Control of Mitochondrial Substrate Oxidation

Mechanism of citrinin-induced dysfunction of mitochondria. IV--Effect on Ca2+ transport

The effect of citrinin on Ca2+ transport was studied in isolated kidney cortex and liver mitochondria, and baby hamster kidney cultured cells. The mycotoxin significantly inhibited the activity of 2-oxoglutarate and pyruvate dehydrogenases in both kidney cortex and liver mitochondria. Citrinin promoted a decrease in the velocity and in the total capacity of Ca2+ uptake, in both mitochondria. Apparently, citrinin acts by a mechanism similar to ruthenium red. In intact cultured cells, citrinin also had a preferential effect on mitochondrial Ca2+ fluxes. Citrinin promoted a marked decrease in the Ca2+ level in the mitochondrial matrix, whereas that of the extramitochondiral fraction became less affected. All the observed effects were dependent on the citrinin concentration.

Proton-translocating transhydrogenase and NAD- and NADP-linked isocitrate dehydrogenases operate in a substrate cycle which contributes to fine regulation of the tricarboxylic acid cycle activity in mitochondria

H(+)-transhydrogenase (H(+)-Thase) and NADP-linked isocitrate dehydrogenase (NADP-ICDH) are very active in animal mitochondria but their physiological function is only poorly understood. This is especially so in the case of the heart and muscle, where there are no major consumers of NADPH. We propose here that H(+)-Thase and NADP-ICDH have a combined function in the fine regulation of the activity of the tricarboxylic acid (TCA) cycle, providing enhanced sensitivity to changes in energy demand. This is achieved through cycling of substrates by NAD-linked ICDH, NADP-linked ICDH and H(+)-Thase. It is proposed that NAD-ICDH operates in the forward direction of the TCA cycle, but NADP-ICDH is driven in reverse by elevated levels of NADPH resulting from the action of the transmembrane proton electrochemical potential gradient (delta p) on H(+)-Thase. This has the effect of increasing the sensitivity to allosteric modifiers of NAD-ICDH (NADH, ADP, ATP, Ca2+ etc), potentially giving rise to large changes in the net flux from iso-citrate to alpha-ketoglutarate. Furthermore, changes in the level of delta p resulting from changes in the demand for ATP would, via H(+)-Thase, shift the redox state of the NADP pool and this, in turn, would lead to a change in the rate of the reaction catalysed by NADP-ICDH and hence to an additional and complementary effect on the net metabolic flux from isocitrate to alpha-ketoglutarate. Other consequences of this substrate cycle are, (i) the production of heat at the expense of delta p, which may contribute to thermoregulation in the animal, and (ii) an increased rate of dissipation of delta p (leak).

Prevention of postischemic canine neurological injury through potentiation of brain energy metabolism by acetyl-L-carnitine

BACKGROUND AND PURPOSE

Mechanisms of ischemia/reperfusion brain injury include altered patterns of energy metabolism that may be amenable to pharmacological manipulation. The purpose of this study was to test the effectiveness of postischemic acetyl-L-carnitine administration on potentiation of metabolic recovery and prevention of neurological morbidity in a clinically relevant model of complete, global cerebral ischemia and reperfusion.

METHODS

Neurological deficit scoring as well as spectrophotometric and fluorescent assays of frontal cortex lactate and pyruvate levels were used in a canine model employing 10 minutes of cardiac arrest followed by restoration of spontaneous circulation for 2 or 24 hours.

RESULTS

Dogs treated with acetyl-L-carnitine exhibited significantly lower neurological deficit scores (p = 0.0037) and more normal cerebral cortex lactate/pyruvate ratios than did vehicle-treated control animals.

CONCLUSIONS

Postischemic administration of acetyl-L-carnitine potentiates normalization of brain energy metabolites and substantially improves neurological outcome in a clinically relevant model of global cerebral ischemia and reperfusion.

Bound and determined: a computer program for making buffers of defined ion concentrations

A computer program that allows the preparation of buffers containing known concentrations of metal-ligand complexes at defined pH values and temperatures is described. Ligands are defined as compounds that bind metals and may include AMP, ADP, ATP, GMP, GDP, GTP, EGTA, EDTA, BAPTA, phosphate, sulfate, chloride, monocarboxylic acids, dicarboxylic acids, organophosphates, and/or citric acid. Metals may include sodium, potassium, magnesium, calcium, and/or manganese. The program uses association constants corrected for temperature and ionic strength so that solutions between 0 and 40 degrees C and between pH values of 4 and 10 can be defined. The program can perform the following: (i) calculate the concentration of all metal-ligand complexes when total metal and total ligand concentrations are known, (ii) calculate the concentration of metal ion required to make a solution of known free metal ion concentration when total ligand concentrations are known, (iii) calculate the concentration of ligand required to make a solution of known free metal ion concentration when total metal concentrations are known, and (iv) calculate the total concentrations of metal and ligand required to make a buffer of known metal-ligand concentration. Options i-iii are useful for making buffers of defined free metal ion concentrations; option iv is useful for making buffers of defined metal-nucleotide concentrations.

