renaissance of interest in metabolism has focused mostly on techniques to measure massive numbers of metabolites. Yet, established metabolic theory has been abandoned. Here, I highlight one misconception: how ATP (and ADP and AMP) are currently understood. A critical point was established early on: cellular [ATP] is constant.

Transthyretin Suppresses Amyloid-β Secretion by Interfering with Processing of the Amyloid-β Protein Precursor

In Alzheimer's disease (AD), most hippocampal and cortical neurons show increased staining with anti-transthyretin (TTR) antibodies. Genetically programmed overexpression of wild type human TTR suppressed the neuropathologic and behavioral abnormalities in APP23 AD model mice and TTR-Aβ complexes have been isolated from some human AD brains and those of APP23 transgenic mice. In the present study, in vitro NMR analysis showed interaction between the hydrophobic thyroxine binding pocket of TTR and the cytoplasmic loop of the C99 fragment released by β-secretase cleavage of AβPP, with Kd = 86±9 μM. In cultured cells expressing both proteins, the interaction reduced phosphorylation of C99 (at T668) and suppressed its cleavage by γ-secretase, significantly decreasing Aβ secretion. Coupled with its previously demonstrated capacity to inhibit Aβ aggregation (with the resultant cytotoxicity in tissue culture) and its regulation by HSF1, these findings indicate that TTR can behave as a stress responsive multimodal suppressor of AD pathogenesis.

Sites of superoxide and hydrogen peroxide production by muscle mitochondria assessed ex vivo under conditions mimicking rest and exercise

The sites and rates of mitochondrial production of superoxide and H2O2 in vivo are not yet defined. At least 10 different mitochondrial sites can generate these species. Each site has a different maximum capacity (e.g. the outer quinol site in complex III (site IIIQo) has a very high capacity in rat skeletal muscle mitochondria, whereas the flavin site in complex I (site IF) has a very low capacity). The maximum capacities can greatly exceed the actual rates observed in the absence of electron transport chain inhibitors, so maximum capacities are a poor guide to actual rates. Here, we use new approaches to measure the rates at which different mitochondrial sites produce superoxide/H2O2 using isolated muscle mitochondria incubated in media mimicking the cytoplasmic substrate and effector mix of skeletal muscle during rest and exercise. We find that four or five sites dominate during rest in this ex vivo system. Remarkably, the quinol site in complex I (site IQ) and the flavin site in complex II (site IIF) each account for about a quarter of the total measured rate of H2O2 production. Site IF, site IIIQo, and perhaps site EF in the β-oxidation pathway account for most of the remainder. Under conditions mimicking mild and intense aerobic exercise, total production is much less, and the low capacity site IF dominates. These results give novel insights into which mitochondrial sites may produce superoxide/H2O2 in vivo.

Respiration characteristics of mitochondria in parental and giant transformed cells of the murine Nemeth-Kellner lymphoma

Respiration characteristics of mitochondria of the parental and giant cells of murine NK/Ly (Nemeth-Kellner lymphoma) were studied. The giant cell-enriched ascites were obtained by serial intraperitoneal injections of vinblastine in tumour-bearing mice. Ascites containing >70% giant cells were used. Their diameter of was over 17 μm (~2800 μm(3)), while the diameter of the parental cells was 12.7 μm (1100 μm(3)). The respiration rate of mitochondria in situ was measured by oxygen consumption in intact and digitonin-permeabilized NK/Ly cells. Endogenous respiration of intact giant NK/Ly cells was three times higher compared to the parental ones, roughly in agreement with the volume change. The giant NK/Ly cells were far more resistant to permeabilization with digitonin than the parental cells, as shown by Trypan Blue and LDH (lactate dehydrogenase) release tests. After digitonin permeabilization, oxygen consumption was reduced to a minimal level (0.06 ng atom O/(s × 106 cells) in both types of cells. Addition of α-ketoglutarate or succinate to the incubation medium increased oxygen consumption in the parental cells by 46 and 164% respectively. In the giant NK/Ly cells, the corresponding increases were 164 and 276%. Addition of ADP to α-ketoglutarate- or succinate-supplemented medium further stimulated oxygen consumption of the permeabilized NK/Ly cells; however, the effect of ADP was more pronounced in the giant cells. In addition, indices of respiratory control were significantly higher in the giant cells. Oligomycin suppressed considerably the respiration of the intact giant cells but had a much weaker effect on parental cells. Thus, giant NK/Ly cells possess much higher respiration rates and show tighter coupling between the respiration and oxidative phosphorylation compared with parental cells.