Indexing tricarboxylic acid cycle flux in intact hearts by carbon-13 nuclear magnetic resonance

Although the tricarboxylic acid (TCA) cycle is the prime means of carbon metabolism for energy generation in normal myocardium, the noninvasive quantification of TCA cycle flux in intact cardiac tissues is difficult. A novel approach for estimating citric acid cycle flux using 13C nuclear magnetic resonance (NMR) is presented and evaluated experimentally by comparison with measured myocardial oxygen consumption over a wide range of cardiac contractile function in intact, beating rat hearts. Continuous series of 13C NMR spectra, obtained after the introduction of [2-13C]acetate as substrate, quantified the time course of 13C appearance in the carbon positions of myocardial glutamate, which are sequentially enriched via citric acid cycle metabolism. A TCA cycle flux parameter was calculated using the premise that TCA cycle flux is inversely proportional to the time difference between 13C appearance in the C-4 and C-2 positions of glutamate (glutamate delta t50 [minutes]), which are enriched in subsequent "turns" of the TCA cycle. This TCA cycle flux parameter, termed KT, correlated strongly with myocardial oxygen consumption over a range of developed pressures in hearts perfused with 5 mM acetate (r = 0.98, p less than 0.001), as well as in separate studies in hearts perfused with 5 mM glucose and 0.5-0.8 mM acetate (r = 0.94, p less than 0.001). Results of numerical modeling of 13C glutamate kinetics suggest that this TCA cycle flux parameter, KT, is relatively insensitive to changes in metabolite pool sizes that could occur during metabolism of other substrates or during conditions of altered oxygen availability. Additional studies in separate hearts indicated that the time course of 13C appearance in citrate, which is predominantly mitochondrial in the rat heart, is similar to that in glutamate, further supporting the premise that the described 13C NMR parameters reflect mitochondrial citric acid cycle activity in intact cardiac tissues.

The administration of triiodothyronine to rats results in a lowering of the mitochondrial membrane potential in isolated hepatocytes

Although thyroid status has been shown to influence the magnitude of the membrane potential in isolated rat-liver mitochondria, there is variation in the reported size and direction of the thyroid hormone-induced change relative to the normal state. Measurement of the mitochondrial membrane potential in intact hepatocytes isolated from hyperthyroid and euthyroid rats reveals that hyperthyroidism results in a decrease of approximately 30 mV in the magnitude of this potential relative to that in the euthyroid state. As well, the magnitude of the plasma membrane potential of hepatocytes from hyperthyroid rats is reduced by 6 mV compared with that in cells from euthyroid rats. The thyroid hormone-induced decrease in these potentials may reflect reported changes in the lipid composition of the membranes.

Dehydrogenase activation by Ca2+ in cells and tissues

The activation of intramitochondrial dehydrogenases by Ca2+ provides a link between the intensity of work performance by a tissue and the activity of pyruvate dehydrogenase and the tricarboxylate cycle, and hence the rate of ATP production by the mitochondria. Several aspects of this model of the control of oxidative phosphorylation are examined in this article, with particular emphasis on mitochondrial functioning in situ in cardiac myocytes and in the intact heart. Recent use of the fluorescent Ca2+ chelating agents indo-1 and fura-2 has allowed a more quantitative description of the dependence of dehydrogenase activity upon concentration of free intramitochondrial Ca2+, in experiments with isolated mitochondria. Further, a novel technique developed by Miyata et al. has allowed description of free intramitochondrial Ca2+ within a single cardiac myocyte, and the conclusion that this parameter changes in response to electrical excitation of the cell over a range which would be expected to give substantial modulation of dehydrogenase activity.

Distribution of control of oxidative phosphorylation in mitochondria oxidizing NAD-linked substrates

The flux control distribution of the net rate of state 3 respiration was determined in heart and kidney mitochondria incubated with low concentrations of pyruvate (0.5 mM) or 2-oxoglutarate (1 mM), and in conditions that led to activation of NAD-linked dehydrogenases, i.e., high substrate or Ca2+ concentrations. Control of flux was exerted by the ATP/ADP carrier (flux control coefficient, ci = 0.37) and Site 1 of the respiratory chain (ci = 0.28) when dehydrogenase activity was low. Control of the process shifted to the ATP synthase (ci = 0.32) and the Pi carrier (Ci = 0.27) when dehydrogenases were activated by high pyruvate and high Ca2+. The changes in the control exerted by the ATP/ADP carrier and the ATP synthase were not due to changes in the transmembrane potential, nor to a modification of intramitochondrial ATP/ADP ratios. Applying the summation theorem of the control analysis, it was found that at low Ca2+ and pyruvate concentrations the dehydrogenases shared the control of state 3 respiration with other steps. The NAD-linked dehydrogenases did not exert any significant control at high Ca2+ or high pyruvate concentrations.

Characterization and molecular properties of 2-oxoglutarate decarboxylase from Euglena gracilis

2-Oxoglutarate decarboxylase was purified to homogeneity, as judged by polyacrylamide gel electrophoresis. It had a molecular weight of 250,000 and consisted of four identical subunits of molecular weight 62,000. The enzyme was specific for 2-oxoglutarate, but not for other 2-oxo acids such as pyruvate and oxalacetate. Thiamin pyrophosphate and MgCl2 were required for maximum activity. The Km values of the enzyme for 2-oxoglutarate, thiamin pyrophosphate, and MgCl2 were 330, 56, and 93 microM, respectively. 2-Mercaptoethanol and NADP+ augmented significantly the enzyme activity. The amino acid composition and amino acid sequence of the amino-terminal region of 2-oxoglutarate decarboxylase were determined. On ouchterlony double-immunodiffusion gels, the anti-2-oxoglutarate decarboxylase antibody gave sharp precipitin lines against the mitochondrial fraction of E. gracilis and the purified 2-oxoglutarate decarboxylase, but not against pyruvate decarboxylase from Saccharomyces cerevisiae. On Immunoblots of the crude extract of Euglena, the antibody recognized two polypeptides whose molecular weights were 62,000 and 65,000, respectively. The polypeptide with the molecular weight of 62,000 was found only in mitochondrial fractions. In vitro translation of Euglena polyadenylated RNA in a cell-free rabbit reticulocyte lysate system explained the formation of a single polypeptide with a molecular weight of 65,000, suggesting that a putative precursor of 2-oxoglutarate decarboxylase which is about 3000 larger than the subunit of the mature enzyme is synthesized in Euglena cells.

Carbohydrate supplementation attenuates IMP accumulation in human muscle during prolonged exercise

The effect of carbohydrate (CHO) ingestion on metabolic responses to exercise has been investigated. Subjects cycled at approximately 70% of maximal oxygen uptake to fatigue [135 +/- 17 (+/- SE) min] on the first occasion (control, CON) and at the same work load and duration on the second occasion but with addition of ingestion of CHO during the exercise. Biopsies were taken from the quadriceps femoris muscle before and after exercise. The sum of the hexose monophosphates (HMP), as well as lactate and alanine, in muscle was higher after CHO exercise (P less than or equal to 0.05, P less than or equal to 0.05, and P less than or equal to 0.01, respectively). Acetylcarnitine increased during exercise but was not significantly different between treatments after exercise (CON, 6.6 +/- 1.7; CHO, 10.0 +/- 1.2 mmol/kg dry wt; P = NS). The sum of the tricarboxylic acid cycle intermediates (TCAI; citrate + malate + fumarate) was increased during exercise and was higher after CHO exercise (2.34 +/- 0.32 vs. 1.68 +/- 0.17 mmol/kg dry wt; P less than or equal to 0.05). IMP was less than 0.1 mmol/kg dry wt at rest and increased to 0.77 +/- 0.26 (CON) and 0.29 +/- 0.11 mmol/kg dry wt (CHO) (P less than or equal to 0.05) during exercise. It was recently found that during prolonged exercise there is initially a rapid and large expansion of TCAI and glycogenolytic intermediates in human muscle followed by a continuous decline in TCAI and glycogenolytic intermediates [K. Sahlin, A. Katz, and S. Broberg. Am. J. Physiol. 259 (Cell Physiol. 28): C834-C841, 1990].(ABSTRACT TRUNCATED AT 250 WORDS)

Control of respiration and oxidative phosphorylation in isolated rat liver cells

NAD(P)H fluorescence, mitochondrial membrane potential and respiration rate were measured and manipulated in isolated liver cells from fed and starved rats in order to characterize control of mitochondrial respiration and phosphorylation. Increased mitochondrial NADH supply stimulated respiration and this accounted for most of the stimulation of respiration by vasopressin and extracellular ATP. From the response of respiration to NADH it was estimated that the control coefficient over respiration of the processes that supply mitochondrial NADH was about 0.15-0.3 in cells from fed rats. Inhibition of the ATP synthase with oligomycin increased the mitochondrial membrane potential and decreased respiration in cells from fed rats, while the uncoupler carbonyl cyanide p-trifluoromethoxyphenylhydrazone had the opposite effect. There was a unique relationship between respiration and membrane potential irrespective of the ATP content of the cells indicating that phosphorylation potential controls respiration solely via phosphorylation (rather than by controlling NADH supply). From the response of respiration to the mitochondrial membrane potential (delta psi M) it was estimated that the control coefficients over respiration rate in cells from fed rats were: 0.29 by the processes that generate delta psi M, 0.49 by the process of ATP synthesis, transport and consumption, and 0.22 by the processes that cycle protons across the inner mitochondrial membrane other than via ATP synthesis (e.g. the passive proton leak). Control coefficients over the rate of mitochondrial ATP synthesis were 0.23, 0.84 and -0.07, respectively, by the same processes. The control distribution in cells from starved rats was similar.