Inhibition of acetyl CoA carboxylase by GTP gamma S

The effect of nonhydrolyzable guanine nucleotides on mammalian acetyl CoA carboxylase (ACC) activity was examined. Using porous rat adipocytes and crude fat cell homogenates to study metabolic pathway flux, GMPPNP and/or GTP gamma S inhibited [14C]fatty acid formation by up to 95% when either [6-14C]glucose-6-phosphate or [1-14C]acetyl CoA was used as substrate. If [2-14C]malonyl CoA initiated flux, however, no inhibition was apparent. These pathway flux studies suggested that ACC was the locus of inhibition, and that the mechanism might involve a disruption of guanine nucleotide hydrolysis by the nonhydrolyzable analogues. Using partially and avidin-sepharose-purified ACC preparations from rat fat, liver and mammary tissue, citrate-stimulated ACC activity was inhibited by 25-75% with 50 microM GTP gamma S. Related compounds and nucleotides had absent-to-minimal effects on ACC. ATP gamma S was inhibitory (10-30% at 5-15 microM), but always to a lesser degree than equimolar GTP gamma S. Filter binding assays with [alpha-32P]GTP or [35S]GTP gamma S were negative, but low-level GTPase activity was detected. Using photoaffinity labelling techniques, [alpha-32P]GTP was found to bind ACC and not pyruvate carboxylase. The hypothesis that citrate-responsive ACC activity may be modulated by an intrinsic or associated GTP binding site is explored. Since ACC forms polymers, as does the cytoskeletal protein beta-tubulin, amino acid sequence comparisons between ACC and atypical GTP binding domain of beta tubulin are presented.

Labeling of mitochondrial proteins in living cells by the thiol probe thiobutyltriphenylphosphonium bromide

Alterations to the redox state of mitochondrial thiols is a particularly important response to oxidative stress, because mitochondria are a major source of reactive oxygen species within cells. To investigate these changes we designed and synthesized a novel probe for mitochondrial thiols, thiobutyltriphenylphosphonium bromide [TBTP; R. J. Burns, R. A. J. Smith, and M. P. Murphy (1995) Arch. Biochem. Biophys. 322, 60-68]. This lipophilic cation was accumulated several hundred-fold into the negatively charged matrix of isolated mitochondria, where it equilibrated with endogenous thiols and during oxidative stress formed disulfide bonds to exposed protein thiols. In this paper we show that TBTP can also selectively react with mitochondrial protein thiols in living cells. Since TBTP localizes specifically to mitochondria and forms disulfide bonds selectively with mitochondrial proteins during oxidative stress, we conclude that TBTP has utility for investigating changes in mitochondrial thiols, independently of alterations occurring in the cytoplasm.

Interrelation between mitochondrial respiration, substrate supply and redox ratio in perifused permeabilized rat hepatocytes

A one-step perifusion technique is described for studying the regulation of energy metabolism in intact hepatocytes and in mitochondria of the same cells after their permeabilization by digitonin. Cell count and activities of glutamate dehydrogenase, the latter being used as an indicator of mitochondrial integrity, were found to be nearly unchanged after permeabilization and perifusion for at least 40 min at 37 degrees C. The residual activity of lactate dehydrogenase after permeabilized indicated that permeabilized cells were almost depleted of soluble cytosolic components. The composition of the perifusion medium was chosen so that various metabolic states could be adjusted of both intact and permeabilized hepatocytes without the need to change the perfusion medium. Oxidative phosphorylation of mitochondria within permeabilized hepatocytes remained intact throughout the perifusion as indicated by the response of respiration to the addition of ADP, carboxyatractyloside and uncoupler. The application of the perifusion technique allows us to sample indicator metabolites in the effluent medium like acetoacetate (AcAc) and 3-hydroxybutyrate (HB) for calculating the mitochondrial redox ratios and rates of ketogenesis. In the presence of octanoate and ADP, an improvement of substrate supply by glutamate and malate led to increases in the intramitochondrial HB/AcAc ratio and the respiration rate. Glutamate/malate concentrations of 1 mM resulted in maximal respiration rates, whereas concentrations of 5 mM further enhanced the HB/AcAc ratio. Mitochondria responded to increasing ATP/ADP ratios in the perifusion medium by decreased respiration rates at higher HB/AcAc ratios. By comparing respiration rates and redox ratios of mitochondria in permeabilized cells with those before permeabilization (gluconeogenic conditions of hepatocytes), it is concluded that in the intact cell oxidative phosphorylation is limited with respect to substrate supply as well as by the ATP demand.