Activation of pyruvate dehydrogenase by electrical stimulation, and low-Na+ perfusion of guinea-pig heart

(1) Electrical stimulation (2 Hz) of guinea-pig hearts, perfused with medium containing 11 mM D-glucose plus 0.1 mM octanoate as substrate, resulted in an increase in the percentage of pyruvate dehydrogenase in the active form (PDHa) from 16 to 68%. (2) Rapid isolation of mitochondria by a technique designed to minimize net loss or gain of Ca2+ revealed an increase in mitochondrial Ca2+ content of the stimulated hearts, as measured with 45Ca (2.74 +/- 0.27 versus 1.37 +/- 0.11 nmol/mg protein; stimulated versus rested). (3) Perfusion of rested hearts with a medium containing a reduced Na+ concentration (20 mM, with the remainder replaced with Li+) also gave increased values of PDHa content (30.9% versus 16% for the normal, physiological medium). This procedure is known to raise cytosol Ca2+ concentrations and would be expected to give mitochondrial Ca2+ loading. (4) These results are consistent with a role of mitochondrial Ca2+ in activating pyruvate dehydrogenase in the intact heart.

The contribution of the leak of protons across the mitochondrial inner membrane to standard metabolic rate

This paper presents and assesses the hypothesis that the proton leak across the mitochondrial inner membrane is an important contributor to standard metabolic rate, and that increases in the amount of mitochondrial inner membrane may be important in causing changes in proton leak and in the standard metabolic rate. The standard metabolic rate of an animal is known to be a function of body mass, phylogeny and thyroid status, and is largely attributed to the metabolically active internal organs. The total area of mitochondrial inner membrane in these organs correlates well with standard metabolic rate over a wide range of body masses in both ectotherms and endotherms. In hepatocytes isolated from rats, proton leak across the mitochondrial inner membrane accounts for about 30% of the resting oxygen consumption, and the distribution of control over respiration suggests that changes in mitochondrial inner membrane surface area will be accompanied by significant changes in the proton leak. This change in the leak will result in significant changes in resting oxygen consumption, but changes in ATP demand may also have a role to play in determining resting respiration rate. Extrapolation of these results to other tissues and other animals suggests that the hypothesis has the potential to explain a substantial proportion of the variation in standard metabolic rate with body mass, phylogeny and thyroid status. However, in most cases the quantitative contribution of proton leak compared to cellular ATP turnover has yet to be experimentally determined.

Regulation of oxidative phosphorylation in the mammalian cell

The cell is capable of maintaining a steady-state flux of energy from mitochondrial oxidative phosphorylation, producing ATP, to the cytosolic adenosinetriphosphatases (ATPases), performing work. Considerable effort has been devoted to investigating the individual mechanisms involved in these two processes. However, less effort has been directed toward learning how these reactions of energy metabolism interact through the cytosol to maintain the observed steady state in the intact cell. The "classical" model for the cytosolic interaction of these two processes involves the feedback of ATP hydrolysis products, ADP and Pi, from the ATPases to oxidative phosphorylation. This model is based on data from isolated mitochondria in which the rate of oxidative phosphorylation is controlled by the concentration of ADP and Pi. Yet, recent data from intact tissues with high oxidative phosphorylation capacities (i.e., heart, brain, and kidney) indicate that the cytosolic concentration of ADP and Pi do not change significantly with work. These data imply that this simple feedback model is not adequate to explain the regulation of energy metabolism in these tissues. Other sites within the oxidative phosphorylation process must be playing a regulatory role or the kinetics of ATP synthesis must be very different than currently believed to establish the steady state. This review covers the potential sites within oxidative phosphorylation which may be regulated through cytosolic transducers to result in the necessary feedback network regulating the steady-state flow of energy in the cell. These sites will include substrate delivery to the cytochrome chain, the processes involved in the phosphorylation of ADP to ATP, and the delivery of oxygen.

Muscle carnitine metabolism during incremental dynamic exercise in humans

The changes in muscle content of carnitine and acetylcarnitine have been studied during incremental dynamic exercise. Six subjects exercised for 10 min on an ergometer at 40 and 75% of their maximal oxygen uptake (VO2 max) and to fatigue at 100% of VO2 max (about 4 min). Muscle samples were taken from the quadriceps femoris muscle at rest and after exercise. Muscle content of free carnitine was (means +/- SE) 15.9 +/- 1.7 mmol kg-1 d.wt (dry weight) at rest and remained unchanged after exercise at low intensity but decreased to 5.9 +/- 0.6 and 4.6 +/- 0.5 mmol kg-1 d.wt after exercise at 75 and 100% of VO2 max respectively. Acetylcarnine content at rest was 6.9 +/- 1.9 mmol kg-1 d.wt and increased during exercise in correspondence with the decrease in free carnitine. Muscle content of pyruvate and lactate was unchanged after exercise at 40% of VO2 max but increased at the higher intensities. The parallel increases in acetylcarnitine, pyruvate and lactate indicate that formation of acetylcarnitine is augmented when the availability of glycolytic three-carbon metabolites is high and is consistent with the idea that acetylcarnitine provides a sink for pyruvate and acetyl CoA. This could be of importance for the maintenance of an adequate level of CoA and thus function of the tricarboxylic acid cycle.