Effect of mitochondrial protein concentration on the efficiency of outer membrane removal by the cholesterol-selective detergent digitonin

The effects of different mitochondrial protein concentrations on the efficiency of digitonin titration of the outer mitochondrial membrane were investigated in isolated rat liver mitochondria. Isolated mitochondria were subjected to treatment with digitonin concentrations ranging from 0 to 0.40 mg digitonin per mg protein. This digitonin concentration range was used in incubations containing 5 to 50 mg mitochondrial protein per ml. Significant differences in the efficiency of outer membrane removal by digitonin titration were noted at protein concentrations of less than 20 mg per ml. Estimation of the effective concentration of digitonin required to remove 50% of the outer membrane indicated that in general, as the protein concentration decreases, the amount of digitonin required to remove the outer membrane increases. Significant differences were also noted in the amount of digitonin needed for removal of 95% of the outer membrane between 5, 10 and 20 mg/ml with the effect of protein concentration disappearing above 20 mg/ml. No effect of protein concentration was found on the disruption of the inner membrane by digitonin as judged by leakage of matrix marker enzyme activity and by release of inner membrane marker enzyme activity. The conclusions of these studies indicate that at relatively low mitochondrial protein concentrations (< 20 mg/ml), the efficiency of digitonin in removing the outer membrane is substantially reduced.

Effect of treatment in vivo of rats with bacterial endotoxin on fructose 2,6-bisphosphate metabolism and L-pyruvate kinase activity and flux in isolated liver cells

The effect of treatment of rats with bacterial endotoxin on fructose 2,6-bisphosphate (Fru-2,6-P2) metabolism was investigated in isolated liver cells prepared from 18 h-starved animals. The results obtained support the hypothesis that a stimulation of 6-phosphofructo-1-kinase (PFK-1) activity and an inhibition of fructose-1,6-bisphosphatase (Fru-1,6-P2ase) may be one mechanism underlying the inhibition of gluconeogenesis from lactate and pyruvate by endotoxin. We suggest that the stimulation of PFK-1 and inhibition of Fru-1,6-P2ase activity is the result of a 2-3-fold increase in Fru-2,6-P2. The latter is not due to changes in the total activity or phosphorylation state of the bifunctional 6-phosphofructo-2-kinase (PFK-2)/fructose-2,6-bisphosphatase, but appears to be the result of a decrease in the cytosolic concentration of phosphoenolpyruvate (PEP), an inhibitor of PFK-2 activity. The effect of endotoxin is resistant to the presence of glucagon, which has comparable effects in cells prepared from both control and endotoxin-treated animals. The mechanism by which endotoxin treatment of the rat decreases PEP and gluconeogenesis remains to be established. However, it does not involve alterations in either the total activity or the phosphorylation state of pyruvate kinase, nor does it involve increased flux through this enzyme in the intact cell, which is in fact decreased in this model of septic shock. It is suggested that the decreased flux may result from a lower rate of formation of PEP, suggesting that the prime lesion in sepsis is an inhibition of one or more of the steps leading to PEP formation.

The effects of ethanol concentration on glycero-3-phosphate accumulation in the perfused rat liver. A reassessment of ethanol-induced inhibition of glycolysis using 31P-NMR spectroscopy and HPLC

The dose-dependent effect of ethanol on the hepatic metabolism of the perfused rat liver has been investigated by (a) 31P-NMR spectroscopy for the follow-up of intracellular phosphorylated metabolites and (b) HPLC for compounds released in the effluents. Perfusion of livers from fed rats with ethanol induced an increase in the level of sn-glycerol 3-phosphate and net accumulations of 3.30 +/- 0.33 and 0.69 +/- 0.15 mumol x g-1 wet liver were reached after 20 min, for 70 mM and 0.5 mM ethanol, respectively. sn-Glycerol-3-phosphate accumulation was fully detected by 31P NMR as indicated by comparing quantitations based on NMR and biochemical assays. Ethanol administration up to a concentration of 10 mM induced a dose-dependent decrease in the release of lactate + pyruvate by the liver. Lactate release decreased from 1129 +/- 39 to 674 +/- 84 nmol x min-1 x g-1, while pyruvate decreased from 230 +/- 9 to 6.2 +/- 0.4 nmol x min-1 x g-1, after 20 min of perfusion with 10 mM ethanol. Nevertheless, the flux through 6-phosphofructo-1-kinase, as measured by both the accumulation of sn-glycerol 3-phosphate and release of lactate + pyruvate, was not affected in the early phase of ethanol oxidation. Finally, data obtained from oxygen consumption, the release of acetate and the accumulation of sn-glycerol 3-phosphate do not support the involvement of the microsomal ethanol-oxidizing system in the catalysis of ethanol oxidation, even at high doses of alcohol.

Metabolism of adenosine through adenosine kinase inhibits gluconeogenesis in isolated rat hepatocytes

In isolated hepatocytes from fasted rats, 0.5 mM adenosine inhibited gluconeogenesis from glutamine, lactate and pyruvate. This inhibition was due to adenosine conversion through adenosine kinase. An increase in ketone body release was only observed in the presence of lactate or pyruvate, and the two phenomena (i.e. inhibition of gluconeogenesis and increased ketone-body release) were linked. With alanine, dihydroxyacetone or serine as substrates, adenosine did not change gluconeogenesis; however, its conversion through adenosine kinase also inhibited gluconeogenesis. With asparagine as substrate, 0.5 mM adenosine increased gluconeogenesis; this increase was due to adenosine conversion through adenosine deaminase. However, adenosine conversion through adenosine kinase inhibited gluconeogenesis from asparagine. Thus, whatever the substrate used, adenosine conversion through adenosine kinase inhibited gluconeogenesis. The inhibitory effect of adenosine on gluconeogenesis cannot be related to the decrease in Pi concentration and to the increase in ATP pool. Beside its effect on gluconeogenesis, adenosine inhibited ketogenesis measured without added substrate; adenosine conversion through adenosine kinase was also involved in the inhibition of ketogenesis.

Ca2+-dependent activation of the malate-aspartate shuttle by norepinephrine and vasopressin in perfused rat liver

The role of Ca2+ in stimulation of the malate-aspartate shuttle by norepinephrine and vasopressin was studied in perfused rat liver. Shuttle capacity was indexed by measuring the changes in both the rate of production of glucose from sorbitol and the ratio of lactate to pyruvate during the oxidation of ethanol. (T. Sugano et al. (1986) Amer. J. Physiol. 251, E385-E392). Asparagine (0.5 mM), but not alanine (0.5 mM) decreased the ethanol-induced responses. Norepinephrine and vasopressin had no effect on the ethanol-induced responses when the liver was perfused with sorbitol or glycerol. In the presence of 0.25 mM alanine, norepinephrine, vasopressin, and A23187 decreased the ethanol-induced responses that occurred with the increase of flux of Ca2+. In liver perfused with Ca2+-free medium, asparagine also decreased the ethanol-induced responses, but norepinephrine and vasopressin had no effect. Aminooxyacetate inhibited the effects of norepinephrine, A23187, and asparagine. Regardless of the presence or absence of perfusate Ca2+, the combination of glucagon and alanine had no effect on the ethanol-induced responses. Norepinephrine caused a decrease in levels of alpha-ketoglutarate, aspartate, and glutamate in hepatocytes incubated with Ca2+. The present data suggest that the redistribution of cellular Ca2+ may activate the efflux of aspartate from mitochondria in rat liver, resulting in an increase in the capacity of the malate-aspartate shuttle.

The mechanism by which adenosine decreases gluconeogenesis from lactate in isolated rat hepatocytes

Incubation of hepatocytes from 24 h-starved rats in the presence of 0.5 mM-adenosine decreased gluconeogenesis from lactate, but not from alanine. The inhibition of gluconeogenesis was associated with a stimulation of ketone-body production and an inhibition of pyruvate oxidation. These metabolic changes were suppressed in the presence of iodotubercidin (an inhibitor of adenosine kinase), but were reinforced in the presence of deoxycoformycin (an inhibitor of adenosine deaminase); 2-chloroadenosine induced no change in gluconeogenesis from lactate. These data indicate that the inhibition of gluconeogenesis by adenosine probably results from its conversion into adenine nucleotides. In the presence of lactate or pyruvate, but not with alanine or asparagine, this conversion resulted in a decrease in the [ATP]/[ADP] ratio in both mitochondrial and cytosolic compartments. Adenosine decreased the Pi concentration with all gluconeogenic substrates.