The regulation of the matrix volume of mammalian mitochondria in vivo and in vitro and its role in the control of mitochondrial metabolism

The purpose of this article is to describe briefly the methods by which the intra-mitochondrial volume may be measured both in vitro and in situ, to summarise the mechanisms thought to regulate the mitochondrial volume and then to review in more detail the evidence that changes in the intra-mitochondrial volume play an important part in the regulation of liver mitochondrial metabolism by glucogenic hormones such as glucagon, adrenaline and vasopressin. It will be shown that these hormones cause an increase in matrix volume sufficient to produce significant activation of fatty acid oxidation, respiration and ATP production, pyruvate carboxylation, citrulline synthesis and glutamine hydrolysis. These are all processes activated by such hormones in vivo. I will go on to demonstrate that the increase in matrix volume is brought about by an increase in mitochondrial [PPi]. This is able to stimulate K+ entry into the matrix, perhaps through an interaction with the adenine nucleotide translocase. The rise in matrix [PPi] is a consequence of an increase in cytosolic and hence mitochondrial [Ca2+] which inhibits mitochondrial pyrophosphatase. In the final section of the review I provide evidence that changes in mitochondrial volume may be important in the responses of a variety of tissues to hormones and other stimuli. I write as a metabolist with a working knowledge of bioenergetics rather than the converse, and this will certainly be reflected in the approach taken. If I cause offence to any dedicated experts in the field of bioenergetic by my ignorance or lack of understanding of their studies I can only offer my apologies and ask to be corrected.

Myocardial contractile function during ischemia and hypoxia

There is good evidence that elevated [Ca2+]i, produced by an influx of Ca2+ in exchange for Na+, is the underlying pathology in reperfusion or reoxygenation damage. Further measurements of [Na+]i and [Ca2+]i during ischemia and reperfusion, coupled with information about metabolic levels, are needed to confirm or refute this hypothesis. Contributions to cell damage by other mechanisms, e.g., oxygen free radicals, certainly cannot yet be excluded.

Regulation of energy metabolism in pancreatic islets by glucose and tolbutamide

The kinetics of insulin secretion and oxygen uptake in response to D-glucose and tolbutamide were compared in mouse pancreatic islets. In addition, the role of decreased ATP as a driving force for secretagogue-induced oxygen consumption was examined. D-glucose (10-30 mmol/l) triggered a biphasic insulin release which always coincided with a monophasic increase in islet oxygen uptake. In the presence of D-glucose (5-30 mmol/l), tolbutamide (3-500 mumol/l) consistently elicited an initial peak of insulin secretion which was followed by a continued decline. Tolbutamide-induced secretory profiles were accompanied by similar respiratory profiles. Oxygen consumption per ng of insulin released during the test phase was higher after elevation of the glucose concentration than after addition of tolbutamide. In conjunction with 5 or 10 mmol/l D-glucose, but not with 15 or 30 mmol/l D-glucose, tolbutamide (30-100 mumol/l) lowered islet ATP content significantly (p less than 0.02). Phosphocreatine was not found in isolated islets, although they contained substantial creatine kinase activity. It is concluded that the driving force for tolbutamide-induced oxygen uptake is a decrease in the phosphorylation potential caused by the work load imposed by stimulation of the secretion process. However, a major proportion of the respiratory response to glucose also results from enhancement of biosynthesis.

31P NMR saturation transfer measurements of phosphorus exchange reactions in rat heart and kidney in situ

31P NMR spectra of rat kidney and heart, in situ, were obtained at 97.2 MHz by using chronically implanted radio-frequency coils. Previous investigators have used magnetization transfer techniques to study phosphorus exchange in perfused kidney and heart. In the current experiments, saturation transfer techniques were used to measure the steady-state rate of exchange between inorganic phosphate (Pi) and the gamma-phosphate of ATP (gamma ATP) in kidney, and between phosphocreatine (PCr) and gamma ATP, catalyzed by creatine kinase, in heart. The rate constant for the exchange detected between Pi and gamma ATP in kidney, presumably catalyzed by oxidative phosphorylation, was 0.12 +/- 0.03 s-1. This corresponds to an ATP synthesis rate of 12 mumol min-1 (g wet weight)-1. Comparison of previously published O2 consumption and Na+ reabsorption rates for the intact kidney with the NMR-derived rate for ATP synthesis gave flux ratios of JATP/JO2 = 1.6-3.3 and JNa+/JATP = 4-10. The rate constants for the creatine kinase reaction, assuming a simple two-site exchange, were found to be 0.57 +/- 0.12 s-1 for the forward direction (PCr----ATP) and 0.50 +/- 0.16 s-1 for the reverse direction (ATP----PCr). The forward rate (0.78 +/- 0.18 intensity unit/s) was significantly larger (p less than 0.05) than the reverse rate (0.50 +/- 0.16 intensity unit/s). This difference between the forward and reverse rates of creatine kinase has been previously noted in the perfused heart. The difference has been attributed to participation of ATP in other reactions.(ABSTRACT TRUNCATED AT 250 WORDS)

Mitochondrial respiratory control in the myocardium

The heart muscle has proved to be a practical model for studying respiratory control in intact tissues. It also demonstrates that control at the level of the respiratory chain is augmented by metabolic control at the substrate level as exemplified by the very narrow range of changes in the redox state of the mitochondrial NADH/NAD couple even during extensive changes in ATP and oxygen consumption. The behaviour of mitochondria when isolated can largely be duplicated in the intact myocardium. Moreover, the high intracellular concentrations of enzymes, coenzymes and adenine nucleotides create conditions of high reaction rates, enabling the formation of a near equilibrium network of certain main pathways. This equilibrium network in connection with metabolic regulation of the hydrogen pressure upon the matrix NADH/NAD pool is a prerequisite for the regulation of cellular respiration at a high efficiency of energy transfer. Experimentation on the intact myocardium also seems to be capable of resolving some of the uncertainties about prevailing mechanisms for the regulation of cellular respiration.