Metabolic state of the rat liver with ethanol: comparison of in vivo 31phosphorus nuclear magnetic resonance spectroscopy with freeze clamp assessment

In vivo 31phosphorus nuclear magnetic resonance spectroscopy was used to measure the hepatic metabolic state in various groups of rats given ethanol, a control liquid diet or a solid chow diet. The use of selective presaturation pulses applied to the broad phosphorus resonances of immobile phospholipids permitted reliable determination of ATP/ADP ratios by quantitation of the ATP-beta and ATP-gamma peak areas. ATP/ADP ratios were depressed by both techniques in rats chronically ingesting ethanol compared to pair-fed animals consuming the control liquid diet. These differences were observed regardless of whether ethanol feeding was continued up to the time of investigation or whether it was discontinued for 24 hr prior to study. Acute alcohol administration in chow-fed rats, not previously ingesting ethanol, did not lower hepatic ATP/ADP ratios by either methodology. In all cases, liver ATP/ADP ratios assessed by 31phosphorus nuclear magnetic resonance spectroscopy were higher than those measured by high-performance liquid chromatography. However, parallel decreases in hepatic ATP/ADP ratios were observed with chronic ethanol consumption by both 31phosphorus nuclear magnetic resonance spectroscopy and the biochemical method, confirming the utility of in vivo 31phosphorus nuclear magnetic resonance spectroscopy for assessment of the hepatic bioenergetic status. The difference in absolute ATP/ADP ratios by the two methods may to some degree be explained by binding effects of ADP with proteins or mitochondrial membranes, rendering it partially invisible to nuclear magnetic resonance or alternatively, by breakdown of high energy phosphate bonds with freeze clamp extraction.

Effects of alanine on malate-aspartate shuttle in perfused livers from cold-exposed rats

The role of glucocorticoids in the increase by cold-exposure of the effect of alanine on the malate-aspartate shuttle was studied in perfused rat liver. The capacity of the shuttle was evaluated by measurement of changes in both the rate of glucose production from sorbitol and the ratio of lactate to pyruvate during ethanol oxidation (Biomed. Res. 6, Suppl., 1986). The effect of alanine on the shuttle capacity was decreased by adrenalectomy. When 1.5 mg/kg dexamethasone sulfate was administrated to adrenalectomized rats kept at 24 or 4 degrees C, once daily for 5 days, the effect of alanine on the shuttle increased its capacity to the level of sham-operated rats that had been exposed to 4 degrees C for 5 days. The effects of dexamethasone were blocked by the coadministration of tetracycline with the agent. Cold exposure and steroid replacement had little effect on the alanine-induced elevation of the levels of aspartate, glutamate, and alpha-ketoglutarate in liver cells. The increase of the effect of alanine could not be explained only by changes in the activity of NAD+ malate dehydrogenase and aspartate aminotransferase. The results suggest that cold exposure and replacement treatment with glucocorticoids modulate equally the effect of alanine on the capacity of the malate-aspartate shuttle.

Expression of classical mitochondrial respiratory responses in homogenates of rat forebrain

Respiratory studies of brain mitochondria have, in general, been limited to purified preparations. Conventional procedures for mitochondrial isolation yield relatively small and potentially selected subfractions of mitochondria. Examination of respiratory responses of homogenates of rat forebrain indicated that key respiratory properties of mitochondria are fully expressed in these preparations. In a high K+ buffer, comparable to those commonly used for purified mitochondria, forebrain homogenates exhibited many of the characteristics of oxygen uptake by "free" mitochondria: requirement for both pyruvate and malate for maximal respiration, stimulation (over threefold) by ADP, stimulation by uncoupling agent [carbonyl cyanide m-chlorophenylhydrazone (CCCP)], but little effect of digitonin. In a modified Krebs-Ringer phosphate buffer (a physiological buffer), respiratory responses were primarily due to mitochondria enclosed in synaptosomes: respiration with glucose was markedly stimulated by CCCP, further stimulated by pyruvate, and extensively inhibited by digitonin (which disrupts the cholesterol-rich synaptosomal membranes). Studies with purified mitochondria and synaptosomes supported the specificity of these responses. These data indicate that classical mitochondrial responses are expressed in whole brain homogenates and, under appropriate conditions, provide functional measures of the total pools of free and synaptosomal mitochondria.

Formation of hexose 6-phosphates from lactate + pyruvate + glutamate by a cell-free system from rat liver

A cell-free system prepared from rat liver containing cytosol and mitochondria as well as a number of cofactors and gluconeogenic intermediates at near-physiological concentrations was shown to form hexose 6-phosphates linearly from lactate + pyruvate + glutamate at a rate of 0.82 +/- 0.05 mumol/min per g of liver (mean +/- S.E.M., n = 8, 37 degrees C). The indicated rates were measured between 20 min and 60 min incubation time, when the system was near steady state. Experiments with either [1-14C]lactate or [U-14C]glutamate revealed that the incorporation of radioactive label into hexose 6-phosphates was proportional to the utilization of lactate + pyruvate and of glutamate during incubation and that both served as gluconeogenic substrates at a ratio of about 2:1. When the [ATP]/[ADP] ratio was lowered from 60 to 19 by addition of ATPase, the rate of hexose 6-phosphate formation fell to one-third. This decrease in gluconeogenic flux was mainly due to a decreased flow through the phosphoglycerate kinase step. Hexose 6-phosphate formation could also be decreased by increasing the ratio [NADH]/[NAD+], either by addition of ethanol or by increasing the initial concentration of lactate + pyruvate at a fixed ratio of 10:1. The observed inhibition was linked to a limitation in the availability of oxaloacetate for the phosphoenolpyruvate carboxykinase reaction and to an increased formation of sn-glycerol 3-phosphate. Finally, the rates of hexose 6-phosphate formation in incubations with cytosols from fed rats were only 50% of those observed with cytosols from animals starved for 48 h. One of the limiting steps was found to be the flow through the phosphoenolpyruvate carboxykinase step.

Incorporation of the 1-pro-R and 1-pro-S hydrogen atoms of ethanol in the reduction of acids in the liver of intact rats and in isolated hepatocytes

Ethanol oxidation causes redox effects. The coupling of this oxidation via NADH to intermediary metabolism was studied in order to reveal the underlying mechanisms. Isolated rat hepatocytes were incubated with [1,1-2H2]-, (1R)-[1-2H]- and (1S)-[1-2H]-ethanol and the 2H incorporation was measured in lactate, beta-hydroxybutyrate, fumarate, malate, succinate, alpha-oxoglutarate and citrate. The results differed in the following ways from results obtained in intact rats. Lactate became labelled to an increasing extent, and in more than one position, indicating labelling of pyruvate. A small and constant fraction of malate and fumarate was formed without access to [2H]coenzyme. Addition of aspartate increased this fraction, which was concluded to be formed in the mitochondria. Citrate was essentially unlabelled. The 2H from (1R)-[1-2H]ethanol contributed to malate to a larger extent and to beta-hydroxybutyrate to a smaller extent, and 2H from (1S)-[1-2H]ethanol contributed to lactate to a smaller extent. These results indicate that the exchange via shuttle system was less efficient in isolated hepatocytes than in intact rats. The 2H incorporation was independent of concentration of [1,1-2H2]ethanol when this was above 5mM. Additions known to increase ethanol elimination, and cyanamide, which decreases it, had no marked effect on the 2H incorporation. This indicates equilibration of the NADH bound to alcohol dehydrogenase with free NADH. Disulfiram and cyanamide caused a decrease in the relative incorporation from (1S)-[1-2H]ethanol into malate in liver of intact rats. Addition of cyanamide to incubations with hepatocytes resulted in a decrease of the contribution of 2H from (1S)-[1-2H]ethanol in lactate, beta-hydroxybutyrate and malate. This indicates that acetaldehyde was only oxidized in the mitochondrial compartment.