Inhibition of 2-oxoglutarate oxidation in plant mitochondria by pyruvate

Oxidation of 2-oxoglutarate (in the presence of malonate) by mitochondria isolated from turnip, pea leaf and cauliflower tissue was dramatically inhibited by micromolar concentrations of pyruvate. Pyruvate, however, had little or no effect on 2-OG oxidation when carried out in the absence of malonate. The inhibition was reversed by alpha-cyano-4-hydroxycinnamic acid, indicating pyruvate uptake into the matrix was required for the inhibitory effect. In contrast, pyruvate had no effect on 2-oxoglutarate oxidation by mitochondria isolated from rat heart. The possible significance of the effect in terms of the control of 2-oxoglutarate dehydrogenase activity during the operation of a malate/aspartate shuttle in plant mitochondria is discussed.

Inhibition and activation of bovine heart NAD-specific isocitrate dehydrogenase by ATP

In the absence of added calcium, inhibition of NAD-specific isocitrate dehydrogenase by ATP occurred without ADP (I0.5 = 1.8 mM) and with 0.2 mM ADP3- (I0.5 = 1.0 mM) at subsaturating substrate concentrations at pH 7.4. Inhibition by ATP was competitive with NAD+ in the presence and absence of ADP and was not reversed by magnesium citrate. No reversal of ATP inhibition by free Ca2+ was observed in the presence of ADP (0.2 mM). However, when ADP was absent, increasing Ca2+ first caused progressive reversal of ATP inhibition followed by activation by ATP. Without ADP, the S0.5 for calcium activation was 80-140 microM at ATP concentrations between 0.6 and 3.0 mM. The S0.5 for ATP activation, in the absence of ADP, was 1.1 and 2.1 microM when free Ca2+ was held constant at 0.1 and 1.0 mM, respectively. As in activation by ADP, ATP decreased the S0.5 for magnesium isocitrate without affecting V. However, in contrast to ADP, the activation by ATP occurred without lowering the Hill coefficient for the substrate. GDP activated the enzyme at relatively high concentrations of Ca2+ but not without added Ca2+.

A minimal hypothesis for membrane-linked free-energy transduction. The role of independent, small coupling units

Experimental data are reviewed that are not in keeping with the scheme of 'delocalized' protonic coupling in membrane-linked free-energy transduction. It turns out that there are three main types of anomalies: (i) rates of electron transfer and of ATP synthesis do not solely depend on their own driving force and on delta mu H, (ii) the ('static head') ratio of delta Gp to delta mu H varies with delta mu H and (iii) inhibition of either some of the electron-transfer chains or some of the H+-ATPases, does not cause an overcapacity in the other, non-inhibited proton pumps. None of the earlier free-energy coupling schemes, alternative to delocalized protonic coupling, can account for these three anomalies. We propose to add a fifth postulate, namely that of the coupling unit, to the four existing postulates of 'delocalized protonic coupling' and show that, with this postulate, protonic coupling can again account for most experimental observations. We also discuss: (i) how experimental data that might seem to be at odds with the 'coupling unit' hypothesis can be accounted for and (ii) the problem of the spatial arrangement of the electrical field in the different free-energy coupling schemes.

A re-evaluation of conditions required for an accurate estimation of the extramitochondrial ATP/ADP ratio in isolated rat-liver mitochondria

The values reported in the literature for the extramitochondrial ATP/ADP ratio in resting rat-liver mitochondria (State 4) vary widely. The conditions required for an accurate determination of this parameter were therefore investigated. In experiments with rat-liver mitochondria incubated under State-4 conditions, it was found that the extramitochondrial ATP/ADP ratio, as calculated from the values measured in neutralised perchloric acid extracts, was lower than that estimated from the concentrations of creatine and creatine phosphate, using the metabolite indicator method. The discrepancy is due to hydrolysis of ATP occurring in the presence of perchloric acid. Conditions are described for minimising ATP hydrolysis in the presence of perchloric acid, and include the use of low concentrations of perchloric acid, short times of exposure to the acid before neutralisation, low temperatures and the presence of excess EDTA. Under these conditions, the values obtained for the extramitochondrial ATP/ADP ratio agreed with those calculated by the metabolite indicator method, provided ratios do not exceed the value of 100. In cases where the extramitochondrial ATP/ADP does exceed 100, phenol/chloroform/isoamyl alcohol must be used to quench the reactions, as described by Slater et al. (Slater, E.C., Rosing, J. and Mol, A. (1973) Biochim. Biophys. Acta 292, 534-553). With this method, the extramitochondrial ATP/ADP ratio was found to have a value of more than 1000 in rat-liver mitochondria incubated with succinate + rotenone in the resting state (pH 7.0; T = 37 degrees C), in agreement with Slater et al.

Inhibition of bovine heart NAD-specific isocitrate dehydrogenase by reduced pyridine nucleotides: modulation of inhibition by ADP, NAD+, Ca2+, citrate, and isocitrate

The activity of NAD-dependent isocitrate dehydrogenase from bovine heart was inhibited by NADH (apparent Ki about 4.3 microM) and NADPH (Ki about 9.8 microM) at subsaturating substrate concentrations at pH 7.4. The inhibition by NADH or NADPH was reversed competitively by magnesium isocitrate in the presence of ADP, but not without ADP. Reversal of inhibition by NADH or NADPH with respect to NAD+ was competitive or of the linear mixed type depending on whether ADP was absent or present. ADP3- (0.2 mM) increased the Ki(app) for NADPH from 9.8 to 27.1 microM; further addition of Ca2+ (0.2 mM) raised the Ki(app) to 127 microM. For the modification of NADPH inhibition by ADP, S0.5 for Ca2+ was approximately 48 microM. This compares to the Km for Ca2+ of 0.3-1 microM for the activation of the enzyme without NADPH [Denton, R. M., Richards, D. A., & Chin, J. G. (1978) Biochem. J. 176, 899-906; Aogaichi, T., Evans, J., Gabriel, J., & Plaut, G. W. E. (1980) Arch. Biochem. Biophys. 204, 350-360]. ADP did not affect the Ki for NADH. Magnesium citrate, which was about 100-fold more effective as a positive modifier of the enzyme with ADP than without ADP [Gabriel, J. L., & Plaut, G. W. E. (1983) Fed. Proc., Fed. Am. Soc. Exp. Biol. 42, 2082], reversed competitively the inhibition by NADPH in the presence of ADP, but not without ADP. Magnesium citrate did not reverse NADH inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

Six blind men explore an elephant: aspects of fuel metabolism and the control of tricarboxylic acid cycle activity in heart muscle

Many aspects of the interactions of energy providing substrates and the operational control of the tricarboxylic acid cycle are still unclear. This statement is well supported by the sometimes conflicting observations in heart muscle. The paper discusses some of the knowns and unknowns of tricarboxylic acid cycle regulation and discusses mechanisms of metabolite accumulation in this tissue. It is concluded that the data available at this time are insufficient to propose a unifying concept.

Steady-state kinetics of the overall oxidative phosphorylation reaction in heart mitochondria. Determination of the coupling relationships between the respiratory reactions and miscellaneous observations concerning rate-limiting steps

The linear sequence of steps involved in the oxidation of extramitochondrial succinate by O2 in bovine heart mitochondria was examined by a steady-state kinetic method to determine whether or not freely diffusible intermediates occur between the various inhibitor-sensitive steps. The kinetic method is based on the facts (1) that if two inhibitor-sensitive steps within a sequence are linked by a freely diffusible intermediate, inhibition of one will make the other less rate limiting in the overall reaction and thus will increase the amount of inhibitor of the other step required for half-maximal inhibition of the overall reaction, and (2) that if the two steps are not linked in this manner, inhibition of one will make the other more rate limiting and thus will decrease the amount of inhibitor of the other required for half-maximal inhibition. These two types of "coupling relationships" between steps were designated as "sequential" and "fixed," respectively. The results indicate the existence of freely diffusible intermediates (sequential coupling relationships) between the succinate transport and succinate dehydrogenase reactions, between the succinate dehydrogenase and cytochrome bc1 reactions, and between the cytochromes bc1 and aa3 reactions. Uncoupling respiration from phosphorylation results in the coupling relationship between the bc1 and aa3 reactions becoming partially fixed. This change is accompanied by marked decreases in the degrees to which the bc1 and aa3 reactions limit the overall reaction and appears to account for the large uncoupler-induced releases of inhibition at the levels of the bc1 and aa3 reactions observed previously by others. It is suggested that cytochrome c is the freely diffusible intermediate between the bc1 and aa3 reactions and that the uncoupler-induced changes occur as a result of formation of functional and highly efficient supercomplexes between cytochrome c and the cytochromes bc1 and aa3 complexes.