Phosphorylation status of liver by 31P-n.m.r. spectroscopy, and its implications for metabolic control. A comparison of 31P-n.m.r. spectroscopy (in vivo and in vitro) with chemical and enzymic determinations of ATP, ADP and Pi

An investigation into the measurement of Pi and ADP in rat liver in vivo and in freeze-clamped extracts by 31P-n.m.r. spectroscopy was carried out. The concentration of Pi estimated in vivo is less than 25% [1 mM (mumol/ml of cell water)] of the value obtained from freeze-clamped liver (4 mM), whereas ADP in vivo is undetectable (1.4 mM in vitro). At 5 min after infusion of 750 mg of fructose/kg, the Pi content of liver extracts fell to 1.3 mM, whereas Pi is undetectable in vivo under these conditions [Griffiths, Stevens, Gadian, Iles & Porteous (1980) Biochem. Soc. Trans. 8, 641]. The results indicate that the lower Pi and ADP concentrations found in vivo may be due to compartmentation or binding rather than to degradation of labile organic phosphates during extraction. The results are discussed with reference to previous measurements of liver phosphates and investigations of compartmentation in the liver, as are some of the possible consequences for metabolic control in the liver of low ADP and Pi concentrations.

The mechanism of inhibition of ureogenesis by acidosis

In perfused rat liver a decrease of cytosol pH, determined with pH-sensitive microelectrodes, from 7.2 to 6.85 is associated with a 50% fall in ureogenesis from ammonium chloride. In isolated rat hepatocytes the fall in ureogenesis due to acidosis is associated with decrease in the mitochondrial and cytosolic concentration of citrulline. Limitation of carbamoyl phosphate synthesis and thus citrulline supply could be responsible for the inhibition of ureogenesis observed.

Possible role of Pi supply in mitochondrial actions of glucagon

Glucagon is able to diminish the net release of inorganic phosphate (Pi) occurring on incubation of isolated hepatocytes from 48-h-starved rats. Concomitantly the hormone increases the cellular Pi content. This is associated with a rise of Pi in the cytosolic fraction. Other hormonal effectors like phenylephrine, vasopressin and angiotensin II exert a smaller and transient effect as compared to glucagon. It is proposed that this increase in Pi availability to the mitochondria, by favouring substrate level phosphorylation at the succinyl-CoA synthetase step plays a role in the development of the metabolite pattern found in the mitochondrial matrix space after exposure of hepatocytes to glucagon or the above agents. With regard to the glutamate level this view is evidenced by the finding that its hormone-dependent decrease was inversely correlated to the respective increase in the cytosolic Pi concentration. Further evidence is provided by experiments with isolated mitochondria incubated under state-3 conditions at medium Pi concentrations corresponding to those metabolically active in the cytosolic compartment of control and glucagon-stimulated hepatocytes, being 2 mM and 3 mM, respectively. Increasing medium phosphate concentration from 2 mM to 3 mM caused a marked decrease in the level of succinyl-CoA and increased the rates of 2-oxoglutarate utilization and of malate and phosphoenolpyruvate production. Citrulline synthesis also was found to be stimulated at 3 mM Pi. Taken together our results suggest a role of Pi supply in mitochondrial actions of glucagon in intact hepatocytes. Moreover, they could contribute to a better interpretation of glucagon effects on isolated mitochondria from hormone-pretreated liver cells.

Concentration of free oxaloacetate in the mitochondrial compartment of isolated liver cells

The concentration of metabolically active (i.e. 'free') oxaloacetate in the mitochondrial compartment of isolated liver cells was investigated by two independent approaches. On the basis of mitochondrial aspartate aminotransferase maintaining equilibrium and the direct measurements of mitochondrial aspartate, 2-oxoglutarate and glutamate, the concentration of free oxaloacetate was calculated to be 5 microM after incubation of hepatocytes in the presence of 1.5 mM-lactate and 0.05 mM-oleate. Gradually increasing oleate up to 0.5 mM decreased the free oxaloacetate to 2 microM. Very similar results were obtained when free oxaloacetate concentration was derived from the CO2 production of hepatocytes as a measure of citrate flux through the tricarboxylic acid cycle, and the kinetic data on citrate synthase in situ. The decrease in free oxaloacetate on increasing oleate concentration was associated with lowered rates of cycle-dependent CO2 output and O2 uptake, indicating a decrease in the disposal of acetyl-CoA into the tricarboxylic acid cycle. This decrease could explain 25-30% of the increase in ketone-body production occurring at elevated fatty acid supply. This work documents on a quantitative basis the role of free oxaloacetate in the regulation of ketogenesis.