Reconstruction of steady state in cell-free systems. Interactions between glycolysis and mitochondrial metabolism: regulation of the redox and phosphorylation states

A reconstituted "open" system comprising respiring mitochondria and actively glycolyzing muscle extract was devised for studies of vectorially mediated interactions. Glycogen particles were the substrate for the glycolyzing enzymes. Purified soluble (F1) ATPase was added in varying quantities to establish a range of energetic steady states. The data generally confirm our recent conclusions (Wu and Davis, (1981) Arch. Biochem. Biophys. 208, 85-89) on the relative efficacy of the adenine nucleotides and their ratios, and of inorganic phosphate on flux through rate-controlling steps of glycolysis. When mitochondrial ATP synthesis was blocked, glycolytic flux was relatively rapid, and the lactate/pyruvate ratio increased with time to values up to greater than 300. If functional mitochondria were present, glycolytic flux was very strongly suppressed, provided the energy state (ATP/ADP) was high, and the phosphate concentration[Pi] was low. Adenine nucleotide control of glycolysis was to a large extent lost when the steady-state ATP/ADP was below about 10, or if [Pi] was elevated. In the two-phase system containing respiring mitochondria and components of the malate-aspartate shuttle, the ATP/ADP and both extra- and intramitochondrial NAD+/NADH ratios were maintained constant, and to various perturbable levels with varying energy load (ATPase). The gradient in reduction potentials attained values (delta Gredox) of up to about 2.5 kcal. The extramitochondrial redox state was not positively correlated with the external phosphorylation potential ([ATP]/[ADP] X [Pi]). The following conclusions are drawn on the basis of the present data, together with other reports (Davis, Bremer, and Akerman (1980) J. Biol. Chem. 255, 2277-2283) and (Klingenberg and Rottenberg (1977) Eur. J. Biochem. 73, 125-130): (a) the gradient in reduction potential is driven by the membrane potential (delta psi), mediated by the electrogenic glutamate-aspartate exchange, and the poise or set point of this gradient is a function of delta psi; and (b) the gradient of ATP/ADP ratios across the membrane is also driven principally by delta psi, mediated by the electrogenic ATP-ADP exchange. Hence, segregation of phosphorylation and reduction potentials is linked through a mutually shared electrical driving force.

Metabolic control and compartmentation in single living cells

Microspectrofluorometry of cell coenzymes (NAD(P)H, flavins) in conjunction with sequential microinjections into the same cell of metabolites and modifiers, reveals aspects of the regulatory mechanisms of transient redox changes of mitochondrial and extramitochondrial nicotinamide adenine dinucleotides. The injection of ADP in the course of an NAD(P)H transient produced by glycolytic (e.g. glucose 6-phosphate, G6P) or mitochondrial (e.g. malate) substrate leads to sharp reoxidation (state III, Chance and Williams, 1955), followed by a spontaneous state III to IV transition, and an ultimate return to original redox steady state. The response to ADP alone is biphasic, i.e. a small oxidation-reduction transient followed by a larger reverse transient. Similarities between responses to injected ATP and ADP suggest possible intracellular interconversions. Sequential injections of glycolytic and Krebs cycle substrates into the same cell, produce a two-step NAD(P) response, possibly revealing the intracellular compartmentation of this coenzyme. A two-step NAD(P)H response to sequentially injected fructose 1,6-diphosphate and G6P indicates the dynamic or even structural compartmentation of glycolytic phosphate esters in separate intracellular pools. The intracellular regulation and compartmentation of bioenergetic pathways and cell-to-cell metabolic inhomogeneities provide the basis on which the quantitative biochemistry of the intact living cell may be reconciled with these in situ findings.

Selective increase in pericentral oxygen gradient in perfused rat liver following ethanol treatment

The oxygen gradient across the liver lobule was measured in isolated, hemoglobin-free perfused livers from control and ethanol-treated rats using micro-light guides and miniature oxygen electrodes. Both techniques yielded similar values for the lobular, periportal and pericentral oxygen gradients. Oxygen uptake increased 33% in livers from rats treated with ethanol for 4 to 6 weeks. In addition, the pericentral oxygen gradient increased by 100%, but the periportal gradient was unchanged. These observations suggest that chronic ethanol treatment may increase the rate of cellular respiration in pericentral regions of the liver lobule selectively. One possible explanation for a region-specific effect of ethanol may be ethanol-induced inhibition of glycolysis which predominates in pericentral hepatocytes.

Ketone-body utilization by homogenates of adult rat brain

The regulation of ketone-body metabolism and the quantitative importance of ketone bodies as lipid precursors in adult rat brain has been studied in vitro. Utilization of ketone bodies and of pyruvate by homogenates of adult rat brain was measured and the distribution of 14C from [3-14C]ketone bodies among the metabolic products was analysed. The rate of ketone-body utilization was maximal in the presence of added Krebs-cycle intermediates and uncouplers of oxidative phosphorylation. The consumption of acetoacetate was faster than that of D-3-hydroxybutyrate, whereas, pyruvate produced twice as much acetyl-CoA as acetoacetate under optimal conditions. Millimolar concentrations of ATP in the presence of uncoupler lowered the consumption of ketone bodies but not of pyruvate. Indirect evidence is presented suggesting that ATP interferes specifically with the mitochondrial uptake of ketone bodies. Interconversion of ketone bodies and the accumulation of acid-soluble intermediates (mainly citrate and glutamate) accounted for the major part of ketone-body utilization, whereas only a small part was oxidized to CO2. Ketone bodies were not incorporated into lipids or protein. We conclude that adult rat-brain homogenates use ketone bodies exclusively for oxidative purposes.