Sequential removal of outer bilayer and apical plasma membrane from the surface epithelial syncytium of Schistosoma mansoni

The outer and inner bilayers of the apical membrane complex of Schistosoma mansoni were sequentially stripped from adult worms by two incubations in 0.1% digitonin solutions. Membrane removal was evaluated by electron microscopy of worms and bilayer material, using Con A-ferritin as a marker for the outer bilayer. Amounts of Con A removed by the digests were measured with a tritiated Con A marker. To measure the purity of the fractions membrane markers were characterised and quantitated for both bilayers. In the absence of the usual enzymatic markers for plasma membrane diazotised [125I]-iodosulfanilic acid was used as a marker for the outer bilayer. Alkaline phosphatase and a Na+, Mg2+-ATPase were localised to the inner bilayer. From these results we can deduce that the inner bilayer is analogous to the typical, apical plasma membrane of other animal epithelia. The outer bilayer does not share these enzymatic similarities. The integrity of the syncytium after removal of the outer bilayer and the increased levels of lactate dehydrogenase in the supernatant after removal of the inner bilayer suggests that the outer bilayer is secondary in maintaining the permeability barrier of the apical membrane complex, with respect to soluble proteins. The possible significance of these results in terms of the destructive action of complement on the parasite are discussed.

Role of the mitochondrial metabolism of pyruvate on the regulation of ketogenesis in rat hepatocytes

In hepatocytes isolated from fed rats the inhibition of lipogenesis (-80%) by 5-tetradecyloxy-2-furoate (an inhibitor of acetylCoA carboxylase) and alpha-cyano-3-hydroxycinnamate (an inhibitor of pyruvate entry into mitochondria) increases the oxidation of 0.35 mM oleate respectively by 70% and 90%. 5-tetradecyloxy-2-furoate increases ketone body production from oleate only by 30% and has no effect on ketogenesis from octanoate, whereas alpha-cyano-3-hydroxycinnamate mimics the effects of fasting on ketone body production: It increases ketogenesis from 0.35 mM oleate by 90%, from 0.78 mM oleate by 25% and from 1.57 mM butyrate by 37%. alpha-cyano-3-hydroxycinnamate also decreases the activity of tricarboxylic acid cycle and the production of malate and citrate. In hepatocytes from fasted rats, alpha-cyano-3-hydroxycinnamate does not modify ketogenesis from oleate, unless cells are incubated with a mixture of lactate and pyruvate. A lactate and pyruvate mixture decreases ketogenesis from oleate and octanoate and increases citrate and malate production without modifying the uptake of fatty acids. This effect is potentiated by 3-mercaptopicolinate, an inhibitor of phosphoenolpyruvate carboxykinase. The results cannot be interpreted only by the effects of malonylCoA on carnitine acyltransferase. They are discussed with respect to the possible involvement of mitochondrial oxaloacetate concentration in the regulation of ketogenesis.

Gluconeogenesis in vitro. Formation of glucose 6-phosphate from malate by a cell-free rat-liver system consisting of cytosol and mitochondria

A cell-free system prepared from rat liver containing cytosol and mitochondria as well as a number of cofactors at near physiological concentrations was shown to form glucose 6-phosphate from malate + 3-phosphoglycerate at a rate of 1.11 +/- 0.09 mumol . min-1 . g liver-1 (mean +/- SEM, n = 9, 30 degrees C). At least 70% of glucose 6-phosphate formed was derived from malate as calculated from experiments with [U-14C]malate. The indicated rates were measured between 10 min and 30 min incubation time when the system was near steady state with respect to the lactate/pyruvate ratio and to most of the gluconeogenic intermediates. In the absence of mitochondria, the rate of formation of glucose 6-phosphate from malate was about seven times lower than in their presence. A comparison between incubations carried out in presence or absence of mitochondria revealed that mitochondria decreased the lactate/pyruvate ratio and increased the ratio of (ATP + ITP)/(ADP + IDP). It could be shown that under the present incubation conditions, formation of glucose 6-phosphate was closely linked to the ratio of (ATP + ITP)/(ADP + IDP) whereas changing redox ratios had little influence on the gluconeogenic rate.